Characterization of Ashes from Different Wood Parts of Norway Spruce Tree

Transcription

1 A puliction of 37 CHEMICAL ENGINEERING TRANSACTIONS VOL. 37, 2014 Guest Editors: Eliseo Rnzi, Kthrin Kohse- Höinghus Copyright 2014, AIDIC Servizi S.r.l., ISBN ; ISSN The Itlin Assocition of Chemicl Engineering DOI: /CET Chrcteriztion of Ashes from Different Wood Prts of Norwy Spruce Tree Ling Wng*, Jnk Didikov SINTEF Energy Reserch, Sem Sælnds vei 11, Trondheim, Norwy Norwegin Forest nd Lndscpe Institute, NO-1431, Ås, Norwy Woody iomss from the forest sector is n undnt resource for renewle energy genertion. Conventionl woody iomss mterils such s timer nd stem re normlly high qulity solid fuels for comustion pplictions in terms of sh relted opertionl prolems. Recently, new rw woody mterils such s forest residue re gining interests for energy production purpose. Forest residue is the remining frction fter hrvest nd outtke of the wood timer, including tree tops, rnches nd rks. Compred to conventionl woody iomss, the forest residue hs wide vrition of sh content nd concentrtion of sh forming mtters. The im of this work ws to chrcterize nd investigte different prts from Norwy spruce trees regrding sh content, sh composition nd sh melting nd slgging ehviors. Different prts from spruce tree were studied in present work including stem wood, rk, rnch nd twigs. The sh content nd sh melting temperture of the four fuel smples were mesured through following stndrd procedures. Concentrtions of min sh forming elements were nlyzed y n inductively coupled plsm opticl emission spectroscopy (ICP-OES). The shes from stem wood, rk nd twigs were further investigted y scnning electron microscopy equipped with energy dispersive X-Ry nlysis (SEM- EDX) nd X-Ry diffrctometry (XRD). The results showed tht the rnches nd twigs contin higher contents of sh forming mtters thn tht of the stem wood. Chemicl compositions of shes from four prts of the spruce tree re dominted y C, K, nd Si. The K nd N contents in the rnches nd twigs re significntly higher thn tht of stem wood nd rk, indicting high tendency of sh melting nd slgging. The melting points of shes from rnch nd twigs were C lower thn those of the shes from stem wood nd rk, respectively. SEM-EDX nd XRD nlysis, melting of shes from rnch nd twigs re minly ttriuted to formtion nd fusion of low temperture melting lkli silictes. 1. Introduction Utiliztion of woody iomss for het nd power production hs een minly restricted to timer, swdust, wood chips nd shvings from swmills nd wood working industry (Boström et l., 2011). However, with the rpid increse of demnd of ioenergy, woody iomss mterils from forest residues re gining continuous interests recently. Forest residues re derived from hrvest of forests nd outtke of timer process. In Norwy, 37% of the Norwegin minlnd is covered y forest (119,000 Km 2 ). Aout 1.5 million m 3 re produced nnully in Norwy with lrge mounts ville for energy production. Norwy hs set gol to produce new 14 TWh ioenergy y 2020, nd extend use of forest residues will certinly help to meet the gol (Ling et l., 2013). With development of collecting nd undling technologies, the efficient recovery nd utiliztion of the forest residues ecome more importnt. Currently, comustion is still the most widely used technology for converting woody iomss into het nd power (Ling et l., 2013). However, s group of solid fuel for comustion pplictions, in contrst to ordinry woody iomsses, the forest residues hve lrge vritions in totl fuel sh content nd composition of sh forming elements (Werkelin et l., 2005). Therefore, the sh trnsformtion chemistry during comustion of forest residues my e considerly different thn those of conventionl woody iomss fuels (Ehsn et l., 2013). In previous studies, shes in pure smple of stem wood, rk nd folige from trees of pine, spruce, irch Plese cite this rticle s: Wng L., Didikov J., 2014, Chrcteriztion of shes from different wood prts of norwy spruce tree, Chemicl Engineering Trnsctions, 37, DOI: /CET

