Hydrothermal carbonization enthalpy using differential scanning calorimetry: Assessing the accuracy of the exhaust sample method

High pressure differential scanning calorimetry (DSC) is suitable to study the thermodynamics and kinetics of hydrothermal carbonization (HTC). To isolate the thermal effect of the process from the heat required to increase the DSC system temperature, a recently proposed method uses an additional run on the already reacted sample as the blank. The method assumes that the blank system is in equilibrium during the whole run; it neglects the potential for secondary reactions in the blank run, which may affect the results. This work assesses the effect of such an assumption on the final enthalpy results by performing additional re-runs on the same specimen and adopting longer isothermal segments to push the system composition closer to equilibrium. The study of HTC of cellulose at 250 degrees C demonstrates that the thermal contribution of reactions in the second run is negligible compared to the primary HTC effects in the first run and within the experimental error. A deeper investigation of the thermal effect during re-runs shows that the biggest effect of reactions in such runs is to change the system composition rather than a direct exothermicity. Further, the exothermic activity disappears after 12 hours, suggesting that the sample has reached thermal equilibrium.

Automated summary

High pressure differential scanning calorimetry is suitable for studying hydrothermal carbonization. A method using an already reacted sample as the blank assumes equilibrium in the blank system throughout the run. However, this assumption affects the final enthalpy results, as shown in our study.