A TG METHOD FOR EVALUATION OF LIMESTONE SORBENTS USED IN CIRCULATING FLUIDIZED-BED COMBUSTORS

A thermogravimetric (TG) technique has been developed to study the sulphation of limestone sorbents destined for use in circulating fluidized bed (CFB) combustors. This technique accommodates larger sample sizes (1-5 g) than the established TG method and hence more representative samples in terms of particle size distribution. The sample is dispersed in a quartz wool matrix to simulate the typical CFB void fractions encountered by the limestone particles and to minimize the bulk phase diffusion resistance. Gas mixtures of typical CFB composition (0.5% SO2, 2-5% O2, 10-15% CO2, balance N2) are passed through the calcined limestone sample. The sulphation rates of six Canadian limestones were studied using this technique. The results indicate that the sulphation of the limestones is well described by a power law dependence on the fraction of unconverted material available. Results obtained for overall sulphation capacities compare favourably with the results from the fluidized bed reactor (FBR) method, indicating that the technique is an effective and simple method for screening sorbents.