How Coal Pollution Is Being Tackled Today

Coal is a major contributor to climate change and local pollution, and is responsible for 44% of global CO2 emissions. Burning coal releases harmful substances that contribute to air pollution, acid rain, and greenhouse gas emissions. However, there are ways to reduce coal pollution, including pre-combustion and post-combustion approaches. Pre-combustion approaches include cleaning coal using physical and chemical means, while post-combustion approaches include flue-gas desulfurization, selective catalytic reduction, electrostatic precipitators, and fly ash reduction. In addition, carbon capture and storage technologies (CCS) are emerging as a way to capture and store CO2, preventing it from being released into the atmosphere.

Cleaning coal before combustion

Coal can be cleaned physically and chemically before combustion. Physical cleaning involves gravimetric processes, often in conjunction with froth flotation, which remove minerals and other non-combustible components of coal by exploiting their greater density compared to coal. This technology is widely practised.

Chemical cleaning involves treating crushed coal with acids or bases to remove deleterious components, leaving combustible material behind. This technology is expensive and rarely moves beyond the demonstration phase. During World War II, German industry removed ash from coal using hydrofluoric acid and related reagents.

Coal washing, a standard practice in developed countries, is another method of cleaning coal before combustion. It reduces emissions of ash and sulfur dioxide when coal is burned.

The Clean Air Act and The Clean Water Act in the US require industries to reduce pollutants released into the air and water. The coal industry has found ways to reduce sulfur and other impurities from coal, as well as more effective ways of cleaning coal after it is mined.

Flue-gas desulfurization

FGD has been used to reduce SO2 emissions from coal-fired power plants in several countries, including the United States, China, and India. In the US, the 1990 Clean Air Act Amendments led many power plants to install FGD units or switch to low-sulfur coal. As of 2010, power plants with FGD units accounted for 60% of the electricity generated from coal in the US. By 2013, 95% of China's coal-fired generating capacity had FGD units installed, while India, which relies on coal for 76% of its electricity, began regulating SO2 emissions from coal-fired power plants in 2015.

There are two main types of FGD technologies: wet scrubbing and dry scrubbing. Wet scrubbing is the more common method, with about 85% of FGD units in the US using this approach. It involves treating flue gases with a slurry of alkaline sorbents, usually limestone or lime, or even seawater. This process can remove up to 90% of SO2 from the flue gases and also produces marketable by-products like gypsum. On the other hand, dry scrubbing technologies, such as spray-dry scrubbing and dry sorbent injection systems, are less commonly used but can still achieve SO2 removal efficiencies of up to 90%. These systems introduce powdered hydrated lime or other sorbent materials into the exhaust ducts to eliminate SO3 from emissions.

FGD plays a crucial role in reducing air pollution and the health impacts associated with coal-fired power plants. Studies have linked SO2 emissions to serious health issues, including respiratory illnesses, ischemic heart disease, stroke, lung cancer, and acute lower respiratory infection. By removing SO2 from flue gases, FGD helps improve air quality and reduce the incidence of these health problems in populations living near coal-fired power plants.

Carbon capture and storage

Capturing CO2

The first step in the CCS process is capturing the CO2. This is typically done by using a chemical solvent or a porous solid material to separate the CO2 from other components of a plant's exhaust stream. The most common method is to use an amine solvent, which binds to the CO2 molecule. The CO2-rich solvent is then heated to release the CO2, which is then compressed and transported for storage or end-use.

Transporting CO2

Once the CO2 has been captured, it needs to be transported to a storage location. Pipelines are the cheapest and most common method for transporting CO2, but ships and road tankers can also be used for smaller-scale applications. As of 2024, there are over 8000 km of CO2 pipelines in North America, with smaller pipeline networks in Europe and the Middle East.

Storing CO2

The final step in the CCS process is storing the CO2 deep underground in geological formations. This is typically done in depleted oil and gas reservoirs or deep saline aquifers, which are located at least 0.62 miles (1 km) underground. The CO2 is injected into the reservoir, where it is trapped by impermeable "caprocks" and other geochemical trapping mechanisms.

CCS has been proposed as a way to reduce emissions from coal-fired power plants, and there are a number of large-scale CO2 storage sites in the US, such as the Citronelle Project in Alabama. However, CCS is expensive and energy-intensive, and it has been estimated that CCS can increase the cost of electricity from a power plant by 21-91%. As a result, CCS has seen limited adoption, with only 44 plants worldwide using the technology as of 2024.

