hm the spare tank is being replaced for you
but seriously, the idea is not bad, but however it should be (conditionally) 5 seconds or something like that

Name: The Fumigator (My take)
Class: Pyro
Slot: Primary
Stats:
+ Alt-Fire creates a lingering chemical cloud that can be set alight. Clouds linger for 10 seconds, and all enemies that walk into a lit cloud will be set alight, dealing 30 damage on initial ignition.
+ Clouds deal +30% extra burn damage (when compared to the stock flamethrower)
= Stores 3 clouds that are spent on alt-fire instead of ammo. Clouds can be replenished from all ammo sources.
= Pressing Alt-Fire while aimed at an unlit cloud will suck the cloud back into the flamethrower and restore the cloud to your supply.
- Clouds replace the Airblast
- -25% direct damage with Primary-Fire
- -25% flamethrower range
- -25% afterburn duration

I'm completely open to suggestions and constructive criticism so don't be afraid to tell me what you think :)