OCR Text |
Show nitrogen in the air can be viewed as being replaced by the diluent associated with the fuel mixture. Fig. (2) shows typically the concentration of carbon dioxide at equilibrium following the adiabatic combustion of various stoichiometric fuel mixtures of methane and carbon dioxide in air at constant pressure for different initial preheating mixture temperatures. It can be seen that for any initial temperature, since pure methane produces the highest adiabatic flame temperature the concentration of C 0 2 , with dissociation effects, will be very low for mixtures with high initial temperature. The concentration of carbon dioxide decreases virtually linearly with increasing the initial temperature as dissociation effects are increased in their intensity. For fuel mixtures at low preheating initial temperatures that contain initially very high proportions of carbon dioxide, the concentrations of the gas in the equilibrium products will be very high with only a relatively small contribution from the dissociation processes. This would indicate that at relatively low initial preheat mixture temperatures and with high carbon dioxide concentrations, much of the gas remains essentially intact to act largely as a diluent. This will also indicate correspondingly a reduction in the relative concentration of carbon monoxide in the products. The absolute value of the concentration of carbon monoxide in the products, however, is increased. Some kinetic considerations The combustion of methane-air mixtures involves the simultaneous interaction of numerous reaction steps, each with different reaction rates and reacting species. There are numerous kinetic scheme formulations for methane combustion varying in detail and complexity. These can be used in simple zero dimensional modelling of combustion systems, such as to provide useful indications of the autoignition process including the presence of carbon dioxide with the fuel or air. The net effect of these reactions will establish the characteristic parameters of the combustion process such as the rates of fuel and oxygen composition, the energy release rate and the production rates of the different reacting species, both stable and unstable. O n e commonly used simplifying approach is to consider the reaction rate on a global basis while ignoring details of the reaction activity of the mixture. The results of any relevant experimental observations can then be employed to produce an optimized relationship for an apparent single step reaction for the conversion of the fuel to products by combustion . The relevant apparent kinetic data, such as the empirical activation energy, order of reaction and rate constants are obtained through a best fit of the experimental data available. Such a general formulation can be presented typically as: d[CH4]/dt = k[CH4]a [02]b [Diluent]0 e"^7 (1) where [ ] is the molar concentration, often relates to that of the initial mixture, and k, a, b, c are constants. Since the actual combustion reaction cannot be represented universally by such a formulation, these constants will apply only to part of the combustion process and will vary significantly depending on the conditions under which the experimental data were obtained. For common diluents, including carbon dioxide, the index "c" of equation (1) tends to be generally much smaller than unity. Thus, the reaction rate under isothermal conditions will be affected only little by the diluent addition. It is the energy released by the reaction and the consequent temperature rise with time that will be affected and reduced very significantly by the increased presence of the diluents in the mixture. These factors are mainly responsible for the substantial reduction observed in the oxidation rates of combustion processes involving the presence of carbon dioxide. 3 |