Numerical analysis for CO2 absorption and regeneration behaviors in porous solid sorbent by modified unreacted-core model

Takahiro Tanaka1 , Eiki Tabata1, Takao Nakagaki1, Mamoru Mizunuma2, Yasuko Y. Maruo2

1Department of Modern Mechanical Engineering Waseda University,2NTT Energy and Environment Systems Laboratories

CO2 separation

Pure CO2

RGN

750oC

ABS

550oCSOFC

Heatrecovery

OperationT = 700 ~ 800oC

Reformer, Heat outputCO2

30 ~ 40%

City gas

CO2 < 20%

Capturing CO2 from Solid oxide fuel cellsConclusions/Summary

Unreacted core model of porous pellet

sCObCOfsCO CCkRN ,,

2

1, 2224

eqCOcCOcCO CCkSN ,,3, 222'

sc

cCOsCO

effCORr

CCDN

114

,,

2,22

2

1. Rate of CO2 mass transfer through the gas film

2. Rate of CO2 mass transfer through the product layer

3. Rate of CO2 absorption at the surface of the unreacted core

2COC

R0rcRs

bCOC ,2

sCOC ,2

cCOC ,2

eqCOC ,2

1. Gas film

2. Li2CO3+Li2SiO3

3. Li4SiO4

Unreacted core

Over all CO2 absorption rate

32

max

231

max

,,

2

1'

411

1

4222

w

w

kS

R

w

w

D

R

k

CCMR

dt

dW

c

s

eff

S

f

eqCObCOCOs

Lithium ortho-silicate (Li4SiO4) is a suitable solid sorbentfor capturing CO2 from solid oxide fuel cells. CO2

absorption reactors packed with porous-solid sphericalpellets of Li4SiO4 show unsteady temperature distributionand capture ratio behavior owing to the unsteady CO2

absorption rate and highly exothermic process. The CO2

absorption rate of this sorbent reportedly depends ontemperature, CO2 concentration, and CO2 accumulation,expressed as the weight change of the sorbent.

In this study, the modified unreacted core model isproposed to simulate the mechanism of CO2 absorption ofa porous-solid spherical pellet. Important properties suchas the reaction rate constant of the unreacted core surface(kc), the coefficient of mass transfer through the gas film(kf), and the coefficient for effective diffusion through theproduct layers (Deff) that characterize the behavior of thesorbent were empirically derived using thermogravimetryand a diluted packed-bed reactor. The reaction rate S’kc

and the mass transfer coefficient kf can be expressed bythe Arrhenius law and forced convection mass transfercorrelation for flow in packed beds, respectively. Thediffusivity Deff obtained by unreacted core model has apeak value at a certain temperature to fit experimentaldata and is underestimated at other temperatures of slowreaction rates. The Bruggeman model is commonly usedto Deff and a modified model using linearly decreasingporosity e with CO2 absorption is proposed in this work.

Numerical analysis by applying these parameters to themodified unreacted core model adequately explained thecomplicated CO2 absorption and regeneration behaviors.

3232244 COLiSiOLiCOSiOLi

Experimental formulation

CO2 absorption/regeneration behavior of lithium silicate Thermogravimetric analysis of lithium silicate under various conditions

0

10

20

30

0 200 400 600 800 1000

Wei

ght c

han

ge

wt%

Temperature oC

100 vol%-CO2

5 oC/min

Temperature oC

Wei

ght

chan

ge

wt%

0

10

20

30

0 10 20 30

Wei

ght c

han

ge

wt%

Elapsed Time min.

450℃ 500℃

550℃ 600℃

650℃

10vol%-CO2

Elapsed time min.

Wei

ght

chan

ge

wt%

0

10

20

30

0 10 20 30

Wei

ght c

han

ge

wt%

Elapsed Time min.

30

20

10

550oC

vol%-CO2

Elapsed time min.

Wei

ght

chan

ge

wt%

• Dependency oftemperature

• Dependency of CO2 concentration

RGNABS

CO2 accumulation wt%

8.0

6.0

4.0

2.0

0.0

Def

fm

2 /S

×10-6

0 10 15 255 200

0.1

0.2

0.3

0.4

0.5

0.0

2.0

4.0

6.0

8.0

0 5 10 15 20 25

0.5

0.4

0.2

0.1

0.0

Po

rosi

ty e

0.3

10 vol%, 550 oC

• Effective diffusion coefficient: DeffModified Bruggeman model

knNCOpore DDD

111

22 ,

poreeff DD e

,

*Plots: Obtained from dW/dt at 10% weight change lines: Calculated by Modified Bruggeman model

X15.00ee

maxw

wX

,

Prediction of CO2 absorption behavior

vol%-CO2oC

20 600

20 550

10 550

*Solid lines: experimental (diluted packed bed reactor)Dashed lines: analytical (e = e0 - 0.15X)Chain line: analytical (e : constant)

RTR

kS

S

c

4

2

' 1045.1exp1047.2

4

Arrhenius plot of S’kc by thermogravimetry

Temperature oC

ln S’k

c -13.8

-14.0

-14.21.0

1000/T K-11.1 1.2 1.3 1.4

650 550 450-13.6

100 vol%-CO2

-14.2

-14.0

-13.8

-13.6

1.0 1.1 1.2 1.3 1.4

• Reaction rate: S’kc• Gas film mass transfer coefficient: kf

316.0155.076.2 ScReSh p

Experimental apparatus for diluted packed bed

5 vol%-CO2

550 oC

MFC

CO2

N2

CO2 meter(Vaisala GM70)

Ventilation

LS diluted withalumina ball

Electricfurnace

(LS : Al2O3 = 1 : 3)

Packed bed in detail

Photo: φ=5mmspherical LS pellet from Toshiba Corporation

Changes in LS structureSEM images of the LS sorbent

30 mm 30 mm

(a) Before absorption (b) After absorption at 550oC

0

5

10

15

20

25

0 5 10 15 20

CO

2ac

cum

ula

tio

n

wt%

Elapsed time min.

25

20

15

10

5

00 5 10 15 200.0

2.0

4.0

6.0

8.0

400 500 600 700Temperature oC

4.0

2.0

0.0

Def

fm

2 /S

400 500 600 700

8.0

6.0

×10-6

vol%-CO2 wt%

10 1015

30 20