Attrition of calcining limestones in circulating fluidized-bed systems

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 CO2 looping cycles are proposed. CO2 looping cycles involve repeatedly calcining the CaCO3 component of the limestone to drive off a pure stream of CO2 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.