Current status and pillars of direct air capture technologies

Abstract

Climate change calls for adaptation of negative emission technologies such as direct air capture (DAC) of carbon dioxide (CO₂) to lower the global warming impacts of greenhouse gases. Recently, elevated global interests to the DAC technologies prompted implementation of new tax credits and new policies worldwide that motivated the existing DAC companies and prompted the startup boom. There are presently 19 DAC plants operating worldwide, capturing more than 0.01 Mt CO₂/year. DAC active plants capturing in average 10,000 tons of CO₂ annually are still in their infancy and are expensive. DAC technologies still need to improve in three areas: 1) Contactor, 2) Sorbent, and 3) Regeneration to drive down the costs. Technology-based economic development in all three areas are required to achieve <$100/ton of CO₂ which makes DAC economically viable. Current DAC cost is about 2-6 times higher than the desired cost and depends highly on the source of energy used. In this review, we present the current status of commercial DAC technologies and elucidate the five pillars of technology including capture technologies, their energy demand, final costs, environmental impacts, and political support. We explain processing steps for liquid and solid carbon capture technologies and indicate their specific energy requirements. DAC capital and operational cost based on plant power energy sources, land and water needs of DAC are discussed in detail. At 0.01 Mt CO₂/year capture capacity, DAC alone faces a challenge to meet the rates of carbon capture described in the goals of the Paris Agreement with 1.5-2°C of global warming. However, DAC may partially help to offset difficult to avoid annual emissions from concrete (∼8%), transportation (∼24%), iron-steel industry (∼11%), and wildfires (∼0.8%).

Keywords: Chemical engineering; Energy sustainability; Environmental technology; Mechanical engineering.

Graphical abstract

Figure 1

CO₂ captured from air using liquid and solid sorbent DAC plants, storage, and reuse The ambient air is sucked in through large fans which is then treated with a chemical sorbent (Liquid or Solid) and heated to extract CO₂. This CO₂ is then either sequestrated or used in other industries as shown.

Figure 2

The five pillars of direct air capture Factors like carbon capture sorbent technology, electrical and thermal energy demand, cost, the environmental impact, and political support from different regimes affects the adoption of the DAC technology. These pillars need to be balanced well for the successful deployment of diverse DAC technologies.

Figure 3

DAC and CCS plants around the globe Green, Red, and Orange circles denote the operational status of the plants - Operating, Under Construction, or Planning, Nonoperating, respectively.

Figure 4

Effect of industry growth rate on NET CO₂ removal (A) Net CO₂ removal with varying industry growth rate (in % per year). The unmarked black line denotes the nominal growth rate (20% per year) and the varied growth rates are denoted with marked colored lines and labels. (B) Percent change in net CO₂ removal relative to the nominal case. Markers show the mean of all scenarios by year. The funding is from the club of democracies. (The Figure is taken from the reference (Hanna et al., 2021) with author's permission).

Figure 5

DAC plant land area requirement The left column represents Climeworks and the right column represents Carbon Engineering. Both of them require an area of 0.2 km² per one million of CO₂ removal for the given sorbent and thermal energy source combination.

Figure 6

DAC technologies (A) Liquid-Precipitate Cycle. (B) Liquid Adsorbent. (C) Solid Adsorbent Cycle. Steam and vacuum values in (C) are included in only one of their respective boxes. Not included in (B) are the energy requirements from recirculation pump and blower work, which gives the overall system an energy requirement of 5.23 GJ/tCO₂ (Broehm et al., 2015; Kiani et al., 2020; National Academies of Sciences, 2018; Socolow et al., 2011).

Figure 7

DAC cost breakdown and comparison All systems are presented with data that assumes a plant with removal capacity of one MtCO₂/year and a fixed charge factor of 12%. (A) Liquid Solvent DAC Capital Cost with low and high range. (B) Liquid Solvent DAC Operating Cost with low and high range. (C) Solid Sorbent DAC Capital Cost with low, mid, and high range. (D) Solid Sorbent DAC Operating Cost with low, mid, and high range. Low and high bounds are the result of the type of material used for a specific part, factoring in new technology, and varying costs from vendors (National Academies of Sciences, 2018).

Figure 8

Energy requirements for solid and liquid DAC systems ‘Low’ is used for the lower range values, and ‘high’ is used for the higher range values. Values found from reference (Chatterjee and Huang, 2020; Kiani et al., 2020; Krekel et al., 2018; Luis, 2016; National Academies of Sciences, 2018; Realmonte et al., 2019; Rochelle et al., 2011).