How is Fuel Formed ?

  • Vignesh 
  • 22 min read

The topic of how fuel is formed is dependent on the type of fuel being considered. Fossil fuel is made up of hydrocarbons, hydrogen fuel is made up of both fossil fuel and water electrolysis, and nuclear fuel is distinct from all other uses of radioactive energy.

A fuel is a material that interacts with another to create energy that can be used to perform useful tasks. We must understand how these fuels are produced. Fossil fuels are a kind of energy that comes from the decomposition Is a term used to describe a fuel produced by the anaerobic decomposition of geological or prehistoric living organisms. In terms of chemistry, all organic matter is made up of carbon, and these fossil fuels are made up of hydrocarbons with various compositions and chemical formulas.

In boat, the most popular fuels are fossil fuel derivatives (HFO, diesel oil, low Sulphur and extremely low Sulphur fuel (VLSF) & marine oil gas). We obtain these fuels by a method known as refining. The fuels are refined in refineries, where fractional distillation is used to refine each fuel at a different temperature.

Fig. Fractional Distillation Process

The fractional distillation process involves heating crude oil in a furnace and sending the gas to a distillation chamber, where different fuels are produced at different temperatures.

The following image depicts a more accurate version of the fuel storage in the vessel as bunker fuel and its association to residues.

Fig. Transferring of Marine Bunker Fuels

Residual fuels are made up of residues from various refining operations. The fuel is heated and fed into an ambient distillation column, where the lighter hydrocarbon compounds are distilled out as gases such as diesel, kerosene, and gas oil, leaving approximately half of the petroleum as residual. Any of these residue is fed into the vacuum distillation column; where it is refined further to create vacuum gas oil, waxy distillates, and residue.

This residue can be used in naval bunkers as part of the mix or fed into thermal crackers. Lighter hydrocarbon molecules assemble and are extracted as light distillates under high pressure and temperature. Often, the heavy gas oil from the vacuum distillation column can be subjected to catalytic cracking, in which lighter hydrocarbon molecules are formed and split into even lighter distillate fractions as a result of a chemical reaction.

Factors on Which Fuel Quality Depends

The bulk of the fuel oils we use are derived from crude oil. Although the composition of crude oils varies, the major constituent levels are consistent; for example, carbon is 83-87 percent and hydrogen is 11-14 percent, with the rest being sulphur, oxygen, nitrogen, and other trace elements.

However, although the chemical components are identical, the atomic structures that can be created differ significantly. There are three main types of hydrocarbons in crude oil:

  1. Parraffin or alkane series
  2. cyclo-alkanes or Naphthene
  3. Aromatics or arenes

Parraffin or alkane series

These are the most fundamental hydrocarbons. They begin with the simplest member, methane, and gradually increase in size and complexity by adding H- c-H units to the centre of the compound. The molecules often form straight chains, and the longest molecule units, which have 5-16 carbon atoms, have more interlinking between them and are more viscous. The liquid state is formed by the numbers 5-16. As the number of carbon atoms in a molecule rises above 16, a semi-solid material called wax is formed.

Pentane, hexadecane (last of the liquids at 16 C atom) are all part of the straight chain totally saturated (i.e. each carbon atom is connected to two hydrogen atoms) paraffin family that end in the suffix ‘ane’. Self-ignition temperatures of these groups are from 220 °C to 250 °C.

Cyclo-Alkanes or Naphthenes

The completely saturated carbon and hydrogen mixture forms a crossed cyclic or ring arrangement in this structure.

The ring structure can have anywhere from three to seven carbon atoms, but six is the most common, as in cyclohexane C6H12. While cyclic units can bind together, straight chain paraffin, such as C6H12 changing to dimethyl cyclohexane C8H12, is more common. This hydrocarbon group’s self-ignition temperature varies from 380°C to 400°C.

Aromatics or arenes

Aromatics, also known as arenes, are a ring-structured group of polyunsaturated hydrocarbons. The non-localized double bond format is used to describe why the unsymmetrical, chemically unstable unit has such high stability. These ‘delocalized’ electrons from double bonds serve as grabbers, attracting other elements to form replacement products by binding to the primary benzene ring.

These hydrocarbons have a self-ignition temperature of 500°C to 550°C.

