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

2. Outlines Introduction Objective Technical approach Simulation results
Model validation

3. 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

4. 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.

5. 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)

6. 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)

7. 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.

8. 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

9. 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
H O2 → H2O
r9=1.63×109exp(-3,240/T) [H2]1.5 [O2]
H2 partial combustion
R10
CH O2 → CO + 2H2O
r10=1.58×1010exp (-24,157/T) [CH4]0.7 [O2]0.8
CH4 combustion

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

11. 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)

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

13. Thank you!
Any questions