Simultaneous solution of the conservation equations for energy and chemical species in conjunction with radiative transfer equation was carried out by coupling a previously developed and tested system model of fluidized bed combustion (FBC) to an existing radiation model. The predictive accuracy of the coupled code was assessed by applying it to 0.3 MWt METU Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) Test Rig burning lignite in its own ash and comparing its predictions with the measured temperatures and concentrations of gaseous species along the combustor and radiative heat fluxes incident on the refractory-lined freeboard walls on two combustion tests, with and without recycle. The predictions of the coupled code were found to be in good agreement with the measurements. For the investigation of the significance of coupling of the radiation model to the system model, temperature predictions of the coupled code were compared with those obtained by the original system model. It was found that the effect of incorporating a radiation model into the system model on the predictions was not significant because the high temperatures of refractory-lined freeboard walls and high surface to volume ratio of the test rig under consideration cause the incident radiative heat fluxes to be dominated by walls rather than the particle laden gas emissions. However, in industrial boilers, freeboard is surrounded by water-cooled membrane walls and boilers have much lower surface to volume ratio. In order to examine the effects of both on radiation in industrial boilers, an investigation was carried out on 16 MWt Stationary Fluidized Bed Boiler (SFBB) by applying radiation model, in isolation from the system model, to the freeboard of the boiler. It was found that in the boiler, incident radiative heat fluxes were dominated by particle laden gas emissions. In brief, the coupled code proposed in this study proves to be a useful tool in qualitatively and quantitatively simulating the processes taking place in an atmospheric fluidized bed boilers.