Wet vs. Dry Industrial Scrubbers for Air Pollution

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Wet and dry scrubbers both work to reduce air pollution by treating exhaust gases. However, there are differences between the mechanisms each type of scrubber uses and the pollutants they best neutralize. Choosing which type of scrubber to invest in for your air pollution control requires knowing how these systems work and their pros and cons.

The Difference Between Wet and Dry Scrubbers

Wet Scrubbers

There are a variety of wet scrubbers that remove pollutants from exhaust gases. These different types of wet scrubbers all work by wetting the gas to separate contaminants. Wet scrubbers are highly effective at acid and contaminant removal.

Wet scrubber types differ in how they expose the exhaust gas to the liquid. These variations of wet scrubbers include the following types:

  • Packed bed scrubbers
  • Venturi scrubbers
  • Spray towers
  • Cyclone spray chambers
  • Orifice scrubbers

For all wet scrubbers, the proportion of exhaust gas to liquid is vital, as it relates to the surface area contact of the gas with the liquid. The pressure and speed of the incoming exhaust gas also influence how well the scrubber works. The scrubbing liquid also determines what the unit removes from the exhaust gas. While water is common as a liquid, caustic scrubbing liquid may be an option for facilities that must remove acids from the exhaust gas.

Dry Scrubbers

Dry scrubbers offer an alternative to wet scrubbers, which require an additional wastewater disposal system. The cost-effectiveness of these scrubbers makes them popular for use in many facilities that need treatment of polluting exhaust gases.

Dry scrubbers do not use wet products to treat the exhaust gas. Instead, these systems use a dry reagent called a sorbent to either neutralize or separate the acids from the gas. As in wet scrubber systems, dry scrubbers need to maximize surface area contact between the sorbent and the gas to remove as much acid from the exhaust as possible. Filters in the system filter out particulate matter the sorbent cannot impact. After the sorbent passes through the gas, it becomes a hazardous material that requires special disposal.

While wet scrubbers perform better at removing more pollutants, the extra weight and disposal of the wastewater from them increases their operating costs. Dry scrubbers provide a lower-maintenance alternative thanks to their lighter-weight powder waste.

How Do Dry Scrubbers Work?

Dry scrubbers have a three-step process to remove acids from exhaust gases. By taking out these pollutants, dry scrubbers can reduce the incidence of acid rain and lower harmful air pollution levels.

  • Step 1 – The first step requires cooling the exhaust gases to make treating the pollutants easier. Typically, an evaporative cooling unit cools and dilutes the exhaust gas during this stage.
  • Step 2 – The next stage exposes the gas to a combination of alkaline reagents in the sorbent mixture. Usually, sorbents are either hydrated lime, sodium sesquicarbonate or sodium bicarbonate, or a mixture of them. This process can occur in one of two ways, depending on the structure of the dry scrubber.
  • Step 3 – The third step filters out the pollutant-containing powder through a fabric filter or with an electrostatic precipitator. The former is highly effective and does not require power for operation. The latter can effectively remove particulate matter, too — however, it may use up to 4% of a power facility’s output to maintain its electrostatic charge. Due to the difficulty of cleaning the waste powder to remove pollutants, most plants discard it.

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How Do You Choose the Right Scrubber for Your Needs? 

Advantages of Wet Scrubbers

Wet scrubbers have several advantages for their use — most notably, their ability to process out more pollutants compared to dry scrubbers. Plus, they reduce fire hazards by keeping flammable pollutants wet inside the tower. These scrubbers are also the only means of pollution control that remove both particulate matter and gases from the exhaust in the same process. Different types of wet scrubbers will perform at varying levels when processing out both particulate and gaseous materials. For instance, spray towers don’t operate as well at removing particulate matter as Venturi scrubbers.

Disadvantages of Wet Scrubbers

The most significant downside to wet scrubbers is the need to dispose of the liquid waste from them. Due to pollutant contamination of this liquid, it requires treatment before disposal. Additionally, wet scrubbers can experience corrosion and require regular maintenance to keep them operating at peak efficiency.

Advantages of Dry Scrubbers

Dry scrubbers work well to remove sulfur dioxide and hydrochloride. They have lower use costs because the waste they produce does not require additional treatment and weighs less than the wastewater from wet scrubbers. For existing facilities that require additional exhaust processing, dry scrubbers require less space. In some cases, you can even retrofit them into existing areas. For removing particulate matter, fabric filters work very efficiently with dry scrubbers. Plus, these models have very low maintenance costs.

Disadvantages of Dry Scrubbers

Dry scrubbers are not ideal for all facilities. The greatest disadvantage to these scrubbers is their inability to remove all pollutants. Plus, they do not remove as many acids from gases as wet scrubbers. While sufficient, dry scrubbers do not perform as well as wet scrubbers for air pollution control.

Common Applications

Wet Scrubbers

Common uses of wet scrubbers are for facilities that process propane and natural gas. Additionally, any type of facility that requires the removal of particulate matter and treatment of gaseous pollutants in the most thorough manner possible should use wet scrubbers.

Dry Scrubbers

Industrial applications of dry scrubbers include the removal of acids from their exhaust gases. Typically, facilities that lack the space or funds for a separate wastewater treatment system to handle water waste from wet scrubbers benefit most from using dry scrubbers. Therefore, these types of scrubbers find use in most types of plants, including energy production, manufacturing and waste treatment facilities.

Other Types of Industrial Scrubbers

While wet and dry scrubbers are the two most commonly used types of industrial scrubbers, electrostatic precipitators are another option. Often, this device combines with a dry scrubber to act as a particulate filter. However, it can also work as a standalone scrubber in some applications, with a removal rate of up to 99% of particulate matter from gas.

An electrostatic precipitator (ESP) requires construction and a static charge to match the type of dust or contaminants it removes. The exhaust gas stream passes through perforated discharge plates to control the flow. Discharge electrodes charge the gas with either a positive or negative charge, depending on what particles the system removes.
As the charged gas passes by the collection pipe or plate, the charged particles stick to the pipes or plates because the collection surfaces carry an opposite charge. The final step requires cleaning off the collection plates or pipes to prepare them for more processing.

ESPs can remove the collected pollutant with a dry or wet method. The most common method is the dry process which uses a rapper to vibrate the collection plates or pipes to knock off the pollutant. In wet models, water rinses off the particles. Facilities that have sticky particulate matter that may not come off effectively using a rapper benefit more from using a wet ESP.

Disposing of the pollutants removed with ESP depends on the type of particle and what method you use to remove it. For example, dry ESPs that remove particulate matter from coal-fired power plants gather soot and ash, also known as fly ash. In the United States, facilities recycle up to 43% of this fly ash to cut down on greenhouse gases and eliminate disposal problems.

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Contact MACH Engineering for Your Wet Scrubber Design, Components and More

MACH Engineering can design the right wet scrubber for your facility or help you determine if a rental scrubber will better meet your needs. With so many factors at play, the specifics of the wet scrubbers require expert design to achieve the desired results. Trust us at MACH Engineering to assist with design, installation supervision, tower internals, packing and service. Contact us today for more information.

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