Fischer-Tropsch based Power-to-Liquid process - Technical, economic, uncertainty and sensitivity analysis

Abstract

Liquid synthetic fuels are an important pillar for the defossilization of the hard-to-electrify modes of transport such as aircrafts, ships and heavy trucks. One possibility to produce these liquid fuels is from water, carbon dioxide and electricity via reverse water-gas shift (RWGS) reaction and Fischer-Tropsch synthesis in a Power-to-Liquid process. High carbon efficiencies are only feasible through recycling of unwanted by-products and unconverted reactants to a reformer1. This increases the process’ complexity as changes in the RWGS reactor influence the process performance and design. Yet, an optimum RWGS reactor operation has not been studied in literature so far. Therefore, this work targets to fill this gap with a systematic analysis of the impact of varying RWGS operating conditions on the process performance with respect to both, efficiencies and net production costs, while also taking into consideration the uncertainty in the cost calculations. The resulting confidence intervals are further examined in a global sensitivity analysis to identify the main contributors to the uncertainty. Low costs as well as high efficiencies result at pressure dependent optimum RWGS temperatures (plateaus). Both, a high Power-to-Liquid efficiency and low net production cost, can be achieved at around 5 bar and 800 °C. This optimum is robust to changes in technical parameters, for example changes of Fischer-Tropsch operating conditions or electrolyzer efficiency, as well as to economic uncertainties, especially hydrogen cost. No unwanted graphite formation occurred in the equilibrium calculations in the temperature region of the described plateaus. Experiments further indicate that a suitable catalyst may aid to push the carbon formation boundaries to broaden the possible RWGS operating window. Electricity cost is the main contributor to the net production cost. Reducing the full load hours of the electrolyzer may reduce the average electricity price. However, oversizing of the electrolyzer outweighs the benefit of cheaper electricity resulting in higher net production cost at reduced electrolyzer full load hours. The results of the uncertainty and global sensitivity analysis indicate that seven of the 60 input variables for the economic assessment have a relevant impact on the uncertainty (95 % confidence interval: 2.8-5.3 €2019/kgC5+). For the assessment of the sensitivity indices in the global sensitivity analysis, a combination of the Azzini estimators is recommended over the other investigated estimators in order to minimize the computational effort. Overall, the conducted systematic analysis allows to identify the optimal design and operation of the FischerTropsch based Power-to-Liquid process with particular focus on the RWGS operating conditions to yield robust low net production cost with quantified uncertainty.