Flying with 100% sustainable aviation fuels (SAF)

Airbus and Rolls Royce recently announced that they were testing 100% sustainable aviation fuels in flights. These flights will test the impact of 100% SAF on operability and emissions.
Current ASTM D7566 certification allows the use of SAF in blends of 50% for most technology pathways, including HEFA, Fischer-Tropsch and alcohol-to-jet.
One of the major reasons for only allowing 50% blends is due to the lack of aromatics in the SAF produced through these pathways. A minimum of ~8% aromatics is required in jet fuel, up to a maximum of ~25%, as the elastomer seals in aircraft and engines need aromatics to prevent leakage. Current commercially available SAF, HEFA, produced through the hydrotreatment of fats, oils and greases (lipids), are entirely paraffinic in nature and contain no aromatics. The aromatics are therefore incorporated through the blending of HEFA with conventional jet fuel.
The project announced by Rolls Royce and Airbus uses 100% HEFA fuels which therefore contain no aromatics at all.
Why are they flying on fuel with no aromatics, how will this impact the seals in the aircraft, and what are the implications for ASTM compliance?
The main reason for flying on 100% SAF without aromatics is due to the high soot formation during combustion of aromatics. These soot particles form nuclei for the condensation of water vapour and, under the right temperature and humidity conditions, can form ice particles directly leading to cirrus cloud formation.
Cirrus clouds impact the net radiative force and climate impact of aviation, much greater than the impact of CO2 formation from fuel combustion. Reduced aromatics lead to a reduction in soot and a reduction in cirrus cloud formation, thereby significantly reducing the climate impact of aviation. Flying on 100% paraffinic SAF can therefore have a dramatic impact to reduce the climate effects of aviation (also see article).
But how are the seals in the aircraft affected? According to Rolls Royce, modern engines and aircraft contain seals that are not affected by the lack of aromatics and therefore no leakage is observed. However, this will not be the case in older aircraft.
This could likely be problematic for amendment of the ASTM D7566 standard from 50% blends to 100% as safety could be impacted if the fuel is used by older aircraft. At this stage, however, no application is in the pipeline for amendment of the ASTM certification. If companies intend to regularly fly on 100% SAF, this will have to be addressed.
My immediate reaction on reading about the use of 100% SAF was slightly cynical as I questioned when there will likely be enough SAF to fly with a 100% SAF load. Commercial production and availability of SAF is still less than 0.1% of the global jet fuel demand.
However, according to Steven Csonka at CAAFI, the other reason for demonstrating the safe operation of aircraft on 100% SAF is the potential application of the book-and-claim system.
A book-and-claim system will essentially separate the physical SAF volume from its sustainability characteristics, allowing airlines to purchase SAF and the associated emission reductions without physically using that specific batch of fuel on the specified flights. The system will allow production of SAF and local use at an airport near the production facility, rather than transporting the SAF to every single airport, however remote. By implication, large volumes of SAF could therefore be used at one airport, while none is used at other airports. The overall impact will still be a reduction in overall emissions in the aviation sector. By allowing very high blends of SAF, up to 100%, a greater impact on emissions can be achieved as commercial availability of SAF increases.