Use of coal combustion by-products in mine reclamation: review of case studies in the USA

With the continued use of coal to generate electricity for the world's power needs, coal combustion by-products (CCPs) will be produced in greater quantities during the ensuing decades. About 130 million tons of CCPs are produced annually from the 600 coal-fired power plants currently operating in the USA, with estimates of 500 million tons produced worldwide. Five major types of CCPs exist: bottom ash; boiler slag; fly ash; fluidized bed ash; flue gas desulfurization ash. Bottom ash does not generally constitute a disposal problem because it is extensively used as aggregate fill material for construction projects, filler in construction materials (wall board and dry wall) and de-icing solids for roads. Boiler slag is used for similar purposes as bottom ash, but it can be used as a glassy grit material for sand blasting. Fly ashes constitute 70% of the by-products generated and these ashes are produced in several ways in a power plant depending on the boiler type and the emission control system employed at the power plant. These fine-textured ash materials may be dry fly ash from conventional coalfired boilers, dry ashes collected in flue-gas desulfurization or other collection devices (bag houses or scrubber filters), or they may be collected in wet scrubber systems producing a fly ash slurry. Coal combustion by-products can be beneficially used as: (1) an alkaline seal or fill material to contain acid producing materials and to prevent the formation of acid mine drainage; (2) an agricultural amendment to create artificial soil on abandoned mine lands where native soils are not available; (3) an alkaline amendment added to spoils to neutralize acid producing materials in the spoil; (4) a flowable fill that seals and stabilizes abandoned underground mines to prevent subsidence and the production of acid mine drainage (5) a non-toxic fill material for final pits within the spoil area to reduce reclamation costs. Case studies in this paper demonstrate that CCPs can be used to improve reclamation, revegetation and water quality on reclaimed areas. Results to date have shown the effective neutralizing capacity of these ashes and stabilization of reclamation sites.