Carbon and oxygen recycling strategies in CO2-to-sustainable synthetic fuel production: Recycling route, techno-economics and carbon intensity

Abstract

Anthropogenic carbon utilization and recycling into fuel using renewably sourced or nuclear energy and water is a potentially valuable strategy for replacing fossil fuels. The high-level elemental balance of the CO2-to-Fuel system involves oxygen rejection, with CO2 and H2O reduction dependent on crucial process steps that determine performance metrics such as energy efficiency, element utilization, economics, and carbon footprint. This study elaborates on carbon and oxygen recycling strategies for producing sustainable synthetic hydrocarbon fuels. It covers electrolysis routes, internal recycling pathways, and additional processes to increase synfuel yield. An element balance was derived for four process schemes with systematic recycling route variations, facilitating a comparison of elemental efficiencies: (1) carbon utilization (80.7–90.6 %), (2) hydrogen efficiency (4.8–30.8 %), and (3) oxygen gas rejection rate (56.8–63.1 %). A process flow diagram was proposed to discuss techno-economics through the existing route via the Fischer-Tropsch process. The energy intensity ranged from 118.5 to 177.4 GJ per tonne of synfuel, and the energy conversion efficiency varied from 0.24 to 0.36. We further evaluated the carbon intensity, considering the primary energy efficiency to show the different energy supply scenarios, including (i) 100 % fossil-based (506–1416 kgCO2eq/GJFuel), (ii) grid power with fossil fuel (246–687 kgCO2eq/GJFuel), (iii) hybridization of renewable sources (biomass and wind power) (42–78 kgCO2eq/GJFuel), and (iv) nuclear energy (7–18 kgCO2eq/GJFuel). This work considers the interplay between oxygen rejection, syngas conversion, and combustion, undertaking a novel analysis to bridge the high-level elemental analysis with detailed metrics calculations.