A process and apparatus for drying material, particularly sludges, with indirectly heated driers includes a system for the thermal decontamination of exhaust air from the drier. Excess exhaust air from the drier loop is fed as secondary air to the heating boiler or furnace separately from the primary air for combustion of the fuel and to thermally clean the exhaust air from the drier. The exhaust drier gas from the drier remains in the furnace for a period to decontaminate the drier gas, after which it is mixed with the flue gases. The mixture is then removed from the furnace. The system is suitable for operation under normal operating temperatures and at low, below normal operating temperatures.