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As we have seen heavy fuel oil is a residue from the crude oil refining process and as such is the dregs of the process.
It is used as a marine engine fuel because of its cheapness, but it does contain a lot of impurities and must be kept at a high temperature both during storage and use in ships main engines.
Many years ago when I was a young engineer at sea, I was in charge of taking bunkers either from a bunker barge or shoreside supply. I was always nervous as an oil spill, even in those days was subject to a huge fine.
This is an article in Marine Engineering and in particular the properties of heavy fuel used in marine engines.
We begin with a quick overview of the crude oil refining process and move on to the properties of heavy fuel oil.
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Overview of Crude Oil Refining
The crude oil arrives at the oil refinery from the oil production offshore platform subsea pipeline landfall point, or from oil tankers.
It will have most of the gas, water, and sand/grit removed by the production platform facilities before being processed at the crude oil refinery.
On arrival at the refinery it is again subjected to dewatering and salts removal before being heated to around 700ᴼF and fed into an atmospheric distillation column. This is a vertical column with trays situated at specific heights inside the column.
As the crude oil vapors boil off and pass upwards, the lower boiling point factions such as diesel, kerosene, and naphtha are drawn off at the trays at various temperatures for further refining.
Meanwhile the higher boiling point crude oil residue has dropped to the bottom of the distillation column. It is from these “bottoms" that the heavy fuel oil is drawn off, literally the bottom of the barrel. If no further processing is required it is stored in tanks ready for distribution.
Note: Heavy Fuel Oil can sometimes have addition of lighter fuel oil to reduce excessive viscosity. But it is always advisable to have samples of bunker oil checked to ensure no other “blending" has taken place i.e. addition of lube oil or addition of used motor oil!
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Properties of Heavy Fuel Oil Used In Marine Diesel Engines
The major properties of heavy fuel oil are as follows;
Density is the relationship between mass and volume at a stated temperature, and the SI unit is Kg/m3. This gives value of 800- 1010 kg/m3 for marine heavy fuel oil.
The density of HFO is limited to 991kg/m3 to facilitate efficient centrifuging. (Centrifuges use centrifugal force to separate water from HFO, so the bigger the difference in density the greater removal of water ). However the density of HFO can be been increased to 1010kg/m3 if clarifiers are used, as water seal can be maintained during clarification. Any higher density than this will inhibit HFO water removal treatment through centrifuging.
Viscosity is a measurement of the resistant of a liquid to shear or flow and is measured in Centistokes (CST) with a quoted reference temperature. Knowing the viscosity of the heavy fuel oil is required the temperature range required for satisfactory injection at the fuel atomizer, and efficient combustion.
3. Flash Point
The flash point of the liquid is the lowest temperature at which sufficient vapor is given to produce a flash on application of a flame under specified test conditions.
The minimum flash point for marine fuels in the machinery space of merchant ship is governed by the IMO; being set at 60o C to minimize fire risk during normal storage and handling.
4. Pour point.
The pour point is the lowest temperature at which a marine fuel oil can be handled without excessive amounts of wax crystals forming out of solution. At a lower temperature the fuel will gel, thereby preventing flow.
5. Carbon Residue.
The carbon residue of a fuel is the tendency to form carbon deposits under high temperature condition in an inert atmosphere, and may be expressed as either Conradson Carbon Residue (CCR) or Micro Carbon Residue (MCR). The maximum limit of carbon residue content in fuel is 22 %.
Normally the level of water in the fuel is very low, since an effort is made by the supplier to deliver the fuel as dry as possible. 0.1% - 0.2 % by volume is typical. However, Water can come from a number of sources on the bunker barge and in ships bunker tanks. These include tank condensation, leakage of steam from heating coils, and bunker tank leakage. The maximum limit of water content in fuel is 1 %.
7. Ash content
The ash content is related to the amount of inorganic material in the fuel. For distillate fuel the ash content is defined as the residue remaining after all the combustible components of the oil have been burned and is negligible. It is the incombustible material which remains after the combustion, which mainly consists of the material such as vanadium, sulphur, silicon, aluminum, nickel, sodium, and iron content present in the fuel. The maximum limit of ash content in the fuel is 0.2% m/m.
