Lifecycle Emissions, Regulatory Compliance, and Cost Competitiveness of Ethanol as a Marine Fuel Retrofit Candidate

Bioethanol is evaluated as a retrofit marine fuel candidate through a comparative assessment against liquefied natural gas (LNG) and methanol (MeOH) under the Energy Efficiency Existing Ship Index (EEXI), Carbon Intensity Indicator (CII), and Greenhouse Gas Fuel Intensity (GFI) frameworks. Fuel consumption profiles are derived from a multi-year empirical dataset using a dual-fuel (DF) engine retrofit, and lifecycle emissions are quantified across corn, sugarcane, and green algae feedstocks under IMO lifecycle guidelines. All DF configurations achieve EEXI compliance, whereas very-low-sulphur fuel oil (VLSFO) fails. A structural divergence emerges between regulatory metrics: under the tank-to-wake CII framework, MeOH and ethanol degrade to a marginal ‘D’ rating by 2027, while under the well-to-wake GFI framework the same configurations generate compliance surpluses from 2028 onward. Feedstock selection proves as consequential as fuel type, with bioethanol GFI ranging from 7.5 gCO2eq/MJ for green algae to 56.4 gCO2eq/MJ for corn. Cumulative surpluses reach 70,556 t CO2eq for electro-fuels and 30,952 t CO2eq for green algae bioethanol, whereas grey MeOH and VLSFO incur remedial costs of USD 3.91 million and USD 2.61 million, respectively. Despite a favourable price ceiling of 557.06 to 615.03 USD/t, the most favourable bioethanol pathway remains above this threshold. Ethanol therefore emerges as significant but not yet commercially viable.