Holistic sustainability assessment of biomethane systems

European states, including Ireland must ensure that an increasing portion of energy from renewable sources. This is a particular issue for transport, which in comparison to electricity and heat has very low levels of renewable penetration. Electric vehicles (EVs), liquid and gaseous biofuels are the most likely sources for future energy in transport. However, renewable does not automatically mean sustainable. For example the sustainability of biofuels sourced from food crops has been queried in the context of land use change emissions. This thesis has an ambition of assessing sustainable options for advanced biomethane production in Ireland, a country with a temperate oceanic climate, using various methodologies (life cycle assessment, energy system modelling and cost analysis). Biomethane is a versatile gaseous biofuel that is considered advanced when produced from second and third generation feedstocks such as wastes, residues, grasses, and seaweed, but a simplified and unified framework for biofuels LCA is required to compare different options. Under a low-level land use change emissions scenario, biomethane from grass could play a major role in the Irish energy system for transport in 2050, requiring only 5-11% of Ireland’s agricultural land. With high land use emissions, however, the model would suggest using hydrogen, residues-based biodiesel, and EVs. Biomethane from seaweed could be deemed unsustainable if the system is not optimised. However in an optimal configuration it could achieve 70% greenhouse gases (GHG) savings as compared to gasoline. Such reductions in GHG emissions can be achieved in an optimal system: integrating seaweed cultivation with fish farming; using innovative growing techniques; ensuring optimal seaweed composition; reusing digestate; and using renewable electricity to power plant operations. Biomethane from landfill gas was shown to require a subsidy to allow financial sustainability. Thus in conclusion, biomethane can be a sustainable transport biofuel, but requires system optimisation and state subsidies

Beyond carbon and energy: the challenge in setting guidelines for life cycle assessment of biofuel systems

Life cycle assessment (LCA) is one of the most suitable tool for a uniform assessment methodology of biofuels’ sustainability. However, there are no binding guidelines for LCA of biofuel systems. Published LCAs use a range of methodologies, different system boundaries, impact categories and functional units, various allocation approaches, and assumptions regarding by- and co-products, as well as different reference systems to which the biofuel system is compared. The European Renewable Energy Directive and the US Renewable Fuel Standard focus on greenhouse gas (GHG) emissions. However, previous LCAs of biofuel systems have shown that a reduction of GHG emissions does not lead automatically to a decrease in other environmental impacts, and might in fact be associated with an increase in impacts such as acidification, eutrophication, and land use change. In order to enable effective comparison of biofuel systems, the authors propose a framework for biofuel LCA. System boundaries should be expanded to include the life cycle of by- and co-products. Results should be reported using more than one functional unit. Burden shifting can be avoided by considering an array of impact categories including global warming potential and energy balance, along with eutrophication and acidification potential, and a land use indicator

Life cycle assessment of seaweed biomethane, generated from seaweed sourced from integrated multi-trophic aquaculture in temperate oceanic climates

Biomethane produced from seaweed is a third generation renewable gaseous fuel. The advantage of seaweed for biofuel is that it does not compete directly or indirectly for land with food, feed or fibre production. Furthermore, the integration of seaweed and salmon farming can increase the yield of seaweed per hectare, while reducing the eutrophication from fish farming. So far, full comprehensive life cycle assessment (LCA) studies of seaweed biofuel are scarce in the literature; current studies focus mainly on microalgal biofuels. The focus of this study is an assessment of the sustainability of seaweed biomethane, with seaweed sourced from an integrated seaweed and salmon farm in a north Atlantic island, namely Ireland. With this goal in mind, an attributional LCA principle was applied to analyse a seaweed biofuel system. The environmental impact categories assessed are: climate change, acidification, and marine, terrestrial and freshwater eutrophication. The seaweed Laminaria digitata is digested to produce biogas upgraded to natural gas standard, before being used as a transport biofuel. The baseline scenario shows high emissions in all impact categories. An optimal seaweed biomethane system can achieve 70% savings in GHG emissions as compared to gasoline with high yields per hectare, optimum seaweed composition and proper digestate management. Seaweed harvested in August proved to have higher methane yield. August seaweed biomethane delivers 22% lower impacts than biomethane from seaweed harvested in October. Seaweed characteristics are more significant for improvement of biomethane sustainability than an increase in seaweed yield per unit area

Beyond carbon and energy: the challenge in setting guidelines for life cycle assessment of biofuel systems

Life cycle assessment (LCA) is one of the most suitable tool for a uniform assessment methodology of biofuels’ sustainability. However, there are no binding guidelines for LCA of biofuel systems. Published LCAs use a range of methodologies, different system boundaries, impact categories and functional units, various allocation approaches, and assumptions regarding by- and co-products, as well as different reference systems to which the biofuel system is compared. The European Renewable Energy Directive and the US Renewable Fuel Standard focus on greenhouse gas (GHG) emissions. However, previous LCAs of biofuel systems have shown that a reduction of GHG emissions does not lead automatically to a decrease in other environmental impacts, and might in fact be associated with an increase in impacts such as acidification, eutrophication, and land use change. In order to enable effective comparison of biofuel systems, the authors propose a framework for biofuel LCA. System boundaries should be expanded to include the life cycle of by- and co-products. Results should be reported using more than one functional unit. Burden shifting can be avoided by considering an array of impact categories including global warming potential and energy balance, along with eutrophication and acidification potential, and a land use indicator