reduced by replacing some of the combustion air by pure oxygen less energy is lost in the flue gas and more useful heat becomes available another effect of oxygen enrichment is to increase the flame temperature and increase the heat transfer rate to the water cooled furnace walls this usually results is a lower flue gas temperature at the boiler exit the higher oxygen concentration combined with the higher combustion temperature also accelerates the rate of combustion allowing firing with a lower excess air while maintaining the concentration of unburned species low in a recovery boiler a higher temperature in the bottom part of the furnace has a positive effect on the smelt reduction efficiency because more energy is available for the endothermic reaction of converting of sulfate to sulfide application of the invention to a three air level recovery boiler to quantify the affect of oxygen enriched air combustion on a recovery boiler operation the case of a recovery boiler designed for 3 0 million lbs day of virgin dry solids operated at 3 3 million lbs day equipped with three levels of combustion air injection was considered such a boiler is schematically illustrated in fig1 for boiler 1 primary air 2 is introduced at level a secondary air 3 is introduced at level b and tertiary air 4 is introduced at level c liquor guns for black liquor injection are located at level d typically level b is located 4 to 6 ft above level a and level c is located about 10 ft above level d of the liquor guns the upper part of boiler 1 has a superheater section 5 a.