The choice of which technology to use and the type of power plant to build depends on many factors, such as purpose of the plant, capacity required and fuel availability. The fuel is one of the most important elements to consider also in terms of air pollutant emissions. Table 3.1 shows the most common fuels used, by type of power plant. RFO: residual fuel oil
Table 3.1. Fuels used by Type of Power Plant In Figure 3.1 the relative proportion of pollutants emitted during the combustion of fossil fuels is shown. In this figure, coal has been taken as a reference; therefore, for each pollutant, emissions from coal are given a value of 100%. Natural gas is said to be the "cleanest" of the three types of fossil fuels, due to the significantly lower emissions generated before the post-combustion control equipment. Emissions of NO_X are highly dependent on firing configuration, type of burners, flame temperature and to some extent on the nitrogen content of the fuel, although NO_X is formed even when the fuel contains no nitrogen. In addition to the pollutants shown in Figure 3.1, there are other pollutants emitted by fossil fuel power stations, such as organic compounds that include volatile organic compounds (VOCs), semi-volatile organic compounds, and condensable organic compounds. There are also emissions of some metallic compounds (of which mercury is the only one considered in this report); other greenhouse gases besides CO₂, such as methane and nitrous oxide; and some halogenated compounds.