Fly ash quality, past, present and future, and the effect of ash on the development of novel products

A number of the more important environmental impacts of the operation of large coal-fired power plants are associated with the very large quantities of solid materials resulting from these processes. These are, principally: coal ash residues, and solid residues of flue gas desulfurisation processes. By far the greater proportion of these materials, generally around 80% of the total solid discards from the electricity supply industry, comprise fly ash from the particulate collection equipment and bottom ash from the hoppers under the economisers and air preheaters of large pulverised coal boilers. The utilisation and disposal of the fly ash residues has been the subject of a great deal of development activity over many decades. In the main, the emphasis has been on the increased use of fly ash materials in high volumes, as constituents of cements and mortars, or for the manufacture of concrete and aggregate materials. The unsold ashes are used for land reclamation and for beneficial landfill, wherever possible. In recent years, there has been increasing interest in the processing of boiler fly ashes to improve ash quality and consistency, and in the manufacture of high-value secondary products. The utilisation of power station fly ashes depends on their physical and chemical properties, and there have been a number of recent developments in power plant design and operation, which have resulted in significant changes in the characteristics of fly ashes, eg the progressive introduction of NOx emission control technologies and in-furnace NOx reduction technologies. The increasing interest in the co-firing of non-conventional fuels such as petroleum coke and biomass/waste materials has resulted in the production of mixed and more complex materials. The paper will present a review of the major developments, which can affect coal fly ash quality and discuss the impact of these issues on the utilisation of fly ashes as feed materials for the manufacture of high-value secondary products. (C) 2002 Society of Chemical Industry.