For two years, Ilona Sárvári Horváth, associate professor at the University of Borås, together with other researchers from RISE and Sweden’s agricultural university (SLU) in Uppsala investigated how straw, in the shape of pellets and briquettes, can be used in the production of biogas. Using straw in the biogas process is something that several actors have asked for, politicians, straw manufacturers, as well as farmers.
“Straw has a positive impact on the biogas process. Pellets work a bit better than briquettes as they are more easily digested because the material structure changes more during pelleting than briquetting. Briquettes have a larger particle size making it digest more slowly, and the straw in the reactor may float up to the surface, which causes mechanical problems”, Ilona Sárvári Horváth says.
Easy to transport and a better gas production
“In our research, we have chosen to look at straw products. They do not take up much space and are easier to transport. During briquetting and pelleting, there is a certain pre-treatment with grinding, heat, and pressure, which changes the structure of the straw and makes it easier to transform into biogas. This also creates sales opportunities for companies that produce straw products.”
Straw as a substrate for biogas production can provide better profitability, both in the biogas sector and in agriculture. Straw, however, works best in a co-digestion process, as it has high carbon content but low levels of trace elements, which inhibits degradation. In a co-digestion process, different materials are fed together into the chamber, and since different materials often have different compositions and structures, better nutritional balance may be achieved for the microorganisms that ‘work’ together in the biogas reactor.
Among other things, the researchers have conducted anaerobic digestion experiments in labs for one year, both at the University of Borås and at RISE in Uppsala.
“In the experiments, we have added straw to food waste in different proportions. At the university we investigated pellets, and in Uppsala investigated briquettes in a similar way. The results show that the biogas production gets better because straw has a higher dry matter content, which means that you can feed more material into the reactor that produces the gas. This makes you yield more gas per volume and time unit.”
The project ends in August 2017 and is part of the Biogas2020 project, which includes approximately 30 actors from the academia, companies, and municipalities from Sweden, Norway, and Denmark, with the purpose of building a Scandinavian biogas platform. The research on straw to biogas is partly financed by Biogas2020 and partly by the Swedish Energy Agency.
“What remains in the project is to do microbiological analyses. We want to investigate how adding straw affects the microbiological composition of the anaerobic digestion process”, Ilona Sárvári Horváth says.