A process for combusting sulfur and nitrogen containing carbonaceous materials in a two stage combustion process wherein sulfur and nitrogen containing carbonaceous materials and sulfur fixation agent particulates are introduced into a first combustion stage single fluidized bed having a density/size selective solids withdrawal conduit. The carbonaceous materials are combusted in the fluidized bed under substoichiometric oxygen conditions at temperatures of about 1600.degree. to about 2000.degree. F. producing ash and reducing gases forming a reducing region in a major portion of the single fluidized bed, the reducing gases comprising gaseous sulfur compounds. In a reducing region of the fluidized bed, the gaseous sulfur compounds are reacted with oxides of the sulfur fixation agent forming an intermediate solid metallic sulfur compound and nitrogen contained in the carbonaceous materials is converted to molecular nitrogen. The intermediate sulfur compound is reacted in an oxidizing region maintained at a base portion of the fluidized bed to form a stable, solid, environmentally acceptable sulfur containing compound which is withdrawn through the selective solids withdrawal conduit with agglomerated ash.