Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering

4044 Chem. Rev. 2006, 106, 4044 4098 Synthesis of Trnsporttion Fuels from Biomss: Chemistry, Ctlysts, nd Engineering George W. Huber, Sr Iborr, nd Avelino Corm* Instituto de Tecnologí Químici, UPV-CSIC, Universidd Politénic de Vlenci, Avd. de los Nrnjos, s/n, Vlenci, Spin Received Februry 3, 2006 Contents 1.0. Introduction 4044 2.0. Biomss Chemistry nd Growth Rtes 4047 2.1. Lignocellulose nd Strch-Bsed Plnts 4047 2.2. Triglyceride-Producing Plnts 4049 2.3. Alge 4050 2.4. Terpenes nd Rubber-Producing Plnts 4052 3.0. Biomss Gsifiction 4052 3.1. Gsifiction Chemistry 4052 3.2. Gsifiction Rectors 4054 3.3. Supercriticl Gsifiction 4054 3.4. Solr Gsifiction 4055 3.5. Gs Conditioning 4055 4.0. Syn-Gs Utiliztion 4056 4.1. Hydrogen Production by Wter Gs Shift 4056 Rection 4.2. Methnol Production by Methnol Synthesis 4057 4.3. Alkne Production by Fischer Tropsch 4058 Synthesis 4.4. Other Syn-Gs Rections 4059 4.5. Anlysis of Syn-Gs Processes 4059 5.0. Bio-Oil Production 4060 5.1. Bio-Oils by Fst Pyrolysis 4061 5.2. Bio-Oils by Liquefction 4063 5.3. Bio-Oil Chemistry 4063 5.4. Bio-Oil Problems 4066 5.5. Economics nd Therml Efficiencies of 4066 Bio-Oil Production Methods 6.0. Bio-Oil Upgrding 4068 6.1. Hydrodeoxygention 4068 6.2. Zeolite Upgrding of Bio-Oils 4068 6.3. Bio-Oil Mixtures 4070 6.4. Stem Reforming of Bio-Oils 4070 6.5. Stem Reforming of Chrs 4071 6.6. Economic nd Therml Anlysis of Processes 4071 for Bio-Oil Upgrding 7.0. Biomss Monomer Production 4071 7.1. Pretretment 4071 7.2. Hydrolysis 4073 7.3. Levulinic Acid 4075 7.4. Hydrogention/Hydrolysis 4076 8.0. Sugr Conversion into Fuels 4076 8.1. Ethnol Production 4076 8.2. Zeolite Upgrding of Sugrs 4078 * Corresponding uthor. E-mil: corm@itq.upv.es. 8.3. Aqueous-Phse Processing 4078 8.4. Supercriticl Reforming of Sugrs 4081 8.5. Biologicl Hydrogen nd Methne Production 4082 9.0. Conversion of Nonsugr Monomers Derived from 4083 Lignocellulose 9.1. Lignin Conversion 4083 9.2. Levulinic Acid Conversion 4084 9.3. Furfurl Conversion 4084 10.0. Triglyceride Conversion 4085 10.1. Trnsesterifiction 4085 10.2. Pyrolysis nd Zeolite Upgrding 4089 10.3. Hydrotreting 4090 10.4. Microemulsions nd Cosolvent Vegetble Oil 4090 Blends 10.5. Glycerol Utiliztion 4090 11.0. Ethicl Considertions nd Conclusions 4091 11.1. Ethicl Considertions 4091 11.2. Overll Conclusions 4091 12. References 4094 1.0. Introduction Prior to the discovery of inexpensive fossil fuels, our society ws dependent on plnt biomss to meet its energy demnds. The discovery of crude oil, in the 19th century, creted n inexpensive liquid fuel source tht helped industrilize the world nd improved stndrds of living. Now with declining petroleum resources, combined with incresed demnd for petroleum by emerging economies, nd politicl nd environmentl concerns bout fossil fuels, it is impertive to develop economicl nd energy-efficient processes for the sustinble production of fuels nd chemicls. In this respect, plnt biomss is the only current sustinble source of orgnic crbon, 1-3 nd biofuels, fuels derived from plnt biomss, re the only current sustinble source of liquid fuels. Biofuels generte significntly less greenhouse gs emissions thn do fossil fuels nd cn even be greenhouse gs neutrl if efficient methods for biofuels production re developed. 4-7 The U.S. Deprtment of Agriculture (USDA) nd Ok Ridge Ntionl Lbortory estimted tht the U.S. could sustinbly produce 1.3 10 9 metric tons of dry biomss/ yer using its griculturl (72% of totl) nd forest (28% of totl) resources nd still meet its food, feed, nd export demnds. 8 This mount of biomss hs the energy content of 3.8 10 9 boe (brrels of oil energy equivlent). 2 (The U.S. consumes 7 10 9 bbl/yer or brrels of oil/yer. 9 ) According to the Europen Biomss Industry Assocition 10.1021/cr068360d CCC: $59.00 2006 Americn Chemicl Society Published on Web 06/27/2006

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4045 George W. Huber is currently n Assistnt Professor of Chemicl Engineering t University of Msschusetts-Amherst. Prior to his ppointment s professor, he did postdoctorl sty (2005 2006) with Avelino Corm t the Instituto de Tecnologí Químic (UPV CSIC) where he focused on biofuels production using trditionl petrochemicl technologies. He obtined his Ph.D. in chemicl engineering from University of Wisconsin-Mdison (2005) where he helped to develop novel queousphse ctlytic processes for biofuels production under the guidnce of Jim Dumesic. He obtined his B.S. (1999) nd M.S. (2000) degrees from Brighm Young University, where Clvin H. Brtholomew ws his M.S. dvisor. His current reserch interests re in biofuels, biochemicls, nd heterogeneous ctlysis. Sr Iborr ws born in Crlet (Spin) in 1959. She studied Phrmcy t the Universidd de Vlenci nd received her Ph.D. in 1987. In the sme yer, she joined the chemistry deprtment of the Technicl University of Vlenci s Assistnt Professor, becoming Lecturer 1992 t the sme deprtment where she teches orgnic chemistry. In 1991, she ws ppointed member of the Institute of Chemicl Technology, joint center of the Spnish Ntionl Reserch Council (CSIC) nd the Technicl University of Vlenci where she joined the reserch group of Professor Avelino Corm. The min focus of her work is the ppliction of heterogeneous ctlysts (cid, bse, nd redox solid ctlysts) in the synthesis of fine chemicls. Avelino Corm Cnos ws born in Moncófr, Spin, in 1951. He studied chemistry t the Universidd de Vlenci (1967 1973) nd received his Ph.D. t the Universidd Complutense de Mdrid in 1976. He ws postdoctorl in the Deprtment of Chemicl Engineering t the Queen s University (Cnd, 1977 1979). He is Director of the Instituto de Tecnologí Químic (UPV CSIC) t the Universidd Politécnic de Vlenci since 1990. His current reserch field is ctlysis, covering spects of synthesis, chrcteriztion, nd rectivity in cid bse nd redox ctlysis. Avelino Corm is couthor of more thn 600 rticles nd 100 ptents on these subjects. He is member of the editoril bord of the most importnt journls in the field of ctlysis, physicl chemistry, nd mterils chemistry. He hs been wrded with the Dupont Awrd on New Mterils (1995), the Spnish Ntionl Awrd Leonrdo Torres Quevedo on Science nd Technology (1995), Burdiñol (1997), Premio Iberdrol de Químic (1998) nd F. Cipett Awrd of the North Americn Ctlyst Society (1998), Iptieff Lecturer t Northwester University 2000/ 2001, Rey Jime I Awrd on New Technologies (2000), Frnçois Gult Lectureship (EFCATS) (2001), Eugene J. Houdry Awrd in Applied Ctlysis of the North Americn Ctlysis Society (2002), Breck Awrd of the Interntionl Zeolite Assocition (2004), Gold Medl of the Royl Society of Chemistry of Spin (2005), nd Doctor Honoris Cus of Utrecht University (2006). (EUBIA), Europe, Afric, nd Ltin Americ could produce 8.9, 21.4, nd 19.9 EJ of biomss per yer with n energy equivlence of 1.4 10 9, 3.5 10 9, nd 3.2 10 9 boe, respectively. 10 The worldwide rw biomss energy potentil in 2050 hs been estimted to be between 150 nd 450 EJ/ yer, or 25 10 9 to 76 10 9 boe. 11 Biofuels lso cn hve positive effect on griculture, nd the USDA recently estimted tht the net frm income in the U.S. could increse from $3 billion to 6 billion nnully if switchgrss becme n energy crop. 12 (Note: All costs in this review re reported in U.S. dollrs with conversion from U.S. dollrs to Euros of 1.0 to 1.2. For the purposes of this review, we ssume tht the energy content of 1.00 metric ton of dry lignocellulosic biomss is equivlent to 3.15 brrels of oil, nd 1 brrel of oil hs 5.904 GJ s reported by Klss. 2 ) The current cost of delivered biomss is significntly cheper thn crude oil in mny ntions. However, the cost of biomss vries ccording to type nd region. According to EUBIA, the cost of biomss per boe in the Europen Union (EU) rnges from $11 for solid industril residues to $39 for energy crops such s rpeseed. 10 In the U.S. it hs been estimted tht the cost of lignocellulosic biomss is $5 to 15/boe, 2,13 which is significntly below the current cost of crude oil of $56/bbl (verge cost in 2005). 14 The U.S. Energy Informtion Assocition hs predicted tht the world oil price will continue to rise through 2006, then decline to $47/bbl s new suppliers enter the mrket, nd slowly rise to round $54/bbl in 2025. 9 Furthermore, the price difference between biomss nd petroleum will be even greter if negtive geostrtegicl considertions re dded into the cost of crude oil. In the mid-1800s, biomss supplied more thn 90% of U.S. energy nd fuel needs. 1,2 In the lte 1800s to erly 1900s, fossil fuels becme the preferred energy resource. 1,2 In mny developing countries, biomss is still mjor energy source. 1,2 Other countries tht use biomss to meet lrge percentge of their energy demnds include Sweden, 17.5%; Finlnd, 20.4%; nd Brzil, 23.4%. 1 The Rodmp for Biomss Technologies, 15 uthored by 26 leding experts from cdemi, industry, nd government gencies, hs predicted grdul shift bck to crbohydrte-bsed economy, such tht by 2030 20% of trnsporttion fuel nd 25% of chemicls in the U.S. will be produced from biomss.

4046 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. As discussed in this review, the trnsition to the crbohydrte economy is lredy occurring with mny compnies, including trditionl oil nd chemicl compnies, such s Shell, 16 UOP, 17 Petrobrs, Conoco-Phillips, 18 Dupont, 19,20 Dow nd BP, developing the technology nd infrstructure for biofuels nd biochemicls production. Governmentl leders re lso recognizing the importnce of this fledgling industry by providing tx breks, money, nd mndtes. The Europen Commission hs set gol tht by 2010, 5.75% of the trnsporttion fuels in the EU will be biofuels. It hs been estimted tht this gol requires 4-13% of the griculturl lnd in the EU be used for biofuel production. 21 A number of EU countries, including Austri, Itly, Polnd, Spin, Germny, nd Sweden, nd other countries including Frnce, give full tx exemption for biotrnsporttion fuels, nd the U.K. gives prtil tx exemption. 21 The EU even provides crbon credit of $54/h for frmers who grow energy crops used for biodiesel nd bioethnol production. 21 The U.S. government lso supports biofuels nd gives subsidies of $0.14/L for ethnol production. For the trnsition to the crbohydrte economy to continue, it is vitl tht lowcost processing technologies be developed for conversion of low-cost biomss into fuels nd chemicls. Chemists, scientists, nd engineers will ply key role in developing these processes. The lws of economics dictte tht s petroleum reserves dwindle, the price of petroleum products will increse, nd biofuels eventully will be cost-competitive nd even cheper thn petroleum-derived fuels. The purpose of this review is to discuss current methods nd future possibilities for obtining trnsporttion fuels from biomss. We will present the review in n integrted wy by including not only the chemistry nd ctlysis involved in the process but lso engineering solutions nd chllenges becuse these lso cn hve n importnt impct on the globl process. Life cycle nd economic nlyses re presented for the vrious processes to help reserchers select res where they cn focus. These types of nlyses cn vry considerbly nd re dependent on the ssumptions mde with current regionl informtion; therefore, these nlyses should be viewed only s first order indictors. Figure 1. Sustinble production of trnsporttion fuels from biomss in n integrted biomss production-conversion system. Figure 1 shows n idelized biomss growth nd mnufcturing scheme in which CO 2,H 2 O, light, ir, nd nutrients re the inputs for biofuel production, nd energy to power trnsporttion vehicles nd food re the outputs. The three min technologies necessry for crbohydrte economy re (1) growth of the biomss feedstock, (2) biomss conversion into fuel, nd (3) fuel utiliztion. In this review, we focus on biomss conversion into fuel, while recognizing tht reserch in biomss production nd fuel conversion re lso very importnt. Idelly, it would be desirble to use highyield crops tht required little nutrients, fertilizers, nd energy input. It would lso be desirble to hve biomss conversion process tht is ble to convert ll the energy in the biomss to trnsporttion fuel tht could esily be fit into existing infrstructure nd without ir pollution. In prctice, it is impossible to convert ll the energy in the biomss into fuel just s it is impossible to convert ll the energy in crude oil into gsoline nd diesel fuels. Conversion technologies hve wide rnge of energy efficiencies s will be discussed in this review. Some current biomss technologies hve been criticized becuse they hve low overll therml conversion efficiencies, in which only smll prt of the energy in the plnt is converted into the finl fuel product. The biofuels industry is only in its infncy, nd it is likely tht dvnces in conversion technology nd process integrtion will ultimtely improve overll energy nd economic efficiency. Novel biomss conversion technologies re being developed tht hve higher therml efficiencies thn trditionl technologies, 22 nd it is vitl tht we continue to develop novel routes. In ddition, plnt breeding is producing plnts tht hve higher yields, require less wter, cn grow on rid lnd, nd hve lower fertilizer inputs. Energy to power trnsporttion vehicles is produced from the biofuel, nd while we currently use sprk ignition nd diesel fuel engines for utomobiles, 23,24 other types of energy conversion devices for trnsporttion vehicles re being developed such s polymer electrolyte membrne (PEM) fuel cells, hybrid electric vehicles, nd homogeneous chrged compression ignition engines. Importnt ir qulity control nd infrstructure issues lso need to be ddressed in choosing the optiml biofuel. The choice of biomss feedstock will ultimtely depend on crops yields, regionl conditions, food coproduction, economics, nd the life cycle therml efficiency (LCTE). Biomss, which is typiclly in low density form, must be collected nd trnsported to centrl processing fculty so tht it cn be converted into trnsporttion fuel. The edible nd nonedible prt of biomss cn be seprted, nd the nonedible frction cn then be converted into fuel. The nutrients from the biomss lso cn be seprted nd reused for further biomss growth (Figure 1). Biomss combustion produces electricity nd het. Furthermore, lthough number of other renewble options for sustinble electricity nd het production re vilble such s solr, wind, nd hydroelectric, plnt biomss is the only current renewble source of crbon tht cn be used directly for liquid fuels nd chemicls. Our view is tht the longterm optiml use of biomss is for fuels nd chemicl production, nd other forms of renewble energy should be used for sttionry power genertion. We will first discus the chemicl composition of biomss nd growth rtes of vrious species (Section 2) becuse the first step in producing biofuels is to hve chep nd bundnt biomss feedstock. Lignocellulose (or cellulose) is the chepest nd most bundnt source of biomss, nd therefore we first begin with discussing its conversion. High yield lignocellulosic energy crops such s switchgrss cn be grown. Another strtegy is to use lignocellulosic biomss residues, such s griculturl, industril, nd forest wstes. The production of liquid fuels from lignocellulosic biomss involves removl of some oxygen, s CO 2 or H 2 O, nd conversion into higher-density liquid fuel. Lignocellulosic

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4047 Figure 2. Strtegies for production of fuels from lignocellulosic biomss dpted from Huber nd Dumesic. 22 biomss typiclly hs 40-45 wt % oxygen, nd oxygen removl increses the heting vlue. The more oxygen removed, the higher the energy density of the fuel; however, to improve fuel combustion chrcteristics it my be desirble to leve some of the oxygen in the fuel. Lignocellulosic mteril cn be converted into liquid fuels by three primry routes, s shown in Figure 2, including syngs production by gsifiction (Section 3), bio-oil production by pyrolysis or liquefction (Section 5), or hydrolysis of biomss to produce sugr monomer units (Section 7). Synthesis gs cn be used to produce hydrocrbons (diesel or gsoline), methnol, nd other fuels (Section 4). Bio-oils must be upgrded if they re to be used s trnsporttion fuels (Section 5). Trnsporttion fuels such s ethnol, gsoline, nd diesel fuel cn be produced from sugr nd ssocited lignin intermedites (Sections 8 nd 9). Another method of producing biofuels is to grow energy crops which hve high energy density structures tht re esily converted into liquid fuels such s vegetble oils (Section 10) or hydrocrbon-producing plnts (Section 2.5). Biodiesel produced from trnsesterifiction of rpeseed or other triglycerides represents 80% of the current biofuel mrket in Europe nd will be discussed in Section 10. 21 Hydrogen production will be discussed in this review, even though H 2 is currently not being used s trnsporttion fuel. Hydrogen, which is the feedstock for PEM fuel cells, cn lso be used s n intermedite for biofuels production, just s it is for gsoline nd diesel production. Therefore, processes to produce hydrogen my be n integrl prt of the future biorefinery, just s they re n integrl prt of the current petroleum refinery. It hs previously been pointed out tht the full benefits of hydrogen economy re only relized when hydrogen is derived from renewble resources such s biomss. 25 Biomss nd biofuels pper to hold the key for supplying the bsic needs of our societies for the sustinble production of liquid trnsporttion fuels nd chemicls without compromising the needs of future genertions. A mjor 21st century gol for cdemi, industry, nd government should be the emergence of efficient nd economicl utiliztion of biomss resources. 2.0. Biomss Chemistry nd Growth Rtes The optiml type of biomss for biofuels production will depend on regionl issues such s soil qulity, precipittion, nd climte. Biomss cn be produced not only on griculturl lnd but lso on forest, qutic, nd rid lnd. Nture produces wide rnge of structures from biomss. However, most biomss is built from few bsic monomer units, nd in this section we describe the chemistry of different types of biomss long with biomss growth rtes. 2.1. Lignocellulose nd Strch-Bsed Plnts Plnts use solr energy to combine crbon dioxide nd wter forming sugr building block (CH 2 O) n nd oxygen s shown in eq 1. The sugr is stored in polymer form s cellulose, strch, or hemicellulose. Most biomss is pproximtely 75 wt % sugr polymer. nco 2 + nh 2 O + light98 chlorophyll (CH 2 O) n + no 2 (1) The first step for biofuels production is obtining n inexpensive nd bundnt biomss feedstock. Biofuel feedstocks cn be chosen from the following: wste mterils (griculturl wstes, crop residues, wood wstes, urbn wstes), forest products (wood, logging residues, trees, shrubs), energy crops (strch crops such s corn, whet, brley; sugr crops; grsses; woody crops; vegetble oils; hydrocrbon plnts), or qutic biomss (lge, wter weed, wter hycinth). Tble 1 shows the growth rte or productivity, the lower heting vlue, the totl production energy, nd the chemicl composition of different types of biomss. 26 Plnt growth rtes vry, with typicl rnge from 6 to 90 metric tons/h-yer or 19 to 280 boe/h-yer. 2 Plnts typiclly cpture 0.1 to 1.0% of solr energy, with the percentge of solr energy cptured proportionl to the plnt growth rte. The energy inputs reported in Tble 1 include the energy required to mke fertilizer s well s the trnsporttion energy ssocited with crop growth. The growth rtes of plnts nd

4048 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Tble 1. Chemicl Composition, Energy Content, nd Yield of Vrious Terrestril Biomss Species biomss component corn grin corn stover switchgrss sugrcne sweet sorghum euclyptus pine productivity (dry metric tons/h-yer) 2,26 7 13-24 8-20 73-87 43.8 40.0 11.6 lower heting vlue (MJ/dry kg) 26 17.0 17.5 17 16.8 17.3 18.1 18.6 energy inputs (MJ/dry kg) 26 1.35 1.20 0.346 2.82 5.57 7.43 energy content (GJ/h-yer) 120 228-420 136-340 1230-1460 760 720 210 energy content (boe/h-yer) 20 40-70 23-58 210-250 128 123 37 representtive components (dry wt %) celluloses 3 36 40-45 22 35 48 46-50 hemicelluloses 6 23 31-35 15 17 14 19-22 extrctives (strches, terpenes) 72 6 0 43 23 2 3 lignins 2 17 6-12 11 17 29 21-29 uronic cid 0 0.00 1 4 3 proteins 10 5-11 sh 10 5-6 9 5 1 0.3 Adpted from Towler et l., 26 Lynd et l., 13 nd Klss. 2 Figure 3. Structures of different biomss frctions (lignocellulose, cellulose, lignin nd hemicellulose) before nd fter rections. (Lignocellulose structure dpted from Hsu et l. 29 ) the energy requirements for plnt growth re dependent on plnt species (Tble 1). Plnt breeding, biotechnology, nd genetic engineering promise to develop more efficient plnt mterils with fster growth rtes, which require less energy inputs. Strches re glucose polyscchride tht hve R-1,4 glycoside linkges. 27 Strches lso hve lrge mount of R-1,6 glycoside linkges. These R-linkges mke the polymer morphous. Humn nd niml enzyme systems cn esily digest strches due to the R-linkges. Strches re commonly found in the vegetble kingdom (e.g., corn, rice, whet, bens, nd pottoes). When treted in hot wter, strches form two principle components: wter-soluble mylose (10-20 wt %) nd wter-insoluble mylopectin (80-90 wt %). Amylose contins only R-1,4 glycoside linkges, wheres mylopectin contins both R-1,4 nd R-1,6 glycoside linkges with n pproximte R-1,4 to R-1,6 linkge rtio of 20:1. The structured portion of biomss is composed of cellulose, hemicellulose, nd lignin. Cellulose ( crystlline glucose polymer) nd hemicellulose ( complex morphous polymer, whose mjor component is xylose monomer unit) mke up 60-90 wt % of terrestril biomss (Tble 1). Lignin, lrge polyromtic compound, is the other mjor component of biomss. Extrctives (Tble 1) re defined s those compounds tht re not n integrl prt of the biomss structure. 27 Extrctives re soluble in solvents such s hot nd cold wter, ethers, or methnol nd cn include different types of crbohydrtes such s sucrose from sugrcne nd mylose from corn grins. Ash listed in Tble 1 is biomss mteril tht does not burn. Uronic cids re sugrs tht re oxidized to cids. 27 Other minor components of biomss include triglycerides, lkloids, pigments, resins, sterols, terpenes, terpenoids, nd wxes. Importntly, certin plnts, such s rpeseed or soybens, cn hve lrge mounts of these minor components. Cellulose, s shown in Figure 3, consists of liner polyscchride with β-1,4 linkges of D-glucopyrnose monomers. 3 Unlike strch, cellulose is crystlline mteril with n extended, flt, 2-fold helicl conformtion. 3 Hydrogen bonds help mintin nd reinforce the flt, liner conformtion of the chin. The top nd bottom of the cellulose chins re essentilly completely hydrophobic. The sides of the cellulose chins re hydrophilic nd cpble of hydrogen bonding, becuse ll the liphtic hydrogen toms

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4049 Figure 4. Common lignin linkges dpted from Chkr et l. 32 Tble 2. Annul Seed nd Oil Yields from Oil-Producing Plnts seed yields oil yields verge potentil verge potentil common nme species (kg/h) (kg/h) (kg/h) (L/h) (kg/h) (L/h) cstorben Ricinus communis 950 3810 428 449 1504 1590 chinese tllow tree Spium sebiferum 12553 5548 6270 corn (high oil) Ze mys 5940 596 650 rpeseed Brsic npus 2690 1074 1220 sfflower Crthmus tinctorius 1676 2470 553 599 888 940 soyben Glycine mx 1980 3360 354 383 591 650 sunflower Helinthus nnuus 1325 2470 530 571 986 1030 Adpted from Klss. 2 re in xil positions, nd the polr hydroxyl groups re in equtoril positions. The degree of polymeriztion of cellulose is pproximtely 10 000 to 15 000 glucopyrnose monomer units in wood nd cotton, respectively. 28 Upon prtil cid hydrolysis, cellulose is broken into cellobiose (glucose dimer), cellotriose (glucose trimer), nd cellotetrose (glucose tetrmer), wheres upon complete cid hydrolysis it is broken down into glucose. 29 Hemicellulose is sugr polymer tht typiclly constitutes 20-40 wt % of biomss. 27 In contrst to cellulose, which is polymer of only glucose, hemicellulose is polymer of five different sugrs. This complex polyscchride occurs in ssocition with cellulose in the cell wlls. It contins five-crbon sugrs (usully xylose nd rbinose) nd sixcrbon sugrs (glctose, glucose, nd mnnose), ll of which re highly substituted with cetic cid. The most bundnt building block of hemicellulose is xyln ( xylose polymer linked t the 1 nd 4 positions). Hemicellulose is morphous becuse of its brnched nture nd it is reltively esy to hydrolyze to its monomer sugrs compred to cellulose. Ten to twenty-five weight percent of biomss is typiclly composed of lignin which is highly brnched, substituted, mononucler romtic polymer found in the cell wlls of certin biomss, prticulrly woody biomss. Lignin is often ssocited with the cellulose nd hemicellulose mterils mking up lignocellulose compounds. The mnner in which it is produced from lignocellulose ffects its structure nd rectivity. Figure 3 shows the structurl monomer units of lignin. Softwood lignins re formed from coniferyl lcohol. Hrdwood lignins hve both coniferyl nd sinpyl lcohol s monomer units. Grss lignin contins coniferyl, sinpyl, nd coumryl lcohol. 30 Lignin is n irregulr polymer, which is formed by n enzyme-initited free-rdicl polymeriztion of the lcohol precursors. The bonding in the polymer cn occur t mny different sites in the phenylpropne monomer due to electron delocliztion in the romtic ring, the double bond-contining side chin, nd the oxygen functionlities. 31 Some lignin structurl linkge units re shown in Figure 4. 32 2.2. Triglyceride-Producing Plnts High-energy density liquid molecules, which cn be used to mke liquid fuels, re produced in plnts s triglycerides or terpenes (Section 2.4). Triglycerides, or fts nd oils, re found in the plnt nd niml kingdom nd consist of wterinsoluble, hydrophobic substnces tht re mde up of one mole of glycerol nd three moles of ftty cids. Fts nd oils re used minly for cooking nd food purposes, s well s for lubricnts nd rw mterils for sop, detergents, cosmetics, nd chemicls. From the more thn 350 known oil-bering crops, those with the gretest potentil for fuel production, ccording to Peterson, re sunflower, sfflower, soyben, cottonseed, rpeseed, cnol, corn, nd penut oil. 33 Tble 2 lists triglyceride crop nd oils derived from oilproducing plnts. The nnul yields of oil seeds re 1000-2000 kg/h nd potentilly could rnge from 2500-6000 kg/h. The exception is the Chinese tllow tree, ntive of subtropicl Chin nd from the Euphorbicee fmily, which hs tremendous potentil due to its high growth rte. 2 Vegetble oils hve higher heting vlue of pproximtely 40 MJ/kg; 34 thus, the nnul energy yield of the plnts listed in Tble 2 rnges from 6.8 to 13.6 boe/h-yer. The nnul

4050 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. energy yields of lignocellulosic mteril (Tble 1) rnges from 19 to 280 boe/h-yer, which is significntly greter thn the energy yields of oil seeds. However, s will be discussed lter, oil seeds cn be efficiently converted into liquid fuel. The problems with vegetble oils s feedstock re tht they re more expensive thn cellulosic biomss, nd there re limited quntities. Currently, vegetble oils re being used for biodiesel production by trnsesterifiction (Section 9). The most common feedstocks for biodiesel production re rpeseed nd sunflower in the EU, plm oil in tropicl countries, nd soyben oils nd niml fts in the U.S. 35 Eighty percent of trnsporttion biofuels in the EU re biodiesel produced primrily from trnsesterifiction of rpeseed nd to lesser extent sunflower seeds. 21 Approximtely 20% of the rpeseed produced in the EU is used for biodiesel production. 21 All oil-producing plnts contin triglycerides, crbohydrtes, protein, fiber, nd sh. As shown in Tble 3 soyben Tble 3. Composition of Soybens component wt % protein 40 triglyceride 20 cellulose nd hemicellulose 17 sugrs 7 crude fiber 5 sh (dry weight) 6 From Erickson et l. 36 plnt only contins 20 wt % triglyceride. 36 The first step in the production of vegetble oils is extrction of the oils from the plnt. A pretretment step tht involves clening, drying, nd dehulling must be done prior to extrction. The oils re then extrcted by one of three methods: hydrulic pressing, expeller pressing, or solvent extrction. 36 Two min products re produced in this process: vegetble oil nd the dry solid residue known s mel. The mel hs high mount of protein nd is used s protein supplement for niml feeds. All triglycerides cn be broken into one glycerol molecule nd three ftty cid molecules. The crbon chin length nd number of double bonds in the ftty cids vry, s shown in Tble 4, depending on the source of vegetble oil. A number of wste triglycerides re vilble including yellow greses (wste resturnt oil) nd trp grese (which is collected t wstewter tretment plnts). 37 Yellow grese is used in the mnufcturing of niml feed nd tllow, lthough concerns bout md cow disese re limiting its usge s n niml feed. Trp grese hs zero or negtive feedstock cost but is contminted with sewge components. 37 A recent study of 30 metropolitn res in the U.S. indicted tht the U.S. produces 4.0-6.0 kg/(yer-person) of yellow nd trp grese, respectively. 37 Multiplying this number by the popultion of the U.S. indictes the potentil production of biodiesel of 1.3 billion nd 1.9 billion L/yer from yellow nd trp grese, respectively. 33 The U.S. consumed 160 billion L of diesel fuel in 2003 in the trnsporttion sector; 9 therefore, biodiesel derived from yellow nd trp grese could only supply up to 2% of the nnul diesel fuel consumption in the U.S. However, trp grese must be disposed of, nd converting it into biodiesel would be n efficient wy of using n inexpensive wste mteril. 2.3. Alge Aqutic lge re nother source of triglycerides s well s crbohydrtes nd lignin. The dvntge of using microlge is tht they hve very high growth rtes, utilize lrge frction of the solr energy (up to 10% of the solr energy), nd cn grow in conditions tht re not fvorble for terrestril biomss growth. From 1978 to 1996, the U.S. Deprtment of Energy funded progrm to develop renewble trnsporttion fuels from lge, nd the results of this progrm re reported by Sheehn et l. 38 Over 3000 strins of microlge were collected s prt of this progrm, nd ccording to Sheehn et l. currently 300 species, mostly green lge nd ditoms, re still housed t the University of Hwii in collection vilble to reserchers. 38 Microlge re one of the most primitive forms of plnts nd re microscopic photosynthetic orgnisms. While the photosynthesis mechnism in lge is similr to other plnt mteril, they cn convert more of their solr energy into cellulr structure. Mcrolge re commonly known s seweed. Both microlge nd mcrolge re fst-growing mrine nd freshwter plnts. Commercil production of triglycerides from microlge hs been estimted to be 72 000 L/h-yer (390 boe/h-yer), nd it hs been estimted tht rtes s high s 130 000 L/h-yer (700 boe/h-yer) could be ccomplished with continued reserch. 39 Thus, lge hve triglyceride production rtes 45-220 times higher thn terrestril biomss (Tble 2). Other estimtes indicte tht 2000 h of lnd would be required to produce 1 EJ/yer of fuel with microlge. 38 (The U.S. consumed 42 EJ of Tble 4. Chemicl Composition of Ftty Acids in Vegetble Oils ftty cid composition (wt %) vegetble (no. of crbons: CdC bonds) oil 8:0 10:0 12:0 14:0 16:0 18:0 18:1 18:2 18:3 22:1 cnol 1.2-6 1-2.5 52-66.9 16.1-31 6.4-14.1 1-2 110-126 188-193 coconut 4.6-9.5 4.5-9.7 44-51 13-20.6 7.5-10.5 1-3.5 5-8.2 1.0-2.6 0-0.2 6-12 248-265 corn 0-0.3 7-16.5 1-3.3 20-43 39-62.5 0.5-13.5 103-140 187-198 cottonseed 0.6-1.5 21.4-26.4 2.1-5 14.7-21.7 46.7-58.2 90-119 189-198 olive 0-1.3 7-20 0.5-5.0 55-84.5 3.5-21 75-94 184-196 plm 0-0.4 0.5-2.4 32-47.5 3.5-6.3 36-53 6-12 35-61 186-209 penut 0-0.5 6-14 1.9-6 36.4-67.1 13-43 0-0.3 80-106 187-196 rpeseed 0-1.5 1-6 0.5-3.5 8-60 9.5-23 1-13 5-56 94-120 168-187 soyben 2.3-13.3 2.4-6 17.7-30.8 49-57.1 2-10.5 0-0.3 117-143 189-195 sunflower 3.5-7.6 1.3-6.5 14-43 44-74 110-143 186-194 tllow (beef) 2.1-6.9 25-37 9.5-34.2 14-50 26-50 35-48 218-235 Adpted from Knothe et l. 307 iodine vlue spon vlue

