An Euler-Euler CFD Model

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


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


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]


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


A Lab-scale Fluidized-bed Reactor

3.81 cm

34.2

9 cm

10.6

2 cm

air (N2+O2) / steam

biomass

syngas

coal


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


Simulation Cases

Cases air/biomass ratio biomass moisture (wt%)

0.4 00.8 0

base 1.5 02.1 01.5 151.5 25


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


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


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


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


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


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


Acknowledgement

Support for this work was provided by USDOE-NETL (DE-AC02-07CH11358).


An Euler-Euler CFD Model

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