Conference Agenda

With 2.5% of current global CO₂ annual emissions¹, aviation is one of the most challenging sectors to decarbonize, due to the high-energy-density fuel requirements. As such, according to the International Air Transport Association (IATA), more than 99% of the fuel used today comes from fossil origin². Ongoing decarbonization efforts include scaling the use of sustainable aviation fuels (SAF), which are drop-in replacements to fossil kerosene. They are expected to provide most of the sector’s carbon abatement until 2050, contributing to over 60% of the reduction in emissions required for aviation to reach a net-zero scenario^2,3.

SAFs can be produced from different carbon feedstocks, such as biomass, organic waste and captured CO₂, which, combined with the available array of synthesis technologies, results in several potential pathways. Nevertheless, there is limited literature on the simultaneous assessment and comparison of the alternatives for SAF production, while existing studies provide limited insights into the global environmental implications of their large scale deployment.

In this work, we analyzed different renewable carbon sources and pathways to produce SAF and compared them to fossil jet fuel production routes based on absolute sustainability criteria. Differently from standard life cycle assessments (LCA), which are suitable for comparing different products or processes, absolute sustainability assessments can provide insights into their environmental performance relative to the planet’s ecological limits and carrying capacity⁴. These limits, known as planetary boundaries (PBs), define together the safe operating space (SOS) for anthropogenic activities⁵. As environmental assessment studies of SAF production have focused on relative LCA so far, this is the first time, from the authors’ knowledge, that PBs are incorporated in a SAF production study.

Different pathways for SAF production, differing in the carbon feedstocks and production technologies, were evaluated through the PBs framework and the transgression levels relative to the SOS were quantified. The processes were simulated in Aspen Plus v12 and the respective life cycle inventories were modeled using the mass and energy balances results. The environmental assessment was carried out in Brightway2 v2.4.6 using the Ecoinvent database v3.10 and the Environmental Footprint (EF) method v3.1 1 in combination with the LANCA v2.5.

Our results showcase the trade-offs between the different strategies for SAF production and the benefits compared to the fossil routes. Moreover, they highlight the importance of policy support to promote SAF production.

References

1. Ritchie, H. & Roser, M. What share of global CO₂ emissions come from aviation? Our World in Data (2024).

2. International Air Transport Association (IATA). Executive Summary Net Zero Roadmaps. (2023).

3. McCausland, R. Net zero 2050: sustainable aviation fuels. (2023).

4. Sala, S., Crenna, E., Secchi, M. & Sanyé-Mengual, E. Environmental sustainability of European production and consumption assessed against planetary boundaries. Journal of Environmental Management 269, 110686 (2020).

5. Rockström, J. et al. A safe operating space for humanity. Nature 461, 472–475 (2009).

To reach long-term carbon neutrality in aviation, transitioning from fossil-based jet fuels to Sustainable Aviation Fuels (SAF) derived from renewable resources is essential. This study provides a comprehensive Techno-Economic and Life Cycle Assessment of a SAF production process utilizing renewable hydrogen (H2) and carbon dioxide (CO2) through the Methanol To Olefins (MTO) and Mobil Olefins to Gasoline Distillate (MOGD) pathway.

We investigated the methanol formation kinetics, comparing various models and reactor types and designs. Results indicated that models including both CO2-to-methanol conversion and the Reverse Water-Gas Shift reaction showed excellent accuracy. In addition, a water boiling reactor with external utility achieved a higher single-path conversion (22.6% at 314 m³) compared to an adiabatic reactor (20.9% at 314 m³), with optimal conditions identified at 450 K and 75 bara. Whereas previous studies mainly examined temperature and pressure effects, the influence of reactor design parameters appeared to be overlooked for industrial implementation. This in-depth study provides new insight into how the methanol reactor design impacts its conversion rates. Furthermore, it highlights the significant impact of the methanol reactor design on Capital and Operational Cost.

