The case for direct air capture

By Abhinav Kumar

It is being increasingly recognized that even with deep emission cuts and accelerated transition to renewables, it is next to impossible for the world to meet its 1.5° C goal. That is, without some level of carbon removal (IPCC, 2018). Carbon removal refers to methods of removing CO₂ from the atmosphere.

Planting trees is not enough
Trees naturally remove CO₂ from the air and store carbon in the soil. So, it’s natural to wonder if we could just plant more trees and dig ourselves out of this climate mess? The answer is a little less optimistic. Trees, as good as they have been at removing CO₂, have some significant limitations when being used as large-scale permanent CO₂ sinks. Based on the type of tree, it takes about 31 to 46 trees to sequester about one ton of CO₂ from the atmosphere every year. To put that in perspective, the world is on track to emit roughly 36.6 billion tons of carbon dioxide this year. So, if we were to just go by that math, about 1.4 trillion trees would do the trick. We have 3 trillion trees on the planet, and we are still seeing CO₂ concentrations rise. The reason is multifold; trees are subject to seasonality and can only capture CO₂ when they are leaf-bearing, trees emit CO₂ back into the atmosphere when they die and decay, and deforestation for agriculture and land development not only releases the CO₂ in the trees but also in the soil.

Soil emissions can also increase during droughts and flooding, and with rising temperatures, droughts are becoming more frequent. Finally, wildfires are huge carbon emitters, and climate change is increasing their frequency as well. Unsettling recent estimates suggest that the Amazon rainforest has become a net emitter of CO₂ rather than a CO₂ sink. The fact is that while planting trees will remain an essential element and a symbolic one in our fight against climate change, that alone will not get us to our climate goals. A recent technical study by carbonplan.org, which stresses the importance of accurate measurement and verification of permanent carbon dioxide removal, reached similar conclusions about the effectiveness of terrestrial carbon sinks.

Artificial carbon removal
A range of technologies collectively known as CCUS (Carbon Capture Utilization and Storage) artificially remove carbon dioxide at the source or from the air, and then utilize that CO₂ towards carbon applications (achieving net neutral emissions) or store it permanently deep within the earth (net carbon negative). When carbon is removed at the source, it is referred to as point source capture (PSC). 

PSC uses infrastructure that captures CO₂ at plume stacks for carbon intensive locations such as refineries, steel mills, cement plants, and power plants. The captured CO₂ in the exhaust or plume is then transported to a permanent storage location, where it is pumped thousands of feet underground. PSC, however, can only reduce CO₂ emissions at high intensity sources, typically locations with high scope 1 (direct emissions from burning fossil fuels at the facility) and scope 2 (secondary emissions from utilities consumed, such as power and water) emissions. Scope 3 emissions, which are dispersed over the planet through activities such as driving a car or agriculture, constitute up to 75% of all corporate emissions and are unaffected by PSC. Therefore, PSC is only an emission reduction approach and does not provide net carbon removal. Furthermore, PSC does not capture all CO₂ emissions from plume stacks. The Petro Nova coal plant Carbon Capture retrofit, which was done in 2017, can only capture a third of all generated CO₂. And finally, there is the challenge of transporting the captured CO₂ to a location which is suitable for permanent storage, and the chance of leaking CO₂ in the process.

Direct air capture (DAC) 
DAC is a technology that removes CO₂ directly from the air and thus is non-discretionary to the emission source. Currently, DAC plants are operating only at small scales. DAC is achieved by capturing CO₂ from the air through the use of especially formulated solid or liquid mediums. The captured CO₂ is released and captured through pressure or temperature manipulation and is then stored permanently deep under earth’s geological formations. Currently, there are about 18 DAC units operating worldwide, and the number is growing every year. 

While these large-scale air purifiers seem to be the answer the world is looking for, there are a few things to consider. Firstly, at-scale development of DAC plants is extremely costly. The largest planned DAC unit, which is capable of capturing up to 500,000 tons of CO₂ per year, would cost almost a billion dollars in investment (Johnson, 2022). Secondly, DAC units are extremely energy intensive. Per certain estimates, if all energy for a DAC unit came from a fossil fuel power plant, it would emit the same amount of CO₂ in scope 2 emissions as it would capture. So, DAC becomes viable only when running on renewable energy. On average, DAC units use about 400 kwh of electricity per ton of CO₂ captured. At 8 cents per kwh of renewable (carbon free) electricity, that amounts to about $32 per ton of CO₂. Thirdly, there are operating costs associated with transporting and storing the captured CO₂. Altogether, DAC costs about $250–$600 for removing one ton of CO₂ from the atmosphere. Compare that to about $80–$100 per ton of CO₂ for point source capture (Anderson, 2020) and $20–$50 per ton of CO₂ for offsets through reforestation (Trove Research, 2021). While DAC is costly, the next closest option to DAC in terms of permanence of removal would be through the conversion of biomass into biochar. And while this technology is promising, it is still very much in infancy, and estimated costs of removal are comparable to DAC at $222–$584 per ton of CO₂.

Further benefits of DAC
We have already established that from a purely technical “zero emission” pursuit, DAC has a pretty strong case when it comes to widespread hard-to-abate emissions. DAC offers a permanent and immediate carbon offset solution while the land use case is debatable, especially when you factor the amount of solar and wind needed to power DAC systems. DAC systems are regionally unbiased. This allows them to be located in regions where land use penalty is small, as pointed out by Patricia Loria, VP of Business Development at Carbon Capture Inc. and Executive MNR alum. 

"There are opportunities to put DAC in places like the global south which may have lower energy costs and more land available for use," says Patricia. 

None of these benefits can be achieved through traditional reforestation efforts, or any proven technology, for that matter. As for the economics, there is pressure mounting on large corporations and governments to do more to speed up transition towards a net zero scenario. The question is not whether DAC would be part of the solution, as much as how to make DAC economically viable. 

The good news is that the cost of carbon capture technology is coming down and will continue to come down as it scales up. If current trends are to hold, the technology may cost under $200 per ton of CO₂ by the end of the decade. The ambitious DOE “Carbon Negative Earth Shot” aims to bring together diverse actors from the government, local communities, industry, and academia to collaborate creatively and bring down the cost of permanent removal to under $100 per metric ton of CO₂ equivalent removed within a decade. The larger onus for making DAC economically feasible lies within public policy, and recently the IRA (Inflation Reduction Act of 2022) has done just that. The government is offering up to $180 in credit per one ton of captured CO₂ to technologies developing DAC plants. DAC providers can use that money to make the carbon capture offset market more affordable.

Abhinav Kumar is a Mechanical Engineer with over 15 years of combined work experience across the chemical and oil and gas industries. He strongly believes that climate goals cannot be reached without an honest effort on the part of private organizations to develop new technologies and find creative ways to cut their own emissions across the entire value chain. He is passionate about decarbonization, circular economy, and low carbon intensity energy sources.