The Burner is the engine of the hot air balloon. It is what propels the hot air up into the envelope to make the balloon rise. There are many different types of burner available – single, double, up to quad systems, depending on the size of the balloon flying.
Modern hot air balloons use propane in the burner to heat the air. The propane is highly compressed in the cylinders and flows to the burner in a liquid form. When the pilot starts the burner up, the propane flows to it and is ignited by a pilot light. As the flame burns, it heats up the metal in the surrounding tubing and when the tubing becomes hot it heats the propane flowing through it.
The most popular burner is the double burner which is made up of 2 burner coils. A balloon can be powered by just a single burner, it all depends on the size of the envelope. On smaller balloons, with 1 or 2 passengers, a single system will normally suffice.
For more detailed information about specific types of burner and the technology involved, click this link which will take you to the Burner section of the Cameron Balloons website. There is a great deal of useful information here.