OCR Text |
Show From an operational standpoint, the general opinion of the operating staff was that Unit 4, with the reburn system in service, was easily operated, and slag tapping was unaffected by reburn system operation with the boiler operating on load dispatch. No slagging was observed at the platen superheater. From an emissions performance standpoint, a series of tests have been performed to quantify the influence of excess air level (02 after economizer) on NOx, influence of reburn fuel (natural gas) quantity on NOx, and influence of FGR distribution in the furnace on NOx. Figure 9 shows the relationship between N O x and 0 2 measured downstream of the economizer after reburn retrofit. Clearly, economizer 0 2 level has a significant influence on NOx, with N O x increasing as 0 2 is increased. This trend is typical of most staged combustion processes. N O x values as low as 255 p p m at an 0 2 level of 3.7 percent increased to 370 p p m at 5.7 percent 02. From these results, one concludes that economizer 0 2 should be minimized in order to minimize NOx. Based on the approximate rate of change in Figure 9 for each percentage point change in economizer outlet 0 2 level, there is an accompanying 50-60 p p m change in measured N O x emission level. Practically, economizer outlet 0 2 level reduction is limited by associated increases in C O and carbon in flyash emissions. This issue will be reviewed later in this paper. 400 CO o o E Q. X O 350 h~ 300 250 200 \- 150 100 [- 50 - 300 MWe 12% of Total Boiler Heat Input as Rebum Fuel 1 All Burnout Air and Reburn FGR Dampers 100% Open 1 0 1 2 3 4 5 6 02 After Economizer (%) Figure 9: Post-Retrofit NOx Emissions vs. Percent O after the Economizer Figure 10 presents the relationship between N O x emissions and reburn fuel quantity. These tests were conducted to define optimum levels of reburn fuel mass flow rate. The injected reburn fuel mass flow rate is plotted in Figure 10 as a function of percent of boiler total heat input as reburn fuel. N O x decreased as reburn gas flow increased. This result is consistent with previous reburn laboratory and field demonstrations^. Increasing reburn gas flow reduces reburn /.one stoichiometry, which is a key element in reducing NOx. 13 11-12 |