An Euler-Euler CFD Model forBiomass Gasification in Fluidized Bed
Qingluan Xue and Rodney Fox
Ames Laboratory, Iowa State UniversityDE-AC02-07CH11358
NETL Conference on Multiphase Flow ScienceMorgantown, WV, May 22-24, 2012
Objectives
Provide MFIX-based Euler-Euler CFD models forpolydisperse, reacting, variable density solidparticles
Facilitate the numerical simulation and scale up ofenergy systems such as gasifier
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 2 / 19
Background: Multiphysics &Multiscales
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 3 / 19
Physical ModelsSpherical particle assumption
Porosity and moisture modeled
Uniform conversion model for micro-particlesConstant particle diameter (dp)
Variable particle density (ρp) with increasing pores
Intra-particle transport ignored for micro-particle
Couple continuity and species equation to updateρp
1ρapparent
=
N∑
n=1
Xs,n
(ρn)true
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 4 / 19
Chemical Models
Devolatilization: Biomass → light gas (CO, CO2, H2, CH4) + tar + char + H2OTar reaction: Tar (g) → light gas (CO+ CO2 + H2 + CH4 + H2O) + inert TarVolatile reaction:Carbon monoxide oxidation CO + 1/2 O2 → CO2
Hydrogen oxidation H2 + 1/2 O2 → H2OMethane oxidation CH4 + 2 O2 → CO2 + 2 H2OWater-gas shift CO + H2O ↔ CO2 + H2
Char combustionPartial combustion C + 1/2 O2 → COChar gasification:Boudouard reaction C + CO2 → 2 COSteam gasification C + H2O → CO + H2
Hydrogen gasification C + 2 H2 → CH4
Reaction rate based on min [k = exp[−E/RT ],C(2Sij Sij)1/2]
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 5 / 19
Implementation in MFIX-ContinuumTime-splitting approach coupling hydrodynamicsand kinetics
φ(t)transport−−−−→ φ∗(t + ∆t)
chemical reaction−−−−−−−−−→ φ(t + ∆t)
Synchronized time-step for transport and reactionTime step: t
n
Back up variables at tn
Transport calculation iteration
Reset variables valuesto t
n; reduce ∆t
∆t > dt_min
∆t <= dt_min
Converged ?No
Reset variables values
Source calculation (reaction)
Advanced to tn+1
= tn
+ ∆t
Yes
∆tn
to tn-1
; reduce ∆tn
Back up variablesat t = t
n + ∆tconv
converged ?
yestreact
No
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 6 / 19
A Lab-scale Fluidized-bed Reactor
3.81 cm
34.2
9 cm
10.6
2 cm
air (N2+O2) / steam
biomass
syngas
coal
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 7 / 19
Simulation Conditions
Phase Species (n) Xn T (K)ǫg , ǫsm
ρtrue (g/cm3) d (cm)bed, freeboard
gas (g)
N2 1
1123 0.41, 1 EOSa -O2 0light gasb 0tarc 0
solid (s1) sand 1 1123 0.59, 0 2.649 0.05
solid (s2)
biomassd 1
300 0, 0
0.585
0.05moisture 0 0.649char 0 0.45ash 0 0.45void 0 same as gas
aquation of state for an ideal gas
bincludes CO, CO2 , H2O, H2 , CH4
cincludes active tar and inert tar
dair fuel ratio (mair/mbiomass) is 1.5
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 8 / 19
Simulation Cases
Cases air/biomass ratio biomass moisture (wt%)
0.4 00.8 0
base 1.5 02.1 01.5 151.5 25
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 9 / 19
Convergency History
Gas Temperature and mass inside reactor
1100
1150
1200
1250
1300
0 50 100 150 200
T (
K)
Time (s)
bio-fed locationbed center
upper bedbed surface
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 50 100 150 200
Mas
s (g
)
Time (s)
char in bed char outlet
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 10 / 19
Biomass Apparent Density
0.1 s 0.5 s 1.0 s 1.5 s 2.0 s 10 s 15 sQ. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 11 / 19
Time- and Spatial-Averaged Densityand Temperature (160-200 s)
0
5
10
15
20
25
30
35
0.05 0.1 0.15 0.2 0.25 0.3
-20 -10 0 10 20 30 40
Axi
al h
eigh
t in
bed
(cm
)
Averaged apparent density (g/cm3)
y-component of velocity (cm/s)
apparent densityy-velocity
0
5
10
15
20
25
30
35
800 1000 1200 1400
Axi
al h
eigh
t in
bed
(cm
)
Averaged temperature (K)
gas phasesand phasebiomass phase
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 12 / 19
Gas Molar Fraction @180 s
O2 N2 CO H2 CO2 CH4 TarQ. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 13 / 19
Biomass Mass Fraction @180 s
Bio Char Pore AshQ. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 14 / 19
Mass Flow Ratio (Air/Biomass)
0 0.5 1 1.5 2 2.5Mass flow ratio (air/biomass)
0
0.2
0.4
0.6
0.8
Gas
com
posi
tion
(mol
e)
COCO
2H
2CH
4TarN
2
Gas composition vs. mass flow ratio
0 0.5 1 1.5 2 2.5Mass flow ration(air/biomass)
0
0.2
0.4
0.6
0.8
1
Prod
uct y
ield
(m
ass
frac
tion)
TarCharGas
Product yield vs. mass flow ratio
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 15 / 19
Bed Temperature
1000 1050 1100 1150 1200 1250Temperature at mid-bed (K)
0
0.05
0.1
0.15
Gas
com
posi
tion
(mol
e)
COCO
2H
2CH
4Tar
Gas composition vs. temperature
1000 1050 1100 1150 1200 1250Middle bed temperature (K)
0
0.2
0.4
0.6
0.8
1
Prod
uct y
ield
(m
ass
frac
tion)
TarCharGas
Product yield vs. temperature
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 16 / 19
Moisture Content
0 5 10 15 20 25Moisture content (wt%)
0
0.05
0.1
0.15
Gas
com
posi
tion
(mol
e)
COCO
2H
2CH
4Tar
Gas composition vs.. moisture content
0 5 10 15 20 25Moisture content (wt%)
0
0.2
0.4
0.6
0.8
1
Prod
uct y
ield
(m
ass
frac
tion) Tar
CharGas
Product yield vs. moisture content
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 17 / 19
Summary
Variable particle density implemented for modelingbio-particle transport
Biomass (wood) gasification kinetics implemented
The model captures the key features of gasification
Different conditions tested
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 18 / 19
Acknowledgement
Support for this work was provided by USDOE-NETL (DE-AC02-07CH11358).
Q. Xue & R.O. Fox (A Lab/ISU) Polydisperse Biomass Gasification May 22-24 19 / 19