Solid sorbents for carbon capture

Solid sorbents for carbon capture include a diverse range of porous, solid-phase materials, including mesoporous silicas, zeolites, and metal-organic frameworks, that have the potential to function as more efficient alternatives to amine gas treating processes for selectively removing CO2 from large, stationary sources including power stations.[1] While the technology readiness level of solid adsorbents for carbon capture varies between the research and demonstration levels, solid adsorbents have been demonstrated to be commercially viable as CO2 adsorbents for life-support and cryogenic distillation applications. While solid adsorbents suitable for carbon capture and storage are an active area of research within materials science, significant technological and policy obstacles presently limit the development of all carbon capture and storage technologies.

Overview

The combustion of fossil fuels for energy and heat generates over 13 gigatons of CO2 per year.[2] Concerns over the effects of CO2 with respect to climate change and ocean acidification has led governments and industries to investigate the feasibility of implementing technologies that allow fossil fuels to be continued to burned yet prevent the resultant CO2 from entering the carbon cycle. For new power plants, new technologies including pre-combustion and oxy-fuel combustion may simplify the gas separation process. However, for existing power plants the post-combustion separation of CO2 from the flue gas with a scrubber will likely be necessary. In such a system, fossil fuels are combusted with air and CO2 is selectively removed from a gas mixture also containing N2, H2O, O2, and trace sulfur, nitrogen, and metal impurities. While exact separation conditions are fuel and power plant dependent, in general CO2 is present at low concentrations (4-15% v/v) in gas mixtures near atmospheric pressure and at temperatures of approximately 40-60 °C.[3] Sorbents for carbon capture are regenerated using temperature, pressure, or vacuum, so that pure CO2 can be collected for sequestration or utilization and the sorbent can be reused to capture more CO2.

The most significant impediment to the implementation of carbon capture is the large cost of electricity increase that would result upon adoption of the technology.[4] Without policy or taxation incentives, the production of electricity from power plants using carbon capture technology is not competitive with other sources of energy.[5] The largest operating cost for power plants with carbon capture is the reduction in the amount of electricity the power plant can produce while the capture is in operation.[6] Known as parasitic energy, steam is diverted from making electricity in turbines to regenerating the CO2 sorbent. Thus, minimizing the amount of energy required for sorbent regeneration is the primary motivation behind much carbon capture research.

Metrics

Significant uncertainty exists around the total cost of post-combustion CO2 capture because full-scale demonstrations of the technology are only now beginning to come online.[7] Thus, individual performance metrics are generally relied upon when comparisons are made between different adsorbents.[8] Regeneration energy: Generally expressed in energy consumed per weight of CO2 captured (e.g. 3,000 kJ/kg). These values, if calculated directly from the latent and sensible heat components of regeneration, require the total amount of energy required for regeneration.[9] Parasitic energy: A similar metric to regeneration energy, but from the perspective of how much usable energy is lost. Owing to the imperfect thermal efficiency of power plants, not all of the heat required to regenerate the sorbent would actually have produced electricity.[10] Adsorption capacity: The amount of CO2 adsorbed onto the material under the relevant adsorption conditions.

Working capacity: The amount of CO2 that can be expected to be captured by a specified amount of adsorbent during one adsorption–desorption cycle. This value is generally more relevant than the total adsorption capacity.

Selectivity: The calculated ability of an adsorbent to preferential adsorb one gas over another gas. Multiple methods of reporting selectivity have been reported in the literature and in general values from one method cannot be compared to values from another method. Similarly, values are highly correlated to the temperature and pressure.[11]

Comparison to aqueous amine absorbents

Main article: Amine gas treating

Aqueous amine solutions absorb CO2 via the reversible formation of ammonium carbamate, ammonium carbonate, and ammonium bicarbonate.[12] The formation of these species and their relative concentration in solution in dependent upon the specific amine or amines being utilized as well as the temperature and pressure of the gas mixture. At low temperatures, CO2 is preferentially absorbed by the amines and at high temperatures CO2 is desorbed. While liquid amine solutions have been used industrially to remove acid gases for nearly a century, amine scrubber technology is still under development at the scale required for carbon capture from flue gas.[13]

Advantages of solid adsorbents

A significant number of advantages of using solid sorbents have been reported. Unlike amines, solid adsorbent can selectively adsorb CO2 without the formation of new chemical bonds (physisorption). The significantly lower heat of adsorption for solids requires less energy for the CO2 to desorb from the material surface. Also, two primary or secondary amines are generally required to absorb a single CO2 in liquids. For solid surfaces, very large total capacities of CO2 can be adsorbed. For temperature swing adsorption processes, the lower heat capacity of solids has been reported to reduce the sensible energy required for sorbent regeneration.[8] Many of the serious environmental concerns of using liquid amines can be eliminated by the use of solid adsorbents.[5]

Disadvantages of solid adsorbents

The cost of manufacturing solid sorbents for CO2 capture is expected to be significantly greater than the cost of simple amines that can be easily produced on large scales. Because flue gas contains trace impurities that cause sorbent degradation, solid sorbents may prove to be prohibitively expensive to use. Significant engineering challenges also prevent solid sorbents from being used for carbon capture. Sensible energy required for sorbent regeneration cannot be effectively recovered if solids are used, offsetting the significant heat capacity savings associated with using solids. Additionally, heat transfer through a solid bed is slow and inefficient, making it difficult and expensive to cool the sorbent during adsorption and heat the sorbent during desorption. Lastly, many promising solid adsorbents have only been measured under ideal conditions, which ignore the potentially significant effects H2O can have on working capacity and regeneration energy.

