In this study, influence of spectral particle properties on radiative heat transfer in the freeboard/dilute zone of fluidized bed combustors (FBCs) is investigated. The aim is to identify how important it is to involve spectral particle properties and to determine the predictive accuracy of gray Mie and gray Geometric Optics (GOA) approximations in optically thin and thick media by benchmarking their predictions against spectral solutions. For that purpose, input data required for modelling radiative heat transfer and validating its predictions are provided from three combustion tests carried out in lignite fired 300 kWt Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) and 150 kWt Circulating Fluidized Bed Combustor (CFBC) test rigs. The results show that gray particle assumption leads to accurate radiative heat flux predictions with one order of magnitude less CPU time for both optically thin and thick media while it gives acceptable accuracy in source term predictions for only optically thin medium and the error becomes significant as the optical thickness increases. Assessment of GOA in FBCs, on the other hand, reveals that applicability limit of GOA should be based on cumulative cross sectional area distribution rather than surface mean diameter or cumulative weight distribution of particles. If the majority of cumulative cross sectional area is constituted by large particles which fall into geometric optics limit, gray GOA yields satisfactory results compared to spectral solution even if the medium is optically thick without going through cumbersome spectral calculations. (C) 2017 Elsevier Masson SAS. All rights reserved.