Is direct air capture nature-inspired?
September 8-13, 2019
The very concept of carbon dioxide removal from the atmosphere is nature-inspired. Plants take CO2 from the atmosphere and convert it to reduced carbon. Combining direct air capture with renewable energy to produce synthetic fuels mimics photosynthesis. However, the scale of agriculture, silviculture or aquaculture that could balance the anthropogenic carbon budget dwarfs current world agriculture. This points to the need for process intensification and non-biological, technical approaches to remove excess carbon from the environment.
Closing the anthropogenic carbon cycle without direct air capture is difficult. While air capture could be nature-inspired, it would have to outperform natural processes. The IPCC hints at the difficulty of this challenge. The Fifth Assessment Report and the Special Report on Global Warming of 1.5°C both emphasize the need for negative emissions, but only discuss bioenergy with carbon capture and storage (BECCS). They do not offer a transition from natural processes to advanced, nature-inspired technologies. Early implementations of direct air capture show little inspiration from nature. Fans, blowers and thermal swings mimic flue gas scrubbing rather than natural processes. However, a new generation of designs takes its cues from natural systems. Passive systems that avoid blowers and let wind flow over surfaces face mechanical problems akin to those that nature solved in the design of a tree. Not only is it necessary to make good contact between gas-solid exchange surfaces, but contactors must also operate over a wide range of natural wind speeds. Low wind speeds limit air exchange; high speeds stress structural supports. Passive collection of CO2 from air also sidesteps Sherwood’s Rule, which would have the cost of direct air capture equal the cost flue gas capture multiplied by the ratio of CO2 concentrations. Sherwood’s Rule, if it were to apply, would render direct air capture hopelessly expensive.
Passive systems will likely scale up in numbers rather than size, raising challenging issues of how to control and integrate the output of thousands if not millions of subsystems without armies of operators. This calls for process intensification, automation and feedback controls well beyond those in conventional process engineering. It will require inspiration from the exquisite feedback and control strategies exhibited in natural systems.
With regard to chemical capture mechanism, nature-inspired technologies offer additional opportunities from the study of interactions of water and CO2. A particularly interesting class of sorbents use the presence or absence of water to modulate the affinity of the sorbent to CO2. These moisture swing sorbents control the availability of hydroxide ions for CO2 sorption by controlling the behavior of water in small hydrophobic pores inside the structure of a polymer sorbent. A case can be made that carbonic anhydrase performs a similar task and that one therefore can learn from biological analogs. More generally, these moisture swing sorbents shed light on complex properties of water which equally govern biological systems.
Recent developments, moving well beyond conventional sorbent technology, aim for actively pumping membranes made from moisture swing sorbents. These membranes take advantage of moisture differentials across a membrane and the resulting chemical potential to pump CO2 into the interior of a hollow fiber. The exergy of evapotranspiration moves the fluid and pushes the CO2 against a chemical potential into the fiber. The design has a remarkable similarity to the vascular systems of a tree. Leaves, via evapotranspiration, pull up a carbonate brine, load it with CO2 and return it to the root of the tree, where the captured CO2 is removed.
Direct air capture can help close the world’s carbon budget. By sequestering the captured CO2, it can reduce the CO2 concentration in the atmosphere. It also can provide a chemical feedstock to produce fuels from renewable energy. These fuels create the storage capacity necessary for the deep penetration of intermittent renewable energy into the world’s energy infrastructure. The introduction of synthetic liquid fuels enables a gradual evolutionary approach to the global energy transition. Long after fossil fuels will be phased out, the carbon cycle has been closed and the climate is stabilized, synthetic methane, gasoline, diesel and jet-fuel can support the existing infrastructure. These fuels will then be gradually replaced with energy carriers that owe nothing to petroleum. Evolutionary principles tested in nature suggest that such gradual transformations offer more stability than a radical and sudden overhaul of the entire energy infrastructure.
Klaus Lackner, "Is direct air capture nature-inspired?" in "Nature-Inspired Engineering", Marc-Olivier Coppens, University College London, United Kingdom Bharat Bhushan, Ohio State University, USA Eds, ECI Symposium Series, (2019). https://dc.engconfintl.org/nature_inspired/31