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Show NOx. ppm (03" Oxygen) 1,200 1,000 NOx vs Air Preheat Temperature Convention., au",.,. ~ ---------------------------- ~ ------------------------- -------- ~ -------------------- 200 o 1,500 Figure 3 The second pathway to NO, the "prompt" mechanism, is relatively independent of temperature. It occurs as long as there are hydrocarbon radicals, principally CH for natural gas. A complex series of chemical reactions between C, H, N, and O-containing species are involved in the formation of prompt NO. Since the process is not sensitive to temperature, it can only be controlled by managing the concentration and residence times of the reactive species. This can be achieved by controlling the mixing offuel and air, and the time/temperature profile so that the desired results can be achieved. By properly controlling the reactions, NO that has already been formed can be reconverted to molecular nitrogen. This process, known as "reburning," is being used to minimize NOx emissions from coal-fired boilers. It can also be realized in the burner near-field through mixing control. The third source of NO involves the formation ofN20 by a three-body reaction between a nitrogen molecule, an oxygen atom, and a neutral species. This is favored at higher pressures. Since the vast majority of industrial combustion is conducted at atmospheric pressure, the N20 mechanism will not be discussed further. There are two ways to control NOx' The type first involves treating the flue gas and is comparable to using a catalytic converter in an automobile to reduce undesirable emissions. These post-combustion flue gas treatments can use catalysts or reagent addition to reduce NO to 5 |