Physical adsorbents

Carbon dioxide adsorbs in appreciable quantities onto many porous materials through van der Waals interactions. Compared to N2, CO2 will adsorb more strongly because the molecule is more polarizabable and possesses a larger quadrupole moment.[8] However, stronger adsorptives including H2O often interfere with physical adsorption mechanism. Thus, discovering porous materials that can selectively bind CO2 under flue gas conditions using only a physical adsorption mechanism is an active research area.

Zeolites

Main article: Zeolite

Zeolites, a class of porous aluminosilicate solids, are currently used in a wide variety of industrial and commercial applications including CO2 separations. The capacities and selectivities for many zeolites are among the highest for adsorbents that rely upon physisorption. For example, zeolite Ca-A (5A) has been reported to display both a high capacity and selectivity for CO2 over N2 under conditions relevant for carbon capture from coal flue gas, but like other promising zeolites was not tested in the presence of H2O.[14] Industrially, CO2 and H2O can be coadsorbed on a zeolite, but high-temperatures and a dry gas stream are required to regenerate the sorbent. Thus, while a large number of hypothetical zeolites have been predicted to outperform aqueous amine solutions,[10] the ability of zeolites to do in the presence of water has not yet been demonstrated.

Metal-organic frameworks

A rapidly growing class of porous materials, a large number of metal-organic frameworks (MOFs) are promising adsorbents for CO2 capture.[8] Owing to the nearly limitless diversity of the structural and chemical composition of MOFs, sorbents utilizing a diverse set of properties have been reported. MOFs with extremely large surface areas are generally not among the best forming MOFs for CO2 capture.[8] The best performing MOFs for CO2 capture generally include materials with at least one adsorption site that can polarize CO2. For example, MOFs with open metal coordination sites function as Lewis acids and strongly polarize CO2.[15] Owing to the greater polariability and quadrupole moment of CO2, CO2is preferentially adsorbed over many flue gas components such as N2. However, flue gas contaminants such as H2O often interfere with such materials. MOFs with specific pore sizes, tuned specifically to preferentially adsorb CO2 have been reported.[16]

Chemical adsorbents

Amine impregnated solids

Frequently, porous adsorbents with large surface areas, but only weak adsorption sites lack sufficient capacity for CO2 under realistic conditions. To increase low pressure CO2 adsorption capacity, adding amine functional groups to highly porous materials has been reported to result in new adsorbents with very high capacities for CO2. This strategy has been reported for polymers, silicas, activated carbons, and metal-organic frameworks.[1] Amine impregnated solids utilize the well-established acid-base chemistry of CO2 with amines, but dilute the amines via containing them within the pores of solids rather than as H2O solutions. In comparison to other solid adsorbent, amines are reported to maintain their adsorption capacity and selectivity under humid test conditions.

Notable adsorbents

Name Type 0.15 bar Capacity (% weight) Reference
PEI-MIL-101 Amine 17.7 [17]
mmen-Mg2(dobpdc) Amine 13.7 [18]
Mg-MOF-74 MOF 20.6 [15]
SIFSIX-3(Zn) MOF 10.7 [16]
HKUST-1 MOF 11.6 [19]
Ni-MOF-74 MOF 16.9 [20]
Co-MOF-74 MOF 14.2 [20]
mmen-CuBTTri MOF 9.5 [21]
Zn(ox)(atz)2 MOF 8.3 [22]
Zn-MOF-74 MOF 7.6 [23]
CuTATB-60 MOF 5.8 [24]
bio-MOF-11 MOF 5.4 [25]
FeBTT MOF 5.3 [26]
MOF-253-Cu(BF4) MOF 4.0 [27]
ZIF-78 MOF 3.3 [28]
NH2-MIL-53(Al) MOF 3.1 [29]
CuBTTri MOF 2.9 [30]
SNU-50 MOF 2.9 [31]
en-CuBTTri MOF 2.3 [32]
USO-2-Ni-A MOF 2.1 [29]
MIL-53(Al) MOF 1.7 [29]
MIL-47 MOF 1.1 [20]
UMCM-150 MOF 1.8 [20]
MOF-253 MOF 1.0 [27]
ZIF-100 MOF 1.0 [33]
MTV-MOF-EHI MOF 1.0 [34]
ZIF-8 MOF 0.6 [20]
IRMOF-3 MOF 0.6 [20]
MOF-177 MOF 0.6 [20]
UMCM-1 MOF 0.5 [20]
MOF-5 MOF 0.5 [20]
13X Zeolite 15.3 [35]
Ca-A Zeolite 18.5 [14]

