Desulphurization by a calcium oxide particle cluster and an isolated calcium oxide particle in a circulating fluidized bed (CFB) combustor was numerically analyzed. The gas flow field, the sulphur retention and the nitrogen oxide emission of the cluster were predicted. Computed results showed that the SO(2), capture rate by a calcium oxide particle in the cluster is less than that of the isolated calcium oxide particle out of the cluster. The captured SO(2) and NO emissions decrease with the decrease of the cluster porosity. The maximum SO(2) capture rate by the cluster is at a temperature between 1025 and 1055 K, whereas more NO emissions were found with the increase of the gas temperature. The sulphur removal and the NO emission increase with the increase of the inlet gas velocity. (C) 2007 Elsevier Ltd. All rights reserved.