Capturing hope

Is carbon capture and storage the answer to our problems? (8 minute read)

Removing carbon

Helps net-zero, but beware:

Side-effects abound.

The above haiku is part of New Zealand climate scientist Andy Reisinger's haiku series summarising one of the reports from the Intergovernmental Panel on Climate Change. It intrigues me, because in seventeen syllables it says more than either of the recent synthesis reports which I talked about in last week’s article.

Reisinger is referring to removing carbon dioxide either from the atmosphere or at the source of the emissions, when burning fossil fuels. In the recent synthesis reports, this is referred to as carbon dioxide removal. The obvious methods, the ones which are widely used already, involve us allowing plants to do the job, such as by planting trees. But sitting in the Summary for Policymakers is one tantalising possibility – using fossil fuels in combination with something called carbon capture and storage. It’s tantalising because it suggests that there’s a way that we can keep using fossil fuels, at least for some purposes, and yet reduce climate change.

The synthesis reports don’t get into what exactly carbon capture and storage is, nor how easy or difficult it may be. Nor do they say how advanced the technology is. Are there places where this is being done already? And what are the disadvantages? Reisinger’s haiku hints that it may not be quite as simple as it first appears. Criticisms of the Summary for Policymakers also highlight difficulties with this technology. So, I’ve decided to look more closely at it.

The first part of carbon capture and storage is the capture – getting hold of the carbon dioxide. In theory, this can be done by removing carbon dioxide from the atmosphere itself. This sounds simple, and trees do a great job of it, but it’s not so easy to do on an industrial scale (unless you happen to be a South American rain forest). The main reason is the low concentration of carbon dioxide in the atmosphere. For every million molecules in the atmosphere, 420 are carbon dioxide. That’s a lot more than 150 years ago, when it was just under 290 molecules per million. But if you are trying to separate carbon dioxide from other gases, it makes it difficult. It’s like separating a tiny pinch of sugar from a cup of salt.

I mentioned trees, so here are some trees (Åland Islands, Finland: image credit Melanie Newfield)

As a result, removing carbon dioxide from the atmosphere is energy-intensive and expensive. There are a few facilities operating, but, collectively, they remove about seven seconds worth of emissions from all the world’s energy producers. Mostly, these facilities are capturing carbon dioxide in order to use it, for example for carbonating drinks, rather than as a solution to climate change.

It is the energy-intensive nature of capturing carbon dioxide from the air which is most troubling. In particular, the chemical reactions which remove the carbon dioxide from the air need to happen at high temperatures. Depending on the exact chemical reactions involved, these temperatures could be around 100^oC, but in some cases they could be as high as 900^oC. The energy to reach those temperatures must come from somewhere, and if it comes from fossil fuels, it’s not going to help the climate. There is the potential to use renewable energy or waste heat from industrial processes, but we need to look at the whole picture of any facility extracting carbon dioxide from the air.

The alternative to removing carbon dioxide from the air is to remove it where it is produced, for example at power plants, steel mills or cement factories. There are precedents for this kind of technology, such as the industrials scrubbers which remove a range of different pollutants from factory chimneys or the catalytic converter on your car. There is much more carbon dioxide in the waste gases from a power plant than there is in the atmosphere. It’s been hard to find reliable sources for how much carbon dioxide is found in this kind of waste gas, but as an example, the waste gas from a natural gas power plant may be 3-4% carbon dioxide.

The most widely-used method for removing carbon in this way is to pass waste gases through a liquid containing chemicals known as amines. These are chemicals based on the structure of ammonia, but which also contain carbon. Although many of us have never heard of them, some are familiar, such as histamine, a natural amine which plays an important role in our immune response (and the reason we need anti-histamines to manage allergic reactions).  There are also synthetic amines, such as aniline, used in the production of dyes, drugs and explosives.

Once the amines have absorbed the carbon dioxide, they are transferred to a unit called a stripper, where steam is used to remove the carbon dioxide from the amines. The amines are sent back to absorb more carbon dioxide and the mix of steam and carbon dioxide is cooled. When steam cools, it turns into water, and once the water is removed, what is left is pure carbon dioxide. The carbon dioxide is compressed and then it is ready for storage. If you want a visual explanation, this page from the Massachusetts Institute of Technology has a good, clear diagram.

The process for removing carbon dioxide using amines is decades old – it was patented in 1930. Nonetheless, turning it into a process which is useful on the scale needed to fight climate change has proved difficult. In September 2022, the International Energy Agency reported that there were around 35 commercial facilities capturing carbon dioxide where it is produced. Diplomatically, they state that deployment of this technology has been behind expectations.

So far I’ve talked about the challenges of capturing carbon, but there is also the problem of storage. After all, there’s no point in extracting carbon dioxide from the atmosphere or where it is being produced if it is going to escape back into the atmosphere again. But where can we store all that carbon dioxide safely?

The answer may be to send it back to where it came from. The compressed carbon dioxide can be injected into porous rock formations, deep under the ground. In fact, this is already being done, but not to mitigate climate change. When most of the oil from an oil well has been extracted, it becomes difficult to extract the remaining oil. But carbon dioxide can be injected into the reservoir to push the oil towards the well, allowing more of the remaining oil to be extracted. Clearly, this isn’t helping climate change, since it just makes more fossil fuel available to be burned. But it does show us that the technology is feasible.

In fact, there are a number of different kinds of geological formations which can be used to store carbon dioxide. As well as depleted oil wells, we can use other formations, such as deposits of very salty water trapped underground, and coal seams. The International Energy Agency estimates that there is vastly more capacity for storage than we need.

There have been problems, though, applying carbon capture and storage at the scales we need to solve our problems. One of the world’s most ambitious carbon capture and storage projects is part of the Gorgon Project, a massive natural gas drilling programme in Western Australia. The natural gas in the Gorgon field contains 14% carbon dioxide. So, even before the natural gas is burned, the Gorgon Project is a major problem for the climate. To overcome this, the Gorgon Project includes the world’s largest carbon capture and storage facility.

If you look at the publicity material from Chevron, you can read that 7 million tonnes of carbon dioxide have so far been removed by carbon capture. What this doesn’t mention is the millions of tonnes of carbon dioxide that weren’t captured in the two and half years from when natural gas extraction began until they finally got the carbon dioxide removal working. Not only was carbon capture delayed, but even once it was operating it consistently failed to meet its targets. Chevron has been buying carbon offsets in order to make up for what it hasn’t been able to achieve with carbon capture and storage.

This is not to say that there are no successful carbon capture and storage projects. Existing facilities are capturing almost 45 million tonnes of carbon dioxide every year. That’s better than nothing, but it’s nowhere near enough. We emitted around 1000 times that figure in 2022 (depending on which source you use, it could be a bit more or a bit less). Oceanographer David Ho, writing in the journal Nature, explained the problem in terms of time travel. Speaking about four direct air capture “hubs” in the USA, each expected to remove one million tonnes of carbon dioxide each year, he said:

At that rate, for every year of operation at its full potential, each hub would take the atmosphere back in time by almost 13 minutes, but in the time it took to remove those 13 minutes of CO₂, the world would have spewed another full year of CO₂ into the atmosphere.

This doesn’t mean that carbon capture and storage, either directly from the air or at the point of production, is no use. But, as with other methods of removing carbon, such as planting trees, it’s not something that we can do instead of drastically reducing carbon emissions. If we get better at carbon capture and storage, it will help us deal with some of the most difficult areas for reducing emissions, like concrete production. But there is no carbon capture method which allows us to continue with business-as-usual.