In this study, banded one gas spectral line-based weighted sum of gray gases (banded SLW-1) model is coupled with a 3-D radiation code based on method of lines (MOL) solution of discrete ordinates method (DOM) for freeboard of METU 0.3 MWt atmospheric bubbling fluidized bed combustion (ABFBC) test rig containing non-gray gas, non-gray particle mixture bounded by non-gray walls. Spectral parameters of banded SLW-1 are estimated by the approach based on two emissivities calculated at two different path lengths L-1 and L-2 . The predictive accuracy of banded SLW-1 model is tested by comparing its incident wall heat flux and source term predictions with measurements and predictions of banded SLW. The results indicate that band-wise selection of L-1 and L-2 based on spectrally averaged mean beam length provides the most accurate radiative heat transfer predictions. Furthermore, based on a sensitivity analysis on the test rig under consideration, utilization of path lengths L-1 and L-2 which are 0.10 and 1.10 times of the spectrally dependent mean beam length is found to yield the best radiative heat transfer predictions in terms of accuracy. Incident heat flux and source term predictions of banded SLW-1 are found to be in reasonable agreement with those of banded SLW under both air and oxy-fired conditions. Furthermore, CPU time requirement of banded SLW-1 is about 20-25 times lower than that of banded SLW. With the introduction of fly ash recycling, particle load increases an order of magnitude leading to further improvement in the accuracy of banded SLW-1. On the other hand, gray wall assumption leads to accurate radiative heat flux predictions whereas this assumption leads to considerable inaccuracies in the source term predictions in the upper region of freeboard. However, those inaccuracies become insignificant with the introduction of fly ash recycling. Application of the model to different wall conditions common in the industry indicates that discrepancies between the source term predictions of gray and non-gray cold water wall and those of gray and non-gray cold slag covered water wall are insignificant for the combustion test rig under consideration which is attributed to the combined effects of low wall temperatures and similar gray and non-gray wall emissivities. (C) 2020 Elsevier Ltd. All rights reserved.