Abstract: A downflow hearth furnace includes a refractory-lined furnace lid. A burner is thermally coupled through the lid. A combustible material conveyor system communicates with an entry port. A variable speed motor driven rotatable combustible material disperser is positioned under the entry port and coupled to a rotation shaft. A refractory-lined furnace shell has a top edge mating with a bottom edge of the furnace lid and can be raised and lowered between an open position and a closed position. A bed of gas-permeable heat resistant material suspended on a layer of filter material defines a bottom end of a hearth disposed above a plenum. An outlet duct communicates with the plenum and has a horizontal outlet flange. A fixed scrubber input duct has an inlet flange positioned to mate with and form a seal with the outlet duct flange when the furnace shell is in the closed position.