OCR Text |
Show The best NO emissions obtained, at a commercially acceptable operating x condition, was 25 ppm (corrected to 3% 02)' a 64% reduction from baseline. The operating conditions were 1% excess 02 with 30% total OFA with 20% FGR. Overfire air (OFA) parameters such as velocity, injection angle, and elevation affected NO emissions only slightly within the tested range. x However, CO emissions increased substantially at certain OFA angles and velocities. Furnace outlet gas temperature was linearly reduced with increased FGR mass flow rate, and essentially unaffected by OFA mass flow rate. Oil Firing Results Oil firing results are shown in Figures 9, 10, and 11. Baseline NO x emissions were 140 ppm (corrected to 3% 02) at a nominal operating excess 02 of 1.5%. In contrast to natural gas firing, NO is formed both through x the thermal fixation of atmospheric nitrogen (thermal NO ), . as described x by Zeldovich(4) , and through the oxidation of fuel· bound nitrogen (fuel NO ) when firing fuel oil (or coal). Fuel NO formation is predominantly x x sensitive to oxygen availability and not temperature. As a result, total NO emissions can be effectively limited by controlling the availability x of oxygen during the combustion process with little regard toward temperature. For this reason, NO emissions were reduced substantially with OFA (Figure x 9). However, because of the dominant fuel NO formation mechanism and the x combustion characteristics of oil, NO emissions were found to be only x moderately reduced by flue gas recirculation (Figure 10). During operation with large quantities of OFA (~30%), NO emissions were x unaffected by increases in FGR flow rates (Figure 11). The best NO emissions obtained at a commercially acceptable operating x condition was 83 ppm (corrected to 3% 02)' obtained with 27% total OFA (19% separated, 8% close-coupled) no FGR, and 1.5% excess O 2 , This represented a 41% reduction from baseline oil firing NO emissions. x 6 |