The United States. rejoined the Paris climate agreement a few months ago, which means that – along with 194 other countries – it must now find ways to seriously curb greenhouse gas emissions. Many argue that renewable energy sources such as sun and wind are the way to go. But another way to reduce air pollution is to trap carbon dioxide (CO₂) while it is being produced before it can even reach the wider atmosphere.
There are a number of ways to achieve carbon capture. “Carbon capture after combustion” is the simplest method, and – as the name suggests – it happens after a fossil fuel, such as coal or natural gas, has been burned.
"The most common form of carbon dioxide capture is to divert the gas that normally goes down the chimney to an afterburner installation, which uses chemicals that react with the carbon dioxide and trap it," says Peter Clough, a lecturer in energy engineering at Cranfield University in the United Kingdom. "These chemicals with the trapped carbon dioxide can be transferred to another reactor, where they release the carbon dioxide and concentrate it."
Another method of capturing carbon is to burn the fossil fuel with oxygen instead of air. This is known as the "oxi-fuel" process and it ultimately produces an off-gas consisting mainly of CO₂ and water vapor, which can then be easily separated from each other by means of a cooling process.
There is also relief for pre-combustion. This is done by heating the fossil fuel in oxygen before it is burned, creating carbon monoxide and hydrogen. This mixture is then treated in a catalyst with water vapor, which produces hydrogen and CO₂. Finally, amine is added to bind with the CO₂, causing it to fall to the bottom of the chamber where it can then be isolated.
Now comes the storage part, and for that you need a suitable underground cave. "You look for a stable geologic structure a few miles underground and map it carefully, so you can be sure there are no leak points," says Niall Mac Dowell, a professor of energy systems engineering at Imperial College London. "That's where you put the carbon dioxide in."
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If you imagine the cave as a dome, Mac Dowell says, then you drill into the bottom rim and inject the CO₂: “It will rise to the top of the dome and just sit there. According to the laws of physics, it cannot leak. "
Some people mistakenly compare this to storing nuclear waste, that is, it is safe and stable until it is no more. That comparison is not correct, say Clough and Mac Dowell, because once the CO₂ is in the cave reservoir, it reacts with the rock to form stalagmites and stalactites. In other words, an end game is in sight – while nuclear waste remains in its radioactive form for thousands of years. “That's the long-term fate of carbon dioxide and that's where the analogy to nuclear waste falls apart,” says Mac Dowell.
CO₂ leakage is also highly unlikely. “It's not a hope or an assumption that it will stay there,” says Clough. "We've done a lot of trials and tests to confirm that it stays there – in the long run it turns into rock." The length of this process depends on the cave's rock type, but it can take place in less than a decade.
So, what's keeping us from rolling out this technology en masse to cut fossil fuel emissions along with raising the ante for renewable energy production? Well, it's not science. "There is a lot of technical experience to do this. There is nothing earth-shattering new," says Mac Dowell. "It is a very mature technology." But it costs money and at the moment there is simply not the political want to make it happen on a large and meaningful scale, he added.
Clough agrees, but is optimistic that politics is changing: “Until recently, there was no deterrent to release CO₂ into the atmosphere. Now we have clear decarbonisation targets that cannot be achieved by switching to fuel or simply building more renewable energy sources. "