Life cycle emission reduction benefits of alternative marine fuels under supply chain constraints.

As shipping decarbonization transitions from ambition to implementation, alternative marine fuels have become central to policy and industry debates. Yet the extent to which their unconstrained mitigation benefits can be realized under port-level supply chain constraints remains uncertain. This study develops a well-to-tank (WTT) assessment framework that distinguishes unconstrained from supply chain constrained emission reduction benefits. Static ship data and Automatic Identification System records are used to estimate port-level energy demand. GREET-based WTT inventories are then regionalized for the selected production-pathway mixes and transportation assumptions. Upstream fuel availability and port storage/bunkering deploy ability are represented through transparent scenario coefficients. Qingdao, Shanghai, and Shenzhen are examined, and Monte Carlo simulation propagates uncertainty in the adopted constraint coefficients. Results indicate that LNG and Used-cooking-oil biodiesel generally provide the largest positive benefits, whereas the coal-dominated methanol pathway remains unfavorable. Although electrolysis-based ammonia has the greatest unconstrained benefit, its scenario-adjusted contribution is small because of limited upstream availability and port deploy ability. This study provides methodological support for port alternative-fuel infrastructure planning and low-carbon shipping policy design.