When biofuels are used in both demonstration and commercial flights, it is common for them to be mixed with standard Jet A fuel. Is this due to safety reasons, cost or due to specific properties (e.g. freezing point of biofuels) that may not enable them to be used solely all the time?
The issue with the biofuels is the low aromatic content, which changes the behavior of the fuel tank sealants. That's why blends are needed for the time being.
(...) very low aromatic contents may cause seals and sealants to shrink and harden leading to acute or chronic failure. Unfortunately, most of the material qualification tests are more concerned with excessive swelling than shrinkage and there is little guidance offered as to an acceptable level of shrinkage or other changes in physical properties related to low aromatic content (NASA).
At this stage, approval has been given for airlines to operate flights using the BtL and HEFA processes up to a limit of 50% biofuel and 50% conventional fuel. This is a precautionary measure enabling the industry to start using biofuels while additional assessments are undertaken on the need to maintain required levels of aromatic content in fuels. Aromatics are hydrocarbons found naturally in fossil-based fuels and are a necessary component for conventional jet fuels, forming up to 25% of the volume. They are not found in biomass-sourced fuels and this restricts the use of jet biofuels to 50% blends in order to guarantee aromatic content. The restriction allows time for an assessment of whether a synthetic aromatic source needs to be developed (Air Transport Action Group).
Biofuels and FT fuels are very poor in aromatic content. This causes problems with seal shrinking and thus aromatics must be added to drop-in fuels in flight tests such that a concentration of 8% is achieved (European Commission).
(...) aromatic hydrocarbons are more aggressive and cause more swell in seal materials than do paraffinic hydrocarbons
After the second cycle [of a jet fuel analog], the volume swell dropped from +22% to +13% as would be expected from the lower aromatic content in the fuel. However, this second cycle also caused the compression set to increase dramatically from 3% to 43%. Upon drying, the material again returned to its original volume, and compression set improved slightly. The reason for this significant increase in compression set is not obvious, but it is concerning (SAE International).
Trivia: there's been at least one demonstration flight on a jet that used 100% biofuel.