Well-to-wake greenhouse gas assessment of alternative marine fuels (LNG, methanol, and ammonia) compared with conventional marine fuel for bulk carrier decarbonization

This study aims to quantify and compare the well-to-wake (WtW) greenhouse gas (GHG) emissions of alternative marine fuels for large ocean-going vessels. Four fuels—marine gas oil (MGO), liquefied natural gas (LNG), methanol, and ammonia—are evaluated under identical propulsion assumptions and a representative voyage profile for a 200,000 dead weight tonnage (DWT) bulk carrier on a Taiwan–Australia route. The tank-to-wake (TtW) stage is modeled using vessel-specific Energy Efficiency Design Index (EEDI) calculation and verified sea-trial reference data, aligned with manufacturer-derived Computerized Engine Application System (CEAS) engine performance maps under a representative Taiwan–Australia voyage profile. The well-to-tank (WtT) stage is based on literature-derived life cycle inventories with Monte Carlo uncertainty analysis. Methane slip for LNG (0.20–0.28 g/kWh) and nitrous oxide (N₂O) emissions for ammonia are explicitly included. The assessment follows an attributional life cycle assessment (LCA) framework with system expansion and associated emission credits. Results show that optimal operation occurs at 65–70% specified maximum continuous rating (SMCR), minimizing emission intensity. LNG achieves ∼25% lower TtW emissions than MGO, while ammonia provides up to ∼70% reduction, conditional on N₂O mitigation. However, upstream fuel production dominates lifecycle emissions. Fossil-based methanol and ammonia exceed conventional fuels by more than twofold in WtW emissions, whereas renewable pathways significantly reduce emissions and may achieve near-zero or negative WtT values. Sensitivity analysis indicates that ±5% variation in fuel lower heating value (LHV) does not alter fuel ranking. Pilot fuel contributes 2–3% of total emissions in dual-fuel systems. Overall, effective maritime decarbonization requires optimized engine operation, low-carbon fuel pathways, and emission control technologies, particularly for N₂O in ammonia systems.