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Show volume. Another benefit is that the exhaust gases become more concentrated by the removal of N 2 from the oxidizer. This usually makes post gas treatment equipment more efficient as it is easier to treat more concentrated streams. The curve shows a rapid initial decline in the flue gas volume with a much slower decline as the 0 2 purity increases near 100%). Again, this suggests there may be economic advantages to using lower purity oxidizers, between air and pure 02. N Ox As is well known, thermal NOx is exponentially dependent on the gas temperature. Glassman [9] gives a simplified equation for the maximum rate of formation of N O due to the Zeldovich mechanism: _^_]_4xl0>V^[olN2] (3) From this it can be seen that thermal NO formation is strongly dependent on temperature T, atom concentration [O], and molecular nitrogen concentration [N2]. MacKinnon [10] developed an empirical formula for N O formation: d[NO](. ppm -68180 „ linS=3-82xl°15e ' V « <4> and a similar equation for the concentration of the NO formed: -72380 [NO](inppm) = 5.2xl017/_ T P02 l/2Pm (5) where / is the residence time in seconds, T is the absolute temperature(K), and P02, Pm are partial pressures of oxygen and nitrogen, respectively. Figure 8 shows the theoretically predicted amount of NO resulting from the stoichiometric, adiabatic combustion of ambient temperature and pressure C H 4 with an oxidizer consisting of various concentrations of oxygen (balance N2): CH4 + 202 + xN2 <-> C02, 2H20, N2, NO, etc. (6) The curve shows two competing effects. One effect is the reduction in the nitrogen concentration as the oxidizer purity increases. The other effect is the increase in adiabatic flame temperature (see Figure 5) as the oxidizer purity increases. Reducing the nitrogen concentration reduces N O while increasing the temperature increases N O , as shown in eq. (5). The competition between these two effects causes the curve values to maximize at about 4 5 % 0 2 in the oxidizer. As the oxidizer purity increases from 21%), there is a rapid rise in N O due to the dominance of the increase in the flame temperature, even though the N 2 concentration is decreasing. Beyond about 4 5 % 02 in the oxidizer, the decreasing N 2 concentration becomes dominant, even though the adiabatic flame temperature continues to rise to its peak at 100% 02. It was shown in the 7 |