Air Purifiers: Fighting Pollution, One Breath At A Time

Air pollution control devices are used to prevent harmful pollutants from entering the atmosphere, primarily from industrial smokestacks. These devices can be divided into two categories: those that regulate the amount of particulate matter escaping into the environment and those that control acidic gas emissions.

Some of the most common devices used to control air pollution include:

- Electrostatic precipitators: These use static electricity to remove soot and ash from exhaust fumes.

- Cyclone separators: These use the concept of inertia to remove particulate matter from flue gases.

- Fabric filters: These use physical means, such as felt, to trap dust particles from the air.

- Scrubbers: These are systems that remove harmful pollutants from industrial exhaust gases, including those that contribute to acid rain.

- Incineration: This process converts volatile organic compounds (VOCs) into carbon dioxide and water through combustion.

- Carbon capture and storage: This process involves capturing carbon dioxide and storing it underground or in forests and oceans to prevent its release into the atmosphere.

Electrostatic precipitators

History

In 1824, M. Hohlfeld, a mathematics teacher in Leipzig, first described the process of removing smoke particles using electricity. However, it wasn't until almost a century later that the first commercially successful process was patented. In 1907, American chemist Frederick Gardner Cottrell of the University of California, Berkeley, applied for a patent on a device for charging particles and then collecting them through electrostatic attraction—the first electrostatic precipitator. Cottrell first applied the device to the collection of sulphuric acid mist and lead oxide fumes emitted from various acid-making and smelting activities, which were adversely affecting wine-producing vineyards in northern California.

How it works

ESPs work by applying energy only to the particulate matter being collected, without significantly impeding the flow of gases. Particles in the gas stream are given an electric charge as they enter the ESP and are then removed under the effect of an electric field. The charged particles are attracted to and deposited on plates or other collection devices. The treated air then passes out of the ESP and through a stack into the atmosphere. When enough particles have accumulated on the collection devices, they are shaken off by mechanical rappers and fall into a hopper at the bottom of the unit, from where they are transported away for disposal or recycling.

Types of ESPs

ESPs can be classified as dry ESPs and wet ESPs, depending on the method used to clean the collector plates. Dry ESPs, the most commonly used type, clean the collector plates by applying mechanical impulses or vibration to knock loose the collected particulate matter. Wet ESPs, on the other hand, clean the collector plates by rinsing them with water. Wet ESPs are typically used when gas streams contain sticky particles with low resistivity.

ESPs can also be categorised according to the voltage they use: high-voltage, single-stage, and low-voltage, two-stage. The former combines an ionisation and collection step and is commonly referred to as Cottrell precipitators. The latter uses a similar principle but is followed by collection plates.

Performance

ESPs can achieve collection efficiencies greater than 99%. Their performance can, however, be affected by particle resistivity, which influences the deposition and removal of particles from the collection plates. The ideal situation is to have particles with moderate resistivity—they should conduct away some of their charge once they reach the plate but retain enough of their charge to lightly hold them to the plate.

Applications

ESPs are available in many different sizes and types, designed for various dust and water droplet characteristics and gas volume flows. They can be used to:

• Remove dirt from flue gases in steam plants
• Remove oil mists in machine shops
• Remove acid mists in chemical process plants
• Clean blast furnace gases
• Remove bacteria and fungi in medical settings and pharmaceutical production facilities
• Purify air in ventilation and air conditioning systems
• Recover material from gas flow (including oxides of copper, lead and tin)
• Separate rutile from zirconium sand in dry mills and rutile recovery plants

Wet scrubbers

The design of a wet scrubber is guided by several factors, including packed bed height, scrubbing liquid composition and pH, liquid flow rate, target gas velocity, and materials of construction. The packed bed height relates to the contact time needed for mass transfer between the polluted gas stream and the scrubbing liquid. The composition and pH of the scrubbing liquid can be adjusted to target specific contaminants. The liquid flow rate, or the proportion of solvent liquid to gas treated, impacts the volume of droplets available for collecting pollutants. Target gas velocity refers to the speed at which the contaminated gas moves through the scrubber, with higher velocities requiring more energy. Finally, the materials used to construct the scrubber should be chosen based on factors such as temperature, corrosion resistance, and compatibility with the scrubbing liquid.

Overall, wet scrubbers are a crucial tool for reducing air pollution and protecting human health and the environment.

Packed scrubbers

The basic design of a packed scrubber involves a large tower filled with various types of packing media, such as specially shaped plastic or ceramic elements. The scrubbing liquid is distributed evenly throughout the packing material, and the contaminated gas enters the system from either the bottom or the side of the tower, depending on the layout. As the gas passes through the tower, it interacts with the packing material, and chemical reactions or solubility with the scrubbing liquid remove chemical and particle impurities. Finally, the gas passes through a mist filter to separate any remaining liquid droplets before being released into the atmosphere.

