Abstract
This thesis provides new insights into the fundamental thermodynamics and kinetics of hydrothermal carbonization (HTC) and some of its practical applications. HTC has gained importance in the scientific community due to its potential to valorize problematic biological wastes with high moisture content. Such materials would otherwise be discarded at high environmental and economic costs. The process of HTC consists in heating a biomass with up to 98 % water content between 180 and 250 °C under autogenous pressure, over a few minutes to several hours. The main products are a coal-like solid (called hydrochar), an aqueous phase that may contain a variety of organic and inorganic compounds, and a gas mostly composed of carbon dioxide.
The thermodynamic nature of this process (i.e. the assessment of the enthalpy change between reactants and products and thus its exothermicity or endothermicity) is the subject of several works in the literature. These works are not always agreed in their results. The reason for this lays in the different techniques used to assess the enthalpy change of the process: the direct application of Hess’s Law versus the use of differential calorimetry. The first option is dependent on the adopted hypotheses, and indeed provided very scattered results among different works in the literature. In the second option, carried out using a differential scanning
calorimeter at high pressure, issues were still present and low reproducibility was often found. To overcome this, a novel technique is proposed to assess the enthalpy change of the HTC of biomass using high-pressure calorimetry. This methods is applied to three substrates at one HTC temperature to assess the heat release profile of HTC and, by integration, its enthalpy change, demonstrating the exothermic nature of HTC.
This same method is then used to assess the heat release profile of HTC of two biomass substrates at three different HTC temperatures. The thermal curves obtained are then used to tune the parameters of a Arrhenius-like kinetic model of the HTC process. This model relates the heat release of the HTC process to time and temperature, and can be easily incorporated into more complex simulation tools for its detailed description.
The study of the hydrochar formation is another necessary step towards a detailed description
of HTC. The hydrochar mainly forms via two pathways. The first is the chemical modification of the solid matrix in the feedstock that reduces its oxygen and hydrogen contents, increasing its carbon fraction. The second happens through the polymerization and condensation of dissolved chemicals in the water phase that form micro-spheres which adhere onto the solid matrix. The products of the first and second paths are named primary and secondary char, respectively. The secondary char formation has been studied both using pure components, e.g. sugars, and complex substrates, e.g. food waste, as feedstocks. For complex substrates, the isolation of the secondary char relies on solvent extraction.
A systematic comparison on the use of different solvents for secondary char extraction is then presented, using different food-waste substrates as HTC feedstocks. The amount and composition of secondary char as a function of the solvent used is assessed, providing insights on the secondary char nature and on the selectivity of each solvent towards particular compounds in the hydrochar.
As ethanol shows the best performance among the tested solvents, it is used to extract the secondary char from hydrochar produced from food waste at various conditions both in the HTC range, and in the hydrothermal liquefaction (HTL) range. The HTL process is conceptually identical to HTC, but performed at higher temperatures (up to 370 °C) and higher pressures, shifting the chemical reactions in the media towards the production of an oily-phase and reducing hydrochar yield. The analysis of the secondary char from samples produced in the two process regimes sheds light on the transition between these two processes.
The last part of this thesis is dedicated to lab scale and practical applications of the HTC process. An overview of the literature concerning the coupling between anaerobic digestion and HTC is provided, with the aim of assessing the most critical aspects that have to addressed in the future.
Lastly, different approaches for the HTC product valorization are then discussed. Firstly, the results of the experimental activity performed at the EME2 Lab (Cornell University) are presented. In this case, HTC is applied to different food waste substrates using a bench scale reactor. Individual feedstocks and their mixtures are treated at several different conditions. The effect of the HTC severity and of the feedstock composition on the HTC products are assessed and the occurrence of synergistic effects due to the co-HTC of different substrates is proven. Secondly, the results of the experimental activity performed at the Bioenergy and Biofuels Lab (Unibz) are presented. Using digestate as feedstock, hydrochar and aqueous HTC liquids
are produced using a lab scale reactors. Then, the opportunity to valorize the aqueous HTC liquid produced during HTC of digestate through supercritical water gasification is presented as supercritical water gasification enables the conversion of a wet biomass substrate into a high heating value gas stream by heating it up to 500-600 °C under very high pressure.