In a previously developed system model, bed and freeboard models were coupled for the continuous combustion of lignite particles of widely varied size distributions, burning in their own ash in a fluidised combustor. This was modified to incorporate a) a procedure for faster computation of particle-size distributions (PSDs) without any sacrifice of accuracy; b) an energy balance on char particles, for determining temperature variation with particle size; and c) assumption of plug flow for the interstitial gas. The improved computer code was evaluated by being applied to prediction of the behaviour of a pilot-scale fluidised-bed combustor, followed by comparison of its predictions with measurements and also with the results predicted by the previous code. The computer code replaces the conventional numerical integration of the analytical solution of population balance with direct integration in ODE form, by using a powerful integrator LSODE; this results in a reduction of CPU time by two orders of magnitude. For the prediction of physically expected variation of char hold-up with excess air, an energy balance on char particles must be incorporated into the system model. Comparison of the predicted and measured temperature and concentration profiles shows that the present code produces better agreement than the previous one in bed-concentration profiles.