| OCR Text |
Show -7- CO emissions are plotted versus economizer O2 in Figure 6. Data taken at 155 and 115 MW show that CO emissions increased with decreasing O2 levels, as expected. At minimum load, CO emissions increased with increasing O2 levels. This is another indication of possible quenching in the flame zone due to the high O2 levels required at minimum load to maintain superheat and reheat steam temperatures. CO emissions varied from 25 to 35 ppm at nomi nal O2 levels across the load range. The primary variable affecting OFA performance is OFA flow. OFA flow was varied by changing the OFA port configuration and varying the windbox to furnace differential pressure. The effect of OFA flow on NOx emissions is shown in Figure 7. Note that with the OFA dampers closed, the cooling air flowing through the dampers is equivalent to nominally five percent OFA at full load. The full load data showed that 20 percent OFA decreased NOx emissions by 42 percent relative to the LNB baseline. At intermediate load, the reduction decreased to 36 percent. Reductions were limited at minimum load, due to both limits on OFA flow and high excess O2 levels. With 12 percent OFA, NOx reductions at minimum load were about four percent. LOllevels are plotted versus OF A in Figure 8. Baseline (l&., no OFA) LOI levels ranged from 17 to 21 percent over the load range tested. At 155 MW and 115 MW, LOI levels increased with OFA. Maximum levels were in excess of 35 percent at these loads. At minimum load, LOI emissions decreased Slightly (Le., from 17 to 12 percent) as OFA was increased to 12 percent. This decrease in LOI with increasing OFA at minimum load is believed to be due to a reduction in burner zone quenching. As OFA is increased, burner zone stoichiometry decreases. At minimum load, this decrease reduces flame zone quenching which results from high O2 levels, and subsequently, an increase in combustion efficiency. Figure 9 shows the effect of OFA flow on CO emissions. The data show that CO emissions varied between 20 and 40 ppm, except at 155 MW with 20 percent OFA. In that instance, CO emissions increased to nearly 60 ppm. Baseline (Le., no OFA) CO emissions varied from 20 to 30 ppm. This shows that the use of OFA did not significantly increase CO emissions. |
