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The production of waste materials during energy recovery processes is an unavoidable consequence of the need for energy; consequently, safe and efficient disposal or reuse alternatives for these waste materials is essential for sustainable development. For waste streams that must be geologically disposed, the largest volumes of energy related waste include Coal Combustion Products (CCPs) such as fly ash, coal mining wastes, and processed water and drill cuttings from oil and gas exploration, with relatively small amounts of silica resulting from pipe scaling in geothermal energy production. The fate of the vast majority of these energy-related wastes is geologic disposal, which ranges from placement in landfills (lined or unlined) or surface impoundments, to deep injection within geologic units. Applications for productive reuse of energy-related wastes are cost effective alternatives to disposal, and are gaining popularity as sustainability of processing becomes more critical. This review paper examines geologic disposal and reuse of energy-related waste streams within the U.S., and provides insight into fuel-to-waste production ratios, preferred disposal or productive reuse alternatives, and associated geotechnical/environmental considerations.

High-carbon-content fly ashes with biomass particles are typically landfilled in accordance with the ASTM C618 regulation. To quantify their geotechnical properties relating to storage and disposal, this study evaluates the small to intermediate strain properties of fly ashes with various carbon and biomass contents. Tested fly ashes had carbon contents ranging from 1.1% to 9.6%, resulting from co-combusting coal with biomass (biomass contents ranging from 0% to 8.2% by weight). The small-strain stiffness and intermediate-strain constrained modulus were evaluated using consolidation tests performed in a modified oedometer cell equipped with bender elements. It was found that initial void ratio governed the compressibility (or constrained modulus) of fly ashes, and with an increase in carbon and biomass contents, the small-strain stiffness of fly ashes decreased due to the decrease in number of direct contacts between microspheres. In addition, the interfine void ratio, ef, was employed instead of global void ratio to capture the alteration of interparticle contact or interparticle coordination between microspheres, due to the change in carbon and biomass contents. Finally, the stiffness in an overconsolidated state and the stiffness anisotropy of fly ashes were evaluated.