Converting CO2 emissions to natural gas

“It may not seem particularly convincing in a time when the government is telling us all we need to come off the gas,” says Annemiek ter Heijne. As a senior researcher at Wageningen University & Research, she is the driving force behind the research on the conversion of CO₂ into methane. “The ultimate goal is to recycle the extensive CO₂ streams we have at present, for example from coal-fired power stations, the steel industry and the food industry, and turn them into methane.”

Ter Heijne, who is an environmental technologist affiliated with the Wageningen Institute for Environment and Climate Research (WIMEK), uses microorganisms in the conversion. More specifically, she uses archaea, whose name means ‘ancient things’. These microorganisms are found in water treatment plants, in soils and in the oceans. Some species can partially convert CO₂ into methane, producing a mixture called biogas that is made up of about 60 per cent methane (CH4) and 40 per cent carbon dioxide (CO₂). This biogas is a sustainable energy source because of its biological origins.

Triggered by energy transition

Indeed, the transition to more sustainable energy sources that is happening worldwide was the trigger for the bio-electrochemical research being conducted by Ter Heijne’s group. “About ten years ago, no one was really interested in the bio-electrochemical conversion of CO₂ into methane. But now there is a huge demand for sustainable energy.” Not just that, the question of how to store energy has also become urgent. “Take the fluctuations in the supply of wind and solar energy.

We have to be able to store that energy because we lose it otherwise. Batteries discharge slowly over time. You could use some of the solar and wind energy for the CO₂-CH₄ conversion. What is more, methane is a completely stable energy source: you can store it and then draw on it when you need it. You could use iet to fill up empty gas fields, for instance. It is also a practical solution because we already have the infrastructure.”

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Even so, it sounds a bit peculiar to deliberately set out to convert CO₂ into methane, a much more potent greenhouse gas. “That is true, but natural gas is also basically methane. The idea is that you use the gas as an energy source. To be sure, you burn the natural gas and thereby release more CO₂, but that is then used again to produce methane. You aren’t sequestering CO₂ for the long term, but you can recycle it rather than releasing new CO₂ into the atmosphere.”

Faster methods

What makes the research carried out by Ter Heijne and her Environmental Technology colleagues so interesting? The CO₂-CH₄ conversion is not a spontaneous reaction. Normally, when implemented by research institutes and companies, it requires two steps. First, electrolysis of water produces oxygen and hydrogen. The hydrogen is then fed into a setup with archaea and CO₂, in which a biochemical reaction takes place that produces methane.

The Wageningen scientists in the Environmental Technology group have developed a new system in which only a single step is needed for converting CO₂ into CH₄. Ter Heijne expects this new method will save about 20 per cent in electricity because it does not require electrolysis. The experimental setup is simple yet elegant. A small tube introduces bubbles of pure CO₂ into a reactor, a sophisticated beaker containing archaea cultures. Two electrodes connected to a power source are suspended in the beaker.

Electrons are released at the negative terminal. The electrons contain the energy the microorganisms need for their own energy systems, for growth and for converting CO₂ into methane gas. The methane is collected in a gas sampling bag, and a gas chromatograph is used to check for impurities. The aim is pure methane gas, but there can be some carbon dioxide left in amongst the methane.

Scaling up

The amount of methane obtained from the reactor in Ter Heijne’s laboratory is still minuscule. “I would like to scale it up to a reactor where the conversion reaction is faster and we produce more methane. At present, we’re looking at millilitres and I’d like to move up to litres. That would let me investigate the relationship between the reaction efficiency and the speed at which methane is formed. If you do experiments on a larger scale, you may also find other, unexpected side reactions that don’t occur at the scale we are currently operating at. Scaling up lets you explore how to tackle or eliminate such undesirable processes.”

Reducing CO₂ emissions is a key measure in efforts to prevent global warming. Reducing those CO₂ emissions and at the same time generating and storing renewable energy sounds like the perfect solution. But there are still some obstacles that need to be tackled first. The material used for the electrodes must be conductive and must not react with other components in the reactor. The surface of the electrode is also important: the larger the surface area, the more microorganisms can ‘feed’ themselves with electrons. “At present, we are experimenting with activated carbon granules – just like the health product ones. The nice thing about this material is that each individual granule has an immense surface area. The idea now is to make electrodes shaped like plates and to spread the activated carbon granules on them like powder to make the surface area as big as possible.”

Ter Heijne says it will be a while before substantial volumes of natural gas can be made from CO₂. “It simply takes a long time to turn a good idea into a large-scale application.” Even so, she is optimistic about the future of the sustainable bio-electrochemical process for the production of natural gas. “I have every reason to be positive if you look at what our group has achieved in the past. The water purification process was developed here, the fermentation process was invented and developed here and now there are fermentation plants all over the world. We face huge environmental problems as a society. I want to make the world a genuinely better place so that there is still a planet left for future generations. I am not saying technology can solve everything, but I do believe new inventions can speed up natural processes.”

This article was previously published in the magazine WIMEK's Ecological Recycling Stories.