|Engineering and Technology; Environmental Engineering; Energy Systems; Teknik och teknologier; Naturresursteknik; Energisystem
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Climate change is one of the major challenges of the 21st century. The energy sector represents the main contributor to global greenhouse gas emissions, due to its reliance on fossil fuels. Renewable energies arise as current solutions. Nevertheless, they are still facing two central difficulties: the lack of large-scale energy storage technologies to deal with their intermittent nature (e.g. wind and solar power), and the absence of energetically dense fuel alternatives for the transportation sector. Additionally, biogas technologies are indispensable for achieving sustainable societies. They result in energy and nutrients recovery from waste, mitigating greenhouse gas emissions and other kinds of pollutions. These technologies are required in circular economies, characterised by the nonproduction of disposable wastes. However, biogas needs to be upgraded to optimise its properties as energy carrier. Indeed, biogas upgrading results in a broader use for the gas, besides combined heat and power generation; enabling its efficient transport, large-scale storage, and use as vehicle fuel. This project shows how electricity and gas systems can be integrated through an innovative Power-to-Gas technology which is able to partially solve these problems. The technology is based on the synergy of coupling biogas plants to hydrogen generation systems powered by off-peak electricity surpluses from intermittent renewable energies (e.g. solar and wind power), and subsequent biological methanation of the CO2 from the biogas and the produced H2 in an ex-situ anaerobic reactor. At first, this thesis presents a detailed definition of the overall innovative system and its different components. Subsequently, focus is put on the search for the most suitable biological methanation technology for industrial purposes. Through experimental work, this thesis examines and compares four different anaerobic reactor configurations, aiming to determine the most effective technology among the ones studied. Expressly, the experiment investigated different diffusion techniques for injection of the gases in the liquid media, together with diverse pore-sizes for the mentioned diffusers. The leading reactor configuration transformed 98.4% of the injected H2 at the highest loading rate tested (3.6 LH2/LR.d), upgrading biogas from a CH4 concentration of 60% to 96% in volume. The performance of the different setups is examined, and origins for the biological efficiency variations are elucidated, in order to help with the selection of subsequent experimental prototypes. Given its early stage of development, this biomethanation unit process forms the pivotal technology of the overall system. As soon as this technique is developed, a fully commercial system will be available to initiate major environmental and socio-economic benefits.