Advancing synthetic fuel technology: A model study for the integration of direct air carbon capture and diesel synthesis

Direct air capture (DAC) integrated with solid oxide electrolysis (SOEC) and Fischer–Tropsch (FT) synthesis is a promising way to produce carbon-neutral liquid fuels. However, the high demand for renewable electricity, particularly from electrolytic hydrogen production, and limited cross-process integration pose key challenges to this mode of production. This study addressed these constraints by modeling a fully integrated DAC–SOEC–FT diesel system using a commercial, equation-oriented simulation platform under steady-state conditions and assuming that renewable power supplied the SOEC unit. The process design incorporated thermal and process-level integration with waste heat from the calciner, FT reactor, and SOEC burner repurposed for internal heating and feed conditioning. System-derived byproducts (e.g., naphtha, purge gases) were used as internal fuels to minimize external energy inputs and avoid additional emissions. Results showed that under ideal thermal integration scenarios, the theoretical internal recovery of up to 78% of total process heat could substantially reduce reliance on external utilities. While SOEC remained the primary electricity consumer (29.8 MWh/t-diesel), internal energy recovery mitigated auxiliary demands. Cradle-to-gate CO 2 emissions were net-negative and reached –1.20 kg-CO 2 /kg-diesel in Japan and –1.56 kg-CO 2 /kg-diesel in Canada. These results emphasized the strong synergies unlocked by integrated system design and offered a pathway toward energy-efficient, carbon-negative synthetic diesel suited for hard-to-abate transport sectors. • Integrated system enables energy-efficient e-diesel from air-captured CO 2 . • Co-electrolysis and FT synthesis form a thermally integrated conversion route. • Heat recovery reduces external demand and improves process circularity. • System achieves net-negative emissions with minimal external utilities.