Selection of Optimal Solid Sorbents for CO2 Capture Based on Gas Phase CO2 Composition - presentation by Adam berger of EPRI at the UKCCSRC Natural Gas CCS Network Meeting at GHGT-12, Austin, Texas, October 2014

1. Adam H. Berger and Abhoyjit S. Bhown
Electric Power Research Institute (EPRI)
Palo Alto, California
UKCCSRC Natural Gas CCS Panel
International Conference on Greenhouse Gas Technologies (GHGT-12)
Austin, Texas, October 5-9, 2014
Selection of Optimal Solid Sorbents for CO2 Capture Based on Gas Phase CO2 Composition

2. © 2014 Electric Power Research Institute, Inc. All rights reserved. 2
About The Electric Power Research Institute
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Established 1973 as independent non-profit for collaborative research in electric sector
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Major locations in Palo Alto, CA; Charlotte, NC; Knoxville, TN
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Nearly every area of electricity: generation, delivery, use, health, environment, and efficiency
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Members include 450+ participants in more than 30 countries

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CO2 Capture from Existing CO2 Streams
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Carbon capture is the only way to remove CO2 emissions from existing sources
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But CO2 streams have different compositions
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What is the best way to capture CO2 from these different streams?
Gas Mixture
CO2 Concentration
Air
400 ppm
Flue Gas from Natural Gas Turbine
3-5%
Flue Gas from Coal Combustion
10-14%
Steel Production
10-30%
Cement Kilns
15-30%
Ethanol Production
98+%

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Thermodynamic Minimum Work revWE=min
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Thermodynamic minimum work for separation is equal to the work required to overcome entropy of mixing
()()[]Σ−−+−=−=Δ−=Δ= iCOCOCOCOiimixmixxxxxRTxxRTSTGE2222min1ln1lnln

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Thermodynamic Minimum Work for Separation and Compression
Σ−=Δ−=Δ== iiimixmixsepxxRTSTGEWlnmin   = 21lnPPRTWcompsepcomptotalWWW+=
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Varies with inlet CO2 concentration
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Real processes will be above minimum work value
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T set to 40°C

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Solid Sorbents for CO2 Capture
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Temperature swing adsorption (TSA) process
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Operation of TSA process can be modeled for different sorbent materials Assumptions
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Langmuir isotherm with IAST
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Heat and mass transfer are fast (equilibrium)
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Binary gas stream (CO2 + N2)
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Contactor type unimportant (fixed bed or moving bed)
Flue Gas
CO2 rich
Flue Gas
N2 rich
N2 rich
CO2 rich
4 Cooling
1 Adsorption
2 Heating
3 Purge
Clean bed
Flue Gas
N2 rich
1 Adsorption

7. © 2014 Electric Power Research Institute, Inc. All rights reserved. 7
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Class of zeolite-like sorbents with varying heat of adsorption
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Adsorption: 1 atm, 13% CO2, T = 40°C,
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Regeneration: 1 atm, 99% CO2, vary Tregeneration
Thermal Energy for TSA Separation
Log(Henry’s Coefficient)
Heat of Adsorption
Correlation used for
heat of adsorption

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Total Work Required for CO2 capture
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Total work is compression work + impact of thermal load
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Compression work: ηs = 85%, 40°C intercooling, PR <2.5
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Thermal energy is converted to work in power plants at an efficiency that varies with the temperature
0.1
0.15
0.2
0.25
0.3
0.35
80
100
120
140
160
180
Efficiency [Wlost/Q]
Saturation Temperature [C] at Extraction Pressure
Imposed Load from Steam Extraction
Linear Fit
Simulated Result
ELost = Q*(-0.08037+0.002326*T)
lostcomptotalEWW+=
For a TSA system

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Theoretical minimum energy for separation + compression
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Regeneration at 1 bar, followed by compression to 150 bar
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Varying Tregeneration and sorbent ΔH
Minimum Energy for Separation and Compression
Σ−= iiisepxxRTWln   = 21lnPPRTWcompsepcomptotalWWW+=

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Calculated Energy for Separation and Compression
Solid lines: Calculated
Dashed lines: Thermodynamic minimum
% CO2
Minimum
Calculated
0.04
682
1317
1
628
1183
3
562
1017
5
531
937
13
472
784
15
463
760
20
444
712
35
406
616
Energy numbers in kJ/kg CO2
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Compression work dominates at high CO2 concentrations
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Thermal impact dominates at low CO2 concentrations
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Thermal energy requirement causes majority of difference between thermodynamic and calculated values

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Optimal Separation Conditions
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Low CO2 concentration requires high heats of adsorption and temperatures of desorption, leading to high energy requirements
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Natural gas CCS: optimal ΔH: -57 to 60 kJ/mol CO2, Tregen: 120-129°C
% CO2
ΔH [kJ/mol]
Tregen [°C]
0.04
-72
159
1
-67
147
3
-60
129
5
-57
120
13
-52
101
15
-51
98
20
-49
91
35
-45
77

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Effect of Increasing CO2 concentration
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Enriching CO2 mole fraction has greatest impact at low CO2 concentrations
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Demonstrates how flue gas recycle impacts total energy penalty
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Eg. Increasing CO2 mole fraction from 3% to 15% lowers calculated energy penalty by 257 kJ/kg CO2 (>25%)

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Summary and Conclusions
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Optimal material properties and performance modeled for natural gas CCS
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The tradeoff between increasing CO2 concentration and the performance of the rest of the plant can be estimated
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Low concentrations of CO2 require greater temperatures and heats of adsorption to desorb CO2 at 1 atm
Emin [kJ/kg CO2]
E (TSA)
Tregen [°C]
ΔH [kJ/mol]
Natural Gas (3-5%)
531-562
937-1017
120-129
-57 to -60
Coal (13-15%)
463-472
760-784
98-101
-51 to -52

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