The efficiency of packed scrubbers can depend on various factors, such as gas contamination levels, expected purification levels, and operating pressures. However, testing has shown that most packed scrubbers have a removal efficiency of over 90%, with some combinations of absorbers, liquids, and gases achieving purification levels of up to 99.9%.

One advantage of packed scrubbers is their relatively low-pressure drop, which helps to maximize efficiency. The pressure drop refers to the change in pressure as the gas travels through the scrubber, and minimizing this drop is crucial for maintaining high efficiency. Packed scrubbers also have small space requirements, as a single scrubber can remove multiple types of pollutants. Additionally, they are versatile and can be designed in both vertical and horizontal configurations, making them adaptable to different industrial settings.

Overall, packed scrubbers play a crucial role in reducing air pollution by effectively removing a wide range of contaminants from industrial exhaust gases before they are released into the environment.

Flue gas desulfurization

Flue-gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide (SO2) from the emissions of fossil-fuel power plants and other sulfur oxide-emitting processes such as waste incineration, petroleum refineries, and cement and lime kilns. FGD is often treated in a dedicated wastewater facility, allowing facilities to meet strict FGD wastewater discharge limits.

History

The first FGD unit was installed in 1931 at Battersea Power Station in London, with two more major FGD systems installed in the UK before the Second World War. However, these early large-scale FGD installations were suspended during the war, as the characteristic white vapour plumes they produced could have helped enemy aircraft locate the power stations.

FGD is a scrubbing technique that uses an alkaline reagent (typically a sodium- or calcium-based alkaline reagent) to remove SO2 from flue gas. The reagent is injected into the flue gas in a spray tower and absorbed to neutralize and/or oxidize the SO2, forming solid sulfur compounds such as calcium sulfate (gypsum) or sodium sulfate. These are then removed from the waste-gas stream using downstream equipment.

Types of FGD systems

FGD processes can be classified as either once-through or regenerable, depending on how the sorbent is treated. In once-through processes, the spent sorbent is disposed of as waste or utilized as a byproduct, whereas regenerable processes release the sorbed sulfur dioxide to generate other products such as elemental sulfur, sulfuric acid, or liquid SO2.

Both once-through and regenerable technologies can be further categorized as wet, semi-dry, or dry systems. Wet processes duct the flue gas to a spray tower where an aqueous slurry of sorbent is injected, while semi-dry systems inject an aqueous sorbent slurry at a higher concentration. Dry systems directly inject powdered sorbent into the furnace, economizer, or downstream ductwork.

Wet FGD systems tend to utilize sorbent more efficiently than dry processes and can typically reduce SO2 emissions by more than 90%. Dry FGD systems, on the other hand, are more easily retrofitted onto existing combustion facilities.

FGD byproducts

The solids produced by FGD systems represent the second-largest coal combustion product by volume, after fly ash. FGD gypsum, a high-value product that is commonly used in wallboard and panel products, is the most common byproduct. It can also be used in cement and concrete, agriculture, and structural fills.

Other air pollution control devices

Other air pollution control devices include:

• Fabric filters/baghouse filters: a popular method for removing particulate matter from a gas stream due to their high effectiveness and versatility.
• Electrostatic precipitators

Carbon sequestration

Biological carbon sequestration is a natural process where plants absorb CO2 from the air and bind it into biomass. This process can be enhanced through deliberate human actions, such as changes in land use and agricultural practices, known as carbon farming. For example, reforestation and sustainable forest management can increase the amount of carbon stored in forests, which act as carbon sinks. However, these biological stores may be temporary due to events like wildfires, disease, and economic pressures.

Geologic carbon sequestration involves storing CO2 underground or in the Earth's crust, such as in depleted oil and gas reservoirs or deep coal beds. This method ensures long-term sequestration, with the carbon remaining locked away for thousands to millions of years.

Carbon capture and storage (CCS) is a geoengineering proposal that involves capturing CO2 from industrial emissions, compressing it, and transporting it to a suitable location for long-term storage. CCS can also include the use of ""artificial trees"" to remove CO2 from the surrounding air. While CCS offers a promising solution, there are economic and technical challenges to its large-scale implementation.

Other technologies that contribute to carbon sequestration include scrubbers, which remove harmful pollutants from industrial exhaust gases, and catalytic converters in vehicles, which convert dangerous air pollutants into less harmful ones. Additionally, the development and use of alternatives to chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) have helped reduce the depletion of the ozone layer.

Overall, carbon sequestration plays a vital role in reducing carbon emissions and mitigating the impacts of climate change. By combining natural processes with technological innovations, we can work towards limiting the amount of CO2 in the atmosphere and slowing down global warming.

Frequently asked questions