The three methods are
wet gas desulfurisation(W)
dry gas desulfurisation(D)
seawater gas desulfurisation.(S)
In method W Calcium carbonate, CaCO3 material is used and material is obtained by quarrying. The solid waste is sold for use in buildings.
In method D Calcium oxide, CaO is used and the material is obtained by Thermal decomposition of calcium carbonate. Solid waste is sent to landfill.
In this method material is obtained from sea water. Liquid waste is returned to the sea.