CO2: From climate burden to valuable resource?

Environment and sustainability 18. nov 2021 4 min Professor Anders Bentien Written by Sabina Askholm Larsen

The otherwise maligned gas CO2 has great potential if we can become more proficient at capturing and recycling it before it reaches the atmosphere. With the right processing, it can replace several fossil-based raw materials such as oil and methane so that we do not have to extract them underground. This episode of the Forskningsfortællinger podcast (in Danish) takes a closer look at how to imitate a well-known process from the human body to capture CO2 and how it can be converted to a building block for the chemical industry by combining electrochemistry and microbiology.

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CO2 has a massive impact on our climate when released into the atmosphere. Researchers and companies are therefore working to find methods to capture CO2 from factories and incineration plants on a scale that can seriously help to reduce emissions of CO2 and other greenhouse gases.

However, the technology still has a long way to go before reaching the necessary scale that can significantly contribute to reducing CO2 emissions at the rate needed to achieve the climate goals outlined by the United Nations.

“About 40 billion tonnes of CO2 are currently emitted per year. The United Nations Intergovernmental Panel on Climate Change (IPCC) has estimated that we must remove 14% of these emissions by capturing CO2, about 5–6 billion tonnes annually. Right now we are capturing about 40 million tonnes, so we are nowhere near achieving this goal,” says Christina Lunde, Senior Science Manager, Novozymes A/S.

Similar to a well-known process in the body

Christina Lunde is involved in developing a method that uses enzymes for capturing CO2. Enzymes are proteins that help to catalyse processes in our bodies and in other organisms. When we eat, for example, enzymes help to break down the foods we ingest. Novozymes imitates this process in its efforts to develop enzyme-enabled carbon capture.

“We are fortunate that enzymes in nature can capture CO2. They do this in our red blood cells, and they also have a function in our lungs. For example, when we do physical work, breaking down fuel and excreting CO2, the carbonic anhydrase family of enzymes captures the CO2, converts it into carbonate and conveys it to the lungs, where it is again converted into CO2 and then released. We therefore have a family of enzymes with an important biological role that we can now use for capturing CO2,” explains Christina Lunde.

An enzyme-enabled carbon capture plant has an absorber unit and a stripper unit. In the absorber unit, the CO2 is blown through a liquid, and the enzyme converts the CO2 into carbonate, a component of hard water. The CO2 is then transferred with the liquid to the stripper unit, and the enzymatic CO2 is released through heating. The product is a pure CO2 stream, which can then be compressed to produce liquid CO2 and then either be stored or used in another way.

Christina Lunde adds that, because the enzymes are a natural part of our biological world, the method will not leave harmful chemicals in the wastewater discharged from the carbon capture process. This is advantageous when the enzymes are used to extract CO2 from flue gas from incineration plants.

Another benefit of the method is reduced energy consumption. The enzyme-enabled carbon process can release CO2 at a lower temperature than conventional carbon capture solutions, which are based on chemicals such as amines.

“Releasing CO2 requires heating the liquid and releasing the gaseous CO2. Conventional solutions use temperatures of 100–120°C, requiring a lot of energy to heat the liquid. The advantage of the enzyme-enabled carbon capture process is that the liquid only needs to be heated to 80°C. This saves energy, and the excess heat of many processes such as producing cement or steel can be used to release the CO2 to provide a solution that is more sustainable,” adds Christina Lunde.

Microorganisms can convert CO2 from biogas plants into methane

Capturing CO2 to avoid harming the atmosphere is one thing. Another is to take the CO2 collected and process it into a raw material for other products. One such example is the CO2 formed in biogas plants. With the right processing, the CO2 that would otherwise be emitted from these biogas plants can be converted into methane that can be reused in the chemical industry – thereby eliminating the need to extract methane from underground.

Three researchers from Aarhus University and Aalborg University have combined their knowledge of electrochemistry and microbiology in the research project Redox Mediated Microbial CO2 Reduction (ReMeSh) to develop a process using microorganisms that can rapidly convert CO2 from biogas plants to methane.

“Biogas comprises both CO2 and methane, and to extract the methane, the CO2 is currently filtered out and emitted into the atmosphere. The ReMeSh project bioelectrochemically upgrades biogas, converting the CO2 into methane, so that the gas produced by the biogas plant is nearly pure methane,” says Anders Bentien, Professor and Head of Section, Department of Biological and Chemical Engineering, Aarhus University, and one of the three researchers behind the ReMeSh project.

Existing technologies use hydrogen and microorganisms to convert the CO2 from biogas plants into methane. These are the same components the researchers behind the ReMeSh project will use, but they will use alternative technologies to speed up the process.

Current technologies use electrolysis to create hydrogen, which is then pumped into the microorganisms in the biogas plant, but Anders Bentien and his colleagues will make the process more efficient by skipping this first step. Instead, they will develop a hybrid technology in which smaller molecules are used to transport the hydrogen molecules, which can then be transferred directly to the microorganisms used to convert the CO2 into methane. The new hybrid technology is expected to increase the conversion rate by up to a factor of 1000 compared with the existing technologies and thus more efficiently convert CO2 to methane.

A key building block

But why does it even make sense to convert CO2 to methane, which is an even more potent greenhouse gas than CO2? The reason is that methane, when not released into the atmosphere, is a key building block of the chemical industry. Methane is a platform chemical that is the starting-point for several everyday products such as fuel.

The problem with methane is that it is currently extracted from natural gas underground, with major environmental and climate effects as a result. There is therefore great potential in developing an efficient method that can convert CO2 from biogas plants into methane, since this avoids extracting methane from underground – and producing a key building block in the chemical industry.

“Reducing CO2 emissions is a prerequisite for achieving our climate goals, and the easiest way to achieve this is to stop pumping more crude oil and methane from underground. But then the problem is that this requires finding substitutes for this crude oil and methane. Methane from biogas plants is a very important piece of that puzzle,” concludes Anders Bentien.

Podcast guests:

  • Christina Lunde, Senior Science Manager, Novozymes A/S
  • Anders Bentien, Professor and Head of Section, Department of Biological and Chemical Engineering, Aarhus University

The Novo Nordisk Foundation has awarded an Exploratory Interdisciplinary Synergy Programme grant of DKK 4,965,291 to Anders Bentien together with Michael Kofoed, Researcher, Department of Biological and Chemical Engineering, Aarhus University and Jeppe Lund Nielsen, Professor, Department of Chemistry and Bioscience, Aalborg University for the project Redox Mediated Microbial CO2 Reduction (ReMeSh).

The research project will be implemented at the Novo Nordisk Foundation CO2 Research Center, which is being established at Aarhus University. The Novo Nordisk Foundation has awarded a grant totalling DKK 630 million over 7 years for the Center, which is scheduled to be operational from January 2022.

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