Electrostatic precipitators

How Electrostatic Precipitators Work

ESPs use electrical energy to charge particles, move them to collecting surfaces, and then remove them through the application of high-voltage direct current. The basic components of an ESP include:

• Gas distribution plates: Perforated plates that help maintain proper flow distribution of the incoming gas stream.
• Discharge electrodes: Energized by a power supply to create ions that collide with particles and apply an electrical charge.
• Collection surfaces: Plates or pipes that attract the charged particles, which then build up and are removed through rapping or vibration.
• Rappers: Mechanisms that knock loose or vibrate the collected particles from the collection surfaces.

Types of Electrostatic Precipitators

There are two main types of ESPs:

• High-voltage, single-stage: These combine ionization and collection in a single unit. They are commonly referred to as Cottrell precipitators, after their inventor, Frederick Gardner Cottrell.
• Low-voltage, two-stage: These use a separate ionizing section followed by collection plates. This design minimizes ozone production, making it suitable for enclosed spaces.

ESPs can also be classified as dry or wet, depending on the method used to clean the collector plates. Dry ESPs, the most common type, use mechanical impulses or vibration to dislodge particles, while wet ESPs rinse the plates with water and are typically used for sticky particles with low resistivity.

Factors Affecting Performance

The performance of ESPs is influenced by particle resistivity, which affects how particles interact with the collection plates. Particles with moderate resistivity are ideal, as they retain enough charge to adhere to the plates while still allowing new particles to deposit. Very high resistivity particles can inhibit deposition and cause a buildup of charge, while very low resistivity particles may be repelled back into the gas stream. Other factors, such as gas temperature, moisture content, and particle size distribution, also impact ESP efficiency.

Applications of Electrostatic Precipitators

ESPs are widely used in power plants, chemical processing plants, and oil refineries to remove fly ash and other contaminants from exhaust gases, reducing air pollution and improving air quality. They are particularly effective for small particles below 10 μm (PM10), which can pose health risks if inhaled.

Scrubbing

The use of scrubbers is one of the main strategies implemented by plant owners to meet sulfur dioxide (SO2) emission reduction goals. In the US, coal-fired electric power plants are the largest source of national sulfur dioxide emissions. The Cross-State Air Pollution Rule (CSAPR) calls for a 53% reduction in SO2 emissions from the electric power sector by 2014. As a result, many plants have installed flue gas desulfurization (FGD) scrubbers to reduce their SO2 output.

FGD scrubbers can achieve SO2 removal rates of up to 98%, according to EPA estimates. The removal rate depends on factors such as the specific equipment type, age, and the sulfur content of the coal being burned. Newer systems tend to be more effective at removing SO2 than older ones. Additionally, the type of coal being burned also plays a role in the effectiveness of scrubbers, as different types of coal have varying sulfur content. For example, bituminous coal and lignite coal typically have higher sulfur content than subbituminous coal.

The use of scrubbers has been shown to significantly reduce SO2 emissions from power plants. In 2010, plants with FGD equipment generated 58% of the total electricity produced from coal while only producing 27% of total SO2 emissions. This indicates that the use of scrubbers can help reduce SO2 emissions without necessarily reducing the amount of electricity generated.

In addition to reducing SO2 emissions, scrubbers can also be effective in removing other harmful substances from power plant emissions. For example, electrostatic precipitators and fabric filters can remove up to 99% of fly ash from flue gases. Scrubbers can also be used in conjunction with other technologies, such as selective catalytic reduction, to reduce emissions of nitrogen oxides (NOx).

While scrubbers have been successful in reducing certain emissions, there are some challenges and limitations to their use. One issue is the cost associated with installing and operating scrubbers, which can be significant. Additionally, scrubbers may not be effective in removing all types of pollutants, and other technologies may be needed to address specific pollutants.

Overall, scrubbing, or flue gas desulfurization, is an important technology in the effort to reduce coal pollution and improve air quality. By removing sulfur dioxide and other harmful substances from power plant emissions, scrubbers play a crucial role in meeting emission reduction goals and protecting public health and the environment.

Frequently asked questions