A three-dimensional comprehensive furnace model capable of predicting gas flow, coal-combustion; and radiative heat transfer is described. The model has been validated using operational data from a tangentially coal fired 500-MW(e) boiler. The model validation and the results of two investigations which have been conducted by Pacific Power by applying the model to large coal-fired boilers are described. The results demonstrate that the model has practical application for both the solution of operational problems and the optimization of the combustion performance of utility furnaces. For a swirl burner wall-fired furnace the model was able to illustrate the aerodynamic mechanisms which contribute to burner throat ash deposition. A combustion optimization investigation on a tangentially fired furnace indicates that potentially significant improvements are possible via modification of boiler operating parameters.