Biorefinery development for the conversion of softwood residues into sustainable aviation fuel: Implications from life cycle assessment and energetic-exergetic analyses

Biofuels present a strong potential to support the rapid decarbonization of the mobility sector and substitution for fossil fuels. In the aviation sector, sustainable aviation fuels (SAF) are currently produced from various feedstocks and conversion pathways to achieve sustainability targets. A new SAF production pathway has been recently developed, which is based on enzymatic hydrolysis of softwood residues (saw dust), fermentation of wood sugars into isobutene, and subsequent conversion to SAF isoparaffins by oligomerization and hydrogenation. This pathway is currently under consideration for inclusion as an additional annex to ASTM standard D7566. In this study, several biorefinery set-up scenarios including various process energy provisions and co products valorization were considered in order to assess the environmental impact of SAF production. First, a life cycle assessment (LCA) was conducted to estimate the greenhouse gas (GHG) emissions of the conversion pathway. Second, the GHG reduction potential was evaluated according to the frameworks of EU RED 2018/2001/EC and CORSIA. Third, energetic and exergetic analyses were performed to evaluate the efficiency of the biorefinery. Inefficiencies of upstream processes, such as for electricity provision, were not considered. Depending on the plant layout, the GHG emissions vary between 18.7 and 56 gCO2eq/MJ. Thus, compared to the fossil reference, GHG emission reductions of up to 80.1% and 79% can be achieved for both frameworks, respectively. Plant set-up comparisons revealed that the highest reduction in GHG emissions can be achieved when using the by-product lignin for thermal energy provision and renewable energy sources (RES) to cover electricity demand. The energetic and exergetic efficiency analyses of SAF as a single product were 11.7%–14.9% and 11%–13.8%, respectively. A lignin-CHP plant set-up revealed the highest efficiencies and has the additional benefit of covering up to 82.3% of the total primary energy demand (PED) via RES. Taking all by-products into account, the energetic system efficiency ranged from 39.4% to 50.1% and the exergetic system efficiency from 40.4% to 56.9%, respectively. The highest efficiencies were achieved with the natural gas boiler set-up and electricity consumption from the public grid. The analysis revealed the importance of utilizing all biorefinery products (main and by-products) to increase the system efficiency of the biorefinery.