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Show NOx FORMATION MECHANISMS Nitrogen oxides are actually present in two chemical forms, nitric oxide (NO) and nitrogen dioxide (N02). At normal thermal oxidation temperatures nitric oxide predominates. However, when gases containing nitric oxide enter the atmosphere, the nitric oxide is converted to nitrogen dioxide as the gas cools. Therefore, NOx emission calculations usually assume all of the NOx is in the N02 form because of its transformation in the atmosphere. NOx is formed from one of three sources in a combustion process: fuel bound NOx, prompt NOx, and thermal NOx. Fuel bound NOx is generated from nitrogen compounds present in the waste or in the fuel itself. Generally, gaseous fuels such as natural gas or propane are free of nitrogen compounds. However, a significant amount of fuel bound NOx can be generated from liquid fuels such as fuel oils which can contain as much as 1 % of nitrogen by weight. Fuel or waste nitrogen compounds are only partially converted to the equivalent amount of NOx. The rate of conversion is much less than 1/1 in most cases. The exact conversion rate is a complex function of stoichiometry, temperature, and the specific nitrogen compound oxidized. However, for most compounds and conditions, the conversion rate is in the range of 20 - 70 %. A discussion of methods for minimizing NOx formation from thermal oxidation of wastes containing chemically bound nitrogen can be found elsewhere (Ref. 1). A lesser known type of NOx formation is termed "prompt NOx". Here, hydrocarbon radicals (CH, CH2, etc.) formed from fuel fragmentation react with nitrogen in the combustion air to form a hydrogen cyanide (HCN) intermediate. The HCN then reacts with oxygen and nitrogen in the combustion air to form nitrogen oxides as shown below. CH* + N2 < > HCN + N* HCN + OH* < > CN* + H20 CN* + 02 < > NO + CO The formation of prompt NOx is proportional to the number of carbon atoms present in the fuel and has a weak temperature dependence and a short lifetime. Prompt NOx is only significant in fuel rich flames which inherently produce low NOx levels. Thus, prompt NOx is usually a minor contributor to overall NOx emissions. V-30 3 |