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Show 4 2.0 EMISSIONS FROM GAS TURBINES Gas turbines fuelled by natural gas are one of the cleanest types of engines available for power production. They produce little or no sulphur dioxide (S02) or carbon monoxide (CO). Overall efficiency increases will contribute to lower carbon dioxide (C02) emissions per unit of energy output. However the trend towards high power and thermal efficiency, derived through high firing temperatures and air compression ratios, has been translated into increased emissions of nitrogen oxides (NO and N02, termed NOx). This pollutant combines with volatile organic compounds and sunlight to form ground level ozone, or smog. Elevated concentrations of ozone are known to have serious effects on respiratory systems and on vegetation. NO"" also contributes to the effect of sulphur dioxide in the format~on of acid rain. The emissions can be transported over long distances, resulting in harmful effects in other geographical areas. . Nitrogen oxides are produced by the high temperature ( 20000C) oxidation of nitrogen contained in the large quantities of air swallowed by the engine. The air entering the combustor from the compressor is at high pressure (15 to 30 atm.) which additionnally contributes to NOx formation. Even though most of the air bypasse8 the actual combustion chamber, there are still comparatively large amounts of excess oxygen and nitrogen available for NOx formation ( ie. five times the 02 in turbine exhaust compared to a boiler). sometimes long residence time of the very hot mixture, which ensures complete combustion of fuel to minimize CO and smoke, also tends to promote NOx production. Over most of the operating range for existing combustors CO emissions will rise as NO~ emissions decline. This represents one of the difficult cha.l.lenges in developing low-emission combustion systems for full power range operation. Additional NOx is formed from liquid fuels such as distillate oil, which have higher local flame temperatures and some entrained nitrogen compounds. Liquid fuelled units also produce some S02 emissions, depending on fuel sulphur content. eml •• lons ppm Combustion temper.ture ---. Influence of temper.tw. on CO and NOx aml •• 1one |
