TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The drying zone and/or the torrefying zone exhibit several hearths arranged one above the other. A mechanical transferring device is provided between the drying- and the torrefying zone for transferring the dried, carbon-containing material flow. The transferring device is formed in a gas-tight manner between the drying- and the torrefying zone to prevent mixing of the two gas circuits. The hearth furnace exhibits a transporting device for transporting the material flow that is driven via a driving shaft, where the driving shaft is divided between the drying- and the torrefying zone, and respectively equipped with a separate drive to alter the residence of the carbon-containing material flow independently in the two zones via the respective rotational speed. Preferred Method: The drying zone is operated in a concurrent flow, and the torrefying zone is operated in a countercurrent flow. The temperature of the carbon-containing material flow during transferring into the torrefying zone is less than 150 degrees C. The drying gas flow is filtered in the gas circuits. The steam-containing drying gas flow is overheated in the heat exchanger. The recirculated amount of vapor in the drying circuit is high, so that an inert atmosphere with an oxygen content of less than 10%, preferably less than 8% is adjusted. The recirculated amount of exhaust gas in the torrefying circuit is high, so that an inert atmosphere with an oxygen content of less than 10% is adjusted. The temperature of the exhaust gas recirculated into the torrefying zone is more than 300 degrees C, and the temperature of the recirculated drying gas flow is adjusted at 150-300 degrees C. The temperature, the gas flow, the amount of gas and/or the pressure are measured in the drying- and/or the torrefying zone, and used for regulating the residence time of the material in the two zones. The residence time in the drying- and the torrefying zone is adjusted by altering the number of the hearths, reducing hearth surfaces, altering the design and the number of the transporting devices, or varying the rotational speed of the transporting devices.