This study is concerned with the prediction of the fluid-flow, chemical reactions and heat transfer processes in an industrial off-gas ducting system. A mathematical model is developed and then applied to predict the processes occurring in the off-gas ducting system. Particular attention is focussed on the two-phase thermal behaviour and the chemical reactions. A three-dimensional, two-phase numerical solution technique is used to solve the governing time-averaged partial differential equations. The model includes equations for turbulence, chemical reactions and two-phase thermal radiation. The calculations are performed for a particulate phase comprising non-reacting particles and a gaseous phase comprising chemically reacting gases. Both exothermic and endothermic reactions are considered. The effects of thermal radiation, particle solidification, chemical reactions and heat transfer on the two-phase flow are introduced and examined in detail. Predictions are made for an extensive range of parameters. The effects of these parameters on the off-gas ducting system are quantified. Comparisons are made between predicted results and experimental data when available and agreement is reasonable. The models developed can be easily incorporated into general-purpose fluid-flow packages. The procedure is general, and allows two-phase, two- or three-dimensional computations. Industrial plant can be modelled realistically on minicomputers at moderate costs. Convergence can normally be obtained with ease. It is concluded that for the cases studied, thermal radiation is a dominant factor in the calculation of the heat losses and that the particle contribution to these losses is small compared with that of the gases. The model indicates that the strongly temperature dependent reaction rates have a dominant influence in determining optimal operating conditions.