Dry scrubbing systems control acid gas emissions (SO2, HCl, HF, etc.) and are used primarily
on utility and industrial boilers, municipal waste combustors, medical waste incinerators, and
some refinery processes. Of course, wet scrubbing systems can also function effectively as
acid gas collectors. Regardless of whether scrubber acid gas control systems operate wet or
dry, they have a mechanism for introducing alkaline material into the exhaust gas to react
with the acid gases present.
Up to this point, you have been learning about wet scrubber designs. In wet scrubbers, liquid droplets provide the primary targets for collecting particles and gases. To facilitate this process, gas streams are saturated with moisture; therefore wet scrubbing systems release a steam plume when exiting the stack. Also, wet scrubbers require a system of pipes and pumps for collecting, treating, and recirculating the scrubbing liquid. In contrast, as their name implies, dry scrubbers either operate completely dry or use much smaller amounts of liquid than wet scrubbers. In some semi-dry designs, liquid is added to the alkaline material, creating a slurry. High scrubber temperatures evaporate the moisture before the gases and reaction products leave the scrubber. Therefore, dry scrubbing systems do not have a stack steam plume or waste water handling/disposal requirement.
There are a number of different dry scrubbing systems designs. However, all consist of two main sections or devices: (1) a device to introduce the acid gas sorbent material into the gas stream, and (2) a particulate-matter control device to remove reaction products, excess sorbent material and any other particulate pollutants in the flue gas. Dry scrubbing systems can be categorized as dry sorbent injectors (DSIs) or as spray dryers [also called semi-dry scrubbers or spray dryer absorbers (SDAs)]. Since dry scrubbing systems only remove gases, a separate device is always required to remove particles. The particulate control devices are generally fabric filters or electrostatic precipitators (ESPs).
Dry sorbent injection involves the addition of a dry alkaline material (usually hydrated lime or soda ash) into the gas stream to react with any acid gases that are present. The sorbent can be injected directly into the flue gas duct ahead of the particulate control device or into an open reaction chamber. The acid gases react with alkaline sorbents to form solid salts which are removed in the particulate control device.
In spray dryer absorbers, the flue gases are introduced into an absorbing tower (dryer) where the gases are contacted with a finely atomized alkaline slurry [usually a calcium-based sorbent such as Ca(OH)2 or CaO]. Acid gases are absorbed by the slurry mixture, and react to form solid salts. The heat of the flue gas is used to evaporate all the water droplets leaving a non-saturated (i.e. dry) flue gas exiting the absorber tower. The effect of cooling and humidifying the hot gas stream increases collection efficiency over simple dry injection.
Up to this point, you have been learning about wet scrubber designs. In wet scrubbers, liquid droplets provide the primary targets for collecting particles and gases. To facilitate this process, gas streams are saturated with moisture; therefore wet scrubbing systems release a steam plume when exiting the stack. Also, wet scrubbers require a system of pipes and pumps for collecting, treating, and recirculating the scrubbing liquid. In contrast, as their name implies, dry scrubbers either operate completely dry or use much smaller amounts of liquid than wet scrubbers. In some semi-dry designs, liquid is added to the alkaline material, creating a slurry. High scrubber temperatures evaporate the moisture before the gases and reaction products leave the scrubber. Therefore, dry scrubbing systems do not have a stack steam plume or waste water handling/disposal requirement.
There are a number of different dry scrubbing systems designs. However, all consist of two main sections or devices: (1) a device to introduce the acid gas sorbent material into the gas stream, and (2) a particulate-matter control device to remove reaction products, excess sorbent material and any other particulate pollutants in the flue gas. Dry scrubbing systems can be categorized as dry sorbent injectors (DSIs) or as spray dryers [also called semi-dry scrubbers or spray dryer absorbers (SDAs)]. Since dry scrubbing systems only remove gases, a separate device is always required to remove particles. The particulate control devices are generally fabric filters or electrostatic precipitators (ESPs).
Dry sorbent injection involves the addition of a dry alkaline material (usually hydrated lime or soda ash) into the gas stream to react with any acid gases that are present. The sorbent can be injected directly into the flue gas duct ahead of the particulate control device or into an open reaction chamber. The acid gases react with alkaline sorbents to form solid salts which are removed in the particulate control device.
In spray dryer absorbers, the flue gases are introduced into an absorbing tower (dryer) where the gases are contacted with a finely atomized alkaline slurry [usually a calcium-based sorbent such as Ca(OH)2 or CaO]. Acid gases are absorbed by the slurry mixture, and react to form solid salts. The heat of the flue gas is used to evaporate all the water droplets leaving a non-saturated (i.e. dry) flue gas exiting the absorber tower. The effect of cooling and humidifying the hot gas stream increases collection efficiency over simple dry injection.
In some semi-dry designs, liquid is added to the alkaline material, creating a slurry. High scrubber temperatures evaporate the moisture before the gases and reaction products leave the scrubber. Therefore, dry scrubbing systems do not have a stack steam plume or waste water handling/disposal requirement.
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