Wet and Dry Scrubbers

Washer cooler with central nozzles without internals 

Wet scrubbers are particularly intended to wash fume- or dust-laden gases with a spray or sheet of water to remove such contaminants as are present, leaving only clean air or gas to be exhausted from the system. They are capable of handling many fume and dust control problems arising from industrial processes, and are particularly applicable to hot gas processes, where it is possible to quench the gas stream by direct introduction of water as close to the process as possible. Quenching in this manner makes the dusty gas stream very much easier to handle. 

Scrubbers may be open or fully enclosed. An example of the former type is the venturi scrubber used on a process vessel or storage tank, which needs to breathe, but as a consequence can release objectionable fumes. The action of water sprayed into a venturi section creates suction, drawing the fumes into the scrubber, where they are removed and carried down by the scrubbing water, leaving only cleaned gas together with entrained water to be exhausted. 

Entrained moisture in the exhaust gases will be in droplet form or as a mist, which may or may not be acceptable, hi some processes, for example, the scrubbing liquid may not be water but an aggressive fluid, such as an acid, when it is obviously necessary to eliminate this from the final exhaust by some form of demister, such as mist eliminator pads or blades. The latter are more efficient, have a lower pressure across the eliminator, and are better able to handle any dust remaining without clogging. Alternative dedusting units include elementary centrifuges, cyclones etc. 

Other types of wet scrubbers in common use include spray towers, vertical and horizontal packed towers, jet ejectors, extended surface scrubbers, fan spray scrubbers and various individual designs, some of which may be described as cleaners. 

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Wet and dry scrubbers Dry scrubbers are generally preferred today. Scrubbers can be used for the removal of both particulate matter and toxic sulfur gases (dry scrubber or spray dryer desulfurizers). 

The description of the workings of the control technologies is beyond the scope of this article. However, it is worth noting that many of these technologies create substantial amounts of solid or liquid waste that needs to be disposed of properly. For example, in the USA the total amount of fly ash produced from coal combustion alone is about 57 Mt/y (Kalyoncu 2000). About one-third is utilized as secondary raw material (e.g., for aggregate and asphalt), but the rest is usually disposed of in landfills. The wet and dry scrubbers for S02 control produce a sludge or dry waste that finds little secondary use, and a large amount is disposed in landfills. 

Exhaust gases are treated to minimize SO2 emissions into the atmosphere. Removal of SO2 is achieved by the utilization of a scrubber system and will be discussed below in greater detail. Although numerous variations of both wet and dry scrubbers are commercially available, most rely on the adsorption of S02 onto A1-, Na-, or Ca-based adsorbents. Ca-based adsorbents, such as lime (CaO) or calcite (CaC03), are usually preferred due to their low cost and high availability. 

The Clean Air Act Amendments of 1970, which followed the original Clean Air Act of 1967, set national air quality standards for six criteria air pollutants NOx, SOx, ozone, carbon monoxide (CO), particulates and lead. The result was the removal of lead from gasoline and the installation of emission control technologies, including baghouse filters for particulate control, wet and dry scrubbers for SOx control and automobile exhaust catalysts for controlling hydrocarbons (HC), CO and NOx. As a consequence, lead emissions have been dramatically reduced, SOx emissions are being controlled, and automobile CO, HC and NOx emissions have decreased by nearly a factor of 10 (over uncontrolled emissions). In spite of these dramatic improvements, in 1989 approximately 130 million people in the U.S. lived in 96 areas which did not meet air quality standards either in ozone, in carbon monoxide, or in both [2]. 

Particulate removal equipment includes electrostatic precipitators, fabric filters, or mechanical particulate collectors, such as cyclones. SOx removal equipment includes sorbent injection technologies and wet and dry scrubbers. Both types of scrubbers result in the formation of calcium sulfate and sulfite as waste products. 

The production of acid rain starts when atmospheric sulfur dioxide is oxidized to sulfur trioxide in a complex series of reactions. SO3 is, in turn, hydrolyzed to sulfuric acid. Anthropogenic sources of sulfur dioxide include the burning of coal, the refining and burning of oil, and the smelting of copper ores. Until control measures started to come on line, the pH of rainwater in the northeastern United States and other areas downwind of such sources had fallen to values between 3 and 4. The best control measures have proven to be wet and dry scrubbers. As a result of installation of these flue-gas desulfurization (FGD) devices, the amount of sulfur and nitrogen oxides released to the atmosphere has measurably decreased in the last several decades. 

Of the contents of this section, 7B (Sedimenting Separators), 7C (Centrifuges) and 7D (Cyclones and Hydrocyclones) are solely concerned with sedimentation, 7E (Coalescers), 7G (Mist Eliminators) and 7H (Electrostatic Precipitators) are largely so, but 7E (Wet and Dry Scrubbers) is as much concerned with filtration as well. 

Most postcombustion cleaning systems make use of scrubbers to remove sulfur dioxide. Scrubbers are devices tbat contain some chemical that will react with sulfur dioxide in flue gases. Two kinds of scrubbers are used, wet and dry. As their names suggest, the two... 

The degree to which FGD scrubber material is treated influences its physical properties. Basic physical properties include solids content, moisture content, specific gravity, and wet and dry density [67]. When dewatered, the calcium sulfite FGD sludges become a soft filter cake with a solids content typically in the 40-65% range. Calcium sulfate FGD sludges can be dewatered much more easily and may achieve solid contents as high as 70-75% after dewatering [67]. 