References

  1. ↑ 1.0 1.1 A. H. Lu, S. Dai, Porous Materials for Carbon Dioxide Capture Springer, 2014.
  2. ↑ International Energy Agency, CO2 Emissions from Fuel Combustion: Highlights IEA, 2013.
  3. ↑ A. Samanta, A. Zhao, G. K. H. Shimizu, P. Sarkar, and R. Gupta Industrial & Engineering Chemistry Research 2012 51 1438-1463.
  4. ↑ A. Adragna, "CO2 Capture Could Raise Wholesale Energy Price Eighty Percent" Bloomberg News, Feb. 12, 2014.
  5. ↑ 5.0 5.1 NETL, "Cost and Performance Baseline for Fossil Energy Plants" Volume 1: Bituminous Coal and Natural Gas to Electricity. http://www.netl.doe.gov/energy-analyses/pubs/BitBase_FinRep_Rev2.pdf
  6. ↑ H. Herzog, J. Meldon, A. Hatton, "Advanced Post-combustion CO2 Capture" https://mitei.mit.edu/system/files/herzog-meldon-hatton.pdf
  7. ↑ Global CCS Institute, http://www.globalccsinstitute.com/projects/browse.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 K. Sumida, et al. Chem. Rev. 2012, 112, 724-781.
  9. ↑ S. Sjostrom, H. Krutka, Fuel 2014, 89, 1298-1306.
  10. ↑ 10.0 10.1 L.-C. Li, et al., Nature Materials 2012, 11, 633–641.
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  12. ↑ M. E. Boot-Handford et al., Energy Environ. Sci. 7, 130 (2014).
  13. ↑ G. Rochelle, Science, 2009, 325, 1652-1654.
  14. ↑ 14.0 14.1 Bae, T.-H.; Hudson, M. R.; Mason, J. A.; Queen, W. L.; Dutton, J. J.; Sumida, K.; Micklash, K. J.; Kaye, S. S.; Brown, C. M.; Long, J. R. Energy Environ. Sci. 2013, 6, 128-138.
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  16. ↑ 16.0 16.1 P. Nugent, et. al, Nature, 2013, 495, 80–84.
  17. ↑ Y. Lin, Q. Yan, C. Kong, L. Chen, Scientific Reports, 2013, 3, Article number: 1859.
  18. ↑ McDonald, T. M.; Lee, W. R.; Mason, J. A.; Wiers, B. M.; Hong, C. S.; Long, J. R. J. Am. Chem. Soc. 2012, 134, 7056-7065.
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  20. ↑ 20.0 20.1 20.2 20.3 20.4 20.5 20.6 20.7 20.8 Yazaydin, A. O., et al. J. Am. Chem. Soc 2009, 131, 18198.
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  24. ↑ Kim, J.; Yang, S.-T.; Choi, S. B.; Sim, J.; Kim, J.; Ahn, W.-S. J. Mater. Chem. 2011, 21, 3070.
  25. ↑ An, J.; Geib, S. J.; Rosi, N. L. J. Am. Chem. Soc. 2010, 132, 38.
  26. ↑ Sumida, K.; Horike, S.; Kaye, S. S.; Herm, Z. R.; Queen, W. L.; Brown, C. M.; Grandjean, F.; Long, G. J.; Dailly, A.; Long, J. R. Chem. Sci. 2010, 1, 184.
  27. ↑ 27.0 27.1 Bloch, E. D.; Britt, D.; Lee, C.; Doonan, C. J.; Uribe-Romo, F. J.; Furukawa, H.; Long, J. R.; Yaghi, O. M. J. Am. Chem. Soc 2010, 132, 14383.
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  29. ↑ 29.0 29.1 29.2 Arstad,B.;Fjellva�g,H.;Kongshaug,K.O.;Swang,O.;Blom,R. Adsorption 2008, 14, 755.
  30. ↑ Demessence,A.;D’Alessandro,D.M.;Foo,M.L.;Long,J.R. J. Am. Chem. Soc 2009, 131, 8784.
  31. ↑ Prasad, T. K.; Hong, D. H.; Suh, M. P.. Chem.–Eur. J. 2010, 16, 14043.
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  34. ↑ Deng, H.; Doonan, C. J.; Furukawa, H.; Ferriera, R. B.; Towne, J.; Knobler, C. B.; Wang, B.; Yaghi, O. M. . Science 2010, 327, 846.
  35. ↑ Li, B., Wang, H. and Chen, B., Chem. Asian J., 2014, doi: 10.1002/asia.201400031.