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4051 Tble 5. Composition of Microlge s Dry Wt % Grown under Different Conditions growth conditions orgnic component (dry wt %) species NCl level (molr) nutrients sh lipid (triglyceride) protein crbohydrte glycerol unknown Botryococcu brunii 0 enriched 5.6 44.5 22.0 14.1 0.1 19.3 0 deficient 7.8 54.2 20.6 14.3 0.1 10.8 0.5 enriched 59.6 46.3 15.0 13.3 0.1 25.3 Dunliell brdwil 2.0 deficient 14.7 10.4 9.7 40.4 16.4 23.1 Dunliell slin 0.5 enriched 8.6 25.3 29.3 16.3 9.4 19.7 0.5 deficient 7.7 9.2 12.5 55.5 4.7 18.1 2.0 enriched 21.7 18.5 35.9 12.5 27.7 5.4 Ankistrodesmus sp. 0 enriched 4.5 24.5 31.1 10.8 0.1 33.5 Isochrysis sp. 0.5 enriched 12.0 7.1 37.0 11.2 0.1 44.6 0.5 deficient 52.0 26.0 23.3 20.5 0.1 30.1 1.0 enriched 65.9 15.3 34.7 15.5 0.1 34.4 Nnochloris sp. 0 enriched 13.6 20.8 33.1 13.2 0.1 32.8 Nitzschi sp. 1.4 enriched 20.4 12.1 16.8 9.2 0.1 61.8 Adpted from Klss. 2 Tble 6. Cpitl nd Operting Costs in 1987 U.S. Dollrs for n Open Pond System for Alge Production on 400 h System with Nutrient nd CO 2 Recycle from Anerobic Digesters cpitl costs ($/h/yer) operting costs ($/h/yer) 112 m ton/h/yer 224 m ton/h/yer 112 m ton/h/yer 224 m ton/h/yer growth ponds operting costs erthworks 10135 10135 CO 2 (2 kg/kg of biomss) 6290 12580 wlls & structurl 8304 8304 N (5.3% in biomss) s NH 3 370 750 mixing systems 4919 4919 P superphosphte, Fe s FeSO 4 530 1070 crbontion system 1830 2978 flocculnts 1120 2250 instrumenttion 500 500 power mixing (10,730 kwh/h) 700 700 primry (settling ponds) 7479 7479 1E hrvest (1,770 kwh/h) 120 120 secondry (centrifuges) 3958 6467 2E hrvest (1,770 kwh/h) 370 600 system-wide costs wter supply (8750 kwh/h) 570 570 wter supply/distrib 4426 4426 other (1562 kwh/h) 110 110 co 2 distribution 260 421 power production (6.5 /kwh) (-2250) (-5100) nutrient supply 781 781 slt disposl ($67/m ton) 1130 1130 slt disposl 833 833 mintennce 1970 2940 buildngs 573 573 lbor 1390 1390 rod nd dringe 521 521 totl operting cost 12420 19110 electricl distr./supply 1924 2215 cpitl costs (25%/yer of totl) 18238 22491 mchinery 417 417 totl cost 30658 41601 eng. + contg (25%) 11715 12742 lnd costs ($1,250/h) 2500 2500 gen-set (eng + const.) 8250 16500 nerobic digestion 3627 7254 totl cpitl cost ($/h) 72952 89965 totl biomss costs ($/m ton) 273 185 Adpted from Sheehh. 38 petroleum products in 2003. 9 ) Microlge re ctegorized into four mjor clsses in terms of their bundnce: ditoms, green lge, blue-green lge, nd golden lge. Tble 5 shows the composition of vrious microlge grown under different conditions. Microlges cn contin from 7 to 60 dry wt % triglycerides. 2 Pilot plnt tests, conducted over six-yer period, demonstrted tht microlge could be produced t productivity rtes s high s 500 kg lge /h-yer in 1000 m 2 pond for single dy. 38 The ponds were n open fce shllow wter design where the wter nd lge re circulted round the pond. Nutrients nd CO 2 were continully dded to the lge pond. The productivity ws dependent on temperture nd sunlight, which vried over the course of the experiments. Idelly, lge could be produced on lge frms in open, shllow ponds where wste source of CO 2, for exmple, from fossil fuel power plnt, could be efficiently bubbled into the ponds nd cptured by the lge. The current limittion of microlge is the high production cost. 38 Tble 6 shows the production cost of lge on lrge lge frm of 400 h. 38 Two scenrios were used for cost estimtion with lge growth rtes of 112 nd 224 metric tons/h-yer. The totl biomss lge cost ws $273 nd $185/metric ton, which is considerbly higher thn the cost of lignocellulosic biomss (less thn $40/metric ton). The cost for CO 2 is 20-30% of the totl cost, nd using wste CO 2 from fossil fuel power plnts would decrese the cost of lge production. One of the conclusions from the cost nlysis is tht lterntive engineering designs for microlge production would not significntly reduce the cost of microlge production. 38 The limiting fctor in cost nlysis is microlge cultivtion issues, nd ccording to Sheehh future reserch work should focus on the biologicl issues regrding microlge production. 38 Microlge cultivtion issues re limited by the vilbility of wter, CO 2, sunlights, nd flt lnd. The development of low-cost hrvesting processes could lso reduce the cost of lge.

4052 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. 2.4. Terpenes nd Rubber-Producing Plnts Some plnt species convert crbohydrtes into mixture of isomeric hydrocrbons of moleculr formul (C 5 H 8 ) n clled terpenes. Terpenes re clssified by the number of isoprene units (C 5 H 8 ) such s (C 5 H 8 ) 2, monoterpenes; (C 5 H 8 ) 3, sesquiterpenes; (C 5 H 8 ) 4, diterpenes; (C 5 H 8 ) 6, triterpenes; nd (C 5 H 8 ) x, polyterpenes. 2 Terpenes re open cyclic chin, monocyclic, bicyclic, tricyclic, etc., nd more thn 23 000 structures of terpenes re known. These nturl hydrocrbons cn be used s trnsporttion fuels if they cn be economiclly produced. Nturl rubber, cis-1,4-polyisoprene with moleculr weight from 500 000 to 2 000 000, is produced commercilly from the ltex of the HeVe brsiliensis tree, member of the Euphorbicee fmily. 40 In 1993, 5.3 million metric tons of nturl rubber ws produced minly in Mlysi, Indonesi, nd Thilnd. 40 The verge yield in the rubber-producing countries vries from 0.4 to 1.2 metric ton/h-yer. This corresponds to pproximtely 2.0-9.4 boe/h-yer, which is below the production rte of vegetble oils nd lignocellulosic biomss. Nturl rubbers cn lso be produced from Guyule, member of the sunflower fmily, nd in 1910 50% of ll commercil U.S. rubber ws mde from wild guyule. 2 During World Wr II, guyule plnttions were used to mke nturl rubber in the U.S. Terpene feedstocks cn lso be used s building blocks for the fine chemicl industry. 41 While rubber is high vlue product, it cn lso be converted into fuels by depolymeriztion processes. Buchnn et l. evluted over 206 species from 57 different fmilies nd 141 geners, tht cn be grown in the U.S., for hydrocrbon nd rubber-producing potentil. 42,43 The plnt mterils hve between 0.1 to 7 dry wt % oil content. Buchnn et l. climed tht the species Ccli triplicifoli nd Sssfrs lbidum hve the best potentil for producing nturl rubber in the U.S. t rte of 2.0 metric ton/h-yer. Melvin Clvin, who won the Noble Prize for his work on photosynthesis, developed plnttions in the U.S. to produce low moleculr weight hydrocrbons (less thn 10 000) from the Euphorbi tree, which is reltive of the nturl rubber-producing trees. 44,45 The plnttions in the U.S. used the species Euphorbi lthyris (gopher plnt) nd Euphorbi tiruclli (Africn milk bush). 2 These plnts were hrvested every 6-7 months nd grew to bout 4 feet high. When the plnts were hrvested, they were crushed nd the oil ws extrcted. The Itlins hd plns in 1938 to build Euphorbi gsoline refinery, nd the French hve plnted nd hrvested Euphorbi in Morocco. 45 Euphorbi plnts cn be grown on semi-rid lnd, which is not suitble for food production, with minimum mount of wter. Initil experimentl results showed tht Euphorbi lthyris could produce 8-12% of its dry weight s oil or pproximtely 20 boe/h-yer over 7-month growing period with unselected seeds. 2 It ws felt tht plnt breeding would be ble to gretly increse the yield to up to 65 boe/h-yer. Other species of plnts, like the Brzilin tropicl tree Copifer multiju, cn produce oil tht cn be used directly s diesel fuel. A single tree of this type could produce 40-60 L of oil/yer, which is obtined by drilling hole in the tree to collect the oil. 44 The hole is plugged, nd every six months cn be drined to collect more oil. According to Klss, the min difficulties with the concept of nturl hydrocrbon production from biomss re tht most of the species tht hve been tested exhibit low liquid yields compred to the mss of biomss tht must be hrvested nd the nturlly produced liquids re complex mixtures nd not pure hydrocrbons. 2 Field studies of E. lthyris indicte tht the biocrude hs to sell for $100-200/bbl to be economicl. 3.0. Biomss Gsifiction Gsifiction is process in which solid or liquid crbonceous mteril, such s biomss, col, or oil, rect with ir, oxygen, nd/or stem to produce gs product clled syngs or producer gs tht contins CO, H 2,CO 2,CH 4, nd N 2 in vrious proportions. 2,46-49 The principle difference between producer nd syn-gs is tht ir is used to mke producer gs, which hs higher levels of N 2 nd lower concentrtions of CO, H 2,CO 2, nd CH 4 thn syn-gs. Producer gs is usully combusted to electricity nd/or het. Biomss gsifiction is n old technology, nd in the mid-1940s it ws used to power over million vehicles in Europe. 50 Biomss gsifiction is similr to col gsifiction with few differences. Biomss gsifiction occurs t lower temperture thn col gsifiction becuse biomss is more rective thn col. Biomss lso contins potssium, sodium, nd other lkli tht cn cuse slgging nd fouling problems in conventionl gsifiction equipment. A number of commercil biomss gsifiction units exist minly to produce het nd electricity, nd in the 1970s nd 1980s bout 40 worldwide compnies offered to build biomss gsifiction plnts. 2 As discussed in Section 4.0 syn-gs is used for production of fuels nd chemicls, nd mny industril routes for utiliztion of syn-gs exist such s production of H 2 by the wter gs shift rection, diesel fuel by FTS, methnol by methnol synthesis, nd methnol-derived fuels. Syn-gs is produced industrilly from col nd nturl gs. 51,52 3.1. Gsifiction Chemistry A complex combintion of rections in the solid, liquid, nd gs phses occurs during biomss gsifiction including pyrolysis, prtil oxidtion, nd stem gsifiction. Tble 7 shows some exmples of the gsifiction rections. Pyrolysis is the therml decomposition of the feedstock into gseous, liquid, nd solid products without oxygen or stem. Prtil oxidtion processes use less thn the stoichiometric mount of oxygen required for complete combustion. Stem reforming involves the rection of wter with the biomss-derived feedstock to produce CO, CO 2, nd H 2. The wter-gs shift (WGS) rection (wter nd CO rect to form H 2 nd CO 2 ) nd methntion (CO nd H 2 rect to form CH 4 nd H 2 O) re two other importnt rections tht occur during gsifiction. Het to drive gsifiction rections is generted in two wys: indirect gsifiction, where het is generted outside the gsifier nd trnsferred into the gsifier, or direct gsifiction, where the het is generted by exothermic combustion nd prtil combustion rections inside the gsifier. Evns nd Milne observed three mjor rection regimes during the gsifiction process identified s primry, secondry, nd tertiry regimes s shown in Figure 5. 53 This thermochemicl process cn be optimized to produce solid, liquid, or gseous products depending on residence times, temperture, nd heting rte s discussed in Section 5. During the primry stge of gsifiction solid biomss forms gseous H 2 O, CO 2, nd oxygented vpors nd primry oxygented liquids (Figure 5). The primry oxygented vpors nd liquids include cellulose-derived molecules (such

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4053 Tble 7. Fundmentl Rections nd Enthlpy of Selected Cellulose Gsifiction Rections clssifiction stoichiometry enthlpy (kj/g-mol) ref temp 300 K pyrolysis C 6H 10O 5 f 5CO + 5H 2 + C 180 C 6H 10O 5 f 5CO + CH 4 + 3H 2 300 C 6H 10O 5 f 3CO + CO 2 + 2CH 4 + H 2-142 prtil oxidtion C 6H 10O 5 + 1 / 2 O 2 f 6CO + 5H 2 71 C 6H 10O 5 + O 2 f 5CO + CO 2 + 5H 2-213 C 6H 10O 5 + 2O 2 f 3CO + 3CO 2 + 5H 2-778 stem gsifiction C 6H 10O 5 + H 2O f 6CO + 6H 2 310 C 6H 10O 5 + 3H 2O f 4CO + 2CO 2 + 8H 2 230 C 6H 10O 5 + 7H 2O f 6CO 2 + 12H 2 64 wter-gs shift CO + H 2O f CO 2 + H 2-41 methntion CO + 3H 2 f CH 4 + H 2O -206 Adpted from Klss. 2 Figure 5. Gsifiction nd pyrolysis rection pthwys dpted from Milne et l. 50 s levoglucosn, hydroxycetldehyde), their nlogous hemicellulose-derived products, nd lignin-derived methoxyphenols. 50 No chemicl interctions re observed mong the orgnic compounds during primry pyrolysis rections, which re substntilly free of secondry gs-phse crcking products. 53 Primry pyrolysis vpors re of rther low moleculr weight, representing monomers nd frgments of monomers. (A more complete discussion on primry chemistry is discussed in Section 5.3, since bio-oils re primry pyrolysis products.) Chrcol, which retins the morphology of the originl lignocellulose, is lso mjor product formed during slow pyrolysis. During secondry rections, the primry vpors nd liquids form gseous olefins, romtics, CO, CO 2,H 2,H 2 O, nd secondry condensed oils such s phenols nd romtics. The primry vpors undergo crcking (secondry rection regimes) when heted bove 500 C, nd the secondry rection temperture regime is from 700 to 850 C. Further heting to 850-1000 C results in tertiry rections from secondry products forming CO, CO 2,H 2,H 2 O, nd polynucler romtics (PNA) compounds including methyl derivtives of romtics such s methyl cenphthylene, methyl nphthlene, toluene, nd indene. Some tertiry products, including benzene, nphthlene, cenphthylene, nthrcene/phennthrene, nd pyrene, condense to form liquid tertiry phse. Soot nd coke re formed during these secondry nd tertiry processes. Coke forms from thermolysis of liquids nd orgnic vpors. The homogeneous nucletion of high-temperture decomposition products of hydrocrbons in the gs-phse produces soot. 53 The inorgnic components of the gsifiction feedstock re usully converted into bottom sh, which is removed from the bottom of the gsifiction rector, or into fly sh, which leves with the product gs. 49 The composition of the sh includes CO, K 2 O, P 2 O 5, MgO, SiO 2,SO 3, nd N 2 O. 2 Ash melts round 1000 C, nd it is importnt to keep the operting temperture below this temperture to void sh sintering nd slgging. 47 The ctul outlet gs composition from the gsifiction rector depends on the biomss composition, gsifiction process, nd the gsifying gent. 46,48,54 Higher moleculr weight hydrocrbons re clled trs nd re problemtic becuse they condense in exit pipes nd on prticulte filters leding to blockges nd clogged filters. Trs re defined s ny mteril in the product strem tht is condensble in the gsifier or in downstrem processing equipment. 50 Trs cuse further downstrem problems nd clog fuel lines nd injectors in internl combustion engines. The mount of trs cn be reduced by choosing the proper gsifiction conditions nd rector. 55 The chemicl structure nd formtion of trs in biomss gsifiction is the subject of report by Milne, Abtzoglou, nd Evns. 50 According to this report, tr is the most cumbersome nd problemtic prmeter in ny gsifiction commerciliztion effort. 50 Tr removl, conversion, or destruction hs been reported to be one of the gretest technicl chllenges for the successful development of commercil gsifiction technologies, 56 nd mny times new biomss gsifiction projects end becuse the cost of removing the trs is greter thn the cost of project. 50 The chemicl components of trs, which re strong function of temperture, re shown in Tble 8. The composition of the trs chnges s the tempertures increses in the following order: mixed oxygentes, phenolic ethers, lkyl phenolics, heterocyclic ethers, polyrmotic hydrocrbons, nd lrger polyrmotic hydrocrbons. 57 One pproch to decrese the tr concentrtion is to dd solid ctlysts inside the gsifiction rector. 58-60 Ctlysts tht hve been dded into the gsifiction rector include Pd, Pt, Ru nd Ni supported on CeO 2 /SiO 2, nd dolomite. Rh/CeO 2 /SiO 2 ws the most effective ctlyst for reducing tr levels. 59 Nickel-bsed ctlysts hve lso been tested by Bker et l. in the gsifiction rector, but they dectivted rpidly due to coke formtion nd ctlyst ttrition. 60 Another pproch to reduce trs is to mix lkli metl ctlysts with the biomss feedstock by dry mixing or wet impregntion. 61 Some of the lkli slts dded to the biomss include K 2 CO 3, 62,63 N 2 CO 3, 62,63 N 3 H(CO 3 ) 2, 62 N 2 B 4 O 7 10H 2 O, 62 CsCO 3, 63 NCl, 64 KCl, 64 nd ZnCl 2, AlCl 3 6H 2 O. 64 While lkli slts decrese tr formtion, they lso enhnce chr yields s hs been shown by severl fundmentl studies of cellulose nd biomss pyrolysis compounds. 65-68 According to Dyton, lkli metls re unttrctive s commercil

4054 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Tble 8. Chemicl Components in Biomss Trs conventionl flsh pyrolysis mixed oxygentes 400 C f phenolic ethers 500 C high-temperture flsh pyrolysis f lkyl phenolics 600 C f heterocyclic ethers f PAH f lrger PAH 700 C 800 C 900 C conventionl stem gsifiction high-temperture stem gsifiction 450-550 C 600-650 C 700-800 C 900-1000 C cids benzenes nphthlenes nphthlene ldehydes phenols cenphthylenes cenphthylene ketones ctechols fluorenes phennthrene furns nphthlenes phennthrenes fluornthene lcohols biphenyls benzldehydes pyrene complex oxygentes phennthrenes phenols cephennthrylene phenols benzofurns nphthofurns benznthrcenes guicols benzldehydes benzopyrenes syringols 226 MW PAHs complex phenols 276 MW PAHs Adpted from Elliott. 57 gsifiction ctlysts becuse of poor crbon conversion, incresed sh content, nd the difficulty in recovering lkli metls. 56 3.2. Gsifiction Rectors The following steps re importnt in the conversion of biomss to syn-gs: biomss storge nd trnsport, size reduction, drying, feeding, gsifiction, product gs conditioning, nd sh disposl or recycling. Biomss prticle size ffects the gsifiction rection rte nd the product gs composition. Size control is expensive nd energy intensive, nd there is trdeoff between the optiml biomss prticle size nd the gsifiction process. Specilized equipment is used to feed the solid biomss into gsifier. Screw feeders, where the screw forms compct, pressure-retining plug, re used for tmospheric gsifiers, nd lock-hopper feeder or lock-hopper/screw-piston feeder for pressurized gsifiers. Inside the gsifiers the following sequence of events occurs: drying, heting, therml decomposition (combustion nd pyrolysis), nd gsifiction. 47 The high moisture feedstock content of the feedstock hs negtive influence on the therml process efficiency nd is usully the most energyintensive prt of the gsifiction process. There re hundreds of different types of gsifiers in the ptent literture. However, they cn be divided into three principles types: 50 (1) Updrft gsifier (Figure 6A) where biomss enters from the top of the rector nd ir/oxygen/stem enter from the bottom of the rector, flow upwrd, nd the product gs leves from the top. In this rector, minly primry trs form t level of pproximtely 100 g/nm 3. The dvntges of updrft gsifiers re tht they re mture technology for het production, cn be used for smll-scle pplictions, cn hndle feeds with high moisture content, nd there is no crbon in the sh. The disdvntges of updrft gsifiers re tht they hve feed size limit, high tr yield, nd slgging potentil. (2) Downdrft gsifier (Figure 6B) in which the ir or oxygen nd the solid biomss enter t top of the rector flow downwrd, nd the product gs leves t the bottom of the rector. The product gs contins the lowest concentrtion of prticultes nd trs (pproximtely 1 g/nm 3 ) becuse most of the trs re combusted in this rector. The flme temperture in this rector is 1000-1400 C, nd the trs produced re lmost exclusively tertiry trs. This rector is idel when clen gs is desired. Disdvntges of this type Figure 6. Gsifiction rectors. of gsifier include lower overll therml efficiency nd difficulties in hndling higher moisture nd sh content. (3) Fluidized-bed gsifier (Figure 6C) where the biomss, which is previously reduced to fine prticle size, nd ir, stem, or oxygen enter t the bottom of the rector. A high velocity of the gs stem forces the biomss upwrd through bed of heted cermic or silic prticles. Both pyrolysis nd chr gsifiction occur in this process. This gsifier is good for lrge-scle pplictions, hs medium tr yield, nd the exit gs hs high prticle loding. The typicl tr level, 10 g/nm 3, is n intermedite level between the updrft nd the downdrft gsifier, nd the trs re mixture of secondry nd tertiry trs. 3.3. Supercriticl Gsifiction Gsifiction of biomss to produce mixture of H 2, CO, CO 2,CH 4, nd chr cn lso be ccomplished in supercriticl nd ner-supercriticl wter. 69 Modell nd co-workers were the first to use supercriticl wter to gsify biomss when they gsified mple swdust nd wter to produce high BTU gs contining CO, CO 2,H 2, nd CH 4 s the mjor components. 70,71 The combustible product gs is minly used

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4055 for sttionry power nd het ppliction from wste biomss sources. 69 A number of wste biomss feedstocks hve been used s feeds, including mnure solids, sw dust, corn fiber, nd wood residue. The product gs cn be converted into either more H 2 -rich strem by the wter gs shift rection (Section 4.1) or into syn-gs by stem reforming. More ner term pplictions of this technology is to produce mixtures of H 2 nd CH 4 tht cn be used s substitute nturl gs. One dvntge of this process is tht the wter in the biomss is not vporized, thereby improving the process therml efficiency (PTE). Therefore, wet feedstocks cn efficiently be processed with super/subcriticl wter. The product gs from this process is vilble t high pressure. Supercriticl gsifiction occurs t both high temperture 500-800 C 72-74 nd lower tempertures rnging from 350 to 600 C with the ddition of heterogeneous ctlyst such s Ru/TiO 2. 75,76 Crbon lso cn be used s ctlyst for high-temperture supercriticl tretment of biomss. 77 The importnt rections tht occur in supercriticl wter re the sme s those tht occur in gsifiction, including pyrolysis, hydrolysis, stem reforming, WGS, nd methntion. 78 Supercriticl gsifiction ppers to be unique technology, which will require further development. Some res of future reserch include the development of highly ctive, stble, nd selective novel ctlysts, rection chemistry studies, nd rector designs. 3.4. Solr Gsifiction Concentrted solr energy cn supply the energy to drive the gsifiction process. 79-84 Solr gsifiction decreses the mount of biomss tht needs to be burned in the gsifiction process, thus improving the PTE. Het is provided to the gsifiction unit using concentrted solr gsifiers nd specilly designed solr rectors. Two different rector configurtions re used for solr gsifiction including direct irrdition of the rectnts through trnsprent window, usully mde of fused qurtz, nd indirect heting through n opque wll, in which the solr energy is bsorbed by nontrnsprent wll nd trnsferred to inside prticles. Solr energy is lso used to dry wet biomss prior to the gsifiction process. Figure 7. Solr gsifiction rector concept. (Figure dpted from Adinberg et l. 84 ) Figure 7 shows the concept of solr gsifiction rector bsed on design by Adinberg et l. 84 The rector is centrl sphericlly or cylindriclly shped rector. An rry of verticl tubes re evenly distributed round the rector. Incoming solr rdition is bsorbed in these tubes, which contin molten slt. The tubes provide therml storge of the solr energy s well s rection chmber. Secondry concentrting optics (compound prbolic concentrtor) cn be dded to enhnce the therml concentrtion nd reduce therml losses. The bsorbed rdition cn het the molten slt up to pproximtely 850 C. 3.5. Gs Conditioning Trs must be removed by gs conditioning, which is generl term for removing unwnted impurities from the product gs tht usully involves multistep, integrted pproch. 85 A combintion of three min strtegies re used for gs conditioning (see Figure 8): hot gs conditioning, Figure 8. Gs clening strtegies for gs from biomss gsifiction rectors dpted from Milne et l. 50 wet scrubbing, or dry/wet-dry scrubbing. We will not cover ll the technologies for gs clening in this review but will present some of the more common ones. Trs cn be destroyed by therml destruction, but this typiclly requires very high tempertures of greter thn 1000 C. This high temperture cuses mteril nd economicl problems nd lso produces soot. Therefore, it is usully desirble to remove the trs t lower temperture, which requires the ddition of ctlyst nd often stem nd/or oxygen to the product gs. Ctlysts re used to rect the hydrocrbon trs with H 2 O, CO 2, nd/or O 2 producing CO, CO 2,CH 4,H 2, nd H 2 O. The CO/H 2 /CO 2 rtio is djusted during this rection, nd this rtio is very importnt for downstrem processing of the syngs. Ctlytic tr destruction voids the cost of ccumulting nd disposing trs by converting them into useful gseous products. If the syn-gs is to be used t high temperture then some method of hot gs clening t high temperture is desirble, since cooling nd reheting the gs, s occurs with wet scrubbing, decreses the PTE. Recent reviews hve been published on ctlytic reforming of trs, 50,56,61,86 which hs been shown to be n effective method of tr removl. According to Sutton et l. desirble ctlyst for hot gs conditioning should hve the following chrcteristics: 61 (1) The ctlyst must be effective for tr removl. (2) If the desired product is syn-gs, the ctlyst must reform the methne. (3) The ctlyst should provide suitble CO/H 2 rtio for downstrem processing. (4) The ctlyst should be resistnt to dectivtion s result of crbon fouling nd sintering. (5) The ctlyst should be esily regenerted. (6) The ctlyst should be strong. (7) The ctlyst should be inexpensive.

