Environmental impact assessment of renewable hydrogen and methane production technologies and hydrogen transport by hydrogen carriers

<p dir="ltr">Energy transition towards net zero emission target has driven increasing production of renewable energy, and implementation of emission reduction measures in high polluting industrial sectors. Together with increasing production of wind and solar energy, scaling up of biogas and hydrogen production is also important for the clean energy future. Hydrogen is a clean energy carrier when it is produced from renewable sources, however, current production of hydrogen mostly relies on natural gas reforming, which have considerable amount of emissions. Hydrogen can be produced from fossil or renewable sources with a wide range of biomass and waste materials as possible feedstocks for the renewable hydrogen production, as well as water electrolysis powered by renewable electricity. Important role of hydrogen can be found in numerous hydrogen export and import projects. However, distribution of hydrogen is challenging due to risks during storage and low volumetric density of hydrogen, which lead to the distribution in the form of liquefied or compressed hydrogen, or using chemical hydrogen carriers, such as methanol, ammonia and liquid organic hydrogen carriers (LOHC). Hydrogen can also be converted to renewable methane which can directly substitute natural gas because of similar chemical composition (> 97% CH₄ content).</p><p dir="ltr">This thesis aims to investigate environmental impacts of renewable hydrogen and renewable methane which are produced from various energy sources and waste materials. First, the environmental impacts of natural gas-produced hydrogen were analysed to understand the impacts of current hydrogen industry. Later, the environmental impacts of renewable hydrogen produced by wind and solar electricity, biomass gasification, and biogas upgrading were compared through literature review which enabled identifying renewable hydrogen options with lower environmental impacts. The renewable hydrogen with direct air-captured CO₂ were used to produce renewable methane which could be a better option for distribution using existing natural gas infrastructure. Sewage sludge, pig and cattle manure, food waste and landfill gas were also selected as feedstock of the renewable methane with applying anaerobic digestion and biogas upgrading technologies. The impact assessment performed for the renewable methane production confirmed that energy consumption during the production processes and environmental credits provided to by-products had the greatest influence to the impacts.</p><p dir="ltr">Life cycle assessment of hydrogen transport by liquid organic hydrogen carriers (LOHC) was conducted by modelling production of renewable hydrogen carriers, hydrogen transport by the carriers, and separation of hydrogen (dehydrogenation) at the destination. Two renewable hydrogen carriers, renewable toluene and dibenzyltoluene, were applied for the first time among the hydrogen transport studies, where the carriers were produced from bioethanol and biomethanol using bagasse and wheat straw. Similar to the renewable methane production, energy-intensive dehydrogenation was the most impactful process, while the environmental impacts and transport distance presented non-linear relationship. Ammonia was also applied as a hydrogen carrier, which was synthesised using wind-powered hydrogen and air-captured nitrogen through Haber-Bosch process. At the destination, decomposition of the delivered ammonia was performed to obtain pure hydrogen via two catalytic decomposition processes using either ruthenium- or nickel-activated catalyst, followed by pressure swing adsorption (PSA) for separation and purification of the hydrogen. Vanadium membrane was applied as a novel way of the separation that can replace PSA, and found to have lower global warming impact per kg of the produced hydrogen than PSA due to higher hydrogen recovery rate. The analysis performed in this thesis provides information on sustainability of various renewable energy options which can be useful for clean energy policies, while suggests potential of renewable energy production from waste materials.</p>