2 38 nd spen were nlysed. Both totl fuel sh content nd concentrtions of sh forming elements re significntly different for three prts of ech tree species (Werkelin et l., 2010). Compred to stem wood smple, the folige contins much higher contents of sh nd elements such s K, P nd S (Werkelin et l., 2011). Due to rther higher contents of fuel sh nd criticl sh forming elements in forest residues, sh relted prolems such slgging nd fouling re expected to occur during comustion process (Öhmn et l., 2004). Such sh relted opertionl prolems my cuse unscheduled shutdowns nd costly mintennces of comustion pplinces (Lindström et l., 2010). It ws stted tht sh rich rk nd logging residues hve high sh sintering tendency, which resulted in severe slgging prolems during comustion in different types of urners (Lindström et l., 2010). Ashes from comustion of the rk nd logging residues were investigted vi comintion of different nlyticl methods. The nlysis results reveled tht sh slgging tendency correlted well to content of Si nd K in the fuel (Öhmn et l., 2004). Formtion nd melting of different lkli silictes is one of min resons leding sh sintering nd slgging (Wng et l., 2011). However, ecuse of lrge heterogeneity of forest residues, the knowledge of sh chrcteristics is still limited. The ojectives of the present work were: 1) chrcteriztion of shes from different prts of Norwy spruce tree, 2) investigtion the studied fuel sh melting nd sintering ehviours t elevted tempertures, 3) studying the sh sintering chemistry through comintion of XRD nd SEM-EDX nlysis. 2. Experimentl section The stem wood nd forest residues were otined from Norwy spruce forest in Est Norwy (Hoøl, Ltitude N nd Longitude E). The Norwy spruce tree were first cut down nd divided into four prts long the tree trunk from se level, ottom crown level, middle crown level nd top crown level. The stem wood, stem rk, rnches nd twigs from ech level were seprted fterwrds. Along the stem to needle top rdil direction, the rnch ws divided into four prts including se section, middle section, top section nd needle top. All fuel smples were collected nd ir dried first, which were milled to prticle size smller thn 1 mm nd dried gin t 105 C to get stle weight. The sh content of ech fuel smple ws mesured through following ASTM stndrd D Chemicl compositions of ech sh smple were nlyzed y n inductively coupled plsm opticl emission spectroscopy (ICP-OES). The smples for ICP-OES nlysis were prepred y following the stndrd CEN-TS Aout 50 mg sh smple produced t 550 C ws digested y mixture of cid H 2 O 2, HNO 3 nd HF in the first stge, nd H 3 BO 3 in the second stge. The melting temperture of one 550 C sh smple ws mesured ccording to procedures descried in the stndrd. ISO 540: Ech sh smple ws first shped into cuicl specimen. The sh specimen ws then put in n sh fusion nlyzer, in which it ws heted from room temperture to 1,500 C t heting rte of 2 C/min in n oxidizing tmosphere. Shpe chnges of the sh specimen t elevted tempertures were recorded, which were used for determine sh fusion chrcteristic tempertures. Four sh fusion chrcteristic tempertures were determined including initil deformtion temperture (IDT), softening temperture (ST), hemisphere temperture (HT) nd fluid temperture (FT). For ech sh smple, 6 tests were performed nd the verge test vlues re presented in Tle 3. In ddition, 50 g of ech fuel smple ws urned t 1000 C for 5 h. After comustion, the sh residues from ech fuel smple were visully evluted in terms of sintering degree. Prt of the sh residues were nlyzed y Bruker D8 Advnce X-ry diffrctometer using Cu k-lph rdition nd LynxEye detector. Identifiction of the crystlline components in ech smple ws done through n instrument integrted TOPAS evlution progrm nd the ICDD-PDF2 dtse. In ddition, the sh residues from four fuel smples were nlyzed y scnning electron microscopy (SEM) equipped with Energy Dispersive X-ry spectrometry (EDX). The sh residue smples were collected nd stick on cron tpe on smple holder. The SEM ws operted in ckscttered electron mode for illustrting distriution of detected elements in scnned re. EDX semi-quntittive spot/re nlyses were crried out for interesting res to get more detiled microchemistry informtion. 3. Results nd discussion 3.1 Ash content Ash contents of ech prt of the Norwy spruce tree re presented in Tle 1. The sh content in the stem wood ws %. The sh content of the rk ws %, which is 8-10 times of those of the corresponding stem wood. The rnch nd top hve high sh contents out %.

