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Show 4197299444 CORP. HQ 489 P05 MAR 17 '94 12:08 CO which lower NOx concentrations in the reaction zone even when process temperatures are kept high. Chemical equilibrium studies were conducted to study the effects of air-fuel ratios for combustion of commonly used natural gas in the range of 5:1 (50 percent o~ stoichiometric air) to 15:1 (150 percent of stoichiometric air) at different combustion air preheat temperatures varying from 3S'e to S1S·C (100'F to 1500·F). The adiabatic flame temperatures and reaction products generated during rich combustion of natural gas are shown in Figures 1 and 2. As shown in Figure 1, there is a significant drop in flame temperature as the air-fuel ratio is lowered from stoichiometric conditions. Figure 2 shows compositions of reaction products for a wide range of air to fuel ratios. The total combustibles, defined as the sum of reducing gases (CO and H2) dan be as high as 26 percent at 5:1 air-fuel ratio. Values given in Figure 2 are calculated at a reaction temperature of 1370·C (2500~F) . . There is a slight variation in percentages of Hz, CO, C02' and H20 at a given air-fuel ratio as the air preheat temperature is changed. However, the total combustibles remain practically constant at a specific air-fuel ratio, irrespective of the con~ustion air temperature. Effects of rich combustion of natural gas on formation of combustibles were investigated by using chemical equilibrium equations of the reactions involved. For practical heating systems, major parameters which affect NOx formation are: fuel composition, Oz content of combustion air, fuel and combustion air temperature, and the furnace or process temperature. For these studies it is assumed that ambient air with 21 percent oxygen is used for combustion of natural gas and the fuel is introduced at the ambient temperature. Combustion air temperature is varied from 3S·C to 815'C (100·F to 1500·F). This covers a large number of industrial applications for which NOx emissions are becoming a major concern. It is realized that NOx formation will be greatly reduced as the air-fuel ratio for the combustion system is lowered further. However, for practical considerations and for applications to high 'temperature processes, the study was limited to air-fuel ratios of 50 percent of stOichiometric air (approximately 5 to 1 volume ratio of air to natural gas) to 50 percent excess air. Figure 3 shows NOx formation at different air-fuel ratios and at adiabatic flame temperatures generated with a range of air preheat temperatures. Theoretical reaction temperatures are , \ , \ , \ |