Capturing carbon not easy

Many years ago, when I was performing "chemistry magic shows" in schools and other venues, one demonstration involved a very simple process. I would get a volunteer to blow through a straw into a solution of lime water.

The carbon dioxide in their breath would permeate the solution and react with the calcium hydroxide resulting in the production of calcium carbonate. Essentially, we were capturing their carbon dioxide emissions and converting them to minerals.

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Capturing carbon dioxide is easy - sort of.

One of the subtle points of this demonstration is it requires calcium hydroxide or slaked lime. To capture all of the excess carbon dioxide in the atmosphere we could just make large quantities of slake lime but where do we get it from? From calcium oxide which, in turn, comes from heating calcium carbonate to very high temperatures, on the order of 900 C.

The breakdown of calcium carbonate to calcium oxide releases - you guessed it - carbon dioxide. And perhaps more to the point, some form of fuel is used to get the mineral to the required temperature resulting in even more carbon dioxide emissions. The atomic economics are such that we would be using lime to capture one carbon dioxide by releasing two others. Not a winning proposition.

But just because this particular version of carbon capture is not economical and is environmentally damaging doesn't mean carbon capture isn't possible. Indeed, if we are to keep global temperature rise below 2 C by the end of the century, we must invest in some form of carbon capture.

In the United States, the National Academy of Science and Engineering has recently released a report entitled "Negative Emissions Technologies and Reliable Sequestration: A research agenda" outlining, in broad terms, how the United States might move forward in this area.

A negative emission technology, as its name implies, is an enabling technology which consumes more greenhouse gases than it generates. For example, using solar energy to convert carbon dioxide to other compounds which could be stored in some fashion. The latter - storage - is the critical issue and finding reliable methods of sequestering carbon is an important component of any strategy.

The report focuses on a number of approaches, some of which are already in place and some are waiting development. They all have pluses and minuses so central to the theme is the need to consider the complete life cycle analysis.

The simplest idea is to chemically capture carbon dioxide from the air.

This is technology we know how to do as it is used in submarines and spacecraft to keep sailors and astronauts alive. Similar approaches have been effectively utilized on some coal-fired power plants and such.

But these techniques have at least a few major drawbacks as a worldwide solution. The first is they are most effective for point sources, such as smokestacks, and not particularly cost effective for the atmosphere as a whole.

The second is that once the carbon dioxide has been captured, it needs to be desorb from the material and then disposed of in some fashion. This requires heat which, in turn, implies carbon dioxide emissions. And the gas needs to be dealt with through some form of sequestration.

While the cost isn't particularly high when attached to an industrial plant - around $50 to $100 per tonne - scrubbing the atmosphere would like cost a whole lot more - around $1,000 per tonne.

The major avenues for dealing with the gas are either burying it within basalt formations or converting various calcium- and magnesium-rich silicate minerals to carbonates through mineralization.

Burying would require compressing the carbon dioxide gas to form a super-critical liquid which is not particular difficult. The technology for doing this is used in decaffeinating coffee beans, for example. It would then involve injecting the liquid into areas in the Earth where the high pressure would be maintained by the overburden. Unfortunately, the Earth gets a lot hotter the deeper you drill which could lead to issues with the pressurized gas.

In places where this is being tried, there has been evidence of the gas re-emerging through aquifers in the surrounding land. This means the gas is only temporarily trapped.

Mineralization leads to a much longer sequestration and there is a large capacity of suitable deposits but the whole process is not yet well understood. A recent study in Science holds out some hope as it demonstrated chemical reactions can convert carbon dioxide to minerals in less than two years.

At present, we are injecting a staggering 50 billion tonnes of greenhouse gases into the atmosphere each year and of that 35 billion tonnes are carbon dioxide. That is five tonnes for every man, woman, and child on the planet. It is not sustainable and yet it will only increase as the population grows.

Carbon capture is one possible route to a solution.

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