Flue gas desulphurization
Flue gas ducts connect flue gas system to absorber and from absorber to stack.
A series of dampers are foreseen to intercept flue gases and eventually to by-pass the absorber.
In order to compensate the additional pressure drops, at the FGD plant inlet flue gases pressure is increased by means of a booster fan.
After the flue gas cleaning a reheating system for clean gases is required before they enter the stack.
The demand for the energy will be withdrawn from the raw gas by a Ljungstrom gas-gas heater (regenerative heat exchanger).
The absorber consists of a vertical cylindrical vessel, with a flue gas inlet and outlet opening. The part of the absorber between the gas inlet and gas outlet is called ‘’the gas section” which may be subdivided in “the spray section” and in “the mist eliminator section”. The part of the absorber below the gas inlet contains the absorber slurry and is called “the sump”.
In the spray section, the flue gas to be treated is brought in intimate contact with a fine spray of limestone slurry droplets, as produced by the slurry spray banks, equipped with spray nozzles in sufficient quantity to ensure complete coverage of the absorber cross-sectional area and fed by slurry recycle pumps.
The SO2 is absorbed to a large extend in the slurry droplets and react with the limestone present in the slurry to form gypsum as described in the following reaction:
The process absorbs also other acid gases like HCl, HF and removes the fly ash present in the flue gas.
In order to obtain nearly 100% oxidation of sulphite to sulphate, the absorber is provided with an oxidation air injection system for the injection of a certain flow of oxidation air, supplied from a compressor system.
Above the spray section, the mist eliminator section is installed for the separation of the entrained slurry droplets from the ascending flue gas flow; the droplets fall down to the sump.
The slurry in the sump consists of an aqueous solution of dissolved salts in which approximately 10 to 15 wt% solids are suspended. In order to keep these solids suspended in the slurry, the sump is provided with side entry agitators.
The produced gypsum must be removed from the absorber, otherwise the gypsum would accumulate in the absorber, leading to an absorber slurry with a high concentration of suspended solids.
The bleed system consists essentially of an absorber bleed pump and a gypsum cyclone battery, supplied with absorber slurry by the absorber bleed pump.
The underflow of the cyclones flows to the mechanical dewatering (vacuum belt filter or centrifuge) that produces dry commercial gypsum with a maximum water content of 10%.
Most of the cyclone overflow flows back to the absorber together with the filtrate of the dewatering section. A part of this collected overflow is pumped to the 2nd hydrocyclone. The overflow of this hydrocyclone, containing a small quantity of solids, is sent to disposal as waste water for the purpose of discharging fly ash, inerts of the limestone and chlorides; the underflow, containing larger particles of limestone and gypsum, is recovered to the absorber.
MAIN CHARACTERISTICS OF FGD
- High removal efficiency (> 96%).
- Limestone consumption closely approximating stoichiometric requirements
- Reduced space requirements due to compact construction
- Scrubbing tower complete with integral absorption, oxidation, crystallizing and mist separation stages
- Low residual moisture in effluent gas by optimal location of the demister
- High gypsum purity in the final product as a result of high oxidation efficiency