What is the difference between MDO and MGO ?

Difference between MDO and MGO

The term marine diesel oil (MDO) generally describes marine fuels that are composed of various blends of distillates (also called marine gasoil) and heavy fuel oil. Unlike diesel fuels on land that are used for cars and trucks, marine diesel oil is not a pure distillate. The different blending ratios of marine diesel oil can be controlled directly by processes in the refinery or by blending ready-made marine fuels. Marine diesel is similar to diesel fuel, but has a higher density. Unlike heavy fuel oil (HFO), marine diesel oil does not have to be heated during storage.

Marine diesel oil is sometimes also used synonymously with the term “intermediate fuel oil” (IFO). In the strict sense, the term marine diesel oil mainly refers to blends with a very small proportion of heavy fuel oil. This type of marine diesel oil is therefore also classified as a distillate in some textbooks, which means it is also categorized as a middle distillate. Intermediate fuel oils, on the other hand, have a higher proportion of heavy fuel oil. Accordingly, IFO types with particularly high proportions of heavy fuel oil are sometimes classified as heavy fuel oils in some textbooks, standards/norms and publications. This results in the following brief descriptions:

Marine diesel oil in general: a blend of distillates and heavy fuel oil
Marine diesel oil in a narrow sense: Blend of distillates and heavy fuel oil, but with very low heavy fuel oil content 
Intermediate fuel oil (IFO): Marine diesel with higher proportions of heavy fuel oil

According to the ISO 8217 "Petroleum Products – Fuel (class F)" international standard, marine diesel oil with a lower proportion of heavy fuel oil include the DMB and RMA 10 fuels. The heavy fuel oil components in DMB marine diesel oil – which is classified as a distillate according to ISO 8217 – come mainly from residues of heavy fuel oil from tanks where DMB is stored. Due to this pollution by heavy fuel oil, its color changes can range from light brown to black. As emission limits for Emission Control Areas (ECAs) become ever stricter, DMB with its relatively high sulfur content of about 2% is being used much less frequently.

According to ISO 8217, marine diesel oil RMA 10 already counts as a residual fuel. It has a similarly low viscosity to DMB, but a higher maximum permissible sulfur content of 3.5%, and also a higher proportion of heavy fuel oil. Therefore, this type of fuel is also usually darker than DMB. Its color spectrum starts from dark brown and likewise ends in black.

Intermediate fuel oils are black due to their higher proportion of heavy fuel oil. In ISO 8217, IFO fuels with RME, RMG and RMK designations and viscosities of 180 mm²/s or 380 mm²/s count as residual fuels. These marine diesel oils are already so viscous that they must be heated so that they can be pumped at all.

Marine diesel oil is sold with different sulfur content levels. For example, IFO 180 and IFO 380 can have a maximum sulfur content of 3.5% according to ISO 8217. They are also sold in a low-sulfur variant, which has a sulfur content of less than 1%. Ships could even enter an Emission Control Area (ECA) with the later. However, if ship-owners use an engine-fuel combination with a high sulfur content, the emission limit values can also be met by using additional technology (filter systems, scrubbers).

Their different blending ratios make it possible to use marine diesel oil in many different engines. Lighter versions like DMB and RMA 10 are used to power smaller medium- to high-speed marine engines and auxiliary power units, as well as auxiliary engines on very large ships, while the viscous IFO 380 is mainly used in large aggregates. Lightweight and low-sulfur marine diesel oil is sometimes burned in larger engines as well – whenever an area with stricter emission limits is being crossed. Once outside the area, the ship will switch back to a marine fuel with higher emissions.

Marine gas oil (bunker fuel or bunker oil) is any type of diesel which is used in seagoing vessels. The term ‘bunker’ is used for a vessels fuel storage tank and a marina’s fuel storage tank which is where the name ‘bunker fuels’ comes from.

The Marine Gas Oil (MGO) can be used in four-stroke machines and generators. This fuel meets the ISO-F DMA specifications and the sulphur limit required for all European ports.

