OCR Text |
Show The second method of reducing the flame temperature is by introducing a sensible heat load to moderate the temperature. Water or steam injection into the flame is thermally inefficient and therefore not considered practical. One frequently used method recirculates part of the combustion products back to the burner. Generally known as flue gas recirculation (FGR), this method also reduces NOx by reducing the oxygen concentration in the flame envelope. The focus of this paper is the application of an ultra low NOx burner to thermal oxidation of waste gases. The burner design uses the principle of low flame temperature to minimize thermal NOx formation. This low temperature is achieved by using high excess air rates while maintaining excellent flame stability. A schematic representation of this burner is shown on Figure 2. Proprietary design details are intentionally omitted. The adiabatic flame temperatures of natural gas combustion versus excess air rate is plotted on Figure 3. As the excess air level increases the peak flame temperature decreases and consequently the rate of NOx generation decreases. Equilibrium NOx concentrations as a function of flame temperature are shown on Figure 4 (Ref. 4). Although equilibrium cannot be used to predict NOx quantitatively, the observed effect of temperature follows the same trend. 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 exceeded slightly 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. The rate of thermal NOx generation increases rapidly as the peak flame temperature exceeds 2400 F. At 15 % excess air, the theoretical flame temperature of natural gas is about 3300 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 F. Operating a burner having a flame at this temperature reduces the NOx generation rate by more than 90 %. The ultra low NOx burner shown in Figure 2 can be operated in two modes; excess air and flue gas recirculation (FGR). Actual laboratory test data for operation in the excess air mode is shown on Figure 5. At the nominal excess air rate of 100 %, NOx emissions from the burner are less than 10 ppmv even when corrected to 3 % 02. The burner has been operated at excess air rates of up to 110 % but CO levels begin to increase. But at 100 % excess air, CO emissions are approximately 10 ppmv. V-30 5 |