Why carbon, not carbon dioxide?
If we want to capture carbon, we should replicate its coal production process, not imitate photosynthesis.
We have made the point before, but with the Biden administration announcing a note of intent to subsidize Direct Air Capture (DAC) to the tune of 3.5 billion dollars after private investment, we have to make the point more forcefully: Don’t.
There are many promising (and some not so promising) approaches to creating a circular carbon economy and to reduce the global carbon footprint, but DAC has to be at the bottom of that list, so the intense infatuation in certain techie circles (and now by extension, in government circles) continues to befuddle.
It's the kind of technology approach where after ten years everybody will step back, take a second look, and wonder how they got into that mess. In the case of DAC this would mean not only billions of dollars of investment lost, but also ten years of progress.
Why are we so sure about this?
Direct Air Capture (DAC) is the idea of removing carbon dioxide from the ambient atmosphere and storing it deep underground. Pilot plants exist and have received quite a bit of initial media attention. It certainly appeals on an emotional level since it literally vacuums CO2 out of thin air, nevermind that CO2 is only the fourth most common constituent of dry air at around 420 ppm or 0.04% (and rising) after Nitrogen, Oxygen, and Argon, which makes the chemical process technologically tricky.
We don’t intend to stifle competition among technologies, but large public investments into any one contender should follow sound principles. DAC pretty much violates all of them.
Some very fundamental principles include:
Positive energy balance. DAC is massively energy intensive, which is why its only deployed where energy is ubiquitous, cheap, and fossil-free.
Simple, replicable technology. Both elements of DAC, getting CO2 out of the air and storing it deep underground, are cartoonishly complex, requiring both high up-front investments and high running costs.
Decentralizable technology. The complexity of DAC pretty much kills all chances of it being run is small operations, close to the source. Transportation costs are a major additional factor.
Traceable technology. It’s far from straightforward to document the amount of CO2 captured and sequestered.
The best way to remove carbon dioxide from the air is a billion-year old process known as photosynthesis. It's cheap and has many positive side effects. There is no reason to replicate this natural process using expensive technology.
Store the carbon to ameliorate soil or form massive deposits.
The process to replicate is the way nature creates, stabilizes, and stores carbon for the long haul. Grow plants. Use the edible parts for food and convert the rest into stable carbon. Grow forests, recreate moors. Stabilize carbon via char production and put coal back into the ground.
Do things that make economic and ecological sense. But don't waste energy, money, and public goodwill that can be put to other good uses to mimic photosynthesis.
DAC sounds appealing if you assume that energy is abundant and cheap, so massive electricity bills are not an issue. This severely limits its scope now, and becomes pointless if we ever get such a source of cheap, abundant energy because then we want to be able to reuse carbon in a sustainable manner.
Once you also consider water consumption, land use, or the toxic effects and disposal of the chemical binders used, DAC does not make sense. Moreover, safe and long-term CO2 storage cannot be guaranteed.
When DAC plus CCS is used, the captured CO2 is liquefied by pressure and transported to sites where it can be pumped into geological formations – theoretically for long-term storage. However, this technology is associated with a whole range of risks, a significant danger being, for example, leakage of carbon dioxide out of wells.
And never ever think about sequestering CO2 in the oceans! They already contain too much carbon dioxide and are already turning acidic.