What is CCUS? – How Carbon Capture plays a role in the hydrogen economy – Dr. Julio C. Garcia-Navarro.
Some time ago I came across an interview given by the CEO of the company Element 2 about their efforts in the hydrogen economy. I found one of the quotes from the article quite interesting:
“The challenge in capturing carbon at this scale is a big one, and something that is yet to be successfully demonstrated anywhere in the world”
I asked myself “is this true?” and then I questioned everything I thought I knew about blue hydrogen. We all know that blue hydrogen has a bit of a reputation issue, partly fueled by a recent article where they criticize blue hydrogen in account of its lifecycle emissions.
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Blue hydrogen is essentially gray hydrogen but instead of emitting the CO2, it undergoes CCUS to separate the CO2 from the hydrogen. I would like to dedicate this article to show a bit of what I know about CCUS, including the state of the art of the technologies as well as the ongoing projects.
CCUS – overview
CCUS stands for Carbon Capture, Utilization, and Storage. As such, it is not one single process but the result of different stages in the carbon life cycle. CCUS is a bit of a misnomer because one would think that CCUS means that the carbon is first captured, then utilized, and subsequently stored, while in reality the processes are either CCU (Carbon Capture and Utilization) or CCS (Carbon Capture and Storage).
Carbon Capture
The capture of carbon consists of the separation of CO2 from a mixture (e.g., a flue gas from a combustion process or a CO2-rich natural gas stream) via physical or chemical means. The separation of carbon from the fuel is typically done using one of three methods:
- Pre-combustion – the carbon is separated from the fuel before being combusted. An example of this is the SMR process: methane reacts with steam to form CO, CO2 and H2. The end result is that, prior to utilization, the fuel (methane) is stripped from the CO2 and only H2 remains.
- Post-combustion – the carbon is separated after the fuel has been used to generate energy.
- Oxyfuel combustion – this technology is similar to post-combustion but oxygen is used instead of air to combust the carbon-rich fuel, leading to a more complete combustion that releases less CO and has an inherently higher efficiency.
There are different processes to separate the CO2; most carbon capture processes used relate to post-combustion technologies. The table below that showcases how post-combustion carbon capture this is done, as well as the strengths and challenges of each technology. For more information, you are welcome to consult the article of Osman et al. (2020).
| CO2 separation method | Description | Strengths | Challenges | TRL (Technology readiness level, in a scale from 1 to 9) |
| Absorption | CO2 is absorbed in a fluid (called absorbant) that selectively binds to the CO2. The CO2 is then released from the absorbant (regeneration) Typical absorbants: monoethanolamine (MEA) | Mature technology, has been in use for 100 years; also known as “amine sweetening” or “amine gas treating” (used to remove H2S and CO2 from natural gas) Good CO2 selectivity High absorption rate Easy to retrofit existing plants to include it | Absorbant regeneration is energy-intensive Corrosiveness of absorbant Loss of solvent | 9 |
| Adsorption | CO2 is adsorbed on the surface of a highly porous solid (called adsorbant) and subsequently released from it (regeneration) Typical adsorbants: activated carbon, zeolites, metal-organic frameworks, carbon nanotubes | Lower costs Lower energy consumption to release CO2 from adsorbant | Adsorbant regeneration is energy-intensive No widespread industrial applications yet | |
| Biological | Use of photosynthetic organisms for CO2 capture. Algae have better carbon stabilization than land plants. | Lower costs | Costly | 7-8 |
| Other technologies | Membrane separation – uses a membrane that is CO2-permeable Chemical looping – using a chemical (e.g., a metal oxide) to form carbonates, which can later be used to produce methane | Lower energy consumption, Easier to adapt than other CO2 separation technologies | Scale-up not yet there | 4-6 |
Carbon Utilization
Carbon utilization refers to using the separated CO2 in an existing production process. The current demand of CO2 is 200 Mt/year which, considering that the global CO2 emissions are 32,000 Mt/year (160x higher), would make it challenging to create a circular economy around CO2. Below you can see an overview of the different uses of CO2, both in existing production processes and as alternatives to enhance new ones.
An increased penetration of carbon capture in the economy might have as a side effect that CO2 valorization ideas (such as CO2-enhanced processes, as seen on the figure above) might become more relevant, especially if incentives arise to create a revenue stream spanning from the carbon prices in the EU ETS (European Emissions Trading System), for example. A large enough carbon tax might make it interesting for production companies to utilize CO2 in their processes to create an additional revenue stream. Increasing the CO2 consumption rate to 160x might seem like a wide-stretching idea but with a good incentive system, it might be possible to make a more sizable dent in CO2 emissions via carbon utilization.
