(200a) Renewable Liquefied Natural Gas As Marine Fuel: Process Simulation-Based Analysis of Different Production Routes Including Energetic, Environmental and Economic Aspects

Maritime transportation contributes to about 3% of global anthropogenic greenhouse gas (GHG) emissions, adding up to 1.5 billion tons of CO_2,eq per year, including emissions from fuels production as well as those derived from their combustion. These emissions need to be reduced by at least 50% in absolute value by 2050, to contribute to the ambitions of the Paris Agreement. Compared to conventional fuels such as Marine Gas Oil (MGO) and Heavy Fuel Oil (HFO), using Liquefied Natural Gas (LNG) can lower GHG emissions by 20-25%, thanks to the low sulfur and nitrogen content, and to the favorable hydrogen-to-carbon ratio that reduces particle emissions. For this reason, a transition towards LNG engines has been occurring in the latest years. However, to effectively reduce GHG emissions, zero-carbon fuels produced from renewable sources are to be envisaged.

LNG can in fact be produced from renewable sources according to different strategies. Electro-LNG, or e-LNG, can be obtained from renewable electricity, which is exploited in the production of green hydrogen by means of water electrolysis. Hydrogen can then be reacted with carbon dioxide captured from the air or from some other emission sources, allowing the production of synthetic methane. In the case of capture from the air, the CO₂ balance is neutral, because the CO₂ produced by the combustion of e-LNG is sequestered and reused as a process reactant, thus closing the cycle.

Alternately, LNG can be produced by anaerobic digestion of organic matter, such as urban, agricultural and industrial organic waste (bio-LNG). In this case, the biogas obtained from anaerobic digestion (mainly a mixture of CH₄ and CO₂), has to be upgraded to meet the appropriate methane purity. The resulting CO₂ balance is neutral, as the CO₂ obtained as a secondary product of the anaerobic digestion process and produced by the combustion of bio-LNG comes from the photosynthesis process.

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 Energy (LCOE), and the Life Cycle Assessment (LCA). The estimated bunkering capacity of an average-size port in North-East Italy will be taken as reference. 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.

Results indicate that, due to the large energetic burden of green hydrogen production by means of water electrolysis, the most convenient pathway appears to be bio-LNG production, whose levelized cost of production is however 2-3 times larger than that of the fossil counterpart. In this case, however, the main issue is related to feedstock availability to meet the required production capacity.