OCR Text |
Show 4.0 THE POTENTIAL FOR NO CONTROL Both the boiler simulator and the tunnel furnace were operated under staged conditions to examine the influence of fuel characteristics on controlled emissions. Figure 8 presents a comparison of results obtained in the tunnel furnace under staged combustion conditions for two shale and two coal-derived liquid fuels. While the uncontrolled levels were substantially different, the minimum total NO levels achieved under staged combustion conditions were quite similar. It is interesting to note that minimum NO for a 2.0 percent nitrogen shale fuel is similar to that obtained with an 0.97 percent nitrogen coal-derived fuel. Figure 9 allows a comparison to be made between NO emissions obtained in the boiler simulator under staged combustion conditions with nozzles A and B for petroleum and shale-derived fuels. In both Instances it can be seen that nozzle A gave higher NO emissions under excess air conditions, but lower emissions under staged combustion conditions. Smoke emissions from the petroleum-derived residual fuel increased dramatically as NO emissions decreased, and nozzle design had the most significant influence upon exhaust smoke number. Smoke emissions from the shale-derived liquid were acceptable with both nozzles under all primary zone stoichio-metries, yet nozzle A produced significantly lower NO emission levels. Nozzle A produced a smaller mean drop size than nozzle B, and in general, it can be stated that: Under excess air conditions smaller droplets increase fuel NO conversion. Under staged combustion conditions smaller droplets minimize exhaust NO emissions. Figure 10 compares minimum NO emissions achieved under staged combustion conditions for a range of liquid fuels in both the boiler simulator and the tunnel furnace. In general, minimum emissions from the boiler simulator were higher than those from the tunnel furnace. This can be attributed to the slightly longer residence time in the primary zone of the tunnel furnace, and perhaps to variations in mixing rates due to different scales. 7-16 |