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Show , ,;t~ :. , - t... .... '; The balance of fuel is introduced above the main combustion zone (cyclones) in the reburning zone through reburning burners. To protect the tubes around the reburning burners in the rebuming zone from flreside corrosion, some air is introduced through these burners. The burners are operated in a similar fashion to a standard wall-frred burner. The furnace reburning zone is operated at stoichiometries in the range of 0.85 - 0.95 in order to achieve maximum NOll reduction based on laboratory/actual boiler application results (lX3). The balance of the required combustion air totaling 15 - 20% excess air at the economizer outlet is introduced through overfrre air (OFA) ports. These ports are designed with adjustable air velocity controls to enable optimization of mixing for complete fuel burnout prior to exiting the furnace. Pilot- and field-scale sruclies by BHK and other researchers (l.S) have defined acceptable limits of residence times in the reburning zone. A 50 - 60% reduction can be achieved at residence times greater than 0.45 second. In order to complete combustion (based on B&W/BHK experience with staged combustion), about a 0.65-second residence time is required. Thus, a total of about 1.1 seconds is required between the rebuming ports and furnace exit. PILOT REBURNING TEST PROGRAM Technicgl Objectives The technical objectives of this project are to demonstrate NOll reductions of nominally 50 - 60% while maintaining acceptable cyclone/boiler operating conclitions. Three rebuming fuels were evaluated while operating under various anticipated full-scale simulated reburning conclitions. Table 2 summarizes the various ranges of reburning criteria which were evaluated for NOll reduction capability. Main cyclone/reburning burners fuel splits, reburning fuel type, furnace stoichiometries, and furnace residence times were varied. Adclitional variables which are evaluated include mixing, corrosion potential, frreside deposition, and combustion efficiency. The results will be utilized to confmn/expand the Phase I engineering feasibility study. Table 2 SUMMARY OF REBURNING CONDITIONS EVALUATED DURING PILOT TESTS Fuel Fuel Split Stoichiometry Residence Time (Assume Plug Flow) Main Combustion (Primary) Zone Kittanning Coal 70 - 85% 1.0 -1 .2 0.1 second Reburning Zone Natural Gas. No. 6 Fuel Oil, Kittanning Coal 15 - 30% 0.85 - 0.95 0.5 - 0.8 second Burnout Zone 1.05-1.2 0.6 - 0.9 second The major areas of technical uncertainty which were identified in the feasibility case stuclies and were evaluated during the pilot tests for all reburning fuel types include: NO. reduction potentials of the reburning fuels when operating in a cyclone boiler environment of high initial primary NO. levels and low char carryover to the main furnace (high char carryover increases available, unconsumed oxygen in the rebuming zone). 5 |
