World Bunkering >Articles > Fuel management > Just a Lucky Strike?

Jan K Paulsen

Even when delivered according to ISO8217 specifications, bunker fuel is not a uniform product. Various streams of heavy residues and lighter cutter stocks are used to make the final blend, and the product that eventually ends up in the bunker tanks on board the ship will be a result of optimisation between production costs and compliance with specifications. The composition of fuel delivered to ships has changed over the past few years.

The main driving factor has been increasing demand for low-sulphur products. Other factors like high crude oil prices and increasing demand for light distillate products like gasoline and diesel, has also encouraged the refiners to use more resources on sophisticated refinery processes that enables them to produce more of the high-yield fractions. The effect of this has been that both the residue and the lighter cutter stocks have changed.

Purpose of blending

CCAI does not give a good prediction of actual ignition properties.

The main commercial specification for bunker fuel is viscosity, which is reflected in the product names like IFO180, IFO380 etc. This is not at all related to the engine’s ability to burn the fuel, but to the ship’s ability to handle the fuel (pumping, heating, filtering etc). The supplier blends in order to deliver the requested viscosity. Other parameters are of course the well-defined ISO8217 that specifies limits for catfines, density, water content etc. These parameters ensure that the fuel can be pumped, has the right energy content and doesn’t harm the injection system etc.
But what about probably the most important property – how the fuel burns? Does this recipe automatically guarantee that the fuel will ignite and burn properly in the engine – or is that just a Lucky Strike? When using the traditional (but more expensive) cutter stocks like diesel, gas oil or kerosene, ignition and combustion properties are normally not an issue. The calculated carbon aromaticity index (CCAI) has been used for many years as a good indicator to avoid ignition problems. The industry consensus was that as long as CCAI was below 840-850 everybody was happy.
That is, until unexpected engine damage started to increase in frequency as the consumption of low-sulphur fuel increased. One initial thought was that lack of sulphur was the problem – just like the ban on leaded gasoline created a problem for the original gasoline engines. Well, low-sulphur may be a problem for the engine – you will need the right type of lubricating oil to match the acidity of the fuel oil. That challenge was quite rapidly solved, and a whole range of correctly balanced lubricating oils have been launched for use on engines burning low-sulphur bunker fuel. But the real challenge is to be found in the chemical composition of the blending components used. The refineries have a large supply of inexpensive, low-viscous, low-sulphur blending stocks, for example the heavy cycle oils (HCO) and light cycle oils (LCO). These are by-products of the catalytic cracking (FCC) process, which is used amongst other things to increase output of gasoline. The more gasoline produced, the more HCO and LCO will be available.
This is perfect for blending with HFO to get the right viscosity, density etc. Incidentally, the high level of catfines (AL+Si particles) can be a problem since these metals are used as catalysts in the FCC process, and have a tendency to follow the stream into the cycle oils, and eventually end up in the bunker fuel. If the concentration of catfines is too high, the separators and filters on board the ship will not be able to remove enough of them to prevent harmful wear and damage to the high pressure fuel pumps. Unfortunately, the LCO and HCO may have very poor self ignition properties: “Poor cetane number – often 15 or 20; sometimes unmeasurable,” according to ExxonMobil Research in 2006. Similar findings were published by Shell at the CIMAC (International Council on Combustion engines) Congress in 2007. The paper showed that the ECN (estimated cetane number) of a fuel blend based on vacuum residue (ECN=15), was reduced proportionally with increasing content of LCO. Shell also emphasised the importance of knowing the composition and properties of the components used in the final blend in order to avoid such negative effects.

Normally a good fuel for the user, but giveaway for the suppliers

Heavy residue blended with a light cutter stock. Low-sulphur content may give problems for some engines if ignition properties of cutter stock are bad.

