Reducing Air Pollution From Coal-Burning Facilities: Strategies And Innovations

Coal-fired power plants are a major source of air pollution, causing severe harm to both the environment and public health. Burning coal releases toxic airborne pollutants and heavy metals, such as mercury, lead, sulfur dioxide, nitrogen oxides, and particulate matter, which can have detrimental effects on human health, including asthma, respiratory illnesses, neurological problems, and even premature death.

To reduce air pollution from coal-burning facilities, various strategies and technologies can be employed. One approach is to implement pre-combustion measures, such as cleaning coal before it is burned to remove impurities. Additionally, post-combustion techniques, such as flue-gas desulfurization, selective catalytic reduction, electrostatic precipitators, and fly ash reduction, can help capture and remove pollutants from the emissions.

Furthermore, transitioning to carbon-neutral fuels, improving energy efficiency, and adopting circular economy practices can also play a crucial role in reducing air pollution from coal-burning facilities. Advanced carbon capture and storage technologies, such as pre-combustion, oxyfuel combustion, and post-combustion methods, offer promising solutions to mitigate carbon dioxide emissions.

It is important to note that the selection of appropriate techniques depends on various factors, including the age of the power plant, socio-economic considerations, and local regulations. While reducing air pollution from coal-burning facilities is challenging, it is crucial to protect human health and the environment.

Mercury emissions control

Mercury Emissions from Coal-Burning Facilities

Mercury is naturally present in coal, and when coal is burned, mercury is released into the atmosphere. Coal-fired power plants are a major source of mercury emissions, accounting for about 42% of mercury emissions in the United States. The burning of coal has tripled since 1970, making it the largest anthropogenic source of mercury air emissions globally, with approximately 2220 tonnes of mercury released annually. Mercury emissions from coal-burning facilities have severe environmental and health impacts and are linked to asthma, cancer, heart and lung ailments, neurological problems, and global warming.

Mercury's Toxic Effects

Mercury is a potent neurotoxin that can cause damage to the nervous, digestive, and immune systems. It is particularly harmful to fetuses, children, and vulnerable populations. Methylmercury, a form of mercury that is more easily absorbed by the body, can cross the blood-brain barrier and the placental barrier, leading to neurological damage and impaired brain function and development. Exposure to mercury has also been linked to issues with fine motor function, verbal memory, and cardiovascular disease in adults.

To reduce mercury emissions from coal-burning facilities, various strategies and technologies can be employed:

Activated Carbon Injection (ACI)

ACI is a preferred method for removing mercury from coal-fired power plants. It involves injecting activated carbon, which contains halogens like bromine, into the flue gas. The halogens oxidize the mercury, making it easier to absorb and remove from the flue gas. ACI has been successfully implemented in many full-scale systems in the United States.

Flue Gas Desulfurization (FGD)

FGD is a technology used to remove sulfur dioxide (SO2) from flue gas. It can also be used to remove mercury emissions by oxidizing and solubilizing mercury in the flue gas. There are two types of FGD: wet FGD (WFGD) and dry FGD (DFGD). WFGD uses a slurry or wet by-products to scrub the pollutant, while DFGD uses dry by-products.

Selective Catalytic Reduction (SCR)

SCR is a technology typically used to reduce nitrogen oxide (NOx) emissions. However, it can also be used to remove mercury by oxidizing and removing it along with NOx. SCR involves injecting a reducing agent, such as ammonia, into the flue gas, which reacts with NOx to form harmless nitrogen and water.

Particulate Matter Control Technologies

Particulate matter control technologies, such as electrostatic precipitators (ESPs) and baghouses, can also help reduce mercury emissions. These technologies remove particulates and heavy metals from the smoke, including mercury particles. By combining these technologies with other mercury removal methods, such as DFGD or WFGD, the removal of mercury can be enhanced.

Advanced Techniques

Some advanced techniques for multi-pollutant control, including mercury, are being explored. For example, activated coke can remove SO2, NOx, and mercury simultaneously. Additionally, electrocatalytic oxidation (ECO) is a pilot-scale technology that removes multiple pollutants, including mercury, from flue gas. These advanced techniques show promise in effectively controlling mercury emissions from coal-burning facilities.

Sulfur dioxide emissions control

Sulfur dioxide (SO2) is a harmful byproduct of burning coal, which has been linked to asthma, bronchitis, smog, and acid rain. SO2 is produced when the sulfur in coal reacts with oxygen and combines with other molecules in the atmosphere to form small, acidic particulates that can penetrate human lungs.

Flue Gas Desulfurization (FGD)

Flue gas desulfurization (FGD), also known as scrubbers, is a technology used to remove sulfur from the smoke exiting coal-fired power plants' smokestacks. FGD systems are further classified as wet or dry. Wet FGD produces slurry or wet by-products, while flue gas in dry FGD is not saturated. Dry FGD is predominantly used for low to medium sulfur-based coal but can be applied to high-sulfur coal as well.

Sorbent Injection

In the sorbent injection method, sorbents in powder or slurry form (usually alkaline reagents such as lime) are injected upstream of the flue gas to remove sulfur.

Fluidized Bed (FB) Method

In the fluidized bed method, limestone is used to precipitate pollutants during the coal combustion process. This method is often used in conjunction with scrubbers.