Fuel Oil Properties

  1. Density:- It refers to the mass of the fuel in relation to its length. Kg/cub.m is the unit of measure.
  2. Viscosity:- The frictional resistance between layers of fluid to withstand a change in shape as a result of an applied force is known as viscosity. In a flowing fluid, it refers to the resistance that exists between neighboring layers.
  3. Specific viscosity:- It’s the average of 200 cubic cm of gasoline efflux time at 20 to 50 degrees Celsius to 200 cubic cm of pure water efflux time at 20 degrees Celsius, as determined by a viscometer with a 2.8 mm orifice. ‘Degree of basic viscosity’ is the unit.
  4. Dynamic viscosity:- It is the viscosity of a fluid in a laminar steam lined flow with layers spaced one centimeter apart that requires a tangential force of one dyne per sq.cm to be transported at velocities that vary by one centimeter per second.
    • 1P = 1 poise   = 0.1N-sec/sq.
    • 1 cP = 1 centi-poise = 0.001 n-sec/sq.m
  5. Kinematic viscosity:- It is the ratio of dynamic viscosity and density of the fluid at same temperature. The unit are stokes, centi stokes, saybolt seconds, or redwood seconds.
    • 1 stoke    = 1 st = 0.0001 sq.m./sec
    • 1 centi stoke = 1 CST= 0.000001 sq.m./sec
  6. Viscosity index:- It is the index of oil measures the change of viscosity due to a change in temperature. It has no units.
  7. Carbon residue:- It is the tendency of a fuel to form carbon residue deposits. Its unit is coke value which should not exceed 0.05 to 0.1 %. It affects piston ring, liner wear, plugging of injector, fouling of gas, etc. The testing for carbon residue is done by Conradson test or micro carbon residue test.
  8. Conradson carbon residue:- It is the residue quantity of carbon measured as a percentage of the original mass of fuel, after carrying out the conradson test.
  9. Sulphur:- It is an undesirable corrosion-inhibiting constituent of fuel. It forms sulphur dioxide which combines with water vapor at low temperature, resulting in formation of sulphuric acid.
  10. Flash point:- It is the minimum temperature that oil has to be heated, to produce sufficient volatile vapor capable of ignition when in contact with an open flame. It is the main fire hazard classification of oil. All diesel fuels on ship should have a flash point greater than 60deg.C. The two types of flash point open flash point and closed flash point.
  11. Closed flash point:- It is the minimum temperature for enough flammable mixture to give a flash when a test lamp source of ignition is introduced in a closed container. Closed flash point is measure in a pensky-martin closed tester where the outside atmosphere does not influence the oil vapors.
  12. Open flash point:- Here, there is no lid on the container. Therefore no vapor is lost, but the temperature is sufficient to give a flash, when a test lamp source of ignition is introduced in an open container. Open flash point is approx. of 15 deg.C higher than closed flash point.
  13. Flash point examples:- For temperatures above 15 deg.c, the test used is pensky-Martin closed flash point test, or else the Abel test is used. Flash point examples are:
    • Less than 22 deg C                       gasoline, benzene (dangerous liquids)
    • 22 to 66 deg C                               kerosene, vaporizing oils
    • Above 66 deg C                             oil safe for marine use
    • Diesel oil                                         95 deg C
    • HFO                                                100 deg C
    • Lube oil                                          230 deg C
    • Petrol                                             17 deg C
  14. Fire point:- It is a temperature that oil has to be heated to produce sufficient volatile vapors, capable of ignition by a flammable application and continuing to burn thereafter. It is approx. 40 deg C higher than the closed flash point.
  15. Self-ignition point:- It is the min temperature at which a fuel is capable of ignition on its own accord, without an external application of heat or flame. It is used when the choosing the compression ratio to match the fuel grade.
  16. Pour point:- It is the lowest temperature at which the oil ceases to flow, or can be poured. It is important when considering storing, heating, pumping, wax crystallization, or solidification of oil.
  17. Calorific value:- It is the amount of heat produced by complete combustion of one unit mass of fuel. For one kg burnt, diesel fuels have a high calorific value i.e. 10,100 to 10,300 kcal, while residual fuels produce 9500 to 10,000 kcal. It is used while measuring the thermal efficiency of an engine.
  18. Cetane number:- It is an index of the ignition quality (ignition delay characteristics) of the diesel fuel which defines the way combustion proceeds in the engine. It is determined by comparing the ignition quality of standard solution (which is a mixture of two hydrocarbons called cetane and alpha methyl naphthalene) which the ignition quality of the fuel tested. It is the percentage of cetane contained in the standard solution which has an ignition delay equaling the ignition delay of fuel tested. Cetane which has a very good ignition quality is assigned the number of ‘100’, alpha-methyl naphthalene is assigns the number of ‘0’, due to its poor ignition quality. The higher the cetane number better is fuel shorter is the ignition delay, and easier is the starting of combustion. The cetane number of diesel fuels varies from 35 to 55. If the density increases, the cetane number also increases.
  19. Octane number:- It is a measure of knock rating of the fuel combustion in the engine. Iso-octane is assigned a number of 100, because of its excellent anti knock characteristics. Better the fuel, higher is the octane number.
  20. Specific gravity:- It is used for denoting the weight of the oil while handling or storage.
  21. Ash:- It is the quantity of inorganic incombustible impurity in the fuel. It consists of sand and metal oxide like sodium or vanadium. It causes abrasive wear.
  22. Vanadium:- It is an undesirable impurity in the fuel. During combustion of fuel vanadium products like vanadium pentoxide are formed, which are deposited on surrounding surface. These deposits are highly corrosive above 700 deg C
  23. Vanadium and Sodium:- When both these impurities are present in a Na:Va ratio of 1:3, vanadium Pentoxide which is formed combines with sodium to a form a very hard compound whose melting point is around 630 deg.C. This compound eats into the metal surface, leaving the surface exposed to corrosion.
  24. Total Sediment Test:- It measures the stability of the asphaltene phase of the fuel sediment accumulates at the bottom of the storage tank and has a very high asphaltness content. This affects filters and components.
  25. Catalytic Fines:- After vacuum distillation, catalytic cracking is often carried out. Catalytic cracking is done to crack the oil vapors by reheating with silica and alumina as catalysts. These catalysts are used in power form in an oil vapor. Some of these catalysts break up to form dust known as catalytic fines. They cause abrasion wear in the engines.
  26. Air/Fuel Ratio:- The stoichiometric ratio for proper combustion is 14.5 kg air to 1 kg of fuel. The actual air ratio is 30 to 44 kg per 1 kg fuel. Excess air is 36.5 kg per 1 kg fuel.
  27. Wax:- It is a residue formed due to high paraffinic content. It is soluble in a petroleum oil base. It crystallizes at its cloud point which may be as high as 35 deg.C.
  28. Other Fuel Impurities:- Other impurities in the fuel include water, iron, phosphorus, nickel, lead, calcium, etc.
  29. Calculated Carbon Aromacity Index (CCAI):- It is a rating of the fuel which indicates ignition quality, because ignition directly depends on the aromatic content in the fuel. Aromatics are compact benzene ring structures present in the fuel which affects the ease of which a hydrocarbon fuel molecule can bum. A low CCAI rating means better ignition, better fuel quality and less ignition delay. Low ratings are up to a CCAI ratio of 850. High ratings are from 850 to 950, and 870 is the limit for its use. It does not affect ignition in modem 2-stroke low speed marine engines, but it mostly affects ignition in medium speed engines.

