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Show 1.6.2 L INTRODUCTION Since its discovery by Lyon CI], the possibility of using NH3 for the noncatalytic reduction of NO in combustion effluents has received a great deal of attention [2-81 The process, often referred to as "Thermal De-NOx", holds promise for application to utility boilers. With suitable modifications such as the addition of H^o, it also can provide some reduction of NOx in the exhaust gases from gas turbines [63. The level to which NO is reduced by reaction with NH3 is highly dependent on temperature. In the absence of OH-producing additives such as H2 or H202, the final NO-concentration in oxygen-rich combustion products has a narrow dip near 1230 K (1755 °F> [2-43. Below this temperature, the overall reaction is too slow to be completed within the time available. As the temperature is increased above 1230 K, reactions resulting in the formation of NO from NH3 dominate reactions leading to the destruction of NO by NH3. Various analyses have been presented of the chemical kinetic processes involved with the reduction of NO by NH3 [9-193. The processes may be different in the oxygen-rich and the oxygen-lean cases. In this paper, attention is given only to the oxygen-rich case, corresponding to the Thermal De-NOx process following injection of NH3 into postflame gases generated by fuel-lean combustion. For this case, Lyon and Benn [103 proposed a mechanism consisting of 9 different steps. Together, these steps account for chain initiation, chain propagation and branching, chain termination and self inhibition. Not all of the steps are defined in terms of chemical reactions. Furthermore, the mechanism does not provide an explanation for the oxidation of NH3 to NO that occurs concurrently with the reduction of NO by NH3. Seery and Zabielski [113 proposed that the main features of the reduction and formation processes are governed by the reactions NHj+ NO -> N2 • NHj + R -> NO + ..-. NH3 • R -> NHj • ..... with i = 0, 1 or 2, and R being an oxygen containing species (02, OH or 0). Fenimore [123 suggested that 1 = 2 and that R = OH: NH2 • NO -> N2 * H20 (i) NH2 + OH -> -> NO (2) NH3 • OH -> NH2 «• H20 (3) Fenimore [123 also noted that this model implies the existence of a critical concentration [N03c, below which [N03 cannot be reduced. The critical concentration obtains when the rate of NO destruction by reaction (1) just balances the rate of NO formation by reaction |
