OCR Text |
Show excess air needed to reduce NOx emissions would result in an unacceptable decrease in thermal efficiency. To achieve the goal of sub-9 ppm NOx at low stack O2 levels, Alzeta selected several promising techniques to evaluate both experimentally and analytically and applied them to the existing RSB. These techniques were: • External FGR to reduce flame temperatures and corresponding NOx formation rates • Fuel staging, the addition of raw fuel downstream of the lean premixed main burner. • Combined FGR and fuel staging techniques The relative advantages and disadvantages of each technique are discussed below. External FGR The addition of external flue gas to the main flame is an effective and common technique to reduce peak flame temperatures and corresponding thermal NOx emissions. When using FGR, a portion of the flue gas downstream of the convective section of the boiler or heater is pumped back through the burner using the existing blower. In conventional low NOx burners, NOx emissions decrease as the level of FGR increases until the stability limit of the burner is reached. The amount of flue gas recirculated is typically limited by burner stability, with maximum FGR levels being on the order of 20-25%. For conventional low NOx burners, the stability limit is reached before 9 ppm NOx emissions are achieved. The major. benefit of using FGR as a NOx reduction technique on the RSB is that FGR is a well understood and accepted method of NOx control, and its effectiveness with the RSB had previously been demonstrated in Alzeta's laboratory (See Figure 2). Because the RSB is a fully premixed surface combustion burner, it can operate at higher levels of FGR without high CO emissions or stability problems encountered with conventional burners. The disadvantages of FGR are decreased heater efficiency and increased fan power requirements. The addition of flue gas decreases the temperature of the 3 |