Kinetic analysis of hydrothermal carbonization using high-pressure differential scanning calorimetry applied to biomass

Assessing kinetic parameters of hydrothermal carbonization still requires several tests and time-consuming analyses, usually relying on few data points. With a view to containing the experimental effort and testing the accuracy of models based on continuum curves, high-pressure differential scanning calorimetry was used for the first time to assess the heat release profile, the overall enthalpy change, and the kinetic parameters of a hydrothermal carbonization process in two substrates: digestate and sludge. The process-related thermal effect was established from the difference between the calorimetric curves for the unreacted samples during a first run and for the reacted samples obtained through a second run. The parameters of a kinetic model built on the Arrhenius reaction were adjusted on the basis of the calorimetric curves, adopting an nth-order and an autocatalytic reaction model. Activation energy values of 139.16 and 161.68 kJ/mol, pre-exponential factors around 2.15 x 10(11) and 2.52 x 10(14) s(-1), reaction orders of about 2.68 and 2.46, and R-2 values of 0.86 and 0.94 were obtained for digestate and sludge, respectively. Results were consistent with the available literature. Autocatalysis was negligible for both substrates, so the process could be modeled effectively as a single Arrhenius nth-order reaction with significant loss of precision. Coupled with more traditional approaches, this newly-proposed method may pave the way to describing the set of reactions taking place during hydrothermal carbonization by means of their enthalpy values, improving our knowledge of the process's chemistry and kinetics.