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Removing CO2 from the emissions of power stations is going to have to get a lot more experimental. D1V1D/Flickr

Carbon capture can’t rely on fine-tuning old technologies

Carbon capture, for those who don’t know already, is the term given to various different technologies that can “capture” the carbon dioxide in streams of gases that would normally be emitted to the atmosphere. These streams come from any process or device that burns a fuel, from a petrol-powered lawnmower, through cars and trucks, right up to the gas and coal-fired power stations that keep our society humming along.

Ideally, we could use some kind of carbon capture technology to remove the carbon dioxide from all those emissions, since everyone (with a few notable exceptions) knows that carbon dioxide is a greenhouse gas. It causes a small but significant rise in the global average temperature, which in turn has potentially disastrous consequences such as a greater incidence of highly variable weather events (flooding, cyclones, drought and so on).

The reality though, is that capturing carbon dioxide is difficult and certainly prohibitively expensive from small and intermittent sources such as car exhausts. In most cases the capture technology would be as large and complicated as the car itself. Even then, there is no obvious place to deposit the captured carbon dioxide.

It only really becomes technically feasible once the sources are large and not moving, such as a coal-fired power station. Even then the current state-of-the-art technologies would use a large proportion of the power station’s output, just to power the capture and storage technology. The storage question, of where to put captured carbon dioxide, is another discussion in itself.

So if this whole business of carbon capture and storage is so difficult, and the existing technologies are so energy-intensive, then why is anyone bothering with carbon capture research and development at all? Why not just pursue a completely renewable energy future, powered by solar, wind, and other emerging clean energy technologies? That’s a tough question, and there’s no straightforward answer, but there are a few key issues.

The most commercially advanced renewable energy technologies are wind and solar, which both suffer from intermittency (they don’t generate power when the wind isn’t blowing or the sun isn’t shining respectively). To a certain extent wind intermittency can be solved by having large interconnected transmission networks, but these are expensive to build, especially in a country as large and sparsely populated as Australia.

The holy-grail solution for these technologies is cheap energy storage, so that excess power could be stored and then used at times of high demand. At the moment, apart from some clever solutions involving pumping water to higher elevations and then releasing it through turbines, there are no cost-effective storage solutions. So until there are suitable energy storage options in place, there is a limit to how much intermittent energy generation can be used. Some places in the world, such as Germany, have probably already passed that limit.

The second issue though is related to cost and resource availability. Certain regions in the world (such as China, the USA and much of Australia) still have enormous coal deposits that could be used to generate electricity. The governments, corporations and individuals that own those deposits are understandably keen to exploit them, and this is where carbon capture really comes into play. If a truly cost-effective carbon capture technology can be deployed, then there is the potential to generate low-cost baseload electricity (or at least, lower cost than the alternatives), which suits those governments, corporations and individuals eminently well.

All over the world, researchers and organisations are working feverishly to optimise existing carbon capture technologies, especially those that have already been demonstrated at some scale. This includes the absorption of carbon dioxide into liquid solutions, which has been practiced for several decades in the oil and gas industry, and the use of solid materials which adsorb carbon dioxide onto their surfaces.

In both liquid and solid cases though, large quantities of energy must be used to release the carbon dioxide, usually in the form of heat or a pressure change. To put this into perspective, early estimates for a power station are that up to 40% of the power output would be consumed running the carbon capture apparatus.

Other groups are working on the use of thin membranes which can filter the carbon dioxide from emission streams, but these too have a myriad of problems. The membranes usually cannot operate at the high temperatures of the emission gas streams, and typically become soft and swollen after prolonged exposure to carbon dioxide, limiting their effectiveness.

Of course many researchers, myself included, have proposed novel carbon capture technologies. A report just published in the journal Angewandte Chemie International Edition describes a new approach using a metal organic framework.

This is a highly structured material with lots of channels inside it with very specific dimensions and properties. It can absorb carbon dioxide and then release it after exposure to light. This is particularly exciting because the light used is very similar to concentrated sunlight, so it could potentially be used in a process that captures carbon dioxide and releases it without the necessity of high temperatures or a pressure change, both of which are expensive. The material we used is expensive though, and may not be suitable for the very large scale technologies that will be required for coal-fired power stations.

Most research and development funding has been directed at the so-called “near-commercial” technologies, as the power generation industry is notoriously risk-averse and prefers to adopt a mature technology. The reality though, is that there are only marginal improvements to be made to the conventional carbon capture technologies, and in my opinion, it is entirely appropriate to designate them as “so-called” near-commercial. They are nearly commercial now, and they always will be.

In the meantime, my colleagues and I, and other aspiring researchers around the world, will continue working with whatever funding support we can obtain to find the next big thing in carbon capture and storage. Maybe it will involve metal organic framework materials, maybe something else we haven’t even invented yet. Two things are for certain though; it won’t be a slight iteration on the existing technology, and it won’t be funded by the power generation industry.

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