A sustainability analysis for a circular power-to-liquid process for diesel production

The power-to-liquid process is a key emerging technology for fossil-free raw materials and energy systems. In this work, techno-economic, and environmental analyses are carried out for a Fischer-Tropsch process producing diesel and characterized by the recovery of carbon dioxide through direct air capture, as well as the recovery of water and heat. The main aim of this study is to verify with respective analyses the circularity of carbon dioxide, water and heat and to conduct a global sensitivity analysis to identify significant system process parameters for some key performance indicators, when changed simultaneously. Despite the proven circularity based on material and energy balances ensuring a power-to-liquid efficiency of about 44 %, results show that the water closed loop is not ensured from an environmental point of view. The water consumption impact category is, in fact, a positive value (0.58-0.74 m3depriv/kgdiesel), while the climate change impact category is a negative value (-1.22 to -0.28 kgCO2eq/kgdiesel). A heat closed loop is attained according to the pinch analysis. The diesel production cost is competitive with the market price (1.76 and 2.07 $/literdiesel respectively when solar and wind energy are used). Regarding the sensitivity analysis, it is found that only costs and efficiency depend on the geographic location of the plant, in contrast to other key performance indicators. Overall, an additional optimization of the process is hence required to ensure a closed water loop from an environmental point of view and reduce further the production cost.