© 2021 Elsevier Masson SASInfluence of fine particles collected in bag filter on spectral thermal radiation is investigated in the freeboard of a 0.3 MWt atmospheric bubbling fluidized bed combustion test rig where lignite, lignite/olive residue, and lignite/hazelnut shell blends are fired with limestone addition. For this purpose, a 3-D radiation code based on method of lines solution of discrete ordinates method coupled with banded one gas spectral line-based weighted sum of gray gases for spectral gas properties, Mie theory for spectral particle properties and spectral wall properties is utilized. The radiation code is implemented by considering first presence of combustion gases only, then combustion gases with bag filter particles (less than 14 μm) only, and then combustion gases with coarse cyclone and fine bag filter particles. The results of this study reveal that particle absorption and scattering are dominated by coarse cyclone particles during combustion of lignite only while they are dominated by bag filter particles during co-combustion of biomass and lignite owing to higher amount of fine particles originated from biomass combustion. During co-firing, contribution of cyclone particles to thermal radiation decreases while those of combustion gases and fine particles originated from biomass combustion increase.