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Fuel Burning Concept

Hydrocarbon combustion refers to the chemical reaction, which produces carbon dioxide, water and heat by reacting with oxygen. Hydrocarbons are hydrogen and carbon ions. This is why hydrocarbon supplies are also known as petrol. Energy from fuels is derived by fuel combustion.

General Reaction Equation:

C x H y + N (O2) ↔ x (CO2) + y 2(H2O)

X denotes the number of carbon atoms in the hydrocarbon.

The number of hydrogen atoms in the hydrocarbon is denoted by the letter Y.

The number of oxygen atoms needed in the hydrocarbon combustion reaction is denoted by the letter N.

Fig. IN THE ABOVE PICTURE THE BLUE ARROW INDICATING HEAT LIBERATED CAN BE USED FOR WORK

This is when a fuel is burned all the fuel is carbon-composed. In an external spark spark plug spark motor operate as a heat source and the heat is formed during the engine’s compression stroke in the compression ignition engine.

Factors affecting combustion of fuel Excess Air Coefficient

Excess Air Coefficient= Actual air supplied/Stoichiometric air. Excess air is supplied to the ensure complete combustion. Power of the engine also depends on the mass of air supplied.

Fuel evenly distributed

To avoid areas with too low air/fuel ratios, the fuel injector must distribute the fuel sprays evenly without overlapping Note that the air will centrifuge

towards the liner wall under centrifuge conditions. Good, even distribution will reduce the time required to complete combustion.

