Studies into the effects of operating conditions in a wall fired furnace using commercial code FLUENT is presented in this paper. The mathematical model is based on a Eulerian description for the continuum phase and a stochastic Lagrangian description for the coal particles. A 3-D combustor model is used to determine the temperature and heat flux profiles and other thermal characteristics for a typical 500 MW utility boiler firing medium volatile coal. Temperature profiles for a 100% boiler load as well as reduced boiler loads are predicted using the 3-D model. Particle trajectories are analyzed to identify the causes of operational problems such as fouling on burners, regions of high-unburnt carbon and temperatures achieved in various parts of the boiler during the combustion process. The effect of excess air on the furnace temperature pattern is also studied. Model calculations showed good agreement with experimental measurements in both full and pilot scale systems as well as limited literature data. Using the experience obtained by these CFD model studies can potentially improve the operation of a boiler, regarding stability and local material temperature of the walls, which lay a foundation for the boiler operating expert system. (C) 2006 Elsevier B.V. All rights reserved.