Combustion Characteristics of Fuels With Preheated Oxygen Enriched Air

Enriching air with oxygen in furnace combustion systems can significantly increase the thermal efficiency of the system and contribute to a savings in fuel and an increase in productivity. If economic feasibility is proven for a given application, the only other potential barrier to the use of oxygen enrichment in industrial combustion processes could be the increase in NOx formation due to the higher flame temperatures. The objectives of this work were to determine the maximum flame temperatures resulting from the combustion of various solid, liquid and gaseous fuels with preheated oxygen enriched air, to determine the maximum (equilibrium) levels of pollutant (NOx) concentrations and to predict actual levels of NOx that could be expected in typical industrial furnaces. The computer program considered thirteen dissociating species, preheat temperatures up to 2000F and oxygen enrichment levels varying from 21% to 50% oxygen. The results indicate that for natural gas above flame temperature of 5165°F, the equilibrium NO values tend to decrease with high levels of 3xygen and high preheat temperatures because other dissociating reactions predominate at these high temperatures. A methodology was developed, using kinetics, to predict the actual NO expected in a typical furnace. The predicted values of NO were compared with measured values for an experimental furnace and found to be in general agreement. Though there are no New Source Performance Standards for industrial furnaces, the analysis indicates that for combustion of nanrral gas with preheated oxygen enriched air, the NOx formed would be conSiderably more than the NSPS for an industrial steam generator of 0.2 lbs N02/million BTU.