20% More Efficient CCS Cleaning Up Coal Plants – With Rust!

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Among presenters at the recent ARPA-E summit were researchers trying rust to capture carbon dioxide more economically from Ohio State University. Last year’s $5 million grant from ARPA-E had enabled their research to the point where a 250 kilowatt demonstration plant is ready to test how well it works and what it costs.

Instead of exposing the coal to air, using a technology already proved in the lab, the team, led by Professor Liang-Shih Fan of the Department of Chemical and Biomolecular Engineering, is producing a highly concentrated stream of carbon dioxide using chemical looping, making it easier to capture.

In this case, they will use iron, cycling it between iron oxide (rust), and metallic iron – in a chemical loop.

Most coal plants burn coal in air, which is mostly nitrogen. “You have to waste a lot of energy to separate the nitrogen from the carbon dioxide,” says Fanxing Li, a researcher from the group.

Technical process
The coal is first gasified, a common process converting coal to syngas (carbon monoxide and hydrogen gas).

The syngas is exposed to particles of iron oxide which act as an oxygen carrier and the reaction causes the iron oxide to release its oxygen, forming metallic iron. The oxygen oxidizes the carbon monoxide, forming carbon dioxide and hydrogen, steam.

The hydrogen and carbon dioxide can now be separated and the steam condensed, leaving concentrated carbon dioxide.

Then the metallic iron is exposed to the oxygen in air, forming iron oxide again – in a chemical reaction that generates heat, which is used to generate electricity – and returns to the first chamber to react with more syngas, closing the loop.

Export to China
Coal is cheaper than renewable energy in part because coal plants in the US have already been built and paid for, and the current costs reflect just the ongoing operating costs of fueling them – but with any capital expenditures to build additional equipment to capture the greenhouse gases emitted when coal is burned, the equation changes.

So the coal energy industry resists the investment. Adding carbon capture machinery adds about 50% to the cost, and running the machinery can add 30-35%, so carbon capture can add as much as 85% to the cost of the electricity produced.

Most people are just as distrustful of CCS as fossil energy owners, but – because coal can never be clean. The ideal would be to never mine any more coal (which I agree with).

But there are countries that are just adding their first coal plants now, and they are not already entrenched in the dirty power plant processing that we have inherited in the US. So it makes sense to me to solve at least the generation side for them, even though the mining side will never be clean, and export the cleaner technology.

Even if we can’t ever stop mining itself in this country, like everyone in the world, we will all still benefit from lower greenhouse gas emissions in China and India, where they are interested in finding cleaner  coal.

The results so far are encouraging, and suggest this will be 20% more efficient than conventional CCS processes, with 100% carbon capture. First results will be out at the end of March.

Susan Kraemer@Twitter

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