This study proposes a simplified methodology to determine coal ash emissivity for the thermal design of coal-fired boilers. For this purpose, the Konopelko equation expressing spectral emissivities in relation to the wavelength and iron content in ash was modified. Spectral emissivity of coal ash is known to have a positive wavelength dependence; it is low at short wavelengths and increases with a rise in wavelengths which then remains constant (epsilon(max)) over a certain wavelength (lambda(max)). In the modified equation, input parameters are epsilon(max), lambda(max) and spectral emissivity in the shorter wavelength: 1.6 mu m(epsilon(1.6(T))). Based on spectral emissivity data from early studies, we determined epsilon(max), lambda(max) as 0.85 and 8 mu m, respectively. To verify our modification, we compared total emissivities obtained by calculated spectral emissivities with those obtained by measured spectral emissivity for several literature. The modified equation predicts 90% of data records with an error of 15%. Additionally, to obtain epsilon(1.6(T)) without measurements, we investigated dependences of iron content in ash and temperature on epsilon(1.6(T)) using a shield tube emissometer. With reflecting these results, our equation requires only iron content in ash to obtain spectral emissivity of coal ashes. This simplified equation determines total emissivities obtained in different studies with an error of almost 20%. (C) 2011 Elsevier Ltd. All rights reserved.