Biomass Characterization and Gasification

Transcription

1 Biomass Characterization and Gasification CHEN 4470: Process Design Practice Sushil Adhikari, Ph.D. Biosystems Engineering Department January 24, 2013 Biomass Properties Physical Properties Density, size, shape, area Chemical Properties Heating value, proximate analysis, ultimate analysis Biomass Constituents Hemicellulose, cellulose and lignin 1

2 Proximate Analysis Proximate Analysis (weight percentage) Moisture Content (wet basis/dry basis) ASTM E871 Ash Content ASTM D1102 Volatile Matters-- ASTM E872 Fixed Carbon Ultimate Analysis (contd.) Ultimate Analysis (ASTM D ) Carbon (E 777) Hydrogen (E 777) Nitrogen (E 778) Oxygen Other elements-s, Cl.. Carbon, hydrogen and nitrogen are converted into carbon dioxide, water vapor, nitrogen, respectively for quantification. Usually, oxygen is calculated from the difference (100-C-H-N). 2

3 Heating Value Heating value represents the heat released when the chemical compound is stoichiometrically combusted. Heating value is expressed in terms of higher (gross) heating value (HHV) or lower (net) heating value (LHV). While measuring HHV, the products of combustion are cooled to the initial temperature of the compound. In LHV, the water produced during combustion is not condensed. Table: Proximate, ultimate and heating value analyses (dry weight basis) of selected biomass feedstocks Proximate Analysis Fixed Carbon Volatile Matter Ash Switchgrass Hybrid poplar Pine Sugar cane woodchips b bagasse Wyoming Elkol coal Ultimate Analysis Carbon Hydrogen Nitrogen Oxygen Sulfur Chlorine n/a HHV, MJ/kg calculated from difference. n/a= not available. 3

4 Enthalpies of Formation Enthalpies of formation is quite useful for thermodynamic calculations such as Gibbs free energy of minimization. The standard enthalpy of formation of a particular biomass sample is equal to the sum of heats of formation of the products on combustion minus the HHV. If you use the minus sign, then you should use - for the HHV because of exothermicity. Otherwise, you can use plus sign without worrying any sign for the HHV. It is assumed that ash is inert. Standard enthalpies of formation at 298 K of the combustion products are as follows: CO 2 = ; H 2 O=-68.37;NO 2 =8.09; SO 2 =70.95 in kcal/g-mol Biomass Gasification Biomass: Gasification: High Temperature ( o C) Products: Syngas: H 2 CO CO 2 CH 1.44 O 0.66 Insufficient Oxidizing agent (Air, O 2, H 2 O and CO 2 ) CH 4 Small solid or liquid fractions 4

5 Biomass Gasification Partial oxidation of biomass to produce a low calorific-value fuel called syngas or producer gas. Main components of the producer gas are CO, H 2, CO 2,CH 4,N 2,andH 2 O. Chemical transformation can take place in fixed, moving, or fluidized bed or entrained flow gasifiers at temperatures of 1400 to 1800 F with pressures from 1 to 30 atmospheres. Syngas Potential Source: Jenny B. Tennant. NETL Overview of DOE s Gasification Program 5

6 Conversion of Syngas to Fuels Power Gasification Steps 1. Drying (>150 o C) 2. Pyrolysis or Devolatilization ( o C) 3. Combustion ( o C) 4. Reduction ( o C) Processes 1, 2, and 4 absorb heat whereas step 3 releases heat. Source: Prabir Basu, Combustion and Gasification in Fluidized Beds 6

7 Drying Every kg of moisture in the biomass takes away a minimum of 2260 kj to vaporize water (Basu, 2010). Typical moisture content of freshly ranges from 30 to 60% and for some biomass it can exceed 90%. For the production of a fuel gas, most gasification system use dry biomass with a moisture content of 10 to 20%. Pyrolysis Complex physical and chemical processes occur during the pyrolysis process. It starts slowly at 350 o C, accelerating to an almost instantaneous rate above 700 o C. During pyrolysis process, large compounds are broken down and evaporate with other volatile components. Biomass + Heat Char + Gases+ Vapors/liquid (tar or PAHs) 7