3 Tle 1: Ash content of fuel smples from different prts from summer hrvest Norwy spruce tree Ash content (wt %. d ) Stem wood Stem Brk Brnch nd top se Middle top Needle top Bse level Bottom crown level Middle crown level Top crown level : dry sis; : not detected. Tle 1 shows tht the sh content of fuel smples from different prts of tree incresed long trunk of the tree. The sh contents of fuel smples from higher level crown re evidently higher thn those of lower prt of the living crown. The similr correltion etween the sh content nd prts of the tree ws reported (Werkelin et l., 2005). In ddition, long the whole rnch, the sh content of ech smple ctegory incresed, nd the highest sh content ws in the needle top. In previous study, higher sh contents were lso mesured from the needles nd shoots of four tree species, which well-resemled the results showed in Tle Ash forming elements The wood discs contining stem wood nd rk were cut from the tree. The stem wood nd rk were seprted fterwrds. Brnches nd twigs were collected from the sme tree height level. The rnches contin lrge size rnches tht re from se nd middle section of the rnches. The twigs refer to thinner rnches together with ttched folige nd needles. The concentrtion of min sh forming elements in ech fuel smples from ech prt of the tree long the rdicl direction. As shown in Tle 2, the stem wood smple hs the lowest concentrtions of ll sh forming elements. The dominting element in the stem wood sh (550 C) is C, K nd Mn. The stem rk sh hs the similr chemicl compositions s those of the stem wood, ut the significnt higher mounts of Si, P nd Al. The rnches contined oth wood nd rk prts, nd the concentrtions of elements C, Al, Zn nd Mn re etween those of stem wood nd rk s shown in Tle 2. However, the Si content in the rnch sh is significntly higher thn those of the shes from stem wood nd rk. The sh produced from the twigs is dominted y C, Si, K, P nd Mn. The concentrtions of element Si, K nd P re much higher thn those in shes from the stem wood, rk nd rnch. The contents of Si, K, P, nd S in the shes from the four fuel smples incresed in the order stem wood < stem rk < rnch < twigs. The twigs hve considerly high contents of Si, K nd P, which were 100, 15 nd 50 times higher thn those in the stem wood sh, respectively. The K, P nd S re moile elements s reported in litertures of plnt physiology (Werkelin et l., 2010). They redily solve in sp solutions nd re rich in the young nd iologiclly ctive issues. High concentrtions of K, P nd S in prts of the tree (e.g. folige nd shoots) hve een reported (Werkelin et l., 2010). These prts of the tree contin lrge mount of living cells nd high content of K nd P s well. On the other hnd, the C nd Mn re chrcterized s rther immoile elements tht ccumulte esily in the phloem sp, mjor composition of the rk. It could explin the high concentrtions of C nd Mn in the stem rk. In ddition, the rnch contins oth rk nd wood, which is rich in C nd Mn s shown in Tle 2. Si is likely to solve in the certin phloem sp in the tree tissue, which keeps contct with ir (Werkelin et l., 2005). Twigs prt of the tree/rnch is such kind of tissue, which explins tht hs the highest Si content mong four prts of the tree. The lrge vritions of sh forming elements in different prts of the one spruce tree species hve een reported (Werkelin et l., 2010). The similr concentrtions of sh forming elements in different prts of tree were lso oserved from the other three tree species pine, irch nd spen in the sme study. 3.3 Ash melting nd sintering ehviours The sh melting nd sintering ehviors of shes from four prts of the spruce tree were investigted. Firstly, the melting tempertures of shes from the four prts (stem wood, rk, rches nd twigs) produced t 550 C were mesured nd the results re shown in Figure 1. Tle 2: Concentrtions of mjor sh forming elements (mg/kg D.S.) in fuels smples Smple C Si K N Al P S Mg Fe Zn Mn Stem wood 25, ,752 8, ,758 Stem rk 221,064 8,147 70,668 6,217 3,086 25,359 9,710 7, ,054 18,284 Brnch 163,671 26,673 85,830 5,403 2,541 31,290 11,873 4,461 1,111 4,758 16,032 Twigs 120,663 58, ,051 3,564 1,646 52,197 13,320 3, ,486 12,511 39

