Reducing Coal Pollution: Strategies For A Cleaner Future

Coal is a major contributor to pollution and climate change, accounting for about 44% of global CO2 emissions. Burning coal releases harmful substances that contribute to air pollution, acid rain, and greenhouse gas emissions. It is also a leading cause of mercury pollution. The combustion of coal releases diverse chemicals into the air, including water, carbon dioxide, sulfur dioxide, and nitrogen oxides. The residue after combustion, coal ash, often contains arsenic, mercury, and lead. The good news is that we can reduce the amount of pollution caused by coal through a series of systems and technologies that seek to mitigate the health and environmental impacts of burning coal for energy.

Cleaning coal before combustion

Coal is a carbon-rich black rock formed over thousands of years from heat and pressure underground. It is a reliable and affordable energy source, especially in the US, where it generates about 30% of all electricity. However, burning coal releases harmful substances, contributing to air pollution, acid rain, and greenhouse gas emissions, with severe environmental and public health impacts.

Physical cleaning is a widely practiced method for cleaning coal. It is more common due to the high cost and complexity of chemical cleaning, which has rarely moved beyond the demonstration phase. During World War II, German industry used hydrofluoric acid and related reagents to remove ash from coal.

The gravimetric processes involved in physical cleaning can be used in conjunction with froth flotation, a technique that separates minerals and other non-combustible material from coal by exploiting their greater density. This process can effectively remove non-combustible components, but it is not perfect, and some pollutants may remain.

Reducing Pollutants

The combustion of coal releases diverse chemicals, including water, carbon dioxide, sulfur dioxide, nitrogen oxides, and mercury. The residue, coal ash, often contains arsenic, mercury, and lead, and the burning of certain types of coal can also release radioactive materials.

By cleaning coal before combustion, the amount of these harmful substances can be reduced. For example, physical and chemical cleaning can remove minerals and other non-combustible components, reducing the amount of ash produced during combustion.

Additionally, other technologies can be used in conjunction with clean coal to further reduce pollutants. For example, flue-gas desulfurization can remove sulfur dioxide, selective catalytic reduction can reduce nitrogen oxides, and electrostatic precipitators can remove particulates and heavy metals.

Drawbacks of Cleaning Coal

While cleaning coal before combustion can help reduce some pollutants, it is important to note that this process does not eliminate all harmful emissions. Coal contains a variety of pollutants, and a single technology cannot capture all of them. For example, mercury, nitrogen oxide, and other poisonous contaminants may still be produced during combustion, even with pre-combustion cleaning.

Additionally, the cost of cleaning coal is very high, and the process is expensive, especially when retrofitting old plants with new infrastructure. The high cost is a significant barrier to implementing clean coal technology on a large scale, and it may be more economical to transition to cleaner energy sources.

Flue-gas desulfurization

FGD processes can be divided into two main categories: once-through (or non-regenerable) processes and regenerable processes. Once-through processes use the sorbent, a substance used to collect impurities, as a beneficial byproduct or dispose of it as waste. Regenerable processes, on the other hand, release the sorbed sulfur dioxide to generate other products such as elemental sulfur, sulfuric acid, or liquid sulfur dioxide.

FGD methods include wet scrubbing, spray-dry scrubbing, and dry sorbent injection systems. Wet scrubbing uses an alkaline slurry, usually made from limestone or lime, to scrub the gases. Spray-dry scrubbing uses similar sorbent slurries, while dry sorbent injection systems introduce powdered hydrated lime or other sorbent materials into the exhaust ducts to eliminate sulfur trioxide from process emissions.

FGD has been used to limit the release of sulfur dioxide from coal-fired power plants since the late 1960s. It is often treated in a dedicated wastewater facility to meet strict FGD wastewater discharge limits.

Selective catalytic reduction

Burning coal releases a number of airborne toxins and pollutants, including mercury, lead, sulfur dioxide, nitrogen oxides, particulates, and various other heavy metals. These emissions have been linked to asthma, cancer, heart and lung ailments, neurological problems, and acid rain.

