A previously developed and tested mathematical model for the simulation of both the bed and freeboard of atmospheric fluidised-bed combustors (AFBCs) burning high- and low-quality coals was modified to incorporate calcination of limestone and subsequent sulphation taking place in the emulsion phase, in accordance with a deactivation model similar to those for catalysts. The new model was then used to investigate the effect of excess air and Ca:S molar ratio on the behaviour of a pilot-scale AFBC, for which full-scale experimental data are available. The coal used in the experiments was a typical low-calorific-value (HHV = 3386 cal g(-1)), high-ash (44%) and high-sulphur (4.1%) lignite burning in its own ash. The sorbent was limestone containing 97% CaCO3. Predicted concentration and temperature profiles, particle-size distributions in the bed and carryover, combustion and sulphur-retention efficiencies were compared with measured data and found to be in reasonable agreement. A simulator of this type, for the combustion of low-quality lignite with in-situ desulphurisation and its testing with measured data, is not yet available.