Development of a Bunker Norm for Ships

7. Sulphur

As mentioned already at the beginning of this report, the sulphur content has great impact on the environment and, in addition, can give rise to technical problems. Therefore the problems of sulphur will be addressed in more detail in the following:

Sulphur in fuel oil is the result of sulphur in the crude oil. The sulphur largely accompanies the residual oil throughout the refining process and, consequently, is present in considerable quantities in heavy fuel oils. On the other hand, distillate fractions from the cracking process can also contain some sulphur.

It is desirable to keep the content of sulphur as low as possible – not only for environmental reasons, but also in terms of industrial engineering.

During combustion, sulphur produces sulphur dioxide, and this can convert into sulphur trioxide and sulphuric acid, H₂SO₄. This conversion is probably accelerated considerably by the presence of iron oxide and vanadium pentoxide, the latter being formed by the combustion of vanadium, which is also present in the oil. The sulphuric acid thus formed can be highly corrosive to the construction elements of the engine, especially those parts of the engine that are exposed to the flue gas. Investigations have shown that the dew point of the sulphuric acid is between 120 and 160⁰C. To reduce the corrosion it is necessary to ensure that the combustion products will not be cooled sufficiently to approach the dew point of the sulphuric acid.

The following problems may occur when running an engine on fuel oil with a relatively high content of sulphur:

• low-temperature corrosion with abrasion of piston rings and cylinder liners;
• adhesion of a black varnish-like layer on the slide faces of the cylinder liners;
• low-temperature corrosion and abrasion of exhaust valves, gas ducts and turbocharger housings, and unbalanced turbocharger;
• coatings in the fuel oil preheater;
• high-temperature corrosion of exhaust valves (sulphuric corrosion of Na₂SO₄);
• the higher the sulphur percentage in the oil, the lower will the lower calorific value be per kg of oil;
• the higher the sulphur percentage in the oil, the higher must the TBN index of the lubricating oil be. This implies a risk of precipitation on the pistons consisting of hard deposits on the pistons – coming from the additives to the lubricating/cylinder oil;
• damage to the environment by air pollution caused by SO_x;
• deterioration of the working environment on board ships from flue gas, as well as for any repair staff.

The following problems may occur when running an engine on fuel oil with a low content of sulphur:

• scuffing, i.e., seizing of cylinders and abrasion of piston rings;
• may cause abrasion of the plunger and barrels of the fuel pumps, and possibly compound seizure.

Opinions are divided within the industry regarding the abovementioned problems associated with the running of engines on a fuel oil with low content of sulphur. Some can report extremely grave and big problems, whereas others argue that it has no importance whatsoever.

In this connection it might be mentioned that a Norwegian shipping company operating ferry services has used fuel oil with a low content of sulphur for about two years without experiencing any problems. The oil is produced in Norway specially for the shipping company. The contract specified 1% sulphur, but the oil received often has 0.6% sulphur. The company uses 120 cSt fuel for its medium-speed engines and 240 cSt fuel for its slow-speed engines.

Lubricating oil
A lubricating oil must be capable of neutralising the increased quantity of acid produced by combustion of fuel oil with a high content of sulphur. For that reason additives of a highly alkaline nature are added to the oil, so that there is a reserve to neutralise the strong acid. The quantity of these additives is indicated by a TBN index, where TBN stands for Total Base Number. Today, the designation BN is also used.

Over the past few years the TBN index has increased sharply in most oil companies’ marine oils, and a TBN of 70 for cylinder oils and up to 30 for system oils is now common, depending on the sulphur content of the fuel oil used. It is obvious that in practice it is not possible to change to another lubricant to reflect the sulphur content in the relevant bunker oil, but the ship will choose a lubricating oil with a TBN index that will generally meet the requirements for the average type of fuel oil being used.

In recent years there has been a good deal of discussion in shipping and machinery circles as to the impact which any low-sulphur geographic areas would have on the choice of lubricating oil. For example, will bunker oil with a low content of sulphur create a too alkaline environment in the engine, and what will the drawback be?

At the moment it seems that engine designers are unsure about the impact which bunker oil may have on the lubrication of the somewhat bigger engines. Thus, many manufacturers are planning to carry out, in the near future, test runs on slow-speed engines with a fuel oil having 1% sulphur and with lubricating oils having various TBN’s.

But in general terms one might say that for ships that sail worldwide and rarely enter low-sulphur areas, the problem is not great, but if the ship is going to sail in a low-sulphur area for a considerable period of time, the choice of lubricating oil with the correct TBN will be much more important.

In most cases the sulphur content is between 2.6 and 3.0%, but it may vary considerably from one country to the next and even within the same bunker port.

Extracts from the ISO 8217 standard and CIMAC regarding sulphur for the different types of fuel oil are as follows:

Marine Distillate Fuels

Marine Residual Fuels