Low Sulphur Fuel Oil

While various kinds of regulations are being enforced to inhibit the emission of atmospheric pollutants from ships, other controls are being imposed on sulfur content in fuel oils to restrict the emission of sulfur oxides (SOx) and particulate matter (PM) included in exhaust gases of diesel engines, boilers and so on. Because of these controls, the low sulfur fuel oil, which is a distillate of gas oil, etc is being demanded in ships that were using mainly low-grade residual oil until now.

With the use of low sulfur fuel oil, new issues are now being pointed out in machinery in which the use of such oil was not envisaged during the design stage of the inherent properties of this oil. Thus, measures are needed to ensure safe operation.

Low Sulfur Fuel Oil:
Low sulfur fuel oil that generally complies with regulations include Marine Gas Oil (MGO) defined as DMA, DMX, DMZ grade and Marine Diesel Oil (MDO) defined as DMB grade in ISO 8217.

Characteristics of Low Sulphur Fuel Oil:

1. Low Viscosity: The viscosity of low sulfur oil is extremely low compared to the viscosity of heavy fuel oil and marine diesel oil. As a result, internal leakage in FO pump, increases in flowability of fuel from fuel injection nozzle, and so on, are likely to occur and the effect of these on various equipment is a cause for concern.
2. Low Lubricity: Since the lubricity of fuel oil depends on the viscosity, the lubricity of low sulfur fuel oil is low. Consequently, abnormal wear is likely to occur in the sliding/contact parts of pumps, etc.
3. Acidity: The sulfur content in fuel oil changes to acid salt because of combustion, but most of it becomes sulfur dioxide (SO2) while a part of it gets converted to an anhydrous sulfurous oxide (SO3). Anhydrous sulfurous oxide reacts with water in scavenging air or water generated during combustion to form sulfuric acid (H2SO4). With the reduction in sulfur content, the amount of sulfuric acid generates also reduces. When cylinder oil with high alkali base number for heavy fuel oil in diesel engine is used continuously, deposits may be formed and abnormal wear of the cylinder liner may occur.
4. Flash Point: Fire hazard increases because the flash point of low sulfur fuel oil, such as gas oil, is low. The use of fuel oil with flash point below 60'C is prohibited according to the SOLAS convention. On the other hand, the lowest flash point of gas oil of DMX grade defined in ISO 8217 is 43'C, the introduction by mistake of gas oil with such low flash point into ships is a cause for concern.

Precautions When Using Low Sulfur Fuel Oil
Diesel Engine

1. Compatibility of Cylinder Oils With Fuel Oil: The Lubrication of cylinders is an important point to be borne in mind for low-speed diesel main engines. Until now, it was commonplace to use high alkali base number cylinder oil for heavy fuel oil(sulfur content 1.5% or more), but engine and oil manufacturers are recommending the use of low alkali base number cylinder oil for low sulfur fuel oil(sulfur content below 1.5%). As measures for low sulfur fuel oil, it is important to monitor the exhaust gas temperature to check abnormalities of ring/liners and to carefully check the condition of the rings during the maintenance and inspection work. If any sign of abnormality is detected, the most realistic response that can be taken is to adopt the necessary measures after discussion with the engine manufacturer.
2. Low Viscosity & Low Lubricity: Currently, the low lubricity of gas oil is most focused and important issue related to properties of low sulfur fuel oil. Problems associated with lubricity included abnormal wear of plunger/barrel and oil leakage in fuel injection pump for generator engine. Generally, the lubricity of fuel oil is considered to be due oil film formed because of viscosity and due to the lubricity of fuel oil is considered to be due to oil film formed because of viscosity and due to the lubricity of sulfur content itself. Generally, such low viscosity is not considered during the design stage of fuel injection pump having sliding/contact parts. To prevent problems due to low lubricity beforehand, fuel oil cooler may be installed, or agents may be added to fuel oil for improvement of lubricity.
3. Ignition Delay & Defective Combustion: The viscosity and residual carbon in low sulfur fuel oil in recent years are generally low. There is a high probability that clarified oil with high aromatics is used as cutter stock in the manufacturing stages of such fuel oil. Fuel oil with high aromatics tends to become ignitable, combustible and also defective such fuel oil generally hinders combustion because of the abnormal wear it causes in rings/liners. When a sign of abnormality is detected (rise in exhaust temperature etc) the following methods may be considered as immediate on board the ship;

