Carbon Dioxide Removal (CDR) plays a pivotal role in many scenarios of greenhouse gas mitigation. However, the scale of meaningful deployment of CDR is underappreciated.1 A United States National Academies study estimates that 10 to 20 billion metric tonnes (Gt) of CO2 must be removed annually by 2100, an enormous challenge.2
A Direct Air Carbon Capture and Storage (DACCS) system is one method of CDR that removes CO2 from the atmosphere via chemical reactions, then compresses and transports it to a long-term geological storage site. The energy needed for the capture is a combination of electricity and thermal energy that is provided by natural gas or waste heat from industrial processes. In current configurations, the electricity is purchased from the grid or generated on-site with gas turbines. Several pilot plants have demonstrated their technical feasibility at the scale of millions of metric tonnes (MtCO2), but operating costs are currently much higher than existing emissions reduction methods. Many industry analysts expect costs to decline as the technology matures and deployment expands.
A DAC plant built by Carbon Engineering in British Columbia, Canada removes 1 MtCO2 annually and requires 5.25 GJ of natural gas and 366 kWh of electricity per tonne of CO2 captured.3 The CO2 from natural gas burned on-site is captured, so the net capture is 1 MtCO2 if the electricity has zero emissions. If a similar plant were built in the United States and connected to a regional grid, the actual carbon benefit would vary depending on the energy mix of the grid. For example, the SRMW grid in the Midwest relies on coal to generate 62% of its electricity (August 2022). The same Carbon Engineering plant connected to the SRMW grid would therefore provide a carbon benefit of 0.75 MtCO2 removed/yr. Compare this to the NYUP grid serving New York State, where nuclear and hydropower supply 66% of electricity. The same Carbon Engineering plant connected to that grid provides a carbon benefit of 0.96 MtCO2 removed/yr.
The climate benefit of a gigatonne-scale DACCS program is fully realized when the DAC process uses a zero-carbon source of electricity from some combination of renewable and nuclear sources.4 In this way, the energy supply challenges of DAC plants mirror those of electric vehicles (EVs), whose actual carbon footprint also depends on the carbon intensity of electricity used to charge them. Electrification technologies including DAC and EVs are benefitting from the ongoing decarbonization of the grid that is underway in most regions of the country. The thermal energy needed to regenerate the binding agents in the DAC process must come from either on-site natural gas, whose emissions are then captured, or from nearby existing sources of waste heat.
For a more in-depth look at Direct Air Carbon Capture:
Vilallonga, Lucía and Cutler J. Cleveland. 2022. Direct Air Carbon Capture and Storage Market Scan.
Boston University Institute for Global Sustainability, https://www.bu.edu/igs/files/2022/09/Direct-Air-Carbon-Capture-and-Storage-Market-Scan_091522.pdf
1 Anderson, K., & Peters, G. (2016). The trouble with negative emissions. Science, 354(6309), 182–183. https://doi.org/10.1126/science.aah4567
2 National Academies of Sciences, Engineering, and Medicine. (2019). Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259.
3 Keith, D. W., Holmes, G., Angelo, D. S., & Heidel, K. (2018). A Process for Capturing CO2 from the Atmosphere. Joule, 2(8), 1573–1594. https://doi.org/10.1016/j.joule.2018.05.006
4 Deutz, S., Bardow, A. Life-cycle assessment of an industrial direct air capture process based on temperature–vacuum swing adsorption. Nat Energy 6, 203–213 (2021). https://doi.org/10.1038/s41560-020-00771-9
