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From biomass to diesel

Using the power of microbes: biochemists from Leipzig and Tübingen use the combined power of microbes and electrolysis to produce fuels from organic material. This new process uses electricity from renewable resources to produce diesel from organic waste and green cuttings, amongst other things, and can therefore also be used for storing wind and solar energy.

Biochemist Dr. Falk Harnisch combines the power of microbes with electrotechnical processes. © UFZ / Tobias Hametner

Microorganisms are destructive. They break up, mince and chop complex organic matter into smaller compounds and eventually turn them into inorganic substances. This process happens every day in sewage treatment plants, biogas plants and across nature. However, bacteria are also capable of assembling things. This includes, for example, the ability to produce longer-chain fatty acids from shorter building blocks. Biochemists refer to this process as “microbial chain elongation”.

Researchers at the Helmholtz Centre for Environmental Research - UFZ in Leipzig led by Dr. Falk Harnisch worked with Prof. Dr. Lars Angenent from the University of Tübingen and researchers from Cornell University (USA) to develop a clever method of producing fuel by exploiting bacteria’s capacity to construct things. If you take the kind of fermentation tank (bioreactor) used in a biogas production plant and add filtering and electrolysis processes, you have a ready-made system for producing biobased fuels from the most diverse organic materials. Microbes in the bioreactor turn biomass into acids which are enriched by filtration (liquid-to-liquid extraction). Subsequently, a process called Kolbe electrolysis is used to synthesise long-chain alkanes, i.e. diesel-like fuels, using an electric current.

The bioelectrical pathway from biomass to diesel: process sequence of a method developed and patented by a group of researchers led by Falk Harnisch at the UFZ in Leipzig. Fuels produced from biowaste, green cuttings and other organic waste can be used to store wind and solar energy. © UFZ / Carolin Urban and Falk Harnisch

Combining microbiology and electrochemistry has several advantages: unlike the conventional production of fuels from petroleum or biomass, the bioelectrochemical synthesis processes proceed under life-sustaining conditions, i.e. at room temperature, ambient pressure and neutral pH. This minimises energy consumption, cost and occupational safety risks. For practical reasons, the chemists used "corn beer" (by-product of bioethanol production) and corn silage as starting materials for their laboratory experiments. However, Falk Harnisch is convinced that a large number of other substances are also suitable for the process: "We are one hundred percent in accordance with other studies that address the production of acids from organic waste, grass or green waste." Lars Angenent, who has been Humboldt Professor at the University of Tübingen since April 2017, adds: “We used corn beer for our experiments, which is a relatively high-quality raw material. The process has huge potential, both with regard to the variety of starting materials and resulting products as well as with regard to combining microbiology and electrochemistry.”

The ability to use organic waste, which means that the process is not in competition with food production, is another important aspect of the innovative process. In addition, the different speeds of microbial and electrochemical processes can be combined ingeniously: while the work of the microbes in the fermentation tank is a continuous, slow process, electrolysis is relatively fast and flexible. The power required can therefore be accessed from regenerative power generators such as wind turbines or solar modules. Falk Harnisch says with a conspiratorial wink: “Actually, the best thing would be to set up a wind turbine next to a biogas plant. Then we would be able to produce fuel instead of methane.”

In fact, the process could potentially be used to modify existing biogas plants for fuel production. This would only require a small effort. Alternatively, the method can be used for storing “excess” renewable energy in the form of fuels. Carolin Urban, Harnisch’s doctoral student and first author of the study published in the renowned journal Energy & Environmental Science, ran the process for a few weeks in the laboratory. She was able to “harvest” 50 ml of fuel and test it thoroughly. The test results demonstrate that the resulting diesel fuel is of high quality. Although the 50 millilitres are not sufficient to test engines, Harnisch nevertheless claims “that people can pour what we’ve produced into their tank and drive.” The new bioelectrically produced fuel requires neither new engines nor new fuel pumps.

The future of the fuels

High tension in the corn field: the future of regenerative fuels also depends on the political situation. © UFZ / André Künzelmann

But does it make sense to continue investing in technologies whose final product is diesel-like fuels at a time when diesel fumes are polluting our cities and everyone is talking about doing away with combustion engines? Falk Harnisch sees it like this: "We should definitely be thinking about new mobility concepts, especially in cities. But I would be very, very skeptical about achieving this for aviation or even road haulage. I do not think that this could be achieved with battery technology - simply because of the high energy density required in these instances.” Leaving coal, oil and gas in the ground and starting to produce fuels and other products from organic waste does not contribute to less air pollution per se, but it does contribute to achieving climate goals and the transformation from the petroeconomy to the bioeconomy.

Combining microbiology and electrochemistry could potentially lead to the replacement of petrochemicals with regenerative, organic raw materials. But this shift will not happen on its own as biofuel production is currently much more expensive than conventional petrochemical production. “The question is, ‘what are we willing to pay for renewable fuel?’,” says Falk Harnisch, “I would think that waiving part of the energy tax for renewable fuels or raising a CO2 tax for non-renewable resources in order to cut emissions as Australia once did, would promote alternative fuel concepts.”

Harnisch and his team have shown that the shift to renewable fuels is technically feasible. Now the ball is in government’s and industry’s court to give the process a future. Falk Harnisch puts it this way: "The best thing that could possibly happen is an entrepreneur or an engineer knocking at my door and saying ‘we’ll help you build a pilot plant’. However, I’m well aware that they would also need a meaningful market for this type of biofuel. And even if we were able to use the existing infrastructure, which is something that is not possible with other biofuels, our biofuel in the next 20 to 30 years will never be as cheap as petrochemical fuels. It will not be competitive in the marketplace if petrol continues to be as cheap as it is now and if the political conditions do not change.”

Publication:

Urban, C., Jiajie, X., Sträuber, H., dos Santos Dantas, T. R., Mühlenberg, J. Härtig, C., Angenent, L. T., Harnisch, F. (2017): Production of drop-in fuel from biomass at high selectivity by by combined microbial and electrochemical conversion. Energy & Environmental Science 2017,10, 2231-2244, doi: 10.1039/C7EE01303E http://dx.doi.org/10.1039/C7EE01303E http://dx.doi.org/10.1039/C7EE01303E

Credits:

The project was funded by the German Federal Ministry of Education and Research (BMBF) (BMBF initiative "Next generation biological processes - Biotechnology 2020+"), the Helmholtz Association (Young Investigators Group & Research Programme Renewable Energy) and NSF SusChemProgram. 

Website address: https://www.biooekonomie-bw.de/en/articles/news/from-biomass-to-diesel