Introduction

Through the Paris agreement of 2015, the global community agreed to make efforts to limit global warming to 2°C. Our society must reduce emissions of CO2 drastically.

 

One important way in which the industry plans to avoid releasing CO2, is to capture the CO2 and store it underground, also known as carbon capture and storage (CCS). A common way is to inject it into depleted oil and gas reservoirs. In fact, carbon dioxide was already used in the past for enhanced oil recovery, by increasing the pressure of an oil and gas field. It is expected that by 2050, CCS will be storing 10-30 billion tons of CO2 a year [1]. Carbon capture can be applied in a wide variety of industrial facilities such as cement, iron and steel, chemicals, natural gas and power generation.

 

In this post, we will discuss 6 methods of how carbon can be captured.

 

1. Absorption using solvents

The basic principle is that CO2 is absorbed in a mixture of water and a chemical solvent such as an amine. The gas stream from which the CO2 must be removed is fed into the bottom of an absorption column which is filled with either packing or distillation tower trays. The solvent solution is sprayed down in the column from the top and the packing or trays ensure a good contact between gas and liquid in the column. When the gas stream leaves the top of the column, most of the CO2 has been removed.

 

The liquid leaving the bottom of the column now contains all the absorbed CO2. This liquid stream is send to another column: the stripping column. The bottom of that column contains a reboiler in which heat is added to the process which causes the CO2 to be released from the liquid. A condenser is connected to the top of the stripper column to condense mainly water which is send back as a reflux stream to the column. What remains is pure CO2 which was captured and can now be compressed for underground storage. Often multiple plants send their CO2 to a storage site which is close to an oil and gas reservoir.

 

From the bottom of the column leaves the regenerated solvent, which is sent back to the top of the absorption column.

 

 

 

 

 

 

 

 

 

 

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Figure 1: process flow chart absorption process

 

Amine systems for carbon capture in power stations tend to become large industrial facilities. Consequently, also the regenerator reboiler will be very big. The Wave Exchanger provided by Kelvion Thermal Solutions is a perfect thermosyphon reboiler for amine systems. Large heat transfer area can be made in one unit, leading to lower equipment count. Also the weight and plotspace will be a lot smaller.

 

Well established method

 

The absorption of CO2 is a well established method. It was applied for many decades in the oil & gas industry for large-scale industrial plants. Natural gas coming from the well can contain CO2, but also other impurities such as hydrogen sulfide (H2S) and mercaptans. These impurities must be removed to get the gas at the right specification for the natural gas grid. Because of the acidity of H2S and CO2 these absorption systems are normally called acid gas removal unit (AGRU) and the method called gas sweetening. The carbon dioxide was in the past either released to atmosphere, or it was used for enhanced oil recovery (EOR).

 

Today there exist many licensors for CO2 absorption processes. These companies have different variations of solvents and variations in process line-up, all tuned for the specifics of a project in terms of exact gas composition, CO2 concentration, operating pressure etc.

 

A big challenge in CO2 capture from power plants is the energy which is required for the capture process. Amine systems have an energy penalty which can easily be 10% for coal power plants [2]. A lot of research is therefore aimed at reducing the energy consumption of amine systems. In their sales literature licensors express the energy consumption in GJ per ton of captured CO2 [3]. 

 

​2. Membranes

Monsanto was the first company to use membranes for gas separation on a commercial scale in 1979 for the separation of H2 from CH4, N2, and Ar [4]. It is also applied in the capturing CO2. Membrane materials are selected and engineered such that they have a selectivity for CO2. When gas is passed through, the ‘membrane let CO2 move through at a higher rate than the other gases’ [5].

 

An advantage of membrane technology is that it can be added at the end of processes such as power stations without major impacts on the overall process. 

 

3. Adsorption using a sorbent material

In adsorption the CO2 is captured on the surface of a solid material. CO2 is adsorbed after which it needs to be released and the sorbent material can be reused. Releasing the CO2 can be done either by increasing the temperature, which is called temperature-swing adsoptrion (TSA) or the CO2 can be release by reducing the pressure, known as pressure-swing adsorption (PSA).

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4. Cryogenic systems

In cryogenic systems, flue gas from a process is cooled down to cryogenic temperatures and the CO2 solidifies to form dry ice. The dry ice can then be filtered from the gas stream, melted, pressurized and be sent to storage. Various technology providers have developed a process following this path. The advantage of cryogenic systems is that no chemicals such as amines and water need to be used.

A clear disadvantage is the cryogenic temperatures that will require a cold source such as a chiller system.

 

Kelvion Thermal Solutions developed a cryogenic CO2 separation process as well. That system is based on their desublimation technology. KTS has been supplying desublimation equipment since the 1950s for various petrochemical processes. Desublimation is the phase change from vapor directly into solid. We can observe this process in everyday life, when water crystals grow on branches of trees in the winter. In the last decade, KTS invested a lot of R&D to apply desublimation of CO2 capture.

