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New coal technology harnesses energy without burning

06 February 2013

A new form of clean coal technology reached an important milestone recently, with the successful operation of a research-scale combustion system.

Elena Chung (left) and master's student Samuel ayham (right) display chunks of coal along with pulverized coal (bottle, centre) and the iron oxide beads (bottle, right) that enable the chemical reaction (photo: Jo McCulty, Ohio State University)
Elena Chung (left) and master's student Samuel ayham (right) display chunks of coal along with pulverized coal (bottle, centre) and the iron oxide beads (bottle, right) that enable the chemical reaction (photo: Jo McCulty, Ohio State University)

For 203 continuous hours, the Ohio State University combustion unit produced heat from coal while capturing 99 percent of the carbon dioxide produced in the reaction. The technology is now ready for testing at a larger scale.

Liang-Shih Fan, professor of chemical and biomolecular engineering and director of Ohio State’s Clean Coal Research Laboratory, pioneered the technology called Coal-Direct Chemical Looping (CDCL), which chemically harnesses coal’s energy and efficiently 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.”

Though other laboratories around the world are trying to develop similar technology to directly convert coal to electricity, Fan’s lab is unique in the way it processes fossil fuels. The Ohio State group typically 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 research-scale unit, through a similar process called Syngas Chemical Looping (SCL).  Both units are located in a building on Ohio State’s Columbus campus, and each is contained in a 25-foot-high insulated metal cylinder that resembles a very tall home water heater tank.

The researchers claim that no other lab has continuously operated a coal-direct chemical looping unit as long as the Ohio State 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.

At any one time, the units each produce about 25 thermal kilowatts which, in a full-scale power plant, would be used to heat water and turn turbines to produce 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’s National Carbon Capture Centre in Wilsonville, Alabama. 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 coal that’s been ground into a powder, and the metal beads are made of iron oxide composites. The coal particles are about 100 micrometers across and the iron beads are about 1.5 - 2mm across.

The coal and iron oxide are heated to high temperatures, where the materials react with each other. 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 out of the ash, and delivered to a chamber where the heat energy would normally be harnessed for electricity. The coal ash is removed from the system.

The carbon dioxide is separated and can be recycled or sequestered for storage. The iron beads are exposed to air inside the reactor, so that they become re-oxidised for reuse. The beads can be reused almost indefinitely, or recycled.

Since the process captures nearly all the carbon dioxide, it exceeds the goals that DOE has set for developing clean energy. New technologies that use fossil fuels should not raise the cost of electricity more than 35 percent, while still capturing more than 90 percent of the resulting carbon dioxide. Based on the current tests with the research-scale plants, Fan and his team believe that they can meet or exceed that requirement.

Reader comment

From Mr Nick Cook, managing director, ReSus Technology Ltd:
When I saw the headline “New Coal Technology Harnesses Energy Without Burning” I thought this might, at last, be the beginning of the DCFC (Direct Carbon Fuel Cell) commercialisation so I had to check it out. However it turns out that this is an alternative way to react carbon (coal) with oxygen to create heat to drive a conventional thermally powered power station.

To be honest I would consider that this process is still effectively burning, but more akin to the way we burn fuel (carbohydrates etc.) in our bodies. Admittedly this approach does appear to give much improved carbon capture, both in terms of efficiency and cost, but it still doesn’t improve on the coal/heat energy to electricity conversion efficiency which is fundamentally limited by the Carnot or Rankine cycle efficiencies of the power plant. 

By comparison, the DCFC reacts carbon with oxygen electrochemically achieving efficiencies of about 80% (max 100% theoretical) in basically the same way as the better known, but less efficient  (83% max theoretical), hydrogen fuel cell. Additionally the waste stream of the DCFC is pure CO2, captured and ready for the next stage, preferably recycling back into carbon fuels.

Another interesting article “Government Maps out Future of Hydrogen Powered Cars” also caught my attention. The Government's prediction that over 1.5 million H2 cars could be on the roads by 2020 I believe is likely to be optimistic.

For a start, unless these cars are powered by expensive fuel cells they are likely to be significantly more expensive to run than current petrol and diesel models and furthermore, unless we can find some very cheap sources of clean electricity the hydrogen produced will cause its own carbon emissions which could be greater than just burning carbon fuel in current cars. 

Additionally, current developments battery technology I believe are likely to outpace H2 development and EVs can provide a much more efficient route from energy source to wheels than H2 technologies.

The company EOS, for instance, are in the early stages of commercialisation of a zinc air technology that would be about one quarter the cost of current Lithium technology with much longer battery life (10K cycles) and higher energy density. They are also working on a variant that could effectively recharge a car in about 3 minutes. 

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