Heavy fuels can be replaced with marine diesel oil (MDO) which is less carbon intensive and allows for more effective fuel combustion, resulting in better efficiency and lower levels of emitted particulate matter. 

This switch can reduce carbon dioxide emossions from vessels by as much as 5 percent. Such a switch could also reduce nitrous oxide by more than 91 percent, particulate matter by 63 percent and nitrogen oxides as a group by nearly five percent.

Instead of cleaner MDO, the oil currently used by most ocean-going ships is of low quality. This type of “residual fuel” is used because of its low cost, around $550 per metric ton.

Even so, its use presents challenges for ship operators. Residual fuels must be heated to about 140 degrees C before being used. A proportion of the fuel, the sludge which cannot be put though the engine, must be removed, and is regularly burned on board for disposal.

The sulfur content of residual fuels varies according to the crude stock but globally averages about 2.5 percent. In contrast, cleaner fuels such as marine diesel oil contain 0.5 percent sulfur, while marine gas oil contains only 0.1 percent sulfur.

Switching to low-sulfur fuels would reduce emissions of fine particles, including black carbon, as well as carbon dioxide, nitrogen oxides and nitrous oxide, and enable the use of other emissions control equipment that the sulfur levels in residual fuel would otherwise impede.