This document presents research on optimizing solid sorbents for CO2 capture from various gas streams, highlighting the differences in CO2 compositions and the energy required for their separation. It discusses the thermodynamic principles involved in CO2 capture, specifically focusing on temperature swing adsorption (TSA) processes and the trade-offs related to sorbent properties and operational conditions. Key findings indicate that higher CO2 concentrations significantly reduce energy penalties in the capture process, particularly noting optimal energy requirements for both natural gas and coal sources.

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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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

9. © 2014 Electric Power Research Institute, Inc. All rights reserved. 9
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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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