Abstract: A method and an apparatus for treating materials in a combustor are disclosed. A pair of axially aligned slide gates extend longitudinally through the combustion chamber of the combustor. The innermost ends of the gates abut when the gates are closed, with the gates being openable by moving them axially apart to define a treatment aperture. Movement of the two gates permits the aperture to be positioned at a desired location in the combustion chamber, while movement of one gate with respect to the other adjust the width of the aperture.