Ships rely on fuel products in order to sail and deliver cargoes and shipments around the world. Marine engines are known for using low-grade fuel products as they cost cheaper than most of the fuel by-products.
However, using low-grade products such as heavy fuel oil (HFO) causes more pollution due to its composition. Thus, most ships are not allowed to enter emission control areas (ECA’s) unless they change over to fuels with low sulfur content like MDO and MGO.
Reducing air pollution has been the main goal of the International Maritime Organization (IMO), that’s why they implement conventions regarding lessening carbon dioxide CO2, sulfur oxide SOX, and nitrogen oxides NOx.
IMO even started to implement the last January 2019 the use of exhaust gas cooling systems (EGCS) with a scrubber and selective catalytic reduction (SCR) for ships using HFO in order to lessen the emission content of NOx and SOx on smokes.
What Are The Major Types of Marine Fuels?
Marine gas oils (MGO) are considered high-quality oils and are much more expensive than marine diesel oil. MGO consists mainly and exclusively of distillates. Distillates are components of crude oil that evaporate during distillation and then later on condensed from a gas into a liquid.
Meanwhile, marine diesel oil (MDO) is distillates as well but in the lower bracket of the refining process. MDO is produced by heating and evaporating residual oil that is blended by catalytic crackers and/or cokers that processes on medium to heavy sour crudes.
These are often called residual destruction processes to obtain more diesel from residual oil hence, MDO may contain a very subtle amount of black refinery feedstock due to the residual oil.
In short, MDO in general is a mixture of distillate and a little HFO. However, due to the hefty demand for MGO and MDO, refineries are optimizing their production in order to refine residual oil more to produce MGO and MDO since the price of HFO is slowly dropping.
What are the differences between MDO and MGO?
Marine gas oils and Marine diesel oils have quite a lot of differences. Here are some of their characteristics.
Flashpoint is a characteristic of fuel where the temperature at which the vapor of the heated fuel will ignite. MGO when heated has a flashpoint between 63-87oC while MDO has a flashpoint of 61oC. However, both fuels have a minimum flash point of 60oC as per IMO standards for safety regulations.
The density of fuel indicates the ignition quality and is also used to calculate the amount of fuel that is supplied onboard during bunkering. The density of MGO is close to 850 kg/m3 @ 15oC and has a maximum density of 890 kg/m3 @ 15oC. Meanwhile, MDO has a maximum density of 920 kg/m3 @ 15oC.
- Cetane Index
The cetane index indicates the flammability of the fuel. The higher the cetane index, the higher the flammability. As for MGO, it has a cetane index of 40 while MDO has only 35.
Viscosity is a characteristic of a fuel that describes resistance to flow. Kinematic viscosity represents the dynamic viscosity of a fluid per unit of density.
This characteristic is also used to define the ease of atomization and the ease to pump it into the fuel system. Moreover, the viscosity of the MGO is 5-7 CST (centistoke) at 400oC meanwhile as for MDO, its viscosity is around 8-11 CST at 400oC.
- Sulfur content
This is one of the major concerns of IMO, for sulfur content contributes to the non-ending pollution emitted by the ships.
Fortunately, MGO has a low sulfur content which is lower than 0.1%. This enables the MGO to be used in the sulphur emission control areas (SECA) for shipping, under MARPOL (Annex VI).
On the other hand, MDO has a sulfur content of 2% which significantly produces more soot and matter than MGO.
What is marine gas oil used for?
Mostly, marine gas oils are used by small to medium boats, ferries, or tugs that have no space to equip heaters and purifiers to purify HFO. Since MGO is based on lighter distillates, they have low viscosity and can be pumped around 20-40oC.
Marine gas oils have lower sulfur content than other fuels, making them convenient to use on ports and SECA because it has lower emission of NOx, SOx, soot, and matter.
SECA includes the Baltic Sea, North American ECA, North Sea, and the United States Caribbean Sea ECA. Moreover, some ports are starting to adopt regulations to change-over fuel consumption to MGO prior to entering their territorial water.
However, the common problems that the engineers encounter during switching over to MGO are fuel mixing, leakage, and lacquering.
Engineers must be very careful not to exceed the limit of mixing the two incompatible fuels during the change-over process. This will increase the risk of burning two kinds of fuel with different densities and viscosities which will result in building asphalt of the heavy fuel precipitating as heavy sludge and lead to filter clogging.
Moreover, leakage may occur due to a change of viscosity during change-over. MGO has a lower viscosity than HFO resulting in leaks on o-rings and gaskets.
Along with this, burning MGO produces deposits that stick on the liner surface and disturbs the oil film lubrication resulting in lacquering on the liner.
What is the difference between HFO and MDO?
When it comes to HFO, they are the leftovers from the crude oil after utilizing almost everything during the refining process. Also, they are the non-distillable oils from atmospheric distillation which remain after boiling at 500-600oC.
However, modern refineries use vacuum distillation in order to squeeze lighter oils from residual oils to gain more profit. They are often called catalytic cracking, coking, and topping. HFO needs more heating than the MDO because it has a higher viscosity which makes it harder to pump if not heated.
Unfortunately, compared to MDO, HFO has a higher density and sulfur content of 3.5 to 4.5% m/m which needs to be treated and purified to lessen the emission of pollutants in the air.
Anyhow, HFO costs cheaper than the MDO which is most likely what the owners and charterers consider using.
The calorific value of MGO, MDO, and HFO
The calorific value of the fuel indicates the quality of the fuel during combustion. It is the amount of heat produced by the complete combustion of one unit of fuel. MGO and MDO have almost the same calorific value of 42,700 to 43,500 kJ/kg. Meanwhile, HFO has an approximately lower calorific value of 39,000 to 41,000 kJ/kg.
In conclusion, engineers should have complete knowledge in changing over from one kind of fuel to the other during entering and leaving SECA in order to avoid mixing two different grades of fuel which has non-compatibility issues.
Moreover, when being supplied by MGO from bunker tanks, make sure that the MGO storage tanks onboard are clean and free from sludges that cannot be drained during normal operation.
Furthermore, engineers must ensure that the fuel they are using has the right temperature and viscosity to maintain the fuel pump pressure that is needed by the engine.
However, most light distillate fuels don’t need preheating, so maintain the temperatures for HFO to avoid clogging and to be able to provide engine efficiency.