OCR Text |
Show Once the characteristics of the cyclonic combustion were defined, two additional series of tests were carried out to investigate this concept's potential for low-NOx emissions. The second series of tests on the 40-hp burners was carried out using the existing arrangement. For these tests, the combustion air was preheated to about 700oF, representing the extremecase scenario for large watertube boilers. It was recognized that, with ambient air, NOx emissions would be significantly lower at comparable excess air levels or that similar NOx levels would result at significantly lower excess air levels. Figure 5 shows the effect of excess air on NOx and CO emissions. NOx decreased to 15 ppm at 62% excess air and to 10 ppm at 70% excess air. Combustion was stable even at 100% excess air, providing an NOx level below 4 ppm. The CO level increased with increasing excess air, but even with 100% excess air, the CO level was only about 20 ppm. As indicated, NOx emissions would be significantly lower with ambient air because of the reduced peak flame temperatures. Although not measurable on the simulator, the thermal efficiency of the boiler would decrease with increasing excess air. The third series of tests was carried out to evaluate combustion air staging, which lends itself well to this type of burner. For these tests, however, the refractory-lined water-cooled combustion chamber was replaced with bare water-cooled rings, as illustrated in Figures 6 and 7. This simulated the Morison tube of a firetube boiler. With all the air supplied to the primary zone, the NOx emissions with this arrangement were about 30 to 100 ppm at 5% excess air - or about 50% lower at high fire and 80% lower at low fire than the NOx levels with the refractory-lined combustion chamber. This NOx decrease is believed to be caused by the higher heat transfer to the bare water-cooled tube, which would also significantly decrease the temperatures of the internally recirculated gases. Figure 8 illustrates the effect of combustion air staging on NOx and CO emissions. The overall excess air level was held between 5% to 7% at the different primary stoichiometric ratios by simultaneously adjusting the amount of secondary air. The figure shows that NOx decreased from about 70 ppm with no staging to about 20 ppm at a primary stoichiometric ratio of 0.7. The level of CO increased wi th decreas ing primary air, but the maximum CO measured was still only slightly above 50 ppm. It must be noted that these measurements were made at the exit of the combustor with temperatures in the range of 20000 to 23000 F. These temperature levels are believed to be low for any substantial NOx formation but not for CO burnout. The CO level is therefore expected to continue decreasing in an actual boiler with the additional residence time without a simultaneous increase in NOx formation. Other tests wi th natural gas firing using this arrangement in the range tested showed that both NOx and CO decrease with increasing primary air/gas injector velocity, and when the level of excess air is increased. |