This paper describes the modifications and evolution of a finite-difference solution procedure for predicting the aero and particle dynamics of a 500 kW coal-fired non-slagging vertical cyclone combustor incorporating a fragmentation model and modified volatile burning law. Validation of the procedure was carried out from experimental data from a prototype firing natural gas, where an algebraic stress model of turbulence was needed to model cyclonic flows. The fragmentation model was based on experimental work indicating that fragmentation occurred early in the unit and at 80% of the coal diameter. The volatile-burning law was modified to allow for large amounts of soot, tars and higher hydrocarbons formed by gasification within the unit. The programme investigated wall ricochets/impacts of the burning coal particles and found that fragmentation after four ricochets for particles d greater than or equal to 50 mu m correlates well with the original 80%-diameter fragmentation concept. Investigation of scale, inlet velocity and chamber-wall roughness show that inlet velocities of 36 m s(-1) appear optimum for all sizes of unit; carbon burnout increases with scale, and the effect of chamber-wall roughness is reduced with scale.