How Taxonomies Lead to Fuel Pathways - The Light Gas Pathway

THE LIGHT GAS PATHWAY

This category includes fuels comprised of small molecules with low-carbon/hydrogen (C/H) ratio, which helps to reduce carbon emissions, and in the case of methane (CH 4 ) high energy content. However, they require cryogenic storage and more demanding fuel delivery systems.

Such fuels include LNG, bio-LNG, and synthetic natural gas (SNG) or renewable natural gas (RNG), which can be produced from biomass and/or by using renewable energy. The production of the synthetic or renewable fuels from biomass is currently limited in scale and will have to be increased before they can be considered as viable commercial solutions.

LNG → Bio-/ElectroMethane → Hydrogen

LNG is a relatively mature low-carbon fuel, comprised primarily of methane. Its C/H ratio offers a reduction in carbon dioxide (CO 2 ) emissions of up to 21 percent compared to baseline heavy fuel oils 4 . This value does not include carbon release from methane slip, which may be an issue in two-stroke or four-stroke engines that operate on LNG in the Otto cycle.

Minimizing methane slip is critical to the commercial adoption of these renewable fuels. The industry is currently developing in-cylinder emissions control strategies, which could be combined with aftertreatment systems. By minimizing methane slip, fuels such as bio-LNG and SNG/RNG can offer carbon-neutral propulsion.

The two-stroke and four-stroke engine manufacturers already offer solutions for minimizing methane slip from combustion, using high-pressure gas injection in the cylinder. These can be combined with methane oxidation catalysts and other aftertreatment systems used to treat the exhaust gas to further reduce the methane emissions and minimize the carbon output of using LNG.

As a low-carbon fuel, LNG can be combined with new technologies and/or operational measures to meet the 2030 emissions-reduction goals, and it can contribute to further reductions in future, if blended with bioLNG or SNG/RNG. If the latter can be commercialized and made available at large scale in the medium term, the carbon footprint from using LNG would be reduced in proportion to the amount of renewable fuel used in the blend.

Given the carbon neutral promise of bio-LNG and SNG/RNG, significant efforts are currently being made to explore these solutions for commercial use.

At the end of the light gas spectrum, hydrogen may be a solution for future zero-carbon marine vessels. It offers the highest energy content per mass among all candidate fuels, high diffusivity, and high flame speed. However, it also requires cryogenic storage and dedicated fuel supply systems for containment.

Hydrogen as a fuel has been demonstrated in internal combustion engines, gas turbines, and fuel cells, all of which will play a role in marine power generation and propulsion systems. Nevertheless, significant technical advances are needed before hydrogen can be considered a viable, large scale, commercial fuel option.