MARPOL Annex VI "Regulations for the Prevention of Air Pollution from Ships" stipulate that Heavy Fuel Oil (HFO) combustion is not permitted during port stay and in specific areas at sea. The switchover to lighter types like Marine Diesel Oil (MDO) or Marine Gas Oil (MGO) requires monitoring and some modifications to the burner and fuel systems.

 

MDO & MGO - Fuel property comparision

If special fuel such as low-sulphur fuel oil is not available in a port, the obvious choice of fuel would be a good quality Marine Diesel Oil (MDO) or Marine Gas Oil (MGO). These two fuels differ in their properties, however, and should be handled differently as described in the following.

Heat value(calorific value)

The heat value of lighter fuel oils is typically a little higher than the heat value of HFO.The additional heat input to the boiler is, however, considered to have an insignificant influence on the boiler itself. In some cases, it may nevertheless be necessary to re-adjust the air/fuel ratio if the pre-set air amount is beyond the limit and grey smoke is generated.

Viscosity

Lighter oils commonly have lower viscosity and need not be preheated. When operating on fuel oil types of low viscosity, it must be ascertained that the fuel pumps are indeed able to operate with the low viscosity.

It should also be considered that the viscosity mentioned in the specification is indicated with
a reference temperature and that the actual operation temperature might be higher causing the viscosity to drop further.

Heat tracing should be shut off as should oil pre-heaters that have not been bypassed.

Density

Typically the density of lighter fuels is lower than of HFO (and MDO), which may have the result that the amount of (lighter) fuel to the burner will differ from the amount original pre-set and thus for instance cause ignition problems or
increased smoke emission.

This can happen because most marine installations have the oil amount pre-adjusted based on a calculation of the main fuel, and on a manual volume calculation/ adjustment that takes its density into account.

Flash point

The flash point for some lighter fuels is occasionally lower than the requirements from the classification societies, which should be considered when storing in the traditional fuel tanks onboard.

Lubricating properties

Lighter fuel oils traditionally also have lower lubricating properties. The hydrodynamic lubrication can, by using the well-known Sommerfeld number, be described as a function of the viscosity, So = f (v) x k, for a certain pump. This means that when the viscosity is reduced, the lubricating properties are reduced, which is an important aspect to consider too, with relation to fuel pumps

Fuel system

Most of the components in the fuel system will not be affected by operation on a lighter fuel oil, but some considerations have to be made.

Fuel pumps

It must be ascertained that the fuel pump can actually operate with the new fuel under the given operating conditions. As most fuel pumps are either screw pumps or gear pumps, it is important to check if the pumps are able to operate with the lower viscosity of the new fuel, as there is a risk of increased wear and tear as well as breakdown if the pump is unsuited for the viscosity.
Fuel pumps running continuously during periods when the boiler/ burner is in standby position may heat up, causing the temperature of the fuel to increase and thereby the viscosity to decrease. In this case, the control of the pumps should be considered, too. In case the pump control system is not preset to do it already, it may be preferable to adjust the control system in such a way that the pumps are always shut off when not required.

Fuel preheaters

Light fuels rarely need to be preheated, and if heating is unnecessary, the fuel pre-heaters should be bypassed during operation on light fuel to avoid the risk of the oil being overheated.

Fuel pipe tracing

If the fuel does not need preheating, the fuel pipes do not need to be heated, and consequently the tracing of the fuel pipes should be shut off. If preheating is necessary, check if the tracing is sufficient. Be careful that the tracing does not heat the fuel more than necessary.

Change-over between fuels

When changing over/switching between fuels, make sure that fuel in the return/recirculation pipes is returned to the correct fuel tank. Make sure that the new fuel is not preheated in an unintended way.

Burners

The fuel change-over affects the three common burner types normally supplied for marine boiler installations in various ways. It is recommended a post-purge of the furnace whenever MGO has been used.

It should be observed that the lower viscosity for fuel pumps in systems supplied by most manufacturers is normally 4.5 cSt. This means that if a fuel with a lower viscosity is used, the fuel pumps must be changed or modified.