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Figure 2: Desublimation of water crystals on tree branches in the winter 

Desublimation is a batch process and multiple units are used to make it semi-continuous, The gas from which CO2 must be removed is sent through the desublimator, which is cooled on the other side with a cooling medium. The inside of the desublimator has a structure which is tuned for the specific crystal rheology of CO2. Crystals are captured in the desublimator, while the gas moves on and leaves the unit without CO2 (less than 0.5% has been achieved).

 

At a certain moment, the sublimator is completely loaded with CO2 crystals and cannot capture more, so the CO2-containing gas stream is now switched to a second desublimator.

 

The first desublimator now goes into generation mode. Instead of a cooling medium, a heating medium is connected and the CO2 starts to melt of. Since this is done at elevated pressure, the CO2 will come out of the unit in liquid form and can easily processed and inject it in for instance old gas fields.

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Figure 3: Desublimator installed in commercial scale CO2 capture installation for biogas

An explanation of the desublimation process is given in the video below.

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5. Direct air capture (DAC)

As the term suggest, CO2 is directly captured and stored from the ambient air. So in stead of avoiding chemical process from emitting CO2 and increasing the CO2 levels in our planet’s atmosphere, these technologies are aimed at directly reducing the CO2 concentration of our ambient air. Hence, these technologies are classified as carbon negative.

 

The business model of these technologies is in the trading of CO2 credits. Companies that have today difficulties in reducing their CO2 emissions, can buy these carbon credits. This is also known in industry as offsetting.

 

For direct air capture (DAC) two main groups of technologies exist. There are liquid based systems and solid capture technologies.

 

In a liquid based system, the air is brought into contact with a liquid that contains a chemical solution and removes the CO2 from the air. In another step of the process, the liquid must be regenerated and the CO2 released for further processing and can then be stored underground for instance in geological formations. Big amounts of air are being processed, and these will be large-scale installations.

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Figure 4: Rendered picture liquid DAC system (Source: interestingengineering.com) [6]

 

6. Oxy combustion

Oxyfuel combustion processes use pure oxygen to burn a fossil fuel. The results is a flue gas stream consisting only of CO2 and water. After removal of the water, the pure CO2 can be transported and stored.

Solid direct air capture leads the air flow over solid adsorbents that bind the CO2. When the solid sorbent material is fully loaded with CO2, the air flow is stopped. In the next step, the solid sorbent material is heated to release all the CO2 which is then further processes for storage. The regenerated solid sorbent is now used again for capturing CO2.

Oxyfuel combustion processes use pure oxygen to burn a fossil fuel. The results is a flue gas stream consisting only of CO2 and water. After removal of the water, the pure CO2 can be transported and stored.

 

Cement industry

 

The cement industry is a carbon intensive industry that is today looking into the use of oxyfuel combustion. According to the Global Cement and Concrete Association: “Oxyfuel combustion creates a flue gas of highly-concentrated CO2 that is relatively easy to capture and process for geological storage or onward use.” [7] By means of oxyfuel combustion 90-99% capture rates can be achieved in the cement industry.

 

Power industry

 

A very special power production cycle that uses oxyfuel combustion was developed by the company NET Power from Durham, North Carolina. Their technology is based on the Allam-Fetvedt Cycle. An air separation unit separate pure oxygen out of the air. Natural gas is burned using this pure oxygen to form a stream of supercritical CO2 and water vapor. The supercritical CO2 moves a special turboexpander to generate electricity. Another part of the process removes the water from the gas stream and pure CO2 remains, which can than be transported and stored underground (for instance in geological formations). With their process, NET Power can capture over 97% of CO2 [8].

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Figure 5: NET Power power cycle schematic (Source: Power Magazine)

 

Closing

In this post, we gave a brief overview of some different carbon capture technologies. We know it is far from complete. As CO2 capture has a lot of attention, many new technologies are still under development. Also, there is a lot more to say about the technologies we highlighted in this article. There are also different ways to classify carbon capture technologies such as pre- and post-combustion methodologies. One thing is clear though: the industry will use large-scale carbon capture in the near future.

 

We hope that this list gives a nice insight in some of the technologies that are out there and which will be intensively used in the coming decades.

 

Heat transfer 

Like in most industrial plants, also in carbon capture technologies, heat transfer plays an important role. Amine systems require reboilers, condensers and interchangers. Flue gas from power stations may require cooling before they can be treated in any carbon capture installation. 

 

If you are working on any carbon capture project and you have questions related to heat transfer, please reach out to our team!

 

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References

[1] https://www.sccs.org.uk/why-ccs/what-is-ccs

[2] NETL, Carbon dioxide capture from existing coal-fired power plants. 2007.

[3] Global CCS Institute (2022). State of the art: CCS Technologies 2022. Technical Report.

[4] W.S.W. Ho, K.K. Sirkar, Membrane Handbook, reprint ed., Kluwer Academic Publishers, Boston, 2001.

[5] Energy Industries Council (December 2022). Carbon Capture, Utilisation, and Storage (CCUS). EIC Insight Report.

[6] https://interestingengineering.com/innovation/worlds-largest-carbon-capture-facility 

[7] https://gccassociation.org/cement-and-concrete-innovation/carbon-capture-and-utilisation/oxyfuel/