Chapter 16 - Disposal of flue gas desulphurisation waste

Abstract

The production of a sludge of calcium sulphite and calcium sulphate is the major disadvantage of wet nonregenerable processes, which represent the majority of FGD units in use at present. The sludge is usually stored in ponds or used as landfill material depending on the degree of dewatering that is practiced and the geological/hydrological conditions of the site. Concern about the potential pollution of ground water is leading to the development of processes that first stabilize the sludge before disposal. This chapter describes the disposal of flue gas desulfurization waste. Flue gas desulphurization sludges may emit sulphurous gases into the atmosphere even when chemically stabilized. Direct ponding or the storage of sludge in ponds adjacent to the power plant is the most widely practiced method of sludge disposal. Scrubber sludge generally contains about 15% solids as it exits the absorber. It is thickened to 30% solids before being pumped directly to a disposal pond. Excess water from the thickener and pond is recycled to the FGD system. Oxidation of FGD sludge converts CaSO 3 ·1/2H 2 O to CaSO 4 ·2H 2 O or gypsum. The flat plate crystalline structure of the sulfite, which gives sulfite sludge its thixotropic property, is modified to large needle-like sulfate formations. These large crystals exhibit improved settling rates and dewatering characteristics. Gypsum sludge can be centrifuged or dewatered by vacuum filtration to a solids content of between 75 and 85%. The material is suitable as landfill or as a source of gypsum. Dewatered sludge and fly ash are mixed to form a physically stable material. There are few chemical reactions unless the ash is particularly alkaline when partial curing of the sludge may occur. The sludge may be dewatered in centrifuges and vacuum filters to a solids content of between 50% and 80%.