Failed tactic. You're not going to be more than 20ft from them with a flamethrower and when you burn the horde, that's gonna turn their attention to you the minute it starts roaring, as well as every other zombie in the area. How fast you think you're going to run with those bigassed tanks (it's more than one) on your back? So you hide. Now you have a walking inferno (literally) following you and surrounding where you're hiding. Zombies that aren't on fire, quickly catch fire with minimal contact against those who're already burning, causing a fire that not only takes hours to go out (humans burn for a while, like candles), but possibly never given the amount of new zombies showing up for the fire show/bigassed smoke signal. If the fire doesn't kill you, smoke inhalation or oxygen depletion will. Of course, this is assuming they don't catch you while you're retreating, paw at you, breaking the hose from the tank to the ignition causing a leak which catches fire and causes the pack to flare and you to catch fire yourself and die one of the most painful death's possible. Either way... you're dead.