Effect of recycle on radiative heat transfer in the freeboard of a fluidized bed combustor is investigated by applying a previously developed 3-D radiation model to the prediction of incident radiative heat fluxes along the freeboard walls of lignite-fired 0.3 MWt Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) Test Rig and comparing its predictions with measurements. Freeboard is treated as a rectangular enclosure containing gray, absorbing, emitting and isotropically scattering medium bounded by gray and diffuse walls. Radiative properties of the medium are calculated by using Leckner’s correlations for gas and Mie theory for polydisperse particle cloud. Radiative transfer equation for this system is solved by using Method of Lines (MOL) solution of Discrete Ordinates Method (DOM). Experimental data required for application and validation are generated from two runs in which parameters other than recycle ratio was held as nearly constant as possible. Comparisons between predicted incident radiative heat fluxes and measurements with and without recycle reveal that the agreement is excellent and that the effect of recycle on incident radiative heat fluxes is significant. A parametric study is also carried out to investigate the effect of particle load on fluxes. Predictions are found to be relatively insensitive to the particle load but strongly affected by the temperature profile.

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