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New technology improves separation of ultrafine coal particles from water

World Coal,

A new technology that could help release some of the currently unusable energy in an estimated 2 billion t of US coal waste has been successfully demonstrated by a Department of Energy (DOE) 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 8 year cooperative effort between the Office of Fossil Energy’s (FE) National Energy Technology Laboratory (NETL) and the Virginia Polytechnic Institute and State University (Virginia Tech) to use the patented process to effectively remove water from very fine coal slurries, or mixture of waste coal fines and water.

US coal producers discard large amounts of moisture-laden fines (small, coarse coal particles) each year that are typically deposited in containment ponds or impoundments as slurry. In some cases the water is evaporated to stabilise the deposits before they are recovered in surface reclamation; in others, the waste coal – which represents a potentially useful energy resource – is not recovered for a variety of technological, operational, market or other reasons.

The hyperbaric centrifuge technology is aimed at separating the fine coal particles from water, allowing their recovery for energy, while simultaneously cleaning up the environment and providing jobs in the coal cleaning industry. The technology represents a step forward in clean coal separation and could pave the way not only for the use of billions of tonnes of waste, but also the 70 million – 90 million t of fine coal refuse added to slurry impoundments by the US coal industry each year.

As a result of support by NETL through FE’s Hydrogen and Fuels Program, researchers at Virginia Tech developed and patented the hyperbaric centrifuge, as well as other related technologies. Virginia Tech’s Center for Advanced Separation Technologies (CAST) successfully tested its prototype technology at a variety of coal-cleaning plants.

The technology was subsequently sublicensed to Decanter Machine Inc., which built the initial prototype unit that successfully dewatered fine coal to a level of 13 – 19% moisture at a rate of 30/min. Coal recovery from the sludge was greater than 97%.

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

The 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 mm 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 subsequently becomes ash during coal combustion. As a successful example of technology transfer, the Microcel process has been widely used in Australian coal clean plants.

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