Wet scrubbers are the most widely used FGD technology for SO2 control throughout the world. Calcium-, sodium- and ammonium-based sorbents have been used in a slurry mixture, which is injected into a specially designed vessel to react with the SO2 in the flue gas. The preferred sorbent in operating wet scrubbers is limestone followed by lime. These are favoured because of their availability and relative low cost. The overall chemical reaction, which occurs with a limestone or lime sorbent, can be expressed in a simple form as:
SO2 + CaCO3 = CaSO3 + CO2
In practice, air in the flue gas causes some oxidation and the final reaction product is a wet mixture of calcium sulphate and calcium sulphite (sludge). A forced oxidation step, in situ or ex situ (in the scrubber or in a separate reaction chamber) involving the injection of air produces the saleable by-product, gypsum, by the following reaction:
SO2 + CaCO3 + 1/2O2 + 2H2O = CaSO4.2H2O + CO2
Waste water treatment is required in wet scrubbing systems.
A variety of scrubber designs is available including:
- spray tower design where pump pressure and spray nozzles atomise the scrubbing liquid into the reaction chamber providing large particle surface area for efficient mass transfer;
- plate tower design where the gas is dispersed into bubbles, which also provides large sorbent surface area;
- impingement scrubber design where a vertical chamber incorporates perforated plates with openings that are partially covered by target plates. The plates are flooded with the sorbent slurry and the flue gas is accelerated upwards through the perforations. The flue gas and sorbent liquid make contact around the target plate, creating a turbulent frothing zone to provided the desired reaction contact;
- packed tower design where the flue gas flows upwards through a packing material counter-current to the sorbent which is introduced at the top of the packing through a distributor; and
- the fluidized packed tower design or turbulent contact absorber, which is similar to the packed tower, except that the packing is fluidized. The turbulence created keeps the packing material clean and improves the mass transfer between the flue gas and the slurry liquid.
In the simplest configuration in wet lime/limestone/gypsum scrubbers, all chemical reactions takes place in a single integrated absorber resulting in reduced capital cost and energy consumption. The integrated single tower system requires less space thus making it easier to retrofit in existing plants.
The absorber usually requires a rubber, stainless steel or nickel alloy lining as construction material to control corrosion and abrasion. Fibreglass scrubbers are also in operation.
Commercial wet scrubbing systems are available in many variations and proprietary designs. Systems currently in operation include:
- lime/limestone/sludge wet scrubbers;
- lime/limestone/gypsum wet scrubbers;
- wet lime, fly ash scrubbers; and
- other (including seawater, ammonia, caustic soda, sodium carbonate, potassium and magnesium hydroxide) wet scrubbers.
Wet scrubbers can achieve removal efficiencies as high as 99%. Wet scrubbers producing gypsum will overtake all other FGD technologies, especially with the increased cost of land filling in Europe and the introduction of increasingly stricter regulations regarding by-product disposal.
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