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Hydrothermal processes

A process in water at elevated temperatures and pressures is called “hydrothermal”; the term originally comes from geology; the illustration shows a simplified overview of various hydrothermal processes along with the water vapor pressure curve. The higher the temperature required for a certain process, the higher the pressure must be so that the water does not evaporate. If the biomass is to be converted biochemically, it is often first digested hydrothermally. The best-known process of this type is the so-called "steam explosion", in which the abrupt reduction in pressure evaporates the stored water and literally tears apart the structures of the biomass. In the range around 200 ° C, the production of carbon-rich solids, which is called hydrothermal carbonization, is successful. At slightly higher temperatures, a method for producing hydrogen with the help of noble metal catalysts is known (“aqueous phase reforming”). At approx. 300-350 ° C, biomass can be converted into a highly viscous tar, which has a higher calorific value and a higher viscosity as well as a lower water content than liquefaction products from "dry" processes. Syntheses of individual substances, e.g. B. furfural erythrose, glycolaldehyde and lactic acid are known from carbohydrates. From these substances, in turn, further basic chemicals and finally z. B. Plastics can be produced. In the vicinity of the critical point (371 ° C, 221 bar) biomass can be converted to methane and carbon dioxide (catalyzed near-critical biomass gasification) in the presence of a noble metal catalyst, and at higher temperatures of over 600 ° C to predominantly hydrogen and carbon dioxide ( supercritical biomass gasification). Typical of hydrothermal processes are compared to "dry" processes that were developed from coal conversion:

  • lower temperature
  • higher pressure
  • higher yields of the desired product, e.g. B. Coal
  • Wet biomass can be converted directly without prior drying.

further reading

  • A. Sinag, A. Kruse, and P. Maniam,
    Hydrothermal conversion of biomass and different model compounds,
    The Journal of Supercritical Fluids 71 ​​(2012) 80-85.
  • A. Sinag, T. Yumak, V. Balci, and A. Kruse,
    Catalytic hydrothermal conversion of cellulose over SnO2 and ZnO nanoparticle catalysts,
    Journal of Supercritical Fluids 56 (2011) 179-185
  • A. Kruse,
    Supercritical water gasification,
    Biofuels, Bioproducts and Biorefining 2 (2008) 415-437.
  • A. Kruse and H. Vogel,
    Heterogeneous Catalysis in Supercritical Media:
    2. Near-Critical and Supercritical Water, Chemical Engineering & Technology 31 (2008) 1241-1245.
  • A. Kruse and E. Dinjus,
    Hot compressed water as reaction medium and reactant: Properties and synthesis reactions,
    The Journal of Supercritical Fluids 39 (2007) 362-380.
  • A. Kruse and E. Dinjus,
    Hot compressed water as reaction medium and reactant:
    2. Degradation reactions, The Journal of Supercritical Fluids 41 (2007) 361-379.