Reducing Coal's Impact: Strategies To Minimize Air Pollution

Burning coal has a profoundly harmful impact on the environment and public health. Coal-fired power plants are linked to asthma, cancer, heart and lung ailments, neurological problems, acid rain, global warming, and other severe environmental and public health impacts.

Coal is a carbon-rich black rock that releases energy when burned. It is an abundant and relatively inexpensive fuel source, but it affects the environment. The breaking down of carbon molecules releases energy, but it also produces harmful environmental impacts and pollutants that harm public health.

Several principal emissions result from burning coal, including sulfur dioxide, nitrogen oxides, particulates, carbon dioxide, mercury, and other heavy metals. These emissions contribute to respiratory illnesses, smog, haze, lung disease, and global warming.

To reduce air pollution from burning coal, various methods and technologies can be employed:

- The Clean Air Act and The Clean Water Act require industries to reduce pollutants released into the air and water.

- Power plants can use flue gas desulfurization equipment (also known as scrubbers) to clean sulfur from smoke before it leaves their smokestacks.

- Electrostatic precipitators or baghouses can be used to remove particulates and heavy metals from the smoke.

- Carbon capture and storage technologies (CCS) can be used to capture and store carbon dioxide emissions.

- Reusing and recycling waste produced from burning coal can also help reduce environmental impacts.

Use flue gas desulfurization equipment to clean sulfur from smoke

Burning coal releases a number of airborne toxins and pollutants, including sulfur dioxide (SO2), which is a major air pollutant. It is formed when the sulfur in coal reacts with oxygen, and it combines with other molecules in the atmosphere to form acidic particulates that can penetrate human lungs. SO2 is linked with asthma, bronchitis, smog, and acid rain, which damages crops and other ecosystems, and acidifies lakes and streams.

Flue-gas desulfurization (FGD) is a set of technologies used to remove SO2 from exhaust flue gases. FGD units use scrubbers to reduce the amount of sulfur exiting smokestacks. Here's how it works:

Wet Scrubbing

Wet scrubbing is a common method used in FGD systems to remove SO2 from flue gases. It involves using an alkaline-based sorbent, typically a slurry of limestone or lime, or even seawater, to scrub the gases. The flue gas exits the boiler and is drawn into a spray tower by a fan. The gas temperature is usually above 150 degrees Celsius, so it may need to be passed through a heat exchanger before entering the spray tower. The spray tower holds the alkaline-based slurry in its lower third, known as the effluent holding tank (EHT). A centrifugal pump is used to pull and discharge the slurry from the spray headers at the top of the tower. The slurry gets sprayed across the tray area, where it comes into contact with the gas. As the SO2 meets the slurry, it is absorbed by the water, and the limestone within the slurry neutralises it. The remaining slurry drops back into the EHT, and any excess is caught by a mist eliminator. The SO2 gases are now neutralised, and the reaction that has taken place in the slurry produces calcium sulfite (CaSO3).

Spray-Dry Scrubbing

Spray-dry scrubbing is another method that uses similar sorbent slurries to wet scrubbing. It is often used for smaller combustion units as it forms a solution rather than a slurry, making it easier to operate. This process produces a "spent caustic" solution of sodium sulfite/bisulfite or sodium sulfate that must be disposed of.

Wet Sulfuric Acid Process

The wet sulfuric acid process recovers sulfur from flue gases in the form of commercial-quality sulfuric acid.

SNOX Flue Gas Desulfurization

SNOX FGD removes SO2, nitrogen oxides, and particulates from flue gases.

Dry Sorbent Injection Systems

Dry sorbent injection systems introduce powdered hydrated lime or other sorbent materials into exhaust ducts to eliminate SO3 from emissions.

By using these FGD methods, coal-fired power plants can significantly reduce the amount of SO2 exiting their smokestacks, minimising its harmful effects on human health and the environment.

Use electrostatic precipitators or baghouses to remove particulates and heavy metals

To reduce air pollution from burning coal, electrostatic precipitators or baghouses can be used to remove particulates and heavy metals from smoke.

Electrostatic precipitators are large industrial emission control units that use electrical energy to remove dust and other particles from exhausted gases. The process involves charging particles, moving them to collecting surfaces, and then removing them through the application of high voltage direct current. The four main components of an electrostatic precipitator are gas distribution plates, discharge electrodes, collection surfaces, and rappers. The gas distribution plates help maintain the proper flow distribution of the incoming gas stream. The discharge electrodes are divided into fields and energized by a power supply to create ions that collide with the particles and charge them. The collection surfaces, typically plates or pipes, collect the charged particles. Finally, the rappers knock loose or rinse collected particles from the collection surfaces, depending on whether the unit is a dry or wet electrostatic precipitator.

Electrostatic precipitators can achieve collection efficiencies of over 99%, and their performance can be influenced by particle resistivity. Particle resistivity affects how particles are deposited on and removed from the collection plates, with moderate resistivity being ideal. If particle resistivity is very high, a negative charge builds up on the plates, inhibiting the deposition of other particles. On the other hand, if particle resistivity is very low, particles quickly lose their charge when they reach the plate and are repelled back into the gas stream.

Baghouses are another technology used to remove particulates and heavy metals from emissions.

Reduce the amount of coal burned per unit of useful energy produced

The burning of coal for energy production has several negative impacts on the environment and human health. Coal is a carbon-rich black rock that releases energy when burned. It is a fuel source that is relatively inexpensive to produce and convert to useful energy. However, the process of burning coal releases harmful pollutants into the atmosphere, including carbon dioxide, mercury, nitrogen oxides, and particulate matter. These pollutants contribute to global warming, acid rain, and respiratory illnesses.

