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Show diminishes to zero as the flame extinguishes approaching approximately 20.95 percent 02. Guaranteeing N O x , for example, not to exceed 80 ppmv, is a frequent requirement in USA environmental permits. However, when the actual measured N O x exceeds the permit limitation, albeit an infrequent occurrence, considerable additional expense and effort may be incurred. The uncertainty is related to the fact that the guaranteed N O x value in this example is less than ten percent of the thermodynamic equilibrium value preferred by nature. The uncertainty is further exacerbated the temperature sensitivity of the mechanisms which yield N O and N 0 2 . Should the local flame temperature actually be higher than required anywhere in the entire flame volume, a volume which is typically greater than 30 cubic meters in a waterwall boiler furnace (5 cubic meters in a firetube boiler furnace), the N O x formation will exceed the desired design value. Figure 3 shows the change in N O x versus combustion air temperature. Local flame temperatures which are 50°C high, yield significant increases in N O. Should the local flame temperature be higher anywhere, by 100 degrees, for example, the N O concentration will be even greater. The peak stoichiometric adiabatic flame temperature possible at thermodynamic equilibrium for ambient temperature combustion air and the typical hydrocarbon boiler fuels exceeds 2000°C. N O x yields of less than 100 ppmv require mean flame temperatures less than 1500°C. Clearly, there is more than ample room for the flame temperature to be too high by a few hundred degrees somewhere in the large flame volume. OUTLET OXYGEN VOLUME PERCENT Figure 2. 0.3 %w, FBN, AMBIENT TEMP AIR In addition, preheated combustion air guarantees higher flame temperatures. N O x , of course cannot be lower in preheated flames releasing the same specific enthalpy in identical boundary conditions. Figures 4 and 5 illustrate typical relative values of N Ox in hot combustion air compared to Figures 1 and 2 without air preheating. Future boiler burner installations would yield lower N O x when an economizer is employed to cool the flue gas in lieu of an air heater. Consequently, the certainty of N O x guarantees is not precise, nor easily repeated without precision in the burner controlled flame characteristics. 1000 , afa § 100 ill 3 (_ t- Q 111 N _l < £ cc 10 o z 1 J 1 0 500 1000 1500 TEMPERATURE, Celsius Figure 3. COMBUSTION AIR TEMP vs NOx As the effects of MACT serve to lower the NOx guarantee values further, the risk associated with problems resulting from actual N O x measurements exceeding permit limits increases. Further, N Ox guarantees may be required in the order of 10 ppmv, since a certain population of recent sites are achieving N O x results in that order of magnitude already. This author is presently involved in the demonstration of 5 ppmv N O x (referenced to 3 % 02) burners which are likely to be commercially available within two years. Ultra-low N O x installations are especially likely for natural gas but equally desirable for distillate oils for future sites. As these lower levels of N O x are included in the environmental permit, the risk of noncompliance at specific sites escalates. More expense should be anticipated to bring these sites into compliance as minor variations from one site to another will surely demand. As application of M A C T produces lower N Ox on one fuel, natural gas for example, the economic incentive to do it on other fuels, such as distillate oils, becomes great. Many of the burners manufactured in the last two decades have been capable of dual fuel firing either natural gas or distillate, or any other combination of fuels without hardware changes. The desire for dual fuel operation intensified in the 1970s as major price |
