Air-capture dreams

The IMechE appears to have left behind some of the principles that those of us working in the field take for granted

I read that some of the elements of air-capture technology heavily promoted by the IMechE are being displayed (News, PE September). I am surprised that, in an effort to spearhead innovation, the institution appears to have left behind some of the principles that those of us working in the field take for granted.

Direct air capture (DAC) may have a role to play eventually in countering emissions from some decentralised sources of CO2, such as from buildings and vehicles (ships, planes) that prove expensive to remove by other means, but there is other relatively “low hanging fruit” to be picked before addressing these sources.

Examples of this include:

• energy constraint (do we need to use it in the first place?)
• energy efficiency (using less of it to do the same job)
• use of lower-CO2 energy sources (nuclear power stations, wind and wave)
• CO2 capture from large point source emitters (power stations, steelworks, refineries) and storage

Until capture from these sources has been implemented on a global scale, DAC is never going to be a cost-effective alternative.

High CO2-emitting energy sources are not viable options for providing the power for DAC systems, because their CO2 emissions may well exceed the CO2 captured. Compressing CO2 to conditions suitable for long-term geological storage is an energy-intensive process: the sensible place to start is with CO2 at pressure, then CO2 at atmospheric pressure and lastly that under vacuum conditions, because compressor power increases exponentially with pressure differential.

The storage part of CO2 capture and storage would have to be both inexpensive and feasible at a grand scale for any additional CO2 captured by DAC to be economically viable. The current mechanisms to incentivise this are inadequate by several orders of magnitude.

Economic models of global CO2 emissions are stretching over very long periods of time, particularly those that feature “overshoot trajectories” and rely on DAC should be viewed with extreme caution, given the large uncertainties in estimating the cost of DAC, the cost of raw materials and the uncertainties associated with technology roll-out and global uptake.

DAC is not currently an economically viable approach to mitigating climate change. At best DAC is a distant dream, and at worst it is a distraction of funds and engineering effort from more promising technologies, with a consequential delay in their implementation.

Andy Brown, Barnwood, Gloucester

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