1

N2O Decomposition by CaOunder Conditions of

Carbonator of Calcium Looping Cycle

T. Shimizu, , S. Sizuno, K. Ito,R. Houshito, H.j. Kim, L.y. LiNiigata University, Japan

71st IEA-FBC, Seoul, Korea, Nov.4-6, 2015

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Principle of CaL process for CO2 separationCaL process consists of a carbonator and a regenerator. In the cabonator, flue gas from air-blown combustor is introduced. In the regenerator, fuel (coal) is burned to supply heat to decompose CaCO3 to CaO.

Carbonator Regenerator

CaCO3

CO2, H2O

CaO

Fuel O2CO2Flue gas

CO2-free gas

(CO2 10 - 15%)

(a) Ideal CaL process (with CO2 recirculation)

CaO+CO2 CaCO3→

→CaCO3 CaO+CO2

3

Flue gas from air-blown FB combustorCaL process consists of a carbonator (absorber) and a regenerator. In the regenerator, fuel (coal) is burned to supply heat to decompose CaCO3 to CaO.

Carbonator Regenerator

CaCO3

CO2, H2O

CaO

Fuel O2CO2Flue gas

CO2-free gas

(CO2 10 - 15%)

(a) Ideal CaL process (with CO2 recirculation)

CaO+CO2 CaCO3→

→CaCO3 CaO+CO2

N2O in flue gas

Flue gas from fluidized bed combustors sometimes contain N2O, which is known as a strong greenhouse gas (300 times of CO2 for the same concentration).

N2O decomposition?

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Char transportation from regenerator to carbonatorCoal combustion in regenerator.àFormation of char and transportation of char with CaOàFormation of CO and/or CO2

Carbonator Regenerator

CaCO3

CO2, H2O, NOx

CaO

Fuel O2CO2Flue gas

CO2-free gas, CO

(CO2 10 - 15%)

Fuel-N

NOxChar

Char

CharCO

CO2

CaO

CaCO3

Char

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Possible roles of char in carbonatorChar is expected to have different roles in carbonator:● Oxidation of char-N : formation of N2O● C+N2OàN2: N2O reduction

Carbonator

CaCO3

CaO

Decrease

CharNOx, N2O

O2

N-

NOx

Char

Flue gas (CO2, O2, NOx, N2O)

N2, N2O

Increase{, N2O (?)

Coal O2

CO2

Char Volatile matter

CO2Char

Regenerator

Recirculated CO2

NOx

, N2O

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N2O formation from char oxidation/NO reductionN2O emissions from char combustion / NO reduction by char were only slight (<10 ppm = 0.3% CO2-equivalent).

0

5

10

15

20

0 5 10 15 20 25

N2O

em

issi

ons

from

car

b.

[ppm

]

Carb. fluidizing gas O2 conc. [%]

NO feed 196-200 ppmWithout NOfeed

MVB

0

5

10

15

20

0 5 10 15 20 25N

2O e

mis

sion

s fro

m c

arb.

[p

pm]

Carb. fluidizing gas O2 conc. [%]

NO feed 193-205 ppmWithout NOfeed

SA

(Shimizu et al., J. Jpn. Inst. Energy, 94, 841-850, 2015）

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Reduction of fed N2O in carbonatorFor semi-anthracite, about 16% of N2O in fluidizing gas was reduced in carbonator.

Slope = 0.84

0

50

100

150

200

250

300

0 100 200 300

N2O

in fl

ue g

as fr

om

carb

onat

or [p

pm]

N2O fed to carbontor [ppm]

Fuel: SA,O2 in carb. =4%

(Shimizu et al., 6th HTSLC Meeting (Milan, Italy, 2015)

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Expected N2O reduction by CaO in carbonatorCaO is expected to catalyze N2O decomposition and reduction by CO which is formed by char oxidation.

Regenerator

CO2, H2O

Fuel O2 CO2

recycleChar

Carbonator

CaCO3

CaO

Flue gas

Char

O2 NOx

Char

(CO2, O2,NOx, N2O)

, N2O

NOx, N2O reduction

NOx and N2O reactions in carbonator

CO2

to dilute O2

CO2 gasN2

CaOCO

CO2 free gas

VM

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This work

Basic study on N2O decomposition by CaO using quartz FB reactorN2O-containing gas was fed to a bubbling fluidized bed of CaO at 600 oC to evaluate N2O decomposition. Also batch feed of charto the CaO bed was conducted.

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Experimental apparatus

N2O (480ppm), O2(8%), N2

mixture

Filter

GC

Quartz FBID 26mmLimestone (0.35-

0.42 mm) calcinedat 900 oC

Static bed height Hs=7 cm

Temp. 600 oCUgas= 17cm/s(Hs/Ugas=0.4 s)

Heater

Char batchfeed

CaObed

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Char batch feedChar was prepared by heating at 900 oC in N2. Batch feed was conducted.

