An emissions model has been developed by coupling the fluid dynamics of a bubbling fluidised-bed combustor with the 8-reactions heterogeneous and 340-reversible-reactions homogeneous chemistries. The model is used to explain the observed emission trends of nitric and nitrous oxides as a function of bed temperature, excess air and operating pressure. Reduction of nitrogen oxides in the bed results in only a small fraction of the nitrogen oxides formed by coal oxidation being emitted from the bed. At low temperatures the concentration of CO in the bed is higher than at high temperatures; this enhances NO reduction via the catalytic NO/CO destruction reaction. Increases in N2O destruction rates at high temperatures lead to reductions in its emissions. As the excess air ratio is raised, the bed char loading falls, with a consequent fall in the reduction of nitrogen oxides over bed char and higher emissions. At elevated operating pressures both the char loading and the residence time of gases in the bed govern the emissions. The emissions of nitric oxide fall inversely with pressure, whereas the emissions of nitrous oxide, a product formed by NO destruction on char, pass through a maximum. Emissions of nitrogen oxides can be explained in terms of a competition between the formation and destruction reactions. This study shows that the destruction reactions control the net emissions of nitrogen oxides. In turn, this dominance of the destruction reactions over formation suggests that the optimal operating conditions should be evaluated in order to maximise the destruction of nitrogen oxides within the bed.