A special blend of robust bacteria in a new process is paving the way to convert even the stubbornest forms of biomass into methane.
Despite Danish biogas plants operating at full capacity, only about half of the energy content from the raw materials is converted into biogas. This is because a large proportion of the biomass received—e.g. straw and other biomass with a high lignin content—cannot be digested by the biogas plant bacteria. Lignin is a carbon compound that protects plants from being degraded in the wild.
A new process developed at DTU Chemical Engineering now allows biogas plants to both make better use of biomass and at the same time produce methane—which has a significantly higher value than biogas. With the new process, close to 100 per cent of the carbon content of the raw material ends up in the final product.
Initially, biomass is converted into so-called synthesis gas with the help of gasification. Microorganisms subsequently convert the synthesis gas into methane through the process of fermentation.
"Fermentation occurs under mild conditions—i.e. without increased pressure and with only a slightly raised temperature of between 30 and 60 °C. This is a simple and inexpensive process which makes the method particularly attractive to the many small biogas producers in Denmark," says Associate Professor Hariklia Gavala. She heads the biological part of the Synferon project—Optimised Syngas Fermentation for Biofuels Production—which includes a number of research groups at DTU Chemical Engineering.
Robust microorganisms
Hariklia Gavala and her colleagues have developed special blends of microorganisms to achieve the very high conversion of the carbon content of biomass into methane—emphasizing the use of robust bacteria to withstand the harsh conditions in a production plant.
Another prerequisite for the high level of methane conversion is a special reactor where the researchers add hydrogen to the process. The hydrogen reacts with the CO2 in the synthesis gas, producing an even higher methane yield. Following a series of successful laboratory-scale experiments, the researchers have built a reactor on a demonstration scale.
"With the technology we've developed, it will be possible to use virtually all forms of biomass as a starting point for producing methane—for example, straw, wood briquettes, or sludge residue from biogas production. Any type of biomass with a high carbon and a low water content can be gassed and turned into synthesis gas," says Associate Professor Ioannis V. Skiadas, who has led the development of the new reactor.
