OCR Text |
Show -6- unchanged as a result of the retrofit when comparing data with no OFA. Note, however, that since different coals were fired before and after the retrofit, this is not an exact comparison. The use of OFA decreased FEGT. With the OFA dampers closed, FEGT averaged 2176°F. FEGT decreased with OFA, to a minimum of 1974°F when using 3/3 OFA. This decrease in FEGT with increasing OFA is an indication of increased combustion staging. The primary variables evaluated during the LNB testing included air register settings and excess O2 level. Variations in register swirl had minor impacts on NOx emissions, while changes in spreader position impacted LOllevels. Excess O2 level was found to have the greatest impact on emissions and unit performance while changes in windboxlfurnace differential pressure had little effect on performance. Figure 4 shows NOx emissions in Ib/MMBtu plotted versus economizer O2 levels for LNB tests performed with the OFA dampers closed. At full load, NOx emissions were about 0.79 Ib/MMBtu at nominal O2 levels. NOx emissions decreased to 0.69 Ib/MMBtu at 115 MW and increased to 0.81 Ib/MMBtu at minimum load. These data also show that the system is quite sensitive to O2 levels, as illustrated by both the 155 and 115 MW test results. The NOx sensitivity was found to be 73 ppm NOx per percent O2 at 155 MW and 65 ppm NOx per percent O2 at 115 MW. At minimum load, the sensitivity was 30 ppm NOx per percent O2• This high NOx sensitivity to O2 level is not typical of 10w-NOx burners. Figure 5 shows ash LOI as a function of economizer O2 levels. Data taken at 155 and 115 MW show that the LOI levels increased with decreasing O2 levels over the range of conditions evaluated. Maximum LOI levels, recorded at minimum O2 conditions, were in excess of 25 percent. At minimum load, the lowest LOI was recorded at the nominal O2 level. As O2 was either increased or decreased, LOI increased. While the increase in LOI with decreasing O2 was expected, the increase with increasing O2 was not. It appears that increases in O2 at minimum load above the levels normally found (Le., about 8.5 percent) decreased combustion efficiency due to flame zone cooling. |