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Show H C N <-> N O + C H . While this reaction is near the end of the mass reduction chain, it is by no means near the end of the flame burn out zone. It occurs in a very short time, promptly, in the flame near field of the burner. This third mechanism is referred to as prompt N O. The total N O x is the sum of the three sources, thermal, fuel bound and prompt N O. Chemical reactions have forward and back reaction rates. Both are highly temperature dependent, as is the equilibrium concentration of each specie. The forward reaction of the above equation, O + H C N -> N O + CH, is predominate early in the reacting flame. However, the backward reaction, N O + C H -• H C N + O, is predominate in reburn flames in which the fuel rich reaction has a very low O concentration. The benefit is of monumental importance, annihilation of N O . NO which is formed in the fuel lean main burner flame is quite resistant to backward reactions, of for example, the type N O + N O + intermediates -> N 2 + further oxidized intermediates. This is true even for low excess air burners as most low-NOx burners are. The fuel rich rebum zone can yield a high percentage of the reverse N O + N O reactions due to the severe paucity of O. Each reverse reaction eliminates two N O molecules in the flue gas. In the quest for ever lower NOx, low O concentration is desirable in the main burner flames of highly non-adiabatic, boiler burner diffusion flames, just as it is in reburn flames. L o w O is achieved in practice in boiler burner flames by operating with very low excess air, or even better, fuel rich. This is the most effective means of precluding the N necessarily stripped from CxHyN from becoming stable N O . An adiabatic flame with very low O concentration suffers from large yields of thermal N O , which can overwhelm the reduction in fuel derived N O . The boiler burner flame temperature does not increase nearly as much in the slowly diffusing, highly non-adiabatic waterwall chamber surrounding it. Therefore, all three N O formation mechanisms are simultaneously suppressed in a low O boiler burner flame. This technique is not nearly as effective in turbine engine combustors and other nearly adiabatic chambers where F B N is present. Totally discrete burning zones, in the main burner flame as in rebum, are proven means of reversing the N O formation to the non-toxic substance N 2 [11,12]. EMISSIONS REDUCTION As the evolution of more stringent limits is implemented [13], the certainty that a given project will be installed and commissioned in full compliance with emissions requirements diminishes. The principal reason is that oxides of nitrogen are guaranteed at concentration values which are a minuscule portion of their thermodynamic equilibrium value at the temperatures required to have all of the other guaranteed emissions concentration values exceedingly low as well. Minor variations in features outside of the burner can cause N O x to increase dramatically. As the required level of N O x is further reduced, many more 'minor' feature variations may increase N O x beyond the guaranteed limit unless realistic margins are included in the permit, or improved burner technology is realized. Maximum Achievable Control Technology (MACT) requires the use of the "average emission limitation achieved by the best 12 percent of existing sources." M A C T applies to the full 188 entry long list of regulated toxic substances. The primary species, in that list, which are influenced by combustion are N O x , C O, VOCs, P M , PM10, and PM2 5. Basing M A C T on prior actual emissions results is an excellent means of reducing specific site compliance risk that actual emissions could exceed the permitted limitations. However, as noted previously, most pollutants emitted from burner fired boilers are present in minute quantities. N O is one of the most important of them, and which can be controlled by the burner shaped flame to reduce flame temperatures and decrease N O to levels quite low compared to thermodynamic equilibrium concentrations. At typical boiler exit state points, the temperature and pressure of the combustion products would reach equilibrium with N O present in the order of 1000 ppmv, and N 0 2 in the order of 0.1 ppmv. Both of these values are representative of distillate fuel oils and natural gas. Current burners yield total N O x leaving the boiler of less than 100 ppmv in most installations which use ambient temperature combustion air. Figures 1 and 2 illustrate the dependency between N O x and excess air, or furnace outlet oxygen volumetric concentration, for OUTLET OXYGEN VOLUME PERCENT Figure 1. ZERO FBN, AMBIENT TEMP AIR two typical mass concentrations of fuel-bound-nitrogen (FBN). These figures apply to precisely shaped flames which are at their lowest practical temperature limit. N O x is least when 0 2 is least, especially when 0 2 is much less than one mole percent of the flue gas. N Ox increases less rapidly as 0 2 is increased above a threshold value obvious in the figures, and then |
