Abstract: The downward burning of a vertical stack of solid fuel in a vertical furnace shaft is controlled by supplying most of the combustion air to the side of the stack through vertically spaced apart rows of combustion air supply openings in the vertical furnace shaft, each row of combustion air supply openings being generally horizontally oriented and including a plurality of separate openings. Each row of combustion air supply openings is supplied with combustion air through a separate combustion air supply channel, and the combination air fed to each supply channel is controlled by a distribution element which shifts the supply of combustion air to lower rows of air supply openings as the combustion zone moves down to the bottom of the vertical stack.