OCR Text |
Show is compensated for, in real time, with an increase in natural gas firing rate. Operational reliability is thus maintained in spite of the questionable reliability of LFG gu supply. Using natural gas as the swing fuel accommodated the City's demand for electric power generation while preferentially consuming the available LFG first. 3.3 Approach to NOs Control NOx emissions control was based on combustion staging and the use of FGR. Combustion staging was implemented on gas firing only because of a concern with flammability limits with LFG. Combustion staging was achieved by positioning the fuel injection nozzle to create a stratified flame structure. FGR was applied to LFG, natural gas, and fuel oil firing. A typical composition of LFG is contained in Table 2. The fuel includes inerts in the form of nitrogen and CO2 which are known to compromise the fuel flammability limits<l). An analysis was performed, therefore, to assess LFG flammability with and without the use of FGR. The results of the analysis, as presented in Figure 2, show that the application of FGR to LFG may place the fuel outside of the flammable range. At an FGR circulation rate of 15-25 percent, the analysis suggested that LFG may be at the ragged edge of flammability. Based on this analysis, the decision was made to use a small portion of natural gas as support fuel for the stable combustion of LPG. Combustion staging of natural gas was achieved by creating a flame where air-to-fuel ratio is stratified between fuel-rich and fuel-lean conditions. Three fuel-rich regions were created by clustering fuel nozzles in discrete locations. In between the fuel-rich flames, combustion air was made available to provide for complete fuel burnout. -8- |