Abstract: A furnace or generator for reacting a hydrocarbon gas which undergoes a reaction having an exothermic phase followed by an endothermic phase, is disclosed. The generator consists of concentric inner and outer tubes having a reacting gas inlet at the top of the inner tube and a reacted gas outlet near the top of the outer tube. The inner tube has holes adjacent its bottom for communicating the bottom of the inner tube with the annular space between the inner tube and the outer tube. The bottom portion of the generator forms a reactor portion while the top portion forms a recuperator portion. A gas air mixture entering near the top of the inner tube is heated in the recuperator portion by the reacted gas in the annular passage. Additional heating means may be provided.