No one can reasonably ignore it today: carbon dioxide (CO2) is one of the main factors responsible for the greenhouse effect, the phenomenon that contributes to global warming by redirecting reflected radiation towards the lower layers of the atmosphere and the ground. Though the greenhouse effect is essential to maintain a temperature suitable to the development of life on Earth, its excess threatens our climate with serious disruptions in the short to medium term.
The evolution of atmospheric CO2 concentrations shows an alarming increase from the beginning of the industrial era and, more particularly, a real boom since the mid-twentieth century. It has increased from 300 parts per million (ppM) in 1950 to more than 400 ppM today. According to most recent estimates by the experts of the Intergovernmental Panel on Climate Change (IPCC), a drastic and rapid reduction of CO2 emissions is vital to keep global warming within acceptable limits. We must quite simply reduce these emissions from 50 billion tons per year to zero by 2050 (scenario +1,5 °C) or 2075 (scenario +2 °C). We will overcome this challenge by combining a range of solutions.
Direct Air Capture (DAC) of CO2
In nature, especially through photosynthesis, huge amounts of atmospheric CO2 are captured and then very sustainably stored in plants and the animals which eat them. Over time, these will in turn eventually become coal, oil, and gas. This natural capture of CO2 is not the subject here, even though biomass conversion is a promising solution to reduce the concentration of greenhouse gas in the atmosphere.
Among the other solutions, industrial capture of CO2 and its long-term storage – its “sequestration” – could represent up to 20% of emission reductions. Until very recently, the capture of CO2 was only considered in effluents of industries emitting high levels of CO2: coal-fired or heavy fuel oil power plants, cement and steel factories, oil refining, ammonia production, etc. Given the high concentration of CO2 in these effluents, their capture is relatively “easy”, and carbon capture technologies have existed for a long time. However, these concentrated emissions only represent about 50% of the total emissions, the other half includes diffuse emissions due to transportation, construction or small industries.
Direct Air Capture (DAC) of atmospheric CO2 could offer an efficient solution to deal with the problem of diffuse emissions. However, the relatively low concentration of CO2 in the air is a major difficulty. With 400 ppM in air, and assuming a capture rate of 100%, we would indeed need to treat 1.25 million cubic meters of air to capture one ton of CO2. Let’s not forget: the challenge is to capture hundreds of millions, even billions of tons of CO2! That is probably one of the reasons why development plans of DAC have only very recently appeared. Other reasons include the difficulty to find an outlet for the captured CO2 and an economic model to justify the required investments, as well as the very high energy cost of these processes.
In terms of technology, existing projects rely on trusted solutions, based on the chemical reactivity of CO2 (an acidic gas) with basic reagents. The first prototypes developed at the turn of the century did not offer any major innovations. But of note was the demonstrator presented in 2008 by Calgary University made from an absorption column using a sodium hydroxide solution, with a capture capacity of 20 tons of CO2 per year. Since then, technologies have evolved and several industrial actors seem to be moving towards the large-scale development of DAC.
The wet process used in the beginning (bubbling of captured air in a solution of sodium or potassium hydroxide) is now rivalled by dry processes, using for example membranes impregnated with basic reagents. This process is proposed by the Swiss start-up Climeworks, from the federal Ecole Polytechnique in Zurich. The company has 14 operational or planned facilities thus far, among which the biggest commercial DAC factory in the world. The ORCA project, under construction in Iceland, will be able to capture 4,000 tons of atmospheric CO2 per year. But even if progress seems to speed up with growing awareness of the issues at stake, we are still very far from the medium-term objectives.
Whatever the reagent used to capture CO2 is, one of the main issues related to DAC is the energy required for extraction. Energy is essential to obtain pure CO2, both to store it in geological reservoirs or to make use of it as an industrial raw material. This is because even though CO2 reacts quickly with basic reagents, the reverse reaction requires very high temperatures – above 100°C. Hence, whilst this regeneration process makes it possible to recover the basic reagent that can reinjected into the capture cycle, it is energy intensive. Also, this step also results in loss of reagent. Finally, energy is required for packaging of captured CO2 – namely to compress it into a supercritical state at over 80 bars of pressure.
Beyond the economic aspect, these energy costs have a paradoxical effect: the capture process itself has an undesirable carbon footprint. Thus, the quantity of CO2 released by this capture process can amount to 30% of the carbon that it eliminates. To overcome these barriers, more innovating processes are being explored, such as “Electro-Swing-Absorption” (ESA)1; a process based on an electrochemical battery which uses polyanthraquinone as an electrode material. It is a polymer capable of sequestering CO2 when subjected to an electrical potential during charge. During the reverse process, the discharge of the battery releases the CO2 while providing a usable electrical current. Still in the research stages, this process was the subject of techno-economic studies to evaluate the cost of large-scale capture in a range of $50 to $100 per ton of CO2. In comparison, the price of the ton of CO2 on the European emission rights market, which has strongly risen in recent months, currently varies around 55€ ($66) per ton.