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Green coal from plants – CO2 neutral and infinite

It is just a matter of time before coal and oil will run out. However, there are, it would seem, ways to counteract this situation. Plants can be turned into fossil energy carriers, with the added advantage that the combustion of plants on average only releases as much CO2 as the plants have previously absorbed from the atmosphere. Professor Andrea Kruse from the University of Hohenheim is developing methods for using whole green plants for the industrial production of biofuel.

Prof. Andrea Kruse explaining how biomass can be turned into biocoal. © H. Braitmaier

Andrea Kruse was born in Braunschweig and grew up in the city of Darmstadt. As a young girl, she hated chemistry with a passion. She recalls clearly how her father, a chemist, overwhelmed her with his expertise whenever she dared to ask a question about chemistry. She was more interested in physics and wanted to do something related to this field of science. However, life turned out differently thanks to an excellent chemistry teacher who sparked her enthusiasm for the world of molecules during her final year of school. Kruse studied chemistry in Heidelberg, initially specialising in physical chemistry and later turning to applied technical chemistry. She finally found her personal challenge in the utilisation of biomass. “Biomass is not a pure substance, even location and weather conditions have an impact on the ingredients,” says the 48-year-old, “and the goal is to create a uniform product.”

Biorefinery is the technical term that aptly summarises Kruse’s research. “Our aim is to use whole plants to produce many different high-quality products,” said Kruse, adding, “but this is not always possible; so there will also be some products of lower quality.” Kruse’s team is focussing on a method of producing artificial coal from green plants in a process that takes only a few hours compared to the millions of years needed to produce it naturally. The method can also be used to produce oil for processing into fuels, hydrogen for use in fuel cells and chemical syntheses as well as basic chemicals for the production of plastics that are normally produced from petrol.  

Alchemy in a pressure cooker

Biological coal basically looks and behaves like natural brown coal. © Roland Fritz/ Christina Ceccarelli

The application of most methods is currently limited to dry biomass such as hay, straw and wood and must not contain more than ten percent water. However, Kruse’s team is also aiming to convert green plants with an 80 to 90 percent water content into fuel. In order to do this, they are using a method known as hydrothermal carbonisation. The principle of this new biomass conversion method sounds rather simple. Kruse comments: “You put the mushy wet biomass into something that looks like a pressure cooker and leave it to cook for about four hours at a pressure of around 20 bar and a temperature of 200°C.” This results in an aqueous solution of black coal, which captures around 80 percent of the original carbon, and hence energy. The water is then squeezed out of the coal meal, resulting in a biocoal powder. The application of slightly higher temperatures and pressure leads to oils and hydrogen.  

The “biocoal” not only looks like natural brown coal, it also has the same calorific value and could easily be used in power plants for the production of electricity and heat. Moreover, the entire process is climate neutral. At present, the production of biocoal is still more expensive than mining brown coal. “The real attraction is that it is possible to combine the method with biogas plants and carbonise the fermentation residues, i.e. turn them into carbon or carbon-containing residues.” Food residues (e.g. sugar beet leaves, spent grain from breweries and residues from juice production) that might otherwise be thrown away can also be carbonised. “Basically, all green plants can be used for this purpose,” Kruse says. The first companies are already preparing to commercially produce biocoal.

Coal for ovens and soils

Kruse points out that biocoal is also suitable for other applications, e.g. it can be applied to poor soil where it can bind nutrients and water and hence improve soil quality. She is certain that this ancient Amazonian tribal knowledge will in future also be of great interest for Germany. “Ongoing climate change leads to less precipitation, so we also have to think about ways of helping soils store water.” Plants are renewable resources with huge potential and Kruse plans to exploit this potential. 

At present, she divides her working time between her chair at the University of Hohenheim and her research group at the Karlsruhe Institute of Technology (KIT). Kruse considers the biomass expertise at the University of Hohenheim and the engineering and chemical expertise at KIT to be an excellent combination that enables her to cover all production steps from plant to final product.

Despite the time it takes to commute between her home in Bruchsal and her two workplaces, and despite the fact she has currently got a cold, Kruse seems happy with her lot. She finds a nice balance between work and relaxation by growing hydrangeas in pots on her balcony. “I have never carbonised my hydrangeas, although this would be perfectly possible,” laughs Kruse. 

Further information:

Prof. Dr. Andrea Kruse
Institute of Agricultural Engineering
Conversion Technology and Life Cycle Assessment of Renewable Resources
University of Hohenheim
Garbenstr. 9
70599 Stuttgart
Tel.: +49 (0)711/ 459 - 24700
E-mail: Andrea_Kruse(at)uni-hohenheim.de
Website address: https://www.biooekonomie-bw.de/en/articles/news/green-coal-from-plants-co2-neutral-and-infinite