Wet Scrubbers

A wet scrubber, also known as a gas cleaner or smoke cleaner, is an air pollution control device designed to remove specific gases, vapors, and particulate matter from the exhaust gases of industrial facilities. It operates by spraying a cleansing liquid onto the exhaust gases in specialized scrubbing towers to absorb, dissolve, or chemically react with the pollutants before they are released into the atmosphere.

In some cases, a wet scrubber cleaner can also be a dry cleaning type. This depends on the types of gases or chemicals that need to be absorbed or dissolved.

Certain vapors or fumes that are not soluble in water may yield better results with a dry cleaner or dry sorbent injection system. The most common type of dry cleaner is a limestone-based cleaner that uses limestone powder. However, most gas cleaning applications are best suited for using a liquid-based cleaner.

Industrial wet scrubbers are considered as a solution to remove harmful pollutants and odors from process gas streams before discharging them into the atmosphere. The key advantages of industrial scrubbers include:

Wet scrubbers reduce the temperature and volume of saturated high-temperature vapors, allowing for more compact fans and ducts. This also makes them adaptable to existing systems. The ability of a single scrubber to clean both gas and solid particulate matter capability to neutralize corrosive gases

Scrubbers, both wet and dry, are classified as industrial air pollution control devices. Dry scrubbers typically do not achieve the same level of effectiveness but are suitable for applications that lack the infrastructure for wastewater disposal. Dry cleaners remove pollutants from exhaust gases without using a liquid. Instead, they employ a dry reactive material known as an absorbent medium and primarily apply it to contact the acidic gas through an absorptive “bed” to remove acid gas. Wet scrubbers pass the dirty gases through a liquid designed to remove pollutant gases. They are commonly used to remove inorganic pollutants. Wet cleaners feature the characteristic of sending the dirty gas through water, where the water absorbs the pollutants, and the clean gas exits the cleaner. Effective removal of contaminants is achieved using absorbent solutions as the cleaning agent. Various types of solutions are used, including positively charged, negatively charged, or uncharged solutions. The wet scrubber is filled with liquid, and the most common cleaning liquid used for acid gas control (e.g., HCl, SO2, or both) is a caustic solution (sodium hydroxide, NaOH), sodium carbonate (Na2CO3), and calcium hydroxide (lime slurry, Ca[OH]2).

In addition to cleaning particulate matter, wet scrubbers can also capture particle matter by trapping it in liquid droplets. The droplets are then collected with a liquid that dissolves or absorbs the pollutant gases. Any droplets present in the cleaner are separated from the outlet gas stream using a mist eliminator.

The particle collection capability of a scrubber is generally proportional to the amount of input in the scrubber. Additionally, a properly designed and operated mist eliminator is crucial to achieving high cleaning efficiencies. Cleaners that remove gaseous pollutants are referred to as gas absorbers. In wet scrubbers, good liquid-gas contact is essential to achieve high cleaning efficiency. Packed Bed, Packed Tower, or the most popular wet cleaner subcategory known as “acid gas” when used for controlling inorganic gases, are filled with a solid medium. While they also clean solid particulate matter, packed bed scrubbers are typically used for gas processing. Wet scrubbers with packed beds are commonly used in chemical,

aluminum, pulp and paper, food and agricultural, and chrome plating industries, as well as acid plants, fertilizer plants, steel mills, and asphalt plants.

Most Important Reaction Parameters are

When the temperature is too low, there is a risk of NH3 slipping unreacted, meaning it is released without reacting.

The molar ratio of NH3/NOx used typically ranges from 0.5 to 0.9. At higher ratios, there is a possibility of NH3 slipping unreacted, as well as a greater risk of additional reactions with other components in the flue gas, potentially forming aerosols such as ammonium chloride and ammonium sulfate.

To achieve the most effective conversion, both reaction and residence times are important. Insufficient residence time can result in NH3 slip and incomplete reaction.

Wet Scrubbers Can Be Used in Applications to Remove Following Gases

The cleaning liquid selected to neutralize the pollutants is specifically chosen. For example, if the incoming vapor pollutant is an acidic gas, an alkaline cleaning solution will be used to neutralize the acid through a chemical reaction. Similarly, if the pollutant is ammonia gas, diluted sulfuric acid can be used as the cleaning liquid to react with and neutralize the ammonia gas.

In a gas wet scrubber, the tower is typically cylindrical in shape, with the cleaning liquid being sprayed at the top of the tower and flowing downward, while the dirty air enters the bottom of the tower and moves upward through the liquid. The tower may contain a packing material to facilitate the mixing of air and liquid, and these are called packed bed gas scrubbers.

Gas scrubbers can be used to capture multiple types of exhaust gases in a single stream, but they are typically not effective in capturing organic gases (which include most volatile organic compounds (VOCs) and some non-polar odors).

Gas and chemical scrubbers can be used to capture water-soluble gases, such as alcohols and other polar molecules. They are referred to by names like “gas scrubber,” “smoke scrubber,” and “chemical scrubber.” Selective non-catalytic reduction (SNCR) for NOx removal is also possible.

in Selective Non-Catalytic Reduction (SNCR)

An reducing agent is injected into the flue gas stream of a combustion process. Ammonia is commonly used as the reducing agent. In this case, the optimum temperature range is 930-980 °C. Additionally, urea is used at a flue gas temperature around 950 °C.

Specified Temperature Range Indicates the Following Reactions

In the case of urea injection with (NH2) CO, urea thermally decomposes at high temperature first, leading to the formation of NH3, which then reacts with NOx as shown in the reaction diagram above.