combustion

No.

The compressed air, when escaping, will throw the flame across enemy lines. Which is good if your in a battle, but bad in your in a plant. The LPG needs to be mixed with air to burn in the first place, so by the time you add the compressed air, then reduce the pressure back to ambient pressure, you've saved nothing and added the cost of the compressed air.

No LPG is the fuel the oxygen in the air is whats need for it to burn. If you mix the two all you would be doing is supply less fuel. The fuel amount would remain the same to provide the same amount heat.

Both of the answers prior to mine agree, the answer is NO!. But neither of them pointed out a MAJOR problem. Once you mix the LPG with air, you have a flammable mixture. if your flame somehow gets past the orifice into the pipe carrying the mixture, it can then burn all the way back to the point where you mix the air and LPG. If you have a tank of this mixture, you literally have a BOMB. Pure LPG, either as a liquid or as a gas, can't burn until it gets mixed with oxygen of one form or another, so there is no danger of burning back into the pipes.