Fixed bed boilers are widely used to fire coal for heat and power production. However, these systems are often reported to have relatively high unburnout, low efficiency and high emissions, in addition to needing to be optimized and modernized. In this work, the measurements were performed in a test rig, where a combustion front propagates against the airflow. The author decided to use function to describe the simplified profile of emissions. The results obtained from measurements were converted in the form of approximation functions. Furthermore, developed functions were applied for defining the boundary conditions at the interface between the fixed bed and gas phase in FLUENT. The simulations of a domestic boiler have been done and the relative effects of different factors in CFD were evaluated by sensitivity analysis. The validity of the model was verified by measurements which were done in a 25 kW domestic boiler. Model predictions were compared with the experimental gas temperature and species concentration measurements, which were in good agreement. However, at a few measuring positions larger differences between the numerical results and the experiments are still observed.