Abstract: A furnace for burning biomass such as crop residues like cornstalks and other combustibles. The furnace includes a combustion chamber and many chimney stacks extending upwardly therefrom. Loading doors are provided into the combustion chamber to permit massive batches of combustibles to be deposited in the chamber. Centrifugal combustion air draft fans are positioned to deliver forced combustion air from the atmosphere into a lower part of the chamber when needed. A heat exchanger jacket encompasses major parts of the chamber and of the stacks and is provided with an upper opening adjacent the top of the stacks and a lower opening adjacent the bottom of the chamber. Drying air to be heated enters one of the openings and heated drying air discharges from the other. Fan means adjacent the lower opening moves the drying air through the heat exchange jacket and into a crop dryer or other location for use.