OCR Text |
Show MOLECULAR RADICAL REDUCTION In the broadest sense, NSCR, SNCR, and SCR may all be considered radical reduction techniques. Radicals are generated passively in the post-combustion hot gas stream. NSCR depends on the radicals present in fuel-rich operation to reduce NO by reaction with CO (Rl). Application has been restricted to rich-burn engines, however, and is limited by catalyst poisoning and expense (Radian Corp., 1990). Selective reduction processes, which are used widely in Japan, depend on radicals of ammonia for the removal effectiveness. The strict temperature sensitivity of SNCR is due to the effect of the three primary reactions involved. Hot gas temperatures must be sufficient to drive the amidogen producing reactions (R2a,b) which feed the NO reducing process (R3). Temperatures slightly higher, however, will cause NO formation by reaction with 02 (R4). At temperatures significantly below optimum the dissociation of ammonia is not rapid enough to create suitable radical concentrations. The window of effective reduction temperatures is commonly defined as 1250 K ± 100 K, where 1250 K is considered optimum. The external approach to radical generation is designed to eliminate this low temperature limit and thereby increase the applicability and effectiveness of NO reduction. Rl) CO + R2a) NH3 + R2b) NH3 + R3) NH2 + R4) NH2 + NO OH-H-NO D2 => => => => => CD2 + 1/2 N2 NH2 + H2O NH2 + H2 N2 + H2O NO + H2O Radicals of other species, such as nitrogen, methane, and hydrogen have also been investigated. In work with low temperature nitrogen plasmas (Behbahani et al., 1982), NO reduction of up to 45 percent was achieved by direct reaction with the NO molecule (R5). It is important to note, however, that the NO was reduced in flows that were either virtually pure, at least 99.2 % NO in flow, or mixed with D2 or a "few percent" CCh. The rate of the N - atom reaction with D2 has been shown to be small compared with that of N- and NO at low temperature. The reaction of N- with CD2 in typical flue gas concentrations, however, becomes the limiting process which prohibits application of this particular technique ( Boyle et al., 1991). NO is formed by reaction of the nitrogen atom with CD2 (R6). Recent work performed in small scale laboratory tests (De Saro, 1991), with CD2 concentrations of approximately 13%, provided independent confinnation of this mechanism. R5) NO + R6) . N- + N- => CD2 => (). CO Molecular radical reduction, or MRR, involves the addition of external techniques which induce the generation of effective radicals and are therefore less dependent on the conditions of the post-combustion gases. Numerical and experimental investigations were completed to assess the viability and effectiveness of molecular radical injection techniques using radicals of ammonia, methane, hydrogen and nitrogen. Particular emphasis was placed on the utilization of externally induced ammonia radicals for NO reduction. Ammonia slip measurements were taken. NUMERICAL MODELING Chemical mechanisms have been developed for the study of ammonia and related radical producing species, such as urea, in typical combustion gases (Salimian and Hanson, 1980 and Miller, et al., 1981). These reaction sets and corresponding rates have 2 |