Multiple Role of Carbonator of Calcium Looping Process for...

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Multiple Role of Carbonator of Calcium Looping Process for

Abatement of Oxides of Nitrogen

T. Shimizu, Y. Matsuura, R. Houshito, Y. Shimazaki, T. Shimoda, H.j. Kim, L.y. LiNiigata University, Japan

6th IEAGHG HTSLC, Milan, Italy, Sep.1-2, 2015

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CaL processCaL 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%)

CaO+CO2 CaCO3→

→CaCO3 CaO+CO2

Fuel O2

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Anticipated problem: Char transportation to carbonatorCoal combustion Char formation in regenerator Char transportation to carb. Oxidation (CO & CO2)

Carbonator Regenerator

CaCO3

CO2, H2O, NOx

CaO

Fuel O2CO2Flue gas

CO2-free gas, CO

(CO2 10 - 15%)

Fuel-N

NOxChar

Char

Char

CO

CO2

CaO

CaCO3

Char

Coal O2

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Previous works on char transportationIndeed, CO and CO2 were detected in carbonator flue gas due to char transportation and oxidation when inert bed material was used. (Gao et al., 2014).

0

2000

4000

6000

8000

10000

12000

14000

0 2 4 6O2 conc. in flue gas from Reg. [%]

CO

con

c. in

flue

gas

from

Abs

. [pp

m]

SAMVBHVB

(a)

0

1

2

3

4

0 2 4 6O2 conc. in flue gas from Reg. [%]

CO

2 con

c. in

flue

gas

from

Abs

. [%

]

SAMVBHVB

(b)

CO

[ppm

]

CO

2[%

]

O2 in Reg. Flue gas [%] O2 in Reg. Flue gas [%]

Gao et al., Fuel, 127, 38 (2014)

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Possible roles of char in carbonator● Oxidation of char-N : formation of NOx and N2O● C+2NON2: NOx reduction (air pollution control)● C+N2ON2: N2O reduction (GHG reduction)

CaCO3

CaO

Decrease

CharNOx, N2O

O2

N-

NOx

Char

Flue gas (CO2, O2, NOx, N2O)

N2, N2O

Increase{, N2O (?)NOx

, N2O

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This workA bench-scale dual-fluidized bed solid circulation system was operated using inert sand bed. Coal combustion experiments were conducted using oxygen-enriched air to evaluate the role of char in carbonator in NOx formation / reduction N2O formation / reduction

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ExperimentalCoal combustion experiments using a bench-scale dual-fluidized bed solid circulation system

Details of experimental apparatus are available in:C. Gao et al., Fuel, Vol.127, July, pp.38-46, 2014C. Gao et al., Fuel, Vol.121, April, pp.319-326, 2014, and.T. Shimizu et al., J. Jpn. Inst. Energy, 94, 841-850, 2015

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Dual-fluidized bedRegenerator: Fast bedID 2.2 cm, height 1.93 mGas vel. 2.7 m/s at 950 oCO2-enriched air (30% O2)Prim gas : Total gas =0.5 : 1

Carbonator: Bubbling bedID 9.3 cm, bed height 0.3 mGas vel. 0.22 m/s at 600 oCO2-(NO/N2O)-N2 mixture feed

BM: Silica sand (149 m)Primary gas

Coal

Secondary gas (air)

Loopsealgas (air)

Carbonatorgas (air)

Carbonatorflue gas

Regenerator flue gas

Cyclone

Regenerator

Carbonator(Bubbling fluidized bed)

(Fast fluidized bed)

(O2-enriched air)

NOx, N2O

CoalNO, N2O, O2

Sec. air

Prim. gas(air + O2)

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FuelsSemi-anthracite (SA) and medium volatile bituminous coal (MVB), were employed.Different yield/char reactivity of char.

Proximate (air-dried) Ultimate (d.a.f)

Coal V.M. F.C. Ash Moist. C H N O

MVB 26.3 56.2 15.0 2.5 85.9 4.9 1.7 7.0

SA 15.5 70.7 10.9 2.8 89.7 4.1 1.9 3.6

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Results and Discussion

In carbonatorNOx formation / reductionN2O formation / reduction

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NOx formation and reduction in carbonator

NOx formation from char oxidationReduction of NOx in feed gas

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Experimental ProcedureMeasurement of NO conc. in carbonator flue gas

Without NO feed (NO from char)

Coal and

Secondary gas

Loopsealgas (air)

Carbonator fluidizing gas (O2+N2+NO)

Carbonator flue gas


Cyclone

Regenerator

Carbonator


Air

NO

to O2 analyzer

N2

NOx, N2O)

O2

Air

(Bubbling bed)

Bottom fluidizing gas

pneumatictransportation

3-wayvalve

to gas analyzers(CO, CO2,

(air)

gas (air)NO feed to carb. Inlet (NO reduction in carb.)

NO feed to carb. flue gas (inlet NO conc.)

