The essential process stages of this wet desulphurisation method are:
In this method, limestone (CaCO3) or quicklime (CaO) can be used as the absorbent. The selection of an additive which can be added dry or as a slurry is made on the basis of project-specific boundary conditions. To remove sulphur oxides (SOx) and other acidic components (HCl, HF), the flue gas is brought into intensive contact with a slurry containing the additive in the absorption zone. In this way, the largest possible surface area is made available for mass transfer. In the absorption zone, the SO2 from the flue gas reacts with the absorbent to form calcium sulphite (CaSO3).
The limestone slurry containing calcium sulphite is collected in the absorber sump. The limestone used for cleaning the flue gases is continuously added to the absorber sump to ensure that the cleaning capacity of the absorber remains constant. The slurry is then pumped into the absorption zone again.
By blowing air into the absorber sump, gypsum is formed from the calcium sulphite and is removed from the process as a component of the slurry. Depending on the quality requirements for the end product, further treatment is carried out to produce marketable gypsum.
In wet flue gas desulphurisation, open spray tower absorbers have prevailed which are divided into two principal zones. These are the absorption zone exposed to the flue gas and the absorber sump, in which the limestone slurry is trapped and collected. To prevent deposits in the absorber sump, the slurry is suspended by means of mixing mechanisms.
The flue gas flows into the absorber above the fluid level and then through the absorption zone, which comprises overlapping spraying levels and a mist eliminator.
The limestone slurry sucked from the absorber sump is finely sprayed co-currently and counter-currently to the flue gas through the spraying levels. The arrangement of the nozzles in the spraying tower is essential importance to the removal efficiency of the absorber. Flow optimisation is therefore extremely necessary. In the mist eliminator, the drops carried from the absorption zone by the flue gas are returned to the process. At the outlet of the absorber, the clean gas is saturated and can be directly removed via a cooling tower or wet stack. Optionally the clean gas can be heated and routed to a dry stack.
The slurry removed from the absorber sump undergoes preliminary dewatering by way of hydrocyclones. Generally this pre-concentrated slurry is further dewatered through filtration. The water, obtained from this process, can be largely returned to the absorber. A small portion is removed in the circulatory process in the form of waste water flow