OCR Text |
Show health. The Act also provides that the period of such a standard should be from 1 to 3 hours. A review of acute short term NO2 effects and the prevalence of critical levels of NO2 in the atmosphere is underway and will be used as the basis of a decision for the revision of the existing standard. There are two areas under consideration for new or revised NSPS for steam generators. First, the available data are being reviewed to identify demonstrated technology for industrial boilers (less than 73 MW thermal, but greater than a not-yet-established lower limit). NSPS for both gas turbine and reciprocating engines are also being prepared. The gas turbine standard has been proposed at 75 ppm (at 15 percent O2) for nitrogen-free fuels, with a stepped approach for fuel nitrogen up to a maximum of 125 ppm (at 15 percent O2) for fuels with over 0.25 percent nitrogen. Pollutant Formation Mechanisms The primary emphasis in EPA's N0X control technology development program is on combustion modification to prevent pollutant formation. To optimize the control technology for any given fuel, it is necessary to understand the mechanisms by which N0X is formed and destroyed during combustion. Two distinct sources of N0X, identified by the terms thermal N0X and fuel N0X, are discussed below. In addition, it is necessary to ensure that control technology does not adversely affect other pollutants or system efficiency. A brief discussion of these factors is also given below. The fixation of a small fraction of the molecular nitrogen in the combustion air results in the formation of thermal N0X. Since the activation energies of several of the formation reactions are high, the rate of formation of thermal N0X is strongly temperature dependent. Thermal N0X is formed during the combustion of all fuels in the regions of peak temperature that occur in all diffusion flames. |