Ash (dry) Moist. (sample) C (daf) H(daf) N(daf)

23.3 2.0 - 3.5 95.8 0.6 1.2

VM content: 2 - 3% of combustible part. (TGA weight loss in N2 between 200 – 600 oC)

Analyses of char

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Results and Discussion

ØN2O decomposition by CaO without charØChar batch feed to CaO/inert bed

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N2O decomposition without char feedSilica sand bed: nearly 0% conversion

(no thermal decomposition at 600 oC)

CaO bed: nearly 100% decomposition(480ppm at inlet à <1 ppm at outlet)

kt = ln(In/Out) > 6.2*k: first-order rate constant :[1/s]t: contact time [s]

(*: This value includes bubble-emulsion mass transfer resistance)

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Rate of N2O reduction by CaOIn dry gas, rate expression is: kN2O(0) = 2.8x107exp(-1.057x104/T)=154 [1/s].

Gas-solid contact time: t = 0.2 sàkN2Ot = 30àContribution to N2O reduction.

(Shimizu et al., Energy Fuels 2000, 14, 104-111)

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N2O decomposition by CaO in the presence of H2O In the presence of water, rate expression is:

kN2O(0)=k1+k2, k2/kN2O(0) =0.0011T - 0.685kN2O(0) = 2.8x107exp(-1.057x104/T)K1= 43exp(4.19x103/T), K2=2.7

At 600 oC andand PH2O=0.1 atm, kN2O=34 s-1

àContribution to N2O reduction is expected.

OHOHON PK

kPK

kk22

2

21

12 11 +

++

=

10

100

1000

10000

0 0.1 0.2App

aren

t firs

t-ord

er ra

te c

onst

. [1

/s]

Water vapor pressure [atm]

T [K]= 1073T [K]= 1023T [K]= 973T [K]= 873

(Shimizu et al., Energy Fuels 2000, 14, 104-111)

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Char batch feed to CaO bedDuring the experiments, char batch feed (40 – 50 mg) was conducted. Formation of CO, CO2 and change in N2O were observed.

Under the present condition, however, the effect of char feed on N2O decomposition was not clear because sufficiently high N2O decomposition was observed without char feed.

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Anticipated problem with char transportationTransportation of char to carbonatoràLow temperature oxidation of char à Formation of CO

Carbonator Regenerator

CaCO3

CO2, H2O, NOx

CaO

Fuel O2CO2Flue gas

CO2-free gas, CO

(CO2 10 - 15%)

Fuel-N

NOxChar

Char

CharCO

CO2

CaO

CaCO3

Char

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Formation of CO and CO2 in carbonator (dual-FB)

0

1

2

3

4

0 5 10 15 20 25

CO

+CO

2co

nc. i

n ca

rbon

ator

flue

gas

[%]

O2 conc. in carbonator fluidizing gas [%]

SA

MVB

Char transportation to carbonator àCO & CO2 formation when sand was employed as bed material for dual-FB experiments.

(Shimizu et al., J. Jpn. Inst. Energy, 94, 841-850, 2015）

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Previous results of CO emissions from carbonatorWhen sand was employed as bed material, 20 – 40% of carbon oxidized in the carbonator was converted to CO.àCO in gas = 1000 – 2000 ppm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

2 3 4 5 6 7 8

CO

/(CO

+CO

2) in

car

bona

tor f

lue

gas

[-]

O2 conc. in regenerator flue gas [%]

O2 in carb: 21%O2 in carb: 15%O2 in carb: 10%O2 in carb: 4%

c) SA

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

2 3 4 5 6 7 8

CO

/(CO

+CO

2) In

car

bona

tor f

lue

gas

[-]

O2 conc. in regenerator flue gas [%]

O2 in carb: 21%

O2 in carb: 10%

O2 in carb: 4%

a)HVB

(Shimizu et al., J. Jpn. Inst. Energy, 94, 841-850, 2015）

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Present results of CO formation from charSelectivity to CO was reduced from 0.45 for sand bed (similar to dual-FB results) to zero (<detection limit) for CaO bed. CaO bed can suppress CO emissions even when char is transported to carbonator.

Sand bed CaO bed0.45 0

Selectivity to CO

0

0.05

0.1

0.15

0.2

0

400

800

1200

1600

0 1000 2000 3000

CO

2co

nc [%

]

CO

con

c. [p

pm]

Time [s]

Sand bed, O2 8%, Char 42.2 mg, Gas feed 1.7 NL/min

COCO2

Sand bed

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ConclusionsIt is expected that carbonator of CaL process can work as N2O abatement reactor because CaOcan work as N2O decomposition catalyst.

Char particles are known to be transported from regenerator to carbonator and form CO there. The presence of CaO is expected to reduce CO emissions from carbonator.

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Acknowledgements

The authors express their thanks to the financial supports from The Iwatani Naoji Foundation.