© 2022 Elsevier LtdA previously developed 1-D comprehensive model for a bubbling fluidized bed combustor (BFBC) co-firing lignite with cotton residue and limestone is coupled with a 3-D spectral and gray radiation models utilizing method of lines (MOL) solution of discrete ordinates method (DOM). The performance of the coupled models is assessed by comparing their predictions of temperature profiles and gaseous emissions (i) with those of original FBC model without coupling and (ii) with the in-situ measurements from Middle East Technical University (METU) 0.3 MWt BFBC. Comparisons between fluidized bed combustion (FBC) system models with and without radiation reveal that the spectral model improves the predictions only slightly. This is considered to be due to the very small temperature difference between the freeboard medium and hot refractory side walls which results in lower rates of heat transfer from the freeboard to the walls. Furthermore, radiative heat transfer in the freeboard is dominated both by combustion gases and particles.