4056 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Two min types of ctlysts re used for hot gs conditioning: nonmetllic mixed oxide ctlysts nd metlbsed ctlysts. The principle nonmetllic mixed oxide ctlyst tht hs been used is clcined dolomite. 50,56,61 Dolomite is n inexpensive nturl sedimentry rock forming minerl consisting of clcium mgnesium ore with the generl formul CMg(CO 3 ) 2 nd found ll over the world. Clcintion t temperture of 800-900 C removes the CO 2 from the dolomite to form the ctive ctlytic phse, mixed MgO-CO. The clcintion rection is reversible, nd if the CO 2 prtil pressure is too high the inctive dolomite phse will form. Thus, dolomite is not good ctlyst when the syn-gs is highly pressurized. Other problems with dolomite include severe ctlyst ttrition nd the production of fine prticulte mteril in fluidized bed rectors. Delgdo et l., 87,88 Sutton et l., 61 nd Dyton 56 wrote reviews on gs conditioning with dolomite ctlysts. Other reserchers who hve studied dolomite ctlysts include Simell nd coworkers 89-91 nd Corell nd co-workers. 92,93 The operting conditions for using dolomite ctlyst re tempertures from 700 to 100 C nd spce times from 0.007 to 7 s. 56 Other nonmetllic oxide ctlysts used for this rection include MgO, 87 CO, 87 nd olivine ( mgnesium luminosilicte). 58,94,95 Ni-bsed ctlysts re used industrilly for stem reforming of nphth nd methne, 96,97 so it is not surprising tht Nibsed ctlysts hve proven to be very effective for hot gs conditioning of biomss gsifiction product gses. Ni-bsed ctlysts hve high ctivity for tr destruction, methne reforming, nd hve some WGS ctivity. Tr destruction occurs by stem reforming of hydrocrbons, which cn be described by three stoichiometric rections. In the first rection, the hydrocrbon dissocites on the metl surfce (eq 2) to form CO nd H 2. Once the CO nd H 2 re produced, equilibrium concentrtions of CO, H 2,CH 4,CO 2, nd H 2 O re formed ccording to the methntion (eq 3) nd WGS (eq 4) rections. Syn-gs cn lso be produced by dry reforming of methne in which the CO 2 insted of wter rects with the methne. The kinetic limiting step in methne stem nd dry reforming is probbly C-H bond ctivtion. 98 C n H m + nh 2 O f nco + (n + m2)h 2 (2) CO + 3H 2 T CH 4 + H 2 O (3) CO + H 2 O T CO 2 + H 2 (4) Reviews of Ni-bsed ctlysts for hot gs conditioning re published elsewhere. 50,56,61,86 The rection conditions for Ni-bsed ctlysts re tempertures of 600-900 C nd contct times of 0.01-3 s. 56 Most of the Ni ctlysts were supported on low-surfce re lumins. Additives such s MgO, CO, SiO 2,K 2 O, nd CuO hve been dded to Nibsed ctlysts. 60,92,99-108 A number of novel ctlyst compositions hve been tried s well for this rection including Ni/dolomite, 109 Co/MgO, 110 Ni/MgO, 111 LNi 0.3 Fe 0.7 O 3, 112 nd Ni/LO/Al 2 O 3. 113 The stem reforming of hevier hydrocrbons is rpid in the rnge of 500-600 C, while methne stem reforming occurs more slowly t tempertures of 800 C. 96,97 Severl dectivtion mechnisms occur with nickel-bsed ctlysts. These include poisoning by sulfur, chlorine, nd lkli metls, nd coke formtion. The high levels of impurities in biomss such s sulfur, chlorine, nd lkline bring new problems in regrd to ctlyst stbility. The coke cn be removed by oxidtion; however, repeted hightemperture regenertions of nickel-bsed ctlyst led to sintering, phse trnsformtions, nd voltiliztion of the nickel. If the syn-gs is to be used t tmospheric conditions, it is possible to use number of physicl methods to remove the trs such s web scrubbing. A disdvntge of wet scrubbing is the formtion nd ccumultion of wstewter, s well s tr disposl. For wet scrubbing technologies, cyclones re followed by cooling or scrubbing towers s the first units where the hevy trs condense. Venturi scrubbers re usully the second wet scrubbing units. Other tr seprtion units include demisters, grnulr filers, nd wet electrosttic precipittors (ESP). Wet ESP re significntly more expensive thn other tr removl systems. All wet gs clening systems generte contminted wstewter with orgnics, inorgnic cids, NH 3, nd metls, which must be treted downstrem by wet oxidtion, ctive crbon dsorption, nd/or gsifiction process sh crbon dsorption. Hot gs filtrtion with fbric, cermic, or metllic filter to remove ner-dry condensing tr prticles is lso possible nd is usully combined with ctlytic reforming. Figure 9. Pthwys for fuel production from syn-gs dpted from Spth nd Dyton. 114 4.0. Syn-Gs Utiliztion Figure 9 shows routes for trnsporttion fuels nd chemicls production from syn-gs. 114 The fuels produced from syn-gs include hydrogen by the wter gs-shift rection, methnol by methnol synthesis, lknes by Fischer-Tropsch Synthesis, isobutne by isosynthesis, ethnol by fermenttion, or with homogeneous ctlysts nd ldehydes or lcohols by oxosynthesis. Methnol is pltform chemicl used to produce rnge of other chemicls nd fuels including olefins, gsoline, dimethyl ether, methyl tert-butyl ether, cetic cid, nd formldehyde. In this section, we discuss the vrious processes to produce fuels nd chemicls from syn-gs. We then conclude by discussing the economics nd therml efficiency of the vrious processes. 4.1. Hydrogen Production by Wter Gs Shift Rection Industril hydrogen production, which is minly used for mmoni synthesis nd petrochemicl rections, is the mjor use of syn-gs. Hydrogen cn be used s fuel directly in PEM fuel cells. Hydrogen is lso n essentil rectnt for number of biomss conversion strgeties, 18,115 just like it is

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4057 n essentil rectnt in the petrochemicl refinery. Depending on the technology, biomss cn store H 2 in the form of biofuel. 22 The wter gs-shift rection (WGS), where CO rects with wter to form CO 2 nd H 2 (eq 5), djusts the CO/H 2 levels for further downstrem processing. CO + H 2 O f CO 2 + H 2 (5) Industril hydrogen production vi the WGS rection is done in two series of rectors: (1) high-temperture WGS rector t 350-500 C with Fe-oxide-bsed ctlyst, nd (2) low-temperture WGS rector t round 200 C with Cu-bsed ctlyst. 97 The CO concentrtion decreses to bout 2-3% in the first rector nd further to pproximtely 0.2%. Additionl H 2 purifiction cn be done with pressure swing dsorption, preferentil ir oxidtion (PROX), or Pd membrnes if high purity H 2 is required. 116 Recently, it hs been shown tht nnometer-sized gold supported ctlysts hve very high ctivities for CO oxidtion nd WGS rections, nd Au ctlysts my be used for PROX nd WGS rections. 117-121 Zhng et l. hve designed nd operted process for the production of H 2 by gsifiction of switchgrss or discrded seed corn t rte of 180 kg h -1 followed by hot-gs conditioning nd WGS s shown in Figure 10. 122,123 Air Figure 10. Pilot plnt for H 2 production by gsifiction of switchgrss. 122,123 fluidized the switchgrss in pilot-scle fluidized bed rector rted t 800 kw. A slip strem (5 L min -1 ) from the gsifiction rector ws sent to the hydrogen production process. First trce contminnts nd some trs were removed in dolomite gurd bed t 600 C. The unrected trs nd the lighter hydrocrbons were converted into CO nd H 2 by stem reforming with Ni ctlyst t 800 C. The CO then rected with stem to form H 2 nd CO 2 in the hightemperture WGS rector with n Fe-Cr ctlysts followed by low-temperture WGS rector with Cu-Zn-Al ctlysts tht converted more of the CO. No loss of ctlytic ctivity ws observed during opertion over n 8-16 h time period, lthough dectivtion in short time period would be difficult to detect t the high conversions in the study. The ctlysts were chrcterized nd hd deposits of sulfur, coke, nd chlorine s well s chnge in pore size fter 8-16 h time-on strem. An exciting lterntive to the WGS nd/or PROX rection ws recently reported nd tested t the lbortory scle by Kim et l. in which CO is converted to CO 2 nd electricity using queous polyoxometltes t significntly higher rtes thn the WGS rection. 124-126 The overll rection (eq 6) involves oxidtion of CO nd wter to CO 2 nd protons with polyoxyometltes (POM), such s H 3 PMo 12 O 40, in the presence of gold ctlyst. The queous solution of reduced POMs nd protons cn then be used to produce electricity t the node of PEM fuel cell. The POM solution is reoxidized in the process. The rte of CO consumption, defined s turnover frequency of 0.75-5 s -1 (turnover frequency is defined s moles of CO/(moles of metl surfce sites-second)), t room temperture by using POMs is higher thn the rte of WGS t 220 C. CO + H 2 O + 2PMo 12 O 3-40 f CO 2 + 2H + + 2PMo 12 O 4-40 (6) Biologicl methods re lso vilble to do the WGS rection with photoheterotrophic bcteri t mbient temperture nd pressure. 127 The rte for H 2 production with biologicl methods is currently very low nd hs been reported to be 96 mmol of H 2 L -1 h -1. 127 It hs been estimted tht 1250 L biologicl rector would be required to power 5 kw PEM fuel cell. 4.2. Methnol Production by Methnol Synthesis Methnol, which is one of the top 10 chemicls produced globlly, is produced by the methnol synthesis rection from syn-gs feedstocks, usully with Cu/ZnO-bsed ctlysts, t 220-300 C nd 50-100 br. 97 In 1923, BASF built the first synthetic methnol plnt on lrge scle using Zn/ Cr 2 O 3 ctlyst. Prior to this, methnol ws produced by slow pyrolysis of wood. Methnol synthesis is combintion of two exothermic rections, the WGS rection nd hydrogention of CO 2 to methnol, eqs 7 nd 8, respectively. 128-131 The net rection of these two rections is shown in eq 9. Methnol cn be produced from H 2 -CO or H 2 -CO 2 mixtures, but the rte of methnol production is 7 times higher for H 2 -CO-CO 2 mixtures. 132 Trnsient in-situ kinetic experiments suggest tht t industrilly conditions, methnol synthesis occurs vi hydrogention of CO 2. 133 For ctivity nd selectivity resons, the desired stoichimetric rtio for the syn-gs, defined s (H 2 -CO 2 )/(CO+CO 2 ) should be slightly bove 2. 134 CO + H 2 O f H 2 + CO 2 (7) CO 2 + 3H 2 f CH 3 OH + H 2 O (8) CO + 2H 2 f CH 3 OH + H 2 O (9) Methnol synthesis is thermodynmiclly fvorble t low tempertures nd high pressures. Byproducts of the methnol synthesis rection include methne, dimethyl ether, methyl formte, higher lcohols, nd cetone. One of the chllenges in using methnol synthesis is to design rectors tht efficiently remove the het from this exothermic rection. Copper ctlysts for methnol synthesis typiclly lst 2-5 yers nd undergo slow dectivtion by sintering nd poisoning. Copper ctlysts re sensitive to poisoning by sulfur nd the syn-gs should be purified to less thn 0.1% sulfur. 97 The presence of Cl in the gs phse will result in sintering of the Cu ctlyst. In commercil units, the conversion of syn-gs is limited to bout 25% per pss due to thermodynmic constrints. 135 Methnol is strting mteril for number of other fuels nd chemicls including olefins, gsoline, dimethyl ether, methyl tert-butyl ether (MTBE), cetic cid, hydrogen, nd

4058 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. formldehyde. The lrgest industril use of methnol include formldehyde (35% of methnol use), MTBE (25% of methnol use), nd cetic cid (9% of methnol use). 114 Methnol cn be used directly s trnsporttion fuel in internl combustion engines, 136-138 s feed for direct methnol fuel cells, 139,140 or to produce H 2 for fuel cell pplictions by on bord reforming. 141-145 Concerns bout methnol s toxicity, wter solubility, low vpor pressure, nd phse seprtion hve limited its use s direct fuel. 146 U.S. regultions limit the blending of methnol in gsoline to mximum concentrtion of 0.3 vol %. MTBE is used primrily s gsoline blend, nd it oxygentes gsoline thereby decresing ir pollutnts produced during gsoline combustion. MTBE is produced by recting isobutene with methnol in the presence of n cidic ctlyst s shown in eq 10. Solid cids, zeolites (H-ZSM- 5), nd especilly sulfonic cid ion-exchnge resins re some of the ctlysts used for MTBE production. 147,148 Recently, MTBE hs cused environmentl problems, due to MTBE groundwter contmintion owing to leking tnks in gs sttions, nd plns re being mde to phse out MTBE s gsoline dditive in the U.S. i - C 4 H 8 + CH 3 OH f (CH 3 ) 3 COCH 3 (10) Methnol lso cn be converted to olefins or gsoline. 149,150 This process ws first discovered in the 1970s by Mobil scientists who showed tht zeolite ctlyst, such s ZSM-5, could convert methnol into dimethyl ether (DME) followed by light olefins, nd then higher olefins, prffins, romtics, nd nphthenes. A commercil plnt tht produced gsoline from methnol (MTG) ws operted in New Zelnd by Mobil from 1981 to 1984 nd produced 14 500 bbl/dy. The first step in this process is dehydrtion of methnol t 300 C nd 27 tm to yield n equilibrium mixture of methnol, dimethyl ether, nd wter, which is then introduced to rector contining ZSM-5 t 350 C nd 20 tm to produce hydrocrbons nd wter. The selectivity to gsoline is greter thn 85% with the other 15% being light petroleum gs. 135 Approximtely 40% of the gsoline produced by MTG is romtics. This process cn lso be modified to produce lighter olefins such s propylene, ethylene, nd butylenes, nd UOP currently hs commercil process to produce olefins from methnol using silicoluminophosphte (SAPO) ctlysts. 150 Other eight-membered ring zeolites, such s chbzite, 149 ITQ-3, 151 ITQ-29, 152 nd ITQ-32, 153 offer new opportunities for production of olefins from methnol. Dimethyl ether cn be used s diesel fuel nd is produced in two-step process involving formtion of methnol, followed by dehydrtion of the methnol. Recent improvements in DME involve the development of bifunctionl ctlysts to produce DME in single gs-phse step 154,155 or the use of slurry rector. 156,157 Higher lcohols, including ethnol, 2-propnol, nd butnol re mde from syn-gs with ctlysts consisting of Cu, Zn, Mo, or Cr, promoted with lkli metls. Commercil processes for production of mixed higher lcohols hve been developed by Snmprogetti- Topsoe, Lurgi, Dow, nd IFP-Idemitsu. 158 The importnt rections tht occur in higher lcohol synthesis include methnol synthesis, WGS rection, CO bet ddition, ethnol homologtion, higher lcohol homologtion, condenstion, dehydrtion, DME formtion, brnched iso-lcohols, nd methyl ester synthesis. 114 Onbord processing of liquid fuels is one of the most promising methods of supplying hydrogen to protonexchnge membrne (PEM) fuel cells. Methnol due to its high energy density, low sulfur content, nd sfe hndling/ storge cpbilities is one of the leding cndidtes for fuelcell-driven utomobiles. Methnol cn be converted into H 2 by stem reforming (eq 11), or prtil oxidtion of methnol (eq 12). In utotherml reforming of methnol or oxidtive methnol reforming the het from the exothermic prtil oxidtion rection blnces the endothermic reforming rection s shown in eq 13. 143 Aqueous-phse reforming (APR) of methnol, where liquid wter rects with liquid methnol, lso cn be used to produce H 2. 25,159,160 Methnol reforming nd utotherml reforming usully occur t reltively low temperture (150-350 C) nd generte H 2 with low levels of CO. A number of ctlysts hve been used for this rection including Pd/ZnO, Pt/ZnO, nd Cu/ZnO. 141-145 Direct methnol fuel cells lso pper to be promising; however, ccording to Dillon et l. the biggest limittion is tht they hve low kinetic rtes of methnol oxidtion. 140 CH 3 OH + H 2 O f CO 2 + 3H 2 (11) CH 3 OH + 1 2 O 2 fco 2 + 2H 2 (12) CH 3 OH + 1 2 H 2 O + 1 4 O 2 f CO 2 + 5 2 H 2 (13) 4.3. Alkne Production by Fischer Tropsch Synthesis The Fischer-Tropsch synthesis (FTS) is n industril process to produce lknes from syn-gs using Co-, Fe-, or Ru-bsed ctlysts. This technology ws first developed in the erly 1900s nd used by Germny during the 1930s nd 1940s to produce liquid fuels from syn-gs-derived col. 161,162 After World Wr II Ssol, in South Afric, used FTS (nd still uses FTS tody) to produce gsoline nd diesel fuel. 163 Shell lso uses FTS in Mlysin plnt to produce lubes nd diesel fuel. Severl oil compnies re currently using or building FTS units to produce liquid fuels from nturl gsderived syn-gs in remote loctions. The overll rection in FTS is shown in eq 14. The WGS rection, nd the reverse of the WGS rection, occur during FTS (prticulrly on Fe ctlysts) djusting the CO/H 2 rtio, prticulrly when low H 2 /CO feed rtios re used. CO + 2H 2 f (1/n)C n H n + H 2 O (14) The products from FTS re rnge of mostly stright chin lknes rnging from C 1 to C 50 governed by the Anderson- Schulz-Flory (ASF) polymeriztion model. The lkne products re dependent on the chin growth probbility prmeter in the ASF model, nd gsoline or diesel fuel cnnot be mde selectively using FTS without producing lrge mount of undesired byproducts s shown in Figure 11. Methne formtion is usully significntly higher thn tht predicted by the ASF model. Modifying the ctlytic properties cn be used to tune the product selectivity, 164,165 but ttempts to overcome the ASF distribution hve not yet been successful. 166 However, recent results show tht it is possible to directly produce high octne gsoline in FTS process by coupling the Co or Fe ctlyst with ZSM-5 zeolite ctlyst tht crcks the longer chins in-situ producing gsoline rnge fuel high in brnched prffins nd romtics s shown in Figure 12. 167,168 Thus, Fe ctlysts supported on ZSM-5 hd higher lkne crbon distributions for gsoline

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4059 Figure 11. Idelized product distribution from Fischer-Tropsch synthesis bsed on the Anderson-Schulz-Flory model. Figure 12. Hydrocrbon distribution for Fischer-Tropsch synthesis with Fe-Co-K (bse), Fe supported on ZSM-5 with Si/Al rtios of 15, 25, 40, nd 120 (FeZ15, FeZ25, FeZ40, nd FeZ120, respectively), nd Fe supported on nnocrystlline ZSM-5 with Si/Al rtio of 50 (FeZN50). Reprinted with permission from ref 167. Copyright 2005 Elsevier. rnge fuel thn did trditionl Fe supported ctlysts. Currently, the gol in FTS is to produce hevy wxes nd then hydrocrck the wxes to gsoline nd diesel fuel. Fixedbed, slurry-bed, nd fluidized bed rectors re used commercilly for FTS. 97,166 FTS fuels were produced from biomss-derived syn-gs in demonstrtion pilot plnt in The Netherlnds tht successful rn for over 1000 hours with joint venture between Shell nd the Energy Reserch Centre of The Netherlnds (ECN). 169 Severl different concepts were explored for this pilot plnt, which consisted of fluidized bed gsifier with wood s the feed, followed by wet gs clening, gs conditioning, WGS rection, FTS, nd then ctlytic crcking of the FT wxes to produce premium sulfur-free diesel fuel. The yield of diesel fuel from wood by FTS of biomss-derived syn-gs is 120 L dieselfuel /metric ton biomss, 170 which is lower thn the yield of ethnol from wood reported by NRELs process, vi hydrolysis nd fermenttion, which is 320 L ethnol /metric ton biomss ; 171 however, synthetic nturl gs nd electricity re lso produced s byproducts of FTS. Boerringter hs predicted tht future improvements could llow the yield to increse to 210 L dieselfuel /metric ton biomss. 4.4. Other Syn-Gs Rections Fermenttion of syn-gs with the nerobic bcterium, Clostridium ljungdhlii produces ethnol. 172-175 The fermenttion performnce is not dversely ffected by specific CO/H 2 rtio, nd both CO nd H 2 /CO 2 mixtures cn be used simultneously even though the bcteri generlly prefer CO to H 2. Ethnol cn lso be produced from fermenttion of col or nturl gs-derived syn-gs. 172 Acetic cid is byproduct of this fermenttion process. According to Spth nd Dyton, the ethnol yields for syn-gs fermenttion re similr to those for direct fermenttion of corn-derived strches. 176 Other rections tht occur with syn-gs include oxysynthesis nd isosynthesis. Oxysynthesis or hydroformyltion involves rection of syn-gs with olefinic hydrocrbons to form n isomeric mixture of norml nd iso-ldehydes. 114,135 This rection is highly exothermic nd occurs in the presence of homogeneous metl crbonyl ctlysts. Tody, hydroformyltion is the fourth lrgest use of syn-gs nd used in the production of butnol, propnol, isobutnol, nd ethylhexnol. Isosynthesis involves the conversion of syn-gs to isobutene nd isobutene t extreme conditions (450 C nd 150-1000 tm) over thorium or zirconium-bsed ctlysts. This rection is not currently commercilly prcticed, nd current efforts re being mde to develop ctlysts tht work well t less severe rection conditions. 114 4.5. Anlysis of Syn-Gs Processes In this pper, we will use two types of therml efficiency nlysis: the process therml efficiency (PTE) nd the life cycle therml efficiency (LCTE). The PTE is defined s the energy in the product fuel divided by the energy of the biomss feedstock. The LCTE is the energy in the product fuel divided by the energy of the biomss feedstock plus the fossil fuel energy required to grow the biomss, trnsport the biomss, produce the process mchinery, produce ny fossil fuel used, nd trnsport the finl fuel. PTEs re reltively esy to clculte compred to LCTE. Different ssumptions mde during life cycle nlysis cn drsticlly chnge results. Different reserch groups hve rrived t wide rnge of conclusions regrding life cycle nlysis of biofuels. 177 Spth nd Dyton nlyzed the PTE nd economics of syn-gs-derived fuels with feedstock cost of $33/dry metric ton, nd the results of their nlysis re shown in Tble 9. 114 In their economic nlysis, they concluded tht syn-gs production ccounts for t lest 50% nd up to 75% of the finl product cost. As cn be seen from Tble 9, the cost of syn-gs-derived fuels on n energy bsis increses in the order H 2 < methnol ethnol < FTS liquids. The cost of production of ethnol from fermenttion of syn-gs is reported bsed on limited dt nd with high degree of uncertinty. 176 This nlysis is consistent with the results of Hmelinck et l. who hve lso studied the economics of production of FT trnsporttion fuels, methnol, nd hydrogen from biomss nd concluded tht FTS diesel is 40-50% more expensive thn methnol or hydrogen. 178,179 Also included in Tble 9 is the current cost of vrious petroleum-derived fuels. These costs re dependent on crude oil nd nturl gs prices, which cn be voltile. Figure 13 shows the FOB price of diesel fuel nd gsoline (in New York Hrbor) s function of crude oil price for the yers 1994-2005. During this time period, the cost of diesel fuel rnged from 11.6 to 43.3 /L. 14 The cost of diesel fuel derived from petroleum (currently 43.3 /L) is lower thn the cost of diesel fuel vi FTS (0.71-0.95 /L). However, ccording to Hmelinck et l. FTS biomss-derived trnsporttion fuels re currently economicl competitive with fossil diesel in Europe when the biofuels re exempted from excise duty nd vlue dded txes (11.6 nd 3.5 Euros/GJ

4060 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Tble 9. Therml Efficiency nd Selling Price of Syn-Gs-Derived Fuels products process therml efficiency energy product/ energy feed (LHV) life cycle therml efficiency energy product/ energy feed (LHV) $/GJ (LHV) minimum selling price from Spth 114 $/L or $/kg commercil prices 2003 from Spth nd Dyton 114 nturl gs to H 2 0.83 col to H 2 0.44 biomss to H 2 0.36-0.73 0.27-0.55 9-17 1.1-2.0/kg 0.7-1.4/kg b nturl gs to FTS liquids 0.54-0.63 biomss to FTS liquids 0.16-0.43 0.12-0.33 19-25 c 0.71-0.95/L 0.20/L d nturl gs to MeOH 0.61 biomss to MeOH 0.29-0.65 e 0.22-0.49 13-14 0.21-0.24/L 0.08-0.18/L ethnol vi syn-gs 0.35 0.27 14 0.34/L 0.26-0.37/L fermention olefins (propylene) from biomss-derived MEOH 0.21-0.46 0.16-0.35 18-20 0.81-0.92/kg 0.29-0.50/kg Adpted from Spth nd Dyton. 114 Cost of biomss used in the nlysis is $33/dry metric ton. The PTE for syn-gs production is 0.77 ccording to Prins et l. 180 Commercil prices re reported by Spth nd Dyton who wrote their report in 2003 when crude oil prices where pproximtely $25-31/bbl. 14 Life cycle therml efficiencies re estimted with dt from Towler 26 for euclyptus tree, which is lso reported in Tble 1 nd ssumes tht 5.57 MJ fossil fuel/kg wood nd LHV of 18.1 MJ/kg wood. b Hydrogen cost is for on-site usge. If H 2 is to be sold s product, it must be compressed, which increses the cost. The cost of liquefying nd trnsporttion cn increse the cost to $2.2-3.2/kg. c Hmelinck et l. hve estimted the current cost of FTS liquid from biomss to be $19/GJ nd tht the cost could decrese to $11/GJ in the future. 179 d The price of diesel fuel is dependent on the price of crude oil nd in 2005 the current FOB spot price of diesel fuel in the U.S. ws $0.44/L with crude oil prices of $57/bbl. e The process therml efficiency for conversion of wood into methnol hs been estimted by Mofftt nd Overend to be 0.50. 188 $/L or $/kg Figure 13. FOB spots prices for low sulfur diesel fuel nd reformulted gsoline in New York Hrbor s function of crude oil price. (Key: diesel fuel, squres; reformulted gsoline, tringle; nd yer, X. Source: Energy Informtion Assocition. 14 ) in the Netherlnds 2002). 179 The cost of H 2 derived from biomss ($1.1-2.0/kg) is within the sme price rnge of the mrket price of H 2 ($0.7-1.4/kg). Methnol from biomss ($0.21-0.24/L) is slightly more expensive thn the mrket price methnol in 2003 ($0.08-0.18/L). Importntly, these costs re estimtes tht re not bsed on dt from pilot plnt studies. Gsifiction nd other syn-gs rections re lredy estblished commercil processes; however, further process integrtion nd improvement must be mde. The dvntges of production of fuels by this route re tht ll of the biomss is converted into syn-gs, nd these re estblished technologies. The disdvntge of ll of these processes is tht they hve low PTE (typiclly round 16-50%); thus, lrge mount of energy tht ws previously in the biomss is irreversibly lost in the biomss conversion steps. Gsifiction of the biomss hs PTE of 75%, which represents the mximum PTE possible from syn-gs-derived fuels. Adding the energy required to produce nd trnsport the biomss decreses the therml efficiency even further. Prins et l. modeled the production of Fischer-Tropsch fuels from swdust nd reported the PTE (defined s energy in diesel fuel plus electricity divided by energy in biomss) to be 36%, with 34% of the energy in the diesel fuel nd the other 2% in electricity. 180 These results re consistent with those of Spth nd Dyton shown in Tble 9. The mjor exergy losses (exergy is the mount of energy in system tht is ble to do work) in FTS plnt re in the gsifiction (23% loss), stem genertion (9% loss), nd power genertion (24% loss) system. 180 The exergy losses in the gsifier re intrinsic becuse gsifiction is prtil oxidtion process tht decreses the heting vlue; however, these losses cn be minimized by drying the feedstock nd optimizing the gsifiction system. If more of the syn-gs is converted into liquid fuels, then the efficiency of the FTS process will increse, nd the energy losses in the power genertion system will decrese. The mximum possible overll energy efficiency for FTS plnt would be 46.2%, consisting of 41.8% fuels nd 4.4% electricity. 180 Figure 14 summrizes the mjor processes for conversion of biomss into fuels, chemicls, nd electricity by biomss gsifiction. There re number of processes for converting biomss into liquid fuels including gsifiction, prticulte removl, hot gs conditioning, WGS, nd synthesis gs conversion. The fundmentl chemistry in these processes is not well understood, nd it is likely tht hving more scientific understnding of these processes will led to more technologicl brekthroughs. Improved ctlysts re needed for number of these processes. It is likely tht gsifiction will continue to ply mjor role in electricity production from biomss. Production of fuels by gsifiction of biomss nd subsequent syn-gs conversion hs been proven t the pilot plnt scle. The extent to which this technology plys role in the future biofuel industry will depend on whether more economicl nd energy-efficient biomss conversion strtegies re developed. 5.0. Bio-Oil Production In ddition to producing gses, thermochemicl tretment of biomss cn lso produce liquids nd solids. The residence time, heting rte, nd temperture re the prmeters tht determine if thermochemicl biomss tretments produce liquids, gses, or solids (Tble 10). Process conditions tht

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4061 Figure 14. Summry of gsifiction technology for production of liquid fuels nd electricity from biomss. Reprinted from ref 181 with permission. Copyright 2001 Elsevier. Tble 10. Biomss Pyrolysis Technologies, Rection Conditions, nd Products nme residence time temp ( C) heting rte mjor products conventionl crboniztion hours-dys 300-500 very low chrcol pressurized crboniztion 15 min-2 h 450 medium chrcol conventionl pyrolysis hours 400-600 low chrcol, liquids, gses conventionl pyrolysis 5-30 min 700-900 medium chrcol, gses flsh pyrolysis 0.1-2 s 400-650 high liquids flsh pyrolysis <1 s 650-900 high liquids, gses flsh pyrolysis <1 s 1000-3000 very high gses vcuum pyrolysis 2-30 s 350-450 medium liquids pressurized hydropyrolysis <10 s <500 high liquids Adpted from Klss. 2 fvor liquid products re short residence times, fst heting rtes, nd moderte tempertures. The liquids produced by pyrolysis re nonthermodynmiclly controlled products. Optiml residence times nd tempertures re necessry to freeze the desired intermedites. Long residence times t low temperture produce primrily chrcol, nd high tempertures produce minly gs products. The slow pyrolysis of wood (24 h residence time) ws common industril technology to produce chrcol, cetic cid, methnol, nd ethnol from wood until the erly 1900s. According to Klss, 2 the verge product yield per cord of sesoned hrdwood ws 1025 kg of pyrolyligenous cid (contining 80% wter, 9% trs nd oils, 7% cetic cid, nd 4% methnol nd cetone), 454 kg of chrcol, nd 212 m 3 of fuel gs with heting vlue of 9-11 MJ/m 3. In this section, we discuss how to produce liquid oils, clled bio-oils by thermochemicl tretment of biomss. Liquefction nd pyrolysis re the two mjor technologies to produce bio-oils. Pyrolysis oils re wter soluble nd hve higher oxygen content thn liquefction oils. Liquefction occurs t 50-200 tm nd 250-325 C, wheres pyrolysis occurs from 1 to 5 tm nd 375-525 C. Pyrolysis hs lower cpitl cost thn liquefction, nd mny pyrolysis technologies re currently being used commercilly. The dvntge of bio-oil production is tht it requires only single rector, nd lrge frction of the biomss energy (50-90%) cn be converted into liquid. A wide rnge of feedstocks cn be used for bio-oil production, including wood, blck liquor, griculturl wstes, nd forest wstes. Bio-oils re mixture tht cn contin more thn 400 different compounds, including cids, lcohols, ldehydes, esters, ketones, nd romtic compounds. 182 Commercilly, bio-oils re used s boiler fuel for sttionry power nd het production, nd for chemicl production. Biooils must be upgrded if they re to be used s trnsporttion fuels, which is the subject of Section 6.0. 5.1. Bio-Oils by Fst Pyrolysis Bio-oils re produced by pyrolysis processes where the biomss feedstock is heted in the bsence of ir, forming gseous product, which then condenses. Slow pyrolysis produces lrge mounts of coke, which cn be used s solid fuel, wheres fst pyrolysis produces bio-oils in high yields of up to 80 wt % dry feed. Bridgwter nd Pecocke hve recently completed review summrizing fst pyrolysis technology. 183 Another recent review on pyrolysis ws written by Mohn et l. 184 A summry of the developments on direct liquefction of biomss from 1983 to 1990 by the Working Group of the Interntionl Energy Agency, Bioenergy ctivity on direct liquefction of biomss is presented elsewhere. 182 A number of fst pyrolysis technologies hve been commercilized by Ensyn Technologies (six circulting fluidized bed plnts, lrgest is 50 t/dy), Dynmotive (10 t/dy fluidized bed process, nd currently building 100 t/dy

