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Show 2. NOx FORMATION IN PULVERIZED COAL FLAMES A) Theory During the past twenty years numerous researchers have investigated the formation and destruction of NO from pulverized coal flames. The key controlling principles include the following: • Fuel bound nitrogen is exposed to combustion processes in two forms: nitrogen which evolved with the volatile matter upon devo latilizat ion , and nitrogen remaining fixed in the structure of the char after devolatilization has occurred. The fuel nitrogen split between volatile matter and char nitrogen is very fuel specific and dependent on other operational parameters. • Devolatilization/heat up rates largely influence the amount of fuel nitrogen release in the near-burner zone. These rates can also be strongly influenced by local oxygen concentrations. • Conversion of the volatile matter nitrogen to NO is strongly dependent on the availability of oxygen, and therefore can be controlled by staging the combustion process. • Char nitrogen conversion to NO is typically low, and relatively unaffected by burner design parameters. • Furnace temperature history has a predominate influence on thermal NO formation, however, fuel and prompt NO are also significantly affected by the local gas temperature. It is a well-established fact that both volatile and char released nitrogen contribute to the fuel NO production. In a pulverized coal flame, fuel nitrogen (both volatile and char nitrogen) evolves as the smallest possible nitrogen hydrocarbon compound, HeN. The nitrogen is then available for reaction with the local combustion environment. The ultimate process is dependent on the availability of oxygen. That is, in the presence of oxygen, NO is formed; but in the absence of oxygen, the formation of molecular nitrogen is favored. Under rich conditions, the volatile nitrogen compounds are converted to HCN, NH3, NO, and N2. The nitrogen speciation and the fraction converted to NO is dependent on temperature, gas phase stoichiometry, and coal type. For char NO production, the most accepted theory proposes that homogeneous and heterogeneous NO reductions occur within the pores of the char particle (or on the particle surface) and this is one reason that char nitrogen conversion to NO is less than the conversion of the volatile fuel nitrogen fraction (Eddings, et aI., 1994). Under staged combustion conditions, the retention of fuel nitrogen in the solid phase char reduces the effectiveness of staging. Nitrogen bound in the char structure is liberated into the gas phases only as the residual carbon in the char is burned. In order for the char combustion to approach completion, combustion must take place in an oxygen rich environment. Therefore, char nitrogen is, of necessity, liberated in an excess oxygen environment. Despite this seemingly high potential for NO production, 4 |