CO2: From climate burden to valuable resource?

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