Asphaltene, carbon residues and sediments
Asphaltenes are complex suspended solids in fuel that have high melting points and high carbon/hydrogen ratios. The proportion of asphaltenes in a fuel appears to increase as secondary refinery processes are more widely used. They have poor combustion properties and burn very slowly. If these formations precipitate, they can have a negative effect on blended fuel stability and can cause too much sludge into filters and separators. If the fuel is not stable, particles accumulate at the bottom of the tank.
To reduce risks, make sure that bunkers from different suppliers and sources are not mixed in the ship’s storage tanks. Also, be cautious when heavy fuel oil is mixed on board to reduce viscosity. When paraffinic distillate is added to a low-stability heavy fuel oil, it can cause asphaltenes to collect, resulting in heavy sludge.
A high Micro Carbon Residue content could be a sign of a high asphaltene content. The Micro Carbon Residue is typically expressed on fuel specifications as a percentage of weight and in current fuel oils, it can range from 3 to 18 percent, but due to secondary refinery conversion processes, values as high as 22 percent have been reported. If the fuel preparation equipment is properly adjusted, HFO can contain up to 14 percent asphaltenes and will not cause ignition or combustion problems in 2-stroke engines.
Conradson carbon measures the amount of carbon residue that remains after the combustion of a fuel under specified conditions. It is used to describe a fuel’s predilection to form carbon deposits during operation, which can clog fuel nozzle tips, piston ring grooves, and exhaust valves or scavenging ports of diesel engines, as well as boiler burner nozzle tips and deflectors.
Residual fuels with high carbon content burns at a slower rate, which can result in higher exhaust temperatures and fouling of combustion space and fuel injector’s nozzle, as well as the possibility of incomplete combustion, which can result in smoke or the impingement of incompletely burned fuel on the cylinder wall or piston, resulting in the formation of additional hard carbon deposits.
Combustion can sometimes be improved by advancing ignition, resulting in a longer burning time. It may also be necessary, due to increased cylinder deposits, to increase cylinder oil lubrication in order to maintain free piston rings and clean ports.
The ash forming constituents of crude oil tend to be concentrated in the residue left after distillation. Fuel oils, which are usually under blended form of this residue and a cutter stock, have a measurable ash content which rarely exceeds 0.1%. The prediction in future fuels is for this value to increase as different oil fields are exploited and as refinery techniques change.
Ash forming components are those elements present in the fuel as various organic and inorganic compounds that leave the combustion space of the engine in a solid or semi-solid state. Several different elements have been identified in this form, the most common being silicon, aluminum, calcium, iron, nickel, sodium, vanadium, phosphorus etc.
Usually nickel and vanadium are present only in oil soluble forms. Apart from the fuel sodium natural content, contamination may introduce further quantities of sodium from seawater, iron from rusty storage tanks and pipelines and from dust and dirt.
Harmful ash forming compounds may cause damage to the vessel’s engine. Wear damage to the engine will be experienced due to elevated levels of Cat Fines (Aluminium & Silicon) so that efficient purification is require to reduce the levels prior to injection. Vanadium – Sodium compounds will result in post combustion problems and complexes formed with Sulphur addition will be prevalent.
Recent updates on the Marine fuel oils specification ISO 8217 includes new parameters and specification limits to control the mixing of Used Lube Oils into bunkers. Contamination with Used Lube Oils in bunkers causes concern as the lube oil will carry a lower density and lubricants work against the centrifugal forces adversely affecting a ships fuel treatment plant efficiency and fuel oil purifiers. The concern lays on the forming of an emulsion. This is of extreme concern to engine crew or manufacturers because there is a distinct possibility that any water or abrasives present in the fuel would not be reduced.
Removal from the fuel of insoluble matter can be achieved by centrifugal separation. Oil soluble ash components, however, notably vanadium, cannot be removed from the fuel by this means or by any other method which is practical onboard ship. Sodium on the other hand, being water soluble, can be removed in the water phase from the separator. In view of the problems mentioned previously the removal of salt water is important. Additionally, if fuels are suspected of having a high vanadium content, it is necessary to ensure engine valves, pistons, etc. are maintained at normal operating temperatures through good combustion in order to minimize vanadium ash corrosion. Fuel treatment chemicals (e.g Fuel care) can be used to elevate stiction temperatures and hence reduce corrosion attributed to the vanadium compounds.
