Meghan Hodges
Burning coal releases harmful substances that contribute to air pollution, acid rain, and greenhouse gas emissions. Coal is a major source of fine particulate matter (PM2.5) air pollution, which is linked to asthma, cancer, cardiovascular issues, and premature death. Coal plants are responsible for 42% of US mercury emissions and release other toxic heavy metals, including arsenic and lead. To reduce these harmful effects, pollution control technologies such as scrubbers (flue gas desulfurization equipment) are used to capture pollutants and reduce emissions. Additionally, carbon capture and storage technologies (CCS) aim to capture and store CO2 emissions, although this technology is still expensive and unproven at scale. Cleaning coal by physical and chemical means can also reduce pollutants, but chemical cleaning is costly and rarely moves beyond the demonstration phase.
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What You'll Learn
Cleaning coal with physical and chemical treatments
Coal is a major source of energy and has been relied upon for a long time, especially in the US. However, burning coal releases harmful substances that contribute to air pollution, acid rain, and greenhouse gas emissions. Coal is also linked to asthma, cancer, heart and lung ailments, neurological problems, and global warming.
To mitigate the negative impacts of coal, it can be cleaned using physical and chemical treatments. Physical cleaning usually involves gravimetric processes, often in conjunction with froth flotation. These processes remove minerals and other non-combustible components of coal, exploiting their greater density compared to coal. This technology is widely practiced.
Chemical treatments, on the other hand, use acids or bases to remove the harmful components of coal, leaving the combustible material behind. This technology is expensive and has rarely moved beyond the demonstration phase. An example of this is the treatment of coal with hydrofluoric acid, which was used by the German industry during World War II to remove ash.
Other methods to reduce pollutants from coal include the use of scrubbers (flue gas desulfurization equipment), electrostatic precipitators, and fabric filters, which can remove 99% of fly ash from flue gases. Additionally, carbon capture and storage technologies (CCS) are emerging as a way to capture and store CO2 emissions, though this technology remains expensive and unproven at a large scale.
Overall, the cleaning of coal through physical and chemical treatments, as well as the implementation of new technologies, aims to reduce the negative health and environmental impacts of burning coal for energy.
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Electrostatic precipitators and fabric filters
Electrostatic Precipitators
Electrostatic precipitators (ESPs) are devices that use electrical energy to remove particles from a gas stream. The key principle behind ESPs is charging particles, either positively or negatively, and then attracting them to collector plates carrying the opposite charge. The charged particles are collected on the plates and can be removed as dry material or washed off with water. ESPs are highly efficient, with collection rates exceeding 99%. They consist of gas distribution plates, discharge electrodes, collection surfaces, and rappers. ESPs have been widely used in the electric utility industry due to their low capital and operating costs, but stricter emission standards have increased costs for precipitators, making fabric filters a competitive alternative.
Fabric Filters
Fabric filters, also known as baghouses, are another technology used to capture particulate matter from coal-fired power plant emissions. They work by filtering the gas stream through a fabric or cloth medium, trapping the particles within the fabric's fibres. Fabric filters can achieve high particulate removal efficiencies, similar to ESPs. One advantage of fabric filters is their ability to capture fine particles more effectively than ESPs, making them a preferred choice in meeting stringent emission regulations.
Performance Indicators and Costs
The performance of ESPs is evaluated using various indicators, including particulate matter outlet concentration, opacity, secondary corona power, and various voltage and current measurements. While ESPs have been dominant in the industry, the rising costs associated with meeting stricter emission standards have made fabric filters a more economically viable option. Specific tools have been developed to estimate the costs of ESPs for coal-fired power plants, taking into account factors such as purchasing, installation, and monitoring expenses.
Environmental Impact
The use of technologies like ESPs and fabric filters is essential for reducing the environmental and health impacts of coal-fired power generation. These technologies help capture particulate matter, heavy metals, and toxic pollutants, reducing their release into the atmosphere. By mitigating these emissions, ESPs and fabric filters contribute to improved air quality, reduced health risks for nearby communities, and a lower environmental footprint for coal-based energy production.
