Hydrothermal carbonization of grape marc: Experimental results and reaction kinetics modelling

In this paper grape marc, as a candidate organic waste for energy recovery, has been analyzed in terms of the characteristics of the products obtained from the hydrothermal carbonization process. Moreover, the reaction kinetics of the process have been studied and modeled, to obtain interesting information for possible industrial applications. To collect the experimental data, a 50 mL batch reactor has been used at three different temperatures (180 °C, 220 °C and 250 °C) and three residence times (1 h, 3 h and 8 h). Solid, liquid and gaseous masses have been measured and gas composition, elemental analysis and calorific value of solid have been evaluated. The yields of the three phases and the results of the analyses allowed to develop a reaction kinetic model, investigating a two-step kinetic mechanism. The activation energy and the pre-exponential factors of the various degradation reactions have been determined by means of least square optimization versus the experimental data. The results obtained from the analysis of the experimental data are in good agreement with the literature. The amount of hydrochar (that is the solid-carbonized phase) and its calorific value suggest the possibility of grape marc exploitation for energy purposes. In addition, the kinetic analysis showed that a two-step reaction mechanism can be suitably used to describe the evolution in time of the hydrochar yield at different process temperatures.