Residual activity of sorbent particles with a long residence time in a CFBC

To characterize the sulfation capacity of limestone as a sorbent of SO2 in a circulating fluidized-bed combustor (CFBC), long-term sulfation tests were carried out in a thermobalance. In order to cover the range of residence times expected in a large-scale combustor, the experiments lasted 24 h. The residual activity of the sorbent after the first hours of reaction indicated a conversion of similar to 0.1, depending on the particle's size and sulfation conditions. The shrinking core model described well these residual activities. An effective gas-phase diffusion coefficient was derived from plots of the measured conversion vs. time between 3 and 24 h. These diffusivities ranged between 1.7 and 4.1 x 10(-9) m(2)/s at 850 degrees C and depended only on the limestone. It is argued that this simple description could be incorporated into more complex sulfation models of the first few hours of reaction and also account for the subsequent increases in conversion over longer time scales. It is also stressed that, to predict a sorbent's performance in a boiler, characterization techniques should be expanded to experimental time scales similar to those expected in large-scale CFBCs.