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Carbon capture and storage
During the capture step, CO2 is captured from power generation or industrial activity, such as hydrogen production, steel or cement making. It can also be captured directly from the atmosphere. The captured CO2 is then transported via ship or pipeline. Finally, during the storage step, the CO2 is stored deep underground in geological formations, such as saline aquifers or depleted gas fields. Saline aquifers are underground geological formations composed of porous, sedimentary rock filled with saltwater. The CO2 is injected into these formations and stored permanently. Other forms of CCS include mineral storage, where captured CO2 is reacted with naturally occurring iron, magnesium, and calcium minerals, and natural carbon sinks, such as forests, oceans, grasslands, and wetlands.
CCS has been in operation since 1972 in the United States, where several natural gas plants in Texas have captured and stored more than 200 million tons of CO2 underground. CCS technologies could allow coal plants to capture some of the CO2 they would otherwise release, and store it without harming the Earth's climate. However, CCS remains expensive and has not yet been proven at the scale needed to materially contribute to addressing climate change.
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Scrubbers and flue gas desulfurization
Flue gas desulfurization is a process that removes sulfur dioxide (SO2) from the flue gases produced by coal-fired power plants. FGD scrubbers use a variety of reagents, such as limestone (CaCO3), lime, or sodium hydroxide, to react with SO2 and form calcium sulfite (CaSO3) or gypsum (CaSO4·2H2O). These solid products can be disposed of or used in various industrial applications, such as wallboard production. FGD scrubbers can be categorized into wet scrubbers, dry scrubbers, and spray scrubbers, with wet scrubbers achieving the highest SO2 removal efficiencies of over 90%, and newer dry scrubber designs also approaching 90% efficiency.
The configuration of scrubbers can vary, with vertical or horizontal towers and different flow patterns of flue gas and liquid. Spray towers, for example, use spray nozzles to generate droplets for surface contact with the flue gas, while packed towers can become plugged by collected particles or scale buildup when used with certain reagents. Wet scrubbers produce wastewater that requires treatment to meet environmental regulations, but advancements in ion-exchange membranes and electrodialysis systems have improved the efficiency of wastewater treatment.
The International Maritime Organization (IMO) has adopted guidelines for the use of exhaust gas scrubbers on ships to comply with sulfur regulations. Port States have the right to ensure these scrubber systems are functioning correctly and can sanction ships with non-compliant systems.
Overall, scrubbers and flue gas desulfurization play a crucial role in mitigating the environmental and health impacts of coal-fired power generation by reducing sulfur dioxide and other harmful emissions. These technologies help to minimize air pollution, acid rain, and the risk of respiratory illnesses associated with coal combustion.
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Low-NOx burners
John Joyce, an influential Australian inventor, played a pivotal role in the development of low-NOx burners. In the late 1980s, Joyce embarked on a research and development programme aimed at minimising nitrogen dioxide emissions from unflued gas heaters. This endeavour was prompted by media coverage highlighting the adverse effects of indoor nitrogen dioxide levels on individuals with respiratory issues. Joyce's research focused primarily on surface combustion techniques, and his LO-NOx water heater burners have successfully passed tests demonstrating their safety and NO2 reduction capabilities.
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Frequently asked questions
Coal pollution mitigation is a series of systems and technologies that aim to reduce the negative health and environmental impacts of burning coal for energy.
Coal pollution mitigation can be divided into pre-combustion and post-combustion approaches. Pre-combustion approaches include cleaning coal by physical and chemical means. Physical cleaning involves gravimetric processes, often with froth flotation, to remove minerals and other non-combustible components. Chemical cleaning uses acids or bases to remove harmful components, leaving combustible material. Post-combustion approaches include flue-gas desulfurization, selective catalytic reduction, electrostatic precipitators, and fly ash reduction.
Pollution control technologies, such as emissions scrubbers, have been shown to significantly reduce the number of associated deaths from coal power plants. For example, the Keystone facility in Pennsylvania saw a decline from over 600 deaths per year to 80 per year after installing scrubbers.
