One of the primary advantages of gas turbines is that they produce relatively little pollution, at least compared with coal-fired power plants. In the developed countries of the world where emission control has become a high-profile issue this has had a significant effect on the choice of technology for new generating capacity.
Most gas turbine power plants burn natural gas which is a clean fuel. Gas turbines are, anyway, extremely sensitive to low levels of impurities in the fuel, so fuel derived from other sources, such as gasification of coal or biomass, must be extensively cleaned before it can be burned in a gas turbine.
Even so, gas turbines are not entirely benign. They can produce significant quantities of NO*, some carbon monoxide and small amounts of hydrocarbons. Of these, NO* is generally considered the most serious problem.
NO* emissions are generated during the combustion process. The amount of NO* produced is directly related to the temperature at which combustion takes place. The higher the temperature, the more NO* generated. And since gas turbine designers are pushing forever higher-turbine inlet temperatures in order to increase gas turbine efficiency, the problem of NO* generation has become more acute with time.
It became apparent during the 1970s that development aimed at reducing the amount of NO* generated in gas turbines would become necessary. One approach that met with some success was to inject water into the combustion chamber. This was eventually superseded by the use of dry low NO* burners which control the mixing of fuel and air in such a way as to minimise the production of NO*.
Early low NO* burners did not prove as reliable as their manufacturers had hoped. Nevertheless the latter have pursued this line of development, with second generation low NO* burners appearing at the beginning of the 1990s. The latest heavy gas turbine power plants can generally meet NO* emissions targets in the range 15 -25 ppm. New generation turbines, such as the H-Series from GE, expect to reach 9 ppm.
This level of NO* and carbon monoxide emissions will meet the regulations in many parts of the world but not all. One of the countries that imposes more stringent limits is Japan. In order to meet these limits, a gas turbine has to be equipped with a selective catalytic reduction (SCR) system. This employs a metallic catalyst which stimulates a reaction between NO* and added ammonia or urea, reducing the NO* to nitrogen. SCR is expensive, but effective. A 2800-MW combined cycle power plant built by the Tokyo Electric Power Company at Yokohama in Japan employs SCR units to cut NO* emission levels to less than 5 ppm.
Gas turbines also produce large amounts of carbon dioxide. This is an unavoidable product of the combustion of natural gas. But a gas turbine power station produces proportionally less carbon dioxide than a conventional coal-fired power plant of similar capacity.
The reason for the better carbon dioxide performance is to be found in the composition of natural gas, which is primarily made up of methane. Each methane molecule contains one atom of carbon and four of hydrogen. When this burns in air it generates heat, one molecule of carbon dioxide and two molecules of water.
Coal is primarily composed of carbon. Therefore combustion of coal in air produces only carbon dioxide; it generates no water. The actual comparison is complicated by the amount of heat generated in each case and the efficiency of the two types of power station. But overall, the Electric Power Research Institute (EPRI) has estimated that a gas-fired power station produces around half the carbon dioxide of a coal-fired power station for each unit of electricity.
In the short term a switch from coal-fired to gas-fired power generation can, therefore, reduce carbon dioxide emissions significantly. Since carbon dioxide is a major contributor to the global greenhouse effect, switching is one strategy that is enabling some countries to meet (or attempt to meet) the emission targets of the Kyoto Protocol. In the long term, however, it seems probable that the continued use of natural gas as a power plant fuel will require some form of carbon dioxide capture. (Strategies to accomplish this have been outlined in Chapter 3.)
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