8. Calorific Value (CV)
The CV of a fuel is the heat energy given out during combustion. Heavy fuel oil has a net CV of 38.9 MJ/liter and a gross CV of 41.2 MJ/liter; these values being determined by using calorimeter test equipment in a lab.
9. Specific Gravity
The specific gravity of HFO is its ratio to the density of water at a specified temperature. Bunker oil HFO ranges between 0.95-1.03
Sulphur is soluble in water therefore it cannot be removed by centrifuge treatment. However there are chemicals that can be used to lower the sulphur content in HFO.
Sulphur pre-combustion causes corrosion to the pistons and liners and contaminates the lube oil. Post combustion emits SOx to atmosphere in exhaust gases.
11. Calculated Carbon Aromaticity Index (CCAI).
This is a reference index containing a listing of the ignition standard of Heavy fuel oil. Cetane number is the equivalent reference for light fuel oil such as diesel oil.
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Storage of the fuel is an important aspect. It must be stored at a temperature above that of its pour temperature to avoid wax formation and for ease of transfer via pumps for treatment and intermediate storage in settling tanks and day tanks. In this article we also discuss the need for treatment through purification of the heavy fuel oil before it is used. The combustion products of the heavy fuel oils are regulated by MARPOL and EPA regarding the emission of NOx, SOx and particulates.
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The Need for Treatment of Heavy Fuel Oil Before Use
Heavy fuel oil or residual oil is a by-product of crude oil refining process, containing a lot of the contaminants removed from the lighter oils. This makes it much cheaper than other lighter marine fuels and is the main reason it is used in marine engines.
It is very viscous and requires to be kept at a temperature above pour point in bunkers and storage tanks to ensure efficient transfer and combustion.
This is achieved by use of low steam coils in the bunker and storage tanks and a series of heaters between here and the engine fuel pumps and injectors to keep the oil between 104°F in the main bunkers and 250°F at the main engine injectors.
An image of a ship taking bunkers from a bunker barge is shown below, please click on image to enlarge;
Further treatment of HFO to remove some of the contaminants is required before use and this consists of the following processes;
The heavy fuel oil is taken on board and stored in the bunkers. The bunkers contain steam coils and maintain the HFO at 104ᴼF.
From the bunkers the HFO passes through a strainer and tube/shell heater into a positive displacement pump that discharges the oil through a pair of duplex magnetic filters where any ferrous metals are removed.
Transfer pumps supply the settling tanks where any sediment is allowed to drop out and the oil is maintained at 200ᴼF. From here the HFO drops down to the centrifuges and clarifiers that remove dirt particles and water from the HFO. Transfer pumps discharge the oil from here to the day tanks where the temperature is maintained at 200ᴼF.
High pressure pumps now discharge the treated HFO through a final heater that controls the temperature and viscosity automatically. The high pressure HFO is discharged through very fine mesh strainers, onto fuel pumps and into the injectors at around 250°F.
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Combustion Products of Heavy Fuel Oil
Combustion products of the heavy fuel oil are mainly H2O, CO, and CO2 although due to the other impurities of sulphur and nitrogen, combustion will also produce large amounts of NOx and SOx gases to the atmosphere.
SOx and Particulate Matter (PM10’s), along with the heavy metals are considered to be the most dangerous to human health and. the environment. This is compounded by the fact that these emissions increase near the coastline due to the engines slowing down and maneuvering.
The incombustible material that remains after the combustion mainly consists of the metals vanadium, silicon, aluminum, nickel, sodium, and iron that are present in the original heavy fuel oil supply.
MARPOL (short for Marine Pollution) works under guidance from the International Maritime Organization (IMO) in the prevention of air pollution by the ships at sea, and sets limits on the amount of impurities in marine fuels.
The Environmental Protection Agency (EPA) governs the uses and combustion emissions of marine fuels in maritime waters.