Energy carriers and other co-products from microalgae: Sustainability aspects and feasibility of different production pathways

The cultivation and conversion of algae for biofuels emerged as one silver bullet technological solution over the last few years, in order to decrease our dependency from fossil fuels. This thesis is based on an extensive literature review started in 2008 in order to put some clarity in the algae-to-energy sector, to discern overstatements about algae productivity and possible uses and to assess how this technology could contribute to sustainable development. The aim of the thesis is to identify promising technological solutions and algae-based production options, and to assess the viability of different pathways for energy production. The main findings of the review are summarized in the first part of this thesis together with a characterization of promising algae strains. A special focus is given to thermochemical conversion and biorefinery processes, as well as to sustainability impacts. A second part of this thesis is focused on applying this knowledge to a specific case study: the Austrian context. Two promising algae species have been identified and some results of a project comparing different energy production pathways from algae in Austria are reported. Scenarios are drawn for 2020 and 2050. Hydrothermal processes applied to algal biomass such as Scenedesmus obliquus grown in wastewater appears to be a viable solution in the Austrian context, able to achieve a positive energy balance. The main products of the process would be bio-coal and heat or, alternatively, a syngas rich in methane and hydrogen. The research highlights that, although a promising solution, the use of algae for large-scale production of biofuels is still unpractical given some major technological bottlenecks especially at the cultivation and separation phases. The co-production of non-energy products together with bioenergy can improve significantly the overall viability and sustainability of the process but, to this end, their production scales and market potentials must be compatible.