To reduce the amount of coal burned per unit of useful energy produced, the coal industry and governments have been working together to develop and implement technologies that improve energy efficiency and reduce emissions. Here are some ways to achieve this:

• Flue Gas Desulfurization Equipment: Also known as scrubbers, this equipment removes sulfur from the smoke produced by coal-fired power plants before it exits the smokestacks. This technology not only reduces sulfur dioxide emissions but can also help reduce mercury emissions from certain types of coal.
• Electrostatic Precipitators and Baghouses: These devices are used to remove particulate matter and heavy metals from the smoke generated by coal combustion. By capturing these pollutants before they are released into the atmosphere, the amount of coal burned per unit of useful energy produced can be optimized.
• Carbon Capture and Storage (CCS): CCS is an emerging technology that captures the CO2 released during coal combustion, preventing it from entering the atmosphere. The captured CO2 can then be transported and permanently stored underground, mitigating its contribution to global warming.
• Low-Sulfur Coal: The coal industry has found ways to reduce sulfur and other impurities from coal, either by removing them from mined coal or by choosing to use low-sulfur coal sources. This approach can help reduce sulfur dioxide emissions and the associated environmental and health impacts.
• Energy Efficiency Improvements: By making coal-fired power plants more energy efficient, less coal is needed to produce the same amount of useful energy. This can be achieved through technological advancements and process optimizations.
• Transition to Cleaner Energy Sources: While coal is currently a significant source of electricity, transitioning to cleaner and renewable energy sources like wind and solar power can help reduce the overall demand for coal and, consequently, the amount burned.

By implementing these strategies, it is possible to reduce the amount of coal burned per unit of useful energy produced, thereby mitigating the environmental and health impacts associated with coal combustion.

Use carbon capture to separate and store CO2

Carbon capture and storage (CCS) is a way to reduce carbon dioxide (CO2) emissions, which is key to tackling global warming. It is a three-step process: capturing CO2 produced by power generation or industrial activity, transporting it, and then permanently storing it deep underground.

CCS has been in operation since 1972 in the U.S., where several natural gas plants in Texas have captured and stored over 200 million tons of CO2 underground. As of 2022, there were 194 large-scale CCS facilities globally, with a CO2 capture capacity of 244 million tons per year.

The first step of CCS is capturing CO2 from emission sources. This can be done at power plants, natural gas processing facilities, and some industrial processes. There are three techniques to remove or 'scrub' CO2: post-combustion, pre-combustion, and oxyfuel combustion. Post-combustion involves capturing CO2 from the exhaust gases after burning the fossil fuel, and it can be retrofitted to most conventional power plants. Pre-combustion captures CO2 from a synthetic gas formed by partially burning the fossil fuel in a 'gasifier', and it also produces hydrogen which can be used as fuel. Oxyfuel combustion burns the fossil fuel in oxygen instead of air, resulting in flue gas that is mostly CO2 and water vapour; the water is condensed through cooling, leaving almost pure CO2.

The second step is transporting the captured CO2 to suitable storage sites. Pumping it through pipelines is the cheapest and most common form of transport.

The third step is storing the CO2 in deep geological formations. Possible storage sites include saline aquifers, depleted oil and gas reservoirs, and 'unmineable' coal seams. The Intergovernmental Panel on Climate Change (IPCC) states that well-selected, well-designed, and well-managed geological storage sites can trap CO2 for millions of years, retaining over 99% of the injected CO2 over 1000 years.

CCS is an important technology for reducing air pollution from burning coal, but it is expensive and still unproven at the scale needed to address climate change. However, with significant cost, technical, legal, and environmental challenges, CCS could potentially play a crucial role in transitioning to a clean energy future.

Reuse and recycle waste produced from burning coal

Reusing and recycling waste produced from burning coal can help to reduce the environmental impact of coal production and consumption.

Land that was previously used for coal mining can be reclaimed and repurposed for airports, landfills, and golf courses. This helps to restore the natural landscape that was disrupted by mining activities.

Coal combustion waste (CCW) includes by-products such as fly ash, bottom ash, boiler slag, and flue gas desulfurization material. These materials can be recycled and reused in various ways:

• Construction and building materials: Fly ash, bottom ash, and boiler slag can be used in the production of concrete, wallboard, clinker, roofing granules, paving materials, asphalt filler, and gypsum wallboard. Fly ash can replace Portland cement in concrete mixes, reducing energy consumption and CO2 emissions. It can also be used in bricks, ceiling tiles, flooring tiles, and carpet backing.
• Soil improvement: FGD materials can be used to improve soil conditions, increase plant growth, and reduce stormwater runoff when used as soil amendments in green roofs and landscaping.
• Embankment fill and structural fill: Coal ash can be used as embankment fill, and fly ash, bottom ash, and boiler slag can be used as structural fill for foundations and retaining walls.
• Roadways and pavements: Fly ash can be used in asphalt and concrete pavements, improving durability and reducing environmental impacts. It can also be used in granular base and sub-base layers, and as a mineral filler in asphalt.
• Acid mine drainage neutralization: CCW can be used to fill abandoned mines, neutralizing acidic liquids that can otherwise drain into nearby waterways.

By recycling and reusing CCW, we can reduce the need for landfill disposal, decrease greenhouse gas emissions, and create economic and product benefits. However, there are concerns about the potential for heavy metal contamination in some "beneficial uses," particularly in construction materials.

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