Where do I put all this carbon?
Luca Longo's in-depth study on technologies to get rid of excess carbon dioxide
Today we take a leap into the future. A near future in which research centers around the world have invented effective techniques to capture the excess carbon dioxide dispersed in the atmosphere and intercept the one that comes out with the exhaust gases from chimneys and exhaust pipes. A future in which governments have directed and supported this commitment and industries have developed and put into production commercial plants based on these inventions.
Now that we've gotten rid of all the extra CO2 , the sun's rays aren't trapped by excess carbon dioxide in the atmosphere, the greenhouse effect that allows us to live on a temperate planet is back to normal levels. Even the temperature no longer rises: climate change has been halted. We are safe. Very well but… now where do we put all the carbon we have grabbed?
We have caused excessive warming in the atmosphere by releasing all together, in about a century and a half, a huge amount of carbon that nature had taken care of capturing and hiding in oil and gas fields over hundreds of millions of years.
Now, putting things back together is a bit like trying to put the toothpaste back in the tube. And even worse, since our tube is planetary in size.
The problem of removing a large amount of this element from the normal carbon cycle is far from trivial. And many research centers around the world are studying and putting into practice different methods – some simple, others very complicated – to do it. Let's see them a bit.
IN THE VISCERE OF THE EARTH
If we want to put a gas such as CO2 out of circulation, we need an insulated tank that holds it and never lets it escape.
To bring atmospheric carbon dioxide back to pre-industrial values, we need to lower its concentration from about 400 parts per million (ppm) today, to half of this value. Given that the total mass of the earth's atmosphere is about 5 million billion tons, it is a question of finding a good place to store about one trillion tons of CO2. No matter what pressure we could compress it, we would need all the steel on Earth.
A good alternative is to go and look for huge tanks, but ready and available in which to start putting at least a part of all the CO2. These are located underground and are nothing more than the depleted oil and gas fields. If these have been able to hold hydrocarbons for millions of years waiting for us to extract them, then they are more airtight and resistant tanks than any other container that can ever be built by man.
The technologies for pushing carbon dioxide into depleted fields are also relatively simple: it involves pumping a gas underground rather than pulling it out. For decades the natural gas extracted during the summer, when the demand is lower, has been purified and injected into deep aquifers to recover it again when, in winter, the demand increases.
Often some gases – or even water – can be released into fields being exploited to replace hydrocarbons that have already been extracted and push out the oil or natural gas they still hide. It is called "Enhanced Oil Recovery". Recently the IEA estimated that with this process 60 to 240 billion tons of CO2 could be stowed in the bowels of the earth, recovering up to 375 billion barrels of extra oil in the process.
An alternative solution is to exploit salt water basins that are located at great depths where CO2 is pumped at high pressure. Starting from 850 meters below sea level, carbon dioxide is compressed by the overlying rock layers until it passes into a liquid state. Slowly it mixes with the salt water already present, reacts and precipitates as solid carbonate becoming part of the rock itself.
We may feel uncomfortable thinking that these geological CO2 reservoirs do not have well-defined and controllable walls and boundaries. But we don't have to worry about uncontrolled leaks: there are numerous finds of deposits filled with carbon dioxide that have been undisturbed at various depths for millions of years. Furthermore, even before the precipitation process has taken place, the CO2 bubbles are already trapped in the small fractures between the rocks due to capillarity, remaining permanently stuck there.
PETRIFYING CARBON DIOXIDE
A second class of solutions consists in transforming carbon dioxide from gas to solid, creating something where the carbon is more concentrated and less volatile, therefore not able to easily return to circulation. The first solution is to exploit nature and use trees, which grow by capturing CO2 from the atmosphere and transforming it into a trunk, branches, leaves, flowers and fruits thanks to solar energy. But re-forestation can only eliminate the excess carbon produced in historical times by de-forestation.
Another technique is to find a way to insert the solidified carbon directly into the soil. In this case, it is also necessary to find some tricks to prevent the carbon from becoming prey to microorganisms that degrade this organic substance, transforming it back into carbon dioxide and then releasing it back into the atmosphere.
A good idea seems to be what is called "bio-char". If we collect agricultural and forest waste – which contain carbon extracted from the atmosphere – and subject them to pyrolysis (i.e. we burn them in special oxidisers in conditions of low oxygen concentration), gases are developed, which we can then burn as fuels, releasing part of the carbon introduced, but we also get charcoal where a large fraction of carbon has been trapped. At this point, by mixing this charcoal with the earth, we obtain a more fertile soil than the starting one. In fact, the fragments of charcoal mixed with the soil retain the water and nutrients present allowing to extend crops or forests in previously unsuitable areas.
A study by the Pacific Northwest National Lab. Of Washington estimates that up to 1.8 billion tons of carbon dioxide equivalent per year can be intercepted, equal to about 12% of current emissions.
A second solution is to exploit the interactions between rocks and atmospheric carbon dioxide. This natural – and extremely slow – process causes the CO2 that comes into contact with the rock to react by transforming into bicarbonates that mineralize on the surface of the rock itself. To speed up the process, it is a question of extracting igneous rocks such as basalt from the soil, crumbling them finely and distributing them over large surfaces of soil, maximizing the contact area between rock and atmosphere and thus accelerating the process.
A study by the University of Sheffield has calculated that if you finely crumble basalt – or even better a rock called harzburgite – by distributing it in doses of 1-5 kg per square meter over an area of 20 million km2 each year, it would be obtained by 2100 a lowering of the global temperature equal to almost one degree C in the case of basalt, and up to 2.2 degrees C in the case of harzburghite.
The problem is the enormity of the plan: we should be extracting more rock from basalt quarries than coal we extract from mines today. And the crumbling and distribution of the pebbles over a larger area than Russia would entail logistical problems (and far from negligible emissions of new carbon dioxide to operate quarries, fragmenters, trucks and bulldozers).
A variant of this solution, developed at Columbia University , does not involve major travel. It would involve fragmenting the rocks on the spot with a technology similar to fracking which is mainly used in the United States to extract oil and gas from clayey matrices. Once the rocks are fractured, carbon dioxide is injected directly into the fractures. For example, by hydrofracturing peridottite in a slice of oceanic crust that emerges in the Sultanate of Oman, it is possible to dispatch more than a billion tons of CO2 within the borders of that country alone.
In practice, instead of exposing the rocks to the atmosphere, the opposite is done: concentrated carbon dioxide is injected into the rocks themselves and then petrified forever, like the villains in fairy tales.
IN CONCLUSION
It is highly probable that no solution alone will solve the global problem of excess carbon storage. We will have to use a whole portfolio of technologies that includes forestation, storage in the ground and deep deposits. And these solutions will have to be implemented on a large scale to be effective and economically sustainable at the same time.
Furthermore, these carbon dispatching solutions will have to be accompanied by technologies to make our operations more efficient and emit less new carbon, the development of the circular economy and renewable and non-renewable energies that cause the lowest possible carbon production per unit of carbon. energy produced.
It is a global challenge that we can only win if we all work together.
(Extended version of an article published on eni.com)
This is a machine translation from Italian language of a post published on Start Magazine at the URL https://www.startmag.it/energia/anidride-carbonica-atmosfera/ on Sat, 13 Feb 2021 06:03:14 +0000.