The subject of this work is the modeling of recovery boilers. A three-dimensional, steady-state computer simulation of the flow and combustion process in a recovery boiler is presented here. The numerical algorithm of the original code was modified and extended to cater for certain special aspects of recovery boilers, such as the in-flight combustion of black liquor droplets and char bed burning. To represent the fine structure of numerous air ports, a higher level of grid topology than in the conventional, structured grid system is required. In this work, a highly flexible, unstructured and mixed cell topology grid with local mesh refinement was employed. A number of computations were carried out using various firing arrangements including the ROTAFIRE system. The results show considerable potential and reliability in the use of computer simulations for the improvements and optimization of recovery boilers.