OCR Text |
Show 5 Gas turbine NO emissions are typically measured as a fraction of NOz equivalent m~asured in parts per million by volume (ppmv) in the exhaust stack, corrected to dry ISO (15°C) conditions. Older machines built in the 1960' s had lower firing temperatures and pressure ratios, and generally had full-load emissions in the 50 to 100 ppmv range. However today's more powerful and efficient units will have approximately double these emission levels. As the turbine load is reduced with less fuel flow, and with lower compressor air mass flow and pressure ratio, NO levels drop off at a faster rate. For example, typical productionxof NO (kg/hour) at 80 percent load may only be 60-70 percent of full lo;d production. uncontrolled emission levels of some common new types of base loaded units are shown below (assuming 8000 operating hours/yr, half at full load; simple cycle, gas fuel). Average emissions work out to about 2-3 kg per MW-hour of operation. unit Size ppmv g/GJ(out> kg/hour tonnes/yr 1 MW 70 550 2 14 4 MW 120 900 13 80 14 MW 150 800 40 250 25 MW 180 1000 90 550 50 MW 200 1100 200 1250 Although a large gas turbine could turn out to be a major NOx source, most of the largest ones in combined cycle operation use steam or water injection to reduce levels by 70 percent into the 25 to 75 ppmv range (Control methods are summarized in Appendix B). The complex equipment required for these "wet" methods is already available in the growing number of combined cycle/cogen plants. However this is generally less practical in more isolated, simple cycle installations at gas pipeline compressor stations. Since most existing electric generating units are primarily in peaking duty, about 80 percent of the estimated 40 kilotonnes of gas turbine NO emissions in 1994 will come from the gas transmission industry. Addi tional work is underway to improve inventories of NOx emissions. (Note that total national emissions from all sectors are approximately 1900 kilotonnes, roughly equally split between fossil-fuel combustion in the transportation sector and from stationary sources.) |