A hybrid technique has been developed for solving three-dimensional spectral radiation transport equations for absorbing, emitting and anisotropically scattering media. An optimal mix of computational speed and accuracy is obtained by combining the discrete ordinate method (S4), modified differential approximation (MDA) and P1 approximation for use in different ranges of optical thickness. The technique is used in conjunction with a char burnout model and spectroscopic data for H2O, CO2, CO, char, soot and ash, to determine the influence of ash composition, ash content and coal preparation on furnace heat-absorption.