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The German food and paper industry requires approximately 187 PJ yearly for drying processes, corresponding to 7% of the overall German industrial energy demand. The majority of the drying processes still use gas-fired hot air driers. The evaporated water remains in the exhaust gas stream and is rarely recovered. State-of-the-art hot air drying processes require 750 kWh per ton of evaporated H2O. Recently developed electrified driers use high-temperature heat pumps (HTHP) to recover the latent energy from the water vapor in the exhaust stream and reduce the specific energy consumption to 280 kWh/tH2O. Superheated steam (SHS) driers use superheated steam instead of hot air. There are several publications on the advantages of SHS driers regarding their superior efficiency, drying time reduction, the prevention of oxidisation and the capture of volatile gases. However, their application requires exergy recovery close to the process, which often means internal heat recovery with multi-evaporation. This leads to very complex drying processes with higher investment costs compared to regular drying systems.
The proposed “GreenSteam” drying process uses both superheated and saturated steam. The saturated steam is condensed (indirect heat exchange) while the superheated steam evaporates, transports the water from the material, and leaves the drying process as saturated steam. Both exhaust streams (condensate and saturated steam) are recovered in a sub-atmospheric flash evaporator driven by a high-temperature heat pump and generate low-temperature steam. A steam compressor provides the superheated steam for the drying process. This novel concept is developed as part of the GreenSteam project and may lead to a reduction of the specific energy demand for drying processes by 65% (compared to the hot air drying with heat recovery and heat pump) to 90 kWh/tH2O. This significantly reduces CO2 emissions and energy demand and enables water recovery from drying processes.