Hot gases are injected by a gas nozzle located beneath the furnace tube. The furnace tube preferably includes a lower, frusto-conical section terminating in a heat input orifice. The furnace nozzle is adapted to be positioned at a variable, verically spaced-apart distance from the nozzle to effectuate desired flame control. Preferably a flame guide is secured at a selected variable position between the nozzle and the heat input orifice. The flame control includes a central tubular passage way having a lower skirted portion for guiding the flame interiorly of the passageway. An upper, generally. Ring-shaped downwardly tapered rejection plate is se cured coaxially at the top of the flame guide. The outer diameter of the rejection plate is substantially greater than the diameter of the input orifice of the furnace tube. However, the flame guide is positioned in such a manner that heavy, unexpanded waste particles typically falling from the furnace tube abut the inclined edges of the rejection plate, and are thus prevented from falling into the flame guide passageway or the lower furnace nozzle. In this fashion waste residue is positively controlled to insure adequate and dependable internal heat distribution and material treatment, while maximizing operator safety.