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Show -3- Reducing the peak flame temperature in a burner is a well established method of reducing the NOx generation rate. Equilibrium mole fractions of 0* are plotted in Figure 1 versus temperature for combustion of natural gas. Equilibrium NO mole fractions are also included. A direct correspondence between equilibrium 0* mole fraction and equilibrium NO mole fraction is evident. Equilibrium 0* mole fractions and consequently NO mole fractions are much lower at the lower temperatures. BASIC TECHNOLOGY There are two methods of reducing flame temperature in a burner. One extracts the radiant heat from the flame by transfer to cooled surfaces surrounding the flame. There are practical limits to this technique. The loss of heat radiation from the center of the flame is screened by the gases surrounding the center. The outermost gases successfully radiate their heat to the cooled walls but the central gases only radiate to the gases immediately surrounding them. These outer gases are also radiating back to the center. Therefore, the reduction in the maximum flame temperature is limited. The second method of reducing the flame temperature is introducing a sensible heat load to moderate the temperature. This is the principle behind flue gas recirculation as well as reduction of oxygen concentration in the flame envelope. The same result can also be achieved by using high excess air levels. These are the principles on which the ultra low NOx burner described in this paper is based. Table 1 compares adiabatic flame temperatures of natural gas combustion versus excess air. Water or steam injection into the flame is thermally inefficient and therefore not considered practicable. As the excess air level increases, the peak flame temperature decreases and consequently the rate of NOx generation decreases. Of course, there are limits on the amount of excess air which can be used. The lower flammability limit of methane is 5.0 volume %. This translates to 100% excess air or 10% oxygen in the products of combustion. In practice, this excess air level can be Slightly exceeded because natural gas contains other minor constituents. These include ethane, propane, butane, etc. all with flammability limits of 3 volume % or less. Thus the excess air operating range is broadened to some extent. Variations in natural gas composition for selected cities in the United States is shown on Table 2 (Reference 3). Natural gas compositions with a methane content greater than 95% are unusual. The rate of thermal NOx generation increases rapidly as the peak flame temperature exceeds 2500 deg F (Reference 4). At 15% excess air, the theoretical flame temperature of natural gas is about 3300 deg F. NOx levels in the products of combustion at this temperature are about 100 ppmv. At 100% excess air, or with flue gas recirculation this temperature is reduced to approximately 2200 deg F. Operating a burner at this temperature dramatically reduces the NOx generation rate as documented by actual burner test data that will be presented in this paper. |