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Show .~ t .... ' .. \-~ ... .. , ....... carbon may exist since the reburn fuels are introduced into the reducing environment of the reburning zone. Although they volatilize and partially burn, final burnout will be delayed until the burnout zone. If FGR is introduced, unburned combustible levels increase since the burnout zone residence time decreases due to increased mass loading through the furnace and the associated lower gas temperature profile within the reburn zone region. Efficient mixing of the air introduced through the OF A ports will help alleviate this concern and any potential CO emission problems. Numerous measurements were taken to establish a data base and to validate the trends of variation of unburned combustibles with different reburning zone parameters such as fuel split, FGR, and reburning fuel type. Table 5 illustrates the comparison of baseline and reburning tests at optimum conditions with and without FGR. Isokinetic samples of the fly ash were withdrawn from the stack of the SBS and analyzed for combustibles. In addition, total mass loadings of the fly ash were measured. Table 5 shows the carbon content of the fly ash and percentage of ash at the convection pass to the total ash input to the boiler at baseline conditions. During natural gas and oil reburning tests, the ash went down since these reburning fuels did not contain ash. On the other hand, during coal reburning tests, ash loading almost doubled since ash from the reburning coal portion, unlike the cyclone, was not removed as slag. Total combustion efficiencies were calculated from ash percent in the convection pass, carbon content of the fly ash, and coal analysis. The overall change of combustion efficiencies from the baseline condition is less than 0.1 % for natural gas and oil rebuming and 0.13 % for coal reburning. This is a minimal impact and provides a strong justification that the unburned combustible potential associated with the reburning technology could be controlled to acceptable levels. Table 5 COMPARISON OF COMBUSTION EFFICIENCIES BASELINE GAS REBURN NOFGR 100k FGR Oil REBURN NOFGR 10"-k FGR COAL REBURN NOFGR 10% FGR Km ANNING COAL ASH: 9.7;20k ~ARBQN OIl! 0.3 2.3 4.5 3.0 5.4 1.6 3.4 HEA nNG VALUE: 12,580 BTU/LB ASH % IN ~QNVE~TIQN PASS 18.2 14.2 14.2 14.2 14.2 32.7 32.7 TOTAL ~Y~LONE ~QMB!..!snQN Blli ~FFI~IEN~Y ~!JRNO!..!T OIl! 99.99 99.99 99.96 99.95 99.92 99.90 99.95 99.95 99.91 99.90 99.94 99.95 99.87 99.90 REB!JRNINQ E.JJ.ll ~!..!RNQ!..!T -/l! N/A 100 100 99.95 99.93 99.91 99.79 Further analyses were performed to calculate the individual combustion efficiencies of cyclone and reburning fuels. It was assumed that natural gas burns completely. Therefore, the cyclone fuel burnout was calculated from the total combustion efficiency and fuel split during natural gas reburning tests. Knowing the cyclone fuel burnout, then rebuming fuel burnout could be calculated during oil reburning and coal reburning tests. The results indicate that up to 99.79% of coal reburning fuel was burned. CO levels were low at the stack during the baseline tests (less than 30 ppm), and there was no apparent increase when the reburning technology was applied. In-furnace probing at the reburning zone revealed areas of high CO (> 1000 ppm) due to the substoichiometric condition of this region. Upon introduction of OFA, the CO 14 |