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Sixth Annual Conference on

Carbon Capture & Sequestration

Capture Advanced Concepts

CO2 CAPTURE IN IGCC PLANTS

VIA CRYOGENIC SEPARATION

May 7-10, 2007

Sheraton Station Square

Pittsburgh, Pennsylvania

S. Consonni, F. Vigano

-

Politecnico di Milano(*)

T. Kreutz

-

Princeton University

L. De Lorenzo

-

BP Alternative Energy

(*) currently visiting the Princeton Environmental Institute, Princeton University

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Background and Scope

Technologies of choice for the removal of CO2 from shifted

syngas in IGCCs are chemical and physical absorption

With high-pressure gasification, Selexol physical absorption is

apparently preferable

Selexol system is expensive (large absorber vessel, large solvent

flow rates, recycle compressor, refrigeration system, etc.) and

makes CO2 available at low pressure large power requirements

An alternative technology could be cryogenic separation

This presentation discusses thermodynamic issues and plant

configurations

Preliminary performance estimates are presented for a system

applicable to the IGCC planned by BP at Carson (California)

Work is underway to verify simulation results and estimate costs

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CO2 removal by Selexol

togas turbine

from H2Sremoval

CO2 leansolvent

Solvent pump

H2O

Solvent

Dehydrator

~

~~

~

~

Flash drums

CO2Absorber

Hydraulicturbine

Recyclecompressor

Main CO2 compressor

CO2 to

storage

Refrigeration

CO2 pump

Refrigeration

Intercooling

Aftercooling

After-cooling

Aftercooling

Intercooling

~

Saturator

Syngasexpander

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Basic assumptions

Refer to conditions expected for BP DF-2: IGCC with CO2 capturefed with pet-coke, to be built in the Carson refinery (California)

Entrained-flow, oxygen-blown GE quench gasifier followed by

two shifts and cooling to nearly ambient temperature

Mole flow Mol fract Mass flow Mass fract

kmol/hr % kg/s %

H2O 40.1 0.139 0.201 0.121CO 343.9 1.193 2.676 1.612

H2 15,719.5 54.530 8.802 5.304

CO2 12,315.5 42.722 150.556 90.724

CH4 5.9 0.020 0.026 0.016Ar 29.8 0.103 0.331 0.199

N2 100.2 0.348 0.780 0.470

H2S 271.9 0.943 2.574 1.551

COS 0.2 0.001 0.003 0.002Total 28,827.0 100.000 165.949 100.000

Temperature, C 40.0

Pressure, bar 60.0

Density, kg/m

3

48.17LHV, MJ/kg 6.77

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Basic thermodynamics of cryogenic separation

25 bar40C

Condensation temperaturedecreases as a consequence

of lower CO2 partialpressure in the gas phase

First liquidforms atabout -10C

When temperature and pressurereach triple point conditions,a solid phase appears

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Gas/liquid mixtures

For mixtures with other species in addition to CO2,

thermodynamic properties are estimated by Redlich-

Kwong-Soave (RKS) equation of state.

Very similar results would be obtained with the Peng-

Robinson equation of state.

To be conservative, weve neglected the depression of

the freezing point temperature due to the presence of

species other than CO2

Further work is needed to clarify the behaviour of CO2-

based mixtures at low temperature, particularly the

freezing point.

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Basic process: cool syngas to the lowest temperature

that still prevents freezing

At 60 bar:

Removal of H2S is nearly 85%

Removal of CO2 is about 80%

T = - 53C

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Two-stage scheme

58 bar25C159.36 kg/s

31 bar

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Two-stage scheme: CO2 and H2S capture

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Scheme considered in this work

60 bar40C165.95 kg/s

30 bar

31 bar

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Assumptions

Polytropic efficiency of compressors 0.82

Polytropic efficiency of syngas expander 0.85

Hydraulic efficiency of CO2 pump 0.75

Mechanical - electric efficiency of motor drivers 0.90

Minimum T of gas/gas exchangers, C 15.00

Minimum T of liquid/gas exchangers, C 10.00

Minimum T of CO2 refrigerant/condensing gas exchangers, C 3.00

Minimum T of ammonia/condensing gas exchangers, C 5.00

p in main heat exchangers, bar 1.00

Auxiliary power consumption / heat to environment, % 1.50

Refrigeration fluid NH3

T superheating at evaporator exit, C 3.00

T subcooling at throttle valves inlet, C 5.00

p in CO2 refrigerant evaporators/condenser/intercooler, bar 0.20T along ammonia evaporator/condenser, C 1.00

