5790 Published 2010 by the American Chemical Society pubs.acs.org/EF Energy Fuels 2010, 24, 5790–5796 : DOI:10.1021/ef100931v Published on Web 09/24/2010 Sintering and Formation of a Nonporous Carbonate Shell at the Surface of CaO-Based Sorbent Particles during CO 2 -Capture Cycles Vasilije Manovic and Edward J. Anthony* CanmetENERGY, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario K1A 1M1, Canada Received July 22, 2010. Revised Manuscript Received August 23, 2010 The existence and formation of a carbonate shell at the surface of the particles of CaO-based sorbents is investigated in this paper. Two sorbents were tested: natural Kelly Rock (KR) limestone and synthetic pellets (KR-CA-14) prepared from the same limestone and calcium aluminate cement (CA-14). Various different series of calcination/carbonation cycles were carried out in a thermogravimetric analyzer (TGA) apparatus, and the sorbent samples produced after those cycles were analyzed with a scanning electron microscope (SEM). It is shown that sintering during cycles is more pronounced at the surface of sorbent particles, which results in the formation of nonporous areas or even a totally nonporous shell that surrounds a partially reacted CaO core. However, the dependency of shell formation upon cycle number is difficult to elucidate by SEM because increasing cycle numbers achieve lower conversion levels, which reduce the chance of shell formation. Prolonged carbonation after a series of cycles showed that there is a limit in maximum conversion levels, which cannot be solely explained by product layer formation at the interior sorbent surface area. The SEM images of samples after prolonged carbonation periods clearly show the presence of more sintered areas at the outer particle surface and/or carbonate shell/partially reacted particle pattern. This is explained by the phenomenon of more pronounced sintering at the sorbent particle surface than seen in the particle interior because of surface tension and more pronounced loss of pore volume near the exterior of the particle. The formation of a carbonate shell at the particle surface is a different phenomenon from that of the formation of a product layer at the pore surface area and also limits diffusion during carbonation. Introduction It is widely accepted that climate change is being driven by increasing concentrations of greenhouse gases because of human activity, and this key problem requires urgent solu- tions. Carbon dioxide (CO 2 ) is the main greenhouse gas that causes global warming, and fossil-fuel-fired power plants represent a major source of anthropogenic CO 2 . The negative environmental effects of such emissions represent a growing problem because the use of fossil fuels, such as coal, is increasing and can be expected to do so for the near- to medium-term future. 1,2 Therefore, technologies associated with CO 2 capture and storage (CCS) are increasingly con- sidered to be likely contributors to reduce these emissions. 3,4 Although, in principle, the entire gas stream containing low concentrations of CO 2 could be transported and sequestered underground, the energy and other associated costs gener- ally make this approach impractical. Therefore, CO 2 has to be concentrated to be suitable for compression and piping to a storage site, 5 which is the goal of CO 2 capture. There are three main approaches to obtain concentrated CO 2 streams from fossil fuels: (i) postcombustion CO 2 capture from the flue gas, (ii) precombustion separation, which involves gasification and capture of CO 2 before the combustion of the produced gases, and (iii) oxy-fuel combustion, which uses oxygen instead of air during combustion. 6,7 Currently, “scrubbing” of flue gases using amine-based sorbents, such as monoethanolamine (MEA), is the post- combustion technology closest to the market. This technology has been widely used in the natural gas industry for over 60 years, and it is being considered for separation of CO 2 from flue gas. However, this approach is very energy-intensive and has other problems that appear to be likely to result in signifi- cantly higher electricity costs (by 70%). 8 That has led to the investigation of other methods of CO 2 capture, such as the reversible reaction between calcium oxide and carbon dioxide to form calcium carbonate (the calcium-looping cycle). 6,9,10 *To whom correspondence should be addressed. Telephone: (613) 996-2868. Fax: (613) 992-9335. E-mail: banthony@nrcan.gc.ca. (1) Mohr, S. H.; Evans, G. M. Forecasting coal production until 2100. Fuel 2009, 88, 2059–2067. (2) Hook, M.; Aleklett, K. Historical trends in American coal pro- duction and a possible future outlook. Int. J. Coal Geol. 2009, 78, 201– 216. (3) Intergovernmental Panel on Climate Change (IPCC). Special Report on Carbon Dioxide Capture and Storage; Metz, B., Davidson, O., de Coninck, H., Loos, M., Meyer, L., Eds.; Cambridge University Press: Cambridge, U.K., 2005. (4) Herzog, H. What future for carbon capture and sequestration? Environ. Sci. Technol. 2001, 35, 148–153. (5) Bachu, S. CO 2 storage in geological media: Role, means, status, and barriers to deployment. Prog. Energy Combust. Sci. 2008, 34, 254–273. (6) Anthony, E. J. Solid looping cycles: A new technology for coal conversion. Ind. Eng. Chem. Res. 2008, 47, 1747–1754. (7) Yang, H.; Xu, Z.; Fan, M.; Gupta, R.; Slimane, R.; Bland, B.; Alan, E.; Wright, I. Progress in carbon dioxide separation and capture: A review. J. Environ. Sci. 2008, 20, 14–27. (8) Alie, C.; Backham, L.; Croiset, E.; Douglas, P. L. Simulation of CO 2 capture using MEA scrubbing: A flowsheet decomposition method. Energy Convers. Manage. 2005, 46, 475–487. (9) Stanmore, B. R.; Gilot, P. Review - Calcination and carbonation of limestone during thermal cycling for CO 2 sequestration. Fuel Process. Technol. 2005, 86, 1707–1743. (10) Blamey, J.; Anthony, E. J.; Wang, J.; Fennell, P. S. The use of the calcium looping cycle for post-combustion CO 2 capture. Prog. Energy Combust. Sci. 2010, 36, 260–279.