Decarbonising International Shipping - A Life Cycle Perspective on Alternative Fuel Options

This study aimed to compare hydrogen, ammonia, methanol, and waste-derived biofuels as shipping fuels using life cycle assessment, to establish what potential they have to contribute to the shipping industry’s 100% greenhouse gas emission reduction target. A novel approach was taken where the greenhouse gas emissions associated with one year of global shipping fleet operations was used as a common unit for comparison, therefore, allowing the potential life cycle greenhouse gas emission reduction from each fuel option to be compared relative to Paris Agreement compliant targets for international shipping. The analysis uses life cycle assessment, from resource extraction to use within ships, with all GHGs evaluated for a 100-year time horizon (GWP100). Green hydrogen, waste-derived biodiesel, and bio-methanol are found to have the best decarbonisation po tential, with potential emission reductions of 74–81%, 87%, and 85–94% compared to heavy fuel oil; however, some barriers to shipping’s decarbonisation progress are identified. None of the alternative fuels considered are currently produced at a large enough scale to meet shipping’s current energy demand, and uptake of alternative fuel vessels is too slow considering the scale of the challenge at hand. The decarbonisation potential from alternative fuels alone is also found to be insufficient, as no fuel option can offer the 100% emission reduction required by the sector by 2050. The study also uncovers several sensitives within the life cycles of the fuel options analysed, that have received limited attention in previous life cycle investigations into alternative shipping fuels. First, the choice of allocation method can potentially double the life cycle greenhouse gas emissions of e-methanol due to the carbon ac counting challenges of using waste carbon dioxide streams during fuel production. This leads to concerns related to the true impact of using carbon dioxide captured from fossil-fuelled processes to produce a combustible product, due to the resultant high downstream emissions. Second, nitrous oxide emissions from ammonia combustion are found to be highly sensitive due to high greenhouse gas potency, potentially offsetting any greenhouse reduction potential compared to heavy fuel oil. Further uncertainties are highlighted due to limited available data on the rate of nitrous oxide production from ammonia engines. The study therefore highlights an urgent need for the shipping sector to consider these factors when investing in new ammonia and methanol engines; failing to do so risks jeopardizing the sector’s progress towards decarbonisation. Finally, whilst alternative fuels can offer good decarbonisation potential (particularly waste derived biomethanol and bio-diesel, and green hydrogen), this cannot be achieved without accelerated investment in new and retrofit vessels, and new fuel supply chains: the research concludes that existing pipeline of vessel orders and fuel production facilities is insufficient. Furthermore, there is a need to integrate alternative fuel uptake with other decarbonisation strategies such as slow steaming and wind propulsion.