Experiments in a pilot

Cyclic carbonation and calcination reactions were investigated for capturing CO2 from combustion and gasification processes. Sorbent particles in the size range 600−1400 μm were subjected to multiple capture cycles at atmospheric pressure to obtain a surface mapping of conversion based on calcination and carbonation temperatures. Steam hydration of CaO was utilized to increase both pore area and pore volume to improve long-term conversion to CaCO3 over multiple cycles. The steam hydration improved the long-term performance of the sorbent, resulting in directly measured conversions as high as 52% and estimated conversions as high as 59% after up to 20 cycles. It is estimated that the increase in conversion has improved the economics of the proposed process to the point where commercialization is attractive. It has been shown that when carbonating in the temperature range from 700 to 740 °C, calcination temperatures from 700 to 900 °C can be used without seriously reducing the conversion of CaO for CO2 capture over multiple cycles. Processes based on this approach are expected to be able to reduce CO2 emissions from coal- and petroleum coke-fired fluidized bed combustors by up to 85%, while avoiding excessive sorbent replacement.

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Limestone attrition in circulating fluidized-bed combustors (CFBCs) has received limited attention. Although there are a number of early studies on attrition in bubbling-bed systems, most current studies focus on simultaneous calcination and sulfation. However, this subject is increasing in importance as CO 2 looping cycles are proposed. CO 2 looping cycles involve repeatedly calcining the CaCO 3 component of the limestone to drive off a pure stream of CO 2 for storage or sequestration. Here, we have looked at five limestones from across Canada, the United States, and Mexico to determine the extent of their attrition under calcining conditions in fluidized-bed systems. This work shows that attrition varies very significantly from limestone to limestone, and even among different batches. It is clear, therefore, that each limestone will have to be carefully categorized to determine its potential for use in such cycles. Also, since limestones crush differently, even those limestones that are double-sieved may have very different initial size distributions. This will affect the results seen in tests carried out under realistic conditions. This work shows that most of the material loss in multiple calcination/ carbonation cycles is in the first few cycles, and that even a very low level of sulfation can be a very effective means of reducing that material loss.

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