Source: Lintec Testing Services Ltd
In the simplest terms, residual fuel oil is a suspension of asphaltenes in a carrier fluid. Asphaltenes in a compatible fuel remain suspended, but they agglomerate and settle out as sludge in an incompatible fuel.
In practical terms, a fuel blend is called “incompatible” if the quantity of sludge-type sediment formed after blending two oils surpasses the quantities of sludge in the two oils before mixing.
Although the chemistry of this phenomenon is exceedingly complicated, it may be simplified by examining the mixing of two fuels with distinct hydrocarbon groups. The resulting combination, as a result of the altered carbon/hydrogen ratio, may be unstable and incapable of suspending the asphaltenes. The disturbance of the blend’s equilibrium may be relatively “mild,” in which case the oil will layer or “STRATIFY” in the mixing tank, depending on the densities and viscosities of the constituent components.
In extreme situations, the mixture’s asphaltene concentration may precipitate as a carbonaceous sludge. When this occurs, it is claimed that the oil is “INCOMPATIBLE.”
Sludge production, particularly with cracked fuels, is accelerated by heating and oxidation and is very variable depending on the degree of “cracking” performed in the refinery. Fuel mixed at a refinery to achieve a commercial viscosity grade is often manufactured to provide a stable product, as refinery operators are typically aware of the sources of the oils utilized and have control over the cracking processes. However, as economic markets dictate, refineries are increasingly selling residual fuels to one another for additional cracking procedures, resulting in a loss of control over the end product’s stability. As contrast to “refiners” or “suppliers,” oil “vendors” are frequently simply involved in mixing purchased-in stocks to meet client specifications, resulting in a greater risk of unreliable deliveries.
Mixing fuels on board a ship may be challenging, as the sources and refinery processes are unknown to the ship’s crew. Adding a distillate, such as gas oil, to an incompatible mixture of heavy oils in an attempt to spread the sludge may actually aggravate the difficulties, since the fuels’ primary hydrocarbon groups are likely to be different, increasing the instability.
Typical symptoms of incompatible or unstable fuels include problems with bunker tank pumping, sludge buildup in filters and fuel lines, fouling and blockage of separator and engine fuel oil heaters, overloading of separator bowls, viscosity control fluctuations (noticeable as temperature fluctuations on viscotherm units), fouling of combustion spaces, exhaust valves, scavenging ports, fouling of injection equipment, and poor combustion coupled with higher than normal exhaust.
Incompatibility will most likely manifest itself initially as overburdening the separator equipment as a result of the high volumes of sludge deposited in the bowls. If the sludge discharge frequency is insufficient to cope or is not raised, sludge carryover to the main engine is possible. If this sludge finally enters the combustion area, the extended ignition and higher heat loading on the cylinder surfaces caused by the high carbon sludge may result in the disintegration of the cylinder lubrication oil layer and, in extreme situations, severe engine damage.
It’s worth noting that there are other possible reasons of fuel oil sludge; sludge is not conclusive evidence of incompatibility or instability. Sludge may also occur as a result of stable oil/water emulsions, waxy precipitates, or excessive depositions of dirt, sand, iron, gums, and other substances.
The calorific value of a fuel may be determined in two ways, by measurement or by calculation and the method used should always be stated into bunker delivery receipt.
It is worth noting that in general the variation in calorific value over the whole range of fuel oils from light gas oil to heavy residual oil is less than 5%. This small variation is due to the relative amounts of carbon and hydrogen present which affects the density of the fuel, and the amount of non-combustible material, mainly sulphur. Thus, high density fuels tend to have a slightly lower calorific value than low density fuels, while the greater the percentage of sulphur present the lower the calorific value.
For performance monitoring purposes it should be noted that most engine test bed results are obtained and quoted using a diesel oil.
Calculated Carbon Aromaticity Index (CCAI)
“Ignition Quality” it related to a fuel aromaticity, as aromatic fuels are known to increase ignition delay. Aromaticity is not easy to measure but fortunately further studies has established a correlation between carbon – aromaticity and the density and viscosity of the fuel. CCAI value was developed by Shell and is a measure of carbon – aromaticity, the relation with ignition delay being empirically confirmed.
Additionally, the “Boiling Range” or a high asphaltene content, which may or may not be reflected in a high Micro Carbon Residue can be indicative of ignition problems. Combustion properties of residuals are dependent on the distribution of light and heavy hydrocarbons in the fuel, the heavy fractions, such as asphaltenes, needing a longer ignition time and producing higher temperatures. Insufficient combustion results in soot formation and the source of the fuel and the refinery processes radically affect the behavior of the fuel.
Advancing the fuel injection timing can sometimes, by providing a longer ignition period, improve the combustion properties of poor burning fuels.
The “Cetane Index” is generally only quoted for distillate fuels, it is a measure of ignition quality. The higher the cetane number, the better the ignition quality, and the less the tendency to “Knock”. Fuels with a higher cetane number generally have better performance characteristics in the majority of marine diesel engines.
Fuels supplied to the vessel must not only be stable when delivered or blended but must remain stable at the elevated temperatures necessary for pre-treatment and injection. Heavy fuels, depending on viscosity, may have to be heated to 130-140 deg C for correct atomization. The symptom of thermal instability of a fuel, like incompatibility, is the increasing tendency of the oil to precipitate sludge, in this case sludge deposition will generally occur in heaters. This sludge is again the product of asphaltene precipitation.
Unfortunately, clogging of heater surfaces eventually leads to a reduction in fuel flow rate which effectively retains fuel in the heater for a longer dwell time and thus causes overheating with even more carbonaceous deposition with cracking of the fuel at the surface of the heaters. These deposits can be very difficult to remove. Not all sludging in fuel heaters can be attributed directly to the lack of thermal stability. If for example a fuel system is designed so that under normal conditions a single fuel heater is capable of maintaining the desired temperature and then because of difficulties in maintaining this outlet temperature a second heater is brought into use in parallel with the first, the reduction in flow rate may be sufficient to cause sludging due to overheating. The “difficulty” referred to is the effects of heater blockage caused by thermal instability of the fuel, in which case the “cure” actually has the opposite effect and will greatly increase the problem. Reduction in flow rate can also occur if the inlet viscosity of the fuel is too high.