OCR Text |
Show 8 3.3 NOx EMISSION TARGET LEVELS Gas turbines without N0x. controls produce relatively high levels of nitrogen oxides, ln the order of 100 to 400 ppmv depending upon the type of unit. Regulated national EPA limits in the united states are currently 75 ppmv for electric power units larger than 30 MW, and 150 ppmv for other units, based on 25% efficiency correction. various non-attainment regions are requiring ultra-low levels, such as those in southern California (9 ppmv) and the Northeast (9 - 25 ppmv). These levels can usually only be attained by the use of Selective Catalytic Reduction (SCR) techniques as a back-end exhaust cleanup, al though some manufacturers are beginning to guarantee 10 to 25 ppmv in the combustors on some types of very large units. The N307 Guideline has been developed to promote high efficiency applications of gas turbines, with a reasonably achievable low level of emissions of NOx and co. It was not generally felt that there was a need on a national basis to go to ultra-low NOx levels which would require SCR back-end cleanup, and its attendant difficulties. The Guideline reflects levels (30 - 50 ppmv range for most units) that are expected to be widely achievable through the development and use of dry lOW-NOx (DLN) combustors by many manufacturers in 1993-96. They are also achievable with moderate steam or water injection in cogeneration plants. In addition the individual provincial or regional regulatory agencies may choose to develop more stringent regulations in response to local air quality problems. The working group believes that a standard reflecting reasonably attainable emission levels will result in a greater acceptance by industry to install improved combustion systems on existing units where applicable. It may also avoid the situation where the installation of high-efficiency combined cycles may be discouraged due to the efforts invol ved in meeting very low emissions. The high marginal cost per tonne of NOx removed for large catalytic systems may only be justifiable in serious nonattainment areas. For example, the annual cost of control to 30 ppmv by steam injection or using a dry-low NO combustor may be about $2000/tonne NOx removal, whereas marginai cost for an SCR installation for additional control down to 10 ppmv could be 10 to 15 times as htgh. |