8 Combustion Oxidation or combustion is one of the most important reactions in the gasification. All the thermal energy needed for endothermic reactions are provided during this step. Oxygen supplied to the gasifier reacts with combustible products, resulting the formation of CO 2 and H 2 O. Gasification Chemistry Biomass Oxygen Syngas Steam Source: Jenny B. Tennant. NETL Overview of DOE s Gasification Program 8

9 Reactions Combustion Reactions Boudouard Reaction Water-Gas Reaction Methanation Reaction CO shift Reaction (Water-Gas Shift Reaction) Methane Steam Reforming Reaction Source: Prabir Basu, Combustion and Gasification in Fluidized Beds Reactions (cont.) Combustion Reactions C+1/2 O 2 CO CO+1/2 O 2 CO 2 H2 + ½ O 2 H 2 O ( H = -111 MJ/kmol) ( H = -283 MJ/kmol) ( H = -242 MJ/kmol) Boudouard Reaction C+CO 2 2CO ( H = +172 MJ/kmol) 9

10 Reactions (contd.) Water-gas Reaction C+H 2 O CO+H 2 ( H = +131 MJ/kmol) Methanation Reaction C+2H 2 CH 4 ( H = -75 MJ/kmol) Methane Steam Reforming Reaction CH4+H 2 O CO + 3H 2 ( H = +206 MJ/kmol) Reactions (contd.) Water-gas Shift Reaction CO+H 2 O CO 2 + H 2 ( H = -41 MJ/kmol) For real fuel, the overall reaction can be written as: C n H m O p +??O 2 CO +CO 2 +H 2 +CH 4 +H 2 O+tar 10

11 Heating Value of Syngas The higher heating value of the syngas can be calculated by the volumetric fraction and the higher heating values of gas components, which is given by Types of Gasifier Updraft Gasifier Source: Olofsson et al., Downdraft Gasifier Crossdraft Gasifier 11

12 Mobile BIOMAX Features Field deployable. Self contained and doesn t need grid connection. 25 kwe generating capacity. 50 lbs biomass consumed per hour. Mobile BIOMAX (contd.) 12

13 Biomax control system 64 control points (temps, pressures, flows, motors, engine, generator, etc.) 30 auto alarms with text messaging or . Auto remote start up and shut down. Full data logging downloadable. Remote trouble diagnosis / software upgrades. Manual on-site push button start-stop. Fluidization Regimes Source: Introduction to Fluidization Technology by Dr. Karl V. Jacob and Dr. Ray Cocco on April 13, 2011 at ChemE on Demand 13

14 Types of Gasifier (cont.) Bubbling Fluidized Bed Gasifier Source: Olofsson et al., Entrained Flow Gasifier Fig. Gas conditioning system Fig. Auburn University s bubbling fluidized bed gasifier and biomass feeder 14

15 Advantages/ Disadvantages Updraft Gasifier Size, shape and moisture content of biomass particles are less critical than with a downdraft gasifier. Design is simple and results in a fairly high heating value of the gas. The quality of the syngas is generally quite low. High temperature near the reactor grate can cause blocking due to ash fusion Source: Olofsson et al., Downdraft Gasifier Produced gas is generally of relatively good quality and has low level of tars. Up to 99.9% of the formed tar is consumed minimizing tar cleanup. Syngas contains relatively high levels of CO 2 since a large portion of the biomass is oxidised. Heating value is low. Size and shape and low moisture content of biomass particles must be controlled within close limits. Source: Olofsson et al., Advantages (cont.) 15

16 Advantages/ Disadvantages (cont.) Crossdraft Gasifier Design is simple. Quality of syngas is generally poor. Heating value of the syngas is low and the tar content is high. Source: Olofsson et al., Advantages/ Disadvantages (cont.) Bubbling Fluidized Bed Gasifier Reactor allows high rates of throughput, higher than fixed beds. Results in good mixing, optimized kinetics, particle/gas contact and heat transfer as well as long residence time. High carbon conversion rates and, consequently, high yields. Sand bed makes it possible to use in-bed catalytic processing. Syngas is rich in particulates Source: Olofsson et al.,

