Kinetic and thermal modeling of hydrothermal carbonization applied to grape marc

In the present investigation grape marc was processed in a HTC batch reactor at different operating conditions: 180, 220, and 250 °C and 1, 3, and 8 h residence time. The mass yield of the resulting phases (gaseous, liquid and solid) was measured. The hydrochar and gas compositions were evaluated through elemental analysis and gas-chromatography, respectively. The experimental results allowed to calibrate a kinetics model, based on a two-step reaction mechanism. The activation energy and pre-exponential factor of the various degradation reactions were determined by means of least square optimization versus the experimental data. A simplified dynamic analytic model was also built - based on lumped capacitance method - in order to simulate the thermal behavior of the system, using the actual temperature profile imposed by the reactor external heater. A resistance-capacitance network was used to describe the system, taking into account the thermo-physical properties of the systems (i.e. reactor shell, gaseous and liquid phases). This simplified tool supplemented with the calibrated kinetics model represents a first step in the characterization of the HTC process performance under different operative conditions.