A waste heat recovery boiler utilising plant off-gas was numerically modelled, using the code CFDS-FLOW3D (1994), as a part of a consulting study undertaken by the Cooperative Centre for New Technologies For Power Generation From Low-Rank Coal (CRC). The primary aim of the work was to estimate temperatures of char and the dust within the boiler to determine the likelihood of deposition onto the boiler walls. Difficulties in achieving convergence for this problem, because of its complex shape and multiple blocking, were overcome by adopting a series of solution strategies. This paper covers the results of simulating operation in which only plant off-gas and char are combusted (Mode 1) and operation in which natural gas and melter gas are injected (Mode 3). For Mode 1 the model predicted good mixing in the region between the primary and secondary air jets and a nearly 100% char burnout. From the numerically determined particle tracks, most of the char and primary concentrate particles hitting the walls are cooler than the respective critical temperatures identified for sticking at the boiler walls. Based on Flamelet modelling, combustion essentially takes place at and below the primary air jet plane. For Mode 3, the maximum temperature predicted agrees with the expected maximum adiabatic temperature for the natural gas combustion.