The turbulent flow field in large utility boiler furnaces is strongly affected hy heat release from combustion and by the heat extracted from the furnace through the boiler tube walls. To describe and predict these effects with reasonable engineering accuracy, a mathematical model termed COMpac has been developed at the University of Stuttgart. The series of codes is based on a finite difference solution of the turbulence-modelled Navier-Stokes equations in axisymmetric and in fully three-dimensonal coordinates. Radiative heat transfer is presently predicted using flux method approximatians. A simplified mechanism of pulverized coal combustion is described in a Eulerian framework for both the gaseous and particulate phase. In the present work, validation studies are reported and discussed. The combustion model has been tested for single enclosed flames where detailed measurements were available. Prediction results for three-dimensional furnace flows are compared with velocity data from an isothermal perspex model. Finally, the complete furnace code has been applied to predict a 300 MW brown coal fired furnace both under isothermal and fully reacting conditions.