Scattering effects by ash polydispersion on spectral emission from the coal-fired MHD combustion gas are predicted numerically by means of the first (P 1 ) and third (P 3 ) spherical harmonics approximations of a radiative transfer equation in a rectangular cross section of the MHD power generation channel, with consideration of variation in optical properties across a boundary layer, particle diameter distribution of ash polydispersion and wavelength change of ash complex refractive index. Results of P 1 approximation agree well with those of P 3 approximation when an optical thickness is greater than about 2. Scattering effects can be neglected for a large optical thickness and a small scattering albedo, but precise calculation is needed to get the radiation intensity distribution when the absorption coefficient of combustion gas is of the same order of magnitude as the scattering of absorption coefficient of the ash polydispersion. The self-resonant absorption of potassium 4P-4S spectrum measured through an observation port in the presence of a cold boundary layer is not predicted by the P 1 approximation with an emitting and absorbing slag wall.