Jet fuel, (Jet A) is relatively tame as a liquid but becomes explosive when atomized.
A jet engine is basically a series of continuous controlled explosions where nozzles continuously ejects atomize fuel into combustion chambers where it continuously explodes.
Temperatures in the modern jet engine burn chambers can reach as high as 2,000 degrees C, accounting for why internal parts are made of titanium, and why jet engines look so big. The engine part is pretty small, the rest is ducting for cooling air that prevents the engine from melting.
The planes that hit the towers each carried about 20,000 gallons of Jet-A when they hit, which instantly atomized the fuel.
So basically the fuel didn't burn, it exploded with enough heat and energy to melt and bring down the buildings.
By comparison, Flight 800, the 747 that exploded shortly after departing from JFK, supposedly because an electrical spark ignited the fumes produced by just a few gallons of residual fuel in the basically empty center fuel tank.
There are still questions about whether the fuel vapors on Flt 800 were ignited by an electrical spark or a dud surface to air missile, but the consensus is that the exploding vapors in the center tank are what shredded the airplane.
