September 12, 2006
UCD researchers a step closer to finding a new way to reduce greenhouse gas emissions

Greenhouse gas emissions are widely believed to contribute to climate change and global warming. Under the Kyoto Protocol Ireland agreed to reduce its greenhouse gas emissions to 13% above the 1990 levels by 2008 to 2012. However, the EPA reported that emissions in 2004 were 23% above the 1990 levels, indicating that Ireland is a long way from meeting the target.

The most important greenhouse gas is carbon dioxide which is released into the atmosphere when fossil fuels are burned to provide power and heat for industries, transportation, homes and businesses. One way to reduce emissions is to capture carbon dioxide from the exhaust streams of industrial processes or cars before it is released into the atmosphere.

A new technology is being developed by CSCB and UCD Conway Institute investigators, Professor Don MacElroy and Dr Damian Mooney from the UCD School of Chemical and Bioprocess Engineering and Dr Matthias Tacke and his research group from the UCD School of Chemistry and Chemical Biology, which aims to capture carbon dioxide from exhaust streams. This inorganic membrane technology must be capable of separating and capturing carbon dioxide after combustion.

"To date no membranes have been developed to separate carbon dioxide at temperatures of greater than 400°C from combustion or other high temperature process gases," explains Professor MacElroy. "Our preliminary results show that ultra-thin nanoporous membranes can separate carbon dioxide from nitrogen at 600°C."

It is essential to separate carbon dioxide from other gases to facilitate economic storage after capture.

"The separation technique works on the basis of molecular size. The difficulty with separating carbon dioxide from nitrogen lies in the dimensions of the atoms within the molecules," says Professor MacElroy. "There is about 10% difference in size between them so it was a challenge for us to develop a membrane that is selective for carbon dioxide over nitrogen."
Research work carried out by Dr Laurence Cuffe as part of his postdoctoral programme involved developing a composite membrane on Vycor® glass. The pore size of Vycor® glass is too large so it must be chemically modified by coating it with an inorganic nanomembrane.
"The modification to the surface of the Vycor® results in the formation of nanoporous plugs which are permeable to carbon dioxide but form a barrier to nitrogen," continues Professor MacElroy.

The preliminary results showed that these membranes exhibit selectivities for carbon dioxide over nitrogen of more than 36:1 in one case and 75:1 in another case at a working temperature of 600°C. Professor MacElroy explained that the group is now looking at other processes of modifying the glass which are more versatile.

After carbon dioxide is captured, it must then be stored long term or recycled. Oceans and forests act as natural carbon dioxide reservoirs but underground caverns, old gas wells and saline aquifers are also used. Statoil, for example, has undertaken a commercial project of capturing carbon dioxide from the Sleipner gas field in the Norwegian North Sea and storing it 1000 meters under the sea bed in a saline aquifer.

Professor MacElroy concludes that "Carbon dioxide could be recycled by returning it to an artificial carbon cycle. It is a valuable commodity and under appropriate processing conditions there is the possibility of converting it into low molecular weight chemical commodities or recycling it into methanol. Recycling captured carbon dioxide could well be part of the quest for renewable energy sources."


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