| OCR Text |
Show D. Marlow and T. S. Norton The greatest difference between the detailed mechanism and the skeletal mechanism is the neglect of the chemistry of C2 and higher hydrocarbons in the skeletal mechanism. The C2 chemistry has a secondary effect on NOx kinetics through the formation of soot and soot precursors, which lower the temperature and decrease NOx formation through the thermally sensitive Zeldovich mechanism. The only direct effect of C1 chemistry on NOx kinetics occurs through the reaction, HCCO + NO = HCNO + CO, which is an important path to reduce NO to HCN and N2 at lowtemperature, fuel-rich conditions. The reactions associated with N02 formation and destruction in the skeletal mechanism are also neglected since N01 emissions are insignificant in this study. For low-heating-value gas combustion at typical operating conditions, most of the NOx formation results from NH3 oxidation. The chief path for NH3 conversion is through the nitrogen atom. Ammonia is converted to N by reaction with H, 0, and OH. The resulting N then participates in the reactions of the extended Zeldovich mechanism. In the absence of ammonia, N is chiefly produced through the rate-limiting Zeldovich reaction: N2+O=N+NO Another less important but generally significant path from NH3 to NO is through lINO: NH3 ~ NH2" HNiJ-.. NO Again, this path occurs by reactions with 0, H, and OH. Ammonia can also play a very important role in reducing NO through the paths: NH2+Na..N2+· • • NH2 +Na..NNH+NO NNH+Na..N2 +HNO The remaining lINO is also reduced to molecular nitrogen through the path, HNa.. NH~ N" N2 All of these paths are brought about by reactions with 0, H, OH, and N-species. No hydrocarbon species (in fact, no carbon containing species) participates in any of the reaction pathways discussed thus far. When hydrocarbons are present, another important route to NO production and destruction occurs 4 |