Pressure jet burner

Pressure jet burners are typically used on smaller boiler types and run on MDO, MGO and HFO. Installations delivered by most manufacturers will typically be able to operate on both MGO/ MDO and HFO.

A lower viscosity may also cause an increase in the fuel input through the nozzle, and the risk of increasing smoke emission arises.

Rotary cup burner

Rotary burners are used on all boiler types and run on MDO, MGO and HFO. Installations delivered by most manufacturers will typically be able to operate on both MGO/MGO and HFO.

For smaller burners there should be no problem with operation on a lighter fuel; however, the fuel amount (pressure) should be checked/adjusted in order to obtain a reasonably smoke-free combustion.

For larger burners, there is a risk of coke deposit creation in the burner cup, if the installation is not fitted with a suitable heat shield. This happens because the heat radiation into the rotary cup generates a too high temperature of the fuel in the rotary cup causing the fuel to start coking. It is therefore recommended having the rotary cup installation checked and if necessary modified.

Steam atomizing burner

Steam atomizing burners are typically used on medium and larger boiler types and run on MDO, MGO and HFO. As the viscosity of the fuel burned in the steam atomizing burner is commonly in the range 15–30 cSt, the lower viscosity of a lighter fuel may cause over-firing if the pressure alone controls the oil amount to the burner.

For continuous operation with lighter fuels, it is recommended to use, either compressed air as the atomizing medium, or changing the lance to a type that does not heat the fuel in the same way as the traditional lance.

Due to easier evaporation of lighter fuels, it is recommended to  adjust the control system so that the main burner does not accidentally ignite in case of a missing ignition flame/ source, just as unnecessarily vaporized fuel should be avoided.

Modifications to the existing boiler/burner system

When having to operate a boiler/burner system on a lighter fuel than originally intended for the plant, the complete boiler/burner installation should be examined by authorized personnel, as this is not just a matter of complying with legislation but equally so a serious safety matter.

Steam atomized burner lance

There are two options for the atomization (compressed air and steam) and burner lance design, and it is recommended to change  the existing lance to a modified type where internal volume has been reduced to an absolute minimum and specially designed to operate with MDO/MGO and with steam as atomizing medium. The lance is constructed so the steam piping is insulated from the fuel piping in the burner lance thus avoiding preheating.

Flame supervision

On burner plants, usually only one flame scanner is installed (main flame supervision) from the outset. It is recommended to have two flame scanners for the main flame supervision and in addition one separate flame scanner to detect the operation of the ignition burner.

Effects on Main engine fuel oil system

The lowest viscosity suitable for two-stroke diesel engines is 2 cSt at engine inlet. However, this viscosity limit cannot necessarily be used as a fuel specification for purchasing the fuel, as the viscosity in a purchase specification is tied to a reference temperature.  This is due to the fact that the external fuel systems have an individual effect on the heating of the fuel and, thereby, the viscosity of the fuel when it reaches the engine inlet.

The external fuel oil systems on board today have been designed to keep a high temperature for HFO operation. This can make it difficult to keep the fuel system temperature as low as possible, and thereby as high a viscosity as possible, when changing to DO and GO operation.
Many other factors influence viscosity and its influence on the engine, such as engine condition and maintenance, fuel pump wear, engine adjustment, actual fuel temperature in the fuel system, human factors, etc. Although achievable, it is difficult to optimise all of these factors at the same time. This complicates operation on viscosities in the lowest end of the viscosity range.
To build in some margin for safe and reliable operation, engine manufacturers recommends operators to test the engine’s and external systems’ sensitivity to low viscosity. Furthermore, the necessity for installation of a cooler or cooler & chiller should be evaluated before purchasing fuels with the minimum level of viscosity necessary.

In principle, fuels according to the specified grades DMX/ DMA can be purchased, if the engine and external system are designed to keep a minimum viscosity of 2 cSt at engine inlet. If 3 cSt can be obtained, this is preferred to ensure a higher safety margin.

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