Coal Age Magazine - For more than 100 years, Coal Age has been the magazine that readers can trust for guidance and insight on this important industry.
Issue link: https://coal.epubxp.com/i/668539
Many power producers are looking for ways to meet the U.S. Environmental Protection Agency (EPA) federal regula- tions for Coal Combustion Residuals (CCRs) and Effluent Limitations Guidelines (ELGs). One of the thorniest problems involves handling the water pro- duced by many ash-management process- es — particularly the wastewater from the flue gas desulfurization (FGD) process. Regulatory changes and public pres- sure mean that it's just not possible any more to rely on the long-standing practice of ash slurry impoundments. For many industry members, an ideal solution would be "walk away closure" of their CCR disposal facilities. The distance toward this ideal is daunting, given the need for monitoring and management in many current systems. Most critical is the need for a satisfactory solution to mini- mize leachate and seepage from the dis- posal areas, restrict or eliminate the use of transport water, and manage the wastew- ater streams that may carry metals such as selenium and mercury. Some of the success factors for any solution to CCR disposal are: • Working with the characteristics of the waste materials — principally fly ash, bottom ash and FGD wastewater. • The CCR and the wastewater streams, particularly the FGD, must both be d e a l t w i t h a n d b e c o m e p a r t o f t h e solution. • Flexibility in light of the fact that those materials can be either wet or dry, depending on the processes at a partic- ular power plant. • Reducing or eliminating the potential for seepage from the disposal area. • Taking into account the limitations faced by power utilities, such as the need for an on-site solution that mini- mizes the footprint. Paste Technology a Proven Solution for CCR Disposal A possible solution to the CCR disposal problem, which has been successfully applied at several coal-fired power plants, borrows technology from the world of hard-rock mining called "paste" or "thick- ened tailings." Coal-fired power genera- tion and mineral ore processing are alike in that each produces a problematic byproduct — piles of granular waste con- taining metals that cannot be released into the air or water. In hard-rock mining, the granular waste comes from the milling process that grinds the ore to extract the target miner- al. The leftover "tailings" may contain oth- er metals and constituents of concern that have potential for harmful impacts to sur- face and groundwater. Tailings also have the potential to generate and leach acids into the water. An increasingly common way to immobilize the tailings is to mix them with water and other additives to make paste. These materials are engineered into a "recipe" suitable for deposition while making use of the characteristics of the tailings. The result is a product that can resemble toothpaste in consistency, and which can be pumped through a pipeline. Paste then cures into a solid mass that effectively immobilizes the metals in the tailings, and can help prevent acid genera- tion. Properly engineered paste produces minimal bleed water — typically less than 10% of the volume of water in the paste. If stacked on the surface, it can be shaped into landforms and vegetated, a giant step toward the elusive goal of walk- away closure. Paste is also used to backfill worked- out underground mine workings (see "Could Paste Technology Be a Cost-effec- tive Solution for Subsidence?," February 2015, Coal Age. This article includes a case study for using paste to backfill a former underground coal mine to allow develop- ment of the site for a housing project in Alberta, Canada). Paste is a mature technology, and its growth has brought greater efficiencies in operation, a wide range of equipment able to produce paste, and growing ranks of professionals worldwide with experience 38 www.coalage.com April 2016 o p e r a t i n g i d e a s Locking Away Coal Combustion Wastewater Problem with Paste Technology B Y S U E L O N G O Paste is pumped through pipelines and deposited in layers so it cures into solid form.