A solar fuel plant via supercritical water gasification integrated with Fischer–Tropsch synthesis: System-level dynamic simulation and optimisation

This paper evaluates the annual dynamic performance and techno-economic feasibility of a solar fuels process driven by concentrated solar power (CSP). The process integrates solar supercritical water gasification (SCWG) of microalgae, syngas storage, and downstream Fischer–Tropsch (FT) synthesis, to produce renewable drop-in liquid transport fuels. Performance curves for key components, derived from design-point and off-design simulations in steady-state flowsheet models established in the previous work of the authors, are incorporated into a dynamic energy-based model of the overall plant in OpenModelica. Control units are implemented to model the high-level state transitions of the plant relating to ramp-up and ramp-down of solar-SCWG and FT units as well as syngas storage and dispatch. There is significant uncertainty about the achievable ramping time for FT plant, owing to the dynamics of the synthesis of long-chain hydrocarbons in the FT reactor, so this is modelled as a variable-duration ramp-up with no useful product output. A perfect one-day-ahead forecast scheduler is developed to regulate the supply of solar syngas to the FT unit to minimise these ramp-up events. The levelised cost of fuel (LCOF) for the plant is minimised using a genetic algorithm to adjust the solar multiple and syngas storage capacity of the plant, with weather data and location of Geraldton in Western Australia. The optimal configuration has a solar multiple of ∼4 and syngas storage of ∼20 h, and achieves an LCOF of ∼3.5 AUD/L (∼2.5 USD/L) and capacity factor of ∼72%. Sensitivity analysis shows that the price of the farmed microalgae feedstock has the strongest influence on the LCOF. The one-day-ahead syngas dispatch scheduler contributes an LCOF reduction of 19% compared to an immediate dispatch strategy. The importance of the FT ramping time in these simulations suggests that fast-ramping FT reactors (e.g. microchannel reactors) will be essential for the techno-economic success of solar-gasified FT fuels, and motivates further research and development on this topic.