Ash is produced in combustion of biomass. Sonic part of this matter is called fly ash and is carried by the flow and causes not only air pollution and erosion, but also call affect the thermal radiation. The effects of fly ash particles oil the thermal radiation are considered ill this investigation. By analyzing sampled data in all electrostatic filter, a realistic particle size distribution is found. Although the optical data oil biomass fly ash are not available, however, similarity between coal and biomass ash compositions showed that the optical constants of the low Fc coal fly ash can be applied for file biomass fly ash. The Mie theory is used to predict scattering and absorption coefficients and phase function. The mean Planck scattering and absorption coefficients and phase function are predicted by averaging over the particle size distribution and Planck function, respectively. The effects of fly ash particles oil thermal radiation are evaluated by a three-dimensional test case. It is assumed that the medium is a mixture of non-grey gases and different level of particle loading. Predicted results from the test case showed that the fly ash can be influential oil the thermal radiation. In addition, in selected fly ash volume fractions, the effect of scattering by particles is not so important oil the radiative heat Source and radiative heat flux to the wall whereas their absorption effect is important and call increase the radiative heat Source and wall heat fluxes. (C) 2007 Elsevier Ltd. All rights reserved.