4 40 Figure 1: ) Ash fusion chrcteristic temperture of shes from four prts of the spruce tree, ) The initil deformtion temperture is more interesting, since one sh strts to melt nd ecome sticky, cusing sh relted opertionl prolems. As Figure 1 shows, the stem wood sh hs the highest initil melting temperture, indicting the lowest sintering tendency. During the sh fusion test, only the sh initil deformtion temperture ws identified t out 1,400 C. The other three sh fusion chrcteristic tempertures (ST, HT nd FT) were not le to e mesured, since not evident shpe chnges were hrd to e oserved t elevted tempertures. The sh fusion test results from the stem wood greed with oserved results from comustion tests. The stem wood sh hs n evidently porous structure without oservtion of sintering nd melting. On the contrry, the sh from twigs hs the lowest sh initil deformtion temperture out 1,100 C tht is considerly lower thn those of the other three shes. As the temperture higher thn 1,100 C, the swelling nd uling of the twig sh were oserved, indicting the sh ws pssing through sever molten stge nd ehved s liquid. After mesuring the sh fusion temperture, the sh specimen melted completely nd residues left s thin lyer. After the fuel comustion test, even no sintering, the slightly ggregtion of twig sh grins ws lredy oserved. Microscopic oservtion showed vritions of prticle shpe, structure nd morphology of ll sh residues fter fuel comustion t 1,000 C. Figure 2 shows one representtive SEM imge of stem wood sh with porous structures. The sh residue contins long flkes like grins nd some smll prticles with irregulr shpes. The former one is proly the sh left comustion of wood tissue, which somehow keeps tissue like structure with lyers nd pores s shown in Figure 2-. From the flke like prticle, C ws detected s dominting element with smll mount of Mg nd Mn. Therefore, detection of these elements implies presence of metl oxide, which ws confirmed y XRD nlysis (Tle 3). However, from the smll irregulr prticles, rther smll mount of S ws lso detected, with detection of C s mjor element in the sme re. The strong correltion etween the S nd C cn e found from the EDX element mpping indicting formtion of CSO 4 in the stem wood sh. The stem rk sh is comprised of medium nd smll sized prticles s shown in Figure 3-. Some of sh grins hve porous structures with smll pores on the surfce. Elements C, K nd P were detected from the porous sh grins, indicting presence of potssium clcium phosphtes. On the other hnd, there re some cuicl grins cn e oserved from the lrger sh grin surfces s shown in Figure 3-. Over thn 95 wt% of clcium ws detected from these cuicl grins ( 20 µm). Therefore these grins re smll C rich crystls tht were detected y XRD nlysis (Tle 3). The similr clcium rich crystls in the rk sh hve een oserved nd reported y Werkelin et l (Werkelin et l., 2011). During the urnout stge, the clcium dispersed in chr structure will diffuse to chr prticle surfce s smll crystllites. With further migrtion nd colescence, lrge clcium rich crystls will form nd sintered together s shown in Figure 3-. Figure 4 displys the twig sh contins two kinds of grins in terms of morphology nd structure. Some of sh grins ppered s lrge fused, homogenous structures with minor sh prticles emed. The K nd Si were detected s min elements from the fused portion tht re melted potssium silictes ccordingly. The fused silictes in the sh tht explins presence of morphous phse mterils s reveled y XRD nlysis, which ppers s hump in the se line of the spectrum of the twig sh. Due to the high concentrtion of K nd Si in the twig, different potssium silictes redily form during the comustion process. Some of potssium silictes hve melting points s low s 600 C. Formtion nd melting of the potssium silictes mke the sh grins sticky nd ggregte, inititing sh sintering nd slgging consequently. It hs een reported tht K-silictes chemistry plys min role for sh slgging during comustion of K nd Si rich forest ssortments such s needle nd thinning from trees (Werkelin et l.,