17 Advantages/ Disadvantages (cont.) Entrained Flow Gasifier Almost tar free syngas Leach-resistant molten slag A high percentage of energy is converted into sensible heat. Production of biomass powder is an extra cost. Source: Olofsson et al., Composition of Gas Yield Fuel Composition Gasifying Medium Operating Pressure Temperature Moisture Content of the Fuel Mode of Bringing the Reactants into Contact Source: Prabir Basu, Combustion and Gasification in Fluidized Beds 17

18 Gas Composition (cont.) Component Composition, % Nitrogen CO CO H CH Heating Value, kj/m 3 Gas composition presented here is from downdraft gasifier operated at 20% MC. Source: Wood gas as engine fuel. FAO pp.19 Effect of Operating Parameters Temperature Pressure Feed Characteristics Fuel Reactivity Volatile Matters Ash Moisture Content Source: Prabir Basu, Combustion and Gasification in Fluidized Beds 18

19 Volatile Matter Fuels with high volatile matter content are easier to gasify. Also, char produced from gasification process is more porous and easier to gasify. Biomass has high volatile matters and produces high tar content. High tar content makes gas clean-up process difficult. Source: Prabir Basu, Combustion and Gasification in Fluidized Beds Ash Content Ash content does not have direct influence on the gas composition. However, it affects the practical operation of gasifier. Ash can be removed either in solid or liquid form. In fixed and fluidizing beds, ash is removed in solid form. If the ash is removed in the solid form, feedstocks should have high ashmelting/softening temperatures and the gasifier should be operated at well below melting temperature. 19

20 Ash Content (cont.) The relationship between ash melting temperature and composition is a complicated. It mainly depends on SiO 2 -Al 2 O 3 -Cao-FeO. High in silica and alumina will result high in ash-melting temperature. But, the ratio of silica/alumina is also equally important. It is reduced by the presence of CaO and FeO. Ash-melting temperature of coal is more than 1200 o C but biomass can have significantly lower than 950 o C. Ash Content (cont.) 20

21 Syngas Composition from Different Feedstocks Constituents Fraction (N 2 balance) Peanut hulls Saw dust Poultry Litter Wood chips Higher Heating Value %vol MJ/m O2 CO CO2 CH4 H2 0.0 Peanut hulls Saw dust Poultry Litter Wood chips Gautam et al. (2009), ASABE Annual International Meeting. June 21-June 24, 2009, Reno, NV Gasification Processes and Methanol Production Process Condition Circulating fluidized bed Gasifier Type Bubbling fluidized bed Entrained Feedstock (wood), t/d Steam, t/t dry feed Oxygen, t/t dry feed Air, t/t dry feed Gas. Temp., o C Gas. Press., psi Exit gas (dry) H 2 (vol.%) CO (vol.%) CH 4 (vol.%) CO 2 (vol.%) H 2 /CO Source: Klass (1998). 21

22 Design Consideration Gasifier Efficiency Cold gas efficiency Hot gas efficiency Carbon Conversion Equivalence Ratio Cold gas efficiency = (Heating value of product gas/heating value of feedstocks)x100 % It is important to specify whether the heating values are on higher heating value or lower heating value basis. Design Consideration (contd.) The gas is not cooled before combustion and the sensible heat is also useful. Therefore, sometimes, hot gas efficiency is also used for such applications. Hot gas efficiency = (Heating value of product gas + H sensible /Heating value of feedstocks) x 100 % 22

23 Design Consideration (contd.) Carbon conversion = {1 Carbon in gasification residue/carbon in feedstocks} x 100 % or {Carbon in gas composition/carbon in feedstocks} x 100 % Care is required to interpret the data. Higher methane concentration could result in higher cold efficiency and good for power application but it is not the optimum choice for a synthesis gas applications to produce fuels and chemicals. Design Consideration (contd.) Equivalence Ratio (ER): = (A/F) actual /(A/F) stoichiometric The quality of syngas depends upon the value of ER. A low value of ER (<0.2) results in several problems including excessive char formation. A high value of ER (>0.4) results in excessive formation of CO 2 and H 2 O. Typical range of ER is ~