Activated Carbon Injection (ACI)

Activated carbon injection is a preferred method for removing mercury emissions, which is another harmful byproduct of coal-fired power plants. ACI physically absorbs mercury, and when combined with halogens, it can further oxidize mercury to form HgO.

Low NOx Burner (LNB) Technology

LNB technology is used to control nitrogen oxide (NOx) emissions, which are another harmful emission from coal-fired power plants.

Selective Catalytic Reduction (SCR)

Selective catalytic reduction is often used in combination with LNB technology to further reduce NOx emissions. SCR uses a catalyst to reduce NOx emissions.

Regulations and Standards

The installation of FGD systems and other pollution control technologies is driven by regulations such as the Clean Air Act and the Mercury and Air Toxics (MATS) rule in the United States. These regulations have led to significant reductions in SO2 emissions, with a 26% drop in the United States from 2014 to 2015, the largest annual drop in the previous decade.

Fuel Switching

Another way to reduce SO2 emissions is by switching to fuels with lower sulfur content, such as sub-bituminous coal or natural gas.

Carbon Capture and Sequestration

Carbon capture and sequestration technologies can also help reduce SO2 emissions by capturing CO2 and other harmful emissions before they are released into the atmosphere.

Energy Efficiency and Fuel Switching

Improving energy efficiency and fuel switching to less polluting energy sources can also help reduce SO2 emissions. For example, combined heat and power (CHP) technology allows for the utilization of waste heat, achieving higher energy efficiency than traditional coal-fired boilers.

Circular Economy Approaches

A circular economy approach can be applied by utilizing waste heat, waste coal, and coal fly ash for other purposes, such as using coal fly ash in the construction industry.

Nitrogen oxides emissions control

Nitrogen oxides (NOx) are a significant contributor to air pollution and respiratory illnesses. They are formed during the combustion of coal and have harmful effects on both human health and the environment. Here are some measures that can be taken to control and reduce NOx emissions from coal-burning facilities:

Combustion Modifications

The first set of strategies to reduce NOx emissions focuses on modifying the combustion process itself. This includes:

• Low NOx Burners (LNB): LNB technology is widely used to reduce NOx emissions by controlling the combustion process. It involves injecting over-fire air (OFA) to create a fuel-rich zone, which helps convert nitrogen into harmless nitrogen gas (N2).
• Burner Modifications: Adjustments to burner design and operating conditions can influence NOx formation. This includes altering the burner tilt angle, fuel staging, and air staging techniques.
• Combustion Air Temperature: Lowering the combustion air temperature can reduce thermal NOx formation, as higher temperatures facilitate the conversion of nitrogen to NOx.
• Steam Injection: Injecting steam into the combustion process can help reduce flame temperature and thermal NOx formation.
• Fuel Switching: Using lower-sulfur coal or co-firing with biomass or natural gas can reduce NOx emissions.

Post-Combustion Treatment

In addition to combustion modifications, post-combustion treatment technologies can be employed to remove NOx from the flue gases after the combustion process:

• Selective Catalytic Reduction (SCR): SCR is a widely used method where ammonia (NH3) is injected into the flue gas, which then reacts with NOx in the presence of a catalyst to form harmless nitrogen gas and water vapour.
• Selective Non-Catalytic Reduction (SNCR): Similar to SCR, but without the use of a catalyst. It is effective at higher temperatures and can be used in combination with SCR for enhanced NOx reduction.
• Electro-Catalytic Oxidation (ECO): This technology combines electrostatic precipitators with catalytic reactors to remove multiple pollutants, including NOx, by converting them into useful by-products like gypsum and fertiliser.
• Activated Coke Adsorption: This process uses activated coke to absorb SO2 and NOx from flue gas, producing valuable by-products like sulfuric acid.
• Membrane Separation: Membrane technologies selectively allow certain gases to pass through, separating NOx from the flue gas.

Particulate matter emissions control

Particulate matter (PM) is one of the many harmful pollutants released into the atmosphere when coal is burned for energy. PM is linked to chronic bronchitis, aggravated asthma, cardiovascular issues such as heart attacks, and premature death.

Removal of Particulate Matter

Particulate matter can be removed from smoke through the use of electrostatic precipitators or baghouses. The former works by using electrostatic forces to remove particles and heavy metals, while the latter uses fabric filters.

Reducing Particulate Matter Emissions

Particulate matter emissions can be reduced by improving emission control technologies. One such technology is the use of ultra-low emission technologies, such as:

• Wet electrostatic precipitators
• Selective catalytic reduction
• Wet dust precipitators
• Limestone-gypsum wet flue-gas desulfurization

Another method to reduce particulate matter emissions is to improve the energy efficiency of coal-fired power plants. This can be done by using:

• Supercritical and ultra-supercritical steam power plants, which operate at higher temperatures and pressures
• Circulating fluidized-bed technology, which burns coal without the use of burners or a flame

Carbon dioxide emissions control

Carbon capture and storage (CCS) technologies are emerging methods that could allow coal plants to capture carbon dioxide (CO2) and transport it to a geological repository for secure storage. However, CCS technology is expensive and has not yet been proven at the scale required to address climate change significantly.

Research is also being conducted on carbon capture, which separates CO2 from emission sources and recovers it in a concentrated stream. The CO2 can then be injected and stored underground, a process known as sequestration.

Another method to reduce carbon emissions is to transition to carbon-neutral fuels and renewable energy sources such as wind, solar, hydro, and biofuels.

Frequently asked questions