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1. GAS ABSORPTION & STRIPPING: This is the standard method for removing any component from a gas stream, but it is only good as long as we can find a liquid solvent in which the gaseous component we want to remove is much more soluble than the other components of the gas stream.
The overall process is very straightforward. The feed gas enters an absorber (a vertical column) where the gas stream flow up and the liquid stream flows down. Normally, a packing material is used inside the column in order to provide a larger surface area for the liquid and gases to come in to contact. Once the gas has been stripped of the target component, it is released into the atmosphere or used for some other purpose. The liquid solvent which now contains the element we wanted to remove, passes to a stripper. Inside the stripper, the solubility of the gas in the solvent is greatly reduced. This allows the gas to come out of the solution. Later, this gas is cooled and sent to storage for future use an d the stripped solvent is returned to the absorber.
2. LIMESTONE WET SCRUBBERS: This method is used primarily with sources who produce SO2 through coal or oil burning. In the limestone wet scrubber, the solid ash particulates are first removed. Then, the flue gas flows to a tower where it travels countercurrent to a scrubbing slurry comprised of water and limestone particles. To increase the efficiency of scrubbing, some designs use a packing amterial with a very high open area in the tower, a spraying and mist eliminator system, or custom-made bubbler designs.
Inside the tower, the SO2 dissolves in the slurry and reacts with the limestone to produce CO2 and solid CaSO3. The CO2 enters the gas stream, while the CaSO3 is oxidized to CaSO4. This oxidation can occur in variety of places. For example, it is partly accomplished by the excess oxygen already present in the flue gases. It can also be done in an effluent holding tank or an additional oxidizing vessel.
Once the process is complete, the slurry is recirculated from the holding tank. An additional stream is sent to a settler and filter to remove solids. Finally , freshly ground limestone is added to an effluent hold tank. The scrubber will operate near the adiabatic saturation temperature of the entering flue gas. The cleaned gas is usually heated to restore plume buoyancy and to prevent acid corrosion of duct work. Then, the gas is released to the stack. The water in the waste slurry is reduced by thickening. The filter cake is usually mixed with dry fly ash from the plant to further reduce the water in the waste stream. The ultimate destination for the final waste stream is a landfill.
Some problems with limestone scrubbers are corrosion of stacks and duct work due to chemical content of exhaust gases, solid deposition, scaling and plugging caused by calcium sulfate, and plugging of mist eliminator equipment.
Alternative wet systems include: using quicklime as an alternative to limestone in wet throwaway processes (i.e. processes that use a reagent only once and then throw it away) and double alkali systems which avoids the solid deposition, scaling and plugging problems caused by limestone.
3. DRY SYSTEMS: Overall, dry systems have fewer corrosion and scaling problems associated with them. Typically, dry systems inject dry alkaline particles into the gas stream, where they react with the gas to remove SO2. The particles containing the SO2 are then collected in the particle collection device that is used to collect fly ash. Dry systems eliminate problems with disposal of wet sludge associated with wet scrubbers, while increasing the amount of dry solids to be disposed of. Sine it is generally more difficult to dispose of the sludge, dry systems are often considered a good alternative.
4. WET/DRY SYSTEMS: These systems are a combination of the feature of both wet and dry systems. The type of wet-dry system that is most often used is the spray dryer. Spray dryers are mainly used in process industries (i.e. industries that produce products like dried milk, instant coffee, laundry detergents, etc.) In theses spray dryers, water containing dissolved or suspended solids is dispersed as droplets into a hot gas stream. The gas stream, usually contaminated with SO2 enters the chamber either through the side or the top and generally exits through the bottom. The temperature of the gas is much greater than that of the water so the water droplets will evaporate quickly. The particles that are formed from the evaporation process are dry before they reach the walls or the bottom of the reactor. Therefore, the particles for a fine powder that is relatively easy to be removed. Generally, the powder is cooled before being removed.
One benefit of this process is that since we are able to control the size of the water droplets and the concentration in the feed stream, we are able to control the size of the particles that are formed. In the end, all of these specifications give us a powder with a size distribution that could not be obtained any other way.