Operation of a multiple solids fluidized-bed black liquor combustor employing only combustion residue solids as both fine and coarse bed solids is disclosed. Under normal operating conditions, a solids distribution of fine particle sizes and coarse particle sizes with a minimal population of intermediate particle sizes is maintained. Heat and chemical values are readily recovered from the black liquor and the necessity for separation of the combustion residue salts from inert fluidized-bed solids is avoided.