My week has been full of learning more about Carbon Dioxide Removal (CDR)!
After reading extensively about CDR and drawing several diagrams, I began understanding CDR and its steps on a more detailed level. Furthemore, I took a more detailed look on the actual biological and chemical processes to take CO2 out of the atmosphere (You can think of these processes as vacuum cleaners for CO2!).
I’m excited to share the learnings! We’ll start by forming clarity around CDR. Then, we’ll dive into the biological and chemical processes to take CO2 out from the atmosphere!
When talking about CDR, it is essential to distinguish between at least the following three steps:
Taking CO2 out of the atmosphere
Storing the CO2
Making useful things out of the CO2
For the sake of clarity, I am not using specific terms for these steps of CDR. Most used terms are still ambiguous... Instead, I gave a description to each step to illustrate what’s happening.
1. Taking CO2 out from the atmosphere
First, the CO2 has to be collected from the atmosphere. Biological processes doing this, most notably photosynthesis. Also, chemical processes like carbon mineralization are capturing CO2.
This step seems to be called interchangeably carbon removal and carbon capture.
2. Storing the CO2
Second, the captured CO2 must be stored somewhere where it cannot escape back to the atmosphere for a long-time. Photosynthesis stores the carbon into biomass (trees, algae etc.) and carbon mineralization into stable carbonates. Alternatively, the collected CO2 can be injected deep underground into geoformations.
This step is called carbon sequestration or carbon storage in literature and media.
3. Making useful things out of the CO2
Thirdly, the CO2 can be further turned into useful products that have an economic value. Examples of these are building materials, fuels, chemicals, and other carbon entailing products.
The caveat in making products out of CO2 is that some products store carbon only for the short-term. For example, when synthetic fuels made out of captured CO2 are combusted, the CO2 is released back to the atmosphere.
This step is called carbon-to-value, C2V, and carbontech.
The following diagram is my attempt to summarize these insights.
This diagram is still a work-in-progress, so if you identify any mistakes or have other comments, I’d greatly appreciate for letting me know of those (firstname.lastname@example.org)!
Vacuum cleaners for CO2
Now let’s have a look at the first vertical of the diagram “Taking CO2 out from the atmosphere”.
I like to think of these as different kinds of vacuum cleaners that can suck the CO2 out of the atmosphere.
Based on my research so far, there are four main underlying methods to take carbon dioxide out from the atmosphere:
Direct Air Capture (Absorption, Adsorption)
We can thank photosynthesis and photosynthetic plants and organisms for our lives on Earth.
Photosynthetic plants and organisms do photosynthesis and convert light energy into chemical energy (carbohydrates like sugars). They take in carbon dioxide and water and turn it into carbohydrates.
Oxygen is the by-product of photosynthesis - the one that enables the lives of our Homo Sapiens species and of many other oxygen-inhaling species on this planet!
In photosynthesis, carbon becomes fixed into sugars and can be used to build further molecules that the plant needs to survive.
This week, I also came across interesting groups of microbes that cannot undergo photosynthesis - chemolithotrophs and electrotrophs. Instead they can do other awesome things!
Chemolithotrophs obtain their energy from reacting with inorganic compounds, like hydrogen, hydrogen sulfide, and reduced metals. In chemistry, inorganic compounds mean compounds that simply have no carbon atoms.
Kiani, A., Jiang, K., and Feron, P. (2020). Techno-Economic Assessment for CO2 Capture From Air Using a Conventional Liquid-Based Absorption Process. Frontiers in Energy Research. Link
Liew, F. et al. (2016). Gas Fermentation - Flexible Platform for Commercial Scale Production of Low-Carbon-Fuels and Chemicals from Waste and Renewable Feedstocks. Frontiers in Microbiology, 7:694. Link
Psarras, P., Woodall, C. M. & Wilcox, J. (2021). The Role of Carbon Utilization. CDR Primer, edited by Wilcox, J., Kolosz, B. , Freeman, J. Link
Song et al. (2004). Tri-reforming of Methane Over Ni Catalysts for CO2 Conversion to Syngas With Desired H2/CO Ratios Using Flue Gsa of Power Plants Without CO2 Separation. Studies in Surface Science and Catalysis, 153, 315-322. Link
TED (2018). A new way to remove CO2 from the atmosphere| Jennifer Wilcox. Link
The National Academies of Sciences, Engineering, and Medicine (2018). Direct Air Capture and Mineral Carbonation Approaches for Carbon Dioxide Removal and Reliable Sequestration. Link
The National Academies of Sciences, Engineering, and Medicine (2019). Negative Emissions Technologies and Reliable Sequestration. Link
Zhang et al. (2020). Recent advances in carbon dioxide utilization. Renewable and Sustainable Energy Reviews, 125. Link