5 2011). In ddition, K in the iomss fuel hs high ffinity to rect with P during comustion processes. It leds to formtion of low temperture melting K-phosphtes tht my melt nd promote sintering of the iomss sh (Boström et l., 2011). As the K-silictes melts encounter lkli erth metls C nd Mg, the C nd Mg my dissolve into the melts nd drive out the K (Boström et l., 2011). As result of this, the mount of K-silictes melts will reduce nd sintering degree of the sh s well. Moreover, C nd Mg in iomss sh will lso incorporte into the K-phosphte, enhncing formtion of K-C-phosphtes with high melting tempertures. Therefore, s the C nd Mg content increse, the sh melting points is lso incresing. Figure 2- shows good correltion etween concentrtions of key sh forming elements (indicted s mole rtio etween Si+K+P/C+Mg) with the sh fusion temperture µm 100 µm Figure 2: SEM imge of stem wood sh produced t 1000 C 500 µm 100 µm Figure 3: SEM imge of stem rk sh produced t 1000 C 500 µm 100 µm Figure 4: SEM imge of twigs sh produced t 1000 C

6 42 Tle 3: Mjor minerl phses oserved from rley strw sh with nd without dditive ddition Mjor minerl phses Stem wood sh CO, MgO, trce mount CSO 4 Stem rk sh CO, MgO, C 5 (PO 4 ) 3, smll mount CSiO 3, Brnch sh K 2 SO 4, C 5 (PO 4 ) 3, CSiO 3, CO, MgO Twig sh CSiO 3, C 5 (PO 4 ) 3 OH, CMgSi 2 O 6, MgO, trce morphous mteril 4. Conclusions The sh contents of different prts from the Norwy spruce tree re different. Compred to the stem wood, the rk, rnch nd twig contining lrge mount living tissues generlly hve higher sh content. In ddition, concentrtions of sh forming elements in different prts of the Norwy spruce tree re significntly different. Contents of Si nd K re low in the stem wood, ut cn e considerly higher in the twig. The sh from the studied twig showed high sintering tendency due to high content of fuel sh nd certin sh forming elements such Si nd K. The SEM-EDX together with XRD nlyses indicted tht melting of the twig sh is minly due to formtion of low temperture melting potssium silictes. Thus, the lrge vrition of mjor sh forming elements in different prts of the tree should e considered for more efficient comustion of different wood iomsses for comustion pplictions. Acknowledgement The uthors cknowledge the finncil support y the Bioenergy Innovtion Centre (CenBio), which is funded y the Reserch Council of Norwy, lrge numer of industry prtners nd seven R&D institutions. References Boström D., Skoglund N., Grimm A., Bomn C., Öhmn, M., Broström M., Bckmn R., 2011, Ash Trnsformtion chemistry during comustion of iomss, Energy Fuels, 26(1), Wng L., Sndquist J., Vrhegyi G., Mts Güell B., 2013, CO 2 Gsifiction of Chrs Prepred from Wood nd Forest Residue: A Kinetic Study, Energy & Fuels, 27, Wng L., Becidn M., Skreierg, O. 2013, Testing of Zeolite nd Kolin for Preventing Ash Sintering nd Fouling during Biomss Comustion. Chemicl Engineering Trnsctions, Werkelin J., Skrifvrs B.-J., Hup M., 2005, Ash-forming elements in four Scndinvin wood species. Prt 1: Summer hrvest, Biomss nd Bioenergy, 29, Werkelin J; Linderg, D; Boström D., Skrifvrs B.-J., Hup M., 2011, Ash-forming elements in four Scndinvin wood species prt 3: Comustion of five spruce smples, Biomss nd Bioenergy, 35, Houshfr E., Wng L., Vähä-Svo N., Brink A., Løvås T. 2013, Experimentl Study of Single Prticle Rector t Comustion nd Pyrolysis Conditions. Chemicl Engineering Trnsctions, Werkelin J., Skrifvrs B.-J., Zevenhoven M., Holmom B., Hup M., 2010, Chemicl forms of sh-forming elements in woody iomss fuels, Fuel, 89, Öhmn M., Bomn C., Hedmn H., Nordin A., Boström D., 2004, Slgging tendencies of wood pellet sh during comustion in residentil pellet urners, Biomss nd Bioenergy, 27, Lindström E., Lrsson S.H., Boström D., Öhmn M., 2010, Slgging Chrcteristics during Comustion of Woody Biomss Pellets Mde from Rnge of Different Forestry Assortments, Energy Fuels, 24, Wng L., Skjevrk G., Hustd J., Grønli M., 2011, Effects of Sewge Sludge nd Mrle Sludge Addition on Slg Chrcteristics during Wood Wste Pellets Comustion, Energy Fuels, 25, Wu H., Cstro M., Jensen P.A., Frndsen F.J., Glrorg P., Johnsen Dm K., Røkke M., Lundtorp K., 2001, Relese nd Trnsformtion of Inorgnic Elements in Comustion of High-Phosphorus Fuel, Energy Fuels, 25,