Pollutant Control Technologies
Coal
Among all the fossil fuels used for power generation, coal requires the most extensive infrastructure for processing, handling, storage, loading and unloading operations (all these facilities generate important environmental impacts). Coal firing requires the use of crushers, pulverizers, ash handling equipment, soot blowers, and dust and emissions control equipment. The most widely used particle controls used for coal combustion are multiple cyclones, electrostatic precipitators, fabric filters and Venturi scrubbers. Various techniques are employed to reduce SO2 emissions from coal-fired plants: physical coal cleaning, chemical coal cleaning, switching to lower-sulfur coals and flue gas desulfurization. Some methods for controlling nitrogen oxides are reducing the peak temperatures in the combustion zone or the gas residence time in the high-temperature zone, the installation of low-NOX burners, selective catalytic reduction, and selective non-catalytic reduction. At some plants in Canada and the United States, devices are used to inject activated carbon or add sorbent to coal specifically to control mercury emissions. Other methods, such as flue gas desulfurization and particulate control, also help to reduce mercury emissions. It has been reported that mercury emissions reductions of 29%, 39% and 45% can be obtained from electrostatic precipitators, baghouses and flue gas desulfurization, respectively [45, 46, 48, 50].
Residual Fuel Oil
Although the uncontrolled air pollutant emissions from oil-fired power stations are substantially lower than those from coal-fired boilers, some emission controls are still required for large oil-fired boilers to meet emissions standards. Control devices similar to those used for coal-fired power plants are used for oil-fired power stations. Mechanical collectors, electrostatic precipitators and fabric filters control particulate matter emissions. Wet scrubbing and spray dryers are common techniques used for reducing SO2 emissions. Due to the low amount of nitrogen present in fuel oils, the techniques for reducing emissions of nitrogen oxides are mainly controls on combustion (for example, flue gas recirculation, staged combustion, etc,); however, post-combustion NOX control techniques are also applied, such as selective non-catalytic reduction and selective catalytic reduction [45, 46, 48, 50].
Natural Gas
Most of the environmental control techniques used in natural gas-fired boilers are for NOX control or abatement and are similar to those described for coal-fired power plants. These include flue gas recirculation, staged combustion, low-NOX burners, selective catalytic reduction and selective non-catalytic reduction [45, 46, 48, 50].