4062 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Tble 11. Key Fst Pyrolysis Design Fetures Pretretment prticle size smll prticles needed; expensive feed drying essentil to 10% wshing nd dditives for chemicl production Rector het supply high het trnsfer rte needed het trnsfer gs-solid nd/or solid-solid heting rtes wood conductivity limits heting rte rection temperture 500 C mximizes liquids from wood rector configurtion mny configurtions hve been developed Product Conditioning nd Collection vpor residence time criticl for chemicls, less for fuels secondry crcking reduces yields chr seprtion difficult from vpor or liquid sh seprtion more difficult thn chr seprtion liquids collection difficult; quench nd ep seem best Adpted from Bridgewter nd Pecocke. 183 plnt), BTG (rotry cone rector 5 t/dy, wnts to build 50 t/dy plnt), Fortum (12 t/dy pilot plnt), nd Bioenergy Prtners (15 t/dy pilot plnt, designed 100 t/dy plnt). 185 Tble 11 shows the key fst pyrolysis design prmeters. These essentil prmeters include 183 (1) A very high heting nd het trnsfer rte tht requires finely ground biomss feed. (2) Crefully controlled tempertures round 450-550 C (3) Rpid cooling of the pyrolysis vpors (residence time of less thn 1 s). Tble 12. Typicl Properties of Wood Pyrolysis Bio-Oil, Liquefction Bio-Oil, nd Hevy Fuel Oil property pyrolysis oil liquefction oil hevy fuel oil moisture content, wt % 15-30 5.1 0.1 ph 2.5 specific grvity 1.2 1.1 0.94 elementl composition, wt % crbon 54-58 73 85 hydrogen 5.5-7.0 8 11 oxygen 35-40 16 1.0 nitrogen 0-0.2 0.3 sh 0-0.2 0.1 higher heting vlue, MJ/kg 16-19 34 40 viscosity (50 c), cp 40-100 15000 (t 61 C) 180 solids, wt % 0.2-1 1 distilltion residue, wt % up to 50 1 Adpted from Czernik nd Bridgwter 185 nd Elliot nd Schiefelbein. 346 Tble 12 lists the properties of wood-derived fst pyrolysis oils, liquefction oils, nd diesel fuel. Pyrolysis-derived oils hve higher oxygen content, moisture content, nd lower heting vlue (17 MJ/kg) thn conventionl fuel oil (43 MJ/ kg). Liquefction oils hve higher heting content, lower oxygen content, nd lower moisture content thn fst pyrolysis oils. Fst pyrolysis bio-oils lso re cidic, hving ph of bout 2.5. The bio-oils re chemiclly unstble, undergoing vrious rections with time nd temperture. A typicl fst pyrolysis system is shown in Figure 15. First, the biomss needs to be dried, which cn be done with lowgrde process het such s the outlet flu gs. The biomss prticles must then be ground so tht they hve the optiml het trnsfer properties. Grinding specifiction re less thn Figure 15. Fst pyrolysis rector system dpted from Bridgwter nd Pecocke. 183 0.2 mm for rotting cone rectors, 2 mm for fluid bed, nd 6 mm for circulting or trnsported fluid beds. The cost of grinding increses when smller prticles re desired. Overviews on drying nd grinding of biomss re given elsewhere. 2,186 At the hert of fst pyrolysis process is the rector. Most reserch hs focused on the rector even though its cost is only 10-15% of the cpitl cost of the entire plnt. Four min rector technologies re currently vilble for commerciliztion including (1) fluidized beds, (2) circulting fluid beds, (3) bltive pyrolyzer, both cyclonic nd plte type, nd (4) vcuum pyrolyzer. 187 However, the two more populr configurtions re fluidized bed nd circulting fluidized bed rectors. A fst pyrolysis rector must hve very high heting nd het trnsfer rtes, moderte nd crefully controlled temperture, nd rpid cooling or quenching of the pyrolysis vpors. 183 Fluid beds or bubbling fluid bed, s opposed to circulting fluid bed hve the dvntges of good temperture control, very efficient het trnsfer, short residence times for vpors, nd being technologiclly fesible. The residence time is controlled by the fluidizing gs flow rte, nd is higher for chr thn for vpors. It is necessry to use shllow bed depths nd/or high gs flow rte to chieve short voltiles residence times. 187 The high gs-to-biomss fed rtio results in lowering of the therml efficiency (which is typiclly 60-70%, see Section 6.6). The control of temperture nd concentrtion grdients in fluid bed rectors requires specil design methods due to the low bed height-to-dimeter rtio. Smll prticle sizes of less thn 2-3 mm re needed for this rector. Rector heting cn be ccomplished by hot wlls, hot tubes, hot gs injection, nd hot snd recycling. The products from this rector hve low concentrtion of chr, since chr is rpidly removed from the rector. A highqulity bio-oil is produced in this rector. Circulting fluid beds nd trnsported beds re similr to fluidized beds except tht the chr residence time is lmost the sme s the vpor nd gs residence time. 187 The hydrodynmics of circulting fluid beds re complex; however, they cn still be used for very high throughputs. Process het is supplied by recircultion of heted snd. The rotting cone rector is similr to the circulting fluid bed, except tht the snd nd biomss re trnsported by centrifugl forces of the rotting cone. Abltive pyrolysis relies on the het trnsfer from hot surfce, such s the rector wll, to the solid biomss

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4063 prticle. 187 Incresing the pressure of the rector increses the heting rte, by pushing the biomss prticle onto the hot surfce with greter force. The het moves through the biomss prticle in single direction. A residul oil film forms nd provides lubriction for successive biomss prticle. The oil film lso evportes forming pyrolysis vpors. A rector wll temperture less thn 600 C is required, nd high reltive motion between prticle nd rector wll is lso desirble. An dvntge of this type of rector is tht lrge prticles cn be used since rection rtes re not limited by het trnsfer. However, the process is limited by the rte of het supply to the rector rther thn the rte of het bsorption by the pyrolyzing biomss. Surfce re of the rector is key design vrible. In comprison with other rectors lrge mounts of tr re produced in bltive rectors. Vcuum pyrolysis hs the dvntge of short residence time for voltiles, with longer residence time for the solids. 187 The disdvntges of vcuum pyrolysis re tht poor het nd mss trnsfer rtes occur. Following the pyrolysis rector, cyclone seprtes the solid chr products. 183 It is desirble to collect s much chr s possible, since chr not removed will collect in the liquid products cusing further downstrem processing problems. Chr cn lso ct s vpor crcking ctlyst degrding the pyrolysis products. Chr seprtion is difficult, nd hot vpor filters, which re currently being developed, lso cn be used with cyclones. The chr is burned to provide process het for pyrolysis nd biomss drying. The liquid-gs products re then seprted. The liquid products must be quickly condensed. Otherwise, they will rect nd crck t high tempertures. Production of chemicls nd food dditives requires vpor residence times of few hundred milliseconds. Longer vpor residence times of round 2 s cn be used if bio-oils re to be used s fuel. Short vpor residence times re n engineering difficulty, nd novel techniques such s quenching nd electrosttic precipittion hve been used. However, creful design nd temperture control re needed to void blockge from differentil condenstion of the hevy products. 5.2. Bio-Oils by Liquefction Liquefction of biomss produces wter-insoluble biooil by tretments t high pressure (50-200 tm) nd low temperture (250-450 C). The overll objective of biomss liquefction is to control the rection rte nd rection mechnisms, using pressure, gses, nd ctlysts, to produce premium liquid oil. The rector feeds consist of slurry contining the solid biomss feed in solvent, reducing gses such s H 2 or CO, nd/or ctlyst. The bio-oil produced by liquefction hs lower oxygen content nd therefore higher energy content thn pyrolysis-derived oils (Tble 12). There re vriety of liquefction processes including hydrotherml processing (wter or queous solvent), hydropyrolysis (no crrier liquid solvent), nd solvolysis (rective liquid solvent). The high-pressure processing tht occurs with liquefction cuses technicl difficulties nd n incresed cpitl cost. A review of previous biomss liquefction reserch from 1920 to 1980 is presented by Moffttt nd Overend. 188 A number of ctlysts hve been used for liquefction including lkli (from the lkline sh components in wood, lkline oxides, crbontes, nd bicrbonte), metls (such s zinc, copper nd nickel, formte, iodine, coblt sulfide, zinc chloride, ferric hydroxide), nd Ni nd Ru heterogeneous ctlysts (which id in preferentil hydrogention). A number of different solvents hve been used for liquefction including wter (the most common solvent), 188 creosote oil, 189 ethylene glycol, 189 methnol, 188 nd recycled bio-oil. 188 Wter is one of the most ttrctive due to its low cost. Aqueous-phse liquefction do not require drying step nd therefore re idel for processing wet biomss. Recycling the product oil into the rector hs been shown to increse the product selectivity. 188 Hydro-pyrolysis involves liquefction of biomss with high-pressure H 2 nd heterogeneous ctlyst. 188 Solvolysis is relted high-pressure process where liquids such s creosote oil, ethylene glycol, simple lcohols, nd phenol re used s solvents. A liquefction process entitled hydrotherml upgrding (HTU) ws originlly developed by Shell nd is currently being commercilized by Shell, BTG, TNO-MEP, Biofuel nd Stork Engineers nd Contrctors. This liquefction process tkes plce t 300-350 C, 120-180 br, nd 5-20 min residence times. 16 A typicl product consists of 45 wt % biocrude, 25 wt % gs (mostly CO 2 ), 20% H 2 O, nd 10 wt % dissolved orgnics, cetic cid, methnol. According to Goudrin et l., the dvntges of liquefction process re the high therml efficiencies for conversion of wet feedstocks, good product qulity/flexibility, the potentil for upscling, nd rpid rte of commercil development. However, the HTU bio-oils do hve high viscosity, nd it is questionble if this technology could indeed be rpidly commercilized. 5.3. Bio-Oil Chemistry Bio-oils re usully drk brown, free-flowing liquid tht hs distinctive odor. During bio-oil production, lrge number of rections occur, including hydrolysis, dehydrtion, isomeriztion, dehydrogention, romtiztion, retro-condenstion, nd coking. The exct composition of the bio-oil is dependent on 190 (1) The feedstock (including dirt nd moisture content) (2) Orgnic nitrogen or protein content of the feedstock (3) Het trnsfer rte nd finl chr temperture during pyrolysis (4) Extent of vpor dilution in the rector (5) Time nd temperture of vpors in the rector (6) Time nd temperture of vpors in heted lines from the rector to the quench zone (7) If the vpors pss through the ccumulted chr during filtrtion (8) Efficiency of the chr removl system (9) Efficiency of the condenstion equipment to recover the voltile components from the noncondensble gs strem (10) If the condenstes hve been filtered to remove suspended chr fines (11) Wter content of the feedstock (12) Extent of contmintion of the bio-oil during storge by leching of continers (13) Exposure of ir during storge (14) Length of storge time (15) Storge temperture Milne et l. hve summrized the chemicl composition of bio-oils, which we report in Figure 16. 191 Milne s nlysis is consistent with more recent study by Brnc et l. 192 More thn 400 orgnic compounds hve been found in biooils. Figure 16 shows the rnge of compositions tht cn be found in bio-oils. The compounds in the bio-oil cn vry by

4064 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Figure 16. Chemicl composition of bio-oils ccording to Milne et l. 191 The grph lso shows the most bundnt molecules of ech of the components nd the biomss frction from which the components were derived. Figure 17. Mechnism of cellulose degrdtion without lkli metls (A) nd glucose degrdtion with lkli-metl-ctlyzed or glycoside rupture pthwys (B) from Evns nd Milne. 53 (Reprinted from ref 53 with permission. Copyright 1987 Americn Chemicl Society.) more thn n order of mgnitude. The bio-oil contins cids (some of the mjor components include cetic, propnoic), esters (methyl formte, butyrolctone, ngelic lctone), lcohols (methnol, ethylene glycol, ethnol), ketones (cetone), ldehydes (cetldehyde, formldehyde, ethnedil), miscellneous oxygentes (glycolldehyde, cetol), sugrs (1,6-nhydroglucose, cetol), furns (furfurol, HMF, furfurl), phenols (phenol, DiOH benzene, methyl phenol, dimethyl phenol), guicols (isoeugenol, eugenol, 4-methyl guicol), nd syringols (2,6-DiOMe phenol, syringldehyde, propyl syringol). The multicomponent mixtures re derived primrily from depolymeriztion nd frgmenttion rections of the three key building blocks of lignocellulose: cellulose, hemicellulose, nd lignin. The guicols nd syringols re formed from the lignin frction, wheres the miscellneous oxygentes, sugrs, nd furns form from the cellulose nd hemicellulose biomss frction. The esters, cids, lcohols, ketones, nd ldehydes probbly form from decomposition of the miscellneous oxygentes, sugrs, nd furns. Pyrolysis of pure cellulose produces minly levoglucosn in yields of up to 60%. 193 Levoglucosn probbly forms by mechnism involving intrmoleculr condenstion nd sequentil depolymeriztion of the glycosidic units s shown in Figure 17A. 53 Inorgnic impurities of the biomss ply key role in terms of the bio-oil product selectivity. The cellulose degree of polymeriztion nd crystllinity do hve some influence on the bio-oil composition, but in generl these effects re not s lrge s the effect of inorgnic impurities. For exmple, the composition of levoglucosn is low in the pyrolysis of most biomss even though the cellulose concentrtions re greter thn 50%. The ddition of minor mounts of lkli (such s K, Li, C) to cellulose shifts the mechnism (nd the finl product selectivity) so tht glycolldehyde is the stble rection intermedite insted of levuglucosn. 53,65 The exct mechnism by which trce quntities of slts nd metl ions influence the pyrolysis course is not known, lthough Evns nd Milne hve suggested probble mechnism s shown in Figure 17B. The presence of lkli slts hs greter influence on the rection mechnism thn temperture. Alkli ctions lso increse the rte of rection during pyrolysis. 194 Lignocellulose cn be pretreted to

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4065 Tble 13. Inorgnic Compounds in Bio-Oils nd Chr feedstock ok southern pine switchgrss hybrid poplr chr in oil (>2 µm) bio-oil chr in oil (>2 µm) chr in oil (>10 µm) chr in oil (2-10 µm) bio-oil bio-oil mteril bio-oil chr removl cyclone cyclone + oil cyclone cyclone + oil cyclone + oil cyclone + oil hot-gs hot-gs method filter (2 µm) filter (2 µm) filter (10 µm) filter (2 µm) filter filter chr % 0.74 0.13 sh % 0.09 0.03 15.3 <0.05 0.01 0.007 impurities (ppm) C 160 4580 160 8100 7100 2.2 2.2 1 Si 112 93 3452 1.4 K 55 1300 10 667 8500 175 2.7 1 Fe 86 47 1772 Al 55 41 2.6 0.3 N 2 60 <0.1 372 690 17 7.2 0.9 S <60 <50 349 P <50 <50 550 3600 3.6 Mg <55 <45 903 0.7 Ni <22 <20 288 Cr <17 <17 524 Zn 28 14 258 Li 25 7 110 Ti 17 5 130 <0.2 Mn 15 6 353 0.063 0.04 Cu 39 B <3 <2 170 V 0.002 <0.01 Cl 10600 1600 7.9 11 Adpted from Diebold. 190 remove lkli slts by ion-exchnge prior to pyrolysis. Pyrolysis of wood nd cellulose fter lkli removl leds to high yields of levoglucosn (27% from wood, 45% from cellulose). 195 The pyrolysis behvior of sugrs is different thn tht of cellulose even though they hve similr chemicl structures. 196 Hydroxycetldehyde my be mjor product from glycosidic rupture pthwy, nd sugrs re known to undergo retro-ldol condenstion in sub- nd supercriticl wter. Cellulose pyrolysis yields more levoglucosn, fewer moleculr weight oxygented compounds (such s glycolldehyde nd cetldehyde), nd fewer furns (such s furfurl nd HMF) thn does glucose pyrolysis. This difference in rectivity could be becuse crbohydrtes hve cyclic or open ring forms, wheres cellulose is in fixed polymer structure. Evns et l. studied the pyrolysis of lignin with moleculrbem mss spectrometery 30 nd observed tht lignins preferentilly form their precursor monomers. The lignin frction undergoes primry pyrolysis by structurlly controlled depolymeriztion. Lignin components tht pper in bio-oils include coniferyl lcohol, sinpyl lcohol, isoeugenol, vnillin, vinylguicol, methyl guicol, guicol, nd ctechol. 53 Coniferyl nd sinpyl lcohol re the first products to form from lignin, while the lower moleculr weight products (guicol nd ctechol) re formed lter. The primry pyrolytic lignin content is mostly oligomeric nd monomer content is smll. Bio-oils contin inorgnic compounds s shown in Tble 13. During bio-oil storge, the inorgnic compounds of biomss ctlyze polymeriztion nd other rections in the bio-oil leding to viscosity increse. Leching of processing nd storge equipment by the cidic bio-oils cn lso cuse inorgnic contminnts in the bio-oils. Therefore, cre must be tken to properly design equipment. In contrst to col nd crude oil, biomss contins low mounts of sulfur, nd most of the sulfur becomes concentrted in the chr (Tble 13). Cellulose pyrolysis kinetics hve been studied by mesuring the weight loss s function of temperture in thermogrvimetric nlyzer (TGA). The rection is endothermic, nd the weight loss cn be fit with first-order rte lw nd n ctivtion energy of 240 kj/mol. 65,197 Cellulose derived from different mnufcturers hve shown lrge difference in the kinetics. 197 The kinetics of pyrolysis of lignin nd xyln cnnot be described by first-order rection model. 198 However, the pyrolysis kinetics of lignocellulosic mteril cn be modeled with three first-order rections of three pseudo-components where the models correspond to the frction of hemicellulose, cellulose, nd lignin. 198 A number of different rection models hve been proposed for cellulose decomposition including the Broido-Shfizdeh, 200,201 Wterloo, 202 Diebold, 203 Várhegyi-Antl, 65 nd Wooten-Seemn-Hjligol model. 199 The mjority of these models hve cellulose being converted into more ctive form of cellulose, which is the rte-limiting step. Figure 18 shows the Wooten-Seemn-Hjligol model where the first Figure 18. Mechnism for cellulose decomposition dpted from Wooten, Seemn nd Hjligo. 199

4066 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Figure 19. Cellulose decomposition pthwys in supercriticl wter. step is formtion of n ctive intermedite form of cellulose (identified by NMR) nd then the cellulose decomposed into levoglucosn, crbohydrtes, or other compounds contining methyl, romtics, ketones, or other functionl groups. Insight into the pyrolysis mechnism cn be lerned from studying the chemistry for the decomposition pthwys of cellulose nd glucose in queous wter, which is shown in Figure 19. 204-206 Glucose undergoes isomeriztion to form fructose, which then cn undergo dehydrtion to form HMF. The mechnism of HMF formtion is reviewed by Antl nd co-workers. 207 Further dehydrtion of HMF yields 1:1 mixture of levulinic nd formic cids. Angelic lctone forms by dehydrtion of levulinic cid. Retro-ldol rections produce glycolldehyde, dihydroxycetone, glycolldehyde, nd erythrose from fructose nd glucose. These intermedites rect further to form pyurvldehyde, glcolldehyde, nd cids. Glucose cn lso form 1,6-nhydroglucose by dehydrtion. Decomposition of HMF in pyrolysis chrs hs been shown to form 1,2,4-benzenetriol. Hemicellulose undergoes nlogous rection pthwys to those shown in Figure 19. 5.4. Bio-Oil Problems The most significnt problems of bio-oils s fuel re poor voltility, high viscosity, coking, corrosiveness, nd cold flow problems. 185 These problems hve limited the pplictions of bio-oils. No qulity stndrds hve yet been mde for bio-oil production. The min concerns for burning biooils in diesel engines hve to do with difficult ignition (due to low heting vlue nd high wter content), corrosiveness (cids), nd coking (thermlly unstble components). Biooils must be upgrded or blended to be used in diesel engines (Section 6.0). Bio-oils polymerize nd condense with time, nd this process is ccelerted by incresing temperture, oxygen exposure, nd UV light exposure. These rections result in incresing viscosity nd phse seprtion in the bio-oil. Diebold hs written review on the chemicl nd physicl mechnisms of the storge stbility of fst pyrolysis biooils. 190 According to Diebold, the probble rections tht occur within bio-oil tht cuse degrdtion re (1) Orgnic cids with lcohols forming esters nd wter (2) Orgnic cids with olefins forming esters (3) Aldehydes nd wter to form hydrtes (4) Aldehydes nd lcohols forming hemicetls or cetls nd wter (5) Aldehydes forming oligomers nd resins (6) Aldehydes nd phenols forming resins nd wter (7) Aldehydes nd proteins forming oligomers (8) Orgnic sulfur forming oligomers (9) Unsturted compounds forming polyolefins (10) Air oxidtion tht forms cids nd rective peroxides (which ctlyze polymeriztion of unsturted compounds) Rections 1-4 form products in thermodynmic equilibrium where chnge in temperture or concentrtion will cuse reversible rection. Rections 5-10 form resins or polyolefins tht re probbly irreversibly produced. 5.5. Economics nd Therml Efficiencies of Bio-Oil Production Methods The mjor chllenges for producing bio-oils re 185 (1) Cost of bio-oil is 10-100% more thn fossil fuel (bsed on the cost of fossil fuels in 2004). (2) Avilbility: there re limited supplies for testing nd development of pplictions. (3) There re lck of stndrds nd inconsistent qulity. (4) Bio-oils re incomptible with conventionl fuels. (5) Users re unfmilir with this mteril. (6) Dedicted fuel hndling systems re needed. (7) Pyrolysis s technology does not enjoy good imge. The economics nd process therml energy efficiency for production of liquid trnsporttion fuels hve been nlyzed by the Working Group of the Interntionl Energy Agency

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4067 Tble 14. Economic nd Therml Efficiency Anlysis for Production of Gsoline nd Diesel Fuels by Pyrolysis nd Liquefction tmospheric flsh pyrolysis (AFP) liquefction in pressurized solvent (LIPS) present potentil present potentil Totl Cpitl Requirement ($U.S. millions) primry liquefction 49.8 26.4 84.2 48.4 crude upgrding 46.6 34.3 26.8 26.0 product finishing 14.5 0.7 15.3 0.7 totl 110.9 61.4 126.3 75.1 Production Costs ($U.S. million/yer) fixed operting costs 14.48 10.77 14.48 10.03 vrible operting 25.74 23.67 33.44 33.60 costs (feedstock costs) (20.00) (20.00) (20.00) (20.00) cpitl chrges 12.96 7.17 14.75 8.78 totl production cost 53.18 41.61 62.67 52.39 Minimum Selling Price ($U.S./GJ) bio-oil 9.32 6.91 13.44 12.27 refined bio-oil 16.24 12.99 19.54 14.77 Process Therml Efficiency (energy liquid products/energy feed+inputs) primry product from liquefction 0.61 0.68 0.55 0.48 finished product 0.52 0.53 0.48 0.49 Life Cycle Therml Efficiency (energy liquid products/energy feed+inputs) finished product 0.40 0.40 0.36 0.37 Results in 1990 U.S. dollrs nd ssuming feedstock cost of $30/wet metric ton (wood chips with 50% moisture content) biomss bsed on 1000 dry metric ton/dy of biomss feed ccording Elliott et l. 208 Current FOB spot price cost of diesel fuel bsed on higher heting vlue is $11/GJ with oil t $57/bbl. 14 Life cycle therml efficiencies re estimted with dt from Towler 26 for euclyptus trees which is lso reported in Tble 1 nd ssumes tht 5.57 MJ fossil fuel/kg wood nd LHV of 18.1 MJ/kg wood. direct biomss liquefction ctivity, nd the results re shown in Tble 14. 208 The ssessment ws done with liquid fuels from tmospheric flsh pyrolysis (AFP) nd liquefction in pressurized solvent (LIPS). Three steps were nlyzed including (1) primry liquefction to crude oil product, (2) ctlytic hydrotreting to upgrde the crude product to deoxygented product oil, nd (3) refining the deoxygented product to gsoline nd diesel fuel. The refining costs were estimted bsed on costs for refined oils derived by liquefction of col nd oil shle nd do not represent ctul experimentl results. The AFP process consists of rpid pyrolysis in snd bed of wood fibers to vpors nd chrs developed t University of Wterloo. The LIPS process is bsed on tests t the Biomss Liquefction Experimentl Fcility in which wood chips re mixed with recycled woodderived oil, sodium crbonte, nd syn-gs in n upflow tubulr rection t 350 C, 20.5 MP with 20 min residence time. Ctlytic hydrotreting ws done to upgrde both of the primry oils in two seprte stges: low-temperture stge (300 C) followed by high-temperture stge (350 C). The economic nlysis is bsed on plnt cpcity of 1000 dry tons/dy of biomss, cost of $30/metric ton of wood chips (50% moisture content), nd 10% interest rte. As shown in Tble 14, the cpitl cost for primry liquefction of oils with LIPS is 70-80% higher thn for AFP. The cpitl cost for the ctlytic upgrding of the oils from the AFP process is higher thn for the LIPS process, since liquefction-derived oils hve higher oxygen content thn do pyrolysis-derived oils. Therefore, the finl cpitl cost for the AFP oils is only 14-22% tht for the LIPS process. For both the AFP nd LIPS processes the feedstock cost represents only 30-50% of the finl production cost. Cheper feedstock costs will significntly chnge the cost of the finl product. The minimum selling price of the pyrolysis-derived oils nd upgrded products is less thn the liquefction-derived oils. The cost of producing refined liquid fuels ($13-20/GJ) is greter thn the cost of gsoline nd diesel fuel in 2005 ($11.5/GJ). However, the cost of the biooil ($7-13/GJ) cn be less thn the current cost of gsoline nd diesel fuel. Economic nlysis of the Shell HTU liquefction process hve estimted tht biocrude product cn be produced t $4.6/GJ if the biomss feedstock cn be obtined t zero cost. 16 The process therml energy efficiency of the primry oil products rnges from 0.61 to 0.68 for the pyrolysis oils to 0.48-0.55 for the liquefction oils (Tble 14). The PTE decreses during ctlytic upgrding nd refining to 0.48-0.52. It hs been climed tht the HTU liquefction process hs n overll PTE of 70-90%. 16 Shell currently hs pilot plnt with climed 75% therml efficiency. Bio-oil process therml efficiencies re higher thn liquid fuels derived by biomss-derived syn-gs followed by FTS (0.16-0.43) s shown in Tble 9. Reserch in bio-oil production hs shifted to focus on production of less costly fst pyrolysis oils minly due to the high cpitl cost involved for high-pressure liquefction processes (Tble 14). According to Elliott et l., upgrding of the high-pressure liquefction-derived bio-oils does not pper to hve ny significnt dvntge in the upgrding re. 182 However, in the long term liquefction-derived biooils my prove to be more beneficil since they hve properties more similr to trnsporttion fuels. It would be desirble to be ble to control the chemistry occurring during pyrolysis nd liquefction by ddition of ctlysts nd controlling the rection prmeters. This mens tht gin the fundmentl chemistry of the processes involved needs to be better understood, nd future reserch on this subject is required. Most of the fuels currently mde from pyrolysis re low vlue products nd require further upgrding; therefore, bio-oil upgrding ppers to be promising reserch re.

4068 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. 6.0. Bio-Oil Upgrding Bio-oils must be upgrded if they re to be used s replcement for diesel nd gsoline fuels. As ws sid in the previous section, the properties tht most negtively ffect bio-oil fuel qulity re low heting vlue, incomptibility with conventionl fuels, solids contents, high viscosity, incomplete voltility, nd chemicl instbility. Bio-oils cn be upgrded into liquid trnsporttion fuel by three different routes: (1) hydrodeoxygention with typicl hydrotreting ctlysts (sulfided CoMo or NiMo) (Section 6.1), (2) zeolite upgrding (Section 6.2), or (3) forming emulsions with the diesel fuel (Section 6.3). Alterntively, bio-oils nd chrs cn be converted into H 2 or syn-gs by stem-reforming (Sections 6.4-6.5). 6.1. Hydrodeoxygention Hydrodeoxygention of bio-oils involves treting bio-oils t moderte tempertures (300-600 C) with high-pressure H 2 in the presence of heterogeneous ctlysts. Reviews on hydrodeoxygention hve been written by Furmisky 209 nd Elliott et l. 182 Most hydrodeoxygention work hs focused on sulfided CoMo nd NiMo-bsed ctlysts, which re industril hydrotreting ctlysts for removl of sulfur, nitrogen, nd oxygen from petrochemicl feedstocks. Pt/ SiO 2 -Al 2 O 3, 210 vndium nitride, 211 nd Ru hve lso been used for hydrodeoxygention. During hydrodeoxygention, the oxygen in the bio-oil rects with H 2 to form wter nd sturted C-C bonds. It is desirble to void hydrogention of romtics in the bio-oils, since this would decrese the octne number nd increse H 2 consumption. The energy content of the fuel is significntly incresed, nd the stbility of the fuel increses during hydrodeoxygention s shown in Tble 15. Prtil deoxygention results in n increse in Tble 15. Properties of Bio-Oils nd Upgrded Bio-Oils highpressure liquefction flsh pyrolysis hydrodeoxygented bio-oils elementl nlysis crbon (wt %) 72.6 43.5 85.3-89.2 hydrogen (wt %) 8.0 7.3 10.5-14.1 oxygen (wt %) 16.3 49.2 0.0-0.7 sulfur (wt %) <45 29.0 0.005 H/C tom 1.21 1.23 1.40-1.97 rtio (dry) density (g/ml) 1.15 24.8 0.796-0.926 moisture (wt %) 5.1 24.8.001-0.008 higher heting 35.7 22.6 42.3-45.3 vlue (MJ/kg) viscosity (cp) 15,000 59 (61 C) (40 C) romtic/liphtic crbon reserch octne 77 number (RON) distilltion rnge (wt %) IBP-225 C 8 44 97-36 225-350 C 32 coked 0-41 From Elliott nd Schiefelbein. 346 1.0-4.6 (23 C) 38/62-22/78 oil viscosity, nd deoxygention to less thn 5 wt % oxygen is required to low viscosity like tht required for fuel pplictions. 212 The disdvntge of hydrotreting is tht it requires high-pressure H 2, which in n integrted biorefinery could be produced from the biomss (see Sections 4.1, 6.4, 8.3, nd 8.4 for H 2 production pthwys). Elliott nd co-workers developed two-step hydrotreting process for upgrding of bio-oils derived from pyrolysis. 212-214 The first step involves low temperture (270 C, 136 tm) ctlytic tretment tht hydrogentes the thermlly unstble bio-oil compounds, which would otherwise thermlly decompose forming coke nd plugging the rector. The second step involves ctlytic hydrogention t higher temperture (400 C, 136 tm). The sme ctlyst, sulfided Co-Mo/ Al 2 O 3 or sulfided Ni-Mo/Al 2 O 3, is used for both steps. This process cn produce yields of 0.4 L refined-oil /L bio-oil-feed with the refined oil contining less thn 1 wt % oxygen. During this process, 20-30% of the crbon in the bio-oil is converted into gs-phse crbon, decresing the overll yield. Ctlyst stbility nd gum formtion in the lines were identified s mjor process uncertinties. The properties of hydrotreted nd untreted bio-oils re shown in Tble 15. Upgrded biooils hve reserch octne number (RON) of 72, nd n romtic/liphtic crbon rtio of 38/62-22/78. The octne number is lower thn gsoline, nd while romtics do hve higher octne number they cuse ir pollution problems. Delmon nd co-workers studied the hydrodeoxygention of model bio-oil compounds with sulfphided CoMo nd NiMo ctlysts to elucidte the min rection pthwys, the influence of the importnt rection prmeters, nd the possible ctlytic poisons. 215-219 The model feedstock ws mixture of 4-methylcetophenone, ethyldecnote, nd guicol s shown in Figure 20. The ketone group is esily nd selectively hydrogented into methylene group bove 200 C. 217 Crboxylic groups re lso hydrogented under hydrodeoxygention conditions, but prllel decrboxyltion pthwy lso occurs t comprble rtes. 217 Crboxylic groups nd guicyl groups re not s rective s ketone groups, nd tempertures greter thn 300 C re required for their conversion. Guicol ws hydrogented into ctechol nd then to phenol. Guicol ws the compound tht cused ctlyst dectivtion due to coking rections. The cidity of the ctlytic support does not chnge the hydrogention rte of the 4-methylcetophenone, but incresing the support cidity does increse rtes of decrboxyltion nd hydrogention of ethyldecnoete nd coke formtion from guicol. Crbon, which hs low cidity, is good ctlytic support for hydrodeoxygention. It is necessry to dd sulfur (dimethyl disulfide) to the feed to keep the ctlyst from dectivting, nd the ctlytic ctivity is dependent upon the H 2 S prtil pressure. Wter decresed the ctlytic ctivity to one-third the initil ctivity. 219 Future work in hydrodeoxygention could focus on developing non-sulfurbsed ctlysts for hydrodeoxygention. 6.2. Zeolite Upgrding of Bio-Oils Zeolites, nd in generl moleculr sieve inorgnic mterils, re the most widely used industril ctlyst used for oil refining, petrochemistry, nd production of fine nd specilty chemicls. 220-222 Zeolites re crystlline microporous mterils with well-defined pore structures on the order of 5-12 Å. 220 Zeolites contin ctive sites, usully cid sites, which cn be generted in the zeolite frmework. The strength nd concentrtion of the ctive sites cn be tilored for prticulr pplictions. Zeolites hve very high surfce res nd dsorption cpcity. Their dsorption properties cn be controlled, nd they cn be vried from hydrophobic to hydrophilic mterils. Bio-oils cn be upgrded using zeolite ctlysts to reduce oxygen content nd improve therml stbility. Tempertures

