All power plants that burn fossil fuel to generate electricity create gases and fine particulate matter as a byproduct. This can include dust, nitrous oxide, sulfur dioxide, and mercury and other heavy metals. All of these contribute to air pollution and can result in health issues. See the OWOE topic "What are the types of air emissions from electrical power plants?"
Burning natural gas generates the least amount of these pollutants, and burning low quality coal generates the most. Early power plants emitted these directly into the atmosphere, but over the years the EPA has continually tightened allowable pollution levels. With the exception of carbon dioxide, all these emissions are controllable using modern emissions control technology that must be built into any new coal-fired power plants and that can be retrofitted to older plants to meet EPA limits. Such technologies and their impact on the various categories of emissions are summarized in Figure 1 and include:
- low NOx burners are designed to distribute fuel and air to minimize their mixing at critical nitrous oxide formation temperatures.
- fluidized bed combustion (FBC) or circulating fluidized bed (CFB) are types of boilers in which the combustion air blows upward and suspends a "bed" of pulverized coal and limestone. This results in a turbulent mixing of gas and solids and more effective chemical reactions during combustion. In an FBC boiler, the bed is stationary. If the velocity of the gas flow increases, the particles separate and the bed begins to behave as a fluid and circulate, creating a CFB boiler.
- flue-gas desulfurization (FGD, or more commonly referred to as "scrubbing") is a set of technologies used to remove sulfur dioxide (SO2) from exhaust flue gases of fossil-fuel power plants. There are two basic forms of FGD - wet and dry. In a wet FGD system, a lime or limestone slurry reacts with the SO2 in the flue gas within a large absorber vessel to capture the SO2, as illustrated in Figure 2. In a dry FGD system, hydrated lime and water (either separately or together as a slurry) are injected into a large vessel to react with the SO2 in the flue gas. The term "dry" refers to the fact that the amount of water added is only just enough to maintain the gas above the saturation (dew point) temperature. The first scrubber was installed on a coal-fired power plant in the US in 1968, and modern scrubbers can remove over 98% of the SO2 generated.
- selective non-catalytic reduction (SNCR) to control NOx by injecting nitrogen containing chemicals, typically ammonia or urea, into the flue gas which then reacts with nitrous oxide and oxygen to form molecular nitrogen (N2) and water (H2O). Although it is a less efficient process than SCR, it eliminates the need for expensive catalysts for the chemical reaction. The first SNCR was installed in 1974, and modern SCRs can remove over 90% of the NOx generated.
- selective catalytic reduction (SCR) to control NOx by injecting ammonia (NH3) vapor into the flue gas stream. The chemical reaction to convert the NOx to N2 and H2O is facilitated by passing the gas through a bed of catalyst - typically containing titanium, vanadium oxides, molybdenum, and/or tungsten. The first large-scale SCR system was installed in a new coal-fired power plant in the US in 1993 and the first retrofit in 1995.
- dry sorbent Injection (DSI) is the injection of dry sorbent reagents, such as sodium carbonate, sodium bicarbonate and hydrated lime that react with SO2 and acid gases (hydrochloric acid - HCL and hydrofluoric acid - HF). The downstream PM control device captures the solid reaction products.
- electrostatic precipitators (ESP) to capture particulate matter (PM). An ESP uses an electrical charge to separate the particles in the flue gas stream. The particles are then attracted to oppositely charged plates or tubes and removed from the collection surface to a hopper by vibrating or rapping the collection surface. The effectiveness of an ESP varies depending on the resistivity of the fly ash and on particle size. In ideal conditions, an ESP can capture over 99 percent of total PM, and up to 80-95 percent of PM2.5.
- fabric filters (FF, or more commonly referred to as baghouses) to capture particulate matter (PM). Baghouses are made of woven or felted material in the shape of cylindrical bags or flat envelopes. Hundreds of these filters are assembled into the baghouse system as illustrated in Figure 3, which also includes a dust collection hopper and a cleaning mechanism for periodic removal of the collected particles. A modern on a coal-fired power plant can capture up to 99.9 percent of total PM and 99.0-99.8 percent of PM2.5.
- processes such as activated carbon injection (ACI) for mercury removal. ACI involves injecting powered activated carbon (PAC) into the flue gas upstream of the particulate control equipment (baghouse or ESP). The mercury attaches itself to the PAC and is then captured with the fly ash particulates. ACI has also been demonstrated to be effective in removing dioxins and furans from combustion gas.
- carbon capture and sequestration (CCS) is relatively new technology that has yet to be proven commercial at power plant scale. For more information on this technology, see the OWOE topic "What is clean coal?"
Among all the fossil fuels used for power generation, coal requires the most extensive infrastructure for managing emissions, and a modern coal-fired plant will include all of the separate types of technologies summarized above. According to the National Energy Technology Laboratory (NETL) in 2008, a new subcritical pulverized coal plant built with modern pollution controls would reduce NOx emissions by 86 percent, SO2 emissions by 98 percent, and PM by 99.8 percent when compared with a similar plant with no pollution controls. Figure 4 illustrates the reduction in NOx, SOx and PM for a plant with modern emissions control technology vs a plant without. And Figure 5 illustrates the reduction in SO2 and NOx emissions per BTU of coal consumption from 1970 through 2006, with SO2 reduced to approximately 1/4 and NOx to approximately 1/3 of the 1970 values.
Oil-fired power plants generate emissions that are substantially less than those from coal-fired plants; however, similar control systems are still required to meet emissions standards. For natural gas-fired plants, the primary concern is with NOx control, utilizing the same technologies as those for coal-fired power plants.