A process of producing a combustible gas from a solid sulfur-containing carbonaceous fuel is provided. In the process, an aqueous solution is provided. A solute of the solution is a carbonate salt of an alkali metal. Particles of the fuel and the aqueous solution are mixed (26) to form a mixture. The mixture is fed into a gasifier (22) that contains molten salts of the alkali metal. The fuel is partially combusted in the gasifier to produce the combustible gas. At least a portion of the carbonate salt in the aqueous solution may be recovered (24) from a molten sulfide salt. The molten sulfide salt may be taken from the molten salts in the gasifier (22).