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Direct Air Capture (DAC)

Banks of fans blow air through a carbon dioxide capturing solution in this rendering of a direct air capture plant. Source: Carbon Engineering
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Carbon emissions can be produced by point sources (e.g. fossil fuel power plants) and nonpoint sources (e.g. automobiles). While point source emissions can be sequestered through CCS, non-point emissions are not suitable for this. Direct Air Capture (DAC) helps fill this gap. DAC separates carbon dioxide directly from ambient air through physical or chemical processes. It is a negative emissions strategy that may play a role in mitigating climate change, particularly in the mid to latter part of the 21st century. An increased demand for DAC may also emerge from the need to ameliorate significant air pollution in large cities such as Beijing and New Delhi. 

DAC has an estimated sequestration potential of 2-11 GtCO2/year. Cost estimates per ton of CO2 removed have dropped from about $600 (Socolow et al. 2011) by a factor of three or more. One method by the company Carbon Engineering, gives a range at $94-235 per tCO2 (Keith 2018), and others project that the cost of DACs could drop to around $55/ton CO2 by 2040 (Fasihi et al, 2019). That said, even at these lower costs, without an adequate price on carbon, a market to motivate deployment of DAC at the scale needed will not develop. 

DAC can be carried out through two processes: 1) Absorption and 2) Adsorption. Absorption is the process of removing CO2 from the atmosphere using a basic solvent (such as sodium hydroxide). Absorption has a major drawback in that it requires high quality, high temperature heat to aid the chemical process. Adsorption is the process through which a solid sorbent which has a high affinity for carbon dioxide is used. Carbon dioxide occupies the pores on the solid sorbent which is then later removed through an increase in temperature or a decrease in pressure. Adsorption requires lower quality heat at lower temperatures and allows for more wide-spread installation. 

A recent study (Realmonte et al. 2019) explored scenarios with both technologies and found that the absorption process lends itself to larger plant size where 30,000 facilities would be required to remove 30 GtCO2/year by 2100, while the adsorption process would require 30 million units to capture the same amount. The authors point to other massive production numbers such as the 73 million cars manufactured in 2017, but also offered several cautions. On top of the massive amount of expected pollution from the sorbents needed, DAC at scale would use one quarter of the total global energy demand by 2100. They also stress the importance of near term mitigation efforts as opposed to reliance on the future potential of DAC.  

Many hurdles must be overcome for DAC to reach technical maturity such as improving methods of capture, new regulatory environments, cost, energy inputs; and the resource requirements of energy, equipment, and chemicals. In addition, sufficient siting of facilities at scale is needed to capture and process the CO2, whether the carbon is sequestered underground or repurposed.

Perhaps the ultimate promise of DAC is the potential to dial down the global average temperature beyond what may be achieved by massive deployment of the efficient use of clean energy. For example, if future climate change of even below 2°C is determined to have too many negative impacts, DAC in combination with other NETs such as afforestation, land management and Bioenergy with Carbon Capture and Storage, could be used to drawdown the concentration of carbon dioxide to a desired concentration and a more favorable climate.