The Feasibility of Carbon Capture and Storage

Introduction

Fig. 1: Keeling Curve. [12] (Courtesy of the Scripps Institution of Oceanography)

In order to prevent critical global warming of 1.5°C - 2°C, scientists have estimated that the world needs to reach net negative carbon dioxide emissions around mid-century. [1] Carbon dioxide emissions are increasing annually, and industries are looking to artificial carbon capture and storage (CCS) to counteract emissions. These systems that can remove carbon dioxide from gas streams have not been widely deployed, but demonstration projects have been planned in countries including the United States, the Netherlands, Belgium, the United Kingdom, Norway, and Italy. [2] A concern of CCS is its economic feasibility at scale, especially when compared to other methods of carbon offset, such as reforestation/deforestation prevention.

Current Carbon Emissions

In 2017, global emissions were recorded at 3.62 × 10¹³ kg CO₂. [3] Current carbon offset through natural processes and technological efforts are far from net negative emission goals, evident in the Keeling Curve (Fig. 1), which tracks carbon concentration in the air at a Mauna Loa observatory in Hawai'i. We estimate the annual increase in atmospheric carbon using the average increase of approximately 2.1ppm y^-1 from 2003 to 2013. [4] Multiplying by the mass of the atmosphere, 5.148 × 10¹⁸ kg, [5], and accounting for molar mass, we find an annual atmospheric carbon increase of

The rest of the CO₂ emission budget is presumably being absorbed by the oceans.

Using Trees for Net Zero Atmospheric Carbon Emissions

Large scale sequestration occurs naturally through plants, evident in the seasonal sequestration that results in the steep oscillations of the Keeling Curve (Fig. 1). One proposed method of carbon sequestration is the selective harvesting and burial (or stowing away) of trees, which would cut off carbon's return pathway to the atmosphere after being absorbed via photosynthesis, thus creating a carbon sink. The sustainable carbon sequestration potential for wood burial is estimated at (10 ± 5) × 10¹² kg C y^-1 (approximately (3.7 ± 1.8) × 10¹³ kg CO₂ y^-1), 6.5 × 10¹³ kg C (2.38 × 10¹⁴ kg CO₂) of which is currently suitable for burial on forest floors. [6] Note that this estimate is significantly more cost-effective and has more carbon sequestration potential than estimates of reforestation without burial. Estimates from a 2019 study indicate that tropical reforestation could remove 8.6 × 10¹¹ kg CO₂ y^-1 at $.10 kg^-1 CO₂ (totaling $86 billion annually) in 2030 and 1.84 × 10¹² kg CO₂ y^-1 at $.10 kg^-1 CO₂ (totaling $184 billion annually) in 2050 with precipitously diminishing cost efficiency at higher capacities. [7] Compare this with estimates for selective harvesting and burial in tropical rainforests indicating a potential of 4.2 × 10¹² kg C y^-1 (1.54 × 10¹³ kg CO₂ y^-1) at a price of $.014 kg^-1 CO₂ based on North American logging industry data (totaling $216 million annually). [6]

Carbon Capture and Storage

In order to help reach global net negative emissions goals, industries are looking to CCS technology. This technology operates most efficiently on gas streams with high CO₂ concentrations. CO₂ is removed from the gas streams, compressed, transported, and stored instead of released into the atmosphere. These systems are not widespread, but demonstration projects are being funded around the globe. The efficiency of CCS technology will be greatest for industrial production plant gas streams. CCS advocates focus on coal-fired power plants, which account for approximately 60% of the world's large (emitting more than 1 × 10⁸ kg CO₂ per year) stationary sources and emit about 8 × 10¹² kg CO₂ y^-1 altogether. [8] In 2018, the Petra Nova plant in Texas was the only U.S. fossil-fueled power plant capturing over 1 × 10⁹ kg CO₂ y^-1 [9], but was shut down in 2020. This plant captured 3.54 × 10⁹ kg CO₂ from January 2017-December 2019, but still fell short of its 4.2 × 10⁹ CO₂ projection. There are no direct reports of the cost per ton of CO₂ captured, but the costs have been estimated at a minimum of $.06 kg^-1 CO₂ in addition to the $300 million investment from power generation company NRG, $310 million in impairment charges, $190 million grant for the U.S. Energy Department, and $250 million in loans from Japanese banks. [10]

The theoretical costs of CCS for ambient air have been estimated at $1 kg^-1 CO₂, though rates may drop to approximately $.30 kg^-1 CO₂ by 2050. [11] At this estimated rate, neutralizing todays atmospheric emissions would cost 1.64 × 10¹³ kg y^-1 × $1 kg^-1 = $1.64 × 10¹³ y^-1 = $16.4 trillion per year.

Conclusion

There have been no successful industrial-scale demonstrations of CCS technology to catalyze widespread implementation. Estimates suggest that selective harvest and burial of trees is currently a much more cost-effective solution for carbon capture and sequestration needs. A comparison with other carbon reduction methods is necessary to determine which strategies in the industrial sector should take priority to reach net negative emissions.

© Blake Villanueva. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

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[11] K. Z. House et al., "Economic and Energetic Analysis of Capturing CO₂ From Ambient Air," Proc. Natl. Acad. Sci. (USA) 108, 20428 (2011).

[12] C. Harris, "Charles David Keeling and the Story of Atmospheric CO₂ Measurements," Anal. Chem. 2010, 78657870 (2010).