Maritime transportation contributes to about 3% of global anthropogenic greenhouse gas (GHG) emissions, adding up to 1.5 billion tons of CO2,eq per year, including emissions from fuels production as well as those derived from their combustion. These emissions should be reduced by 70% in absolute value by 2040 with respect to 2008, to contribute to the recently revised ambitions set by the International Maritime Organization (IMO). Compared to conventional fuels such as Marine Gas Oil (MGO), 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 and decarbonize the maritime sector, zero-carbon fuels are to be envisaged, and LNG should be produced from alternative renewable sources rather than from fossil natural gas.

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 via the so-called methanation reaction. 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. Even in this case, this can be achieved by hydrogenating the CO2 fraction in the biogas.

The methanation step can be carried out according to the Sabatier reaction, which is a thermochemical catalytic process occurring at moderate-to-high pressures, and temperatures above 300°C to favor the kinetics. Alternatively, a biological process (the so-called biomethanation) catalyzed by hydrogenotrophic microorganisms (methanogens), which occurs at relatively mild operating conditions, may be employed. From a general perspective, finding biological alternatives to conventional chemical processes represents a more environmentally friendly option, and is therefore worth investigating.

The LNG-SEA project aims at analyzing different renewable LNG synthesis pathways (e-LNG and bio-LNG) 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 Net Carbon Footprint, accounting for equivalent CO2 emissions related to LNG production and subsequent utilization. In addition, the core of the project will be dedicated to investigating the possibility of carrying out the methanation step biologically, rather than via thermochemical synthesis. Specifically, a kinetic model of the bioprocess will be developed based on experimental data, and it will be used to evaluate the corresponding mass and energy balances at large-scale. 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 will be developed by means of process simulation.

The ultimate goal of the project is that of providing reliable quantitative information that can help decision-makers identifying a convenient pathway towards decarbonization of the maritime transport sector already at an early conceptual design stage, keeping into account technical, energetic, economic, and net overall emission factors.