Increasing the extraction of ethanol from syngas

Environment and sustainability 18. apr 2023 2 min Professor Lars Angenent Written by Kristian Sjøgren

Much larger amounts of ethanol can be produced from the syngas (synthesis gas) produced by the steel industry or by incineration of waste by using acetate, pure carbon monoxide and specific bacteria.

Industry is very interested in methods that can maximise the yield from the various by-products it produces.

This applies to syngas, which is a mixture of hydrogen and carbon monoxide with some carbon dioxide and methane. The steel industry produces large quantities of syngas, as does incineration of waste. Because syngas is toxic, industry is not permitted to discharge it into the environment.

Syngas comprises many different gases and therefore there is great potential for producing ethanol, and some companies are already making money from producing the product for use as a fuel. Recycling syngas is thus part of a circular economy.

A new study shows how using bacteria, acetate and pure carbon monoxide can boost the production of ethanol from syngas.

The study has been published in Bioresource Technology.

“Companies striving to recycle syngas are constantly seeking new methods to optimise their industrial processes, and we suggest a method for increasing the amount of ethanol that can be extracted from syngas,” explains a researcher behind the study, Lars Angenent, Professor, Department of Geosciences, Environmental Biotechnology Group, University of Tübingen, Germany and the Novo Nordisk Foundation CO2 Research Center and the Department of Biological and Chemical Engineering at Aarhus University in Denmark.

Pure carbon monoxide increases ethanol production

Lars Angenent is optimising the process of producing products from syngas that can be useful in industry. The process includes Clostridium ljungdahlii, a bacterium that produces acetate and ethanol from the components of syngas through an enzymatic process.

The researchers studied whether they can boost production of ethanol by using the bacteria and investigated what happens when they are exposed to pure carbon monoxide as a feedstock. The fermentation tanks that both researchers and industry use for the process normally contain only 60% carbon monoxide.

The experiment showed that using pure carbon monoxide drastically changed the production ratio between acetate and ethanol when enough external acetate is present. Clostridium ljungdahlii started making much more ethanol and much less acetate.

“This confirms that pure carbon monoxide boosts the ethanol production rate, but only when external acetate is present. This is promising because producing more ethanol from syngas creates an alternative source of fuel that limits the use of fossil fuel,” says Lars Angenent.

Anaerobic process

However, converting syngas to ethanol is not that straightforward, but the researchers have shown how it can be done in this research paper by optimising the second-stage of a two-fermentation tank system.

Previously, the researchers put Clostridium ljungdahlii in two fermentation tanks, with the bacterium producing acetate in the first tank and ethanol in the second tank.

The two fermentation tanks work in parallel, with acetate from the first tank flowing into the second tank. In the new research paper, the researchers included the external acetate (in a real system from the first tank) and pure carbon monoxide to boost the ethanol production rate.

“The process is 100% anaerobic, so if any oxygen enters the fermentation tanks, syngas will not be converted to ethanol,” explains Lars Angenent.

Production increased by 240%

The results show an increase of 240% in the yield from the bacterial production of ethanol. According to Lars Angenent, this is a huge increase and will be of considerable interest to industry.

However, Lars Angenent and his colleagues have not finished developing the process to make it more efficient. They are currently closely studying the bacteria they use in fermentation to determine how to optimise them so that they do more of what is useful for industry.

Specifically, they are examining whether they can increase the production of ethanol even further by manipulating the enzyme the bacteria use to convert syngas into ethanol.

“Industry is also intensively researching these processes and how to optimise them, and perhaps they already know much of what we show in this study. We are also therefore modest about our results, but we still think that many companies will be interested in using the methods we have shown for increasing the production of ethanol from syngas,” concludes Lars Angenent.

Acetate augmentation boosts the ethanol production rate and specificity by Clostridium ljungdahlii during gas fermentation with pure carbon monoxide” has been published in Bioresource Technology. The project was partly financed by the Alexander von Humboldt Foundation and the Novo Nordisk Foundation CO2 Research Center.

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