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Show Abstract ULTRA-LOW NOx WALL-MOUNTED BURNERS ~ Chad F. Gottschlic1t and Wayne C. Gensler JJf': Selas Corporation of America Dresher, PA 19025-1918 PAPER NO. 33 Combustion modification methods can be used with gas-fired burners to lower the NOx emission to well under 0.025 Ib/MMBtu in high-temperature process heaters. 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 and nitrogen oxides (NOx). NOx is the more difficult of the pollutants to control. A number of control techniques for NOx exist but they often suffer from high capital cost or increased fuel consumption. As gaseous fuels usually have little or no chemically bound nitrogen, the NOx emitted by gas burners comes from the nitrogen in the combustion air. The accepted chemistry for nitric oxide formation from air during combustion is the modified Zeldovich reaction. o + N2 = NO + N N + O2 = NO + 0 N + OH = NO + H Rate constants have been measured for each step of this reaction. 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. At the temperatures typically found in a furnace outside of the flame~ the reaction rate is negligible. 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. Clearly, one approach for reducing NOx emission would be to modify the combustion process to lower the flame temperature. Five methods that have been proposed and investigated are: 1. Mixing steam with either the fuel or the combustion air ahead of the burner. As steam is a heat-absorbing diluent, it lowers the flame temperature and, consequently, the NOx emission. 2. Adding excess air. Its effectiveness depends on the type of burner. With premix burners, i.e., those in which the -1- |
