in Wiley InterScience (www.interscience.wiley.com).The importance of calcium-based sorbents, especially natural limestones, for CO 2 removal necessitates an investigation into the sorbent decay mechanism. This study starts from pore size distributions for samples from tests under various calcination/carbonation cycling conditions. A sintering model is formulated to describe the cyclic behavior of sorbents during cyclic calcination and carbonation. It explains the similar reversibility shown by sorbents under different test conditions. A balance between shorter cumulative sintering time and higher calcination rates appears to be responsible for the similar degrees of sintering and sorbent reversibility. 2007 American Institute of Chemical Engineers AIChE J,

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A three-part experimental program was carried out to investigate possible methods to improve sorbent reversibility during cyclic calcination/carbonation using one limestone and one dolomite. In the first part, the different roles of steam and water are discussed and investigated. Steam addition during carbonation and calcination did not help to significantly achieve good reversibility. Hydration, especially with low-temperature steam and liquid water, is promising to help improve sorbent reversibility by regenerating favorable pore size distributions; however, a carbonate layer inhibited hydration. Preliminary tests were also conducted on possible agents that might improve CO2 capture efficiency and sorbent cyclic performance. An ∼1:1 molar ratio of CaO to Al2O3 showed promising results on a free-lime basis. A series of other tests did not give promising results, but provided information relevant to developing synthetic CO2 sorbents. It was also found that CO can regenerate CaO from CaSO4 formed during co-capture of SO2 and CO2, but the rate of reduction is too slow to be of practical interest.

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Capturing carbon dioxide is vital for the future of climate-friendly combustion, gasification, and steam-reforming processes. Dry processes utilizing simple sorbents have great potential in this regard. Long-term calcination/carbonation cycling was carried out in an atmospheric-pressure thermogravimetric reactor. Although dolomite gave better capture than limestone for a limited number of cycles, the advantage declined over many cycles. Under some circumstances, decreasing the carbonation temperature increased the rate of reaction because of the interaction between equilibrium and kinetic factors. Limestone and dolomite, after being pretreated thermally at high temperatures (1000 or 1100 °C), showed a substantial increase in calcium utilization over many calcination/carbonation cycles. Lengthening the pretreatment interval resulted in greater improvement. However, attrition was significantly greater for the pretreated sorbents. Greatly extending the duration of carbonation during one cycle was found to be capable of restoring the CO 2 capture ability of sorbents to their original behavior, offering a possible means of countering the long-term degradation of calcium sorbents for dry capture of carbon dioxide.