In this study, effect of changing biomass source on radiative heat transfer during co-firing of high-sulfur content lignite in the freeboard of 300 kWt Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) is investigated by using an in-house developed radiation code based on Method of Lines (MOL) solution of Discrete Ordinates Method (DOM). The freeboard is treated as a 3D rectangular enclosure containing gray, absorbing, emitting gas with absorbing, emitting, anisotropically scattering particles surrounded by black/gray diffuse walls. Radiative properties of participating gases are evaluated by using Leckner’s correlations and gray particle properties are calculated based on Planck’s distribution from the spectral Mie solutions. Input data required for the model are provided from six combustion tests which were previously carried out for Çan lignite with 14, 35 and 50% thermal shares of olive residue and hazelnut shells in the fuel mixture for the same Ca/S ratio. The results show that changing the biomass source affects the radiative properties of the particles in the freeboard through the change of particle size distribution rather than optical properties, which may lead to significant variations in radiative wall heat fluxes and source terms.