In this study, influence of limestone addition on radiative heat transfer during co-firing of high-sulfur content lignite with olive residue in the freeboard of 300 kW(t) 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 combustion gases and particles are evaluated by using Leckner's correlations and Mie theory, respectively. Input data required for the model are provided from four combustion tests which were previously carried out sequentially for lignite only, lignite with limestone addition followed by 30 and 50wt % shares of olive residue in the fuel mixture for the same Ca/S ratio. The results show that addition of limestone affects the radiative properties of the particles in the freeboard through the change of particle size distribution and load rather than optical properties and decreases the radiative wall heat fluxes along the freeboard. On the other hand, co-firing with biomass compensates this decrease as less limestone is utilized in almost direct proportion to the coal replacement.