NO

Carb. flue gas

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NOx emissions from char combustionEmissions of NOx were 10 – 30 ppm and increased with increasing O2 concentration in carbonator feed gas.

0

10

20

30

40

50

0 10 20 30

NO

x em

issi

ons

from

ca

rb. [

ppm

]

Carb. fluidizing gas O2 conc. [%]

NO in carb. fluidizing gas = 0 ppm

SA MVB

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

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NO reduction balances with NO formation at [NO]balInlet NO concentration > [NO]bal net NOx reductionInlet NO concentration < [NO]bal net NOx formation

0

50

100

150

200

250

0 50 100 150 200 250

Car

b.flu

e ga

s N

Ox

conc

. [p

pm]

Added NO conc. [ppm]

SA, O2 in carb. feed gas =4%

[NO]bal


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Where should we install SCR (deNOx catalyst)?When DeNOx catalyst is required, installing DeNOxcatalyst in the downstream of CaL process is favorable for efficient NOx reduction.

Air-blown combustor

Air-blown combustor

DeNOxcatalyst

DeNOxcatalyst

CaLprocess

CaLprocess

Low NOx

Increased NOx

HighNOx

HighNOx

NOxRed.

Low NOx

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N2O formation and reduction in carbonator

N2O formation from char oxidation and NOx reduction by char

Reduction of N2O in feed gas

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Experimental ProcedureMeasurement of N2O conc. in carbonator flue gaswithout N2O feed (N2O from char / NO-char reaction)

Coal and

Secondary gas

Loopsealgas (air)

Carbonator fluidizing gas (O2+N2+NO)

Carbonator flue gas


Cyclone

Regenerator

Carbonator


Air

NO

to O2 analyzer

N2

NOx, N2O)

O2

Air

(Bubbling bed)

Bottom fluidizing gas

pneumatictransportation

3-wayvalve

to gas analyzers(CO, CO2,

(air)

gas (air)N2O feed to carb. Inlet (N2O reduction in carb.)

N2O feed to carb. flue gas (inlet N2O conc.)

N2O

Carb. flue gas

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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

]


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]


NO feed 193-205 ppmWithout NOfeed

SA


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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%

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Expected role of CaO in NOx/N2O reduction Char reduces part of NOx and N2O in carbonator.

In addition, CaO is known to catalyze reduction of NOx in the presence of reducing gas (CO) and N2O decomposition (with/without reducing gas).

Order of magnitude of reaction rate of CaO for NOx reduction and N2O decomposition were calculated using literature data.

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Expected NOx reduction in the presence of CaOCaO is a catalyst of NOx reduction by CO as:

2NO + 2CO N2 + 2CO2A rate expression (Tsujimura, 1983):

r = kxCO[NO] (mol・m-3・s-1)k = 3.41×107exp(-2.11×104/T) (ppm-1・s-1)

At carbonator temperature (873 K), k =0.0011 ppm-1 s-1

CO in carbonator flue gas: xCO=2000 ppm (Gao, 2014)kxCO= 2 s-1

Gas-solid contact time: = about 1 s (design value)kxCO = 2 Contribution to NOx reduction is expected.

Tsujimura et al., J Chem Eng Jpn, 16, 132 (1983)Gao et al., Fuel, 127, 38 (2014)

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Expected N2O reduction in the presence of CaOCaO is a catalyst of N2O decomposition as:

2N2O N2 + 2O2A rate expression is: (Shimizu et al., 2000)

kN2O(0)=k1+k2, k2/kN2O(0) =0.0011T - 0.685kN2O(0) = 2.8x107 exp(-1.057x104/T)K1= 43exp(4.19x103/T), K2=2.7

At T= 873 K and PH2O=0.1 atm, kN2O=34 s-1

Gas-solid contact time: = about 1 s (design value).kN2O= 34 Contribution to N2O reduction is expected.

OHOHON PK

kPK

kk22

2

21

12 11

Shimizu, T. et al., Energy & Fuels, 14, 104 (2001)

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Expected NOx/N2O reduction in the presence of CaOCaO is expected to catalyze NOx and N2O reduction in addition to reduction of NOx and N2O by char.

Regenerator

CO2, H2O

Fuel O2 CO2

recycleChar

Carbonator

CaCO3

CaO

Flue gas

Char

O2 NOx

Char

(CO2, O2,NOx, N2O)

, N2O

NOx, N2O reduction

CO2

to dilu te O2

CO2 gasN2

CaOCO

CO2 free gas

VM

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ConclusionsCoal combustion in regenerator of CaL processChar transportation to carbonator

Negative effects: loss of carbon and active CaOPositive effects: NOx and N2O reduction by char

Literature data on reduction of NOx and N2O catalyzed by CaO suggest that the carbonator of CaL process can reduce NOx (in the presence of CO) and N2O (with / without CO).

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Conclusions (continued)The reduction of NOx and N2O in the carbonator may make CaL process more competitive than other post-combustion processes.

Further works are still required to evaluate the contribution of CaL process for abatement of NOx and N2O.

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Acknowledgements

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

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