Based on the analysis of measured data on flow rates, concentrations and temperatures taken during steady state operation of a lignite-fired 0.3 MW, Atmospheric Fluidized Bed Combustor (AFBC) test rig, radiative exchange in freeboard of the combustor was modeled by using a well-stirred enclosure model in conjunction with Radiosity-Irradiation Method (RIM). Radiative properties of the particle laden combustion gases were calculated by assuming grey radiation behaviour for both particles and gas, and using Leckner's correlations for gas and Mie theory for particles. The accuracy of the model was tested by comparing its predictions with incident radiative heat fluxes measured on the freeboard wall. Comparisons between the incident radiative heat fluxes predicted by the present model and those measured show that this engineering approach can be used with confidence by designers for the particle loadings typically encountered in freeboards of bubbling fluidized bed combustors.