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Show -2- INTRODUCTION Nitrogen Oxides (NOx) are one of six chemical species classified as criteria pollutants under the National Ambient Air Quality Standards (NAAQS) of the Clean Air Act. NOx in combination with Volatile Organic Compounds (VOC) present in the atmosphere can combine in the presence of sunlight to form ozone. Ozone has been found to be damaging to human health in concentrations as low as 0.1 ppmv. The EPA has recently reclassified areas of the United States for attainment of NAAQS standards. 185 areas were classified as non-attainment for ozone (Reference 1). Almost all combustion reactions produce NOx via one of two mechanisms. Thermal NOx is produced in high temperature flames from nitrogen present in the combustion air. Fuel or chemically bound NOx is produced from nitrogen compounds present in the fuel or waste materials combusted. Depending on the nitrogen concentration present, fuel or chemically bound NOx generation rates can be orders of magnitude greater than thermal NOx generation rates. EMISSION STANDARDS Both the federal government, through the 1990 Clean Air Act Amendments, and individual states are stringently regulating NOx emissions from combustion processes because of its adverse affect on human health. No regulations are more severe than those applied by the Southern California Air Quality Management District (SCAQMD). Two SCAQMD rules apply to NOx emissions from gas fired burners. Rule 1109 limits NOx emissions from boilers and process heaters with a rated heat input greater than 40 MM BTU/hr to a maximum of 0.03 Lb NOx/MM BTU. For boilers, steam generators, or process heaters with rated capacities less than 40 MM BTU/hr, NOx emissions are limited to 40 ppmv (O.05Ib/MM BTU). The SCAQMD also has the authority to reduce these limits even further based on Best Available Control Technology if new designs can demonstrate lower NOx emissions. THERMAL NOX FORMATION THEORY This paper describes a state-of-the-art burner designed to minimize thermal NOx generation when combusting fuel gases. The generally accepted mechanism of thermal NOx formation is described by the Zeldovich equilibrium reactions shown below. (Reference 2) (1) (2) + + 0* N* <--> <--> NO NO + N* + 0* The forward reaction rate constant for reaction number (2) is much larger than the corresponding rate constant for the forward reaction of equation (1). Therefore, a cursory analysis might conclude that reaction (2) is the dominant reaction producing NO. However, the concentrations of the species involved in the reactions must also be considered. The N* and 0* are produced by the thermal disassociation of N2 and O2 at elevated temperatures. Molecular nitrogen is thermally disassociated at a much slower rate than oxygen. This results in a large population of 0* atoms early in the reaction while the N* atom population remains relatively small. This high concentration of 0* relative to N* may be sufficient to offset the disparity in rate constants between reactions (1) and (2). |