Carbon capture methods, both nature-based and technological, are the subject of the Our Future Planetexhibition at the Science Museum, open until September 2022.
Now an experimental system which removes carbon dioxide directly from the atmosphere with unprecedented performance has been unveiled in the journal ACS Environmental Au by a team from Tokyo Metropolitan University.
Their “liquid-solid phase separation” method is based on a colourless liquid used to make polymers and coatings, called isophorone diamine (IPDA) and could scrub carbon dioxide contained in the atmosphere with 99% efficiency.
The compound IDPA is reusable and at least twice as fast as existing systems and marks “an exciting new development for direct air capture”, according to Professor Seiji Yamazoe of Tokyo Metropolitan University, where he has studied liquid-solid phase separation systems.
Even though carbon dioxide levels are at a record high, and driving climate change the greenhouse gas is only present at around 420 parts per million (though the highest level recorded in human history) , so low that chemical reactions with sorbents are very slow.
There is also the difficulty extracting carbon dioxide once it has been absorbed, which can also be energy intensive.
Traditional methods used for direct air capture, such as those using potassium hydroxide, which reacts with carbon dioxide to form potassium carbonate, have relatively low efficiency and high carbon dioxide recovery costs, spurring the hunt for alternatives.
The problem is that an alkali-earth (such as potassium) hydroxide approach involves bubbling air through the liquid, with a chemical reaction occurring between the liquid and the carbon dioxide, but as reaction product accumulates in the liquid, subsequent reactions slow down.
Although this still works with 40-80% removal efficiency, another issue is that a high temperature of more than 700°C is required in the process to recover the carbon dioxide.
The liquid-solid phase separation systems studied by Professor Seiji Yamazoe offer one possible solution. The reaction product is insoluble and comes out of solution as a solid, so there is no accumulation of product in the liquid, and the reaction speed does not slow much.
The team focused their attention on liquid amine compounds, modifying their structure to optimize reaction speed and efficiency with a wide range of concentrations of carbon dioxide in air, from around 400ppm to up to 30%.
They found that an aqueous solution of IDPA could convert 99% of atmospheric carbon dioxide into a solid carbamic acid precipitate.
Crucially, they demonstrated that the solid dispersed in solution only required heating the solution to 60°C to release the captured carbon dioxide, so the solvent could be reused.
The rate at which carbon dioxide could be removed was at least twice as fast as that of the leading direct air capture lab systems, making it the fastest carbon dioxide capture system in the world, Prof Yamazoe explained:
‘The maximum CO2 removal efficiency (201 mmol/h for 1 mol sorbent) of our system was much higher than that of conventional potassium hydroxide system (13 mmol/h for 1 mol sorbent) and other phase separation systems (5-107 mmol/h for 1mol sorbent).
Scaling up of this IPDA system is next challenging issue. In addition, we must improve the carbon dioxide concentration under carbon dioxide desorption process because highly concentrated carbon dioxide is required if it is to put to further use.’
This could be one of the technologies that will not only cut carbon levels but achieve a “beyond zero” to actively reduce the amount of carbon dioxide in the atmosphere. Prof Yamazoe went on to say:
‘Direct air capture (DAC) is essential to achieve the Paris 1.5°C target. However, the cost of current DAC technology is too high.
Our system has a potential to solve these problems, and the general use of direct air capture could start if we perfect a large-scale system, which could take around a decade.’
‘Amine based solutions provide an effective way of capturing carbon dioxide efficiently, with significantly lower energy requirements for recovering the captured carbon dioxide.
Monoethanolamine (and dosed amine solutions) have been used for some time to recover carbon dioxide from higher concentration post-combustion processes but IPDA appears to be particularly adept at removing the much lower carbon dioxide concentrations encountered in direct air capture, DAC, processes.
This has potential to significantly reduce the operating costs of the next generation DAC systems, addressing one of their current challenges.’
