Balancing Pursuit: Furthering the Quest for Sustainability in the Realm of Sustainable Aviation Fuels (SAF)

Demand for air connectivity coupled with the increasing forecasted passenger growth by 2040, implies an exigency in the aviation sector to adopt sustainable approaches for net zero emission by 2050. SAF is currently the immediate answer, however, ensuring further sustainability lies in the prospect of lowering carbon footprints in the entire life cycle of SAF. Despite the continuing technological advancements, the production of hydrogen used as a reagent to produce SAF cannot be omitted as a major source of fossil greenhouse gas emissions. The processing, conversion and refinement of feed entailing hydrodeoxygenation, decarboxylation, hydrogenation, isomerisation, and hydrocracking, requires substantial amounts of hydrogen for the approved ASTM routes of SAF. These processes saturate the unsaturated components in the bio-based feedstock, remove oxygen and adjust the hydrocarbon chain structure to comply with jet fuel standards. The utilised hydrogen comes from in-situ or ex-situ production processes, and the sources can range from renewables to non-renewables. A call to action has emerged to recognise the emission implications related to hydrogen usage and to overcome the environmental carbon footprint associated with the utilisation of conventional sources of hydrogen. Aside to the predictable proposal for the adoption of water electrolysis to generate hydrogen, other recommendations for environmental performance enhancement from the previous research encompass hydrothermal gasification, biomass gasification (with or without carbon capture) and biomethane with steam methane reforming (with or without carbon capture) owing to the lower greenhouse emissions compared to the fossil-based alternatives, the convincing status of the technology readiness level and the lower acidification potential. Usage of less hydrogen, identifying appropriate catalyst and increasing catalyst life in the deoxygenation process, low-cost iso-propanol as a hydrogen donor (oil-based feedstock), aerobic fermentation of sugar to 1,4 dimethyl cyclooctane with the intermediate formation of isoprene, aqueous phase reforming, single stage hydroprocessing, catalytic and co-pyrolysis of waste oil with solid feedstocks, selection of feedstock with high degree of saturation and use of monometallic and bimetallic sulphide catalyst could contribute to lowering the specific consumption of hydrogen in SAF pathways. Furthermore, endeavours relating to research and development are essential to bolster and enhance the field conducive to the seamless integration of the proposed hydrogen production processes with the existing and future SAF infrastructure. Techno-economic and life cycle assessments will provide evidence for feasibility of any chosen incorporation. Optimising the catalyst system, process conditions and automating process controls are the added benefits for the implementation of the systems. Also, breakthroughs in the material and metabolic science of the photoelectrochemical processes for producing hydrogen will have the potential to upgrade their technology readiness level. Creating a research database of the saturation level and content of sugar, lipid and oil in a range of biomass types will be an added advantage to select the appropriate feedstock for bio-based hydrogen pathways.