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Show 8.4. Behaviour of the N O x emissions 8.4.1. Con version to oth er bases We have herein reported the exhaust-gas NOx levels "as measured", in terms of the mole ratio Xm , expressed in p p m (i.e., the mole fraction, dry basis, in ppm). For our natural gas, burning with 15 % excess air (about X0 = 3 % ) , 1 p p m of N O x is equivalent to an emission index, EINOx, of 21.4 m g N O per kg of fuel, or an emission of 0.448 m g N O per M J combustion heat release. 8.4.2. Causes of the low levels of the emissions The very low level of NOx emission seen with the CGRI burner is undoubtedly due in large measure to the admixture of large quantities of recirculating combustion products that have been significantly cooled, relative to adiabatic combustion conditions, by furnace heat transfer. A s a result, the maximum temperatures reached in the combustion zone are considerably below those that would otherwise be attained, and thermal N O x formation reactions, at least, are correspondingly suppressed. This mechanism explains the fairly strong sensitivity of the emissions to the temperature level of the recirculating gases as indicated by the exhaust gas temperature, Fig. 8. By contrast, Fig. 8, a commercial burner of an earlier low-NOx design gives far higher levels of emission, and the emissions are much less sensitive to exhaust-gas temperature level. The explanation is that the fuel and air streams in this burner are largely shielded, by a substantial burner quarl and by aerodynamics, from entraining cooled products before they mix. Figure 8 demonstrates that the N O x emissions are well below chemical equilibrium levels. This satisfies one of the assumptions of the simplified Zeldovich mechanism. At exhaust gas temperatures above 1800 K, the N O x emissions are also far below the conservative predictions of that mechanism. The reasons for such very low N O x levels likely include N O x "reburning" in which N O x in the combustion products is abated by chemical reduction following entrainment into the fuel jet prior to confluence with the air jet. The product gases entrained by the fuel jet are not, of course, normally devoid of oxygen. They contain oxygen at the level of the excess air, which is enough promote a very fuel-rich reaction, presumably producing C H fragments in some quantity and so driving the reduction of entrained N O x. 8.4.3. Effects of burner parameters The effects of the combustion-air and exhaust-gas temperatures are monotonic As expected, the N O x emissions rise with increasing temperature. The effects of the burner variables that were studied, namely the fuel-port angle 0\ and the air port diameter D2, on the other hand, may, and indeed appear to, exhibit mimima in the N O x emission. This happens principally, it would seem, because of changes in the entrainment of recirculating products up to the point of confluence and in the size of the reaction zone. Increasing entrainment in the pre-encounter zone, and thus increasing dilution with products that are largely inert in character and that have lost enthalpy to furnace heat transfer, should diminish N O x production in the succeeding two-jet mixing.zone where the bulk of the combustion occurs. The effects of enlargement of the combustion zone, 21 |