The sulphur is present as an organic compound and depends entirely on its crude oil origin and the amount of distillate removed and the contents can be up to 4 – 5 % and cannot be removed by any vessel conventional treatment systems.
When a fuel burns, the sulphur content is converted into sulphur dioxide and mostly is exhausted from the engine without any reaction, but a small amount oxidizes in the presence of the other combustion gases products and form sulphur trioxide. Although the sulphur trioxide content of exhaust gases is relatively low, this is a highly reactive gas and a strong oxidizing agent. When it combines with water vapours will form sulphuric acid which has a dewpoint in the region of 140/150 deg C, which will condense therefore on various metal surfaces, including portions of the cylinder liner during normal operation.
Sulphur trioxide also combines with combustion ashes lowering their melting points and causing them to become more adhesive to metal surfaces.
The use of high-quality alkaline lubricating oils decreases the risk of corrosion to cylinder liners, piston rings and associated engine components and increasing operation temperatures further helps by preventing condensation of the sulphuric acid within these areas.
In general, low speed marine diesel engines run quite satisfactorily on fuels with a sulphur content of up to 4% when correctly lubricated.
For each percentage increase in sulphur content of the fuel oil the calorific value reduces by approximately 80 Kcal/kg.
Occasionally, there is another aspect associated with the sulphur content of fuel, and that is the occurrence of fuels with very low sulphur content (for example 0.5% and below), being used for prolonged periods in engines using highly alkaline cylinder oils (commonly a TBN of around 70). The effect is high cylinder and piston rings wear rates due to the formation and build-up of abrasive alkaline ashes in the areas of high thermal load.
Vanadium is a metallic contaminant that is present in all crude oils and the amount of vanadium in any given fuel is dependent on two factors, the source of the crude and the severity of the refining process. In places like Venezuela and Mexico the crudes have a vanadium content of 300-400 ppm whereas in other parts of the world the levels vary up to about 150 ppm. However, these contents can be greatly increased in the refinery, as the light distillates are removed from the crude feedstock the metallic contaminants become concentrated in increasingly smaller volumes of residuals. Unfortunately, there is no economical method to remove the vanadium, and being oil soluble centrifugal separation will not remove it on board ship.
During combustion vanadium is oxidized to various compounds. The most important of these, when the vanadium is not in contact with sodium, is vanadium pentoxide. When in the presence of sodium (and sulphur) vanadium forms sodium-vanadate compounds.
Vanadium pentoxide has a relatively low melting point, 680°C, and in the molten state causes corrosive attack on steel surfaces (“hot” corrosion). The vanadate compounds have a much lower melting point (the “stiction” temperature) which, dependent on the relative amounts of sodium ash (sodium sulphate) and vanadium ash (vanadium pentoxide) can be as low as 330°C. These vanadate compounds are extremely corrosive in the molten state and adhere easily to steel surfaces. It is not solely the vanadium level which is critical, but the combination of vanadium and sodium, the most dangerous ratio being approximately 3:1.
The vast majority of any sodium content of a fuel is inorganic in nature, oil insoluble, and is present as seawater contamination. Sodium in seawater occurs as common salt, sodium chloride, and being water soluble it is possible to largely remove it by onboard purification. This operation is vital if seawater contamination has occurred because of the risk of highly corrosive sodium – vanadium ashes forming during combustion.
Traces of iron can be found in fuel from a number of sources. The most obvious source of contamination with the iron present as inorganic oil insoluble particles is rust flakes or scale from pipelines, tanks, etc. However, iron also occurs naturally in some oils as small amounts of organic oil soluble compounds in the form of complex porphyrins. It should be remembered that some fuel additives added to bunker, settling or service tanks contain iron.
Water can be present in oil either as fresh water or as salt water, depending on its source. Common sources of fresh water contamination are condensation inside a tank or steam coil leakage. Seawater may find its way into fuel from tank leakage, through cracked hull plates or via unsecured sounding pipe caps during heavy weather or even during deck washing operations.
If the water is allowed to remain in the fuel may result into damage to fuel pumps and injectors, poor fuel atomization and consequently reduced combustion efficiency. Salt water is especially harmful as it is more corrosive than fresh water and the sodium component of the salt content produces fouling in combustion spaces and more dangerously may combine with vanadium compounds present in the fuel to form a highly corrosive molten ash.
Apart from the physical difficulty of removing the water this effect will tend to produce reasonably stable oil/water emulsions. An appreciable water content (in the region of 1.0% or above) also intensifies the problems of incompatibility or instability.
-End of Part II-