OCR Text |
Show the H02 has a maximum near ~ = 0.9. Both of these features are the result of a combination of two factors, the high temperatures near stoichiometric favoring large radical (') 10 ...-4 4.0,------------------, * 3.0 ~ ..0... ..,.; () ct1 ~ 2.0 OH c..... -Q) 0 ::;E 1.0 .......... 0.4 0.6 0.8 1 1.2 1.4 1.6 Equi valence ratio Figure 3 Selected species concentrations at equilibrium, plotted as functions of equivalence ratio. concentrations, and the availability or lack of a atoms as equivalence ratio change. Note also that as equivalence ratio increases, the equilibrium levels of incompletely oxidized intermediate s'pecies such as H2 and CO increases dramatically. The concentrations of both H2 and CO exceed B% of the total mixture for ~ = 1.5 which is near the rich flammability limit for methane at atmospheric temperature and pressure. Neglecting these species in computations of adiabatic flame temperature would therefore be especially inaccurate for fuel-rich mixtures. It is particularly interesting to see the manner in which the equilibrium NO level varies with equivalence ratio, reaching its maximum value for ~ = 0.B5, in spite of the significantly lower temperature than at ~ = 1. This is a result of the significantly higher a atom and 02 concentrations at lean conditions than for richer mixtures. This suggests that NOx levels should be greatest for fuel-lean mixtures in most combustion systems burning methane or natural gas in air. However, the peak equilibrium values are only half of the story. The rate of formation of NO in high temperature flames depends strongly on the rate of the reaction a + N2 NO + N (I) The N atom then is available to produce more NO by the reaction N + 02 NO + a (2) 147 This sequence, termed the Zeldovich mechanism, proceeds very slowly at low temperatures, since a great deal of thermal energy is required to break the very strong N=N bond in N2. The rate of reaction (I) is given [BJ by k =7.6 x 1013 exp{-3BOOO/T) cm3mol- 1s-1 where T is the temperature in degrees Kelvin. This rate is nearly a factor of 10 greater for the temperature of about 2226K at ~ = 1.0 than for T = 1997K at ~ = O.B. Therefore, even though the equilibrium level of NO is more than 50% higher at ~ = O.B than at ~ = 1.0, the rate of attaining that equilibrium level is very much slower at ~ = O.B. For typical residence times in practical combustion systems, most often the best strategy for limiting NOx production is to reduce the equivalence ratio and thereby lower the rate of NO production. This is a good example of a situation in which the computation of the adiabatic flame temperature and the equilibrium composition actually provides a somewhat misleading result. Rather than rely on equilibrium considerations to limit NOx emissions, dynamic considerations are generally more important in predictions of NOx production. EXCESS OXYGEN - For the above computations, the oxidizer was "real" air in each case, in which the ratio of molecular nitrogen to oxygen was 3.76. A technique which can be used to increase the product temperatures is to add further molecular oxygen to the air. Since the addition of only oxygen will result in an overall lean mixture, this process usually includes addition of further fuel as well, keeping the ratio of fuel to oxygen fixed. To examine the influence of excess oxygen in the oxidizing gas, a series of adiabatic flame temperature computations was carried out. The fuel was assumed to be methane, and the ratio of methane to 02 was held fixed at 1:2 while the ratio of N2 to 02 was reduced from 3.76 to zero. The pressure is atmospheric, and the reactants are initially at room temperature. The results of these calculations are shown in Figures 4-6, plotted as functions of percent 02 in the oxidizer. Initially, the addition of more fuel and 02 produce a rapid increase in the adiabatic flame temperature from about 3500°F at normal air composition to almost 4500°F when the 02 fraction is doubled. After the 50% 02 point the increase in Tad is quite slow, indicating that the addition of more 02 is not very effective in producing higher temperatures. This type of curve, in combination with economic factors concerning the cost of 02 addition to the air stream, is easily used to estimate the optimum amount of 02 enrichment to be employed for a particular application. |