Carbon Capture and Storage is becoming a mainstream news item as one of the actions the world will need to undertake to achieve net-zero carbon dioxide emissions by 2050. There is a flurry of activity to explore direct air capture (DAC) of carbon dioxide (CO2) out of the atmosphere and either using it in chemical or horticultural applications but more likely intending to sequester it in a sub-surface aquifer.

In an article in July this year Leslie Hook described work in this area in a Financial Times article 15th July. In this article, Leslie explores some of the work being undertaken in Switzerland by Climeworks and by Carbon Engineering in Canada and nearer to home Silicon Kingdom in Ireland.
These companies are all working on direct air capture of carbon dioxide. In Leslie’s video, attached to her article, she visited Climeworks where they have a demonstration plant working at 900t/a capture using adsorption onto membranes. Captured CO2 is released through a heating cycle at 100°C. Climeworks acknowledges that the current cost, $800/t is very expensive but have customers taking some CO2 for growth enhancement in greenhouses. Their aim is to reduce costs to $100/t but they admit a key step is to build a market for the CO2.

This proposed route to capturing carbon dioxide directly from the air, whilst clearly feasible, is very expensive. The carbon dioxide once captured would have some smaller-scale industrial uses such as growing tomatoes or making carbonates/bicarbonates or even re-cycling into liquid fuels. Mostly these represent a short-term holding position for the CO2: tomatoes get eaten, stems composted, fuels burnt and bicarbonates release their CO2 on use. We say smaller scale; the provisional figures for the UK’s CO2 output in 2018 is 364 million tonnes. We could never use that amount of CO2 without some new chemistry and a market for the product.

The direct air capture process is expensive; the written article quotes $200/tonne CO2, $720/tonne carbon equivalent. (As mentioned in the video it is currently $800/t for Climeworks). The article is not explicit but if these costs are just for the capture step then transport by pipeline and pumping into a sub-surface formation will add substantially to the final bill. The technology has to overcome many challenges to yield a working system. In the end, with storage below ground, you have paid a lot of money to store a valueless product and gained an on-going liability for the stored CO2 and the infrastructure that has put it there. The liabilities may be less if the gas is reacted with sub-surface species in the brine that can transform it into a stable mineral. In some models, this may take centuries.

How does Biochar compare to direct air capture?

Biochar, made by heating green or woody materials in the absence of air, is essentially carbon and is similar to charcoal. In this form, it is fairly resistive to biological degradation and stable in the environment for years if not centuries. The green or woody materials, by contrast, will decay in the environment with most carbon returning to the air as carbon dioxide and some methane. Large scale use of biocharring would reduce the contribution the decomposition of green wastes makes to carbon dioxide and methane emissions.

In the UK, biochar is manufactured on a small-scale and the current cheapest bulk price is similar to that at Climeworks at $800p/t, however, this is bought in from the EU. Scale-up of production of biochar could be achieved with few obvious technical barriers but needs investment in small plant and mobile pyrolysis equipment, which will bring down the cost per tonne in the future. Biochar, unlike DAC, is a value-added product and as a stable form of carbon is not a liability, but instead has many potential positive uses.

Its uses are being developed but the most promising is its use in horticulture and agriculture as a soil improver. Biochar shows positive benefits on soil health, plant growth and crop yield. More work is underway to understand where the application of biochar yields the best long-term results and we are conducting our own small scale back garden trials.

There is a tendency for money to rush into new exciting bleeding edge technologies full of promise, but investors shouldn’t discount a technology that has been around for thousands of years; sometimes the simplest ideas are the best ones. The global heating challenge is so vast that in reality, we will need every tool in the arsenal to tackle it.