COP of auxiliary chillers (evap. 20C, conden. 35C) 10.20

Turbomachines

Heat exchangers

External refrigeration cycle and auxiliary CO2 chiller

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Internal refrigeration

60 bar40C

165.95 kg/s

30 bar112C 31 bar10C

24.03 kg/s

135.4 kg/s

-53C-53C

135.4 kg/s

100C135.4 kg/s

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60 bar40C

165.95 kg/s

30 bar112C 31 bar10C

24.03 kg/s

-53C-53C

135.4 kg/s

100C135.4 kg/s

150 bar25C

135.4 kg/s

-33C

-56C

+3.8C

Internal refrigeration

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60 bar40C

165.95 kg/s

30 bar112C 31 bar10C

24.03 kg/s

-53C-53C

135.4 kg/s

100C135.4 kg/s

150 bar25C

135.4 kg/s

-33C

-56C

+3.8C

Internal refrigeration

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E t l f i ti

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60 bar40C

165.95 kg/s

30 bar112C

31 bar10C

23.13 kg/s

136.1 kg/s

-55C-55C

41.38 kg/s

100C23.13 kg/s

-45.6C

-60.0C

External refrigeration

E t l f i ti

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60 bar40C

165.95 kg/s

30 bar112C

31 bar10C

23.13 kg/s

136.1 kg/s

-55C-55C

41.38 kg/s

100C23.13 kg/s

-45.6C

-60.0C

External refrigeration

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Preliminary design of syngas turbo-compressor

EXPANDER:axial, 6 stagesshaft power 23 MW

COMPRESSOR:centrifugal, 5 stagesshaft power 28 MW

1st stage:external diameter 0.25 mblade height at impellerdischarge 0.016 m

5th stage:

external diameter 0.22 mblade height at impellerdischarge 0.01 m

6st stage:external diameter 0.24 mblade height 0.025 m

1st

stage:external diameter 0.19 mblade height 0.01 m

single shaft

33,000 rpm

electric motor

5.5 MWel

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Internal vs

external

refrigeration:

mass balances

internal ref. external ref.

Minimum T of gas being treated C -53.00 -55.00

Maximum P of gas being treated bar 300.00 300.00

kg/s 24.03 23.13

LHV, MJ/kg 44.25 45.78

bar 30.00 30.00

C 10.00 10.00

kg/s 135.35 136.13% mol CO2 97.92 98.01

% mass CO2 99.71 99.72

kmol/s 3.07 3.08

% 91.94 92.48

kmol/s 3.07 3.09

% 89.52 90.04

H2S in ppmvd 19.34 19.34

H2S out (to gas turbine) ppmvd 1.84 1.73

COS in ppmvd 4.98 4.98COS out (to gas turbine) ppmvd 0.06 0.06

MWLHV 13.47 12.94

% 1.28 1.23

MWLHV 1.67 1.63

% 6.18 6.03

MWLHV 15.45 14.88

% 1.43 1.38Total LHV lost in flow to storage

H2 lost in flow to storage

CO lost in flow to storage

Cryogenic system

CO2 captured from syngas

Carbon captured from syngas

Syngas to gas turbine

Flow to carbon storage

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internal ref. external ref.

kg/s 159.36 159.36MWLHV 1078.52 1078.52

kg/s 24.03 23.23

MWLHV 1063.07 1063.64

kg/s 44.26 41.38

MWshaft 28.50 27.96

kg/s 23.90 23.13

MWshaft -23.39 -23.04

Syngas turbo-compressor MWel 5.68 5.47

kg/s 115.11 117.98MWel 1.57 1.38

kg/s 131.82 13.13

MWel 7.29 1.82

kg/s 259.29 40.51

MWel 18.63 14.26

kg/s 338.29 49.94

MWel 15.90 11.24

CO2 refrigeration after compression MWel 2.25 0.00

Auxiliaries for cooling water MWel 0.72 1.13MWel 49.80 35.29

kJel / kg CO2 captured 368.96 259.99Overall power consumption

Cryogenic system

Syngas in

Clean syngas out

Main syngas compressor

Syngas expander

LT refrigeration compressor

MT refrigeration compressor

HT refrigeration compressor

CO2 pump

Internal vs external refrigeration:energy balance

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Selexol+ CO2 drying

+ CO2 compression

Selexol for H2S removal+ drying

+ cryogenic with ext.

refrigeration

kg/s 165.95 165.95

LHV, MJ/kg 6.77 6.77kg/s 26.97 27.59

LHV, MJ/kg 40.07 38.37

kg/kmol 5.44 5.64

bar 30.00 30.00

C 131.30 111.90H2O, % mol 4.89 5.07

Ar, % mol 0.18 0.15

CO, % mol 1.96 1.92

CO2, % mol 4.86 5.38

H2, % mol 92.38 91.96

N2, % mol 0.59 0.56

CH4, ppmv 326.99 294.87

COS, ppmv n.a. 0.06

H2S, ppmv n.a. 1.73

kg/s 140.12 136.13

% mol CO2 99.97 98.01

% mass CO2 99.99 99.72

Syngas to gas turbine

Flow to carbon storage

Composition of syngas to gas

turbine, dry basis

Syngas in

Ext. refrigeration vs Selexol: flows and compositions

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Conclusions

CO2 cryogenic separation appears a viable alternative toconventional absorption techniques

Must remove H2S upstream of cryogenic process

Two-stage schemes can achieve 90% CO2 removal rates

External ammonia refrigeration cycle appears definitely

superior to internal refrigeration

Compared to Selexol, significant reduction of overall power

consumption

No new technology, although turbomachinery must be

specifically designed for cryogenic plant

Further work needed to increase accuracy of these

preliminary performance estimates and to assess costs