A solar-assisted liquefied biomethane production by anaerobic digestion: Dynamic simulations for harbors

This work presents a dynamic simulation model for the production of liquefied biomethane by anaerobic digestion. This layout includes anaerobic digesters supplied by the organic municipal solid wastes, biogas upgrading and biomethane liquefaction units, photovoltaic panels, evacuated solar collectors, electric and thermal storages. Solar collectors provide the heat to anaerobic digesters. Photovoltaic panels supply electricity to the electric devices of the plant. The dynamic model evaluates the electric consumption for the biogas upgrading and biomethane liquefaction units and the thermal load of the anaerobic digesters. The produced liquified biomethane is supplied as fuel to some ships traveling from the mainland to the islands of the Gulf of Naples. The simulation models of the anaerobic digester, biogas upgrading and biomethane liquefaction units are developed in MATLAB® environment and subsequently integrated in TRNSYS18, where the whole plant is modelled. The model of anaerobic digester takes into account both the biological and thermal processes and it considers the digester geometrical and structural features. The biogas upgrading unit consists of a three-stages selective membrane system, whereas the liquefaction system is modelled by the conventional Linde cycle. Significant energy savings and CO2 emissions reduction are obtained, that is a strategic outcome for harbours located in densely populated cities. When the proposed system is compared with the reference system consisting of an existing fleet, equipped with very old ferries using internal combustion diesel engines, the primary energy saving is about 50 %. Considering the present Italian funding policy for biomethane, such system exhibits a good economic profitability.