Technical University of Denmark researchers develop bioreactor making microorganisms convert CO2 into methane

Hariklia Gavala points to a cigar-shaped steel cylinder that is a few meters high and sits behind a glass window in her lab at DTU Chemical Engineering. The cylinder is a bioreactor housing millions, if not billions, of loyal ‘employees’, that can only be seen by the naked eye when they are lumped together. These microorganisms are the invisible heroes that can convert CO2 and syngas into methane, ethanol, or organic acids, which are building blocks that can be used to produce more sustainable alternatives to anything from fuels to chemicals, plastics and food.

“The process is not much different from brewing beer at a microscopic level, but the potential is enormous for the green transition,” says Hariklia Gavala, associate professor at DTU Chemical Engineering.

Close to full utilization
The inside of the bioreactor is coated with pieces of plastic, which provides a large amount of surface on which the microbes can grow. When you add syngas or CO2 in gaseous form, the microorganisms begin to eat away and convert it into methane through fermentation.

Syngas consists of CO2, hydrogen and carbon monoxide and is produced by gasifying biomass such as wood, straw or organic solid waste such as wastewater or food waste. But syngas cannot be used directly as a fuel in the transport sector, nor can it be used directly in the gas grid as the energy content is too low. So there is great benefit in using the microbes to convert it into methane.

“When we produce methane, close to 100% of the CO2 or syngas is converted into methane, and the production rate is ten times higher than in a conventional biogas plant,” says Hariklia Gavala.

By using different types of microorganisms, you can control what the CO2 will be converted into, and even though Hariklia Gavala sees enormous potential in methane, it can also be used to produce ethanol or organic acids.

Whereas conventional bioreactors require higher pressure to ensure that the gas molecules move to the liquid where the microorganisms reside, DTU Chemical Engineering’s bioreactor is designed to operate under regular atmospheric pressure, making it cheaper and safer to operate.

Electricity, heat and fuels for buses
The great strength of the bioreactor is that it can be used in many different contexts. The methane output can be converted into electricity and heat in a gas turbine and thus replace the use of fossil natural gas. The Danish Energy Agency expects biogas to make up 70% of Danish gas consumption in 2030 compared to just 20% in 2021.

Biogas typically only has a methane content of 45-75% whereas the rest is primarily CO2 but since the methane holds the energy, the biogas must be upgraded by cleansing the CO2 before using it in the gas grid. Most times, the upgrading process simply releases the CO2 to the atmosphere. The bioreactor valorizes almost all the carbon and converts it to pure methane and that also makes the costly upgrading process redundant.

“We have produced methane of a grade that can be used directly in the gas grid,” says Hariklia Gavala.

Both methane and ethanol can be used in biofuel. In Sweden – which is one of the EU countries that invests most heavily in biofuels for the public transport sector – a large proportion of buses run on fuels produced from food waste, wastewater and residual products from the paper and forest industries. But conventional bioreactors only extract a limited part of the energy in the biomass, and DTU Chemical Engineering’s bioreactor is far more efficient.

“We can make better use of all types of biomasses, including biomasses that cannot easily be converted into methane in biogas plants. Also, industries that generate off-gases, such as heat and power plants, cement plants, and the steel industry will be able to implement this technology and turn the gas into something useful,” says Hariklia Gavala.

Produces 40 times as much microalgae
Hariklia Gavala and her colleagues have already tested the bioreactor on a scale 35 times bigger than in the lab and proven that the process can work on an industrial scale, and this has gotten several companies intrigued.

Hariklia Gavala has had preliminary talks with Danish companies and the Greek business Solmeyea has also seen great potential in the bioreactor – they’ve already reached an agreement with DTU to use it commercially. Solmeyea manufactures microalgae, which are single-celled algae that use photosynthesis to consume CO2 and produce a range of useful bio products from plant-based foods to biofuels.

So far, they’ve grown microalgae in a way similar to crops by placing the algae in water in large glass vessels where the sunlight makes them multiply. By utilizing DTU Chemical Engineering’s bioreactor they are now able to grow the microalgae much more efficiently – 40 times more microalgae than in conventional production. Another benefit is that the bioreactor takes up much less space than the large glass vessels.

“This bioreactor is a very robust way of getting the microalgae to eat CO2. The productivity is much better, they occupy less space, and the process is not dependent on the sun shining,” says Diego Grumbach, biotechnology engineer at Solmeyea.

For now, the algae are producing lipids which can be used in plant-based foods as alternatives to meat, fish and eggs, but the long-term plan is for the algae to also produce biofuels and bioplastics. Solmeyea has already started a demo plant where they will use the bioreactor.

“The potential is even larger in biofuels than in food. Many biofuels are produced from crops which competes with the food sector, so we need to find a better way of producing biofuels. That’s what the microalgae offer,” says Diego Grumbach.