K.S. Lackner from Columbia University presents a comprehensive review of the progress towards implementing ambient air capture units, and focuses more deeply on capture sorbent development. Lackner discards the notion of using an aqueous sorbent due to the large binding energy required and the corrosiveness of a strong sodium hydroxide solution. Experiments were thus carried out to identify a sorbent with lower binding energy that could still maintain an uptake rate equal or better to that of a sodium hydroxide solution. Ultimately a solid strong-base ion-exchange resin was deemed to be the best sorbent option. Experiments showed that this resin could be loaded with absorbed CO2, and upon exposure to moisture the CO2 would be driven off and the resin would be ready to recommence CO2 uptake after dried. Based on this, Lackner goes on to describe a modular unit that could be easily deployed and could be expected to capture one ton of CO2 per day.
Technology to capture CO2 directly from ambient air has the potential to significantly reduce emissions from non-point sources and to capture historic emissions (Lackner 2009). Sorbents used in flue gas scrubbing can be used much to the same effect in free standing CO2 capture units, which can be designed to compensate for the lower CO2 concentration in the air stream. It is important to carefully choose a sorbent material that will maximize capture, and to consider a practical and marketable unit design and size. Lackner proposes that capture units be made on the scale of capturing one ton of CO2 per day. Given this, and the energy tradeoff associated with the operation and manufacturing of these units, Lackner determines that it would take approximately ten million capture units to make a significant impact on the world’s CO2 emissions. At present, prototypes for these units could break even at $200/ton of CO2 captured, but Lackner predicts that over time this price could feasibly drop to $30/ton. Ambient air capture is at this time technologically feasible, and may in time be economically feasible with the improvement of sorbent materials and as the need for retroactively capturing emissions grows. — Shanna Hoversten
Lackner, K., 2009. Capture of carbon dioxide from ambient air. The European Physical Journal 176, 93–106.
Lackner’s summary of ambient air capture technology exposes both the potential for further innovation and development of this technology, and the great promise for this technology to significantly reduce CO2 emissions. At present, 70% of the cost of the device derives from the development of the resin and the regeneration chamber; a reduction in these costs is essential in order to make the technology viable. Although changes in the filter thickness can compensate for regional differences in CO2 composition of the air stream, and air flow rate over the filter, current technology excludes the deployment of these modules from locations with extremely cold temperatures or locations with high humidity. However, in places where the units would work effectively, large air capture parks could be established directly on top of the designated storage site, eliminating the need for transfer networks. Ambient air capture units would collect CO2 from power plants and transportation sources alike, in addition to capturing past emissions at a rate far exceeding collection rates by trees, thus providing a promising technique for CO2 emission mitigation in the future.