Okay... first off... DO NOT RELY ON THIS INFORMATION as I am just guessing how it may be done, and I don't want anyone to think this would be a safe route without doing a little research first.
That being said, I assume a propane tank could be hooked up to a solenoid valve, like we use for air control. The output of the valve would be aimed over a pilot lite to ignite the flames. You would have to figure out some sort of backflow valve to keep the flames from igniting the inside of the valve or tubes (screen from a bottle of 151, possibly?)
If you used a valve like the washing machine valves we're all familiar with, you could hook the tank to one input and an air compressor to the other input and the pressure could possibly create larger, shooting flames.
Again, please don't yell about this method being stupid or not safe. I know it's not, it's just a possible way it could be done. Of course, you would want to use industrial parts made for this type of setup, not just the parts we find in our garage!