Using an electric arc to produce green methanol from biogas residues

Digestate from biogas plants is quite a wet substance. It consists of more than 90 percent water and is usually spread on fields as fertilizer. However, it also contains organic matter that could be used as a raw material for the chemical industry. Researchers at the TU Bergakademie Freiberg aim to extract a base material for methanol production from these residues. An electric arc, similar to that used in welding, is expected to help them achieve this.

The Freiberg team is part of the biogeniV research alliance in Mecklenburg-Western Pomerania. There, partners from academia, industry, and local governments are working to convert previously underutilized waste materials into green methanol and biomethane. Methanol is a simple alcohol that industry needs for plastics and paints, among other things. It can also be used as a fuel. To date, methanol has been produced primarily from natural gas or coal.

Pellets from fermentation residues

An initial subproject investigated how the water-rich fermentation residues can be dried and processed into small compressed pieces, known as pellets. Biochar is mixed in with them to make them easier to convert into gas. This process produces what is known as synthesis gas, which consists mainly of hydrogen and carbon monoxide. Methanol can then be produced from this.

The next subproject began in early September. The researchers aim to improve the conversion of the pellets into gas. To do this, they are using an electric arc generated between two electrodes in the reactor. Similar to welding, it produces very high temperatures.

In conventional gasification processes, part of the feedstock is burned to generate the necessary heat, a process that produces CO₂. If, instead, an electrically powered arc were to supply part of the heat, more carbon could be left over for methanol production. This would be particularly interesting if the electricity came from wind or solar power plants.

Project leader Martin Gräbner, professor of energy process engineering at TU Bergakademie Freiberg, expects this to result in significantly better utilization of the carbon from the fermentation residues. “This will lead to a massive increase in the carbon utilization rate and thus result in a significant reduction in CO₂ emissions,” he says. Whether these expectations will be met remains to be seen in the experiments.

Electricity costs are a key factor

Gasification using plasma – an extremely hot, electrically conductive gas – is also being researched internationally. For example, scientists have already been able to demonstrate that the process could be economically viable. However, this requires a sufficient supply of affordable electricity from renewable sources. The studies focused on other waste materials, not the digestate from biogas plants.

It is precisely these materials that present researchers with additional challenges. The large amount of water must be removed before gasification, which consumes energy. In addition, the digestate contains mineral components that can cause problems during processing. The environmental impact is also influenced by what happens to the nutrients that have traditionally been used as fertilizer on fields.

The Freiberg project aims to determine how such residues can be converted into synthesis gas as efficiently as possible using an electric arc. The results are expected to pave the way for a larger pilot plant. Only then will it be possible to assess more precisely whether the process is also viable on an industrial scale.