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4069 Figure 20. Hydrodeoxygention pthwys of 4-methylcetophenone, ethyl decnote, nd guicol from Ferrri et l. 215 (Reprinted from ref 215 with permission. Copyright 2001 Elsevier.) Tble 16. Comprison of Different Ctlysts for Zeolite Upgrding of Wood-Derived Fst-Pyrolysis Bio-oils HZSM-5 silic-lumin (SiO 2-Al 2O 3 rtio 0.14) SAPO-5 SAPO-11 MgAPO-36 ctlyst properties pore size (nm) 0.54 3.15 0.80 0.56 0.75 BET surfce re (m 2 /g) 329 321 330 205 196 cid re (cm 2 /g) b 224.9 125.5 76.0 15.5 product yields (wt % of feed) orgnic liquid product 33.6 24.9 22.2 19.9 16.3 gs 6.1 10.3 12.2 10.1 coke + chr c 20.5-30.2 40 30.0 25.5 38.7 tr d 0-4.1 9.5 11.9 10.1 queous frction 25.0 24.2 26.3 23.1 composition orgnic liquid product (wt %) totl hydrocrbons 86.7 45.6 51.0 56.8 51.6 romtics 85.9 e 2.1 27.5 29.1 26.7 liphtics 18.6 43.5 23.5 24.4 23.4 From Bkhshi nd co-workers. 224-226 Rection temperture 370 C. b Acid re is mesured by mmoni TPD nd represents Bronsted plus Lewis cid sites. c Coke is defined s orgnics tht could only be removed from ctlyst by clcintions. Chr is defined s orgnics deposited in the rector due to therml decomposition, nd these compounds were not on the ctlyst. d Tr re the hevy oils deposited on the ctlysts tht were only removed with hexne/cetone wsh. e Toluenes nd xylenes re the most common romtics for HZSM-5, wheres benzene is the most common romtic for SAPO nd MgAPO ctlysts. of 350-500 C, tmospheric pressure nd gs hourly spce velocities of round 2 re used for zeolite upgrde. The products from this rection include hydrocrbons (romtic, liphtic), wter-soluble orgnics, wter, oil-soluble orgnics, gses (CO 2, CO, light lknes), nd coke. During this process number of rections occur including dehydrtion, crcking, polymeriztion, deoxygention, nd romtiztion. Similr rections using zeolite ctlysts lso occur with other feedstocks including methnol (Section 4.2), sugr monomers (Section 8.2), lignin (Section 9), nd vegetble oils (Section 10). The dvntges of using zeolite ctlyst re tht no H 2 is required, tmospheric processing reduces operting cost, nd the tempertures re similr to those for bio-oil production. According to Bridgwter, this offers significnt processing nd economic dvntges over hydrotreting. 223 However, poor hydrocrbon yields nd high yields of coke generlly occur under rection conditions limiting the usefulness of zeolite upgrding. Tble 16 shows the results for zeolite upgrding of woodderived fst-pyrolysis bio-oils by Bkhshi nd co-workers with different ctlysts. 224-226 Between 30 nd 40 wt % of the bio-oil ws deposited on the ctlyst s coke or in the rector s chr. The ZSM-5 ctlyst produced the highest mount (34 wt % of feed) of liquid orgnic products of ny ctlyst tested. The products in the orgnic crbon were mostly romtics for ZSM-5 nd liphtics for SiO 2 -Al 2 O 3. Gseous products include CO 2, CO, light lknes, nd light olefins. Bio-oils re thermlly unstble nd therml crcking rections occur during zeolite upgrding. Bkhshi nd coworkers developed two rector process, where only therml rections occur in the first empty rector, nd ctlytic rections occur in the second rector tht contins the ctlyst. 227 The dvntge of the two rector system is tht it improved ctlyst life by reducing the mount of coke deposited on the ctlyst. The trnsformtion of model bio-oil compounds, including lcohols, phenols, ldehydes, ketones, cids, nd mixtures, hve been studied over HZSM-5 ctlysts, nd the mjor pthwys re shown in Figure 21. 228-230 Alcohols were converted into olefins t tempertures round 200 C, then

4070 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. hlf tht of the bio-oil, nd the viscosity of the emulsion incresed s the frction of the bio-oil incresed. The surfctnt production costs for fuel with zero strtifiction emulsions re reported to be 2.6, 3.4, nd 4.1 /L for 10, 20, nd 30 wt % bio-oil emulsions, respectively. 233 Fuels with higher weight percent of bio-oils (up to 75% bio-oil) were prepred, chrcterized, nd tested by Chirmonti et l. 234,235 Mixtures of methnol nd cetne enhncers cn be used to improve the combustion chrcteristics of bio-oils. Suppes reported cetne number of bio-oil s 27; 185 however, blending bio-oils with 4% cetne enhncer (tetrethyleneglycol dinitrte), 24% methnol nd 72% bio-oils showed performnce similr to tht of diesel fuel in terms of ignition chrcteristics. Figure 21. Products from zeolite upgrding (HZSM-5) of model biomss compounds including propnol, butnol, nd cetone dpted from Gyubo et l. 228,229 to higher olefins t 250 C, followed by prffins nd smll proportion of romtics t 350 C (Figure 21). 228 Phenol hs low rectivity on HZSM-5 nd only produces smll mounts of propylene nd butnes. 2-Methoxyphenol lso hs low rectivity to hydrocrbons nd thermlly decomposes generting coke. Acetldehyde hd low rectivity on ZSM-5 ctlysts, nd it lso underwent therml decomposition leding to coking problems. 229 Acetone, which is less rective thn lcohols, first is dehydrted to i-butene t 250 C (Figure 21) nd then converts into C 5+ olefins t tempertures bove 350 C. These olefins re then converted into C 5+ prffins, romtics, nd light lkenes. Acetic cid is first converted to cetone, which is converted into cetone products. Products from zeolite upgrding of cetic cid nd cetone hd considerbly more coke thn did products from lcohol feedstocks. Thus, different molecules in the bio-oils hve significnt difference in rectivity nd coke formtion rtes. Gyubo et l. recommended tht the oil frctions tht led to therml coking (such s ldehydes, oxyphenols, nd furfurls) be removed from the bio-oil prior to zeolite upgrding. Bio-oils cn be seprted by frctiontion using minly wter nd produce n oil lyer (with mostly ligninderived components) nd n queous crbon contining lyer. 185 The ptent literture lists processes for the selective removl of phenolic compounds from bio-oils by liquidliquid extrction, where the phenolic compounds re then used to mke phenol-formldehyde resins. 231,232 6.3. Bio-Oil Mixtures Bio-oils from fst pyrolysis re not soluble in petroleumderived fuel due to their high wter content; however, blending of diesel with bio-oils cn be ccomplished using surfctnts. 233-235 Bio-oil emulsions hve promising ignition chrcteristics but lso hve high cost due to surfctnt ddition nd high energy cost for emulsifiction. Higher corrosion levels occur in engine pplictions with the biooil-diesel emulsions. 235 Ikur et l. produced emulsions of bio-oil obtined by fst pyrolysis of hrdwood (Ensyn Technologies), from 10 to 30 wt % bio-oil using mixture of Hypermer B246SF, Hypermer 2234 surfctnt, nd No. 2 diesel fuel. 233 The cetne number, which is mesure of the diesel fuel qulity with higher cetne numbers being better for engine use, decresed from 46, 43, 38, to 34 s the bio-oil concentrtion incresed from 0, 10, 20, to 30 wt %, respectively. The corrosivity of the emulsions ws bout 6.4. Stem Reforming of Bio-Oils Stem reforming of bio-oils produces syn-gs, which cn then be converted into rnge of fuels s discussed in Section 4.0. One ppliction of this technology would be to hve number of smller plnts tht produce bio-oils, which re then trnsported to lrge centrl biorefinery where the biooils re converted into syn-gs-derived fuels. 236 The lrge biorefinery could tke dvntge of the economy of scle, nd trnsporting the dense bio-oil is cheper thn trnsporting biomss. Blck liquor, the mjor wste biomss-contining strem from chemicl pulp nd pper production, lso cn be converted into syn-gs by stem reforming. 237 Stem reforming of fossil fuels is well-estblished technology, 51 nd stem reforming of bio-oils is n extension of this technology. Stem reforming rections occur t high temperture (600-800 C) nd high spce velocities usully with Ni-bsed ctlyst. According to Czernik et l., the most importnt prmeters for stem reforming of bio-oils re temperture, stem-tocrbon rtio, nd ctlyst-to-feed rtio. 238 Stem reforming of bio-oils is complicted since some bio-oil components re thermlly unstble nd decompose upon heting. Dectivtion of the ctlysts due to coking is one of the mjor problems, nd bio-oils hve more dectivtion problems thn do petroleum-derived feedstocks. In fct, stem reforming of bio-oils in fixed bed rectors requires ctlyst regenertion step fter 3-4 h of time-on-strem. 238 While bio-oils re more rective thn petroleum oils, high temperture is needed in the rector to gsify coke deposits formed by therml decomposition. High rtios of stem to crbon (greter thn 7) re necessry to void ctlyst dectivtion by coking. Czernik et l. developed fluidized bed rector for stem reforming of bio-oils. 238 Ctlysts were more stble in the fluidized bed rector thn in the fixed bed rector due to better contcting of the ctlyst prticle with stem. 238 The current problem with fluid bed ctlysts is due to ctlyst ttrition, nd ttrition resistnt ctlysts re being developed. Another dvntge of stem reforming of bio-oils is tht higher vlue products in the bio-oils cn be seprted from the low vlue products, which cn then be stem reformed. 238 Bio-oils seprte into n queous nd orgnic frction by the ddition of wter to the bio-oil. The orgnic frction could be used to mke chemicls, such s phenol-formldehyde resins, or could be lterntively converted to romtic hydrocrbons nd ethers tht cn be used s high-octne gsoline blending components (Section 9.2). The queous frction cn be converted into syn-gs by stem reforming. 239 The stem reforming of model biomss compounds, including cetic cid, cetone, phenol, ethnol, cresol,

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4071 dibenzyl ether, glucose, xylose, nd sucrose, hs been crried out with Ni 240 nd noble metl-bsed ctlysts. 241,242 Acetic cid cused coking problems on the Ni ctlyst surfce, while glucose, xylose, nd sucrose thermlly decomposed prior to the ctlyst bed. 240 The sugrs once voltilized did not cuse coking problems on the ctlyst surfce but decomposed in tubing prior to the ctlyst bed. Nobel metl ctlysts, including Pt, Rh, nd Pd supported on Al 2 O 3 nd CeO 2 - ZrO 2 were ble to stem reform cetic cid, cetone, phenol, nd ethnol. 241 Synthesis gs cn lso be produced from gsifiction of biomss-derived oils without ny ctlysts. 243 One potentil wy of improving stem reforming of biooils would be to prtilly hydrogente the bio-oils prior to the stem reforming section. Prtil hydrotreting of biooils t lower temperture will improve the therml stbility of bio-oils nd should decrese the mount of coking on the ctlyst. The hydrogen produced by stem reforming could be recycled for hydrogention purposes. 6.5. Stem Reforming of Chrs During bio-oil production, chrs re produced, which cn be converted into H 2 or syn-gs by stem reforming. 244,245 Alterntively, the chrs cn be burned s solid fuel. Stem reforming of chrs occurs t tempertures from 700 to 800 C, stem flow rtes of 2.5-15 g/(h-g chr ) nd residence times from 0.5 to 2 h without ny ctlyst. The rection tkes plce in bed contining the chr with flowing stem. The concentrtion of the gses is dependent on the rection conditions, but the methne concentrtion is less thn 2 mol % with the remining product gs being H 2, CO nd CO 2. Up to 90% of the chr cn be gsified. One potentil future ppliction of this technology would be the stem reforming of crbon deposited on ctlyst during Bio-Oils upgrding. 6.6. Economic nd Therml Anlysis of Processes for Bio-Oil Upgrding Bio-oils need to be upgrded if they re to be used s trnsporttion fuel. Ech conversion technology hs dvntges nd disdvntges. Hydrotreting of bio-oils cn produce stble, energy dense, noncorrosive oil, but requires high-pressure H 2. Zeolite upgrding does not require H 2, but extensive coking occurs on the ctlyst surfce. Bio-oils cn form emulsions with diesel fuel, but this fuel hs high corrosivity, nd requires expensive emulsifying gents. Stem reforming of bio-oils is techniclly possible route, but s discussed in Section 4.5, the therml efficiency is low for syn-gs production nd the subsequent conversion into fuels. The overll mss nd PTE for production of bio-oils from wood by fst pyrolysis followed by conversion into refined liquid fuels by hydrotreting or zeolite upgrding is shown in Tble 17. 223 The pyrolysis oil contins pproximtely 70% of the energy nd 83% of the mss of the wood feedstock. The energy content of the fuel fter hydrotreting nd zeolite upgrding is 63% nd 53% of the wood feedstock. Further refining of the hydrocrbons reduces the mss nd energy content to 25-27% nd 55% of the wood feedstock, respectively. These process therml efficiencies re higher thn the PTE for syn-gs-derived liquid fuels production using biomss gsifiction nd FTS (Tble 9). Evns et l. hve estimted tht cost of producing H 2 by stem reforming of pyrolysis vpors is $9.51/GJ, $7.78/GJ, nd $6.05/GJ when the biomss feedstock is vilble t $48, $24, nd $0 per metric-ton of dry biomss, respectively. 246 Tble 17. Overll Mss nd Process Therml Efficiencies (PTE) for Conversion of Wood into Liquid Fuels by Pyrolysis nd Ctlytic Upgrding (Hydrotreting nd Zeolite) pyrolysis oil prtilly hydrotreted The cost of H 2 from bio-oil stem reforming is similr to the commercil cost of H 2, which in 2003 ws $5.7-11/GJ ccording to Spth nd Dyton. 114 The cost of H 2 is dependent on the cost of fossil fuels, nd s the cost of fossil fuels increses the cost of H 2 will lso increse. All of the bio-oil upgrding routes should be explored in the future, nd new ctlysts need to be developed for these routes to become economicl. It would be idel to use the functionlity of the bio-oils to produce high qulity trnsporttion fuel. For exmple if the cids nd lcohols in the bio-oil could rect to form esters this might improve the chrcteristics of the bio-oil. Future work will lso require tht the upgrding system be integrted into the biorefinery plnt. Niche mrkets, for high-vlue products from bio-oils re the ner term ppliction of bio-oils. As bio-oil production becomes more efficient, nd bio-oil upgrding technology improves, it is likely tht bio-oils derived from pyrolysis nd/or liquefction could be used s trnsporttion fuel. 7.0. Biomss Monomer Production crude hydrocrbons refined hydrocrbons mss PTE mss PTE mss PTE mss PTE hydrotreting 0.83 0.70 0.50 0.66 0.30 0.63 0.27 0.55 zeolite 0.83 0.70 0.23 0.53 0.25 0.55 Adpted from Bridgwter. 223 This tble does not include the energy dded to the hydrocrbon by the hydrogen. The previous methods for lignocellulose biomss conversion require high-temperture tretments (greter thn 500 C) for production of gses, liquids or solids. In this section, we discuss how to selectively convert cellulosic biomss into monomer units by low temperture rections where the first rection involves cid hydrolysis. The monomer units re then selectively converted into trgeted fuels s discussed in the following sections (8.0 nd 9.0). Biomss conversion into monomer units is function of the biomss type nd some plnt mterils, such s cne sugr nd corn, re esily converted into monomer units. Lignocellulose is difficult to brek up into monomer units due to its reclcitrnt nture, nd significnt mount of reserch hs been done to selectively convert this low-cost mteril into monomer units. Biomss hydrolysis cn llow the selective nd energyefficient conversion of biomss into monomer units, which cn then be selectively converted into fuels or chemicls. 7.1. Pretretment To chieve high yields of glucose, lignocellulose must first be pretreted. The gol of pretretment is to decrese the crystllinity of cellulose, increse biomss surfce re, remove hemicellulose, nd brek the lignin sel. 247 This pretretment chnges the biomss structure nd improves downstrem processing. Pretretment methods include physicl, chemicl, nd therml or some combintion of the three. Pretretment is one of the most expensive processing steps for the production of sugrs from biomss, nd the costs hve been estimted to be s high s $0.08/L ethnol. 247 Pretretment is lso one of the lest understood processing options. A

4072 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Tble 18. Effect of Promising Pretretment Methods on the Structure nd Composition of Lignocellulose Biomss pretretment method increses surfce re decrystlizes cellulose removes hemicellulose removes lignin lters lignin structure unctlyzed *** *** * stem explosion liquid hot wter *** N.D. *** * ph controlled *** N.D. *** N.D. hot wter flow-through *** N.D. *** * * liquid hot wter dilute cid *** *** *** flow-through cid *** *** * *** mmoni fiber *** *** * *** *** explosion (AFEX) mmoni recycled *** *** * *** *** percoltion (ARP) lime *** N.D. * *** *** Adpted from Mosier et l. 247 ***, mjor effect; *, minor effect; N.D., not determined. recent issue of Bioresour. Technol. is dedicted to pretretment methods. 247-250 According to Wymn et l. the following is list of desirble pretretment ttributes: 247 (1) Low cost of chemicls for pretretment, neutrliztion, nd subsequent conditioning (2) Miniml wste production (3) Limited size reduction becuse biomss milling is energy-intensive nd expensive (4) Fst rections nd/noncorrosive chemicls to minimize pretretment rector cost (5) The concentrtion of hemicellulose sugrs from pretretment should be bove 10% to keep fermenttion rector size t resonble level nd fcilitte downstrem recovery. (6) Pretretment must promote high product yields in subsequent enzymtic hydrolysis or fermenttion opertions with miniml conditioning costs. (7) Hydrolyste conditioning in preprtion for subsequent biologicl steps should not form products tht hve processing or disposl chllenges. (8) Low enzyme loding should be dequte to relize greter thn 90% digestibility of pretreted cellulose in less thn 5 dys nd preferbly 3 dys. (9) Pretretment should fcilitte recovery of lignin nd other constituents for conversion to vluble co/products nd to simplify downstrem processing. Physicl pretretment methods include bll milling, comminution (mechnicl reduction of biomss prticulte size), nd compression milling. Solvents such s H 2 O 2, ozone, glycerol, dioxne, phenol, or ethylene glycol hve been used for biomss pretretment, nd these solvents re known to brek prt cellulose structures nd promote hydrolysis. 247 However, solvent pretretments pper too expensive for prcticl purposes. 247 According to Mosier et l., the most cost-effective nd promising pretretment methods re dilute cid, unctlyzed stem explosion, ph controlled hot wter, tretment with lime, nd tretment with mmoni. 247 Tble 18 shows the effect of vrious pretretment methods on the chemicl nd physicl structure of lignocellulosic biomss. Unctlyzed stem explosion is used commercilly to remove hemicellulose for the mnufcture of fiberbord nd other products by the Msonite process. 247 High pressure stem is pplied to wood chips for few minutes without the ddition of chemicls, nd this process is terminted by decompression of the stem. This process increses the surfce re without decrystlizing the cellulose, nd cellulose downstrem digestibility is significntly improved. 247 Wter tretments t elevted tempertures (200-230 C) nd pressures cn increse the biomss surfce re nd remove hemicellulose. 247,249,250 Three types of rectors re used for hot wter pretretment including co-current (biomss nd wter re heted together for certin residence time), countercurrent (wter nd lignocellulose move in opposite directions), nd flow through (hot wter psses over sttionry bed of lignocellulose). 251 The dvntge of hot wter tretment is tht cid ddition nd size reduction re not needed. A disdvntge of these methods is tht hot wter tretment forms sugr degrdtion products (furfurl from pentoses nd HMF from glucose). The degrdtion products cn be minimized by controlling the ph of the hot wter by ddition of bses such s potssium hydroxide. Dilute sulfuric cid tretments cn be used to hydrolyze hemicellulose to sugrs with high yields, chnge the structure of the lignin, nd increse the cellulosic surfce re. 247,249,250 The disdvntge of this process is tht it requires corrosive cid, with corresponding downstrem neutrliztion, nd specil mterils for rector construction. Ammoni fiber/ freeze explosion (AFEX), where nhydrous mmoni is contcted with lignocellulose, cn increse the surfce re of the biomss, decrese crystllinity of cellulose, dissolve prt of the hemicellulose, nd remove lignin. Tretment of the biomss with less concentrted mmoni solution is known s mmoni recycled percoltion (ARP). Ambient conditions cn be used for lime tretments; however, the time required for these tretments is in terms of weeks. This process involves mixing lime with wter nd sprying it onto the biomss. The mjor effect of lime pretretment is removl of lignin. The biomss surfce re is incresed, nd the cetyl nd uronic cid frctions of hemicellulose re removed. Tble 19 shows the results of different pretretment methods followed by enzymtic hydrolysis for production of sugrs from corn stover. 249 Tble 20 lists the rection conditions for the pretretments. 249 Using corn stover feedstocks sugr yields of over 90% were obtined with the vrious pretretments. A hot wter tretment with flow through rector ws the pretretment method with the highest overll soluble product yield; however, the xylose monomer yield ws only 2.4%, mening this method did not produce xylose monomers. A dilute cid pretretment method produced the highest mounts of sugr monomers with 92% yield. Results re expected to be different with other feedstocks.

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4073 Tble 19. Xylose nd Glucose Yields of Corn Stover fter Vrious Pretretments Followed by Enzymtic Hydrolysis xylose yields (%, mx 37.7) glucose yields (%, mx 62.3) totl sugr yields (%) pretretment system stge 1 stge 2 totl stge 1 stge 2 totl stge 1 stge 2 totl dilute cid 32.1(31.2) 3.3 35.3(34.5) 3.9 53.3 57.2 36.0(35.1) 56.6 92.5(91.7) flowthrough 36.3(1.7) 0.8(0.7) 37.1(2.4) 4.5(4.4) 57.0 61.5(61.4) 40.8(6.1) 57.8(57.7) 98.6(63.8) prtil flow pretretment 31.5(2.8) 4.3(4.2) controlled ph 21.8(0.9) 8.9 30.7 3.5(0.2) 54.7 58.2 25.3(1.1) 63.6 88.9 AFEX ND(30.2) ND(30.2) 61.8 61.8 ND/92.0 ND/92.0 ARP 17.8(0) 17.0 34.8(17.0) 0 59.4 59.4 17.8(0) 76.4 94.2(76.4) lime 9.2(0.3) 20.2 29.4(20.5) 1.0(0.3) 59.5 60.5(59.8) 10.2(0.2) 79.7 89.9(80.3) Adpted from Wymn et l. 249 Stge 1 is pretretment of corn stover nd stge 2 is enzymtic hydrolysis fter pretretment with cellulose loding of 60 FPU/g of glucn in the originl corn stover. The vlue reported in ech column is sugrs plus oligomers, while the vlue in prentheses is the vlue for monomers only. A single vlue indictes tht only monomers were observed. Tble 20. Optiml Pretretment Conditions for Ethnol Production from Corn Stover pretretment system chemicls temp ( C) pressure (tm) rection time (min) solid conc (wt %) dilute cid 0.5-3.0 wt % sulfuric cid (0.49 wt %) 130-200 (160) 3-15 2-30 (20) 10-40 (25) flowthrough 0.0-0.1 wt % sulfuric cid (0.0 wt %) 190-200 (200) 20-24 12-24 (24) 5-30 ph controlled wter or stillge 160-190 (190) 6-14 10-30 (15) 5-30 (16) AFEX 100% (1:1) nhydrous mmoni 70-90 (90) 15-20 <5 (5) 60-90 (62.5) ARP 10-15 wt % mmoni (15 wt %) 150-170 (170) 9-17 10-20 (10) 15-30 lime 0.05-0.15 C(OH) 2/g biomss (0.08) 25-60 (55) 1 2-8 weeks (2 weeks) 10-20 Adpted from Wymn et l. 249,250 The optiml rection prmeters re in prentheses. Energy Lbortory (NREL) hs estimted tht the cost of unrefined sugr monomers, in n queous solution, produced from lignocellulose would be 12-14 /kg sugr. 171 Lynd et l. hve projected the price of sugrs could decrese to s low s 5.3 /kg. 13 (C 6 H 10 O 5 ) n + nh 2 O f nc 6 H 12 O 6 (15) Figure 22. Ethnol production cost with vrious pretretment methods from Eggemn nd Elnder. 248 (Reprinted from ref 248 with permission. Copyright 2005 Elsevier.) An economic nlysis of ethnol production using the vrious pretretment methods ws conducted by Eggemn nd Elnder, nd the results re shown in Figure 22. 248 The cost of ethnol production increses s dilute cid < AFEX < lime < ARP < hot wter. The reson hot wter pretretment is so expensive is tht it requires more enzymes to brek down the xylose oligmers. If the oligmers could be successfully converted into ethnol (or other products), then the cost of mking ethnol for the vrious pretretment method decreses for the hot wter, ARP, nd lime method, ll of which mke significnt mount of oligomers. 7.2. Hydrolysis The hydrolysis rection for cellulose conversion into sugr polymers is shown in eq 15. Hydrolysis of cellulose is significntly more difficult thn for strches becuse cellulose is in crystlline form with hydrogen bonding (Section 2.1). The hydrolysis rection cn be ctlyzed by cids or enzymes, nd recent review hs been written by Wymn et l. 3 Cellulse enzymes re ble to ctlyze the rection with yields close to 100% t 50 C. The Ntionl Renewble Erlier cellulose hydrolysis kinetic models, developed by Semn, 252 involve two first-order rections where the first involves cellulose hydrolysis to glucose followed by glucose decomposition (eq 16). Undesired byproducts including 5-hydroxymethylfurfurl (HMF) nd levulinic cid re produced by cid-ctlyzed degrdtion of sugrs. Most hydrolysis dt were fit to this simple model from 1945 to 1990, nd Tble 21 shows the model prmeters from vrious studies. 253 Using these prmeters the mximum yield of glucose is lwys less thn 70%. Enzymtic hydrolysis cn produce glucose yields bove 95% s shown in Tble 19. The cid hydrolysis of cellulose hs lower ctivtion energy thn lignocellulose, thus showing the effect of lignin on the cid hydrolysis rection. cellulose + wter 98 glucose 98 degrdtion products k 1 k 2 (16) More complicted kinetics models hve been developed bsed on mechnistic dt. Oligomer conversion into glucose is 2-3 times fster thn conversion of cellulose to glucose; however, oligomers hve been observed during hydrolysis. 3 These observtions led to the development of two-step model where cellulose is converted into oligomers, which re then converted into glucose. Mok nd Antl observed tht in ddition to the hydrolysis pthwy nother pthwy occurs tht produces modified cellulose tht cnnot be hydrolyzed to glucose s shown in Figure 23. 254 Importntly, this model suggests tht cellulose structurl rerrngements cn occur with high-temperture tretments. The cid hydrolysis rections re heterogeneous with the solid biomss

4074 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Tble 21. Kinetic Prmeters for Acid Hydrolysis of Vrious Biomss Feedstocks with the Semn Model (eq 14) feed temp ( C) cid conc (wt %) K 1 (min -1 ) K 2 (min -1 ) E 1 (kj/mol) E 2 (kj/mol) m n glucose 160-260? 1.85 10 14 136 1.0 cellulose 100-130 5-40 H 2SO 4 1.57 10 14 142 1.42 Dougls fir 170-190 0.4-1.0 H 2SO 4 1.73 10 19 2.38 10 14 180 137 1.34 1.02 Krft pper 180-230 0.2-1.0 H 2SO 4 28 10 19 4.9 10 14 189 137 1.78 0.55 newsprint 200-240 1.0 H 2SO 4 28 10 19 4.8 10 14 189 137 NR NR Solk-floc 180-240? 1.22 10 19 3.79 10 4 178 137 NR NR Cne bgsse 100-130 5-40 H 2SO 4 1.15 10 21 152 1.42 Adpted from Fn et l. 253 Semn model is represented s cellulose + wter 98 glucose 98 degrdtion products where k k 1 k 1 ) K 2 1(Con cid) m exp(-e 1/RT) nd k 2 ) K 2(Con cid) n exp(-e 2/RT) with Con Acid in wt frction of cid. Figure 23. Cellulose cid-ctlyzed hydrolysis pthwys dpted from Mok nd Antl. 254 recting with liquid cid. Thus, mss trnsfer limittions lso cn ply role in hydrolysis. The mechnism for C-O-C bond clevge in cellulose involves protontion of glucoside bonds s shown in Figure 24. The proton cn either ttck the oxygen bond between the two glucose units or the cyclic oxygen, which is defined s pthwys A-1 nd A-2, respectively. 253 The mechnism is thought to involve the rpid formtion of n intermedite complex with the oxygen nd proton, followed by the slow splitting of glucosidic bonds by the ddition of wter molecule. Heterogeneous rections occur during cellulose hydrolysis in the biomss where the cid first penetrtes into disordered cellulose regions leding to n initil rpid decrese in the degree of polymeriztion (DP). 253 After the rpid initil decrese, the DP reches n symptotic vlue where the DP remins t constnt vlue clled the degree of polymeriztion (LOPD). The LODP is dependent on the type of cellulose smples nd is reched when only 2-5% of the smple hs been hydrolyzed. The verge length of crystllite in the cellulose smple is considered to be the sme s the LODP. Oxidtion of cellulose (with oxidizing gents such s H 2 O 2, NClO 2,O 3, KBrO 3, etc.) prior to hydrolysis or during progressive hydrolysis reduces the DP of prtilly hydrolyzed residues. This tretment decreses the ldehyde concentrtion nd increses the crboxyldehyde concentrtion, which prevents recrystlliztion. Recrystlliztion cn occur during cid or enzymtic hydrolysis. 253 Prior to enzymtic hydrolysis, the cellulose structure must be pretreted to open up the structure of biomss for rection of the cellulose with cellulse. Initilly, process ws designed to produce ethnol through enzymtic hydrolysis by seprte hydrolysis nd fermenttion (SHF) steps. This involved using improved enzymes from the fungus Trichoderm reesei. 3 Problems with this methods re tht cellobiose nd glucose inhibit the rection, which incresed enzyme Figure 24. Mechnism of cid hydrolysis of cellulose dpted from Fn et l. 253

