Surface emissivity is an important property of solid surfaces and influences the radiant heat transfer within furnaces, boilers etc. Emissivity depends on the surface roughness and colour. In the case of an ash deposit, surface roughness is influenced by the surface conditions, i.e. whether deposits are particulate, sintered, or fused, and this is determined by the temperature of the deposit. This, together with the fact that the composition of the deposit varies largely with the fuel composition, makes the prediction of ash deposits’ surface emissivity a very complex task.

This “Short Communication” was submitted to the IFRF Online Combustion Journal in response to the publication of the Combustion File 149 (CF149) published in IFRF Online Combustion Handbook. Experimental data of the emissivity of coal ash were presented and these results were compared to the correlation provided in CF149.

Since correlations, given in CF149, were based on experiments, it can be concluded that the surface emissivity data available in the literature are inconsistent. It is clear that additional experiments, performed on the ash samples, using different particle sizes and surface conditions, as well as composition, are needed to obtain satisfactory calculations.

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