OCR Text |
Show 3.0 FUEL NO FORMATION The production of fuel NO in a turbulent diffusion flame is dependent upon: (a) the fuel characteristics; and (b) the fuel/air contacting process, which for liquid fuels is dependent upon the complex interaction of the fuel spray and the combustion air flow field. The emission levels presented in Figure 3 illustrate the importance of both of these factors. The results were obtained in the boiler simulator with four different commercial fuel atomizers and several petroleum-derived fuels at two combustion air swirl levels. All four nozzles involved air assist atomization: - Nozzle A employed ultrasonic atomization - Nozzle B employed external mixing - Nozzles C and D employed internal mixing chambers. It can be seen that under normal excess air conditions NO emissions increase with increasing fuel nitrogen content, and that the emission level is dependent upon both atomizer design and the combustion air flow field characteristics. Atomizer A produced the highest emissions under medium swirl conditions, but emissions measured at high swirl levels are among the lowest. These investigations concerned the influence of fuel characteristics, particularly fuel nitrogen content, on fuel NO formation. The tunnel furnace allows a direct determination of fuel nitrogen conversion by substitution of argon/oxygen for combustion air, thereby eliminating thermal NO . Figure 4 presents a plot of fuel N0X as a function of overall excess air for five alternative fuels. The fuel NO emissions were obtained with an air preheat level of 405 ± 5 K and an atomization pressure of 15 psig. In all the experiments the fuel viscosity was maintained at 12 ±1.5 centistokes (66 ± 8 SSU) by using the appropriate fuel temperature whenever possible. Figure 5 presents a composite plot for total and fuel NO (defined by argon substitution) as a function of weight percent nitrogen in the fuel for a wide range of petroleum and blended distillate 7-9 |