Gypsum or calcium sulphate dihydrate is a form of calcium sulphate. Other forms are calcium sulphate hemihydrate (CaSO4.½H2O) and anhydrite (CaSO4). By thermal treatment gypsum can be converted into calcium sulphate hemihydrate or anhydrite. With this it loses all or part of its crystal water. For example, gypsum and calcium sulphate hemihydrate respectively contain 20.9% m/m and 6.2% m/m of crystal water.
The combustion gases from the combustion of coal and biomass consist largely out of H2O, CO2 and to a lesser extent SO2. This last component is removed from the flue gases through so called wet flue gas desulfurization, where with the use of water and lime, in the form of CaCO3 or Ca(OH)2, gypsum is formed. The combustion gases are led through a scrubber where (simplified) the following reactions take place:
SO2 + H2O →H2SO3
H2SO3 + ½ O2→ H2SO4
H2SO4 + CaCO3 + H2O → CaSO4.2 H2O+CO2
The resulting gypsum crystals are then removed from the slurry through special centrifuges. After this step the gypsum is further dewatered using a belt filter press or centrifuges, after which it is stored, awaiting transport to the customer. In the process drawing the different steps of the flue gas desulfurization are illustrated.
