OCR Text |
Show S o m e thermodynamic considerations The presence of carbon dioxide or other diluents with the methane reduces the heating value of the fuel mixture. The energy released due to oxidation reactions of the fuel component will be shared with the diluent. The fraction of this energy release taken by the diluent will increase as the temperature is increased since the specific heats of carbon dioxide increase with temperature at rates much greater than those for nitrogen or air. The carbon dioxide will tend also to undergo increasingly highly endothermic dissociation reactions that increase in intensity as high combustion temperatures are approached. This will also tend to modify significantly and somewhat adversely the composition of the final products of combustion. The net effect is the substantial reduction of the adiabatic flame temperature through the presence of carbon dioxide with the methane, as shown in Fig. (1). The relative reduction is especially significant for near stoichiometric mixtures where the energy release and consequently the resulting temperature is highest. It can be seen further that for the lean or rich mixtures containing substantial concentrations of carbon dioxide, the calculated peak temperature will be so low that it may not permit flame propagation and combustion to proceed in practice, even in the presence of a source with a substantial ignition energy. Carbon dioxide expectedly is more effective in reducing the calculated final combustion temperature than water vapour or nitrogen with methane. 2400 METHANE ANO CARBON OIOXIOE To -300K 0.5 LO L5 2J0 2_5 3J0 EQUIVALENCE RATIO ^ (CH4+C02*Air.Eq.R-1.P-1atm) 35.0 30.0 2S.0 20.0 15.0 10.0 5.0 0.0 20%CH4 adia.constP reactor 100% 800 1000 1200 1400 1600 1800 2000 initial temperature (K) Fig. 1 Variations of the calculated adiabatic flame temperature at equilibrium with equivalence ratio of methane and carbon dioxide in air at atmospheric pressure and initial temperature of 300K. Fig. 2 Variations of the volumetric concentration of carbon dioxide at equilibrium for stoichiometric mixtures with initial temperature for various concentrations of methane in methane - carbon dioxide mixtures at atmospheric pressure. It is clear that the heating value of the fuel mixture and hence the concentration of the diluents, is not the sole factor that would establish the combustibility of the fuel mixture in air. The types of fuel and diluents need to be considered also. For example, a fuel mixture of methane-nitrogen would be more likely to permit combustion than a similar mixture of methane and carbon dioxide having the same heating value. Moreover, it follows that the use of oxygenated air will enable the combustion of fuel mixtures containing higher concentrations of carbon dioxide. The displaced 2 |