Sulfur is a naturally occurring element in crude oil, concentrated in the residual components of the crude oil distillation process. The amount of sulfur in the fuel oil depends mainly on the source of crude oil, and to some extent on the refining process. Crude oils have a natural sulfur level and this is the primary feature which determines the sulfur level in any particular blend of fuel oil.

 During the combustion process in a diesel engine the presence of sulfur in the fuel can give rise to corrosive wear. This can be minimized by suitable operating conditions and lubrication with an alkaline lubricant for the cylinder liner. Considerable work has been done by the various engine manufacturers to ensure cylinder liner surfaces do not approach the dew point, which is the temperature at which gases condense to liquid. In a diesel engine the sulfur in the fuel first burns to SO2, then combines with excess oxygen to form SO3. In the presence of water vapor the SO3 is converted to sulfuric acid. If the temperature of the cylinder wall is below that of the dew point the acid will become deposited on the wall. This dew point is a function of the sulfur content of the fuel and also the cylinder pressure. Only a relatively small proportion of sulfur is converted to acid and the remaining oxides pass out of the cylinder with the exhaust gases.

 A broad spectrum of measures intended to control air pollution from shipping has been discussed within the International Maritime Organization (IMO) for over a decade. One of these pollutants is sulfur dioxide(SO2) which one causes of acid rain. This can result in deforestation and damage to man-made structures. Some restrictions now exist with respect to sulfur in some marine fuels and these are often on a voluntary basis. Examples are the use of low sulfur fuels on ferries in Scandinavian and Baltic waters, and in tanker movements on the Prince William Sound in Alaska. In 1997 a new treaty was signed with members of IMO adopting Annex VI to 1973 MARPOL. Upon notification of this Annex by sufficient member states (or review by 2002) a maximum sulfur level of 4.5% m/m will be applied to all marine fuels. If regional areas are legislated through IMO it would seem that vessels trading to those areas would have to carry two types of fuel. While such an approach could be incorporated into new construction, it might not be straightforward to convert existing ships. Any new requirements which are imposed on merchant shipping to reduce air pollution could have a major impact on ship-owners.

How to reduce sulfur

The SO2 emissions are a direct function of the sulfur content of the fuel and essentially there are two ways of reducing them:

    • SO2 can be removed from the exhaust gas by water washing.
      The technology of water washing has been in use for several decades in oil tankers for cleaning the exhaust gas of boilers to produce effectively SO2 free inert gas for the cargo tanks.
    • Formation of SO2 can be limited by restricting the sulfur content of the fuel.