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4075 Figure 25. Kinetic model of hemicellulose degrdtion dpted from Wymn et l. 3 cost. This problem cn be reduced by process known s simultneous scchrifiction nd fermenttion (SSF) where the vessel contins both cellululse nd fermenttive orgnisms to convert glucose rpidly to ethnol. This process significntly reduces the concentrtion of glucose. Although the temperture of the SSF process is lower thn the optiml temperture for enzymtic hydrolysis becuse the fermenttion orgnisms re not stble t these higher tempertures, the rtes, concentrtions, nd yields re still better thn for SHF. Cellulses, the enzymes tht ctlyze cellulose hydrolysis, were initilly ctegorized bsed on the rection they ctlyze. More recently, they hve been clssified bsed on structurl properties. Three mjor types of enzymtic rections re reported including (1) endoglucnses or 1,4-β-D-glucn-4- glucnohydrolses, (2) exoglucnses or 1,4-β-D-glucn glucnohydrolses (lso known s celloextrinses), nd (3) β-glucosidses or β-glucoside glucohydrolses. 255 Endoglucnses rect with internl morphous cellulose sites to produce shorter chins of vrying lengths nd expose chin ends. Exoglucnses hydrolyze the ends of cellulose produced by endoglucnose in progressive mtter to produce cellobiose s the mjor product. β-glucosidses convert cellodextrins nd cellobiose to glucose. The hydrolysis mechnism in n enzyme occurs using proton donor nd nucleophile or bse. Cellulse systems ct in coordinted mnner to efficiently hydrolyze cellulose nd consist of more thn just combintion of the three enzyme systems. 255 Recent reviews hve been published on kinetic modeling of cellulose enzymtic hydrolysis. 255,256 Acid hydrolysis of hemicellulose occurs under less hrsh conditions thn cellulose becuse hemicellulose is n morphous polymer. Hemicellulose hydrolysis even occurs in hot wter ( 210 C), where the wter is thought to brek down hemicellulose nd relese cetic cid, which continues to ctlyze the rection. 257 Wter-soluble oligomers form in high yields with hot wter tretments. Dilute cid tretments of lignocellulose t 160 C, 10 min rection time, nd 0.7 wt % cid, yields 85-90% of the hemicellulose sugrs. 258 Kinetic models usully incorporte two types of hemicellulose fst hydrolyzing type nd slow hydrolyzing type s shown in Figure 25. 3 The proportion of fst nd slow frctions is typiclly 65 nd 35%, s determined by fitting kinetic dt. Oligomer intermedites re experimentlly observed but frequently ignored in kinetic models. Wymn et l. sid concerning hemicellulose models tht lthough significnt effort hs been devoted to describing the kinetics of hemicellulose hydrolysis, the models do not predict consistent results. 3 For exmple, the rte of xylose degrdtion in kinetic models is different thn the rte of pure xylose degrdtion. The hemicellulose lso is ssocited with lignin, nd this type of bonding could chnge the kinetics. Future mechnistic work could help clrify the heterogeneous mechnism of cid hydrolysis of biomss leding to further process improvement. 7.3. Levulinic Acid Levulinic cid cn be selectively produced from cellulosic biomss. Levulinic esters nd methyl-tetrhydrofurn, which cn be used s oxygented diesel nd gsoline fuel dditives, respectively, cn be produced by esterifiction nd hydrogention of levulinic cid (Section 9.2). Levulinic cid is the finl cid-ctlyzed dehydrtion product formed from sugrs or cellulose, s shown in Figure 26, with formic cid (in 1:1 molr rtio) nd wter s coproducts. During this rection, lrge mounts of solid products (humics or trs) lso re formed. A mechnism (Figure 26) for levulinic cid formtion ws been reported by Horvt et l. 259 BioMetics Inc. developed the biorefine process to produce levulinic cid t 50-70% yields from cellulosic feedstocks, including pper mill wste, wood wste, nd griculturl residues, using dilute cid hydrolysis. 260-262 This process occurs in two-stge rector (Figure 27), where the first rector is plug flow, nd the second is CSTR rector. The feed contins 2-5 wt % H 2 SO 4, nd the rection conditions re 215 C, 31 tm, nd 15 s residence time for Figure 26. Mechnism for formtion of levulinic cid from HMF ccording to Horvt et l. 259

4076 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. in fuels in winter months for res with high CO levels, nd ethnol oxygentes gsoline. Ethylene hydrtion is nother method used to mke ethnol from petroleum. Sugrs re converted to ethnol by fermenttion usully with the yest Scchromyces cerevisie s shown in eq 17. Although lmost hlf of the mss of sugr is relesed s CO 2, lmost ll of the sugr energy is cptured in the ethnol. S. cerevisie ferments glucose, mnnose, fructose, nd glctose. High theoreticl yields of ethnol re obtined from this rection, nd smll mounts of byproducts including glycerol, cetic cid, lctic cid, succinic cid, nd fusel oil re formed. Yest production requires smll mount of the sugr s feedstock. C 6 O 6 H 12 f 2C 2 H 5 OH + 2CO 2 (17) Figure 27. Production of levulinic cid by the biorefine process dpted from Mnzer. 19 the first rector; nd 193 C, 14.6 tm, nd 12 min residence time for the second rector. A pilot plnt ws operted for 1 yer using pper sludge feedstocks nd producing levulinic cid (70% yield from cellulose), formic cid (50% yield from cellulose), furfurl (80% yield from hemicellulose), nd chr t feed rte of 1 dry ton/dy. BioMetics estimted tht lrge-scle plnt (1000-2000 dry tons/dy) could produce levulinic cid for $0.09-0.11/kg. During the process, the cellulose is first converted into sugrs, which re then converted into levulinic cid, formic cid, nd chrs (Figure 27). Cellulose conversion into sugrs is fst rection, wheres subsequent sugr conversion into levulinic cid is slow rection. Typicl yields were pproximtely 0.5 kg of levulinic cid/kg of cellulose. The process cn use wet feedstocks without drying, improving the overll process energy efficiency. 7.4. Hydrogention/Hydrolysis Hydrolysis of biomss cn be combined with hydrogention to produce xylitol, sorbitol, nd sorbitn from cellulose, spen, switchgrss, nd wood biomss resources t n estimted polyol cost of $0.055-0.070/kg. 263 This is done by dding Ru/crbon ctlyst for hydrogention, with H 3 - PO 4, cid ctlyst for hydrogenolysis, t 155-170 C nd 30-50 tm H 2. Approximtely 50-70% of the cellulose nd hemicellulose were converted into polyols. Xylitol, sorbitol, nd sorbitn cn be used s feedstocks for fuels production by queous-phse processing. 8.0. Sugr Conversion into Fuels This section discusses production of fuels from sugr monomers. 8.1. Ethnol Production Presently, the production of ethnol by fermenttion of crbohydrtes is the primry technology for the genertion of liquid fuels from renewble biomss resources. In 2001, the U.S. nd Brzil produced 6.63 10 9 nd 11.2 10 9 L of ethnol per yer, respectively. 264 Ethnol cn be used directly s fuel, nd in Brzil hydrous ethnol, which consists of 95.5% ethnol nd 4.5% wter, is used to power vehicles. 264 Ethnol is lso blended with gsoline, nd in the U.S. most ethnol is sold s blend of 10% ethnol nd 90% gsoline. Brzil lso blends ethnol with gsoline. The U.S. Clen Air Act of 1990 mndted the use of oxygentes Industrilly, number of different biomss feedstocks re used for ethnol production s shown in Figure 28. The first Figure 28. Block flow digrm for ethnol production from corn, cne sugr nd cellulosic biomss from Wymn. 264 Corn wet mills produce corn oil, corn gluten mel (CGM), nd corn gluten feed (CGF) for food nd niml feed. Corn dry mills produce n niml feed clled DDGS fter the fermenttion process. (Reprinted from ref 264 with permission. Copyright 2004 Elsevier.) step in ethnol production is conversion of the biomss into fermentble sugrs. This conversion step depends on the feedstock. Sugrcne is converted into wter-soluble sugrs or cne juice (30 wt % of sugrcne) nd insoluble lignocellulose or sugrcne bgsse, nd sugr fermenttion does not require extensive pretretment. The sugrcne bgsse is burned to provide het for the process. The sugrs re heted to 105-110 C to reduce microbil contmintion, nutrients such s mmonium sulfte nd other slts re dded, nd fermenttion is crried out t bout 20 wt % sugr concentrtion, ph 4-5, tempertures 30-38 C, nd residence time of 28-48 h. 2,264 Typicl yields for ethnol

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4077 Tble 22. Production Costs, Energy Rtios, nd Therml Efficiencies for Ethnol Production from Sugrcne, Corn Grin, nd Lignocellulose Biomss nd Diesel Fuel feedstock sugrcne corn grin corn stover (lignocellulose) diesel fuel b costs feedstock cost ($/L) 0.127-0.134 0.21-0.25 0.088 coproduct credits ($/L) (0.07-0.11) estimted production costs ($/L) 0.21-0.25 0.25 0.31-0.38 0.44 estimted production costs ($/GJ HHV) 4.9-5.9 5.9 7.25-8.89 11 fossil energy rtio (MJ fossil fuel/mj product) c biomss production 0.09-0.14 0.25 0.06-0.15 1.113 biomss trnsport? 0.03 0.01-0.04 0.016 ethnol production 0.02-0.04 0.62 (0.16-0.37) d 0.00 0.064 ethnol conversion? 0.02 0.01-0.05 0.006 overll 0.10-0.18 0.92 (0.46-0.67) d 0.08-0.24 1.199 process therml efficiency (PTE) 0.20-0.45 e 0.41 (0.46-0.51) f 0.49 (0.53) g 0.94 life cycle therml efficiency (LCTE) 0.19-0.43 e 0.26 (0.28-0.30) f 0.39-0.45 (0.43-0.49) g 0.83 From Shpouri et l. 2002, 177 Shpouri et l. 1998, 267 Wymn, 264 Aden et l., 171 Wooley et l., 268 nd Sheehn et l. 328 b Diesel fuel t cost of $57/bbl. Diesel fuel costs re the FOB spot price of diesel fuel t New York Hrbor. 14 c Fossil energy rtios re estimted with higher heting vlue of ethnol of 23.404 MJ/L ethnol. d Fossil energy rtios nd efficiencies for ethnol production from corn decreses depending on how energy credits re given for coproducts. 177 The vlues in prentheses re the number with coproduct energy credits. e The process nd life cycle therml efficiencies tke into ccount the energy in the bgsse. It is ssumed tht the energy content of bgsse nd the sugrs re equl. Sugrcne contins 12-17 wt % sugrs nd 68-72 wt % moisture. 264 f The process nd life cycle therml efficiencies tke into ccount the energy in the corn stover. It is ssume tht corn stover nd the sugrs hve equl energy content, nd the mss rtio of corn grin to corn stover is 1:1. The process therml efficiency does not include the energy required for fossil fuel production. g The vlues in prentheses count electricity genertion s coproduct. production from sugrcne re 160-190 L/metric ton. 2 The sugrs re then distilled to zetropic levels (95% ethnol) nd cn be dried further using moleculr sieves. Ethnol is produced from corn grins nd other strches by either wet or dry milling. Corn grin contins 70 wt % strch, 10-11 wt % crude protein, 4.5-6.0 wt % oil, 6 wt % hemicellulose, 2-3 wt % cellulose, 1 wt % lignin, nd 1 wt % sh. 264 The first step in dry milling plnt is to mechniclly grind the grin to 40-mesh size to rupture the hull wlls nd expose the strch polymers. The grin is then heted with wter to 85 C, mixed with R-mylse enzyme, held for 1 h nd heted further to 110-150 C to reduce bcteri levels nd liquefy the strch. This is followed by cooling bck to 85 C for 1 h with the ddition of more R-mylse. 264 The strem is further cooled nd gluco-mylse enzyme is dded to finish process. In the wet milling process, the crude strch, gluten, nd corn oil frctions re seprted through series of steeping, milling, nd seprtion steps. 264 The products include gluten mel, gluten feed, corn germ, mel, nd corn oil, which cn be used for humn consumption or niml feed. In dry mill plnt, coproduct protein, known s distillers dried grins with solubles (DDGS) or distillers dried grins (DDG) is recovered fter distilltion. The ethnol solution, t 12-14 wt % ethnol, is then distilled to zetropic levels (95% ethnol) nd dried further using moleculr sieves. The sugrs re fermented in both wet nd dry milling processes with S. cerevisie, nd typicl yields rnge from 460 to 490 L/metric ton corn grin. 2,264 The ethnol yields from corn grin re higher thn those from sugrcne; however, sugrcne hs higher yield per lnd re (Section 2.1). While not currently done commercilly, reserch is being crried out to produce ethnol from lignocellulosic biomss. The NREL hs modeled process for conversion of corn stover (lignocellulose) to ethnol bsed on dilute cid prehydrolysis nd enzymtic hydrolysis. 171 The first step in this process is feedstock size reduction to the pproprite size. The cellulose is then treted in sulfuric cid (1.1 wt %) for 2 min t 190 C nd 12 tm to relese most of the hemicellulose sugrs nd cetic cid. The rection is then flsh cooled to drop the temperture to 100 C, nd the cid is neutrlized with lime to ph of bout 10. The resulting solid frction is sent to scchrifiction unit where the cellulose is hydrolyzized to glucose nd cellobiose. The scchrifiction rection occurs t 65 C for 1.5 dys. Cellulose enzymes ctlyze the hydrolysis rections. These enzymes contin (1) endoglucnses, which reduce the cellulose polymer size, (2) exoglucnses, which ttck the ends of cellulose fibers, llowing it to hydrolyze highly crystlline cellulose, nd (3) β-glucosidse, which hydrolyze cellobiose relesed by exoglucnsesto glucose. Cellulse is produced industrilly from T. reesei. Genencor Interntionl nd Novozymes Biotech re the two lrgest mnufcturers of this enzyme. In the NREL configurtion, the resulting glucose sugr strem is combined with the xylose sugr strem nd fermented to ethnol with the recombinnt Zymomons mobilis bcterium t 41 C for 1.5 dys. The bcterium must be grown in seed fermenttion vessel in seprte process re. Lignocellulose lso contins xylose sugrs, which cnnot be fermented by S. cerevisie without genetic modifiction. Orgnisms hve been developed tht ferment both xylose nd glucose. 265,266 The ethnol wter solutions re distilled to round 95% ethnol, where ethnol nd wter form n zetrope. Ethnol is further purified using moleculr sieves. The solids left re concentrted in triple effect evportor. The first ethnol plnt from cellulose (which is currently not in opertion) ws built in South Crolin in 1910 nd gve 83 L ethnol /metric ton biomss (swdust). 2 It is estimted tht the new NREL design hs yield of 320 L ethnol /metric ton drybiomss. 171 Production costs of ethnol will be highly dependent on the regionl cost of producing biomss. Typicl ethnol production costs from sugrcne, corn grin, nd lignocellulose re shown in Tble 22. 171,264,267,268 The ethnol production cost decreses depending on the feedstock s sugrcne > corn grin > lignocellulose. The feedstock costs re 53 nd 28% of the ethnol production costs from sugrcne nd lignocellulose. Coproducts re sold when corngrin is used s the feedstock, which reduces the overll ethnol production cost. Lignocellulose hs the lowest feedstock cost, nd reserch is in progress to reduce the cost

4078 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. of cellulosic ethnol. 264,268 The projected ethnol production costs do not include trnsporttion, distribution, txes, nd other consumer costs. Ethnol production from lignocellulose currently requires cpitl investment estimted to be $0.28-0.63/nnul-L. 264 Included in Tble 22 is the current cost of FOB spot price of diesel fuel with oil t $57/bbl. This price on per energy bsis is bout twice tht of the ethnol production cost. (The cost of diesel fuel nd gsoline re firly similr.) Therefore, with oil t its current price, bioethnol is projected to be cost competitive with petroleumderived fuel. The fossil energy rtio (FER), defined s the fossil energy required for ethnol production divided by the energy in the ethnol, is shown in Tble 22. The mount of fossil energy required to produce ethnol tkes into ccount ll fossil energy inputs in the ethnol life cycle including the energy required to grow the fertilizer, mine col used s fuel, plnts the crops, hrvest the crops, etc. The portion of the fossil energy rtio to grow corn is two times higher thn the portion of the FER to grow sugrcne or lignocellulose becuse more fertilizer nd irrigtion is required to grow corn. The overll FER for ethnol production is dependent on the feedstock nd decreses in the order corn grin. sugrcne > lignocellulose. The reson for this lrge difference in FERs for ethnol production is tht the biogs nd lignin remining fter ethnol production cn be burned to provide ll of the process het when sugrcne or lignocellulose re the feedstocks, respectively. However, when corn grin is the feedstock, fossil fuels re used to provide process het. The fossil rtio of ethnol-derived corn grin depends on how energy credits re given for the vrious coproducts mde during ethnol production. The lrgest use of fossil fuel energy during ethnol mnufcture from corn grin results from the energy used during the fermenttion-distilltion process. The U.S. medi hs reported some erroneous nd outdted informtion regrding the fossil energy requirements for ethnol production from corn grin. 177 In the mid-1970s, most reserchers concluded tht the FER of ethnol production from corn grin ws slightly greter thn one. This mens tht the energy in the ethnol is less thn the energy in the fossil fuel used to mke it. As ethnol production in the U.S. grew, the ethnol production process improved, nd the fossil energy requirements decresed. 177 In the lst 16 yers, ll but one of the reserch groups who hve done life cycle nlysis for ethnol production from corn grins in the U.S. hve concluded tht ethnol contins more energy thn the fossil energy inputs. 177 The only resercher in the lst 16 yers to clim tht the fossil rtio in ethnol is greter thn one is Pimentel. 177,269 In the most up to dte nd thorough life cycle nlysis, Shrpoui et l. discussed how Pimentel used outdted informtion (from over 20 yers go) in his nlysis. 177 The PTE for ethnol production rnges from 0.20 to 0.53 s shown in Tble 22. The PTE for ethnol production from lignocellulose is similr to tht of liquid fuels produced by fst pyrolysis of biomss followed by upgrding (Tble 17) nd higher thn the PTE for lknes by FTS of biomssderived syn-gs (Tble 9). However, the ethnol PTE is bout hlf the PTE of diesel fuel production from crude oil. The LCTE, which lso includes the energy to grow the biomss, for ethnol nd diesel fuel production is lso shown in Tble 22. Ethnol cn be further converted into other fuels. For exmple, ethyl-tertiry-butyl-ether (ETBE) is produced by the rection of ethnol with isobutylene. One of the concerns of ethnol s gsoline blending gent is the high voltility of the ethnol-gsoline blend. ETBE cn be blended with gsoline (up to 15 wt %). One of the dvntges of ETBE is tht it is less voltile thn ethnol; however, ETBE my lek from gsoline sttions cusing groundwter contmintion similr to MTBE. 8.2. Zeolite Upgrding of Sugrs Reserchers t Mobil discussed the high energy requirements for ethnol distilltion nd tried to discover more efficient method for biomss conversion. 270 They pssed concentrted sugrs, including glucose, xylose, strch, nd sucrose over ZSM-5 t 510 C, 1 tm, nd WHSV of 2 nd observed hydrocrbon, CO, CO 2, coke, nd wter s products s shown in Tble 23. The ddition of methnol to Tble 23. Products from the Rection of Crbohydrtes over ZSM-5 Ctlyst xylose glucose strch sucrose products (wt %) hydrocrbons 10.0 8.2 8.9 4.4 CO 33.3 18.9 16.8 32.8 CO 2 3.7 3.6 1.5 5.6 coke 16.8 24.9 30.4 23.8 H 2O 36.2 44.4 42.4 33.4 methnol feed weight rtio methnol/sugr 4:1 products (wt %) hydrocrbons 18.6 19.0 7.9 7.9 CO 13.2 12.8 6.0 6.0 CO 2 7.2 10.4 10.8 10.8 coke 16.6 14.0 28.7 28.7 H 2O 44.4 43.8 46.6 46.6 At 510 C, 1 tm nd WHSV ) 2. From Chen et l. 270 the feed strem decresed the mount of coke nd incresed the hydrocrbon products. The hydrocrbon products consisted of gseous lknes (methne, ethne, propne), liquid lkenes nd lknes (butene, pentene, hexne), nd romtics (benzene, toluene, C 8 -C 10 romtics). One of the problems with this rection is tht when methnol is not used 40-65% of the crbon is converted into coke. The therml decomposition of glucose, which is not stble in the gs phse, probbly produces most of this coke. The hydrogented forms of sugrs re more thermlly stble nd therefore would produce less therml coke. Importntly, this shows tht sugrs cn be converted to hydrocrbons by dehydrtion, decrbonyltion, nd decrboxyltion rections. Idelly, with this process, one molecule of glucose could be converted into 2/3 molecules of benzene, 2 molecules of CO, nd 4 molecules of wter s shown in eq 18. The oxygen in the sugr is converted into CO nd wter. The CO could lso be converted into hydrogen by the WGS. This process hs only been briefly studied t the benchtop, nd no detiled current PTE is known. C 6 O 6 H 12 f 2 3 C 6 H 6 + 2CO + 4H 2 O (18) 8.3. Aqueous-Phse Processing Dumesic nd co-workers recently developed queousphse ctlytic processes (APP) for the conversion of sugrs, sugr lcohols, nd polyols into H 2 or lknes rnging from

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4079 Figure 29. Rection pthwys nd selectivity chllenges for H 2 production from APR of ethylene glycol. Pthwy I is desired C-C clevge to form dsorbed CO. Pthwy II represents undesired C-O clevge followed by hydrogention to produce ethnol, leding to formtion of methne nd ethne. Pthwy III is the desired WGS rection. Pthwy IV represents undesired methntion nd Fischer- Tropsch rections to produce lknes. (Figure dpted from Huber et l. 275 ) C 1 to C 15. 18,22,25,115,159,160,271-273 Hydrogen, s well s CO 2, CO, nd light lknes, re produced by queous-phse reforming (APR) of the sugr or sugr-derived feed with liquid wter (eq 19) using heterogeneous (solid) ctlyst t low tempertures (200-260 C) in the queous phse (10-50 br). Virent Energy Systems is currently working to commercilize the APR process. One of the dvntges of APR is tht it produces product H 2 gs with low levels of CO (100-1000 ppm) 274 (mking it idel for PEM fuel cells) in single rector, wheres conventionl stem reforming requires multiple rectors to reduce the CO levels. C 6 O 6 H 14 + 6H 2 O f 6CO 2 + 13H 2 (19) The ctlytic pthwys for H 2 nd CO 2 production by APR involves clevge of C-C, C-H, nd O-H bonds to form dsorbed species on the ctlyst surfce (Figure 29). Adsorbed CO species must be removed from the surfce by the wter-gs shift rection to form CO 2 nd H 2 becuse high surfce coverges by CO led to low ctlytic ctivity. Undesired byproducts my rise from prllel nd series pthwys. Prllel rections proceed vi clevge of C-O bonds followed by hydrogention to give lcohols or by rerrngement rections to form orgnic cids. Series rections rise from hydrogention of dsorbed CO nd CO 2 to form lknes. Thus, good ctlyst for production of H 2 by APR must fcilitte C-C bond clevge nd promote removl of dsorbed CO species by the wter-gs shift rection, but the ctlyst must not fcilitte C-O bond clevge nd hydrogention of CO or CO 2. The product selectivity is function of the feed molecules, the ctlyst, nd the rection conditions. Figure 30 shows the H 2 nd lkne selectivity (primrily light lknes, methne, nd ethne re produced) s function of feed molecule for APR of 1 wt % feeds with Pt/Al 2 O 3 ctlyst. 25 As the size of the feed molecule increses, the H 2 selectivity decreses nd the lkne selectivity increses. When the feed chnges from sorbitol (hydrogented glucose) to glucose, the H 2 selectivity decreses even more. Hydrogen cn be mde selectively by APR from queous feedstocks with high glucose concentrtion (10 wt %) in two-rector process where the first rector (t 100 C) hydrogentes the glucose to sorbitol, nd the second rector (t 200-265 C) converts the sorbitol to CO 2 nd H 2. 276 Hydrogen produced from the second rector cn be recycled for the hydrogention rection. Rection kinetic studies were conducted for the APR of ethylene glycol ( probe molecule for sorbitol) t low Figure 30. Selectivities versus oxygented feedstock for queousphse reforming of 1 wt % oxygented hydrocrbons over 3 wt % Pt/Al 2 O 3 t 498 K (open symbols) nd 538 K (filled symbols). Key: H 2 selectivity (circles), lkne selectivity (squres), nd EG: ethylene glycol. Figure dpted from Dvd et l. 160 tempertures (483 nd 498 K) nd moderte pressures (22 br) over silic-supported Ni, Pd, Pt, Ir, Ru nd Rh ctlysts. The overll ctlytic ctivity for APR of ethylene glycol (s mesured by the rte of CO 2 production per surfce site t 483 K) decreses in the following order for silic-supported metls: 272 Pt Ni > Ru > Rh Pd > Ir Silic supported Rh, Ru, nd Ni ctlysts hd low selectivity for H 2 production nd high selectivity for lkne production. In ddition, Ni/SiO 2 showed significnt dectivtion t 498 K. Thus, silic-supported Pt nd Pd ctlysts exhibited higher selectivity for production of H 2, with lower rtes of lkne production. The ctivity nd selectivity of monometllic Pt-bsed ctlysts cn be improved further by supporting Pt on TiO 2, crbon, or Al 2 O 3. 277 A combintion of high-throughput nd fundmentl studies ws undertken to develop better ctlysts for APR. A highthroughput rector ws designed nd built tht llowed rpid screening of lrge number of ctlysts under APR conditions. More thn 500 different mono- nd bimetllic ctlytic mterils were screened using the high-throughput rector, nd inexpensive nonprecious metl ctlysts nd highly ctive precious metl ctlysts were identified. 271,278 The ctivity of Pt ctlysts cn be improved further by dding Ni, Co, or Fe to Pt/Al 2 O 3 ctlyst. 278 Alumin-supported PtNi nd PtCo ctlysts, with Pt/Co or Pt/Ni tomic rtios rnging from 1:1 to 1:9 hd the highest turnover frequencies

4080 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. for H 2 production (TOF H2, defined s moles of H 2 per mole of surfce site counted by CO chemisorption) with vlues of 2.8-5.2 min -1 for APR of ethylene glycol solutions t 483 K, compred to vlue of 1.9 min -1 for Pt/Al 2 O 3 under similr rection conditions. A Pt 1 Fe 9 /Al 2 O 3 ctlyst showed H 2 turnover frequencies of 0.3-4.3 min -1 t 453-483 K, nd these vlues re bout 3 times higher thn Pt/Al 2 O 3 under identicl rection conditions. Ni-bsed ctlysts re ctive for APR; however, they hve poor selectivity nd stbility. The H 2 selectivity of Ni-bsed ctlysts cn be improved by dding Sn to the Ni ctlyst, nd the stbility of Ni ctlysts cn be improved by using bulk Ni-ctlyst (e.g., Rney Ni). 271,275,279 Therefore, Rney- NiSn ctlysts hve good ctivity, selectivity, nd stbility for H 2 production by APR of biomss-derived oxygented hydrocrbons. This inexpensive mteril hs ctlytic properties (ctivity, selectivity, nd stbility) tht re comprble to those of Pt/Al 2 O 3 for production of H 2 from smll oxygentes, such s ethylene glycol, glycerol, nd sorbitol. Rtes of H 2 production by APR of ethylene glycol over R-NiSn ctlysts with NiSn tomic rtios of up to 14:1 re comprble to 3 wt % Pt/Al 2 O 3, bsed on rector volume. The ddition of Sn to Rney Ni ctlysts significntly decreses the rte of methne formtion from series rections of CO or CO 2 with H 2, while mintining high rtes of C-C clevge necessry for production of H 2. However, it is necessry to operte the rector ner the bubble-point pressure of the feed nd moderte spce times to chieve high H 2 selectivities over R-NiSn ctlysts, while it is impossible to chieve these high selectivities under ny conditions over unpromoted R-Ni ctlysts. The Snpromoted Rney-Ni ctlyst is ctlyticlly stble for more thn 250 h time on strem. 279 According to Dvd et l. the dvntges of using APR to produce H 2 re 160 (1) APR elimintes the need to vporize both wter nd the oxygented hydrocrbon, which reduces the energy requirements for producing hydrogen. (2) The oxygented feedstock compounds of interest re nonflmmble nd nontoxic, llowing them to be stored nd hndled sfely. (3) APR occurs t tempertures nd pressures where the wter-gs shift rection is fvorble, mking it possible to generte H 2 with low mounts of CO in single chemicl rector. (4) APR is conducted t pressures (typiclly 15-50 br) where the H 2 -rich effluent cn be effectively purified using pressure-swing dsorption or membrne technologies, nd the crbon dioxide cn lso be effectively seprted for either sequestrtion or use s chemicl. (5) APR occurs t low tempertures tht minimize undesirble decomposition rections typiclly encountered when crbohydrtes re heted to elevted tempertures. (6) Production of H 2 nd CO from crbohydrtes my be ccomplished in single-step, low-temperture process, in contrst to the multirector stem reforming system required for producing H 2 from hydrocrbons. The lkne selectivity cn be incresed by chnging the ctlyst nd rection conditions. Alknes re produced by queous-phse dehydrtion/hydrogention (APD/H) of sorbitol (eq 20) with ctlyst contining metl (e.g., Pt or Pd) nd cid (e.g., SiO 2 -Al 2 O 3 ) sites to ctlyze dehydrtion nd hydrogention rections, respectively. 115 Hydrogen is produced for this rection by APR (eq 21). These two rections cn be performed in single rector or two seprte ones. The net rection is exothermic, in which pproximtely 1.5 mol of sorbitol produce 1 mol of hexne (eq 22). The APD/H process occurs in the liquid phse, thereby eliminting the need to vporize the queous feedstock nd improving the overll therml efficiency of the process. The lknes produced, ccording to eq 17, retin pproximtely 95% of the heting vlue nd only 30% of the mss of the biomssderived rectnt. This rection pthwy hs one of the highest theoreticl therml efficiencies of ny biomss conversion process. However, this process hs only been studied t the benchtop nd no detiled process nlysis, with PTE, is vilble. C 6 O 6 H 14 + 6H 2 f C 6 H 14 + 6H 2 O (20) C 6 O 6 H 14 + 6H 2 O f 6CO 2 + 13H 2 (21) 19 13 C 6 O 6 H 14 f C 6 H 14 + 36 13 CO 2 + 42 13 H 2 O (22) The lkne selectivity depends on the reltive rtes of C-C bond clevge, dehydrtion, nd hydrogention rections. The lkne selectivity cn be vried by chnging the ctlyst composition, the rection conditions, nd modifying the rector design. 115 In ddition, these selectivities cn be modified by co-feeding H 2 with the queous sorbitol feed, leding to process in which sorbitol cn be converted to lknes nd wter without the formtion of CO 2 (since H 2 is supplied externlly nd need not be produced s n intermedite in the process). As nother vrition, the production of lknes cn be ccomplished by replcing the solid cid with minerl cid (such s HCl) tht is co-fed with the queous sorbitol rectnt. One of the dvntges of lkne production from biomss by APD/H is tht the mjority of the lknes spontneously seprte from the queous feed solution, wheres ethnol produced during fermenttion processes must be removed from wter by n energy-intensive distilltion step. It hs been estimted tht the overll LCTE for production of lknes by APP from corn is double the energy efficiency for production of ethnol from corn. 18,22 Alknes produced by the APD/H of crbohydrtes would provide renewble source of trnsporttion fuel tht could fit into the current infrstructure. Unfortuntely, the lrgest compound produced by APD/H of crbohydrtes is hexne, which hs low vlue s fuel dditive becuse of its high voltility. This limittion hs been overcome by combining the APD/H process with bse-ctlyzed ldol condenstion step. This step links crbohydrte-derived moieties through formtion of C-C bonds, to produce lrger liquid lknes rnging from C 7 to C 15. 18 It should be noted tht the C-O-C linkges (s found in discchrides) re broken under APD/H rection conditions. The ldol condenstion process produces lrge orgnic wter-soluble compounds derived from sugrs. These molecules re then converted into lknes in specilly designed four-phse dehydrtion/hydrogention rector. A conventionl APD/H rector cnnot be used to produce lknes from lrge wter-soluble orgnic compounds, becuse extensive mounts of coke form on the ctlyst surfce (e.g., 20-50% of the rectnt is converted to coke). Accordingly, to produce liquid lknes the rector system employed to crry out dehydrtion/hydrogention rections must be modified to four-phse rector system consisting of (i) n queous inlet strem contining the lrge

