PURPOSE: To obtain a gasification furnace gas containing sulfur in the form of H2S in place of SO2 and to enable the desulfurization of the gas with a conventional desulfurization material, by completely burning a fuel in the preheating stage of a coal gasification furnace and changing the fuel to coal in a state of reducing combustion of said fuel.

CONSTITUTION: A fuel having low sulfur-content is subjected to complete combustion in a stage to preheat a coal gasification furnace to a prescribed temperature. Prior to changing the fuel from preheating fuel to coal, the preheating fuel is brought to a reducing combustion state and the fuel is changed to coal in that state.

EFFECT: The process can eliminate an apparatus which is used only in the start-up and shut-down of a plant such as flue gas desulfurization apparatus.