Kiera Jefferson
Coal is a fossil fuel that releases harmful substances when burned, contributing to air pollution, acid rain, and greenhouse gas emissions. Coal pollution mitigation methods aim to minimize the negative impacts of coal combustion and can be categorized into pre-combustion and post-combustion approaches. Pre-combustion methods involve cleaning coal physically or chemically to remove non-combustible components and deleterious elements. Chemical cleaning, although effective, is expensive and rarely implemented. Post-combustion methods include flue-gas desulfurization, selective catalytic reduction, electrostatic precipitators, and fly ash reduction, which help reduce the release of harmful substances into the atmosphere. Additionally, newly built coal-fired power plants can utilize gasification processes to separate CO2 from exhaust fumes, and carbon capture and storage technologies can further reduce carbon emissions. While clean coal technologies have been promoted, there are concerns about their effectiveness, as some power plants burning refined coal have reported increased nitrogen oxide (NOx) emissions. Nevertheless, the development of pollution mitigation technologies remains a priority to reduce the environmental and health impacts of coal combustion.
| Characteristics | Values |
|---|---|
| Cleaning coal | Can be done physically or chemically |
| Cleaning coal physically | Involves gravimetric processes, often in conjunction with froth flotation |
| Cleaning coal chemically | Involves using chemicals to remove harmful components, leaving combustible material behind |
| Cleaning coal with chemicals historically | During World War II, German industry removed ash from coal by treating it with hydrofluoric acid and related reagents |
| Cleaning coal with chemicals today | Rarely used due to high costs |
| Cleaning coal ash | Can be cleaned up from leaking pits in rivers, lakes, streams, and groundwater |
| Cleaning coal ash in the US | Cleanups are underway in North Carolina, South Carolina, Virginia, Tennessee, Georgia, and Alabama |
| Cleaning coal emissions | Can be done using flue gas desulfurization equipment (scrubbers) to clean sulfur from smoke before it leaves smokestacks |
| Can be done using electrostatic precipitators or baghouses to remove particulates and heavy metals from smoke | |
| Can be done using low-NOx burners to reduce nitrogen oxide emissions | |
| Can be done using selective catalytic reduction to clean up NOx emissions | |
| Can be done using carbon capture and storage technologies | |
| Can be done using gasification of coal prior to combustion to separate out CO2 from exhaust fumes |
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What You'll Learn
Cleaning coal before combustion
Coal can be cleaned before combustion by physical and chemical means. Physical cleaning involves gravimetric processes, often in conjunction with froth flotation, to remove minerals and other non-combustible components from the coal. This method is widely practised. On the other hand, chemical cleaning uses acids or bases to treat crushed coal, removing its harmful components while leaving the combustible material behind. This method is expensive and rarely moves beyond the demonstration phase.
Coal washing is a standard practice in developed countries. It reduces emissions of ash and sulphur dioxide when the coal is burned. Coal washing removes unwanted minerals by mixing crushed coal with a liquid and allowing the impurities to separate and settle. Wet scrubbers, or flue gas desulfurization systems, remove sulphur dioxide by spraying flue gas with limestone and water. The mixture reacts with the sulphur dioxide to form synthetic gypsum, a component of drywall.
Another pre-combustion method is gasification, which makes it easier to separate the CO2 from the exhaust fumes, thereby reducing the cost of the process. An air separation unit produces a stream of almost-pure oxygen, which flows into a coal gasifier. Gasifiers are tanks that produce synthetic gas mixtures known as syngas. The oxygen in the coal gasifier reacts with fuel to create syngas made up of hydrogen, carbon monoxide, water, and CO2.
Carbon capture and storage (CCS) technologies can also be used before combustion to capture and store CO2 emissions. This technology has been around since the 1980s and is considered one of the most promising clean coal technologies. CCS technology can be applied at multiple points in the process, including pre-combustion, to capture and store CO2 emissions.
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$145.47 $159
Flue-gas desulfurization
FGD systems have been used to limit the release of sulfur dioxide from coal-fired power plants since the late 1960s. The technology is widely used in developed countries where needed and is often treated in a dedicated wastewater facility, allowing plants to meet strict FGD wastewater discharge limits. Common FGD methods include wet scrubbing, spray-dry scrubbing, the wet sulfuric acid process, SNOX flue gas desulfurization, and dry sorbent injection systems. Wet scrubbing, for example, uses a slurry of alkaline sorbent, usually limestone or lime, or seawater to scrub gases. It is the most common type of FGD in the US, with approximately 85% of FGD units installed falling under this type.
Another example is SNOX Flue gas desulfurization, which removes sulfur dioxide, nitrogen oxides, and particulates from flue gases. It is possible to scrub sulfur dioxide by using a cold solution of sodium sulfite, forming a sodium hydrogen sulfite solution. By heating this solution, it is possible to reverse the reaction to form sulfur dioxide and the sodium sulfite solution. Since the sodium sulfite solution is not consumed, it is called a regenerative treatment.
