Solar fuels from supercritical water gasification of algae: Impacts of low-cost hydrogen on reformer configurations

Liquid transport fuels produced from biomass are of growing importance, due to increasingly ambitious targets for CO₂ emissions reduction. However, a mismatching hydrogen-to-carbon ratio in biomass feedstocks, versus that required for conventional fuels, requires that supplementary hydrogen be added or surplus carbon be removed in the production process, with many possible process configurations. Here, we consider these alternative configurations for a process incorporating a supercritical water gasification reactor, syngas reformer and downstream Fischer–Tropsch liquid fuel synthesis unit. The feedstock is microalgae, process heat is supplied using a concentrating solar-thermal collector, and additional hydrogen is supplied from photovoltaics-powered electrolysers. Using a dynamic techno-economic process model to capture solar resource dynamics, configurations are optimised for lowest produced fuel cost. Three syngas reformer types are considered: steam methane reforming (SMR), with solar heat driving the conversion of CH₄ into syngas; partial oxidation/dry reforming (PO/DR), with added hydrogen instead serving that same purpose; and autothermal reforming (ATR), combining both H₂ and heat. Furthermore, for SMR, both CO₂ dumping and H₂ addition cases are considered. At present-day 9.72 AUD/kg hydrogen costs, SMR with CO₂ dumping is cheapest, yielding gasoline equivalent at 3.76 AUD/L. With cheaper hydrogen, the optimal configuration shifts to SMR with H₂ addition, then ATR, then PO/DR, reaching a fuel cost of 2.99 AUD/L at H₂ cost of 2.1 AUD/kg. The design of future biofuels processes will depend greatly on the cost of hydrogen.