Renewable Liquefied Natural Gas (LNG) as a carbon-neutral marine fuel: analysis of the energetic, economic, and environmental performance of different production routes

Liquefied Natural Gas (LNG) could give a significant contribution in decarbonizing the maritime transport sector, provided it is produced from renewable sources. In this regard, different production processes could be employed, involving either CO2 methanation with green hydrogen (the so-called e-LNG), or biomass degradation through anaerobic digestion and subsequent upgrading (i.e., bio-LNG). This work aims at analyzing different renewable LNG synthesis pathways at industrial scale, and at comparing them in terms of their energetic efficiency, economic profitability, and environmental impacts by means of acknowledged quantitative performance indicators which are respectively the Energy Return on Energy Invested (EROEI), the Levelized Cost of Methane (LCOM), and the Life Cycle Assessment (LCA). In order to have a fair and robust comparison among the different processes investigated, the conceptual process design and the material and energy balances required to evaluate the aforementioned indicators have been developed by means of process simulation. The estimated bunkering capacity of an average-size port in North-East Italy is taken as reference. Results indicate that, due to the large energetic burden of green hydrogen production by means of water electrolysis, among the four renewable LNG production methods compared, bio-LNG is the most effective/efficient method. This route results in better environmental impacts than the other three renewable routes at a cost that is comparable to the fossil counterpart, provided that an income is ensured due to the waste management. However, the main issue is related to feedstock availability to meet the required production capacity.