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4.7 Sulphur
Sulphur in Fuel
Sulphur levels are typically between 2 - 4 %. The level of sulphur has a marginal effect on the specific energy of the fuel (discussed in a later section).
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 using suitable operating conditions and an alkaline lubricant for the cylinder liner.
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 fuel’s 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.
In a crosshead engine, a suitably formulated lubricant is used for cylinder lubrication. The generally accepted alkalinity level for crosshead engine trading world-wide 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 sulphur level will affect the rate of corrosive wear. However, it is 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 the ship or engine plant continuously bunkers low (say 1.5%) sulphur fuel the use of a lower alkaline cylinder lubricant may be advantageous. Conversely if high (say above 4%) sulphur fuel is bunkered the effect of this as far as corrosive wear can be reduced by increasing the cylinder feed rate.
For a trunk piston engine where the same lubricant is used throughout the engine, there is nothing which can be done operationally to the variable levels of corrosive wear caused by a changing sulphur level. High sulphur levels will increase the rate of TBN depletion, especially for engine designs that have a low oil consumption rate. If the fuel deliveries are such that fuels of low sulphur are typically bunkered, there may be some advantage in changing the lubricant to one with a lower alkalinity level but the economics of this action would have to be studied for each case.
Fuel sulphur has a beneficial effect in preventing scuffering of fuel injection components. Very low sulphur distillate fuels are used in environmentally-sensitive areas. A lubricity additive must be used to prevent damage to fuel injection components when operating on these very low sulphur fuels.
Sulphur levels are typically between 2 - 4 %. The level of sulphur has a marginal effect on the specific energy of the fuel (discussed in a later section).
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 using suitable operating conditions and an alkaline lubricant for the cylinder liner.
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 fuel’s 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.
In a crosshead engine, a suitably formulated lubricant is used for cylinder lubrication. The generally accepted alkalinity level for crosshead engine trading world-wide 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 sulphur level will affect the rate of corrosive wear. However, it is 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 the ship or engine plant continuously bunkers low (say 1.5%) sulphur fuel the use of a lower alkaline cylinder lubricant may be advantageous. Conversely if high (say above 4%) sulphur fuel is bunkered the effect of this as far as corrosive wear can be reduced by increasing the cylinder feed rate.
For a trunk piston engine where the same lubricant is used throughout the engine, there is nothing which can be done operationally to the variable levels of corrosive wear caused by a changing sulphur level. High sulphur levels will increase the rate of TBN depletion, especially for engine designs that have a low oil consumption rate. If the fuel deliveries are such that fuels of low sulphur are typically bunkered, there may be some advantage in changing the lubricant to one with a lower alkalinity level but the economics of this action would have to be studied for each case.
Fuel sulphur has a beneficial effect in preventing scuffering of fuel injection components. Very low sulphur distillate fuels are used in environmentally-sensitive areas. A lubricity additive must be used to prevent damage to fuel injection components when operating on these very low sulphur fuels.
Comment: 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 sulphur content.
