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

• Published in: Applied energy 2021-04, Vol.288, p.116620, Article 116620
• Main Authors: Rahbari, Alireza, Shirazi, Alec, Venkataraman, Mahesh B., Pye, John
• Format: Article
• Language: English
• Subjects: Concentrated solar power; Fischer–Tropsch synthesis; Reforming; Supercritical water gasification; Techno-economic optimisation
• ISSN: 0306-2619; 1872-9118
• DOI: 10.1016/j.apenergy.2021.116620

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. •Alternative reforming options to convert CH4 in a solar fuel process are analysed.•Steam methane reforming with dumping of surplus CO2 is cheapest at current H2 costs.•As hydrogen costs drop, adding H2 instead of dumping CO2 becomes preferable.•At lower future H2 costs (2.1 AUD/kg), partial oxidation/dry reforming becomes best.