A scrubber may be called on to handle both wet and dry gases. The components and zones handling both types of gas must be constructed from material resistant to both wet and dry chlorine. This excludes all common metals. Polymeric linings can be used on vessels and external pipework. Internal piping of certain fluorinated polymers is adequate from the standpoint of corrosion, and it can be used if its support and mechanical design are adequate and if there is no problem due to porosity. 

Dust control equipment includes collection hoods, fabric and fiber filters, reverse jet filters, wet and dry precipitators, cyclones, and scrubbers [27]. Selection depends on particle size and density of the material, volume and temperature of air to be treated, efficiency, economic considerations, and dust diaracteristics. Dusts that are sticky, fluffy, erosive, toxic, or combustible demand special consideration. The efficiencies of various types of collectors have been tabulated, along with a guide to selection for various applications [28]. This information is siunmarized in Tables 2-8 and 2-9. A combination of devices often will be more effective than a single t)q)e. 

There are hundreds of desulfurization processes proposed in the literature. These processes may be classified as wet and dry (Patkar, 1981). Spray drier absorption is a semi-wet system in which an alkali slurry is fed to the unit and the reaction product is collected dry. This process is generally applied to power stations. It is reported by Felsvang et al (1988) that spray drier scrubbers are in operation in power plants of over 10,000 MWe in different parts of Ae world. These absorbers are generally coupled with fabric filters or electrostatic precipitators to collect particulates. Spray drier absorption is experienced as a wet-dry system with different reagents such as lime, limestone, trona (natural soda) and ammonia. 

Semidry Scrubbers The advantage of semidry scrubbers is in that they remove contaminants by way of a solid waste that is easier to dispose of (less expensive). Initially, the scrubbing medium is wet (such as a lime or soda ash slurry). Then a spray dryer is used to atomize the slurry into the gas which evaporates the water in the droplets. As this takes place, the acid in the gas neutralizes the alkali material and forms a fine white solid. Most of the white solids are removed at the bottom of the scrubber while some are carried into the gas stream and have to be removed by a filter or electrostatic precipitator (discussed later). Although semidry systems cost 5-15% more than wet systems, when combined with a fabric filter, they can achieve 90-95% efficiencies. Dry scrubbers are sometimes used in a very similar fashion, but without the help of gas-liquid-solid mass transfer, these systems use much higher amounts of the solid alkali materials. 

Piping, Ducting, Dampers and Fans 8300-8399. .Waste Disposal and Recovery 8900-8999. .Hlace1laneoua (Wet Scrubbers) 8500-8599 -Dry Scrubbers 6550-8580. Precipitators... 

Methods of dust removal depend mainly on the particle size of the dust and the temperature and moisture content of the gas. The methods used are broadly divided into dry methods and wet methods. The dry methods involve the use of gravity and baffle chambers, cyclones, filters, and electrostatic precipitators, while the wet methods involve the use of spray towers and venturi scrubbers. In principle, wet cleaning is preferred to dry cleaning because of the excessive wear associated with and the difficulty in handling the fine dusty material removed in the dry methods. The wet methods, however, must be followed by such operations as filtration, drying of filter cakes, and recycling of water. 

Particulate Scrubbers Wet collectors, or scrubbers, form a class of devices in which a liquid (usually water) is used to assist or accomplish the collection of dusts or mists. Such devices have been in use for well over 100 years, and innumerable designs have been or are offered commercially or constructed by users. Wet-film collectors logically form a separate subcategory of devices. They comprise inertial collectors in which a film of liquid flows over the interior surfaces, preventing reentrainment of dust particles and flushing away the deposited dust. Wetted-wall cyclones are an example [Stairmand, Tram. lmt. Chem. Eng., 29,356 (1951)]. Wet-film collectors have not been studied systematically but can probably be expected to perform much as do equivalent dry inertial collectors, except for the benefit of reduced reentrainment. 

Element Fresh bottom ash Ivsimeter extracted pore water and leachates (Chandler el al. 1997 Cranncll et al. 2000) Aged boiiuiit ash extracted pore waters (Mciina 1997) Dry and semi-dry scrubber residues low LS (0.0-0.2) column leachates (Chandler et al. 1997) Wet scrubber residue low LS 10.0-0.2) column leachates (Chandler et al. 1997)... 

Scrubbers are pollution control devices, which remove pollutants from gas streams, particularly from the combustion gases produced by facilities such as coal-fired power plants. Scrubbers may use absorbents in slurry or solution form (wet scrubbers), or in powder form (dry scrubbers), with wet scrubbers being more often used. Modern scrubbers can consist of several cleaning steps such as dust separation, sulfur removal, capture of mercury, lead, or other heavy metals, and breakdown of nitrogen oxides. Scrubbers of the future will also need to capture and sequester carbon dioxide. Currently, the most common application of scrubbers is still the removal of sulfur, frequently combined with capture of fly-ash. 

Spray-dry scrubbers are an alternative to conventional wet scrubbers. In this type of scrubber, an alkaline slurry or solution is sprayed in fine droplets into a reaction vessel, along with the flue gas. The droplefs rapidly react with the sulfur dioxide while drying to a fine powder of sulfite salts. This powder is entrained in the gas stream, and is carried to a dust precipitator where it is collected, as shown in Fig. 7. Most of the sulfur dioxide is collected in liquid-phase reactions while the droplets are drying, but 10-15 /o additional sulfur dioxide can be absorbed in gas/solid reactions, as the absorbent powder is swept through the ductwork and particulate collector. These are cocurrent devices, and so the limestone utilization and sulfur removal efficiency are inherently lower than those of countercurrent devices such as wet scrubbers. Partial recycle of the sorbent is often used to improve the sorbent utilization. 

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