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Show ~r-----------------------------~~~ 560 540 8 :: ~ 480 (") 460 @J :: 07 09 04 OB 06 028 2 4()() ~ 380 0.5 Ib/MBlu 0 I gy~ 0 13R Q. J60 ------!! ~----- ------- 0 o'Ii 3J2400 o· 00 t~I 6 020o ~ Z 300 024 0019 I 280 260 24 28 32 I I I 36 02, % 05 40 At and Below the Line • Venturi Diffuser Concepts Above the Line • Other Tested Concepts 03R O,ZJ' 48 Figure 7: RO·II "Advanced Coal Nozzle Concepts", NOx VS. O2 Figure 7 graphically depicts the NOx emission performance of a number of the tested AO-II coal nozzle concepts. Data in this figure highlights the fact that the coal nozzle design employed had a dominant influence on NOx levels observed. One can summarize the data contained in Figure 7 by directing attention to the solid line plotted in the center of the graph. All data below the solid line represents the NOx performance of the venturi diffuser concept, all data above the line represents alternative tested concepts. Clearly, the venturi diffuser concept generated lower total NOx at any given operating excess air level, as compared with all other tested coal nozzle concepts. Most importantly, at a nominal flue gas O2 concentration of 3.5% (20% excess air), total measured NOx was 360 ppm (corrected to 3% O2 ), meeting the overall project goal of 0.5 Ib/MBtu NOx. The venturi diffuser coal nozzle assembly, as a result of its success in meeting the NOx reduction target established for the project, has been chosen as the coal nozzle design to be utilized in the AO-II burner. Beyond its NOx reduction capability, the AO-II burner met all other established performance targets. These targets were set to ensure that the firing system hardware developed in the laboratory would be retrofitable to most existing wall fired boiler arrangements. Most units, for example, have fan limitations in terms of achievable windbox to furnace delta static pressure. The AO-II coal burner is capable of operation at less than 3.0" W.C. static windbox to furnace delta pressure at MCA. Most existing boiler F.D. fan systems are capable of achieving at least that pressure differential at MCA. In a similar vein, primary (coal transport) air static pressure at the coal nozzle inlet is a critical factor from a retrofit standpoint. Any low NOx burner installation should operate within existing coal feed system pressure limitations. The AO-II burner operated at MCA with a primary air static pressure at the nozzle inlet of less than 4.5 inches W.C., an acceptable operating primary air static pressure for most existing wall-fired installations. Many low NOx coal firing system laboratory tests and field demonstrations to date have reported that, under low NOx conditions, carbon in flyash levels tend to increase (2,3, 4). In some cases, CO emissions also increase under' low NOx conditions. These results are, of course , very dependent on coal type, coal particle size distribution, and furnace configuration. In practical terms, most low NOx coal firing systems must strike an acceptable balance between NOx reductions and carbon in flyash/CO increases, In the case of the AO-II coal burner, operated at 0,5 Ib/106 Btu, both carbon in flyash and CO emissions did increase, however, the final emission levels documented were within acceptable operating ranges. For example, under baseline, high NOx conditions, carbon in flyash and CO were 1-2% and 30-50ppm, respectively. Under low (0.5 Ib/MBtu) NOx conditions, carbon in flyash and CO increased to 3~/o and 40-70 ppm, respectively. These laboratory results indicate that nominal increases in carbon in flyash may be expected in AO-II field applications, again dependent on coal type and furnace configuration. Several low NOx coal firing systems evaluated to date for wall-fired boiler applications have experienced increased flame lengths as compared to preretrofit cases (5,6). As in the case of the relationship between NOx, carbon loss, and CO, one must in most situations strike a balance between NOx reductions and increasing flame length. Operating experience with the AO-II coal burner to date is good in this regard. Baseline (high NOx) conditions produced a luminous, stable flame about 12' long. Under low NOx (0.5 Ib/MBtu) conditions, flame length increased to approximately 16'-18' long. The increase in flame length was deemed acceptable since the field units targeted for the first AO-II coal demonstrations can accommodate a similar increase in flame length without direct flame impingement on rear wall tube surfaces. Future boiler retrofits will be assessed on an individual basis not only to ensure compatibility between furnace depth and the luminous flame volume of the AO-II low NOx coal burner, but also to ascertain potential for changes in post-retrofit boiler thermal performance. FIELD EXPERIENCE Following successful laboratory development trials, the AO-II coal burner has presently been retrofitted to three (3) field installations. Figure 8 is a schematic of the installed AO-II coal burner. The tangential inlet, spirally ,. r I ----~-~ Figure 8: RO-II Burner 5 |
