Coal Scrubbing: Reducing Airborne Pollution And Saving The Environment

Coal is a widely used fuel source, but burning it produces harmful environmental impacts and pollutants that harm public health. Coal plants are responsible for 42% of US mercury emissions and are the largest source of national sulfur dioxide (SO2) emissions. Scrubbers, or flue gas desulfurization equipment, are used to reduce the amount of sulfur exiting smokestacks and remove harmful emissions from flue gases. They are effective in removing sulfur oxides, nitrogen oxides, acid gases, and other pollutants from the air. By implementing scrubber technology, the US has been able to significantly reduce sulfur emissions from coal plants, improving air quality and reducing the harmful effects of coal on human health and the environment.

Scrubbers remove harmful emissions from flue gases

Scrubbers are an effective way to remove harmful emissions from flue gases. Flue gas treatment processes are a generic term for scrubbers, which can be either liquid or solid-type gas treatment processes. They are designed to remove harmful gases produced during the combustion of fossil fuels like coal and oil, which are known to cause air pollution and have detrimental effects on human health and the environment.

One of the key functions of scrubbers is to remove sulfur dioxide (SO2) from flue gases. SO2 is produced when sulfur in coal reacts with oxygen during combustion. It forms small, acidic particulates that can penetrate the human lungs and are linked to respiratory issues such as asthma and bronchitis. Scrubbers use a process called flue gas desulfurization (FGD) to remove SO2 from the exhaust gases of coal-fired power plants. This process involves passing the flue gas through an alkaline solution, which effectively removes the SO2 and other acid gases like hydrochloric acid.

In addition to SO2 removal, scrubbers can also reduce emissions of nitrogen oxides (NOx). These gases are visible as smog and have adverse effects on lung tissue, exacerbating asthma and other respiratory illnesses. By utilising processes such as wet scrubbing or dry scrubbing, where alkaline media is injected into the flue gas stream, nitrogen oxide emissions can be significantly reduced.

Another important aspect of scrubbers is their ability to remove mercury from flue gases. Mercury is a toxic heavy metal released during coal combustion, which can cause nervous system, digestive, and immune system damage. Wet scrubbers are particularly effective in mercury removal, where the elemental mercury is oxidised and converted into water-soluble salts that can be easily collected.

Furthermore, scrubbers are also capable of capturing particulate matter, also known as soot. Particulate matter is linked to chronic bronchitis, aggravated asthma, cardiovascular issues, and even premature death. By employing electrostatic precipitators or fabric filters, scrubbers can remove up to 99% of particulate matter from flue gases, significantly improving air quality.

Overall, scrubbers play a crucial role in reducing harmful emissions from flue gases, specifically targeting SO2, NOx, mercury, and particulate matter. By utilising various processes, such as FGD, wet scrubbing, and electrostatic precipitation, scrubbers help improve air quality and mitigate the negative impacts of coal combustion on human health and the environment.

Scrubbers can be used to remove mercury from flue gases

Scrubbers are air pollution control devices that can be used to remove particulates and gases from industrial exhaust streams. They are one of the primary devices used to control gaseous emissions, especially acid gases.

Wet scrubbers are effective for the removal of soluble mercury species, such as oxidized mercury (Hg2+). However, mercury vapor in its elemental form (Hg0) is insoluble in the scrubber slurry and is not removed. Therefore, an additional process of Hg0 conversion is required to complete mercury capture. Halogens are usually added to the flue gas for this purpose. The type of coal burned, as well as the presence of a selective catalytic reduction unit, will affect the ratio of elemental to oxidized mercury in the flue gas and, thus, the degree to which the mercury is removed.

The US Environmental Protection Agency (US EPA) has classified mercury and its compounds as severe air quality threats due to their adverse impact on health, global distribution, long atmospheric lifetime, and propensity for deposition in the aquatic environment and living tissue.

In addition to wet scrubbers, dry and semi-dry scrubbing systems can also be used to remove acid gases, such as sulfur dioxide and hydrochloric acid, primarily from combustion sources. These systems inject a dry reagent or slurry into a dirty exhaust stream to "wash out" acid gases.

Flue gas desulfurization equipment can remove up to 97% of sulfur dioxide output

Flue gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide (SO2) from the exhaust flue gases of fossil-fuel power plants. FGD is also used to remove sulfur dioxide from the emissions of other sulfur oxide-emitting processes, such as waste incineration, petroleum refineries, cement and lime kilns.

The removal of sulfur dioxide is critical to establishing a safe and clean environment. When breathed in, it can irritate the nose, throat, and airways, with the risk of more severe problems developing over prolonged exposure. It is also a major air pollutant that impacts all life and is a precursor to acid rain, which has significant adverse effects on forests, freshwaters, and soils, in turn killing insects and aquatic life-forms.

FGD scrubbers can achieve sulfur removal efficiencies of between 50% to 98%. Wet scrubbers typically achieve the highest sulfur removal rates, while dry scrubbers tend to achieve the lowest. For a typical coal-fired power station, FGD may remove 90% or more of the SO2 in the flue gases.

