Management of FGD residues (Smith, 08-2006)

The aim of management strategies for utilities is to find applications for all FGD residues, from wet scrubbers, semi-dry and dry scrubbers. This report summarises production and utilisation of FGD residues in different regions. It points out the reasons for high utilisation rates and barriers to achieving improvements in future. A major role is played by the ash associations, cooperating with utilities, various user industries and environmental protection agencies. Local circumstances dictate the optimal utilisation strategy. Understanding the properties of the residues is essential to maximising their utilisation. The chemical, physical and engineering properties are compared since these determine the quality of the residues for specific applications and appropriate means of handling, storage and transport.

High value applications for FGD residues in the construction industry include gypsum for wallboard manufacture, blocks and floor screed. There are uses for wet and dry FGD residues in the cement industry, and in concrete. The benefits of these applications and barriers to greater deployment are discussed for production processes, technical requirements, transport, handling and storage. The mercury issue is also addressed as there are questions about recent mercury control strategies in the USA involving the capture of mercury in FGD systems. Studies indicate that mercury captured by FGD plant may be more in the fines rather than bound to larger gypsum crystals. Mercury and other trace metals may be separated from the sludge produced from treating the wastewater from FGD gypsum. The rest of the sludge can then be refired, reducing disposal costs. Major research projects on the mercury issue are in progress for all coal combustion residues in the USA. The means and distance of transport are increasingly the most important factors affecting the economics of applications for FGD residues. Utilisation is becoming more economic when viewed against escalating landfill costs. Mixes of residues may be deployed for specific purposes in soil stabilisation and fill, or they are sent to landfill. Wet and dry residues are finding more beneficial uses in the agricultural sector, as fertiliser, and as soil conditioners for example to amend acidic, sodic, and high phosphorous soils. Studies of the long-term environmental effects, especially trace element mobilities, have been undertaken over many years and meet environmental requirements. A quality specification would assist the acceptance of FGD residues in agricultural applications.