FGD is an important technology for reducing the environmental and health impacts of burning coal for energy.
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Carbon capture and storage
CCS has been in operation since 1972 in the US, where several natural gas plants in Texas have captured and stored over 200 million tons of CO2 underground. Today, CCS projects are storing almost 45 million tons of CO2 annually, which is comparable to the CO2 emissions of about 10 million passenger cars.
CCS technology involves capturing CO2 emissions from industrial processes, such as steel and cement production, or from burning fossil fuels in power generation. This CO2 is then transported via ship or pipeline to be stored deep underground in geological formations.
Captured CO2 can be compressed into a liquid-like state and pumped into wells over 2,500 feet deep. These wells can be in used-up oil and gas reservoirs or formations with salty water. This process is known as enhanced oil recovery (EOR), which involves injecting CO2 into active oil reservoirs to increase oil recovery.
CCS can also involve reacting captured CO2 with naturally occurring minerals like iron, magnesium, and calcium through mineral carbonation. This process prevents the re-release of CO2 into the atmosphere, but it occurs slowly under normal conditions, requiring additional energy to increase temperature and pressure for ideal reaction rates.
Overall, CCS plays a crucial role in reducing CO2 emissions and mitigating the environmental impact of burning coal for energy.
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Electrostatic precipitators
An electrostatic precipitator (ESP) is a filterless device that removes fine particles, such as dust and smoke, from a flowing gas using the force of an induced electrostatic charge. This is done by charging particles in the gas stream, either positively or negatively, using electrical energy. The charged particles are then attracted to collector plates carrying the opposite charge. The collected particles may be removed from the collector plates as dry material (dry ESPs) or washed from the plates with water (wet ESPs). ESPs are capable of collection efficiencies greater than 99%.
ESPs are important tools in the process of cleaning up flue gases, and they are highly effective at reducing particle pollution. They can handle large volumes of gas at various temperatures and flow rates, removing either solid particles or liquid droplets. ESPs are available in many different sizes and types, designed for various dust and water droplet characteristics and gas volume flows. Some types are designed to work with gas streams with particular temperature and moisture characteristics. Dry electrostatic precipitators operate above the dew point of the gas stream to remove impurities from smoke and dust. Wet electrostatic precipitators, on the other hand, operate with saturated airstreams that have 100% relative humidity.
ESPs are commonly used in fossil-fuel power-generating stations to remove particulates and heavy metals from smoke. They are also used to control industrial particulate emissions, including smoke from electricity-generating utilities. ESPs are highly effective at capturing fine particles, including those smaller than 2.5 microns (0.0001 inches) in diameter, which are especially harmful if released into the atmosphere as they can be drawn deep into the lungs. These fine particles can contribute to climate change and lead to serious health problems in humans, including lung damage and bronchitis.
The basic design of an ESP consists of a row of thin vertical wires and a stack of large flat vertical metal plates. The plates are spaced anywhere from less than 0.5 inches to about 7 inches apart, depending on the application. The gas distribution plates consist of several perforated plates that help maintain the proper flow distribution of the entering gas stream. The discharge electrodes are divided into fields, with most ESPs having three or four fields in series, while very large units may have up to fourteen fields in series.
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Scrubbers
The Clean Air Act of 1970 imposed more stringent pollution control requirements on coal-fired power plants, accelerating research on "flue gas desulfurization units" or "scrubbers". The first full-scale scrubber at a coal-burning power plant began operating in the United States in 1967. The scrubbers converted sulfur into a sludge-like waste product, which can now be treated to produce a dry powder used for commercial purposes.
Wet scrubber setups with fluidized beds employ multi-level trays that generate foam as the airstream flows through, substantially increasing the surface area available for pollutant absorption. This foam layer also helps retain liquid droplets for longer, enhancing overall scrubber efficiency. Wet configurations can remove up to 95-99% of SO2 from the mixture, making them one of the most effective pollution control methods.
The use of scrubbers has been linked to a stark decline in certain types of air pollution and a reduction in hospitalisations for Ischemic Heart Disease (IHD) in people aged 65+. Scrubbers are a proven technology for reducing the environmental and health effects of coal production and consumption.
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Frequently asked questions
Clean coal refers to technologies that seek to reduce the negative health and environmental impacts of burning coal for energy. This includes pre-combustion approaches, such as cleaning coal, and post-combustion approaches, such as flue-gas desulfurization.
Clean coal technologies have been criticized for failing to deliver on their environmental promises. For example, in 2017, most American power plants that received subsidies for burning refined coal failed to reduce nitrogen oxide emissions by 20%, which was the threshold required under the policy.
Some examples of clean coal technologies include carbon capture and storage, coal gasification, and flue-gas desulfurization.
Coal ash is the residue remaining after coal combustion, which often contains arsenic, mercury, lead, and other harmful contaminants. Coal ash pollution can be cleaned up through excavation or recycling, as seen in the largest coal ash cleanup in North Carolina, where 126 million tons of coal ash were removed from leaking, unlined coal ash pollution sites.
