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New technology removes sulfur from coal

By Susan Trulove

Spectrum Volume 19 Issue 01 - August 29, 1996

A coal-cleaning technology developed by Virginia Tech in the early 1980s has generated new technologies and patents, including a process for doubling the amount of sulfur removed from coal compared to techniques currently used.

Microcel(TM), invented by Virginia Tech researchers to remove ash and some sulfur from fine coal, has been marketed across the United States and world-wide for several years. It was developed by Roe-Hoan Yoon, director of the Center for Coal and Minerals Processing at Virginia Tech, when he was working as a summer intern at the Department of Energy's Pittsburgh Energy Technology Center.

In October 1995, the Virginia Tech researchers received a patent for a coal de-watering process that enhances the coal cleaning effectiveness of the Microcel units (Oct. 17, #5,458,786).

This summer, Gerald H. Luttrell, associate professor of mining and minerals engineering, and Yoon received a patent for another coal cleaning process (June 4, #5,522,510) that improves sulfur removal when used in conjunction with Microcel.

In the newly patented process, an "enhanced gravity separator," separates particles based on different densities. Coupling the flotation process used by Microcel with a gravity separator creates a synergism that facilitates the more efficient removal of the sulfur-containing mineral pyrite from the coal, according to the researchers.

When a coal is cleaned using a flotation device only, pyrite rejection is generally poor as the mineral acquires water-repelling properties and behaves similarly to coal. Both coal and pyrite will attach to air bubbles and float. Pyrite can be more readily separated from coal based on differences in density, particularly if coal is subjected to high gravitational forces.

The flotation/gravity separation process was successfully tested on a large scale at the Pittsburgh center in early 1995. The test results showed that the process can remove 85 to 95 percent of the pyrite from high-sulfur coals. An advantage of the process is that it works without coal grinding, a step that is necessary to achieve high levels of sulfur rejection using only flotation devices.

Some of the low-to-medium-sulfur coals can be cleaned by this process to less than the 1.2-pounds-of-SO2-per-million-Btu level mandated by the 1990 Clean Air Act Amendments and, hence, can be burned without using costly scrubbers.

"The price of coal is highly sensitive to sulfur, and coal companies can command higher prices for lower-sulfur coals," Yoon says. "That's why western low-sulfur coals are displacing much of the eastern high-sulfur coals in the U.S."

Luttrell adds, "If we can remove the sulfur, we can increase the value of eastern U.S. coals."

Studies at the Center for Coal and Minerals Processing showed that the patented technology can increase the retail value of coal by $2-3 per ton by virtue of the lower sulfur content in the coal.

The new process is being marketed by Virginia Tech Intellectual Properties Inc.