Abstract: A system is provided for burning gaseous fuel in a burner which defines a first combustion zone and a second combustion zone, the zones being in communication so that materials from one zone can have access to the other. First and second conduits deliver gaseous fuel to the first and second zones, and are provided with first and second flow control valves, respectively. A third conduit allows pressurized air from a source of air to flow through the first combustion zone such as to discourage materials from the second combustion zone from entering the first combustion zone, and a fourth conduit does the same for the second combustion zone. The third and fourth conduits have third and fourth control valves, respectively.