The U.S. Department of Energy's (DOE) Clean Coal Technology Demonstration Program (CCTDP) was established to address energy and environmental concerns related to coal use. The CCTDP has a value of over five billion dollars and seeks to accelerate the commercialization of advanced coal-based power generation and pollution control technologies.

One project under the CCTDP, conducted by Southern Company Services, Inc., focused on the demonstration of SCR for the control of NOx emissions from high-sulfur, coal-fired boilers. The project began in June 1990, with operational testing occurring between July 1993 and July 1995. The results of this project confirmed the applicability of SCR for US coal-fired power plants, and by 2007, a significant number of commercial SCR units were installed and operating successfully in the United States.

SCR technology plays a crucial role in reducing NOx emissions, which contribute to ground-level ozone formation and the creation of fine particles in the atmosphere. By implementing SCR, we can significantly decrease the environmental and public health impacts associated with coal-fired power plants, moving towards a cleaner and more sustainable energy future.

Electrostatic precipitators

The basic principle behind an ESP is the use of electrostatic forces to remove particles from a gas stream. The device charges the particles, moves them to collecting surfaces, and then removes them through the application of high-voltage direct current. The main components of an ESP are:

• Power supply unit: Provides high-voltage DC power.
• Ionizing section: Imparts a charge to the particulates in the gas stream.
• Collection surfaces: Flat metallic dust collectors that attract the charged particles.
• Means of removing the collected particulates: This can be done through dry or wet methods.
• Housing: Encloses the precipitator zone.

There are two main types of precipitators: high-voltage, single-stage, and low-voltage, two-stage. Single-stage precipitators, also known as Cottrell precipitators, combine an ionization and collection step. Two-stage precipitators use a similar principle but have a separate charging section followed by collection plates.

The performance of ESPs is influenced by factors such as collection surface area, gas-flow rate, particle migration velocity, electrical resistivity, and particle size distribution. ESPs work best under normal resistivity conditions, where particles do not rapidly lose their charge upon reaching the collection plate.

ESPs offer several advantages, including high collection efficiency, low energy consumption, and the ability to handle a wide range of gas compositions and temperatures. However, one challenge is the buildup of particles on the charging wires and collection plates, which requires regular cleaning.

Overall, ESPs play a crucial role in reducing pollution caused by coal-fired power plants, making them an important technology for mitigating the environmental impact of coal combustion.

Fly ash reduction

Fly ash is the finely divided residue that results from the combustion of pulverized coal. It is captured after coal combustion by filters (bag houses), electrostatic precipitators, and other air pollution control devices.

Fly ash is one of the most concerning by-products of burning coal, as it contains enhanced concentrations of toxic elements such as arsenic, beryllium, boron, cadmium, chromium, thallium, selenium, molybdenum, and mercury. If not properly disposed of, fly ash can contaminate groundwater, with documented cases of this occurring in the US.

• Use of Pollution Control Equipment: Air pollution control standards now require that fly ash be captured by pollution control equipment before release. Electrostatic precipitators or other particle filtration equipment are used to capture fly ash before the flue gases reach the chimneys.
• Recycling: Recycling fly ash can reduce pollution and provide economic benefits. It can be used in construction and building materials, agriculture, and high-value environmental protection applications. For example, it can be used as a substitute for Portland cement in concrete production, reducing the demand for virgin materials and providing concrete with more protection from wet conditions and chemical attacks.
• Specially Lined Landfills: Much of the current debate around fly ash disposal revolves around creating specially lined landfills that prevent chemical compounds from leaching into the groundwater and local ecosystems.
• Reducing Coal Consumption: Transitioning to cleaner sources of electricity can help reduce fly ash pollution. While this may be difficult for communities that depend on coal mines and plants, it is essential for avoiding the worst impacts of coal.

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