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

Abstract

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.