A theoretical study of the complex transient system of simultaneous sulphur capture and catalytic reactions of N-containing compounds taking place on a single limestone particle is conducted. The numerical technique developed previously by the authors (Kill et al., 1995, Chem. Engng Sci. 50, 2793-2799) based on collocation on moving finite elements is used to solve the model equations. To our knowledge, this is the first serious attempt to model such transient systems in detail. The particle is divided into moving zones, described by the reaction between limestone, O-2 and SO2, and each zone is assigned a certain catalytic activity with respect to each species involved. An existing particle model, the Grain-Micrograin Model, which simulates sulphur capture on calcined limestone under oxidizing conditions is considered in the modelling. Simulation results in good qualitative agreement with experimental data are obtained here for the catalytic chemistry of NH3 during simultaneous sulphur capture on a Stevns Chalk particle. The reduction of NO by NH3 over CaSO4 (which is the product of the reaction between SO2, O-2 and limestone) was found to be important because this reaction can, according to the model, explain to some extent the change in selectivity with increased solid conversion observed experimentally. Simulations also suggested that it may be advantageous with respect to the emission of NO to use small instead of big limestone particles for desulphurization in fluidized bed combustors due to the ways different sized particles capture SO2.