Experimental comparison of hydrothermal and vapothermal carbonization

The difference between hydrothermal carbonization and vapothermal carbonization for the densification of the energy content of biomass has been investigated systematically for the first time. Vapothermal carbonization allows for higher solid content (solid biomass mass (dry basis) per total mass of feedstock) in the reactor because the biomass is subject to saturated steam instead of liquid water. Results from the experiments show that the process efficiency can be increased due to two reasons: the carbon losses in the liquid phase are decreased and less water needs to be heated up during carbonization. It was also observed that the carbon content of the solid product is significantly lower than that of hydrothermal carbonization at the same process conditions. As it is even lower for dry torrefaction, it is concluded that liquid water facilitates the carbonization process. Calculations based on these experimental results reveal that a mechanical dewatering of wet biomass increases the process efficiency of hydrothermal processes and should be considered in practice. Due to the low efficiency of state of the art drying, torrefaction is less efficient than vapothermal carbonization.