Spray dry scrubbers require the use of an efficient particulate control device such as an ESP or fabric filter. A recycling facility would improve sorbent utilisation and disposal of the by-product is the norm. The sorbent usually used is lime or calcium oxide. The lime slurry, also called lime milk, is atomised/sprayed into a reactor vessel in a cloud of fine droplets. Water is evaporated by the heat of the flue gas. The residence time (about 10 seconds) in the reactor is sufficient to allow for the SO2 and the other acid gases such as SO3 and HCl to react simultaneously with the hydrated lime to form a dry mixture of calcium sulphate/sulphite. Waste water treatment is not required in spray dry scrubbers because the water is completely evaporated in the spray dry absorber. The by-product also contains unreacted lime which may be recycled and mixed with fresh lime slurry to enhance sorbent utilisation as not all of the lime reacts with the SO2. Factors affecting the absorption chemistry include flue gas temperature, SO2 concentration in the flue gas and the size of the atomised or sprayed slurry droplets.
The absorber construction material is usually carbon steel making the process less expensive in capital costs compared with wet scrubbers. However, the necessary use of lime in the process increases its operational costs.
Spray dry scrubbers are the second most widely used FGD technology. However, their application is limited to flue gas volume from about 200 MWe plants on average. Larger plants require the use of several modules to deal with the total flue gas flow. This is why in general the technology is used in small to medium sized coal-fired power plants. Spray dry scrubbers in commercial use have achieved removal efficiency in excess of 90% with some suppliers giving >95% SO2 removal efficiency as achievable.
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