Abstract: A sulfur bearing carbonaceous fuel, particularly a low BTU gaseous fuel, is burned in a deficiency of air in the combustion zone of a boiler to form a reducing gas containing H.sub.2 and CO in an amount in excess of that required to reduce the generated SO.sub.x to H.sub.2 S and COS. The oxides of sulfur and nitrogen are, in part, converted to H.sub.2 S, N.sub.2 and/or NH.sub.3 during heat transfer in the boiler. The balance is passed through a catalyst chamber where sulfur species are converted to H.sub.2 S and oxides of nitrogen to N.sub.2 and/or NH.sub.3. The formed H.sub.2 S is extracted from the flue gas prior to venting to the atmosphere and recovered as free sulfur. Catalytic conversion is carried out at a temperature from about 300.degree. to 800.degree. F.