Clean Energy Pathways for Marine Engines: Technological and Environmental Insights into Biodiesel–Alcohol Blends

The maritime industry is responsible for the emission of approximately 940 million tonnes of CO₂ on an annual basis, accounting for roughly 2.9% of the total global anthropogenic greenhouse gas emissions. Achieving the decarbonization objectives set forth by the International Maritime Organization for the years 2030 and 2050 requires an expedited transition towards low-carbon marine fuel alternatives. This review presents a concentrated and quantitative assessment of biodiesel and biodiesel–alcohol blends as viable near-drop-in substitutes for marine gas oil. Non-edible feedstocks, including Jatropha curcas, Pongamia pinnata, used cooking oil, and microalgae, produce biodiesel with heating values ranging from 37 to 42 MJ kg⁻¹ and cetane numbers between 47 and 58, thereby conforming to ISO 8217 specifications. The incorporation of alcohols, most notably n-butanol, in concentrations of 10 to 20% enhances atomization and ignition properties, resulting in a reduction of CO and hydrocarbon emissions by 50 to 60% and particulate matter by as much as 70%, accompanied by a modest increase in NOₓ emissions of 8 to 15%. A comprehensive meta-analysis of 18 studies representative of marine conditions has corroborated that blends containing 20% biodiesel and 10%-butanol-20% biodiesel maintain brake thermal efficiency (g = −0.03, p > 0.05) while simultaneously decreasing CO, hydrocarbon, and particulate matter emissions by 16 to 21%. Lifecycle assessments reveal savings of greenhouse gas emissions ranging from 65 to 85% contingent upon the type of feedstock utilized. At the same time, techno-economic evaluations demonstrate that production costs are competitive with those of marine diesel, particularly when factoring in carbon credits. This review distinctively amalgamates statistical synthesis, insights from lifecycle and techno-economic assessments, and a strategic policy roadmap that aligns with the directives of the International Maritime Organization, the European Union Emissions Trading System, and various Asian maritime frameworks, thereby providing stakeholders with practical guidance for the large-scale implementation and adherence to the objectives of International Maritime Organization Tier III and carbon intensity indicator.