A sulphur capture model is presented that incorporates detailed laboratory data of limestone reactivity together with boiler data of particle-size distribution as well as residence times for the various particle sizes. The reactivity of limestone as a function of conversion is described by an exponential decay approximation and a total of four constants are used to describe the rate as a function of particle size and conversion. The residence time for the different particle sizes and the particle-size distribution are determined from data of three utility circulating fluidized-bed boilers (CFBB) with thermal outputs of 12, 40, and 165 MW. The model was validated by sulphur and calcium analysis of sieved fly-ash and bed-ash samples whereby the degree of sorbent conversion as a function of particle size was determined.