Working Paper

The potential of liquid biofuels in reducing ship emissions

In light of the emissions reduction goals of the International Maritime Organization’s initial greenhouse gas (GHG) strategy, the shipping industry is currently exploring different liquid biofuels that could replace conventional marine “bunker” fuels. This is one component of a suite of solutions that will be needed to eliminate life-cycle GHG emissions from the sector, and in the near term, existing ships will need “drop-in” fuels that can be used in large marine diesel engines with minimal modifications. To help, this study explores the potential contribution of a number of biofuel pathways.

The authors screen liquid biofuels based on qualitative criteria, assess the potential GHG and air pollution reduction benefits of key candidates compared with distillate bunker fuel, and then discuss these fuels in terms of their compatibility with marine engines. Also considered are other barriers to the use of these fuels, including feedstock availability, cost, and competition with other sectors.

Results show that feedstock is more important than conversion technology in determining a fuel pathway’s GHG reductions, and the paper identifies five liquid biofuels with the potential to reduce shipping GHG emissions on a well-to-wake, life-cycle basis relative to distillate marine fuels: (1) fatty acid methyl ester (FAME) biodiesel produced from waste fats, oils, and greases (FOGs); (2) hydrotreated renewable diesel produced from waste FOGs; (3) Fischer-Tropsch (FT) diesel produced from lignocellulosic biomass; (4) dimethyl ether (DME) generated by gasifying lignocellulosic feedstocks followed by catalytic synthesis; and (5) methanol generated by gasifying lignocellulosic feedstocks followed by catalytic synthesis. All five are expected to reduce combustion-related air pollution emissions, regardless of feedstock.

The results also suggest some policy lessons. First, policymakers should adopt rigorous life-cycle assessment methodologies that include land-use change emissions to ensure they promote only those fuels that offer significant life-cycle GHG benefits. Second, because pathways with the highest potential to deliver deep GHG reductions are also the most technologically complex and currently have the highest costs, policymakers should promote policies that focus on addressing the barriers to these sustainable fuels. Third, because of certain engine compatibility limitations, policymakers should recognize that many fuels will need to be blended with conventional fossil fuels and that they can only reduce life-cycle emissions relative to their blending ratio.

Alternative fuels
Fuels GHG emissions
Emissions control