One example of a wet scrubbing process is the use of a spray tower scrubber. In this process, flue gas exits the water tube boiler and is drawn into the spray tower scrubber by a fan. The lower third of the spray tower is where the alkaline-based slurry is held, known as the effluent holding tank (EHT). The slurry is most commonly a mixture of ground-up limestone and water. A heavy-duty centrifugal pump is used to pull and discharge the slurry from the spray headers nearer the top of the tower. The slurry gets sprayed across the entire tray area where the gas meets the slurry. As the sulfur dioxide comes into contact with the slurry, it is absorbed by the water, and the limestone within the slurry neutralises it.

The gas that continues to rise is now at a pH level closer to neutral, and the slurry then drops back down into the EHT. Any remaining slurry that continues to rise is stopped by a mist eliminator. The sulfur dioxide gases are now neutralised, and the reaction that has taken place in the slurry has produced calcium sulfite (CaSO3), which can be turned into calcium sulfate (CaSO4) through forced oxidation.

Another example of a wet scrubbing process is the wet sulfuric acid process, which recovers sulfur in the form of commercial-quality sulfuric acid. Dry sorbent injection systems, on the other hand, introduce powdered hydrated lime or other sorbent materials into exhaust ducts to eliminate sulfur trioxide (SO3) from process emissions.

Scrubbers can be used to remove acid gases, such as hydrochloric acid

Scrubbers are an effective way to remove harmful materials from industrial exhaust gases before they are released into the environment. They are particularly useful for removing acid gases, such as hydrochloric acid, from exhaust streams.

There are two main types of scrubbers: wet scrubbers and dry scrubbers. Wet scrubbers use a liquid substance, usually water, to wash unwanted pollutants from a gas stream. Water is used to remove inorganic contaminants, especially dust, but other solutions can be used to target specific compounds. Wet scrubbers are also used to remove acid gases, water-soluble inorganic contaminants, and volatile organic compounds.

Dry scrubbers, on the other hand, use a solid substance, generally in powdered form, to remove harmful components from exhaust gases. In dry scrubbers, flue gas comes into contact with pulverized limestone, which is mainly calcium carbonate. The chemical reaction between the calcium carbonate and the sulfur dioxide yields calcium sulfite, which is then removed from the gas stream.

Both wet and dry scrubbers are effective in removing pollutants. Wet scrubbers can remove about 90% of pollutants, while dry scrubbers remove about 98% of sulfur from flue gases. However, scrubbers are expensive to maintain and install, and the process of scrubbing requires a lot of energy. Additionally, the waste products from the scrubbing process must be disposed of safely, as they often contain harmful chemicals.

Despite these challenges, scrubbers have played a crucial role in reducing air pollution from coal-fired power plants. By collecting particulate matter and acidic gases, scrubbers help prevent the formation of acid rain and improve air quality, leading to better health outcomes for people who could otherwise be affected by these pollutants.

Scrubbers are effective in reducing emissions of sulfur and nitrogen oxides, toxic stable organic chemicals, and particulates

Sulfur Oxides

Sulfur dioxide (SO2) is a colorless gas that becomes a secondary particle when it meets other gases within the Earth's atmosphere. It is released into the air through various manufacturing processes, particularly the combustion of coal for power generation. Coal contains sulfur, which, when burned, combines with the oxygen in the combustion air to produce SO2. This gas is emitted into the air through a plant's stacks if not removed by a scrubber.

SO2 scrubbers, or flue gas desulfurization (FGD) technology, remove or "scrub" SO2 emissions from the exhaust of coal-fired power plants. The scrubber sprays a wet slurry of limestone into a large chamber where the calcium in the limestone reacts with the SO2 in the flue gas. Some scrubbers may use other chemicals such as lime or magnesium oxide. The SO2 combines with the limestone to form calcium sulfate, commonly known as synthetic gypsum, which is recyclable and used in manufacturing wallboard, cement, and soil amendments.

Nitrogen Oxides

Nitrogen oxides are one of the principal emissions from burning coal and contribute to smog and respiratory illnesses. Equipment such as scrubbers (flue gas desulfurization) can help reduce nitrogen oxide emissions, along with sulfur dioxide and particulate matter.

Toxic Stable Organic Chemicals

Emission scrubbers come in two forms: wet and dry. Wet scrubbers use water and other liquid solvents like limestone and activated alumina to capture particles and drain them away. The electrical charge of the fluid attracts and removes contaminating particles from the gas. Wet scrubbers are effective in removing volatile organic chemicals (VOCs) and hazardous air chemicals (HACs).

Dry scrubbers use dry material to react with the gas and remove dangerous particles. Both types of scrubbers harness natural principles like gravity and steam to clean gas emissions.

Particulates

Particulates are another emission from burning coal that contributes to smog, haze, respiratory illnesses, and lung disease. Scrubbers, particularly wet scrubbers, are effective in reducing particulate emissions. According to the EPA, wet scrubbers can remove up to 95% of particulate matter from industrial gases, which helps to reduce acid rain and protect the environment and human health. Power plants also use electrostatic precipitators or baghouses to remove particulates and heavy metals from smoke.

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