Storing renewable power into liquid fuels: A Pathway for sustainable transport

Abstract

Introduction Liquid fuels will be the fuel of choice for air and cargo transport for the coming decades. Synthetic fuels produced by the power-to-liquid route are a promising alternative to 1. generation biofuels. Sustainable hydrogen can be derived via water electrolysis using renewable power. Sustainable carbon sources are biomass or carbon dioxide taken from air. “Unavoidable” CO2 from industrial processes like steel or cement production might be considered as reused CO2 reducing the societies footprint. The combination of renewable syngas production and Fischer-Tropsch synthesis (FTS) is investigated and the efficiency, costs and realistic quantities of synthetic fuels production is compared for different process routes and feedstocks. However, whether the new fuels will prevail on the market depends on a variety of factors, e.g. total net production costs and availability of feedstocks. Methods Currently, the Biomass-to-Liquid (BtL) and Power-to-Liquid (PtL) are intensely discussed. For techno economic assessment of different process routes, production pathways of BtL and PtL fuels have been modeled in flowsheeting software. Furthermore, a combined process concept called Power-Biomass-to-Liquid (PB-t-L) is developed and presented. Special emphasis is given to heat and material integration of each process route. Results Renewable synthetic fuels can be produced at a cost of 1.50 to 4.50 €/l, depending on the feedstock, process route, equipment cost and scale of plant. Hydrogen generation is the most significant cost factor, drawing the attention towards the renewable power generation cost as well as the electrolyzer type, efficiency and investment cost. Biomass and CO2 availability respectively limit the plant size or will add additional feedstock transport efforts. The total system efficiency calculated for Power-to-Liquid can be more than 60% for optimal conditions and unit operations. Operating water electrolysis with fluctuating power requires an electrolyzer oversize as well as hydrogen storage capacities. Discussion System operational characteristics based on techno-economical analysis with regard to stationary and fluctuating power supply will be discussed. A sustainable pathway for the production of synthetic liquid fuels is technically feasible and able to take huge amounts of renewable excess power. Economic implication compared to present cost of fossil based fuels will be quantified.