Where can we place all this CO₂?

Let’s take a leap forward in time. Let’s imagine we live in a not too distant future where research centres all over the world have perfected effective techniques for capturing the excess carbon dioxide scattered through the atmosphere and already know how to intercept particles released by chimneys and exhaust pipes. A future where governments have addressed the issue and endorsed efforts to resolve it, and industries have developed and put into operation plants implementing these inventions.

In this future, we’ve eliminated all excess CO₂ so that sunlight can filter through without the heat getting trapped, and the greenhouse effect, without which Earth’s temperate climate and therefore life couldn’t exist, is back to its normal levels. And the temperature is no longer rising: climate change has been stopped. We are safe. Excellent. But now where do we put all the carbon we’ve captured? By releasing all at once, over about a century and a half, the enormous quantity of carbon that nature had already stored away in oil and gas fields over hundreds of millions of years, we caused our atmosphere to overheat. And now, attempts to remedy that feel a bit like trying to put the toothpaste back in the tube – worse, actually, as we’re talking about a tube as big as a planet. Removing a huge quantity of carbon from its normal cycle is no easy feat. Indeed, many research centres around the world are investigating and implementing different methods – some easy, some incredibly complex – to achieve just this.

If we want to dispose of a gas such as CO₂, we need an isolated tank able to store it, permanently and without risk of leakage. In order to restore the carbon dioxide in our atmosphere to pre-industrial levels we need to cut its concentration from about 400 parts per million (ppm) to half of that figure. Considering that the total mass of the atmosphere is around 5 million billion tonnes, it all comes down to identifying a suitable place in which to store approximately a trillion tonnes of CO₂. and, regardless of the pressure required to compress it, we’d need all the steel in the world. A viable alternative would be to look for some humongous, ready-made tank and start filling it with at least some of the CO₂ we have. No need to look any further, actually our depleted oil and gas reservoirs make for the strongest and most hermetic containers in existence, much better than anything we could devise, having managed to contain hydrocarbons for millions of years until we dug them up.

Furthermore, the technologies needed to force carbon dioxide back into the depleted reservoirs are fairly simple: it’s just a matter of pumping it in, rather than pumping it out. As a matter of fact, we’ve been extracting, purifying and re-injecting natural gas into deep aquifers during the summer, when demand is low, to retrieve it in winter, when demand is high, for decades now. Gases, and even water, are often injected into active reservoirs in order to replace the hydrocarbon that has been extracted, to force out any hidden oil or natural gas. This is known as “Enhanced Oil Recovery”. A recent estimation by the IEA (International Energy Agency) has shown that, through this process, it’d be possible to store between 60 and 240 billion tonnes of CO₂ in the bowels of the earth while, additionally, retrieving up to 375 billion barrels of oil.

Another alternative would be to exploit existing deep salt-water basins, where CO₂ can be pumped in at high pressure. Starting at 850 metres below sea level, carbon dioxide is compressed under layers of rock until it becomes liquid. Slowly, it mixes with the existing salt water, reacts and precipitates as solid carbonate, becoming part of the layers of rock themselves. We might perhaps not feel completely at ease, thinking that these CO₂ geological reservoirs have no actual walls or well defined boundaries we could monitor, but we needn’t worry about uncontrolled leakages: it’s really not uncommon to find deposits of carbon dioxide that have been lying undisturbed at various depths for millions of years. Moreover, even before the precipitation process begins, the capillary effect permanently traps CO₂ bubbles in the fissures among the rocks.

A second type of approach involves transforming carbon dioxide from gas to solid, creating a substance in which the carbon dioxide is more concentrated and less volatile, and as such unable to easily go back into the cycle. One way forward involves putting nature to work, with trees capturing CO₂ from the atmosphere and through solar energy using it to grow a trunk, branches, leaves, flowers and fruits. However, reforestation can only eliminate the excess carbon that was historically produced by deforestation. Another course of action seeks to insert solidified carbon directly into the ground, obtaining an organic substance, although this would require some clever adjustment to avoid microorganisms breaking the substance down, transforming it into carbon dioxide, and then releasing it back into the atmosphere.

The notion of “bio-char” is certainly a good one. If we collect agricultural and forest waste, which contains the carbon extracted from the atmosphere, and burn it in special oxidisers with low concentrations of oxygen a process called pyrolysis, we obtain gases that can be used as fuel. Whilst burning these gases does release some of the carbon they contain, the majority nevertheless remains trapped in the charcoal produced using this method. At this point, we can mix this charcoal with soil and obtain a much richer soil than the one we started out with. This is because the fragments of carbon mixed in the soil retain water and nutrients that are already present, so that crops or forests can be grown on areas previously considered unsuitable. A study by the Pacific Northwest National Lab in Washington estimates that this method could capture up to 1.8 billion tonnes of carbon dioxide equivalent per year, equal to about 12% of current emissions.

Another solution entails capitalising on the interaction between rocks and atmospheric carbon dioxide. This is a natural – and extremely slow – process that causes the CO₂ to react when it comes into contact with rocky materials and transform into bicarbonates that, through mineralisation, become part of the rock surface itself. For faster results, igneous rocks such as basalt must first be extracted from the soil, crushed into fine fragments and spread over wide areas of land, maximising the contact area between rock surface and atmosphere, thus accelerating the process.

A study by the University of Sheffield established that finely crushing basalt – or, better still, a rock called harzburgite – and spreading a quantity of 1-5kg per square metre over an area of 20 million km² every year would reduce the global temperature by almost one degree Centigrade (with basalt) or up to 2.2 degrees Centigrade (with harzburgite) by the year 2100. The problem lies in the magnitude of such a plan since we’d have to mine more basalt than we do coal today. Moreover, the crushing and spreading of fragments on a surface larger than Russia would entail some logistical problems (and significant emissions of new carbon dioxide in order to operate mines, crushers, trucks and bulldozers).

Nevertheless, Columbia University has developed a variant of this approach that doesn’t entail any major movements. It involves fragmenting the rocks onsite using a technology similar to fracking that is mainly used in the United States to extract oil and gas from clay matrices. Once the rocks are fractured, carbon dioxide is injected directly into the fissures and, as an example, the hydrofracturing of peridotite rock in a slice of the oceanic crust that emerges in the Sultanate of Oman would provide over one billion tonnes of CO₂ just within that one country. In practice, however, rather than exposing rocks to the atmosphere, the opposite is done, and concentrated carbon dioxide is injected into the rock and is subsequently trapped there, in petrified form, forever, like the villain in a fairytale.

It’s very likely that no single solution will be able to solve the global problem of excess carbon storage. Rather, we’ll have to employ a whole range of methods, including forestation, soil storage and deep reservoirs, and these would need to be implemented on a large scale in order to be both effective and financially sustainable.

Moreover, these solutions for getting rid of carbon would have to be accompanied by the technology to optimise our activities, ensuring a lower emission of new carbon and the development of both the circular economy and renewable and non-renewable energy sources. This is a global challenge that we can only overcome if we all work together.

Industrial chemist specialized in theoretical chemistry. He was a researcher for 30 years before moving on to Eni's scientific communication.