Comparison of the numerical investigation results was carried out when the simple gray gas (SGG) and weighted sum of gray gases (WSGG) models are used to model the radiative properties of the gas phase inside the lignite fired furnaces. Comprehensive mathematical model of the tangentially fired furnace by pulverized lignite was made. Gas radiative properties were modeled by the SGG and WSGG models. Radiative heat transfer was modeled by the zonal model. Gas-phase variables and absorbed wall fluxes were compared on the basis of the relative differences that were determined for all control volumes and surface zones. Average relative differences of the gas-phase temperatures were about 1.0%. Average relative differences of the absorbed wall fluxes were from 2.0% to 5.0%. Absorbed wall fluxes determined by the SGG model were bigger than those determined by the WSGG model. Differences of the heat transfer rates of the absorbed radiation through the furnace walls were expressed in percents of heat transfer rates determined by the SGG model and were similar to the average relative differences of absorbed wall fluxes. Results justify application of the SGG model in comprehensive mathematical models of lignite-fired furnaces. (C) 2012 Elsevier Ltd. All rights reserved.