Abstract: There is always a danger of slagging on and/or corrosion of the tubes of the highest temperature superheater bank and, if present, reheater bank when burning heavy bunker fuel oil. Therefore a clean fuel burning furnace is provided which is separate from the heavy bunker fuel oil burning furnace and only the combustion gases from burning the clean fuel are used to heat the highest temperature superheater bank and, if present, reheater bank. The clean fuel is preferably provided by a gasifier in which a portion of the heavy bunker fuel oil is pre-combusted to give a clean combustible fuel gas.