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Researchers convert CO2 into sustainable fuels

The technology will soon be tested at one of Denmark's largest biogas plants, which looks into a future where biogas is not only created from manure and potato peels, but also CO2.

By Birgitte Dalgaard, , 12/2/2020

Biogas plants today emit large amounts of CO2. Approximately 40 percent of the gas created in biogas plants is CO2, which is emitted directly from the chimney. But now researchers from the University of Southern Denmark have successfully converted CO2 into sustainable energy in the form of biomethane.

– We are very satisfied with the quality of our biomethane. We are now up to having 97 per cent methane, says researcher Muhammad Tahir Ashraf from the Department of Green Technology, who for the past two years has refined the technology that can convert CO2 to sustainable energy.

Methane makes up the remaining 60 percent of the biogas generated at biogas plants. Methane has a high calorific value, and it is therefore the gas that smokes further into the natural gas network in Denmark and provides sustainable heat and electricity.

Why is biomethane CO2 neutral?

  • Biomethane is known as renewable natural gas and is produced by upgrading biogas from the biogas plants
  • Methane is a potent greenhouse gas, but biomethane is CO2 -neutral because plants absorb CO2 as they grow
  • When the plants are eaten by a pig, part of the CO2 enters the manure and enters the biogas plant, where it is converted into biomethane and CO2

When scientists can convert CO2 to biomethane, the biogas plants are looking into a whole new era. By capturing the large amount of CO 2, they can boost the production of biomethane and avoid an environmental problem.

– It is really positive that the University of Southern Denmark is at the forefront of development and contributes to the fact that we are now well on our way to a green transition, where not least trucks, ships and aircraft can be supplied with green energy, says Frank Rosager, director of the industry organization Biogas Denmark.

Way to store wind energy

At the University of Southern Denmark, two mini reactors are converting CO 2 to methane by adding hydrogen. In this way, the greenhouse gas CO2 is conjured into sustainable energy, which in the future can also be used for sustainable fuels for trucks, ships or aircraft.

– Here in the laboratory, we add hydrogen directly, but in the future biogas plant, we will utilise excess electricity from renewable energy sources, such as wind and solar energy, for the production of hydrogen, says Muhammad Tahir Ashraf.

He points out that by utilising excess wind and solar energy to produce methane, another major energy problem is being solved.

Energy from wind cannot be stored today, but when there is excess wind energy, the future biogas plant can use it to convert CO2 to biomethane, and biomethane is a gas that is easy to store

Muhammad Tahir Ashraf, Postdoc

– In this way, the system acts as a gas battery. Energy from wind cannot be stored today, but when there is excess wind energy, the future biogas plant can use it to convert CO2 to biomethane, and biomethane is a gas that is easy to store and utilise in the existing natural gas network, says Muhammad Tahir Ashraf.


Electricity from wind and solar can become climate-friendly fuel for planes, ships, trucks. The technology is called Power-to-X and means in its simplest form that you take power, an energy form such as electricity, and convert it to another energy form.

For the Danish biogas plants, the project provides the opportunity for the biogas plants to become central in a well-functioning green energy system, where solar-, wind- and biogas production play together optimally.

– Since Denmark gets a large surplus of electricity from solar and wind, everyone points to Power-to-X as the future solution for green conversion of especially the heavy transport. With the project here, we have got Power-to-Methane, which brings us a big step forward towards the goal, states director Frank Rosager.

Ready for a large biogas plant

Muhammad Tahir Ashraf is looking at the two reactors, each containing approximately one litre in size.

– We have been running trials non-stop for more than 1000 days, and we are now so satisfied with the quality of methane, that at the end of the year we are ready to test the technology in two 1000 litre reactors at Nature Energy Holsted, says Muhammad Tahir Ashraf:

– We have run many tests to find the right procedures for operation and the design of the reactors. There must be an optimal run of the biogas production and at the same time as we convert CO2 to methane.

Money in biogas

In the report, The Energy Crop Analysis that the Department of Green Technology, SDU and Seges, part of The Danish Agriculture & Food Council, have prepared on behalf of the Danish Energy Agency, the optimal biogas potential in Denmark is calculated to be around 90 PJ/year.

The report, which was published earlier this year, concludes that this figure will increase to 160 PJ/year if the potential of converting CO 2 to methane is utilised. This corresponds to about 10 times more than the estimated biogas production in the period 2018-2019.

– When the methane is burned and becomes, for example, heat, you are left with CO2 once again. One can imagine that in the long run, we can also reuse the CO2 in the system - or collect it from other sources. In that case, there is no limit to the production of methane, says Muhammad Tahir Ashraf.

We must not only create climate sustainable solutions but also economically sustainable solutions, otherwise, the solution will not be attractive to private biogas plants.

Muhammad Tahir Ashraf, Postdoc

Thus, the two reactors in the laboratory at the University of Southern Denmark are well on their way to paving the way for biogas plants 2.0, where the future biogas plants will have a completely different foundation of business when manure and food waste are supplemented with CO2 .

– We must not only create climate sustainable solutions but also economically sustainable solutions, otherwise, the solution will not be attractive to private biogas plants.

– For example, our goal has been for our finished solution on a reactor size to be only 10 per cent of the total size of the biogas plant. We have achieved that goal, and the biogas owners are very satisfied with that, emphasises Muhammad Tahir Ashraf.


Photo: Klaus Elkær, University of Southern Denmark.

The science is a part of the project eFuel

  • Participants: NGF Nature Energy Biogas A/S, University of Southern Denmark, Technical University of Denmark (DTU), Biogasclean A/S, Biogasclean Asis Co., Ltd., ERHVERVSHUS FYN P/S
  • Appropriation: DKK 16.59 million from the Energy Technology Development and Demonstration Program (EUDP)
  • More about eFuel
Meet the researcher

Postdoc Muhammad Tahir Ashraf from SDU Biotechnology, Department of Green Technology is Ph.D. from Khalifa University in Abu Dhabi. He researches, among other things, in biofuels and reactor design.


The team

The research has been carried out in collaboration with Executive Officer Lars Yde, Associate Professor Jin Mi Tiolo, PhD Student Brian Dahl Jønson, Professor Henrik Wenzel, and Professor Jens Ejbye Schmidt.

Editing was completed: 02.12.2020