CCUS is definitely not something new, but rather something that we as society have paid little attention to. CCUS offers a way for us to scale-up relatively mature processes and use them to produce blue hydrogen, which in my view is complimentary, not antagonistic, to green hydrogen. We need both types of hydrogen in our mix.
Dr. Julio C. Garcia-Navarro
Carbon Storage
Carbon storage (or sequestration) is the process where the separated CO2 is injected in an underground geological formation. Not all geological formations are suitable to contain CO2; more often than not, depleted oil and gas wells are used (or at least considered) because these have contained pressurized fluids in the past so by definition these wells are suitable to store CO2.
One important characteristic about CO2 storage is that CO2 is not exactly injected in the gas phase. Rather, CO2 is stored as a supercritical fluid. A supercritical fluid is, in essence, a fluid that is beyond its “critical” temperature and pressure, making it have properties of both a gas (such as low viscosity) and a liquid (such as high density). In the case of CO2, its critical conditions are 304 K (31oC) and 73 bar, meaning that the CO2 storage reservoirs are pressurized. The main challenge of operating pressurized gas wells is to ensure that the gas will not escape either by permeation to the nearby soil or by displacement by another fluid (such as water). Here are a few important properties the ideal CO2 storage reservoirs should have:
- Depth – it is recommended that the wells are at least 1 km underground to ensure that CO2 remains in the supercritical phase (i.e., to keep the pressure)
- High capillary pressure – to prevent another fluid from entering the well
- Low permeability – to prevent CO2 from leaving the well
After some research I found the details of the Sleipnir project in Norway. To my knowledge, Sleipner is one of the (if not the) largest ongoing CCS project. A bit about the Sleipner project: it consists of a family of wells (called Sleipner A, R, T, and B) where natural gas is extracted. Since the gas extracted is rich in CO2, there is a facility that separates the CO2 from the natural gas (at Sleipner T) and injects it back into one of the wells (Sleipner A); more info can be found here. Note that this is not a blue hydrogen production facility but rather a CCS removal facility connected to a natural gas extraction platform. The Sleipner project has been running since 1996 and it has an injection rate of around 1 Mt-CO2/year (source).
Doing a quick back-of-the-envelope calculation, I realized that if the CO2 from an SMR process were being injected, the amount of CO2 being injected into the well corresponds to a blue hydrogen production of 0.18 Mt-H2/year or 1.1 GW of a PEM electrolyzer producing the same amount of zero-carbon hydrogen. I put my calculations in the table below to make them more explicit.
| Injection rate in the Sleipner A well | 1 Mt-CO2/year |
| Equivalent hydrogen production if the CO2 were to come from SMR | 0.18 Mt-H2/year |
| Hydrogen production rate of a PEM electrolyzer | 0.16 Mt-H2/year/GW |
| Equivalent low-carbon hydrogen production | 1.1 GW |
Considering that the EU aims at having 40 GW of installed capacity for low-carbon hydrogen production by 2030, these calculations show that the Sleipner project has stored enough CO2 per year to offset the carbon footprint of 2.5% of the complete 2030 European target since 1996. If we wanted to have 40 GW of blue hydrogen produced by 2030, we would need roughly 40 CO2 injection wells similar to Sleipnir, which raises the question if there are enough underground cavities that are suitable to produce blue hydrogen at a large scale. There appear to be other CCS projects led by Equinor; an overview of the projects can be found here.
Take-home message
My main take home message is that CCUS is definitely not something new, but rather something that we as society have paid little attention to. Is this the holy grail of decarbonization? Probably not, but then again neither we are standing in the midst of a paradigm shift where we have to engage local resources to identify potential strengths. CCUS offers a way for us to scale-up relatively mature processes and use them to produce blue hydrogen, which in my view is complimentary, not antagonistic, to green hydrogen. We need both types of hydrogen in our mix, because the alternative is to continue using gray hydrogen, natural gas, and coal, and neither will bring us anywhere close to decarbonizing our society. Instead of rejecting CCUS, let us bring into the discussion table and investigate what CCUS can do for us.
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About the author
Dr. Julio C. Garcia-Navarro is a Hydrogen Project Coordinator at New Energy Coalition. He has worked in the hydrogen industry for nearly a decade, on topics such as hydrogen electrolysis, compression, and transportation. Besides hydrogen, he is passionate about Renewable Energy Systems and the Internet of Things.
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