By Pam Frost Gorder
A new form of clean-coal technology reached an important milestone recently, with the successful operation of a research-scale combustion system on west campus. The technology is now ready for testing at a larger scale.
For 203 continuous hours, the Ohio State combustion unit produced heat from coal while capturing 99 percent of the carbon dioxide produced in the reaction.
Liang-Shih Fan, professor of Chemical and Biomolecular Engineering and director of the Clean Coal Research Laboratory,
‘We carefully control the chemical reaction so it never burns — it is consumed chemically, and the carbon dioxide is entirely contained inside the reactor.’- Liang-Shih Fan
pioneered the technology called Coal-Direct Chemical Looping (CDCL), which chemically harnesses coal’s energy and contains the carbon dioxide produced before it can be released into the atmosphere.
“In the simplest sense, combustion is a chemical reaction that consumes oxygen and produces heat,” Fan said. “Unfortunately, it also produces carbon dioxide, which is difficult to capture and bad for the environment. So we found a way to release the heat without burning. We carefully control the chemical reaction so that the coal never burns — it is consumed chemically, and the carbon dioxide is entirely contained inside the reactor.”
Dawei Wang, a research associate and one of the group’s team leaders, described the technology’s potential benefits. “The commercial-scale CDCL plant could really promote our energy independence. Not only can we use America’s natural resources such as Ohio coal, but we can keep our air clean and spur the economy with jobs,” he said.
Though other laboratories around the world are trying to develop similar technology, Fan’s lab is unique. The group studies coal in the two forms that are already commonly available to the power industry: Crushed coal “feedstock” and coal-derived syngas.
The latter fuel has been successfully studied in a second sub-pilot unit, through a similar process called Syngas Chemical Looping (SCL). Each unit is contained in a 25-foot-high insulated metal cylinder that resembles a tall home water heater tank.
No other lab has continuously operated a coal-direct chemical looping unit as long as Fan’s lab did last September. But as doctoral student Elena Chung explained, the experiment could have continued.
“We voluntarily chose to stop the unit. We actually could have run longer, but honestly, it was a mutual decision by Dr. Fan and the students. It was a long and tiring week where we all shared shifts,” she said.
Fan agreed that the nine-day experiment was a success. “In the two years we’ve been running the sub-pilot plants, our CDCL and SCL units have achieved a combined 830 operating hours, which clearly demonstrates the reliability and operability of our design,” he said.
The units each produce about 25 thermal kilowatts of energy, which in a full-scale power plant would be used to heat water and turn the steam-powered turbines that create electricity.
The researchers are about to take their technology to the next level: A larger-scale pilot plant is under construction at the US Department of Energy National Carbon Capture Center in Wilsonville, Ala. Set to begin operations in late 2013, that plant will produce 250 thermal kilowatts using syngas.
The key to the technology is the use of tiny metal beads to carry oxygen to the fuel to spur the chemical reaction. For CDCL, the fuel is powdered coal, and the metal beads are iron oxide composites. Chung likened the sizes to talcum powder and ice cream sprinkles, though the mix is not nearly so colorful.
The coal and iron oxide are heated to high temperatures. Carbon from the coal binds with the oxygen from the iron oxide and creates carbon dioxide, which rises into a chamber where it is captured. Hot iron and coal ash are left behind. Because the iron beads are so much bigger than the coal ash, they are easily separated and delivered to a chamber where the heat energy would normally be harnessed for electricity. Then the beads are exposed to air inside the reactor, so that they become re-oxidized and can be used again.
The DOE funded this research, and collaborating companies include Babcock & Wilcox Power Generation Group, Inc., CONSOL Energy Inc. and Clear Skies Consulting LLC.