The impact of low-cost H2 on the solar fuel process design: A case study in solar gasified Fischer–Tropsch fuels

This paper focuses on how the low-cost renewable H2 can change a process design configuration in solar fuel plant. As a case study, an industrial process is considered in detail at ANU to convert the algal biomass into Fischer–Tropsch liquid fuels via solar-powered supercritical water gasification (SCWG). The yield gases from the gasifier mainly contain methane, which is then converted into the suitable composition of syngas in the steam methane reforming (SMR) process. Two scenarios are evaluated here to balance the H2:CO ratio for the downstream process: (i) dumping a fraction of carbon in the form of CO2, (ii) supplying make-up H2 from PV-driven electrolysis unit. A detailed steady-state model of the SCWG-SMR and FT plants is developed in Aspen Plus software. The performance curves of gasification/FT units at design and off-design points together with a set of control logics is used to form an energy-based system-level dynamic model in OpenModelica. The levelised cost of fuel (LCOF) as a key parameter for system optimisation is calculated for the considered scenarios. It is revealed that the preferred process design choice of the whole plant is highly affected by the H2 price from a techno-economic standpoint. If the H2 cost falls by 42% (i.e. 5.6 AUD/kg), the SMR-H2 configuration is more economically feasible as compared to the SMR-dumping scenario.