Techno-economic assessment of integrated anaerobic digestion and hydrothermal liquefaction for the valorisation of the organic fraction of municipal solid waste

Hydrothermal liquefaction is a promising technology for producing renewable fuels in Waste-to-Energy processes. Its great potential is being studied for commercial-scale biofuel production. This thesis investigates the technical and economic sustainability of valorising the Organic Fraction of Municipal Solid Waste (OFMSW) by coupling Anaerobic Digestion (AD) with Hydrothermal Liquefaction (HTL) of the digestate to produce different energy vectors (heat, electricity and biocrude). The simulation processes 60000 tons/y of OFMSW, according to data from public waste management plants in the Piedmont area, for a more plausible commercial evaluation. Mass and energy balances were performed for three different solids concentrations in the digestate after centrifugation: 8 wt.%, 17.5 wt.%, and 30 wt.%. The calculations were conducted to analyse product rates, energy demand, operating costs and possible revenue of the plant, aiming to find the optimal condition among the three. The best condition was then compared energetically and economically with the traditional method for OFMSW treatment, which includes anaerobic digestion combined with composting. For both process configurations, the biogas production is significant (647 Nm3/kgVS), and a portion of it allows the plant to be completely self-sufficient via cogeneration. The alternative configuration requires more energy, but it also produces a greater net energy output, resulting in a higher revenue. Indeed, the EROI (Energy Return on Investment) parameter for the net thermal energy produced accounts for 4.3 with respect to 3.2, and both the gross and net EROI and the NTG (Net-to-Gross energy ratio) show a slight but significant increase, confirming the energetic benefit of the proposed configuration. The final economic assessment on the operating costs shows a positive net cash flow for both configurations, demonstrating better profitability for the proposed alternative. Finally, a sensitivity analysis was performed on the uncertain values of the main cost and revenue variables.