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Sulphur in Fuel

Bunker fuel sulphur content is typically between 2 and 4% with a worldwide average of approximately 2.7%. The level of sulphur has a marginal effect on the specific energy of the fuel (as discussed in a later section).LinerSCAN
During the combustion process in a diesel engine, the presence of sulphur in the fuel can give rise to corrosive wear. This can be minimised by suitable operating conditions and lubrication with an alkaline lubricant matched to the fuel sulphur content.
Considerable work has been undertaken by the various engine manufacturers to ensure the cylinder liner surfaces do not approach the dew point. This is the temperature at which acidic gases from combustion condense into liquid. In a diesel engine, the sulphur in the fuel first burns to SO2 then combines with excess oxygen to form SO3. In the presence of water vapour the SO3 is converted to sulphuric acid, which then forms on the cylinder walls if the temperature is below the dew point for the condensation of acid at the prevailing pressure. This dew point is a function of the bunker fuel sulphur content and the pressure in the cylinder. Only a relatively small proportion of sulphur is normally converted in this way and the remaining sulphur oxides pass out of the cylinder with the exhaust gases.
Residual fuel suphur graphIn a crosshead engine, a suitably formulated lubricant is used for cylinder lubrication. The generally accepted alkalinity level for crosshead engines trading worldwide is 70 mg KOH/g. It should be noted that besides the alkalinity level, the rate of neutralisation and the feed rate are also important factors. Variation in the bunker fuel sulphur content will affect the rate of corrosive wear and it has been standard practice to accept this variation. The number of fuel deliveries with a sulphur level greater than 4% is very low, to the extent of being negligible. If however, very high sulphur fuel was to be bunkered, an increase in cylinder oil feed rate could be used to reduce corrosive wear. Conversely the introduction of Emission Control Areas has increased the likelihood of ships using low sulphur fuel oil (LSFO) for protracted periods. If the ship bunkers low (say 1.0%) sulphur fuel on a regular basis the use of a lower alkaline cylinder lubricant may be required, although the decision to change may not be straightforward.
There are a number of factors that need to be considered. These include the operating power of the engine, the cylinder and lubricator design, the actual fuel sulphur content and the length of time the fuel is in use. Given these variables it is important that the decision regarding cylinder oil Base Number (BN) and feed rate is made in conjunction with the engine builder in order to avoid potential problems with piston rings and cylinder liners. These may arise from insufficient alkalinity reserve resulting in corrosive wear or scuffing from deposits of excess unspent alkalinity additive.
For a trunk piston engine where the same lubricant is used throughout the engine, the feed rate to the cylinders cannot be increased. High sulphur levels will therefore increase the rate of BN depletion, especially for engine designs that have a low oil consumption rate. If the fuel deliveries are such that low sulphur fuels are typically bunkered on an ongoing basis, there may be a requirement from the engine builder to change to a lower alkalinity lubricant. This may also be economically beneficial, but the overall costs of fuel, lubricants and maintenance need to be studied for each case.

Fuel sulphur has a beneficial effect in preventing scuffing of fuel injection components. Very low sulphur distillate fuels are used in ECAs. An additive must be used to prevent damage to fuel injection components when operating on these very low sulphur fuels and ISO 8217:2010 now includes a scar test as evidence of a distillate fuel’s lubricity.

Fuel injector

The above image illustrates a fuel injector damaged from operation on very low sulphur fuel (<0.05%). In the medium term, for both crosshead and trunk piston engines, high maintenance costs may arise if the lubricant/lubrication is not matched to the suphur content.

Off-line Bunker Fuel

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