We captured CO2! Where do we put it?

In the last few posts, we discussed how we can capture CO2 from exhaust gas and directly from the air [1,2,3,4]. But we did not explain what is done with the CO2 after it is captured! Here are a few options:

Store it underground: usually, the captured CO2 is put under high pressure, forcing it to turn into a ‘dense fluid’ which flows like a gas but has a density closer to a liquid [8,9]. The fluid CO2 is pumped into geological formations or old oil fields at depths over 750 meters underground [5,11]. This is already happening in Europe, China and Australia [14]. So far it seems that this method works well in some places but not others, depending on e.g. the type of rock [15]. However, it is the best understood storage method [5].

Store it in rocks: minerals containing calcium or magnesium can be reacted chemically with CO2 [13]. The result is “mineral carbonate” rocks that are very stable, holding carbon safely so it doesn’t easily re-enter the atmosphere as CO2 [13]. The mineral carbonates can then be used e.g. in industrial processes and road surfaces [16]. However, the financial cost of this process is currently higher than underground storage of CO2 [13].

Make products: some researchers propose using the captured carbon in the construction industry [7], while others want to turn it into carbon-neutral fuel [1]. Fuel could be made by combining CO2 with hydrogen (produced from water using renewable energy); if the CO2 released on burning the fuel is then captured again, this could in theory make a circular system with no overall CO2 release [16].

There is no shortage of ideas for how to store & use CO2 [6,10,12]. What we need is further analysis of the risks as well as improvements in cost, efficiency, & scalability of each method of carbon storage [6].


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