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4081 Figure 31. Self-sustining biomss-refinery for conversion of biomss into liquid lknes using queous-phse processing. wter-soluble orgnic rectnt, (ii) hexdecne lkne inlet strem, (iii) H 2 inlet gs strem, nd (iv) solid ctlyst (Pt/SiO 2 -Al 2 O 3 ). As dehydrtion/hydrogention tkes plce, the queous orgnic rectnts become more hydrophobic, nd the hexdecne lkne strem serves to remove hydrophobic species from the ctlyst before they rect further to form coke. In n industril setting, the lknes produced from the rection would be recycled to the rector nd used for the lkne feed. This process cn lso be modified to produce lrge oxygented compounds tht re soluble in diesel fuel. Figure 31 shows proposed biorefinery for converting biomss into liquid lknes bsed on queous-phse processing. In the first step, cellulose nd hemicellulose re respectively converted to xylose nd glucose. Prt of the sugr strem is then converted to H 2 by APR for use elsewhere in the plnt. Furfurl nd HMF re produced from the remining sugr strem by cid-ctlyzed dehydrtion. Furfurl nd HMF re then condensed with cetone over solid bse ctlysts to produce lrge wter-soluble orgnic compounds. In the finl rector, 4-PD/H, the condensed products re dehydrted nd hydrogented to produce lrge liquid lknes (rnging from C 7 to C 15 ) over bifunctionl ctlyst contining metl nd cid sites. Aqueous-phse processing, due to its high therml efficiency nd selectivities, ppers to be promising method for converting biomss-derived sugrs into lknes nd H 2. Other products cn lso be mde by APP including oxygented hydrocrbons, like lrge lcohols, which could be used for oxygented fuels. Previous APP reserch hs been used primrily with clen feeds, nd future work should focus on using rel biomss-derived feedstocks. The production of diesel fuel by APP requires tht lignocellulose be converted selectively into HMF nd furfurl. Furfurl cn be selectively produced from xylose; 280 however, HMF production from glucose is not currently possible with high yields. Future ctlysts development work is needed to chieve more ctive nd selective ctlysts. 8.4. Supercriticl Reforming of Sugrs Supercriticl reforming of sugrs cn lso produce H 2 s shown by eq 19. 69 Supercriticl wter conditions occur t conditions bove the supercriticl point of wter (tempertures bove 375 C nd pressures bove 217 tm). Antl performed thermodynmic clcultions for the reforming of glucose t tempertures rnging from 200 to 800 C nd pressure of 1 tm s shown in Figure 32. 81 The products from the rection include CO 2, CO, H 2 nd crbon (which they climed represented tr). At tempertures of 300 C, the equilibrium products re CO 2,CH 4, nd crbon. As the temperture is incresed, the crbon nd CH 4 equilibrium decrese, nd the CO nd H 2 equilibrium increse. No crbon is obtined t tempertures bove 600 C. Experiments hve shown tht stem reforming of glucose (s well s fst pyrolysis oils nd biomss) t tmospheric pressure produces lrge mounts of both therml nd ctlytic coke. High rection tempertures, bove 600 C, re needed to reform the coke. Model showed tht supercriticl reforming of wood swdust ws ble to produce gseous products nd void coke formtion. 70,71 Thus, supercriticl rections cn be used to efficiently gsify glucose (nd other biomss components) without coke formtion. Figure 33 shows the results of supercriticl gsifiction of glucose without ctlyst s function of temperture, pressure, nd concentrtion in cpillry btch rectors. 69 The crbon efficiency is defined s the mount of crbon in the gs-phse divided by the crbon in the glucose. The product gs yield incresed s the temperture incresed. The rection pressure hd little effect on the product gs, while the glucose concentrtion hd significnt effect. Incresing the glucose concentrtion decresed the yield of gs products. Heterogeneous ctlysts hve been used in supercriticl rections nd hve been shown to gretly chnge the product selectivity. The Bttelle single-step supercriticl gsifiction rector produces gs with high methne levels t tempertures round 350 C nd pressures 21 MP with Ru nd Ni bimetllic ctlysts supported on TiO 2, ZrO 2 nd crbon. 69,76 Higher rection tempertures (600 C nd 34.5 MP) for supercriticl rections hve been ble to produce H 2 from supercriticl reforming of glucose. 69 Xu et l. showed tht ctivted crbon is n efficient ctlyst for supercriticl gsifiction of glucose. 77 At WHSV of round 20 h -1 close to 100% of the glucose feed ws gsified with molr gs composition of 22% H 2, 34% CO, 21% CO 2, 15% CH 4,6% C 2 H 6, nd 2% C 3 H 8.

4082 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Figure 32. Thermodynmic clcultions for rection of glucose (1 mole) with wter (7 mol) s function of temperture t 1 tm. (Reprinted from ref 81 with permission. Copyright 1978 Institute of Gs Technology.) Figure 33. Products from supercriticl reforming of glucose (without ctlyst) s function of () temperture, (b) pressure, (c) concentrtion, nd (d) concentrtion nd temperture. Reprinted from ref 69 with permission. Copyright 2005 Elsevier. Other biomss feedstocks including whole biomss cn lso be used for supercriticl gsifiction (Section 3.4). The dvntges of supercriticl reforming re tht high rection rtes re obtined, impure feedstocks cn be used, wet feedstocks cn be processed with high therml efficiencies, product gs is produced in single rector, nd the product gs is vilble t high pressure. The disdvntges of supercriticl reforming re the high cpitl cost of highpressure rector, nd H 2 only cn be selectively produced t high tempertures where lrge mounts of CO re lso produced. Supercriticl reforming is n excellent wy to produce product gses from queous biomss mixtures. 8.5. Biologicl Hydrogen nd Methne Production Sugrs cn be fermented to CH 4 or H 2 with fermenttive microorgnisms. 2 Methne is produced by methne fermenttion or nerobic digestion in the bsence of oxygen with nerobic bcteri. This sme rection tkes plce in the ecosystem nd in the digestive trct. Methne fermenttion is used worldwide, for disposl of domestic, municipl, griculturl, nd industril biomss wstes. Crbon dioxide is lso produced long with the methne in the gs. Hydrogen cn be produced by drk fermenttion processes using nerobic nd fculttive nerobic chemohetrotrophs, which lso produce cetic nd butyric cids, s shown in eqs 22 nd 23. 2,281-283 Glucose, cellulose, strches, nd number of different wste mterils cn be used for hydrogen fermenttion. As shown in eqs 22 nd 23, the mximum mount of hydrogen tht cn be produced from these routes is 4 mol of H 2 per mole of glucose since cetic nd butyric cids re formed, nd theoreticlly 12 mol of H 2 could be produced from glucose. Reported yields of H 2 production rnge from 0.5 to 3.8 mol of H 2 /mol of feed. 282 Hydrogen production is highly dependent on the ph, retention time, nd gs prtil pressure. The rection is inhibited by hydrogen prtil pressure, nd to chieve high yields the H 2

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4083 must be diluted or long residence times re required. The orgnic cids produced must be sold or converted into other products, nd higher yields of H 2 from glucose re possible with APR or supercriticl reforming. The specific biologicl H 2 production rte rnges from 8 to 121 mmol of H 2 L -1 h -1. 127 A biologicl rector of pproximtely 1000-15000 L would be necessry to provide enough H 2 to power 5.0 kw PEMFC. Other processes cn produce H 2, such s queous-phse reforming (5000-50000 mmol of H 2 L -1 h -1 159 ), t higher rtes. For fermenttion technology methods to become commercilly competitive they must develop methods to synthesis H 2 t higher rtes. Optimiztion of biorector designs, rpid removl nd purifiction of gses, nd genetic modifiction of enzyme pthwys offer exciting prospects for improving this technology. C 6 O 6 H 12 + 2H 2 O f 2CH 3 COOH + 4H 2 (23) C 6 O 6 H 12 f CH 3 CH 2 CH 2 COOH + 2CO 2 + 2H 2 (24) 9.0. Conversion of Nonsugr Monomers Derived from Lignocellulose All components of lignocellulose should be utilized in fuel or chemicl production in n integrted biorefinery. A number of non-crbohydrtes cn be selectively produced from lignocellulose, including lignin, furfurl (from xylose), nd levulinic cid (from cellulose). All of these compounds cn be converted into fuels s discussed in this section. 9.1. Lignin Conversion Lignin, which consists of coniferyl lcohol, sinpyl lcohol, nd coumryl lcohol polymers (Section 2.2), represents mjor frction of biomss (10-30 wt %) nd is currently used s low-grde fuel to provide het in the pulp nd pper industry. Designs of ethnol production in lignocellulosic plnt lso show lignin being used to provide process het. 171 However, it would be idel to convert the lignin into higher vlue fuel or chemicl. This would lso require the development of lterntive wys of providing process het to the biorefinery or developing less energy intensive processes. Lignin cn be used s replcement for phenol-cetone resins, nd other uses re being developed. 284 Lignin lso cn be converted into trnsporttion fuel by dehydroxygention or zeolite upgrding. These re the sme methods used to upgrded bio-oils, which contin lrge frction of lignin products, discussed in Sections 6.1 nd 6.2. Previous dehydroxygention experiments of lignin feedstocks hve used sulfided NiMo nd CoMo ctlysts supported on lumin, chromium, nd zeolites t 250-450 C. 285-288 The mjor products from dehydroxygention include phenols, cyclohexne, benzene, nphthlene, nd phennthrene with liquid oil yields of 61% the originl lignin. It is likely tht coking rections occur under rection conditions nd dectivtion due to wter similr to wht ws observed by Delmon nd co-workers for dehydroxygention of guicol. 217-219 The rections tht occur during dehydroxygention include hydrogention of CdC bonds, hydrogention of romtics, nd deoxygention of C-O bonds. Future work in this re should focus on the development of nonsulfided ctlysts, since sulfur is not present in the feed in lrge concentrtions. Thring et l. studied zeolite upgrding of lignin with HZSM-5 ctlyst t 500-650 C nd 2.5-7.5 h -1 WHSV. 289 Tble 24. Zeolite Upgrding of Lignin with ZSM-5 Ctlyst temperture ( C) 500 550 600 600 600 650 WHSV (h -1 ) 5 5 2.5 5 7.5 5 yield of products (%) gs 11 19 51 54 58 68 liquid 39 43 34 30 22 11 chr + coke 50 38 15 16 20 21 mjor liquid product (wt %) benzene 8.6 9.4 9.3 13.6 14.5 14.4 toluene 33.1 36.7 31.0 42.4 41.9 43.7 xylene 31.5 33.0 25.0 22.7 24.8 21.0 ethyl benzene 3.0 2.1 2.2 1.9 1.5 1.3 propyl benzene 4.2 2.5 3.7 1.3 1.5 1.0 C 9+ romtics 9.0 5.1 6.4 6.0 3.1 3.0 gs composition (wt %) methne 8.7 5.3 13.0 4.4 8.3 13.9 ethylene 6.6 19.5 14.7 16.2 19.1 24.3 ethne 4.5 2.6 4.5 2.8 2.6 2.9 propylene 8.2 21.1 8.9 11.4 14.1 13.4 propne 34.6 13.7 4.5 6.6 4.8 2.6 C 4 18.5 13.2 2.9 4.4 5.0 3.0 C 5+ 4.8 2.4 1.6 1.0 1.9 3.9 CO 3.1 9.4 22.0 23.5 23.9 6.6 CO 2 10.9 12.4 27.7 29.7 20.1 19.6 H 2 0.2 0.3 0.1 0.1 0.3 0.1 Adpted from Thring et l. 289 Tble 24 shows the results of this study. The highest liquid yield ws 43%, nd the coke nd chr yields were 15-50%. As the temperture incresed gs yields incresed, chr nd coke yields decresed, nd liquid yields decresed. The mjor liquid components re toluene, benzene, nd xylene. Joseph Shbti nd the NREL designed process to convert lignin into high-octne-oxygented gsoline dditive s shown in Figure 34. 290,291 The first step in this Figure 34. Process for production of gsoline from lignin by bsectlyzed depolymeriztion of lignin followed by hydrotreting developed by Shbti nd NREL. 290,291 process involves bse-ctlyzed depolymeriztion with NOH t 320 C nd 120 tm of 30% solids feedstock consisting of lignin, wter, sh, tr, nd some cellulose with methnol or ethnol t lcohol to lignin weight rtios of 3:1 to 5:1. 290,292 The lcohol helps mintin supercriticl conditions, which help solubilize the lignin. The depolymeriztion steps brek down the lignin into monomer units. The products then go to flsh tnk where some of the wter nd the solids re removed. The insoluble solids re sent to boiler or sold s boiler fuel. The liquid products re neutrlized with sulfuric cid, nd the lignin is extrcted in toluene strem. The lignin, which contins mono-, di-, nd polylkyl substituted phenols nd benzenes with minor mounts of lkyoxyphenols nd lkyoxybenzene, is then seprted from

4084 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Figure 35. Pthwys for production of fuels from levulinic cid. the toluene strem nd fed to the hydroprocessing unit. Hydrotreting consists of two rectors for hydrodeoxygention nd hydrocrcking/ring hydrogention. The ctlysts for these rectors re sulfided NiMo or CoMo ctlysts. The products consists of C 7 -C 11 lkylbenzenes, C 5 -C 11 multibrnched prffins, nd mono-, di-, tri-, nd polylkylted cyclohexnes nd cyclopentnes. The products re 65% romtics with n octne number of 100-110. The production cost of the high octne reformulted fuel dditive, ssuming 100% solubiliztion of the lignin, nd 70% overll yield, is estimted to be $0.28/L. Alterntively, the depolymerized lignin could be converted into ryl methyl ethers (mostly phenyl-, tolyl-, nd dimethylphenyl methyl ethers) by feeding the depolymerized lignin to rector for mild selective C-C hydrocrcking tretment, to completely depolymerize the lignin to monocyclic phenols, nd then etherifiction of the phenols with methnol. 293 Hydrocrcking ctlysts include Pt/SO x /ZrO 2 or Pt/WO x /ZrO 2. Solid cids, such s SO x /MnO x /Al 2 O 3,SO x /MoO x /Al 2 O 3, nd SO x /WO x / Al 2 O 3 re used for the etherifiction step. 9.2. Levulinic Acid Conversion Levulinic cid cn be converted into fuels by dehydrtion/ hydrogention or esterifiction s shown in Figure 35. Methyl tetrhydrofurn (MTHF), which hs n octne number of 87 nd cn be blended with gsoline up to 70%, cn be produced by dehydrtion/hydrogention pthwy of levulinic cid. MTHF (20 wt % oxygen content) cn increse the oxygen content of gsoline nd hs been pproved by the USDOE s component of P Series fuel. Levulinic cid is seprted from other rection products (wter, formic cid, nd furfurl) by vcuum distilltion t 160 C nd 10-50 mmhg to produce ngelic lctone (the dehydrtion product). This rection is reversible, nd wter ddition will promote levulinic cid formtion. Hydrogention of ngelic lctone with PdRe/crbon ctlysts t 200-250 C nd 100 tm H 2 produces first γ-vlerolctone nd then 1,4-pentnediol, which dehydrtes to form MTHF in yields up to 90%. 294 1-Pentnol nd 2-pentnol re lso produced in this process in lower yields. Rection of ngelic lctone with n lcohol in the presence of n cid or bse ctlyst yields levulinic esters. 20 The bse-ctlyzed rection is crried out t 100-150 C, 55 tm N 2, nd with orgnic (Et 3 N), homogeneous (e.g., N 2 - CO 3,K 2 CO 3 ) or supported oxide (e.g., MgO/SiO 2, LiO/SiO 2 ) bsic ctlysts. The choice of ctlyst depends on the lcohols used. Levulinic esters hve high octne number nd high Tble 25. Fuel Chrcteristics of Levulinic nd Formic Acid-Derived Fuels nd Comprision to MTBE compound O 2 content (wt %) oxygen content (Tble 25) mking them idel for gsoline fuels dditives. The oxygen content of levulinic esters is significntly higher thn the oxygen content of MTBE, nd therefore oxygented gsoline requires less levulinic ester thn MTBE. Diesel fuel cn be oxygented by the ddition of levulinic esters. 295 Levulinic esters lso cn be produced by rection of ngelic lctone with olefins t 100-150 C, 55 tm N 2, nd with homogeneous nd solid cid ctlysts. 296 A mixture of levulinic nd formic cid cn produce levulinic nd formic esters by rection with olefins; 297 therefore, levulinic nd formic cid do not need to be seprted prior to the rection. Formic esters cn be used s fuel dditives. Leo Mnzer of Dupont hs estimted tht levulinic esters could be produced on lrge scle t less thn $0.50/L. 19 9.3. Furfurl Conversion required wt % for 2.7 wt % O 2 oxygented gsoline vpor pressure t 38 C (kp) blending octne no. (R + M)/2 MTBE 11 14.9 55 109 methyl formte 3.8 5.1 126 102 ethyl formte 4.6 6.3 55 103 methyl levulinte 37 7.3 106.5 ethyl levulinte 33 8.1 107.5 isopropyl levulinte 30 8.9 105 isobutyl levulinte 28 9.7 102.5 sec-butyl levulinte 28 9.7 102.5 Adpted from Fgn et l. 297 Furfurl, 2-furldehyde, is the triple dehydrtion product of xylose nd is n importnt chemicl obtined from the hemicellulose biomss frction. Industrilly, more thn 300 000 metric tons/yer of furfurl re produced. 298 Furfurl is currently too expensive for use s fuel; however, future production of furfurl in n integrted biorefinery where ll frctions of biomss re used could significntly decrese the cost. The Quker Ot Compny in 1922 developed the first commercil process for production of furfurl from ot hulls using cid ctlyst. 299 During the production of furfurl, superheted stem psses through rector contining the biomss to provide het for the rection nd remove the furfurl product. Furfurl is rective under these conditions, so it is vitl to remove the furfurl before it undergoes

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4085 undesired rections. Typicl rection conditions for furfurl production re 3 wt % sulfuric cid, 2:1 to 3:1 cid solution to lignocellulosic mss rtios, 170-185 C, nd 3 h retention time. 298 Sulfuric cid is not necessry if the temperture is rised high enough. This is becuse cetic cid in the hemicellulose rection is relesed nd cn ctlyze the dehydrtion rection. 299 Typicl furfurl yields re round 45-50%. 298 Higher yields of up to 60% cn be obtined by cid-ctlyzed dehydrtion of pure xylose. 300 According to Zeitsch, the principle yield loss of furfurl is due to rections between furfurl nd xylose, nd by eliminting this problem by proper rector design, significntly better yields cn be obtined. 280,301 Another new method, tested t the pilot-plnt scle, ppers to be promising for producing furfurl t yields s high s 70%. This method involves using continuous tubulr rector t high temperture (250 C), short residence time (5-60 s), cid concentrtions from 0.3 to 2.0 wt %, nd stem injection to rpidly remove the furfurl. 298 Current furfurl rectors re expensive to operte due to the lrge quntities of stem (30 to 50 times the mount of furfurl produced) nd lengthy rection times. 280,301 Typicl yields of furfurl, bsed on current commercil technology, on weight bsis for feedstocks (kg of furfurl/kg of feedstock) re 0.22 for corncobs, 0.12 for bgsse, 0.17 for cornstlks, 0.16 for sunflower hulls, nd 0.16 for hrdwoods. 298 The mechnism for formtion of furfurl from xylose ppers to go through 2,5-nhydride intermedite. 300 Furfurl cn lso be produced from xylose using heterogeneous ctlysts including MCM functionlized sulfonic cid ctlysts, 302 heteropolycids, 303 fujsite, nd mordenite. 304 High yields of furfurl, up to 75%, re obtined with heterogeneous ctlysts in DMSO nd toluene/wter solvents; 302 however, the yield is significntly lower (less thn 30%) when wter is used s solvent. Furfurl by itself cn not be used s motor fuel becuse of its tendency to polymerize. 305 However, s shown in Figure 36, furfurl cn be hydrogented to furfuryl lcohol, Figure 36. Pthwys for hydrogention of furfurl. methyl furn, tetrohydrofurfurl lcohol, nd methyltetrhydrofurn (MTHF), which ccording to Byn hve octne number of 83, 74, 83, nd 74, respectively. 305 The octne number of furfurl is 53. Of the hydrogented forms of furfurl, only MTHF is suitble s motor fuel becuse it will not polymerize nd hs low voltility. 305 As mentioned previously, MTHF, which lso cn be derived from hydrogention of levulinic cid, is pproved by the USDOE for use s gsoline dditive in P Series type fuels. Ahmed developed two-step process to produce MTHF from furfurl. 306 The first step involved hydrogention of furfurl to 2-methylfurn over Cu-bsed ctlyst t 175 C, followed by hydrocrbon of 2-methylfurn to MTHF with Ni-bsed ctlyst t 100 C. Furfurl cn lso be used to produce liquid lknes (n-c8 to C13) by queous-phse processing s discussed in Section 8.3. 10.0. Triglyceride Conversion Triglycerides re the mjor component of vegetble oils nd nimls fts (Section 2.3). They lso cn be produced from qutic biomss such s lge (Section 2.4). Vegetble oils cn be used directly in diesel engines; however, there re number of disdvntges of pure vegetble oils, including high viscosity, low voltility, nd engine problems (including coking on the injectors, crbon deposits, oil ring sticking, nd thickening of lubricting oils). 34,307 These problems require tht vegetble oils be upgrded if they re to be used s fuel. The most common wy of upgrding vegetble oils to fuel is trnsesterifiction of triglycerides into lkyl-ftty esters (bio-diesel). Wste vegetble oils, like frying oils, cn be used s feedstocks; however, chnges in the process need to be mde s wste vegetble oils contin free ftty cid (FFA) nd wter impurities. Vegetble oils cn lso be blended with diesel fuel or upgrded by other methods including zeolite upgrding nd pyrolysis. 10.1. Trnsesterifiction Trnsesterifiction is the rection of triglycerides (or other esters) with lcohols to produce lkyl esters (biodiesel) nd glycerol, typiclly in the presence of cid or bse ctlysts s shown in Figure 37. Methnol, due to its low cost, is the lcohol most commonly used, lthough other lcohols including ethnol or 2-propnol cn produce biodiesel with better fuel chrcteristics. Ethnol is used to produce biodiesel in Brzil becuse of inexpensive ethnol sources. Alkyl esters or biodiesel re lso clled ftty cid methyl esters (FAME). Alkyl esters cn be used directly in diesel engines with minor engine modifictions, nd they re sold in the U.S. s fuel clled B100. Alkyl esters cn be blended with trditionl diesel fuel in up to 20 vol %, with no engine modifiction. Blends of lkyl esters nd diesel fuel re sold s fuel termed B1-20 rnging from 1 to 20 vol % biodiesel. Biodiesel is soluble with petroleum diesel t ll levels. Reviews of lkyl ester production re published elsewhere. 34,307,308 Trnsesterifiction consists of number of consecutive, reversible rections, with diglycerides nd monoglycerides s intermedites (Figure 37). 309 The first step in trnsesterifiction is production of diglycerides nd lkyl esters, followed by monoglycerides nd lkyl esters, nd finlly lkyl esters nd glycerol. All of these rections re reversible, nd excess lcohol solvent is used (typiclly 50-200% excess lcohol) to drive the rection to completion with yields of lkyl esters s high s 99.7%. The lkyl esters lso seprte from the lcohol-ctlyst phse during the rection, further decresing the reverse rection rte. The first step in the rection mechnism for bse-ctlyzed trnsesterifiction involves rection of the bse with methnol to produce n lkoxide species, s shown in Figure 38. 310 The lkoxide then rects with the triglyceride to form tetrhedrl intermedite. This intermedite then decomposes to form n lkyl ester nd the corresponding nion of the diglyceride. This step is followed by the deprotontion of

4086 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Figure 37. Overll nd intermedite rections for trnsesterifiction of triglyceride nd lcohol to produce lkylesters (biodiesel) nd glycerol. Figure 38. Mechnism for bse-ctlyzed trnsesterifiction of triglycerides dpted from Schuchrdt et l. 310 methnol by the diglyceride nion regenerting the lkoxide species. Diglycerides nd monoglycerides re converted by this sme mechnism into lkyl esters nd glycerol. Freedmn nd co-workers observed second-order rection kinetics for ll three reversible rections (triglycerides to diglycerides, diglycerides to monoglycerides, nd monoglycerides to esters) in the trnsesterifiction of soyben oil with butnol nd methnol. 309 Triglyerides only hve limited solubility in methnol, so it is difficult to mesure the rection rte. Mechniclly mixing of the two phses is importnt

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4087 for improving the mss trnsfer between the two phses. 311 The ddition of solvents tht increse the solubility of methnol nd triglycerides, such s tetrhydrofurn (THF), cn significntly increse the rection rte. 312 Wter hs negtive effect on the rection becuse lkyl esters, triglycerides, diglycerides, nd monoglycerides cn rect with wter to form free ftty cids (FFA) (eq 25). FFAs rect with bse ctlyst to form sop, which cuses downstrem problems such s plugging, gel formtion, n increse in viscosity, nd problems with production seprtion. R - COOCH 3 + H 2 O f R - COOH(FFA) + CH 3 OH (25) Both cid nd bse ctlysts cn be used for trnsesterifiction; however, bse ctlysts re 4000 times more ctive nd cuse fewer corrosion problems thn do cid ctlysts. Most biodiesel processes use minerl cid ctlysts for esterifiction of FFA nd lkli bse ctlysts for trnsesterifiction including sodium hydroxide, potssium hydroxide, nd sodium methoxide. Sodium methoxide is the most widely used biodiesel ctlyst with over 60% of industril plnts using this ctlyst. 308 The most ctive nd expensive ctlyst is lso sodium methoxide. Reserch is ongoing to develop heterogeneous nd enzymtic ctlysts for esterifiction nd trnsesterifiction becuse removl of homogeneous ctlysts requires further downstrem processing, incresing bio-diesel production cost. Heterogeneous ctlysts hve the dvntge tht they cn be esily removed from the product nd recycled, nd current reserch indictes number of promising heterogeneous ctlysts for biodiesel production. 313-317 Problems with current heterogeneous ctlysts re tht they re not s ctive s homogeneous ctlysts, nd they require higher rection tempertures (200-250 C) nd pressures. 308 The ptent nd cdemic literture lists number of solid cid nd bse heterogeneous ctlysts tht hve been used for trnsesterifiction. 308 It should be tken into ccount tht the presence of FFA in the feed will strongly poison solid ctlysts. Thus, specil cre should be tken to remove FFA before they encounter the fixed-bed rector contining the solid/bse ctlyst. Alterntively, reserchers hve focused on the development of solid cid ctlysts for the trnsesterifiction rection nd/or to eliminte FFAs. The Institut Frncis de Petrole developed heterogeneous ctlyst, which ws mixture of ZnO, Al 2 O 3, nd ZnAl 2 O 4, for trnsesterifiction of vegetble oils with methnol t 230 C nd 50 tm. 318 Lopez et l. studied the trnsesterifiction of tricetin with methnol on rnge of solid cid nd bse ctlysts including ETS-10 (N, K), SO x /ZrO 2,WO x /ZrO 2, MgO, Nfion, β-zeolite, nd phosphoric cid/silici. 313 The most promising ctlysts were Amberlyst, Nfion, SO x /ZrO 2, WO x /ZrO 2, nd ETS-10(N, K). However, leching of N occurred with ETS-10 ctlyst, wheres the dectivtion rte for WO x /ZrO 2, Nfion, nd mberlyst ws smll. WO x /ZrO 2 nd SO x /ZrO 2 hd ctivities on per-site bsis similr to H 2 SO 4. However, these ctlysts re less ctive on mss bsis thn sulfuric cid becuse they do not hve the sme number of sites. Tod et l. developed sulfonted ctlyst supported on crbonized scchrides. 314 They clim tht the ctivity of this ctlyst is more thn hlf tht of liquid sulfuric cid nd higher thn conventionl solid cid ctlysts. Cntrell et l. studied the initil ctivity of mixed Mg- Al-oxide ctlysts, derived from hydrotlcite, for trnsesterifiction rection of methnol with glyceryl tributyrte t room temperture nd found the ctlyst with the highest ctivity hd 24 wt % Mg loding. 316 Suppes et l. studied the trnsesterifiction of soyben oil with methnol t 60-120 C with NX fujsite zeolite, ETS-10, nd metl ctlysts, with ETS ctlysts being the most ctive. 317 Kim et l. developed N/NOH/Al 2 O 3 tht hd ctivity similr to NOH ctlysts for trnsesterifiction of vegetble oils; however, they did not recycle the ctlyst, indicting concerns bout ctlyst stbility. 315 Development of future highly ctive, selective, nd stble heterogeneous ctlysts for trnsesterifiction rection promises to decrese the biodiesel production cost nd will be n incresing importnt reserch re in the future. Solid bse ctlysts re used for very similr rections such s the glycerolysis rection of vegetble oils nd glycerol to produce monoglycerides t 240 C. 319 Corm nd co-workers studied this rection nd showed the order of ctlytic ctivity decresed s MgO > Mg-Al-oxide > Sepiolite-CS > MCM- 41-Cs. These uthors demonstrted tht these ctlysts could be recycled without significnt loss in ctivity. Solid Bronsted bse ctlysts hve significntly higher ctivity thn Lewis bse ctlysts for trnsesterifiction of oleic cid methyl esters with glycerol. 320 Efforts hve been mde to synthesize lyered mixed solids (Mg/Al hydrotlcites) with controlled crystllite size nd lrge ccessible surfce res. 321 This hs been shown to increse ctlyst ctivity for trnsesterifiction rections. Lipse, enzymtic ctlysts, cn lso ctlyze esterifiction rections. The dvntges of lipse ctlysts re their bility to ctlyze both trnsesterifiction nd esterifiction of FFA in one step, production of glycerol side strem with miniml wter content nd little or no inorgnic mteril, nd recyclbility. 35 However, enzymtic ctlysts hve high costs nd dectivte due to feed impurities. At high pressure (120 tm) nd temperture (350 C) triglyceride trnsesterifiction occurs without ny ctlyst. 322 Some production plnts in Europe use this technology; however, this requires high pressure leding to n incresed cpitl cost. 35 Advntges of this method re tht the esterifiction rection cn occur rpidly (less thn 5 min), 323 nd no ctlyst is required decresing downstrem processing costs. A block flow digrm for production of biodiesel from vegetble oils is shown in Figure 39. 312 The first step in the process is trnsesterifiction of the refined vegetble oils Figure 39. Block flow digrm for biodiesel production by trnsesterifiction of vegetble oils dpted from Vn Gerpen nd Knothe. 312

