Hydrocarbon Oxidation at High Temperatures

Abstract

The detailed knowledge of combustion mechanisms is important, for example for the control of (kinetically determined) pollutant formation (e.g. NO, hydrocarbons, soot), for ignition problems, or for the extrapolation to technologically important but experimentally inaccessible condition. -In this review it is described how by suitable separation and elimination of unimportant reactions a mechanism is developed with the aid of the present kinetic data for the elementary reactions involved. This mechanism explains, without fitting, the currently available experimental results for laminar premixed flames of alkanes, alkenes and acetylene (flame velocity and structure of free flames, concentration and temperature profiles in burner-stabilized flames). These experimental results are simulated by the solution of the corresponding conservation equations with suitable models describing diffusion and heat conduction in the multicomponent mixture considered. -In lean and moderately rich flames the hydrocarbon is attacked by 0, H, and OH in the first step. These radicals are produced by the chain-branching steps of the oxyhydrogen reaction. The alkyl radicals formed in this way always decompose to smaller alkyl radicals by fast thermal elimination of alkenes. Only the relatively slow thermal decomposition of the smallest alkyl radicals (CH, and C,H,) competes with recombination and with oxidation reactions by 0 atoms and (I2. This part of the mechanism is rate-controlling in the combustion of alkanes and alkenes, and is therefore the reason for the similarity of all alkane and alkene flames. -In rich flames of aliphatic fuels, acetylene becomes a very important intermediate leading to soot precursors and to Non-Zeldovich-NO. Details of the reaction mechanisms are not yet known.

Kinetics of the oxidative coupling of methane at atmospheric pressure in the absence of catalyst

“…The importance of the oxygen atom as a chain-branching radical is well established in high-temperature combustion, e.g., the oxidation of hydrogen (Warnatz, 1981(Warnatz, , 1983). However, in contrast to this conclusion, the role of the oxygen atom as C2H4 + OH Q CH3 + CH2O a major chain-branching radical and thus as a major producer of the methyl radical has not been established from this study.…”

Extending the predictions of chemical mechanisms for hydrogen combustion: Comparison of predicted and measured flame temperatures in burner-stabilized, 1-D flames

“…One of the methods for testing the predictive power of chemical mechanisms (as well as for optimizing them [3]) has been to compare numerical and experimental determinations of the free-flame burning velocity of one-dimensional flames. One well-known limitation to this use of burning velocity data is that the rates of only a relatively small number of reactions have significant influence on the calculated burning velocity [4]. When considering a mechanism for a specific fuel, another limitation is that, in general, the only parameters varied for mechanistic purposes are the equivalence ratio, pressure and mixture temperature.…”

Ethylene From Natural Gas by Direct Catalytic Oxidation

“…A set of over 400 reactions, see Figure 7.11.1, could be reduced to the 164 reactions of Table 7.11.1 (Refs. 34,35,36,37,38,39,40,41,42,43,44) by omitting all the reactions which had no significant contribution to the product distribution. In this way, for example the formation of methanol and reactions in which a methyl dioxygen radical is involved were skipped.…”