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Show "\: _. .' emissions were dramatically reduced as stated above, less than 30 ppm CO was measured at the furnace exiL Thus, it is apparent that good mixing between the OF A and combustion gases did indeed exisL Corrosion Potential. Since the reburning zone must be operated under substoichiometric conditions, corrosion potential within this region was investigated. By operating the cyclone in an excess air mode, the majority (if not all) of the sulfur from the coal in the main combustion zone is converted to S02" Due to the reducing atmosphere in the reburning zone, ~S measurements were performed. High concentrations of ~S can be conducive to increased rate of tube corrosion. ~S concentrations at baseline and rebuming conditions are illustrated in Table 6. Multiple measurements were performed in the furnace, and results are presented in a range of ~S concentrations. Up to 60 ppm of ~S were measured at the SBS during baseline conditions. ~S levels did increase up to 90 ppm during gas reburning where no additional sulfur was added with the reburning fuel. Fuel oil utilized for rebuming contained 0.78% sulfur, and ~S levels were comparable to those observed during gas reburning. However, when coal was utilized, up to 265 ppm of ~S was measured. The impact of these levels of ~S on tube wastage has yet to be determined. It is encouraging that only a small percentage of S02 from cyclone flue gases is converted to ~S. In addition, when sulfur-bearing fuels were utilized for reburning, only a small fraction of the reburning fuel sulfur converted to ~S. Up to 200 ppm of ~S for oil reburning and 900 ppm ~S for coal reburning would be detected if all of the reburning fuel sulfur were converted to ~S. Further evaluations will predict corrosion rates within the various furnace regions during reburn operations. Table 6 H2S CONCENTRATIONS AT BASELINE AND REBURNING CONDITIONS MEASURED BASELINE GAS REBURN OIL REBURN COAL REB URN CYCLONE OUTLET o 98 BELOW REB URN 40-55 50 o 0-200 REBURN ZONE 0-60 25-90 ~ 14-93 0-265 DISCUSSION The cyclone-equipped pilot test results show that coal can provide acceptable NOx reduction despite its low volatility and high fuel nitrogen content compared to natural gas and oil. To illustrate the consistency and provide insight, these data were compared to some of the existing data from the literature. Reburning fuel nitrogen content and primary NOx level entering the reburning zone were identified as the controlling parameters for NOx reduction. Figure 9 compares the results of this work with that of other investigators ('>(7)(8). The NOx reduction efficiencies due to reburning are ploued against the primary combustion NOx levels. The data in Figure 9 indicate that for combustion systems producing low primary zone NOx levels, natural gas produces much higher NOx reduction efficiency than pulverized coal. For high primary zone NOx emission levels, the difference in NOx reduction efficiency is much lower. It is apparent that NOx emissions can be reduced with all three types of rebuming fuels for controlling NOx emissions in cyclone-equipped boilers. For retrofit applications, site-specific engineering and an economic feasibility study, including the determination of reburning fuel type, will be required in order to assess the reburn technology potential. 1S |