Ceramic tubes could cut greenhouse gas emissions from power stations

Greenhouse gas emissions from gas-fired power stations could be cut to almost zero by controlling the combustion process with a porous membrane made from an advanced ceramic material. So claims a team of engineers led by Ian Metcalfe and Alan Thursfield at Newcastle University, UK.

The material in question – LSCF – has previously been used in solid oxide fuel cells. Metcalfe, Thursfield and their Newcastle colleagues, along with Kang Li at Imperial College London, have created tiny tubes of LSCF that can filter from the air all gases other than oxygen. By burning methane in pure oxygen rather than air, it is possible to produce a stream of almost pure carbon dioxide.

Burning methane in pure oxygen rather than air produces an exhaust of almost pure carbon dioxide

If the exhaust of a power station is almost entirely CO2, this can be separated and processed into useful chemicals such as methanol. Burn methane in air and the waste is a mixture of CO2 and nitrogen oxides. Separating these gases is not practical owing to the high cost and large amount of energy required to do so.

LSCF is made by mixing lanthanum-strontium-cobalt-ferric oxides in fine powders and heating to produce a solid state reaction. The tubes resemble microscopic drinking straws, and are permeable to oxygen ions. They are also resistant to corrosion or decomposition at the typical operating temperatures of around 800°C found in power plants.

When air blows around the outside of these tubular reaction vessels, oxygen passes through the walls to the inside, where it combusts with methane gas pumped through the centre of the tubes. Nitrogen and the minor constituents of the oxygen-depleted air can be vented to the atmosphere, while the CO2 can be collected following combustion.

The technology could in theory be applied to coal and oil-fired power stations, as long as the fuels are first converted into gas. Burning gassified coal is both more efficient and cleaner than using solid fuel, but it all adds to the cost and complexity of a power station.

“We have managed to run such a system working on a pure methane and air feed on lab scale for over a month,” says Metcalfe. “As for the reaction of our peers, they probably think it is a nice piece of work, but not too surprising. There have been big activities led by the US Department of Energy and Norsk Hydro trying similar things, but they don’t publish. And the big challenge is longer term operation on a large scale.”

And that will be a major challenge. Metcalfe explains that the researchers can achieve high methane conversions and good CO2 yields in the lab, but some carbon appears to be unaccounted for, and most likely forms deposits. However, the membrane performance does not significantly deteriorate over the timescale of the experiments. “Whether the technology is feasible on the scale of a power station is a very difficult question, and I honestly don’t know the answer,” says Metcalfe.

Industrial partners are currently being sought to help answer the outstanding technical questions and commercialise the technology.

“This is a great example of how chemical engineers and materials scientists can together create real solutions to the carbon challenge,” says David Brown, chief executive of the Institution of Chemical Engineers. “It is the sort of lateral thinking and innovative ability that the chemical and materials engineering research community is good at, and we need to ensure that it’s sustained and recognised in both industry and academia.”

Brown adds a pertinent political point: “This technology is only going to make a real difference to climate change if it’s combined with economic incentives – via the necessary level of carbon pricing, maintained over the long term – to persuade the energy industry to invest in it. Technology and economics working together will take us forward.”

Further reading: Methane oxidation in a mixed ionic–electronic conducting ceramic hollow fibre reactor module, Thursfield & Metcalfe, J. Solid. State. Electrochem. 10, 604 (2006).

Figure: In a power station, LSCF membranes (centre) filter oxygen from ambient air, and gaseous hydrocarbon fuel is combusted in pure oxygen leaving almost pure carbon dioxide as the waste product. This CO2 can then be separated and processed into useful chemicals such as methanol, and the oxygen-depleted air vented to the atmosphere (© Ian Metcalfe/Newcastle University).

Article first published in Nanomaterials News.