OCR Text |
Show Examination of details of the set of reactions that go on during the oxidation of methane at constant temperature will indicate that the presence of diluents with methane will not affect significantly the initiation reactions of the oxidation of methane. The presence of the diluents, however, can undermine somewhat the concentration of the pool of radicals available for propagating the reaction and encourages three bodied recombination reactions. These will lead to the slowing down of the overall conversion rate of the fuel and the associated energy release to an extent that will depend on the amount of the diluent involved, the temperature level and equivalence ratio. The use of a comprehensive kinetic scheme for the oxidation of methane [1,2] shows that the increased presence of carbon dioxide with the methane reduces expectedly the reaction rates for the same operating conditions. It would extend the time needed to ignition as well as the time needed to complete the combustion and achieve near equilibrium conditions. During the course of the reaction the concentrations of the various reacting species change very significantly before approaching their final concentrations. The presence of a significant amount of carbon dioxide with the methane affects both the values of such maximum concentrations as well as the times at which their peak values may be obtained . Under typical operating conditions of constant pressure, the commencement of the exothermic reactions will produce temperature increases that will have profound effects on the reaction rates and the concentrations of the reactive species. The reaction rates will accelerate very rapidly as the temperature increases. The time needed to complete the combustion process and approach equilibrium conditions will be reduced very appreciably. The concentrations of the products eventually will be those expected at the very high final temperatures reached. As shown for a typical set of conditions in Fig. (3), almost all the methane is consumed fully eventually as very high final temperatures are reached, even for mixtures containing very significant amounts of carbon dioxide, for the high initial mixture temperature employed. It can be noted that the reaction initiation time occupies a very substantial part, if not the bulk of the reaction time since much of the early part of ignition delay time was associated with relatively low temperature levels. The dominant role played by the changing mixture temperature is evident throughout. The calculated peak values for the concentrations of carbon monoxide and hydrogen during the adiabatic stoichiometric combustion at constant pressure with air of various fuel mixtures containing methane and carbon dioxide initially preheated to an initial temperature of 1000K, are shown typically in Fig.(4). The corresponding equilibrium values are also shown. Significant differences in the two values are evident throughout. For the carbon dioxide, the values of the equilibrium concentrations are the maximum concentrations that could be encountered. These are higher than their concentrations in the initial mixture prior to combustion. It is also evident that interrupting the full course of the combustion reactions can produce higher concentrations of carbon monoxide and lower of carbon dioxide over the whole range of carbon dioxide concentrations in the fuel mixture. The peak concentrations of carbon monoxide are not associated with fuel mixtures that contain very high concentrations of carbon dioxide with the methane. This is of much importance in relation to the production of exhaust emissions and reflects the important role of the resulting temperature of the reactive mixture. For rich mixtures, the initial part of the reaction appears to be associated predominantly with exothermic reactions that will result in temperatures that can exceed the eventual equilibrium values. These high temperatures, after the lapse of some time, will accelerate the rates of the reforming reactions that will result in the temperature of the adiabatic system dropping eventually to approach the final equilibrium value. Thus, for rich mixtures with diluents, it may be desirable not to prolong the combustion process well beyond the time when this peak 4 |