Microorganisms in a membrane bioreactor can convert CO2 from flue gas into valuable products

What are we going to do with all the CO₂ belching out of the smokestacks of factories and powerplants around the world?

CO₂ contributes to climate change, and although most people recognise that it is a major problem, solutions are not yet ideal.

Some countries are talking about capturing CO₂ and storing it underground, but researchers have invented a new bioreactor that can convert CO₂ from flue gas into biomethane and acetic acid and think we should consider CO₂ as a resource rather than a problematic waste product.

This can be accomplished by a bioreactor containing microorganisms and a CO₂-permeable membrane. Although the bioreactor will probably not soon operate on an industrial scale, and the researchers behind the invention consider it a future component of power-to-X which includes flue gas in a circular economy.

“Flue gas should be activated as a carbon source in direct production of biomethane and acetic acid, both of which are useful in industry. Enabling microorganisms to transform CO₂ from flue gas via biocatalysis into industrially usable raw materials will convert CO₂ into a resource that can support the production of CO₂-neutral products, instead of, for example, storing CO₂ underground,” explains Michael Vedel Wegener Kofoed, Associate Professor, Department of Biological and Chemical Engineering, Aarhus University, Denmark.

The research has been published in the Chemical Engineering Journal.

Capturing CO2 is expensive

Michael Vedel Wegener Kofoed is just one of many researchers who want to use CO₂ as a resource rather than consider it a problem.

Microorganisms can convert CO₂ into biomethane and acetic acid. The problem, however, is that CO₂ in flue gas is diluted, and considerable energy is thus required to capture and purify the gas to a concentration useful for further processing.

Carbon capture agents capture the CO₂ and then are heated to release it. They are often used, but this requires substantial energy, making traditional carbon capture an energy-intensive and expensive process.

Researchers from Aarhus University have investigated various options for using microorganisms to release and transform the CO₂, thereby reducing the cost of capturing and converting CO₂.

They have previously developed a bio-integrated carbon capture and utilisation solution with carbon capture agents mixed directly with the microorganisms.

However, this method cannot be used for producing acetic acid; purification will be too expensive because it requires separating the carbon capture agents and acetic acid.

“Extracting the various components in such a mixture, such as the acetic acid, is expensive and difficult, and then using flue gas as a resource is not viable,” says Michael Vedel Wegener Kofoed.

Bacteria pull CO2 over a membrane

To avoid mixing all the components in one large reactor, Michael Vedel Wegener Kofoed and colleagues invented a new bioreactor that separates the components by using membranes.

CO₂ is captured as in existing capture technology – capturing CO₂ in the smoke in a liquid with carbon capture agents – but unlike the traditional method, CO₂ is extracted from the liquid using microorganisms in a three-phase membrane system.

The new process thus separates carbon capture agents and the microorganisms by a CO₂-permeable membrane to avoid contaminating the microorganisms and the end-product with carbon capture agents.

On the other side of the membrane, microorganisms are present in a liquid with nutrients and constantly pull CO₂ out of the capture agent liquid through their normal metabolic processes, using CO₂ in combination with hydrogen as an energy source.

The microorganisms thus utilise the CO₂, converting it into biomethane or acetic acid.

The microbial CO₂ conversion creates a concentration gradient of CO₂ across the membrane. To equalise the concentration gradient, CO₂ is pulled across the membrane, hereby facilitating a constant supply of CO2 from the liquid with capture agents to the microorganisms .

The membrane allows CO2 to pass but prevents the carbon capture agents in the flue gas liquid from passing through it.

“Our proof of concept shows that this process works,” explains Michael Vedel Wegener Kofoed.

Should be integrated with power-to-X

According to Michael Vedel Wegener Kofoed, some time will pass before the membrane-based bio-integrated carbon capture and utilisation reactor will be available for industrial use.

Subsequent development of the process will include optimising reactor design, analysing the optimal flow of liquids in the system, temperature, choice of membrane and other variables.

The idea, however, is that the bioreactor will be included in power-to-X as a component for converting the climate-damaging products of flue gas into raw materials that can be reused.

“Scrubbing CO₂ from flue gas today is an energy-intensive and expensive process. By doing it this way instead, we let microorganisms release and transform CO₂ captured from flue gas. Ideally, this could make using CO₂ as a resource much less expensive,” concludes Michael Vedel Wegener Kofoed.