FUEL CONTAMINANTS

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Within the complex fuel structure, various metal elements exist such as vanadium, sodium and nickel. These contaminants do not aid the combustion process, and high levels can damage engine components.

Vanadium                

This not only acts as a catalyst in the formation of sulphuric acid, but it also oxidises at high temperature to form a series of compounds of which vanadium pentoxide V₂O₅ has the lowest melting point at 675^oC.

There are no economic methods of removing vanadium from hydrocarbons due to its solubility, although some additives are claimed to react with sodium and vanadium to form other oxides and salt with high melting points, which will exit the engine as very fine solids.

Vanadium compounds can react with relatively soft carbon particles to form clinker-like, abrasive agglomerates, which can increase ring/liner wear.

Sodium

This reacts with both oxygen and vanadium compounds to form highly corrosive deposits. The temperature at which a critical mixture of 3:1 vanadium to sodium (Pento sodium vanadate - 5Na₂O V₂O₄ 11V₂O₅) can adhere to a metal surface can be as low as 450^oC. These deposits build-up on hot surfaces such as exhaust valves or turbine blades, necessitating an increase in turbocharger cleaning. The deposit itself is corrosive and can attack many metals (produces characteristic cobblestone effect), and if the deposit were to break away from a small area of exhaust valve, then a gas passage is formed where hot, high pressure (and hence velocity) gases will escape causing wire-drawing erosion.

The amount of sodium occurring naturally in residual fuels is quite low (35ppm max), and the major increase will come from sodium chloride which is present in large quantities in salt water. 1% salt water ≈ 100 mg/Kg (or ppm) Na. Sodium can be removed by centrifuging from the fuel when it is present in large quantities such as when the fuel contains seawater, with results 1550 to 97ppm, & 88 to 42ppm.

Nickel

In itself this does not cause problems, although it acts as a catalyst in corrosive element production.

Nickel is partly soluble in oil and water, thus only small reductions can be achieved by centrifuging.

Aluminium

This will occur naturally in the oil in very small quantities. However aluminium can be used to indicate the level of oil refining catalyst which is present. This catalyst consists mainly of aluminium trioxide (Al₂O₃) and silica (SiO₂) and is extremely hard and abrasive. Fuel containing a large (over 30ppm) quantity of aluminium could cause rapid liner, piston ring & fuel pump wear. 

30ppm Al equates to 250ppm catalyst, and as the substance is not linked with the fuel, it can be nearly totally removed (18 to 4ppm) by correct use of the purifier/clarifier set, especially at low throughputs (hydroscopic).

Sulphur

This is present naturally in the oil chain, with the quantity dependant upon the crude oil base stock, with the `sour' crudes having the higher sulphur levels. Sulphur is converted by combustion into SO₂, then SO₃, before mixing with water to form sulphuric acid H₂SO₄. This is highly corrosive in the liquid state, so if the dew point of the acid/gas mixture is reached, then acid attack on the metal surfaces will result. For 2% sulphur the dew point lies between 154-162^oC, so the low temperature components should be kept above this temperature and other methods used, such as neutralisation of the liner wall by alkaline cylinder lube oil.

Sulphur is combined with the fuel structure and can not be removed by normal methods.

Sulphur can also react with carbonaceous matter at high temperature to form very hard, compact, abrasive deposits especially in the ring zone.

Additives are marketed which claim to eliminate the formation of sulphur trioxide and hence reduce acid production. However on tests the product works on some engines/fuels, whilst not on others.