How to handle bad ignition and combustion properties

The problem of poor ignition and combustion properties is being addressed very thoroughly across the industry. In Norway, the Marine Use of Low-sulphur Fuels (MARULS) joint project between 19 shipoperators, DNV and the Marintek research institute has recently been completed. A large number of samples were collected from ships and analysed with both traditional and unconventional methods. Information about engine condition and damage was also collected, and the results were correlated in order to find one or more key parameters or indicators that could be used to identify potential problem fuels. Amongst the main findings of the project were:

• Critical ECN (Estimated cetane number by IP541) values vary, depending on engine type;
• CCAI is not always an accurate indicator of ignition properties;
• There is a potential screening method to indicate ignition and combustion properties based on parameters like C/H, H, N and O, possibly in combination with CCAI.

Similarly, universities, researchers and leading industry corporations in Japan have carried out extensive research programmes over the past few years. The results also seem to indicate that ECN values need to be above a certain level, preferably also in combination with a minimum requirement to combustion period. Major questions like “What is adequate ignition and combustion quality?” and “How do we measure this?” still remain to be resolved. Factors such as engine type, age, maintenance status, and operating profile need to be considered. CIMAC‘s Working Group on Heavy Fuel, with representatives from all parts of the industry, is also addressing this issue, and the committee is planning to publish a document with practical recommendations and guidelines in the near future.

Distillates – the only option?

There seems also to be a certain level of consensus among regulators that the only way to comply with future PM emission regulations is to ban heavy fuel in favour of distillate fuels. Distillate fuels have been regarded as relatively problem-free, except for the price, and little independent research has been carried out to “look behind the curtain”. Recent in-house experiments have shown that distillate fuels currently supplied to ships (DMA, DMB, DMC) have reasonably good ignition and combustion qualites, although there is a large span ranging from “Autodiesel quality” to “similar to bad heavy fuel”, and even “worse” for a small number of samples. Other interesting findings in these experiments were:

• There is no significant difference between the DMA, DMB and DMC when it comes to ignition and combustion properties;
• There are large variations in combustion properties (combustion period, after-burning) for fuels with the same ignition properties.

The large span in quality can again be put down to “loose” product specifications. The standard specs do not automatically imply good combustion properties, and whether it is good or bad is again a Lucky Strike (however with much better odds than for HFO). What will happen when the heavy fuel supply is to be replaced with similar volumes of distillates? Will the oil industry be able to supply fuels with similar or better quality as today, or will the future distillates have a different composition that creates new and so far unknown challenges related both to engine operation and safety, as well as emissions? It is important to address these questions when future revisions of the standard specifications are being considered.

No significant differences between DMA, DMB, DMC

Specifications for Autodiesel are tight, resulting in close correlation between ignition and combustion properties

CCAI parameter background

The CCAI parameter, which is calculated from density and viscosity of the fuel, was initially developed in the 80s by Shell Research. The parameter is based on two important findings:

a) combustion properties of fuel are correlated to the aromaticity of the fuel; higher aromatic content means more difficult to burn; and

b) there is a correlation between aromaticity and viscosity/ density; parameters that are easily measured from any fuel sample.

Shell then performed a number of experiments with laboratory engines where they measured the pressure increase during the combustion process in the engine. Since “combustion quality” was more difficult to quantify than “ignition delay”, they made the assumption that ignition quality = combustion quality. So, the CCAI that was developed to give an estimate of combustion quality was actually used as a parameter to predict ignition quality. There is normally no contradiction or problem with that for a straight run fuel (plain atmospheric distillation, no cracking), which used to be the most commonly supplied bunker fuel. This is also quite similar to the EU and US requirements for autodiesel, where minimum cetane number requirement is used to ensure a best possible combustion process in the engine, and due to the tight chemical specifications for autodiesel, there is automatic correlation between ignition and combustion properties. However, this relationship or automatic correlation between ignition and combustion properties does not necessarily apply when the fuel is blended from different stocks to meet certain sulphur, viscosity and density requirements.
This can also be seen from the CCAI parameter. Two fuels with CCAI=850 might very well have ECN (estimated cetane number) ranging from 10 to 40, ie a very large gap that really can make a difference for the engine. That means you can only assess the actual ignition and combustion properties of the fuel by burning it, either in the engine or in appropriate test equipment that burns the fuel under similar conditions – “the proof of the pudding is in the eating”.

CCAI does not give a relevant prediction of the real combustion properties of the fuel. Combustion Period (CP) is measured with FIA-100 FCA, method IP541