Atomization

It is the breakup of the liquid fuel into a minute vapor mist, so that these fuel vapor particles possess a very high surface area to self-ignite with hot compressed air. Atomization depends on the small orifices of the injector; the pressure difference between the fuel line and cylinder; and the temperature, mass flow rate and viscosity of the fuel. If too much atomization takes place, then very small particles will not have enough kinetic energy to go through the whole combustion space. They will gather near the injector due to resistance from the dense compressed air. Hence, they will be starved during combustion and afterburning will take place. If too little atomization takes place, larger particles will possess more kinetic energy and get deposited on the liner wall. This causes after burning and poor combustion. Carbon deposits will be seen on the liner walls, the side of the piston crown and the piston rings.

Sufficient high air temperatures

In order that the fuel will ignite the air temperature at the end of compression must be higher than the auto-ignition temperature Low air temperature increase ignition delay, and can lead to diesel knock.

Turbulence

It is a factor that has already been designed during manufacture and can only be influenced by fouling of inlet ports or exhaust ports; and scavenge or exhaust pressures. It is given to improve the air fuel mixing. It is done by giving a swirl to the intake air by means of the inlet valve passage shape or angle; changing the size of scavenge ports; the positioning and alignment of the fuel injectors; the burning of fuel; and the squish from the piston shape.

Knocking

This is the occurrence of a high sudden pressure and an increase in temperature caused by fuel explosion. It causes strong shock waves, increased flame front speed, increased noise and vibration, and shock loading to engine components such as bearings, piston rings, and cylinders, among others. In the

case of the sound of a “knock,” check whether the fuel is cut off or mechanical. Mechanical knock comes from worn out rods; missing or loose parts; or unnecessary play between the piston and the rod (worn rings or a worn liner). Diesel knocking is dependent on engine rpm, load, compression ratio, mixing force, fuel properties, delays, timings of injection, cetane and octane number.

Types of Fuel Used Onboard

Marine Fuels

There are pure distillate oils or combinations of them. They are medium viscous diesel fuels with a high residual value. The only standard for fuel requirements is ISO 8217. Cheaper residual fuels are used in modern engines to cut costs.

Heavy fuel oil

Heavy Fuel Oil (HFO) refers to a kind of fuel oil with a tar-like consistency. HFO, also known as bunker fuel or residual fuel oil, is a byproduct of the petroleum distillation and cracking process. As a result, HFO is polluted with a variety of chemicals, including aromatics, sulphur, and nitrogen, making combustion emissions more polluting than other fuel oils. Because of its low cost in comparison to cleaner fuel supplies such as distillates, HFO is mostly used as a fuel supply for marine vessel propulsion. The usage and transportation of HFO on board vessels raises many environmental issues, including the possibility of oil spills and toxic emissions and particulates including black carbon.

Heavy fuel oil is defined either by a density of greater than 900 kg/m³ at 15°C or a kinematic viscosity of more than 180 mm²/s at 50°C. Heavy fuel oils have large percentages of heavy molecules such as long-chain hydrocarbons and aromatics with long-branched side chains.

Marine diesel oil

At 50°C (122°F), the viscosity of marine fuels ranges from less than one centistoke (CST) to about 700 CST. 1 CST means 1 mm2/s. Furthermore, higher viscosity grades are preheated when in operation to get their viscosity into the range appropriate for fuel injection (8 to 27 CST). [2] Formalized paraphrase However, MDO does not need to be preheated before use. MDO’s sulphur cap ranges from 1 to 4.5 percent by mass for various grades and Sulfur Emission Control Areas (SECAs).

Most types of bunker fuels are as follows:

Marine gas oil

Marine gasoil (MGO) refers to marine oils that are entirely made up of distillates. Many of the components of crude oil that evaporate during fractional distillation and are then condensed from the gas phase into liquid fractions are referred to as distillates. Typically, marine gasoil is a mixture of different distillates. The density of marine gasoil is greater than that of diesel fuel. Unlike heavy fuel oil (HFO), marine gasoil does not need heating when in storage. Marine gasoil, like heavy fuel oil, is manufactured with differing degrees of sulphur content, but the overall allowable sulphur content of marine gasoil is lower than that of heavy fuel oil. The ISO 8217 quality mark allows for a maximum allowable rating of 1.5 percent. Low sulfur marine gasoil (LS-MGO) has a sulfur content of less than 0.1%. Alternatively, this limit can also be achieved by means of suitable equipment (filter systems, scrubbers).