1. Reduce engine output
2. Reduce cylinder cooling water temperature
3. Tentatively increase the cylinder oil quantity
4. Raise the scavenging air temperature
5. Advance fuel injection timing
6. Enhance fuel cleaning
7. Mix with normal fuel
8. Add combustion accelerating agents

Boilers

1. Turn Down Ratio: When low sulfur fuel oil is used in the burner of heavy oil burning system, the turndown ratio increases, and the minimum boiler load increases. As a result, problems may occur, including deterioration of ignitability. Therefore, change over to an atomizer suitable for combustion of low sulfur fuel oil, if necessary.
2. Flame Detection Ability: Sometimes combustion of low sulfur fuel oil cannot be detected in the cadmium sulfide cell type flame eye combustion of heavy oil because of the difference in flame radiation spectrum. In such cases, change over to or install additional photo IC type flame eye to suit the flame generated.
3. Air-Fuel Ratio: Sometimes abnormal smoke may be emitted due to the excess supply of air for combustion because of reduction in the supply of fuel oil from the oil fuel pump and because of the difference in calorific value of heavy oil and low sulfur fuel oil. In this case, the fuel oil injection amount or the air-fuel ratio of the boiler combustion control system needs to be adjusted.
4. Low Flash Point & Volatility: The risk of flashback increases when low sulfur fuel oil leaks within the boiler furnace because the flash point of this fuel oil is low and it has high volatile content. To prevent fuel leaks within the furnace, duplication of fuel oil shut-off valve or duplication of flame eye so as to increase the chances of detecting of fuel shut off valve or duplication of flame eye so as to increases the chances of detecting flame failure is recommended. Especially when unburnt oil enters the furnace appropriate measures must be taken such as cooling the boiler adequately purging the furnace and so on.

Fuel Pump: The flow rate and pressure reduce due to low viscosity that causes an increase in leakage in the contact parts within the pump and sometimes that quantity of fuel supplied becomes deficient. Similarly, leakage from the pump seal may occur, so the seal needs to be replaced. Also, self-lubricating ability deteriorates, leading to concern about reduction in life of the bearing. As a measure, pump suitable for the viscosity of the fuel oil may be additionally installed or a spare pump may be installed.


Change Over of Fuel Oil: If heavy fuel oil is used to operate an engine, the fuel oil is to be heated to above 100'C and used so as to lower the viscosity. However in this condition, if the fuel oil is changed over to low sulfur fuel oil, the low sulfur fuel oil with high volatility flows into the high-temperature oil supply pipes and vaporizes, This may lead to abnormality in the fuel pump or cause a vapor lock. Unstable combustion and moreover, flame failure may occur in the boiler leading to a risk of explosion in the furnace.
Measures include changing over the fuel oil after the temperature of the fuel supply piping has adequately reduced, installing a fuel oil cooler, and separating the supply pipes for low sulfur fuel oil and heavy fuel oil. However, from the aspect of safety, it is important first to establish the change over procedure of fuel oil.

The following precautions are particularly necessary during the change over of fuel oil:

1. Stop the steam supply to the steam tracer for fuel oil supply piping and fuel oil heater and reduce the fuel temperature (generally below 60'C). At this stage, bypass the safety device for fuel oil low temperature, etc, in the boiler.
2. If pump for low sulfur fuel oil has been provided, change over the pump.
3. If necessary, especially in large boilers provided with steam atomizing burner, replace the atomizer and sprayer and change over the combustion control mode to MGO mode.
4. If combustion defect or emission of black smoke occurs from the boiler, adjust the air-fuel ratio by the fuel/air adjuster in the boiler control panel.

Compatibility of Mixed Fuels: When conventional fuel oil and low sulfur are mixed due to change over operation, overflow, return from machinery, etc large amount of sludge may sometimes be generated depending on the extent of matching of these fuel oils. It is preferable to take precautions to avoid clogging of strainer and unwanted mixing of fuels as far as possible.


Low Flash Point: Fuel oil with flash point below 60'C are prohibited according to SOLAS convention. In case of bunkering, the bunker delivery note must be properly checked to ensure that the flash point is 60'C or more.