Process simulation of switch grass gasification using Aspen Plus Xiao Sun Department of Biosystems & Agricultural Engineering Oklahoma State University

Outlines Introduction Objective Technical approach Simulation results Model validation

Introduction Save time and money! Gasification: is a process that converts organic or fossil fuel based carbonaceous materials into CO, H2 and CO2. This is achieved by heating the material at high temperature (>700°C) with controlled amount of air or steam (Google defination). Switchgrass: a dominant species in central North America. Factors affecting syngas composition: Feedstock type, highest heating temperature, heating rate, residence time, gasifier type, feeding rate, equivalence ratio, flow rate of air or steam, etc. Aspen Plus can be used to simulate gasification process and influence of some factors to the product quality can be predicted. Save time and money! Syngas (H2 and CO) Biochar Tar

Objectives Use available information in literature to build a model. Varying steam to biomass ratio (SBR) and equivalence ratio (ER) to predict their effects on composition of producer gas. Use existing gasification model in literature to validate this model.

Technical approach Assumption Gasification composed of three parts: devolatilization (pyrolysis), volatile reactions, and char gasification; Producer gas contains only CO, H2, CO2, N2, CH4, C2H4 and C2H6; Process was steady-state and isothermal; Process was adiabatic; Pressure throughout the process was set to 1 bar; All gases were distributed uniformly; Char was identified as 100% graphene and no ash or tar produced. (Niu et al., 2013)

Technical approach (cont’d) Specification of chemicals: C, CO, CO2, H2, H2O, CH4, C2H4, C2H6, O2 and N2 Specification of switchgrass (unconventional solid) Enthalpy: HCJ1BOIE Density: DCOALIGT 100% organic sulfur Property method: Peng-Robinson Proximate analysis Value (d.b. wt. %) Moisture content 14.63 Volatile matter 81.59 Ash content 4.72 Fixed carbon 13.69 Ultimate analysis Value (wt. %, dry ash free) C 52.74 H 5.91 O 41.05 N 0.24 S 0.06 (Sharma et al., 2014)

Technical approach (cont’d) Flowsheet diagram Reactor block Description RYIELD Models the pyrolysis of biomass. This model is useful when the reaction stoichiometry and kinetics are unknown but the yields distributions are known. RGIBS Models the volatiles reactions via minimizing Gibs free energy under chemical equilibrium. This model is useful when the reaction temperature and pressure are known but reaction stoichiometry is unknown RSTOIC Models the char gasification. This model is useful when reactions and conversion efficiencies are known but the stoichiometry is unknown. RPLUG Models the char gasification. This model is useful when the reaction kinetics are known and solids such as char are participating in the reactions.

Technical approach (cont’d) Gas yield distribution for RYIELD (char yield was calculated based on fixed carbon in proximate analysis Operating conditions Component Value (% mole) H2 8.4 CO 16.4 CO2 13.7 CH4 2.3 N2 57.8 (Sharma et al., 2014) Simulation parameters Value Feed temperature (°C) 25 Feed flowrate (kg/hr) 10,000 Air temperature (°C) Air flowrate (kg/hr) 3,500 Steam temperature (°C) 200 Steam flowrate (kg/hr) 3,000 RYEILD temperature (°C) 600 RGIBS temperature (°C) 900 RSTOIC temperature (°C) 700

Technical approach (cont’d) Reactions in RSTOIC Reactions in RPLUG Reaction # Reaction Heat of reaction (MJ/kmol) Fractional conversion Reaction name References R1 C + 0.5O2 → CO -111 0.2 of C Char partial combustion (Niu et al., 2013) R2 C + H2O → CO + H2 +131 0.8 of H2O Water-gas R3 C + 2H2 → CH4 -74.3   R4 CO + 0.5O2 → CO2 -283 0.8 of O2 CO partial combustion R5 CH4 + H2O → CO + 3H2 +206 Steam-methane reforming R6 CH4 + 2H2O → CO2 + 4H2 +165 0.5 of CH4 R7 CO + H2O → CO2 + H2 -41 0.2 of H2O Water-gas shift Reaction # Reaction Kinetics Reaction name Reference R8 C + CO2 → 2CO r7=4.1×106exp(-29,787/T) [CO2] Boudouard (Liu et al., 2011) R9 H2 + 0.5O2 → H2O r9=1.63×109exp(-3,240/T) [H2]1.5 [O2] H2 partial combustion R10 CH4 + 1.5O2 → CO + 2H2O r10=1.58×1010exp (-24,157/T) [CH4]0.7 [O2]0.8 CH4 combustion

Simulation results Warnings: Atom balance of C, O, H, N in RYIELD is not achieved Some streams have zero flow of certain compounds

Model validation Predictions of gas composition in model by various SBR and ER Simulation results from literature SBR Simulation results (% mole) H2 CO CO2 CH4 N2 No steam 8.0 10.8 14.9 65.2 0.1 9.4 9.5 16 64.2 0.2 10.5 8.1 17 63.3 0.3 11.7 6.7 18.1 62.5 ER Simulation results (% mole) H2 CO CO2 CH4 N2 0.15 9.4 9.5 16 64.2 0.20 8.8 6.3 17.5 66.1 0.25 8.2 3.4 19 67.9 H2 H2 CO H2 CO CO (Beheshti et al., 2015) (Niu et al., 2013) (Niu et al., 2013)

References Niu, M., Huang, Y., Jin, B., Wang, X. 2013. Simulation of Syngas Production from Municipal Solid Waste Gasification in a Bubbling Fluidized Bed Using Aspen Plus. Industrial & Engineering Chemistry Research, 52(42), 14768-14775. Liu, B., Yang, X., Song, W., Lin, W. 2011. Process simulation development of coal combustion in a circulating fluidized bed combustor based on Aspen Plus. Energy & Fuels, 25(4), 1721-1730. Sharma, A.M., Kumar, A., Huhnke, R.L. 2014. Effect of steam injection location on syngas obtained from an air–steam gasifier. Fuel, 116, 388-394. Beheshti, S.M., Ghassemi, H., Shahsavan-Markadeh, R. 2015. Process simulation of biomass gasification in a bubbling fluidized bed reactor. Energy Conversion and Management, 94, 345-352.

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