OCR Text |
Show -2- Depending on site-specific conditions, these combustion modifications may involve one or more of the following techniques: • • • • • • low excess air firing burners out of service (BOOS) overfire air (OFA) 10w-NOx burners (LNB) fuel biasing among burners (FB) flue gas recirculation (FGR) Historically, when dealing with NOx (NO + N02), focus has been primarily on NO with the general rule of thumb being that NOx is comprised of nominally 950/0 NO with the balance being N02• This basically meant that the N02 component of NOx was largely ignored. However, as combustion modifications have been pushed to produce lower NOx levels, this situation can change, particularly for natural gas-fired systems. The application of high levels of FGR and staged combustion (or fuel biasing) can lead to N02 comprising a higher fraction of the NOx' Table 1 shows how the N02 can vary at one of the Pacific Gas and Electric Company's naturaJ gas-fired utility boilers. (This boiler utilizes FGR, OFA and fuel biasing to reduce NOx emissions.) As can be seen, the N02 can constitute over 400/0 of the NOx, and even at full load, the N02 is 180/0 of the NOx' Table 1 NO/NOx Ratios from a Natural Gas-Fired Utility Boiler Employing FGR, OFA and Fuel Biasing for NOx Control Load N O/N Ox 0/0 MCR 0/0 100 18 90 21 60 43 45 36 15 22-47 While the fraction of NOx that is N02 is not an issue in terms of compliance, unless the N02 becomes high enough to result in plume coloration, it can impact downstream |