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Show Abstract Adding fuel staging to a flat-flame, wall-mounted burner can produce NOx emissions below 10 ppm. Fuel efficiency is not reduced. The burner can be retrofitted without shutting down an operating furnace. Introduction Process heaters commonly burn gaseous fuels with a small amount of excess air. For these applications, the only significant combustion-generated air pollutants are carbon monoxide, unburned hydrocarbons, and nitrogen oxides (NOx). NOx is the most difficult one to control. There are three mechanisms for NOx formation in combustion processes. They are designated as thermal NOx, prompt NOx, and fuel NOx. Thermal NOx is produced by the oxidation of ordinary molecular nitrogen, N2, present in the combustion air or the fuel. Prompt NOx is formed in the flame by reactions between N2 and molecular radicals such as CH and CH2. Fuel NOx is produced by the oxidation of fixed-nitrogen compounds, i.e., compounds other than N2. For fuel gases, fuel NOx is seldom a problem. Prompt NOx can be significant in some situations such as low flame temperatures or fuel-rich combustion. Surface combustion, staged combustion, and gas turbines can provide these situations. For temperature ranges and reaction times of interest in combustion, the reaction is rate-controlled, i.e., equilibrium is not reached. Furthermore, the reaction rate increases rapidly with increasing temperature and is negligible at temperatures below 2500"F. Flame temperatures are well above 3000"F. With preheated combustion air, the flame temperature will approach 4000°F. The temperatures found in a furnace firebox outside of the flame are normally below 2300"F. Thus, NOx is formed in the flame. Outside of the flame, the NOx concentration is frozen because the combustion products are quickly cooled by heat transfer and mixing processes. A number of control techniques for NOx exist. Some techniques treat the combustion products to remove the NOx that has been formed, e.g., selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR). Both methods require accurate control and mixing of small flows of ammonia with a large volume of flue gas. The SCR is expensive and difficult to retrofit to an existing furnace. It is effective at reaching low NOx levels. However, there is some indication that unreacted ammonia released into the atmosphere can oxidize to produce NOx. -2- IV-23 |