problem is that bio-diesel would probably freeze or start to gel at higher altitudes unless some sort of additive could be found to stop this from happening.

Turbine fuel systems recirc the fuel through the hot engine chassis to keep it warm. You could modify this system to provide more heating, insulate the fuel tanks, or add heaters. The problem is that either very extensive mods, or a clean-slate design would be required...

For the mid-term, the best solution for large turbine airlines is alternatively produced Jet A (from coal or natural gas). This sort of fuel is so similar to petroleum jet A that no aircraft mods should be required. The US Air Force has actually embarked on wholesale project to shift to 50%+ artificial Jet A in the next few years. They are doing this mainly for national security reasons to ensure they have a fuel supply in case we get cut off, but it's really good news in that they are laying the groundwork for commercial aviation to follow.

Long-term we may be able to come up with an alternative, non Jet A, fuel which is suitable for airliners, but there are several hurdles:

- The industry and/or government must decide what fuel to use.

- New aircraft must be designed and built. Kerosene burners will not be able to function effectively using anything which is much different from Jet A.

- Fuel supply infrastructure must be developed.

All of this is phenomonally expensive (hundred$ of billion$). The only incentive to using non-kerosene instead of alternative Jet A would be emissions I think. It's possible that large turbine aircraft will always use petroleum an/or alternative Jet A...it has so many characteristics which make it ideal for large airplanes.