Encyclopedia of Renewable Energy - James G. Speight

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be designed to collect particulate matter and/or gaseous pollutants. Wet scrubbers remove dust particles by capturing them in liquid droplets. Wet scrubbers remove pollutant gases by dissolving or absorbing them into the liquid. Any droplets that are in the scrubber inlet gas must be separated from the outlet gas stream by means of another device referred to as a mist eliminator. Also, the resultant scrubbing liquid must be treated prior to any ultimate discharge or being reused in the plant.

      There are numerous configurations of scrubbers and scrubbing systems, all designed to provide good contact between the liquid and polluted gas stream. Examples include a venturi scrubber and the mist eliminator for a venturi scrubber is often a cyclone separator whereas the packed tower design has the mist eliminator built into the top of the tower.

      The ability of a wet scrubber to collect small particles is often directly proportional to the power input into the scrubber. Low-energy devices such as spray towers are used to collect particles larger than 5 micrometers. To obtain high efficiency removal of 1 micrometer (or less) particles generally requires high-energy devices such as venturi scrubbers or augmented devices such as condensation scrubbers. Additionally, a meticulously designed and operated entrainment separator or mist eliminator is important to achieve high removal efficiencies. The greater the number of liquid droplets that are not captured by the mist eliminator the higher the potential emission levels.

      Wet scrubbers that remove gaseous pollutants are referred to as absorbers. Good gas-to-liquid contact is essential to obtain high removal efficiencies in absorbers. A number of wet scrubber designs are used to remove gaseous pollutants, with the packed tower and the plate tower being the most common.

      If the gas stream contains both particle matter and gases, wet scrubbers are generally the only single air pollution control device that can remove both pollutants. Wet scrubbers can achieve high removal efficiencies for either particles or gases and, in some instances, can achieve a high removal efficiency for both pollutants in the same system. However, in many cases, the best operating conditions for particles collection are the poorest for gas removal.

      In general, obtaining high simultaneous gas and particulate removal efficiencies requires that one of them be easily collected (i.e., that the gases are very soluble in the liquid or that the particles are large and readily captured) or by the use of a scrubbing reagent such as lime (CaO) or sodium hydroxide (NaOH).

      For particulate control, wet scrubbers (also referred to as wet collectors) are evaluated against fabric filters and electrostatic precipitators (ESPs).

      Wet scrubbers have the ability to handle high temperatures and moisture and in wet scrubbers, the inlet gases are cooled, resulting in smaller overall size of equipment. Also, wet scrubbers can remove both gases and particulate matter and can neutralize corrosive gases. On the other hand, wet scrubbers are subject to corrosion and there is the need for entrainment separation or mist removal to obtain high efficiencies and the need for treatment or reuse of spent liquid.

      See also: Gas Cleaning, Gas Processing, Gas Treating.

      Hydrolysis is any chemical reaction in which a molecule of water ruptures one or more chemical bonds. The term is used broadly for substitution, elimination, and fragmentation reactions in which water is the nucleophile:

Chemical reaction showing acid hydrolysis.
Polysaccharides
(Hydrolytic bacteria*)
Fatty acids
(Acidogenic bacteria*)
Organic acids
(Acetogenic bacteria)
(Methanogens)

      Large amounts of inexpensive and renewable lignocellulose waste are generated each year from forestry and agricultural production. These include fruit processing residues, dairy industry wastes, corn and sugar by-products, and paper industry wastes. Lignocellulosic biomass can be converted to biofuels such as ethanol and hydrogen via simple sugars. Lignocellulose is a composite of cellulose fibres embedded in a cross-linked lignin-hemicellulose matrix. It gives structure and strength to plants. Although abundant, lignocellulosic waste is difficult to convert to fermentable sugars because of its complex chemical structure: its three major components (cellulose (crystalline and amorphous) hemicellulose and lignin) must be processed separately (Lee et al., 2007).

      Cellulose consists of linear, highly ordered chains of glucose and is one of the most abundant biopolymers on the planet. When produced by plants, it has both highly amorphous regions containing large voids and other irregularities as well as tightly packed crystalline regions. It also accumulates in the environment because it is resistant to most forms of degradation. Hemicellulose is a complex polymer of a variety of sugars. This mix of sugars mainly consists of six-carbon and five-carbon sugars. Lignin is a poly-phenolic polymer that fills the spaces in the cell wall between cellulose, hemicellulose, and pectin components. It is covalently linked to hemicellulose and crosslinks different plant polysaccharides, conferring mechanical strength to the cell wall, and by extension, the plant as a whole. The content of cellulose, hemicellulose and lignin in common agricultural residues depends upon the source and origin of the feedstocks. However, because lignocellulosic biomass is so complex, it is difficult to use as a feedstock for biofuel. A variety of physical, chemical, and enzymatic processes have been developed to fractionate lignocellulose into the major plant components of hemicellulose, cellulose, and lignin.

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