From spent grain to “bio-coal”—Is hydrothermal carbonization (HTC) an unvalued key technology?

Technical Session 21: Spent Grains Session
Heinz Dauth, Münster University of Applied Sciences, Steinfurt, Germany
Co-author(s): Juergen Mueller, Tobias Bosse, and Peter Dettmann, Münster University of Applied Sciences, Steinfurt, Germany

ABSTRACT: HTC was described for the first time in 1913 by a chemist named Bergius and gained increasing interest for several applications during the last decade. What does hydrothermal carbonization mean? In easy to understand terms, it stands for a thermo-chemical process for the conversion of solid biomass at an elevated temperature and pressure in the presence of water. The achieved product differs significantly in its chemical and physical properties in comparison to the starting material. During the HTC process mainly water and carbon are dissociated from the biomass. Thus, the energy density is raised significantly and the heating value is approximately that of dry, high quality brown coal. As mentioned previously the HTC process takes place in an aqueous reaction medium so that wet biomass ,like spent grain, can be easily used and is actually preferred. Initial experiments with spent grain show that this biomass is an ideal substrate for conversion into “bio-coal.” The advantages of spent grain are its particle size distribution, chemical properties due to the composition of the biomass, excellent mechanical compactibility, which results in a high load capacity (mass loading) in the autoclave, and a nearly homogeneous distribution of the biomass in the liquid phase inside the autoclave. Due to the chemical composition of the substrate the required energy input into the system after initializing the process is significantly lower compared to other examined substrates. After a treatment of 5 hr under mild process conditions with respect to temperature and pressure the resulting product is a coal-water slurry. The coal fraction can easily be separated and dried. The “bio-coal” gained from spent grain has a heating value of approximately 27.000 kJ/kg (which is higher than the heating value of brown coal) and has an ash content of just 6% in relation of dry “bio-coal.” The crop of “bio-coal” from 1 kg of “wet” spent grain is approximately 12 mass percent. Furthermore, the process can be enhanced in a way that other biomass from the brewery, like label residues or sewage sludge, is mixed with the spent grain. This will be the next step for experimental examinations. This “bio-coal” from spent grain can serve as a CO2 neutral substitute for fossil coal because by burning “bio-coal,, only that amount of CO2 which was trapped in the plants by means of photosynthesis is released, making the HTC process a perfectly climate neutral and sustainable energy process. In summary, the HTC process provides a porous, brittle, and partly dust-like product that is considerably easier to dry and convert to thermal energy in a brewery than the original biomass. This is a clear conceptual advantage for energy use compared to, for example, the burning or gasification of untreated biomass like spent grain.

Heinz Dauth graduated with a Dipl.-Ing. degree in food technology and biotechnology from the Technische Universität München-Weihenstephan in 1993. Afterward he was appointed as a scientific researcher at the Chair of Process Engineering (Karl Sommer) in Weihenstephan. His doctoral thesis was completed in 1999 in the field of mechanical process engineering. He served the Chair of Process Engineering, TU München, as scientific assistant and university lecturer from 2003 until 2011. His main research interests are bulk solids technology, dispensing technology, and hygiene, as well as process engineering for specific problems in the food and beverage industries. During this time he has also been working as an assistant professor at the Weihenstephan University of Applied Sciences, lecturing on mechanical and thermal process engineering. Since October 2011 he has been a professor at the Münster University of Applied Sciences in the Faculty of Chemical Engineering.