The Paris Agreement’s objectives related to climate change put aviation under great pressure and environmental inspection. In particular, the aviation industry is committed to achieving a 50% reduction in CO₂ emissions by 2050 compared to 2005 levels. A shift to alternative aviation fuels seems imperative. The International Air Transport Association (IATA) has identified the production of drop-in sustainable liquid fuels (SAFs) as the most promising strategy, at least short term, to reduce the environmental impact of the sector. Within this review, a critical summary of the current alternative aviation fuels/pathways is presented and a comparative analysis of the dominant technologies is performed considering techno-economic assessment, environmental evaluation, and future projections. The impact of the ‘ReFuelEU Aviation’ initiative on the current dominant policies and market incentives is assessed. Hydroprocessed esters and fatty acids (HEFA), Fischer–Tropsch (FT) synthesis, alcohol-to-jet (AtJ) conversion, and e-fuel pathways are put under the microscope. A wide range of potential fuel selling prices (0.81–5.00 EUR/L) was observed due to the presence of multiple routes, while some pathways seem able to secure more than 90% emission savings compared to the fossil jet reference. The accelerated scale-up of SAF production is a reasonable demand for the aviation industry. The establishment of a sustainable scale-up framework and the alignment of all of the involved aviation stakeholders is an immediate challenge.

Around 65% of the mitigation needed for the targeted net-zero carbon aviation emissions in 2050 is expected to come from Sustainable Aviation Fuels (SAFs). In this study, an alternative gasification-driven Biomass-to-Liquid (BtL) concept for the production of SAFs is introduced and evaluated. In particular, a fuel synthesis scheme based on the double-stage fermentation of the produced syngas (syngas → acetic acid → TAGs) is assessed instead of the conventional Fischer-Tropsch (FT) or Alcohol-to-Jet (AtJ) synthesis. The objective of the present work is the techno-economic evaluation of a large-scale (200 MWth) replication of the mentioned BtL concept, whose performance has been simulated in Aspen Plus^TM (V.11) with reasonable upscaling considerations and models validated at a pilot scale. The estimated baseline Total Capital Investment (TCI) of €577 million lies in the typical range of €500–700 million that many recent techno-economic studies adopt for gasification-driven BtL plants of similar capacity, while the estimated annual operating costs of €50 million correspond to a 15–40% OpEx reduction compared to such plants. A discounted cash flow analysis was carried out, and a baseline Minimum Jet Selling Price (MJSP) equal to 1.83 €/L was calculated, while a range of 1.38–2.27 €/L emerged from the sensitivity analysis. This study sets the biological conversion of gasification-derived syngas into triglycerides (TAGs) as a promising alternative route for the production of SAFs. In general, gasification-driven BtL pathways, led by the relatively mature FT and AtJ technologies, are capable of thriving in the coming years based on their capability of advanced feedstock flexibility.

An integrated thermochemical-biochemical Biomass-to-Liquid (BtL) pathway for the production of aviation and maritime liquid fuels from biogenic residues is introduced. The presence of a semi-commercially proven technology like Dual Fluidized Bed Gasification (DFBG) ensures extended fuel flexibility, syngas of high quality, complete fuel conversion, and optimal heat integration while avoiding CAPEX (Capital Expenditure) intensive equipment like air separation unit. Then, a two-stage biochemical route is proposed: initially syngas fermentation (anaerobic) into acetate and subsequently acetate fermentation (aerobic) into targeted triglycerides (TAGs) that will be finally purified and hydrotreated to form the desired drop-in biofuels. The tolerance of the bacteria to syngas contaminants minimizes the gas cleaning requirements. Moreover, the low-pressure requirements (1–10 bar) along with the mild operating temperatures (30–60 °C) reduce drastically the capital and operational cost of the process. The biological process of syngas fermentation inherently has limited side products, a fact that reduces the risk of deactivation of hydrotreatment catalysts. Heat and mass balances are calculated for the proposed concept via full-scale process simulations in Aspen Plus™ assuming a thermal input of 200 MW_th with crushed bark as feedstock. Three different operational scenarios are examined mainly through overall performance indicators such as carbon utilization (CU) and energetic fuel efficiency (EFE). Competitive performance compared to technologies that exploit similar feedstock (i.e., biogenic residues) was noticed, since values in the range of 22–27% and 31–37% were obtained for the CU and EFE, respectively. The aim of this study is to determine the appropriate key process specifications and assess the potential of the proposed concept compared to other competitive technologies.