Viscosity.
Viscosity can no longer be regarded as the sole criteria for specifying marine residual fuel quality, either in terms of commercial or technical end user requirements.
The shift in grade profile towards the use of higher viscosity residual fuels will continue. However, the heaviest available grade is unlikely to exceed the current limit of 700 mm2/s since the savings in diluent (and thus production costs) beyond this are negligible. For the consumer, savings will continue to be made by utilising the heaviest grade which a vessel can handle. Perceptions remain that lighter grades equate to better quality, yet as a product, the heavy residual grades such as 380 cSt and above have fairly consistent properties, whereas the characteristics and types of diluents used as blend components in 180 cSt grades can vary widely on a day-to-day basis.
Density.
The introduction of centrifuge technology allowing a relaxation in the density limit for higher viscosity motor ship fuel grades to 1010 kg/M3 presented refiners with substantially increased operational flexibility which even now is not being fully utilised. No new "quantum leap" in centrifuge technology which would allow for further relaxation appears likely.
Carbon Residue.
The correlation between carbon residue and diesel engine performance is poor. In the absence of anything more satisfactory, however, it is retained in fuel specifications to characterise fuel combustibility, and fouling tendency in particular. Several major bunker fuel suppliers recognise that these are critical aspects of fuel performance and apply carbon residue limits of 18-19% weight and below. These are significantly lower than the corresponding limits applied in the International Specifications. The reasoning behind retaining these lower limits is that there has been no sustained experience in operating marine diesel engines with fuels having carbon residue values in excess of these levels. In addition, the little research which has been carried out tends to suggest that levels in excess of those stated may cause excessive fouling, especially in older types of diesel engines.
As a result, carbon residue can be a constraint to refiners and refinery flexibility would increase significantly if the limits in the International Specifications could be safely applied, Increasingly, diesel engine manufacturers are claiming that their engines are able to operate satisfactorily on such fuels, but this has yet to be conclusively demonstrated. There is unlikely to be great demand for limits to be relaxed beyond the already existing upper limit of 22% weight.
Ash.
During refinery processing, the ash-forming constituents of the crude oil tend to concentrate in the residue. The nature of the ash and its amount vary very widely, being largely dependent on the source of the crude oil; but it may be increased by contamination with inorganic matter in the course of handling before combustion. The main elements present are usually calcium, iron, nickel, silicon, sodium, and vanadium. Contamination may introduce further quantities of sodium in the form of sea water, iron from rusty storage tanks or pipelines, and dusts and dirt from the atmosphere. Usually, nickel and vanadium are present only in oil-soluble forms.
High temperature corrosion and fouling are phenomena that can mainly be attributed to a fuel oil's content of vanadium and sodium. Both these elements oxidise during combustion and, in a series of complex chemical reactions, form semi-liquid, and sticky low-melting point salts that adhere to exhaust valves and turbo-chargers in particular. The efficiency of these components is reduced as a result; in addition, severe corrosion can take place. The highest values of vanadium currently encountered in the heaviest viscosity grades world-wide are usually below 400 mg/kg, which is only some two thirds of the 600 mg/kg limit permitted for these grades in International Specifications. It is not envisaged that future refining developments will raise vanadium levels sufficiently for existing specification limits to become constraining.
Ash content will nevertheless continue to generate interest from ship owners however. Reflecting this concern, and taking into account the foregoing, at the next revision of ISO 8217 the ash limit will be lowered for the heavier fuel grades.
Revisions to International Standards rarely broaden the limits on parameter values, changes always move towards tightening of the specifications, thus the scope for the "temptation to reduce quality" referenced earlier is, in reality, likely to be countered over period of time by gradual tightening of supply specifications. Nevertheless, lower quality suppliers may continue to maximise use of the broader specification limits currently in place.
Given that all suppliers will be required to supply within a recognised standard what therefore will become the differentiating features distinguishing the low and high quality suppliers? Let us consider those aspects of a bunker fuel which are frequently referenced as quality indicators.
Water content.
By far the simplest, and thus the most common parameter used to distinguish a quality supplier, is the water content. Limited to 1% by current standards, but with bunker fuels as delivered having an average water content below 0.3%, there is obvious scope for some "massaging" of the amount included with the fuel. The potential added margin to the supplier may be measured in millions of dollars per annum, thus the incentive is clear. Water added deliberately to bunker fuel will however, in most countries, be classed as fraud (even if within the 1% limit), but it is often very difficult to prove such practice. The bunker market is perhaps well enough policed by third party testing services so as to be able to identify, by port at least, where such practice may be prevalent. Most on-board fuel treatment systems will readily accommodate 1%, water content with little impact on diesel engine performance, thus the "quality" tag here reflects a commercial perspective - the user is paying a hydro-carbon price for water.
Reflecting the low "average value" water content seen in practice, future specification revisions will reduce the maximum limit to 0.5% from about 2003. The impact of this on the refiner will be negligible-water is primarily a downstream supply chain issue.
