Development of an integrated process between Hydrothermal Liquefaction and Wet Oxidation: from batch experiments to software simulations

Global warming has forced humanity to find sustainable energy resources for the transport sector, which ranks as the second most polluting emitter accounting for more of 25% of total greenhouse gases emissions. In response to this challenge, the European Union (EU) has taken proactive measures by setting ambitious targets aimed at transitioning away from fossil fuels towards electric mobility and the adoption of low-carbon fuels. These ones include biofuels derived from biomass feedstock, with a preference for utilizing waste materials as a sustainable source. One of the most promising technologies for biofuel production is hydrothermal liquefaction (HTL). HTL is a thermochemical process that involves biomass and water at high temperatures and pressures, operating close to the critical point of water for sufficient time, mimicking the natural fossil fuel formation. The main product is a valuable biocrude with good heating value, that after an upgrading step can be used as drop-in biofuel. The HTL process also generates three significant byproducts: a gaseous phase, mainly composed by carbon dioxide; a solid residue and an aqueous phase. The latter one contains a significant amount of the initial carbon and represents one of the main bottleneck for the sustainability of the process from both energetic and environmental point of view. An emerging technology for the aqueous phase treatment is wet oxidation (WO). WO is an exothermic hydrothermal process in which the degradation of the organic compounds happens under an oxidative atmosphere. It can work with highly polluted waters within a range of pressures and temperatures similar to the HTL process. This work investigates the possible heat integration of the two processes, especially looking at the possibility of obtaining an overall autothermic process. The feedstocks under investigation were wheat straw and cow manure, those not only are abundant residues of agricultural industry, but have synergistic effect that results in higher final oil yields. First a batch experimental campaign was conducted to test the HTL at different operative conditions (300-350°C / 10-30 min). In all conditions the various products yields were calculated and carbon and nitrogen balances were evaluated. Subsequently, based on experimental and literature data and through Aspen Plus® software, the wet oxidation process was simulated in different conditions to investigate the amount of heat released and the output stream composition. Finally the software MATLAB was used to calculate the optimal network configuration using as input the data of both HTL tests and WO simulations. The focus was to allocate the main energetic expenditures and the quality of the output products in order to see Energy Sustainability Index (ESI) of the combined process at different reactions conditions.