Fiber-rich biomass as a sustainable source of bioenergy is abundant in Switzerland, such as harvest residues, straw, corn husks and the solids of dairy cattle manure. However, the conversion to biogas is challenging, because of the low hydrolysis rate of these materials. A novel pretreatment method based on the application of low oxygen concentrations should significantly increase the microbial hydrolysis and consequently, the output of biomethane. In 2016, the Zurich University of Applied Sciences (ZHAW) acquired the three-years R&D project termed HYDROFIB (Micro-aerobic hydrolysis of fiber-rich biomass for increased biogas production), which is supported by the Swiss Federal Office of Energy (SFOE) and the Forschungs-, Entwicklungs- und Förderungsfonds der Schweizer Gaswirtschaft (FOGA) and the SCCER BIOSWEET. The HYDROFIB project includes a further SCCER BIOSWEET member, the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) which is preparing a biogenic mass flow study of Swiss fiber substrates. Two industry partners are involved as well; First Biogas Intl. is committed to the technical realization and Swisspower as an energy system implementation partner. Goals of the project are (1) to demonstrate the additional energetic potential of untapped fiber substrates in Switzerland and (2) to test the feasibility and the effectiveness of the new HYDROFIB technology in laboratory studies and by means of a test facility.
Based on previous lab and pilot scale studies, biogas production from fiber-rich biomass can be significantly improved by micro-aeration, both in yield (up to +30 %) as well in gas quality (methane content). Oxygen promotes specific microbes to produce an enzymatic cocktail for hydrolyzing the cellulosic fiber-biomass. The micro-aeration has to be controlled accurately because too much oxygen would be unfavorable. In a second step, the pre-hydrolyzed material is anaerobically converted to biogas. The two processes have to be separated physically, because oxygen can inhibit the biogas microbes in step 2 (Fig. 1)
Fig. 1: Block flow diagram of the micro-aerobic hydrolysis process. In a first stage, fiber substrates are treated with small doses of oxygen, which promotes microbial hydrolysis. In the subsequent fermentation stage, the pretreated substrates are effectively converted into biogas (CH4 + CO2). Figure by J. Krautwald, ZHAW
HYDROFIB pilot plant (test facility)
A container-based test facility located at the Allmig AG (near Baar, Canton of Zug) the micro-aeration technology will be demonstrated with real-world fiber substrates (Fig. 3). Container 1 contains the substrate delivery and -conditioning. In container 2, the substrates will be micro-aerated and afterwards fermented to biogas in three continuously operated 330-liter digesters. Digester line 1 represents the reference, fed with non-treated substrate. All gas flows of the pilot plant are captured and analyzed (Fig. 2).
Fig. 2: Flow chart of the HYDROFIB pilot plant. In container 1, the fiber substrates are prepared, conditioned and cooled. In container 2, the microaerobic hydrolysis takes place by the addition of small amounts of oxygen. Subsequently, the pre-hydrolyzed material is fermented in two fermenters (lines 2 and 3). A further fermenter (line 1) which is fed with untreated material represents the reference. All produced gases are analyzed and recorded. Diagram by J. Krautwald, ZHAW
Fig. 3: The two containers inhabiting the HYDROFIB pilot plant, located near Baar (Canton of Zug). Picture by R. Warthmann, ZHAW
Implications for Swiss energy transition
Conversion of fiber-rich biomass to biomethane will be a relevant parameter for a Swiss biobased energy future. Since classical Swiss biomass for biomethane production is restricted available, fiber-rich biomass is nearly unexplored, because its use is not economic in the moment. Accomplishing the goals of better conversion of fiber-rich substrates will make it attractive for plant operators and investors, followed by an expansion of the use of this untapped biomass. The potential of fiber-biomass is significant; the HYDROFIB technology as an expansion for anaerobic digestion offers an estimated additional biogas potential of 1 – 5 PJ/year and thus can be able to significantly contribute to the overall BIOSWEET energy goals.
Authors: Rolf Warthmann, Judith Krautwald and Urs Baier, Center for Environmental Biotechnology, ZHAW Zurich University of Applied Sciences, Wädenswil