A method and apparatus for heating materials is described. The apparatus is a furnace that includes multiple gravity-feed trays and a heat transfer fluid that heats material by the heat evolved during phase change. The apparatus also includes moving paddles that urge the material through each tray. The method provides for the torrefying of the material using a phase-change heat-transfer fluid by providing the material sequentially to at least two trays, where the at least two trays are substantially horizontal and disposed at different vertical heights; condensing the vapor phase at a temperature; and providing heat from the condensing the vapor phase to the material, where the temperature is sufficient to torrefy the material.