Low sulfur fuel oil (LSFO)

If the sulphur content of a heavy fuel oil is less than 1%, it is referred to as low sulphur fuel oil (LSFO). Typically, these are desulfurized marine fuel types IFO 180 or IFO 380. Ships will also use this kind of marine fuel in Emission Control Areas (ECAs) before the end of 2014.

Ultra-low sulfur fuel oil (ULSFO)

Since January 1, 2015, ship pollution in those protected areas shall contain no more than 0.1 percent sulphur, in compliance with Annex VI of the MARPOL Conventions (ECAs). Due to these tightened constraints, LSFOs no longer play an important role in these areas and have been largely replaced by ultra-low sulphur fuel oil (ULSFO) marine fuel, which meets those limitations. In theory, highly desulfurized IFO fuels could also be used here, but the cost of desulfurization of such heavy fuel oils is prohibitively costly. As a result, the name ultra-low sulphur fuel oil now commonly applies to marine gasoil rather than desulfurized heavy fuel oils, but to marine gasoil, which is already low in sulfur. It is composed exclusively of distillates and has a sulfur content of under

0.1%. This marine fuel is also known as ultra-low marine gasoil. ULSFO is used in medium- to high-speed diesel engines. When converting from LSFO to ULSFO, it must be ensured that the engine technology is compatible with ULSFO.

Fuel Storage & Distribution Onboard

The fuel is contained aboard ship in the bunkering tanks mentioned below.

Bunker tanks

These are the largest tanks on board a ship in terms of size. They are used to store bulk gasoline and diesel oil after bunkering. Bunker tanks are typically located outside the engine room and are either a wing or a double bottom tank. To stop fuel mixing, low sulphur oil and marine gas oil are bunkered in separate dedicated bunker containers.

Settling tanks

In most cases, more than two settling tanks are present and positioned on a ship as part of the engine room bulkhead. The bunker tank’s oil is moved to the settling tank. The diesel oil settling tank can be used in the engine room as a double bottom tank. The settling tank for low sulphur oil and marine gas oil is kept apart from the rest of the fuel oil.

Service tanks

Onboard ships, service tanks are used to store and supply treated oil to the main engine, auxiliary engine, and boilers. There can be one or two of these tanks. Fuel oil and diesel oil service tanks are usually housed in the engine room’s bulkhead. Tanks for low service fuel oil (L.S.F.O.) and marine gas oil (M.G.O.) are separate to prevent contamination.

Over flow tank

In the engine room, an overflow tank is installed for both the petrol and diesel oil systems to collect the overflowed oil from the bunker tank. Return and leak

off lines can be wired to the overflow tank as well. A typical overflow tank for high and low sulphur systems is standard practise.

Emergency generator diesel oil tank

Fuel for the emergency generator is supplied from a separate diesel oil tank with a size determined by SOLAS regulations. The tank is located in the emergency generator room, which is located outside the engine room.

Distribution of Fuel

Fig. Fuel oil distribution system of marine Engine

The fuel to be used is then moved from storage tanks to a settling tank, which is heated to allow any water and sludge to settle out and be drained out by gravity. Since passing through the purification system, the fuel is discharged into a daily tank. Typically, two daily tanks are used alternately, with one in service while the other is being recharged. Heat is conserved by lagging the settling and service tanks.

A recommended standard of care for residual fuel to be used in a large engine involves the use of two centrifuges of sufficient capacity running in succession. The first serves as a purifier, removing water, solubles, sludge, and other impurities, while the second acts as a clarifier, removing solids. To balance the oil density, the purifier must be fitted with the appropriate disc. To aid in efficient separation, the oil is heated prior to purification (maximum temperature 98°C) and the rate of throughput is reduced. Both centrifuges must be washed on a regular basis. Such systems can work successfully for oils with densities as high as 0.99.

The treated oil is pumped from the service tanks to the engine using a pressurized fuel system. With the oil temperatures needed for high viscosity fuel and the risk of a trace of water remaining, the engine pump suction and circulation attachment must be kept under pressure to prevent heating, gasification, and cavitation. It is mixed with fuel from the return line in the mixing column before being pumped into the radiator by the booster pumps. The viscotherm monitors the fuel viscosity after the heater. The heater control valve is activated by an amplified output signal from the viscotherm. The fuel is then routed through a series of back flushing filters to the engine fuel rail. The fuel is continually recirculated down a return line to a mixing column, from which it is reintroduced into the system; this means that the fuel in the fuel rail is held at the proper temperature to preserve the desired viscosity. The device may be lagged to avoid excessive heat loss, and trace heating can be mounted.

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