Comparative study of LNG, liquid hydrogen, and liquid ammonia post-release evaporation and dispersion during bunkering

The use of alternative fuels is a primary means for decarbonising the maritime industry. Liquefied natural gas (LNG), liquid hydrogen (LH2), and liquid ammonia (LNH3) are liquified gases among the alternative fuels. The safety risks associated with these fuels differ from traditional fuels. In addition to their low-temperature hazards, the flammability of LNG and LH2 and the high toxicity of LNH3 present challenges in fuel handlings due to their high likelihood of fuel release during bunkering. This study aims at drawing extensive comparisons of the evaporation and vapour dispersion behaviours for the three fuels after release accidents during bunkering and discuss their safety issues. The study involved the release event of the three fuels on the main deck area of a reference bulk carrier with a deadweight of 208,000 tonnes. Two release scenarios were considered: Scenario 1 involved a release of 0.3 m3 of fuel, and Scenario 2 involved a release of 100 kg of fuel. An empirical equation was used to calculate the fuel evaporation process, and the Computational Fluid Dynamic (CFD) code FDS was employed to simulate the dispersion of vapour clouds. The obtained results reveal that LH2 has the highest evaporation rate, followed by LNG and LNH3. The vapour clouds of LNG and LNH3 spread along the main deck surface, while the LH2 vapour cloud exhibits upward dispersion. The flammable vapour clouds of LNG and LH2 remain within the main deck area, whereas the toxic gas cloud of LNH3 disperses towards the shore and spreads near the ground on the shore side. Based on the dispersion behaviours, the hazards of LNG and LH2 are comparable, while LNH3 poses significantly higher hazards. In terms of hazard mitigations, effective water curtain systems can suppress the vapour dispersion.