COMPONENTS OF FUEL

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The majority of the fuel oils, which we burn, derive their existence from a crude oil base stock. Although all crude oils differ in their make-up, the main constituent levels are similar; eg Carbon  83-87%, and Hydrogen 11-14%,   with the remainder made up of sulphur, oxygen, nitrogen and other trace elements.

But although the chemical ingredients are similar, the atomic structures that can be produced vary greatly.

There are three main types of hydrocarbons in crude oil

1.         Paraffins or alkane series

2.         Naphthaltenes or cyclo-alkanes

3.         Aromatics or arenes

Paraffins or alkane series (C_nH_2n+2)

These compounds are the simplest hydrocarbons. They start with the simplest member METHANE, and by adding H-C-H units to the middle of the compound, increase in size and complexity.

The molecules always form straight chains, and the longer molecule units with 5-16 carbon atoms have increased interlinking with one another and exhibit more `viscous' characteristics. This 5-16 carbon group forms the liquid state. Increasing the carbon number count above 16 forms the semi-solids such as waxes.

All members form straight chain, fully saturated (i.e. each carbon atom is attached to 2 hydrogen atoms) paraffins, which end in the suffix `ane' i.e. pentane, hexadecane (the last of the liquids at 16 C atoms). The Self Ignition Temperature for this hydrocarbon group is 220-250^oC.

Naphthenes or cyclo-alkanes (C_nH_2n)

In this structure the fully saturated carbon and hydrogen combination forms closed cyclic or ring structures.

The number of carbon atoms in the ring structure varies from three to seven, but six is the most common e.g. cyclohexane C₆H₁₂.

Although cyclic units can join together, it is more common for straight chain paraffins to attach themselves; e.g. C₆H₁₂ changes to dimethylcyclohexane C₈H₁₆

The Self Ignition Temperature for this hydrocarbon group is 380-420^oC.

Aromatics or arenas (C_nH_2n-6)

These are a series of polyunsaturated hydrocarbons, having a ring structure. The format of `non-localised' double bonds is used to explain how the unsymmetrical, therefore possible chemically unstable unit, has high stability.

These `delocalised' electrons from the double bonds act as grabbers, which allows other elements to form substitution products by attaching themselves to the main primary benzene ring.

The Self Ignition Temperature for this hydrocarbon group is 500-550^oC.

In order to investigate the composition of fuels formed by the residual of refining, we can study three of the main residue streams, namely Wax, asphaltenes and resins

Wax

These are the semi-solid residuals formed when a high proportion of paraffinic substance is present. Due to the inherent value of waxes to form more saleable products, modern refineries extract the majority of the wax from the base stock. Waxes are readily soluble in a petroleum oil base, and only start to crystallise out when the cloud point is reached, which may be as high as 35^oC.

Asphaltenes

These are highly complex structures, with high carbon/hydrogen ratios, and hence high molecular weights. Asphaltenes can be considered to be condensed aromatic rings linked by paraffinic chains. The high quantity of aromatic rings present means that small but significant amounts of complex combined oxygen, nitrogen, vanadium, sodium, water, and sulphur products are locked into the asphaltene structure.

Due to their complex nature, it is difficult for the oxygen and hydrocarbon elements to mix, and hence combustion is prolonged. This tends to produce black smoke (partly burnt hydrocarbons); fouling and high exhaust temperatures which is an indication of incomplete combustion.

           

Resins

These are low molecular weight asphaltenes with a higher proportion of naphthenic and aliphatic structures. Resins can increase in carbon numbers to form asphaltenes, and are also absorbed into many substances including metals and asphaltenes.

Resinous hydrocarbon compounds, particularly asphaltenes, formed by incomplete combustion, will readily adhere as sticky, semi-solids to metal surfaces and act as a flypaper to trap other deposits.