In order to save fossil fuels, fuels can also be produced from biological sources. Researchers have now developed a new, particularly efficient method of obtaining ethanol fuel from forestry waste. In addition to wood residues, hydrogen from electrolysis is also used for the synthesis. As a result, the process has a significantly higher yield than conventional ethanol production processes.
So far, fossil fuels have been almost indispensable in many areas of transport. But in the course of climate protection – and also in order to reduce dependence on oil and gas imports – traffic has to switch to alternative drives. Synthetic fuels made from renewable raw materials such as biomass are regarded as an interim solution on the way to pure electromobility or fuel cell vehicles. One of these alternative fuels is ethanol. It can be added to the usual car fuel mixture or used as ED95, with 95 percent ethanol, in heavy goods traffic as a diesel substitute.
This alcohol is usually produced by fermenting sugars from starchy raw materials such as corn or from lignocellulosic biomass such as wood or straw. In order for such synthetic fuels to be as environmentally friendly as possible, however, waste materials should be used as a source of biomass wherever possible. Because if you grow energy crops specifically for this purpose, this competes with food production and also promotes intensive agriculture.
From wood to ethanol
Researchers led by Kristian Melin from the Technical University of Lappeenranta-Lahti (LUT) in Finland and the Technical University of Munich have therefore been looking for processes with which waste from forestry can be used as sustainably and efficiently as possible for ethanol production. Their solution: the wood residues are first turned into synthesis gas and chemically used to produce methanol. Then the methanol is converted into acetic acid and converted into ethanol by introducing hydrogen.
“The overall process consists mainly of technically mature sub-processes,” explains co-author Daniel Klüh from the Technical University of Munich. “However, the composition of the process steps and the final step, the hydrogenation of acetic acid to produce ethanol, are new.” The highlight here: The hydrogen required for this comes from electrolysis – the decomposition of water using electricity from renewable energies. This offers the possibility of making sensible use of excess electricity from the sun and wind in the future and saves fossil fuels for the process.
Yield higher than usual fermentation
In their study, the researchers calculated on the basis of initial tests that this process for producing ethanol is no more expensive than conventional processes. However, the yield is significantly higher in comparison to fermentation-based processes based on straw or wood: With the new process, between 1350 and 1410 liters of ethanol can be produced from one ton of dry biomass. With the fermentation-based processes, on the other hand, only between 200 and 300 liters of ethanol can be produced from one ton of dry biomass, as the team explains.
This method of ethanol production could be particularly cheap and useful wherever there is a lot of wood residue: “Countries with a high potential for residual wood and green electricity, such as Finland or Canada, can serve as producers of acetic acid, which is hydrogenated in the last step of the process in order to ethanol,” says co-author Tuomas Koiranen from LUT. “Countries like Germany will hopefully have a green electricity mix in the future and will be able to hydrogenate acetic acid to ethanol in their own country. However, Germany does not have the residual wood potential for large-scale biomass gasification for the synthesis of acetic acid,” adds Matthias Gaderer from the Technical University of Munich.
Overall, the research team comes to the conclusion that their ethanol production process is in principle competitive. However, the technology still needs to be optimized for commercialization. “The next steps would be, for example, further catalyst developments, a reactor design and the construction and operation of a pilot plant,” explains Gaderer.
Source: Technical University of Munich; Specialist article: Frontiers in Energy Research, doi: 10.3389/fenrg.2022.796104