TO AVOID ABNORMALLY LOW TEMPERATURES AND FLOW RATES IN A SULFUR PLANT WHERE HIGH TURN-DOWN RATIOS ARE ENCOUNTERED IN THE RECOVERY OF FREE SULFUR FROM HYDROGEN SULFIDECONTAINING GASES, THE ACID GAS STREAM IS BURNED IN A SEPARATE FIRING TUNNEL WITH ENOUGH AIR TO CONVERT ONETHIRD OF THE HYDROGEN SULFIDE INTO SULFUR DIOXIDE. TO SUPPLEMENT THE HEAT REQUIREMENTS AND NECESSARY GAS VOLUMES, A HYDROCARBON FUEL IN AMOUNTS UP TO ABOUT 20 PERCENT OF THE DESIGN ACID GAS FEED RATE IS BURNED IN A SECOND FIRING TUNNEL. THE PRODUCTS OF COMBUSTION FROM THESE SEPARATE BURNING STEPS ARE COMBINED AND DISCHARGED INTO A BOILER WHERE A PORTION OF THE SULFUR PRODUCED IN THE OVERALL PROCESS IS REMOVED AFTER WHICH THE RESULTING SULFUR DENUDED STREAM IS PROCESSED IN CONVENTIONAL FASHION.