Putting the yield and sustainability debates aside (cwatters is not wrong), there are several key technical issues - the most important in my opinion being cold flow properties for FAME/biodiesel and various other issues such as sticking on standing and effect on catalytic converters for Bioethanol. While the biofuel beads and sandals brigades would have you believe otherwise, there are very real technology limits for the use of biofuels - one of the reasons that Volvo believe that B7 (7% Biodiesel in Diesel) is currently the practical limit with normal engines even though B100 is happily on sale in some parts of Europe.
For aircraft paraffins I suspect that the limiting factor will be lubricity - and that is related to the very low levels of Sulphur in Biofuels. There have already been fleet fuel pump failure incidents (NZ comes to mind) when severly hydrotreated conventional kero caused failures in the domestic fleet (but not the longhaul fleet which of course fills up all over the world).
Another big issue is going to be jet fuels from the Middle East (eg the Qatar Shell/Pearl Gas To Liquids GTL project). The fuels are of extremely high quality but because Sulphur has to be removed from the gas (because it poisons the catalyst used to convert to a liquid) the resultant fuel is very low in Sulphur. Seals corrosivity/swelling/shrinking is also an issue.
None of the above issues are insurmountable with the use of additive packages.
Yusuf Danet hits on an important point - whatever ends up being used it will have to be a "drop-in" replacement. The problem is going to be not that the fuel meets a particular standard but that the compatibility issues between fuels derived from different biomass sources are fungible. While this happens to some extent with conventional fuels (refined from very different crudes around the world) the resulting hydrocarbon fuel is remarkably consistent and compatible. This is not the case at all with biofuels and the immaturity of the industry and the lack of understanding of compatibility issues is a huge concern. There are echoes of BA 038 in this whole debate: we thought we knew conventional Jet inside out - turns out that we don't at all.
Last edited by Pinkman; 10th Feb 2009 at 09:07.