The wet fermentation biogas process : Limitations and possibilities for efficiency improvements

Sammanfattning: The biogas process is known as an environmental friendly and sustainable way of producing energy and fuel but to be fully commercially competitive with other types of processes, efficiency improvements are needed. By doing a case study at the Växtkraft biogas plant in Västerås, Sweden, three specific limitations were identified and studied. Firstly, to improve the capacity of the plant, pre-treatments of the different substrates are needed to disintegrate the substrate and by doing so increasing the gas yield and the speed in which it is produced. Secondly, to improve the fermentation process itself more knowledge is needed around the mixing inside the digester. To be able to create an optimal and stable environment for the microorganisms the mixing is the key, because the mixing affects the mass transfer of all solids, nutrients, gases and other substances in the digester. Thirdly, the water treatment of the recirculated process water cannot reach the desired separation of dry matter (DM) and this is affecting the capacity of the plant negatively. The feed for the digester is produced by mixing the process water and the substrate to get a pumpable slurry with a DM content of 8-10 %. When there is too much DM in the process water to begin with, the mixing ratio between the substrate and the liquid changes, decreasing the amount of substrate that can be added to the mixture and later on fed to the digester. The full biogas potential of most organic materials cannot be extracted during the relatively short retention time of most digesters because of their complex structures. The organic materials are broken down too slowly and the nutrients cannot become biologically available in that time span. This means that a lot of the bound energy in the organic material leaves the biogas plant with the liquid digestate. The efficiency of the process can be improved by pre-treating the material before digestion. Pre-treatment experiments to disintegrate ley crop silage using electroporation, a treatment using electrical fields, were conducted to study its effect on the biogas yield. The experiments resulted in up to twice the amount of biogas being produced from the pre-treated material compared to untreated material. Numerical simulations of the mixing inside a digester were carried out to understand the effect that a gas lift mixing configuration has on the mass transfer in the system. The mixing dynamics were evaluated by testing five different flow rates of the injected gas and the effect that the liquid recirculation system has. The results indicate that there are large unmixed zones and that changing the gas flow rate only has a marginal effect on these areas. The simulation also suggests that the outlet of the liquid recirculation system is situated too close to the gas injectors, resulting in energy losses in form of diminished mixing of the digester. Experiments to reduce the DM content of the recirculated process water were carried out using a ceramic ultrafiltration membrane. The flux through the membrane and the separation efficiency were investigated at different operation temperatures, 70°C, 90°C and 110°C. The results show that 59-63 % of the DM was separated in this temperature interval and that the flux/flow through the membrane increased with the temperature. These results correspond to a 29 % increase in the capacity to add new substrate. The energy required to heat the membrane, if heat recovery is used, is small in comparison to the increased methane yield. In the best case scenario these above identified improvements could increase the methane yield by up to 40%.