In the previous Direct Air Capture Insight I’ve highlighted the crucial, most tricky step for carbon capture systems – the actual CO2 separation from the captured gas. Let’s explore how CO2 can be separated in post-combustion systems.

In order to start this research I opened up Google Scholar Search, typed in “CO2 separation” and received…. 2,160,000 results. 

20,000 publications since beginning 2021 to-date alone!

In light of the sheer expansiveness of the subject matter we’re only skimming the surface in this Insight – nevertheless this introduction to post-combustion CO2 separation methods should serve as a solid foundation to build off of.

Let’s begin;

Separation of CO2 can be done by either chemical or physical means, and the 3 basic methods which are known to be effective are:

  1. Chemical – using solvents / sorbents
  2. Physical – membrane separation
  3. Physical – cryogenic separation

The different CO2 Separation Methods

Chemical – using Solvents

Chemical separation of CO2 with solvents has a well established track record of being effective since its inception and adoption some 60+ years ago by the oil and chemical industries in order to remove Hydrogen Sulphide and CO2 from produced gas.

Amine gas treatment processes – also known as amine scrubbing – in the form of using aqueous amine solutions are the most common. 

We can think of amines simply as various derivatives of ammonia (NH3), where one or more of the hydrogen atoms has been replaced by a substituent.

Amines are well-known for their reversible reactions with CO2 hence theoretically making them ideal for use in CO2 separation processes. There are multiple classifications of amine compounds, each with different characteristics when it comes to CO2 capture.

To-date, the most widely used amine for CO2 capture is mono-ethanolamine [MEA] which reacts quickly and strongly with CO2 even at low CO2 concentrations. It has been shown to achieve CO2 recovery rates of 98% with CO2 purity >99%.

Challenges with using MEA however do exist – specifically around its rate of degradation in an oxidising environment (flue gas) as well as the amount of additional energy required for solvent regeneration for reuse in the scrubbing process.

A number of research projects are ongoing to synthesize amines which are more optimal for use in CO2 separation processes with a focus on reducing regeneration energy consumption as well as reducing amine degradation rates.

Chemical – using Sorbents

Whilst amine scrubbing relies on amine solvents capturing CO2 by dissolving to form a solution, sorbent materials – materials which are capable of absorption / adsorption – can also be used to capture CO2. 

Note for completeness – absorption and adsorption can also be considered physical phenomena but for the sake of keeping things simple we’ll stick with classifying it as a chemical process.

Quick flashback to GCSE chemistry;

  • Absorption is a condition in which something takes in another substance – atoms/molecules undergoing absorption are taken up by the available volume. Picture a sponge absorbing water.
  • Adsorption is when atoms/molecules adhere to a surface, forming a film. Think of sticky tape.

The sorbents most commonly used for CO2 adsorption are:

  1. Zeolites – commonly used as drying agents, in detergents and in water / air purifiers – are crystalline, microporous solids made of silicon, aluminum and oxygen
  2. Activated carbon – commonly used in filtering and purifying water and air – is a form of carbon processed to have small, low-volume pores for the purpose of adsorption
  3. Metal-Organic Frameworks [MOFs] – an emerging and promising nanoparticle technology. MOFs are made-up of metal-containing nodes linked by organic ligand bridges and their porous structure is very similar to zeolites; however their key advantage over both zeolites and activated carbon is the ability to optimize their pore structure and surface function, making them well suited for precise applications. MOFs can be easily designed, synthesized and tuned for selectivity and permeability of CO2, however at a cost.

And the two most common adsorption techniques:

Pressure Swing Adsorption [PSA] – where air flows through a packed bed of adsorbent at elevated pressure until the concentration of the desired gas (CO2) has peaked. The packed bed of adsorbent is then regenerated simply by reducing the pressure.

Temperature Swing Adsorption [TSA] – where air is circulated through a packed bed of adsorbent which adsorbs the intended gas (CO2) and is subsequently regenerated by elevating its temperature.

Both PSA and TSA with zeolites and activated carbon are commercially practiced methods though they are challenging to scale because of the limiting capacity and uncertainty around sorbent CO2 selectivity. Metal-Organic Frameworks have been demonstrated to address this key shortcoming and R&D efforts are ongoing to lower MOFs costs.

Physical – Membrane Separation

Membrane separation can be thought of as using a filter to separate gas mixtures. The membranes act as a permeable barrier through which different compounds move across at different rates or not at all, depending on the molecule size, diffusivity or solubility.

Their main limitation is their poor selectivity and permeability to achieve a high degree of gas separation. Multiple stages and/or recirculating of the gas mixture stream may be necessary to achieve the desired end effect. This in turn results in increased system complexity as well as energy consumption – needed to “push” the gas mixture through the membrane system – which results in higher costs. 

There are developments ongoing to increase membrane separation efficiency; 

  • Solvent-assisted membranes are being developed to combine the best features of membrane systems with solvent scrubbing. 
  • Incorporating certain porous sorbents such as MOFs, zeolites and activated carbons into the membrane systems as fillers to enhance the selectivity and permeability of the membrane.

Physical – Cryogenics

In general, cryogenics deal with the study of very low temperatures. 

Cryogenic CO2 separation is already widely used commercially for gas streams with high CO2 concentration (>90%) but it is not known to be widely used for more dilute CO2 streams such as in post-combustion flue gas where volumetric CO2 concentration can be between 10-25%.

Its key advantage is that it produces a liquid CO2 ready for transportation via pipeline / ship.

The major disadvantage is the amount of energy required to provide the refrigeration as well as the necessity to clean the captured gas mixture of components that have freezing points above the system’s normal operating temperature e.g. moisture, which could freeze and block the process equipment.

The required pre-processing tends to make this method less economical than others for post-combustion applications, however there is a potential for cryogenic CO2 separation to be applied to pre-combustion or oxy-fuel capture processes. 

The above seems to be the general consensus I’ve found in multiple research studies.

Interestingly enough and going against the general consensus there is a company in the US – Sustainable Energy Solutions [SES] (recently acquired by Chart Industries, a $5.5bn market cap giant) – which has patented a post-combustion cryogenic carbon capture process, claiming the energy required to power the process is a fraction of that required for a solvent amine-based system.

Concluding Remarks

This was just a surface level insight into the various CO2 separation methods and I hope that the takeaway from here is that this is a very complex subject matter where unfortunately a one-size-fits-all-applications / winner-takes-all solution does not exist.

When conducting my research for this write-up one thing became very clear to me – the sheer amount of R&D and work that’s already been done on CO2 gas separation, as well as the vast amount of R&D and innovation ongoing around the world to improve existing processes.

Based off looking at the ongoing momentum to advance CO2 separation technologies I think carbon capture will play a larger role in our future energy mix than the IEA’s Net Zero by 2050 vision cares to admit.

References

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