4088 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. (refined by crushing of vegetble seeds) with methnol nd bse ctlyst in CSTR or plug flow rector t 60 C, pproximtely 1 tm, 6:1 methnol/oil molr rtio nd 1 h residence time. Sometimes the trnsesterifiction process is crried out in two different CSTRs where pproximtely 80% of the ctlyst nd methnol is dded to the first rector. In the second rector, the remining lcohol nd ctlyst is dded. The two-step process hs the dvntge of using less methnol thn single step process. The products from the rector re in two phses: glycerol-rich phse nd methyl ester-rich phse. A settling tnk nd centrifuge seprtes the two phses. The glycerol strem contins pproximtely 50 wt % glycerol, most of the bse ctlyst, nd most of the sop. This frction is then neutrlized with cid, nd the sop forms FFAs, which phse seprte from the glycerol strem. The FFAs cn be recycled nd used for biodiesel production. A vcuum flsh process seprtes the methnol nd glycerol phses, with 85 wt % glycerol product, which is then sold. The methyl ester-rich strem, which lso contins 2-3 wt % methnol, smll mount of bse, nd smll mounts of di- nd monoglycerides, is neutrlized prior to methnol removl. The cid removes ny remining ctlyst or sop. The methnol in the methyl ester strem is then stripped by vcuum flsh or flling film evportor. Wter wshing of the methyl ester strem removes slts nd FFA. Any remining wter in the biodiesel is removed during finl drying step in vcuum flsh process. Wter is lso removed from the methnol strem, nd the remining methnol cn be recycled to the process. Wste triglycerides oils, such s cooking oils (2-7 wt% FFA) nd niml fts (5-30 wt %), contin significnt mounts of FFA. 312 If bse ctlysts re used for the trnsesterifiction process, then the FFA will rect with the bse ctlyst forming sop ccording to eq 26. Sop cuses downstrem processing problems including gel formtion, viscosity increses, nd product seprtion difficulty. 34 Freedmn et l. hve recommended tht the concentrtion of FFA in the oil should be less thn 0.5 wt %; otherwise, the FFA will rect with bse ctlysts to form sop nd wter. 324 Free ftty cids cn be removed from vegetble oils by n cid-ctlyzed esterifiction tretment where the FFA rect with methnol to form methyl esters nd wter. Two phses re present in this rection: methnol phse tht contins wter, cid nd some oil nd triglyceride phse tht contins triglycerides, methyl esters, nd unrected FFA. Solid cid heterogeneous ctlysts lso ctlyze the rection of FFA with methnol. 325 R - COOH + NOH f R - COON + H 2 O (26) Bry nlyzed the economics of biodiesel production using two different processes: the CD process which consists of trnsesterifiction in three pcked column rectors in series nd n lkline ctlyzed process from Lurgi PSI where the trnsesterifiction rection occurs in two gitted rectors in series. 308 The Biodiesel production costs re mde up of three mjor components: feedstock costs, cpitl costs, nd byproduct credits. As shown in Tble 26, the biodiesel feedstock (soyben oil, methnol, nd ctlyst) is the single lrgest cost for biodiesel production representing over 70% of the biodiesel cost in this nlysis. The refined production cost of biodiesel ($0.49-0.50/L) is close to the feedstock costs ($0.46-0.47/L) becuse glycerol currently hs high vlue ($1.10-2.20/kg for refined glycerol), which Bry Tble 26. Biodiesel Production Cost for Plnt with Cpcity of 99790 Metric Tons Per Yer nd Soyben Oil Cost of 49 /kg production cost ( /L) CD process lkline process rw mterils 46.34 47.71 by products -10.21-10.41 utilities 0.78 0.63 totl vrible costs 36.90 37.94 lbor (operting, mintennce & control) 2.21 2.06 mintennce mterils 0.70 0.49 operting supplies 0.13 0.13 totl direct costs 39.95 40.60 plnt overhed 1.77 1.65 txes nd insurnce 0.97 0.72 totl plnt csh costs 42.68 42.98 deprecition, 12-yer life, 8.33%/yer 4.06 3.04 plnt gte costs 46.74 46.02 G&A, sles, reserch, 5% PV 3.11 2.91 net production cost (per L) 49.85 48.92 ROI before txes 25%/yer TFC 12.15 9.06 product vlue (PV) 62.08 57.99 Adpted from Bry. 308 The current diesel fuel FOB spot price t New York Hrbor is 44 /L, nd oil prices re $57/bbl. 14 ssumed ws $1.21/kg. It hs been projected tht incresed biodiesel production will significntly decrese glycerol prices. Bry estimtes tht the refined glycerol price could drop to $0.77/kg, which would decrese the credit given for glycerol production to $0.07/L. Hs et l., who lso modeled the biodiesel production cost, hd similr economic results s those of Bry. 326 Hs et l. estimted the biodiesel production cost to be $0.53/L when the feedstock soyben cost ws $0.52/kg, nd with glycerol credit of $0.33/kg for n 80 wt % glycerol-wter solution. The FOB cost of diesel fuel from crude oil is 44 /L when oil prices re $55/bbl. 9 Thus, biodiesel is currently slightly more expensive thn petroleum diesel. However, biodiesel cn be cost competitive with petroleum-derived diesel fuel, in mny countries, if it is exempt from txes. In the EU, the tx on diesel rnges from $0.34-0.99/L, which is higher thn the tx in the U.S. of $0.13/L. Since feedstock is the primry cost of biodiesel, decresing the feedstock cost will significntly decrese the biodiesel cost. Figure 40 shows the cost of biodiesel from cheper Figure 40. Biodiesel production cost s function of feedstock cost (1 gl ) 3.79 L, 1 lb ) 0.454 kg) from Schumcher et l. 327 (Reprinted from ref 327 with permission. Copyright 2004 Elsevier.) feedstocks. The feedstock costs decrese from cnol oil > soy > tllow nd lrd > yellow grese > trp greses. Yellow grese is produced from used cooking oil nd other fts collected from resturnts, cfeteris, nd other commercil cooking industries. Yellow greses tody is used s

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4089 n ingredient in niml feeds; however, concerns bout Md Cow disese cused bn on it for use s n niml feed. 308 Processing of yellow grese is more expensive thn vegetble oils becuse yellow grese hs higher FFA levels. Trp grese or brown grese is collected from grese trps in sewge plnts. Trp grese contins high levels of wter, FFA, nd other impurities. A life cycle wells to wheel nlysis for biodiesel nd petroleum diesel hs been done by Sheehn et l., nd the results, in terms of fossil energy rtio (FER), PTE, nd LCTE re shown in Tble 27. 328 Biodiesel nd petroleum diesel Tble 27. Fossil Energy Rtio nd Therml Effiencies for Production of Diesel Fuel nd Biodiesel from Oil nd Soy, Respectively petroleum diesel biodiesel from soyben fossil energy rtio (MJ fossil fuel/mj product) crude production 1.113 0.149 crude trnsport 0.016 0.034 refining 0.064 0.151 refined trnsport 0.006 0.004 overll energy requirements 1.199 0.311 process therml efficiency 0.94 0.93 life cycle therml efficiency 0.83 0.80 Dt from Sheehn et l. 328 Energy of lignocellulose is not tken into ccount for clculting therml efficiencies of biodiesel. require similr mounts of fossil fuel for production, refining, nd trnsport when the energy content of the finl diesel fuel product is subtrcted from the crude production input. Vegetble oil production nd refining re the two mjor energy consumption steps for biodiesel production consuming 48 nd 49% of the fossil fuel, respectively. The mjor energy consumption step for petroleum diesel is refining (60% of totl energy) followed by production (29% of totl energy). The ethnol refining step (68%) is the mjor energy consumption step for ethnol production. From this life cycle nlysis, it cn be seen tht future work on biodiesel production should focus first on developing high-yield crops tht hve smll mounts of energy inputs nd improving the refining process efficiencies. Biodiesel stndrds hve been developed for number of countries nd re reported elsewhere. 33 Tble 28 lists fuel properties of vrious biodiesel fuels. All of these esters hve high cetne numbers nd flsh points. Kinemtic viscosities re lso in n cceptble rnge for diesel fuels. The cetne numbers of methyl nd ethyl esters cn be correlted with the chin length nd degree of sturtion. 329 Problems with biodiesel include high price, incresed NO x exhust emissions, oxidtive stbility when exposed to ir, nd cold flow properties. The cold flow properties of fuel re defined by its cloud point, pour point, nd cold filter plugging point. Biodiesel is n oxygented fuel nd when blended with petroleum-bsed diesel it reduces CO, prticulte, nd totl hydrocrbon emissions. According to Knothe, biodiesel fuels hve the following dvntges when compred to petrochemicl fuels: 307 (1) Derivtion from renewble domestic resources (2) Biodegrdbility (3) Reduction of most exhust emissions (with the exception of nitrogen oxides) (4) Higher flsh point, leding to sfer hndling nd storge (5) Excellent lubricity, which is prticulrly importnt for low-sulfur petrodiesel fuels, which hve gretly reduced lubricity. Adding bio-diesel t low levels (1-2%) restores lubricity. The EU produces 90% of the biodiesel in the world with production cpcity of 2.4 million metric tons of biodiesel per yer. 330 The EU hs set trget of 5.75% renewble trnsporttion fuels by 2010, nd if this trget is to be chieved with biodiesel fuels then the estimted production in Europe is 7.9 billion gllons/yer. 308 Brzil nd the U.S. re the world s mjor oilseed producing countries. The EU hs chosen to focus on biodiesel production becuse of the superior fuel economy of diesel engines, the griculturl prctices in Europe, nd the fct tht most crs run on diesel fuel in the EU. 308 10.2 Pyrolysis nd Zeolite Upgrding Pyrolysis of vegetble oil cn be used to produce liquid fuel tht contins liner nd cyclic prffins, olefins, ldehydes, ketones, nd crboxylic cids. Pyrolysis products of vegetble oils were used s fuel during the first nd second World Wrs. 331 A possible mechnism for therml decomposition of soyben oil is shown in Figure 41. 332 The exct therml decomposition mechnism is very complex, generting wide rnge of structures. Usully these mechnism proceed through either free-rdicl or crbonium ion mechnism. Homologous lknes nd lkenes series occurs from the genertion of RCOO rdicls from triglyceride clevge nd subsequent loss of CO 2. 332 The R rdicl, upon disproportiontion nd ethylene elimintion, gives the oddnumbered crbon lknes nd lkenes. Unsturtion increses clevge t the R nd β positions reltive to the unsturtion. Tble 28. Fuel Properties of Esters of Oils nd Fts oil or ft ester cetne number H comb (kj/kg) kin visc (40 C; mm 2 /s) cloud pt ( C) pour pt ( C) flsh pt ( C) coconut ethyl 67.4 38158 3.08 5-3 190 corn methyl 65 38480 4.52-3.4-3 111 cottonseed methyl 51.2 6.8 (21 C) -4 110 olive methyl 61 37287 4.70-2 -3 >110 plm ethyl 56.2 39070 4.50 (37.8 C) 8 6 rpeseed methyl 48-56 8850-39780 4.53-4.96-6to-3-9 166-169 rpeseed ethyl 67.5 40663 6.02 1-12 170 soyben methyl 49.6-51.5 37388-40080 3.99-4.30-2to3-7to0 120 sunflower methyl 54-58 38100-38472 4.39 0-1.5-3to3 110 tllow methyl 61.8 37531-39961 4.11-4.99 12-15.6 9-12.8 96-188 yellow grese methyl 62.6 37144-39817 5.16 used frying oil methyl 59 37337 4.50 1-3 >110 wste olive oil methyl 58.7 5.29-2 -6 Adpted from Knothe et l. 307

4090 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Figure 41. Mechnism for pyrolysis of triglycerides dpted from Schwb. 332 Aromtics cn form by Diels-Alder rection of ethylene to conjugted diene. Hydrogen elimintion occurs t high tempertures. The pyrolysis rection cn be done with or without ctlyst, nd number of ctlysts hve been tested including HZSM-5, β-zeolite, nd USY. 333,334 Tble 29 shows the molr Tble 29. Molr Selectivity s Function of Temperture for Pyrolysis of Rpeseed selectivity (molr %) 600 C 700 C 800 C C 1-C 4 18.6 38.7 45.1 C 5-C 9 19.6 13.2 12.6 C 10-C 14 3.5 2.7 1.0 C 15-C 18 0.7 1.1 0.2 romtics 2.0 3.9 11.6 C 3:1-C 8:1 esters 16.6 7.2 4.1 C 9:1-C 16:1 esters 3.2 2.3 0.5 sturted esters 1.2 2.4 3.1 CO 1.2 2.3 3.8 CO 2 0.6 1.1 1.6 coke 3.8 4.7 3.1 other 29.0 20.4 13.3 H 2 0.9 2.7 4.6 Adpted from Billud et l. 335 crbon selectivity for the pyrolysis rection of rpeseed oil from 600 to 800 C. 335 As shown in Tble 29, zeolite upgrding is nonselective process mking rnge of compounds nd undesired coke nd lighter lknes from vegetble oils. Zeolite ctlysts lso cn produce romtic compounds, nd Twiq et l. observed tht upgrding of plm oil with ZSM-5 produced yield of 28, 9, nd 5% gsoline, kerosene, nd diesel fuel, respectively. 334 Lim et l. clim tht pyrolysis products with ZSM-5 ctlyst nd soyben nd plm oil feedstock hve fuel properties similr to Brzilin diesel fuel. 331 An dvntge of the pyrolysis product is tht no methnol or ethnol is required; however, the disdvntge is tht the selectivity to liquid fuel products is not s high s with trnsesterifiction. 10.3. Hydrotreting Vegetble oils cn lso be hydrotreted to produce stright chin lknes rnging from C 12 to C 18. 336-340 These lknes hve high cetne numbers (55-65), nd 10-month on-rod test of six postl delivery vns showed tht engine fuel economy ws gretly improved by blend of petrodiesel with hydrotreted vegetble oil products. 340 The rection conditions for the hydrotreting process re 350-450 C, 40-150 tm, liquid hourly spce velocities 0.5-5.0 h -1, nd sulfided NiMo/Al 2 O 3 ctlysts. 336 The lknes cn lso be isomerized using moleculr sieve or zeolite ctlysts. 337 According to Stumborg et l. the dvntges of hydrotreting over esterifiction re tht it hs lower processing cost (50% tht of esterifiction), comptbility with current infrstructure, engine comptbility, nd feedstock flexibility. 339 It would be desirble to develop nonsulfided hydrotreting ctlysts, since vegetble oils do not hve high sulfur contents. Alterntively, vegetble oils could be mixed with hevy gs-oil nd hydrotreted in petroleum refinery. 10.4. Microemulsions nd Cosolvent Vegetble Oil Blends Studies hve been conducted on blending vegetble oil with petrodiesel, but these studies indicte tht these blends re not suitble for long-term use in direct injection engines. 341 However, mixtures of vegetble oils with methnol or ethnol nd n mphilic compound hve been shown to be one wy of producing vegetble oil fuel blend tht cn be used in direct injection engines. 341 Amphilic compounds re dded to increse the solubility of the lcohol. Alterntively, cosolvent cn be dded to increse the solubility of vegetble oils with methnol or ethnol. Mny vritions of this pproch hve been used to mke hybrid vegetble oil-diesel fuels. 341 10.5. Glycerol Utiliztion Glycerol is currently too expensive to be used s fuel; however, s biodiesel production increses the price of glycerol will decrese. Hydrogen cn be produced from glycerol by queous-phse reforming 25,160,271 (Section 8.3) or gs-phse stem reforming. 342 Another process for fuel production from glycerol is the etherifiction of glycerol with isobutylene nd ion-exchnge resin ctlyst to produce butyl ethers of glycerol, which could be used s n oxygented diesel fuel dditive. 343,344

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4091 11.0. Ethicl Considertions nd Conclusions 11.1. Ethicl Considertions Biofuels should nd cn be produced sustinbly with food nd niml feed s coproducts. 8 Ethicl nd morl questions rise when edible biomss products re converted into biofuels. Regions where mlnutrition exists due to low levels of food production should first focus on producing edible biomss for food use before they produce biofuels from edible biomss. Shell Oil hs policy to only produce biofuels from nonedible frctions of biomss. However, s discussed in this review the lrgest frction of biomss is nonedible lignocellulose such s strw, grsses, corn stover, wood, forest products, etc. Edible biomss is coproduced in plnt mteril with lignocellulose biomss. Currently, the economics re more fvorble for conversion of edible biomss into fuels due to their chemicl structure, which cn be more efficiently converted. Therefore, it is vitl to continue to develop processes for conversion of lignocellulosic biomss into fuels. Agriculturl prctices in industrilized countries re very dvnced, nd most industrilized regions produce more thn enough food for domestic food consumption. Frmers do not pick the crops bsed on how efficiently they produce edible food products. Insted frmers gols re to grow crops tht mximize their income, even though more efficient crops cn be grown. Biofuels lso cn provide frmers in developing countries nother mrket for their products, which could improve the economies of developing countries. The EU hs estblished s the min re of reserch second-genertion biofuels from vrious biomss resources nd wstes. As hs been shown in this review, it is techniclly possible to convert cellulose mterils nd orgnic wstes into biofuels. However, costs hve to be lowered nd technology hs to be demonstrted t commercil scle (greter thn 150 000 metric ton/yer). If this is chieved, second-genertion biofuels will secure higher mrket shre by llowing the use of wider rnge of rw mterils. Moreover, the cultivtion process could be environmentlly less intensive thn for ordinry griculturl crops, with the corresponding cost decrese nd lower greenhouse emission from cultivtion. On these bses, the EU will support reserch on () Biomss to energy conversion processes (b) The bio-refinery concept to find vluble uses for ll prts of the plnt (c) Second-genertion biofuels for improving efficiency nd cost-effectiveness. We recommend tht sustinble policies be developed where food nd niml feed is produced long with biofuels from our griculturl lnds. Biofuels re only prt of the nswer to our dependence on fossil fuels. Other energy sources, such s wind, solr, hydroelectric, nd nucler power, should be used to produce electricity for sttionry power production, nd it is vitl tht we invest in other renewble sources long with biomss. More efficient energy conversion technologies for trnsporttion vehicles, such s hybrid electric crs nd fuels cells, should lso continue to be developed. Finlly, we need to strive to develop lifestyles tht conserve energy. 11.2. Overll Conclusions Regionl issues gretly ffect biomss cost, but biomss costs from chepest to most expensive re typiclly lignocellulose < strches < vegetble oils < terpenes < lges. Lignocellulose is the chepest nd most bundnt form of biomss, nd on n energy bsis is significntly cheper thn crude oil. On griculturl lnd, the growth rte of lignocellulose on per energy bsis is 30-240 boe/h-yer. The limiting fctor is tht low-cost processing technologies tht efficiently convert lrge frction of the lignocellulosic biomss energy into liquid fuels, do not yet exist. 13 Vegetble oils, strches, nd sugrcne currently hve lower cost of conversion into liquid fuels thn lignocellulose feedstocks if feedstock costs re not considered. Terpenes nd lges re currently too expensive to be used s liquid fuel feedstocks. Future work with lignocellulose feedstocks should focus on methods to convert lignocellulose into liquid fuels, wheres reserch on more expensive feedstocks, which re esier to brek down into liquid fuels, such s vegetble oils nd terpenes, should focus on development of better crops through plnt breeding nd genetic engineering. Alge hve tremendous potentil s feedstock becuse they hve very high growth rtes nd cn be grown on nongriculturl lnd. However, lge re currently too expensive to be grown on energy frms, nd future work should focus on the development of cheper lge crops. Figure 42 shows the different routes for production of liquid fuels from biomss discussed in this review. Tble 30 summrizes the development stge, products, nd chllenges of ech of these technologies. Some of the technologies re currently used commercilly, some hve been tested t the pilot scle, nd others hve been tested only t the lbortory scle. Lignocellulose mterils consist of sugr polymers (cellulose nd hemicellulose) nd lignin, nd lignocellulose into liquid fuels involves removl of oxygen s either CO 2 or H 2 O. In this process, functionlity is removed from nonthermlly stble molecule. This is the opposite of petroleum-type rections tht involve dding functionlity to highly thermlly stble molecule with low functionlity. The three bsic technologies for lignocellulosic conversion into liquid fuels, (Figure 42) include gsifiction of lignocellulose to syn-gs followed by syn-gs conversion, production of bio-oils by pyrolysis or liquefction followed by upgrding of bio-oils, nd cid hydrolysis of biomss to monomer units, which cn then be converted into fuels. Gsifiction of biomss to syn-gs nd its subsequent conversion into liquid fuels is well estblished but expensive pthwy tht could rpidly become commercilly prcticed. The most estblished processes for biomssderived syn-gs re production of H 2 (by the WGS rection), methnol (by methnol synthesis), nd liquid lknes (by FTS). Methnol cn be converted into gsoline, olefins, or H 2. During biomss gsifiction 23% of the energy of the biomss feedstock is lost. 180 Further conversion of the syngs to liquid fuels (FTS lknes, methnol, or ethnol) is predicted to hve n overll PTE between 0.2 nd 0.4 (Tble 9); thus, lrge frction of the biomss energy is lost in syn-gs conversion. 114 It is likely tht further dvnces in syn-gs conversion will only show modest improvements in incresing the PTE, since these processes hve been studied for severl decdes. Production of liquid fuels from biomss by the gsifiction route is not currently economicl competitive with production of liquid fuels from petroleum; however, reserchers hve predicted tht in most EU countries syn-gs-derived biofuels cn be cost competitive with petroleum-derived fuels if they re given tx exemptions. 114,178,179

4092 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Figure 42. Known routes for production of liquid fuels from biomss. Production of bio-oils by fst pyrolysis is commercil technology; however, bio-oils re currently not being used for liquid fuels production. (Bio-oils insted re used for primrily for chemicl production.) Bio-oils cn be produced by high-pressure liquefction; however, fst-pyrolysis is cheper technology. 208,223 The PTE for bio-oil production by fst pyrolysis rnges from 61 to 68%. 208,223 Bio-oils degrde with time nd therefore cnnot be used directly s trnsporttion fuel without upgrding or blending. Bio-oils cn be upgrded by hydrodeoxygention 182,209 to produce hydrocrbons nd romtics, or zeolite upgrding (ctlytic crcking), 223,224,227-229 to produce romtics light lknes nd coke. It hs been predicted tht both zeolite upgrding nd hydrodeoxygention of bio-oils decreses the energy content of the bio-oil to bout 50% of the biomss feedstock, 223 which is significntly higher thn the overll therml efficiency of biofuels derived from syn-gs. However, fuels derived from bio-oils hve not been extensively tested, nd the process of bio-oils upgrding must be further developed. The third wy for producing biofuels from lignocellulose involves the selective conversion of lignocellulose into sugr nd lignin monomer units followed by their subsequent selective conversion into trgeted products. It hs been estimted tht sugrs cn be produced from lignocellulose t costs of 5-14 /kg. 13,171 Efficient pretretment methods re needed to fcilitte conversion of biomss into monomer units. 250 Sugrs cn be used to produce ethnol (by fermenttion), 345 lknes (by queous-phse dehydrtion-hydrogention), 18,22,115 romtics (by zeolite upgrding), 270 or H 2 (by stem, supercriticl or queous-phse reforming). 69,81,160,238,271 The C 5 sugrs cn be dehydrted to furfurl, 280 which cn be used s feedstock for n-c8-c13 lkne production (by queous-phse processing) 18,22 or MTHF (methyltetrhydrofurn) production (by hydrogention). 306 Glucose nd other C 6 sugrs cn be used for levulinic cid production, 260-262 which cn then be used to produce levulinic esters (by esterifiction) 20,296 or MTHF (by hydrogention). 294 A recent process hs been developed nd tested t the commercil level to produce levulinic cid from biomss wste t n estimted cost of $0.09-0.11/kg. MTHF is pproved in the U.S. s n oxygented gsoline dditive. Levulinic esters cn be used s n oxygented diesel fuel dditive. Ethnol is the most widely used biofuel. Corn grin nd sugrcne re the feedstocks used in Brzil nd the U.S., respectively. Only the corn grin is used for sugr production, which contins strches tht re esily converted into sugr monomers. The lignocellulose frction of corn, corn stover, is currently not used for ethnol production; however, processes re being developed to utilize corn stover s feedstock. Only the wter-soluble sugrs in sugrcne re used for ethnol production, nd the bgsse (the lignocellulose section of sugrcne) is used to provide process het. The overll PTE for production of ethnol is round 0.20-0.40 345 nd 0.29 177 for cnesugr nd corn grin feedstocks, respectively. The PTE for lignocellulose feedstocks is projected to be 49%, which is significntly higher thn the PTE for sugr production from sugrcne nd corn grin. 171 A lrge frction of the energy for ethnol production is for distilltion to recover ethnol from the wter. Alknes, which cn be blended with gsoline or diesel fuel, produced by queous-phse dehydrtion/hydrtion (APD/H) of sugrs, contin 95% of the energy of the sugr nd only 30% of its mss. 18,22 The dvntge of lkne production by APD/H is tht the mjority of lknes spontneously seprte from the queous-biomss feedstock; therefore, significntly less energy is required for seprtion of lknes from wter thn ethnol from wter. While APD/H ppers to be promising technology for sugr conversion, it is only in the initil stge of development nd will require more study before it cn become commercil process.

Synthesis of Trnsporttion Fuels from Biomss Chemicl Reviews, 2006, Vol. 106, No. 9 4093 Tble 30. Stges of Development for Production of Liquid Fuels from Biomss technology FTS of biomssderived syn-gs products liquid lknes development stge lrge scle pilot plnt chllenges controlling selectivity during FTS process, Tr removl/ conversion from syn-gs fermenttion of biomss-derived syn-gs ethnol lbortory scle development of enzyme ctlysts fst pyrolysis bio-oils commercil bio-oils re unstble liquefction bio-oils pilot plnt bio-oils re unstble hydrotreting of bio-oils mixture of romtics, lknes lbortory scle zeolite upgrding (ctlytic crcking) of bio-oils fermenttion of corn strch or sugrcne ethnol vi fermenttion of cellulosic biomss mixture of romtics, lknes lbortory scle bio-oils re unstble t high tempertures producing coke, requires expensive H 2 lrge mounts of coke produced, qulity of finl product low ethnol commercil requires lrge mounts of energy for distilltion ethnol lrge scle pilot plnt conversion of cellulosic biomss to sugrs, fermenttion of C 5 sugrs, long residence times methyl-tetrhydrofurfurl commercil for not currently economicl for fuels, chemicls need more efficient ctlysts stright chin lknes lbortory scle (C 1-C 16) hydrogention of furfurl (hemicellulose-derived) queous-phse processing need to test with rel biomss of sugr-derived feedstocks, need more efficient feedstocks ctlysts, multiple steps esterifiction of levulinic cid levulinic esters lbortory scle requires low-cost levulinic cid feedstock hydrotreting of lignin lkyl benzenes, prffins lbortory scle conversion of solid lignin residue into liquid; requires expensive H 2 ctlytic crcking of lignin romtics, coke lbortory scle lrge mounts of coke re produced ctlytic crcking of sugrs romtics, coke lbortory scle lrge mounts of coke re produced trnsesterifiction of vegetble oils lkyl esters (bio-diesel) commercil high-cost of vegetble oils; need to replce minerl bse ctlysts. hydrotreting of vegetble oils lknes (n-c 12-C 18) pilot plnt requires expensive H 2 ctlytic crcking of vegetble oils romtics, olefins, prffins lbortory scle lrge mounts of coke re produced; multiple products produced A lrge frction of biomss is lignin, nd in n idel biorefinery the lignin should be used for biofuels nd biochemicls production. Processes exist to convert lignin into liquid fuels including the production of lkyl benzenes nd prffins (by hydrodeoxygention) 285-288 or romtics nd coke (by zeolite upgrding). 289 The lkyl benzenes nd prffins produced by hydrodeoxygention cn be used s high-octne-oxygented gsoline dditive. 290,291 The NREL hs designed process for lignin conversion bsed on hydrodeoxygention nd hydrocrcking technology. 290,291 Triglycerides re high energy content molecules produced from biomss, nd in Europe triglycerides derived from vegetble oils re the most widely used biomss feedstock. While triglycerides re more expensive thn lignocellulosic biomss, they cn efficiently be converted into liquid fuels. The liquid fuels include lkyl esters (or biodiesel by trnsesterifiction), 307 C 1 -C 14 lknes/lkenes (by zeolite upgrding or pyrolysis), 331-334 C 12 -C 18 n-lknes (by hydroprocessing) or be used directly. The direct use of vegetble oil cuses engine problems; therefore, vegetble oils need to be converted prior to combustion. 307 Biodiesel is produced by trnsesterifiction of vegetble oils. Glycerol is byproduct of the trnsesterifiction process, nd the sle of glycerol improves the economics of biodiesel production. 35,308 However, it hs been predicted tht with incresed biodiesel production the cost of glycerol will significntly decrese. Triglycerides cn lso be obtined from qutic lge; however, current methods of lge production re too costly to use lge s feedstock. 38 Biomss lso produces hydrocrbons of moleculr formul (C 5 H 8 ) n clled terpenes. These nturl hydrocrbons cn be used s trnsporttion fuels if they cn be economiclly produced. Nturl rubber, cis-1,4-polyisoprene, with moleculr weight from 500 000 to 2 000 000, is one exmple of terpenes. Nturl rubber is produced commercilly from the ltex of the HeVe brsiliensis tree, member of the Euphorbicee fmily. 40 The mjor problem with using terpenes s biofuel feedstocks is the high-cost of terpene production; therefore, reserch in this re should focus on developing cheper terpene feedstock. Fuels re low vlue commodity produced on very lrge scle, nd therefore development of economicl processes for fuel production requires lrge investment in both money nd time. Most biomss conversion processes re strted with the gol of rpidly developing commercil technologies. The fundmentl chemistry of most of these rections is not well understood, nd it is likely tht further scientific understnding will led us to improved processes. It is vitl tht new more efficient ctlysts be discovered for these rections. It is likely tht heterogeneous ctlysis, which hs been the bckbone of the chemicl nd petroleum industry, will ply key role in the upcoming trnsition to the crbohydrte economy. According to Bridgwter, The use of ctlysts to improve either the yield or qulity of gs nd liquid fuels from thermochemicl biomss conversion processes is still in its infncy. While there is extensive fundmentl work underwy, considerbly more reserch is necessry to explore the wide rnge of conventionl nd unconventionl ctlysts.

4094 Chemicl Reviews, 2006, Vol. 106, No. 9 Huber et l. Of prticulr potentil significnce is the integrtion of ctlytic processes into the therml conversion process to improve efficiency nd reduce costs. 223 Biologicl ctlysts lso pper to be promising; however, they re currently only used for ethnol production. Importntly, we should not limit ourselves to current methods of production of liquid fuels but lso should look to newer technologies nd chemistry. In the short-term, this will require more development work, but in the long term it could yield significntly more energy. However, this will require fundmentl understnding of the chemistry nd development of novel heterogeneous, homogeneous, nd enzymtic ctlysts. We believe tht efficient processes for the sustinble production of biofuels will continue to be developed s the price of crude oil increses. 12. References (1) Klss, D. L. In Encyclopedi of Energy; Clevelnd, C. J., Ed.; Elsevier: London, 2004; Vol. 1. (2) Klss, D. L. 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