Making synthetic fuels for the road transportation sector via solid oxide electrolysis and catalytic upgrade using recovered carbon dioxide and residual biomass

The search for beneficial biofuels with positive energy gains and reduced carbon dioxide emissions is a key target of the road transportation sector. This work focuses on alternative fuels for fossil fuel substitution in the transportation sector and we have examined the production of three different types of synthetic fuels: Dimethyl Ether, Synthetic Natural Gas and the diesel fraction of the Fischer-Tropsch synthesis products. We analyze different pathways to make green synthetic fuels starting from waste carbon dioxide recovered from various sources and residual biomass. Processes for biofuel production that combine high temperature co-electrolysis in a solid oxide electrolysis cell (supplied with renewable electricity) to a downstream catalytic reactor that upgrades further the electrolysis-derived syngas are investigated. The integrated co-electrolysis catalytic upgrade is a way to “recycle” CO₂ to either a gas or liquid fuel depending on the selected catalytic route. We have considered two different sources for waste carbon dioxide recovery: the CO₂-rich off-gas of biogas-to-biomethane plants and CO₂ that could be available from fossil fuel power plants with carbon capture. Residual biomass is the other CO₂ source analyzed in this work. Fuel demand and, the CO₂ and renewable electricity resources available on a regional scale have been assessed to develop a case study of synthetic fuels production in Italy. Results show how synfuel production from biogas off-gas and residual biomass allow to cover a limited percentage of the overall fuel demand (ranging from 1.5% to 37% depending on the synfuel), while using carbon dioxide recovered from fossil power plants could potentially provide enough synfuel to cover completely the fuel demand using Dimethyl Ether or Synthetic Natural Gas. However, an increase of renewable electricity would be needed (ranging from 50% to a three-fold increase depending on the synfuel). Finally, the impact of fuel substitution on the overall carbon dioxide emissions has been investigated showing that a complete substitution of fossil fuels using synfuels produced from waste recovered CO₂ could lead to an emission reduction of almost 40% with respect to the current fossil fuel dominated scenario.