A technology that could help release some of the currently unusable energy in an estimated 2 billion tons of U.S. coal waste has been successfully demonstrated by a Department of Energy supported project.

The full-scale test of the advanced hyperbaric centrifuge technology at a Jim Walter Resources Inc. coal-cleaning plant in Alabama resulted in the successful reduction of moisture from ultrafine coal waste. The test builds on an eight-year effort between the Office of Fossil Energy’s National Energy Technology Laboratory and the Virginia Polytechnic Institute and State University  to use the process to remove water from very fine coal "slurries," or mixture of waste coal "fines" and water.

DOE said U.S. coal producers each year discard large amounts of moisture-laden fines (small, coarse coal particles) that are typically deposited in containment ponds or impoundments as a slurry. In some cases the water is evaporated to stabilize the deposits before they are recovered in surface reclamation; in others, the waste coal  is not recovered. 

The hyperbaric centrifuge technology is aimed at separating fine coal particles from water, allowing their recovery for energy. The technology represents a step forward in clean coal separation and could pave the way not only for the use of billions of tons of waste, but also the 70 million to 90 million tons of fine coal refuse added to slurry impoundments by the U.S. coal industry each year.

Virginia Tech sublicensed the technology to Decanter Machine Inc., of Johnson City, Tenn., which built the initial prototype unit that successfully dewatered fine coal to a level of 13 to 19 percent moisture at a rate of 30 gallons per minute. Coal recovery from the sludge was greater than 97 percent.

Decanter Machine then constructed a full-scale commercial unit capable of handling 600 gallons of slurry per minute. Jim Walter Resources successfully tested the full-scale commercial unit at the greater rate, again dewatering the ultrafine coal to less than 20 percent moisture by applying a combination of air pressure and centrifugal force to significantly reduce moisture. 

Success of the hyperbaric centrifuge has addressed a variety of issues associated with the coal-cleaning process. In the past, removing moisture from very fine coal particles had been difficult. Methods typically used, such as thermal dryers or mechanical dewatering, had either proven too costly or had been unable to dewater ultrafine coal particles of 0.1 millimeters or less.

Through the cooperative agreement with NETL, Virginia Tech’s development of the hyperbaric centrifuge, in combination with its related developments such as a clean coal technology called Microcel, has been able to remove both water and ash from fine coal discarded at impoundments. The Microcel technology uses microbubbles to separate fine coal mineral matter that becomes ash during coal combustion. As a successful example of technology transfer, the Microcel process has been used in Australian coal clean plants.

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