Abstract
The current environmental situation requires immediate action, but the transition to a zero-emission world does not happen overnight. Biofuels will play a significant role in those sectors which are hard to electrify, such as marine and aviation transport. Hydrothermal liquefaction has shown to hold the potential to be an interesting technology to produce drop-in biofuels. There are though still open challenges before a successful scale up can take place. The most critical ones are related to clogging and aqueous phase valorization. To tackle these problems, it is important to efficiently test different solutions. The thesis provides contributions on diverse levels and distinct aspects. The first contribution faces the lack of a continuous, inexpensive, and fast analytical technique, to screen a wide range of substrates and/or hydrothermal treatment conditions. Indeed, it has been demonstrated that high-pressure calorimetry (DSC), can be used for in-situ analysis of hydrothermal processes. This technique has been adopted to continuously assess the heat release profile of cellulose across the hydrothermal spectrum. The results show that both hydrothermal carbonization and liquefaction are exothermic, with enthalpy changes of 0.3–0.9 and 0.9–1.2 kJ g−1 , respectively. The heat release profile curves have been deconvoluted and a set of reactions assigned to the peaks. Furthermore, a thermodynamic transition in overall process enthalpy has been found at the transition from HTC to HTL, which can be explained by the relative importance of hydrolysis, polymerization, and bulk carbonization reactions within the two processing regimes. This approach has the potential to speed up and reduce the costs of an initial screening of potential feedstock, catalysts, or conditions. As an application, different catalysts have been added to the cellulose and their effect discussed. Moving from the micro to the pilot scale, a continuous flow reactor was built to focus on the study of the clogging problem. It is well known that the biocrude produced with HTL cannot be directly used as a drop in biofuel. It shows in fact a high viscosity, high oxygen, and nitrogen content, and if present in the input feedstock, even sulphur. Therefore, an upgrading step is required. After this step, the oil can be distilled to obtain different fuel fractions, such as gasoline, fuel jet, diesel and so on. Unfortunately, the distillation has some residue, composed by all the compounds having a boiling point higher than 350 °C. The amount of residue depends on the HTL process conditions and on the feedstock, but in some cases, as for lignocellulosic biomass it can be up to 40 %. In this regard the thesis provides a contribution, proposing a valorization strategy for the distillation residue. Supercritical water upgrading tests have been performed at the Aalborg University on a residue coming from the distillation of an upgraded sewage sludge oil. It has been identified as optimal condition 450 °C and 60 minutes of residence time. At these conditions, the GC-MS analysis clearly showed a breaking down of the heavy molecules of the residue, yielding lighter compounds as n-paraffins. Combining the mass balance, CHN-O, simulated distillation, and total nitrogen, it was possible to confirm that, at the optimal conditions, 50 % of the residue could be converted in fuel fractions. Moreover, the nitrogen in the residue remains in the heavy fraction and is not transferred to the recovered fuels. Ultimately, the upgraded oil could be very easily separated from the water, making overall the proposed strategy very appealing from an industrial standpoint. As a last contribution, a thermodynamic model based on the Gibbs free energy minimization has been proposed. The model has been compared with literature results showing a good agreement in terms of oil yield and HHV.