While MTO and MOGD reactors were designed based on experimental conversion, this study also emphasizes the final distillation process. We determined that a two-column distillation system was necessary to reach a purity of 97% C8-C16 hydrocarbons for 38 kT/year of kerosene. Detailed sensitivity analyses on distillation parameters, including boil-up ratio, reflux ratio, and column sizing, enabled to achieve optimal parameters of 1.1 for boil-up ratio and 1.7 for reflux ratio.

Economic evaluation established a Minimum Selling Price of $2.46/kg, higher than the current fossil jet fuel cost of $0.68/kg but consistent with prior SAF studies. Additionally, our uncertainty analysis on the Minimum Selling Price showed a standard deviation of 2.30, due to the uncertain H2 price, highly dependent on innovations in performance, cost-breakdown of electrolyzer and renewable electricity production technologies. Moreover, the sensitivity analysis revealed high sensitivity to reactant prices, particularly for H2. Finally, the plant's Life Cycle Assessment was performed, achieving a carbon footprint of 0.67 kgCO2eq/MJ, aligning with European regulations.

This comprehensive study underscores the economic and environmental potential of the SAF production process. By identifying the optimal methanol reactor design configuration, it demonstrates the importance of development of an advanced methanol formation model. These results offer a novel perspective on SAF production optimization and the necessity of realistic industrial design specifications assessment and selection.

The use of sustainable aviation fuels (SAFs) is pivotal in gradually replacing fossil kerosene and lowering the carbon emissions without changing the existing infrastructure. One of the pathways to produce SAFs is through the Fischer-Tropsch synthesis (FTS) process. FTS is a catalytic reaction which converts a mixture of CO and H₂ (syngas) into a variety of hydrocarbons products. The syngas can be produced from reforming process at high temperatures from biogas and/or CO₂ as the carbon sources, which can reduce the lifecycle carbon emissions by 50 to 100% (Peacock et al., 2024).

The present work proposes an integrated process for SAFs production from biogas through reforming process, Fischer-Tropsch synthesis (FTS) and solid oxide electrolysis process. Aspen Plus v14 is used to develop rigorous kinetic models for biogas reforming, FTS and hydrocracking of the resulting heavy fractions, based on established kinetics from literature (Park et al., 2014; Todic et al., 2013), followed by distillation to the final fuels’ cuts (naphtha, kerosene and diesel). Different scenarios for H₂ supply (upstream or downstream of the reformer) and tail gas recycling (to FT or to reformer) are assessed based on the developed integrated process model. The technical evaluation is assessed with several key performance indicators, such as carbon efficiency and process efficiency.

The simulation results showed carbon efficiencies between 79.9% and 95.1%, and process efficiency between 40.2% and 41.7%. The lower process efficiency is due to the high energy consumption required for hydrogen production in the SOEC process. The developed model sets the basis for further optimization of the design and will facilitate the evaluation of the economic and environmental impacts, determining both the production cost of SAFs and their carbon reduction compared to conventional jet fuels. Furthermore, projection cost analyses are essential to predict future decreases in fuel production costs.

References

1. Peacock J, Cooper R, Waller N, Richardson G. Decarbonising aviation at scale through synthesis of sustainable e-fuel: A techno-economic assessment. Int J Hydrogen Energy 2024;50:869–90.
2. Park, N., Park, M.-J., Baek, S.-C., Ha, K.-S., Lee, Y.-J., Kwak, G., Park, H.-G., & Jun, K.-W. (2014). Modeling and optimization of the mixed reforming of methane: Maximizing CO2 utilization for non-equilibrated reaction. Fuel, 115, 357–365.
3. Todic, B., Bhatelia, T., Froment, G. F., Ma, W., Jacobs, G., Davis, B. H., & Bukur, D. B. (2013). Kinetic Model of Fischer–Tropsch Synthesis in a Slurry Reactor on Co–Re/Al 2 O 3 Catalyst. Industrial & Engineering Chemistry Research, 52(2), 669–679.