Investigation on the effects of fly ash particles on the thermal radiation in biomass fired boilers

Abstract

Ash is produced in combustion of biomass. Some part of this matter is called fly ash and is carried by the flow and causes not only air pollution and erosion, but also can affect the thermal radiation. The effects of fly ash particles on the thermal radiation are considered in this investigation. By analyzing sampled data in an electrostatic filter, a realistic particle size distribution is found. Although the optical data on biomass fly ash are not available, however, similarity between coal and biomass ash compositions showed that the optical constants of the low Fe coal fly ash can be applied for the 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 on 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 on the thermal radiation. In addition, in selected fly ash volume fractions, the effect of scattering by particles is not so important on the radiative heat source and radiative heat flux to the wall whereas their absorption effect is important and can increase the radiative heat source and wall heat fluxes.