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Show CHEMICAL KINETICS MODELING FOR SYNTHETIC FUELS The work summarized in the preceding section makes it clear that a chemical kinetics data base is required for a detailed assessment of the combustion characteristics of synthetic (and indeed, conventional) fuels. However, such an assessment also demands an adequate kinetics model, since the details of the combustion of a realistic hydrocarbon fuel are far too complex to be approached by a straightforward application of elementary reaction kinetics. An extremely successful hydrocarbon fuel kinetics model is the quasiglobal model first proposed by Edelman and Fortune (Ref. 4) and described in further detail by Edelman and Harsha (Ref. 5). In this section we describe the application of this approach to the characterization of the kinetics of a conventional liquid hydrocarbon, No. 2 fuel oil, and a synthetic fuel, SRC-II Middle Distillate (MD). The Aromatic/Aliphatic Quasiglobal Kinetics Model The steps involved in the combustion of a complex hydrocarbon can be divided into several major processes. These processes include pure pyrolysis, which involves the decomposition of a complex fuel molecule into hydrocarbon fragments or intermediate fuels, and which can occur without the presence of oxygen; oxidative pyrolysis, in which the fuel molecule reacts with OH and CL to form intermediate fuel species, free hydrogen, and partially-oxidized fuel fragments; partial oxidation, in which the original fuel molecule and the intermediate fuel fragments oxidize to form partially reacted product species; and elementary steps to completion in which the partially reacted product species continue to oxidize, ultimately resulting in the overall products COp and HL0. Coupled to this overall pyrolysis/oxidation process are the reactions that result in soot formation and consumption (gasification) and NO emissions. In the latter case, several mechanisms are involved, including bound nitrogen and/or atmospheric nitrogen conversion in fuel-rich situations and thermal fixation. An analysis of this process for a realistic fuel using a detailed chemical kinetic mechanism would be all but impossible, even if all intermediate reactions and their rates were fully characterized, if for no other reason than the sheer number of species and reactions involved. Since, further, the possible intermediate reactions and their rates are not known, a much more realistic approach to the kinetics modeling problem is through the use of the quasiglobal model. This model provides sets of reactions, whose rate expressions are in general empirically derived, for each of the processes outlined above. The pure pyrolysis, oxidative pyrolysis, 1.2.10 |