Techno-economic optimisation of biological biogas upgrading to biomethane with integrated renewable energy and storages

Biogas is a mixture of CH4 (50-75%vol), and CO2 (25-50%vol) obtained by anaerobic digestion processes. The CO2 from biogas can be converted to further biomethane via biological methanation using externally supplied hydrogen, exploiting hydrogenotrophic methanogens bacteria. This Thesis assesses the possibility of integrating a biological methanation process within an existing agricultural biogas plant. Hydrogen is produced by alkaline water electrolysis powered mainly by solar electricity. To address renewables intermittency, intermediate storage for H2, CO2 and solar electricity are employed. A techno-economic optimisation of biological methanation process performed in GAMS using solver CPLEX is proposed. The optimisation considers mass, energy and economic balances, to minimise the total annual cost. The main optimisation outputs are the hourly profile of each stream, revenues and process units capacity and their costs. Several case studies are taken into account, in particular continuous operation is assessed across four pressure settings (1, 3, 6, and 9 bar) to assess the pressure effect on the process and evaluate the best operating conditions. Also, the possibility of intermittent operation at 9 bar is explored by allowing the reactor to temporarily shut down. The results obtained in continuous operation consider breakeven selling price biomethane higher than 1500 €/tCH4. Among the continuous operation case studies, the 9 bar operating pressure obtains the best results from an economic point of view. The photovoltaic panels and the electrolyser results in the most expensive process units, contributing more the 85% of the total annual costs, followed by hydrogen tanks. Among all the case studies, the intermittent case scenario emerges as the best operating condition from an economic perspective, since it features the lowest biomethane selling price (<1500 €/tCH4).