1.    Introduction

In Combustion File (CF) 186 Petroleum is introduced as a source of industrial fuels, with brief details of its formation and utilisation.

In Combustion File (CF) 62, “What are industrial fuels?”, petroleum derived industrial fuels are broadly categorised as [GLOSS]Petroleum Distillate Fuels[/GLOSS], essentially “light” fuels, and a range of [GLOSS]Petroleum Residual Fuels[/GLOSS], broadly described as light, medium and heavy oils.  These two categories comprise a very wide range of fuels, which are detailed in the present CF, along with information on the derivation of these fuels from crude petroleum.

2.    How are petroleum fuels derived?

Crude petroleum is subjected to a variety of refining and blending processes at a [GLOSS]petroleum refinery[/GLOSS] during production.  Only a very small quantity of petroleum is utilised without refinement, usually directly to provide power during production.

The principal liquid fuels derived from [GLOSS]crude petroleum[/GLOSS] ([GLOSS]crude oil[/GLOSS]) are produced by fractional [GLOSS]distillation[/GLOSS].  Desulphurisation, [GLOSS]hydrogenation[/GLOSS], [GLOSS]cracking[/GLOSS], and other refining processes may be performed on selected fractions before they are blended and marketed as fuels.

Usually gases, dirt and water are removed from the crude petroleum before transportation to the refinery, where the aim is to produce fractions or batches of different hydrocarbons, boiling within certain predetermined temperature ranges, for various applications.

The petroleum products are obtained by separation (e.g. distillation and stabilisation), conversion (e.g. cracking and [GLOSS]reforming[/GLOSS], [GLOSS]alkylation[/GLOSS] and [GLOSS]isomerisation[/GLOSS]) – (See Figure 1).

3.    Distillation of crude oil

The products obtained by distillation of crude oil do not consist of single hydrocarbons, except in the case of simple gases such as ethane and propane.

Each product fraction contains many hydrocarbon compounds boiling within a certain range and these can be broadly classified in order of decreasing volatility into gases, light, middle and heavy distillates and residues.

The gases consist chiefly of methane, ethane, propane and butane. The first two are utilised as fuel or petrochemical feedstocks. Propane and butane may also be liquefied by compression and marketed as liquefied petroleum gas ([GLOSS]LPG[/GLOSS]).  Butane may to some extent be added to motor [GLOSS]gasoline[/GLOSS].

The light [GLOSS]distillate[/GLOSS]s comprise fractions, which may be used directly in the blending of motor and aviation gasolines, or as catalytic reforming and petro-chemical feedstocks; these fractions are sometimes referred to as [GLOSS]tops[/GLOSS] or [GLOSS]naptha[/GLOSS].

The heavier, higher boiling-point fractions in this range are the feedstocks for reforming processes lighting, heating and jet engine [GLOSS]kerosine[/GLOSS].

Heavier distillates are used as [GLOSS]gas oil[/GLOSS] and [GLOSS]diesel fuel[/GLOSS] and also for blending with residual products in the preparation of furnace fuels.

The [GLOSS]residue[/GLOSS] is used for the manufacture of lubricating oils, waxes, bitumen, and feedstocks for [GLOSS]vacuum distillation[/GLOSS] and [GLOSS]cracking[/GLOSS] units, and as [GLOSS]residual fuel oil[/GLOSS]. 

Figure 1: Distillation of Crude Oil to derive Petroleum Products

4. Properties of Petroleum Fuels

The eight most commonly used properties of liquid fuels are shown in Table 1 below:

Table 1: Most commonly used properties for the characterisation of liquid fuels

Whilst considered ‘basic’ in the sense of being well established, these properties are not basic in the fundamental scientific sense.  They invariably facilitate categorisation of fuels through very well defined experimental procedures.

Moreover, several are interdependent, which becomes apparent if one tries to vary these properties independently.

The [GLOSS]relative density[/GLOSS] is often used as a broad indication of liquid fuel type and fuel storage capacity: it is defined as the mass of sample occupying unit volume at a specified temperature.

The [GLOSS]viscosity[/GLOSS] a liquid is a well-defined measure of its internal resistance to flow, and decreases with temperature, whereas the [GLOSS]Pour Point[/GLOSS] is used to characterise the freezing characteristics of fuels.

[GLOSS]Vapour pressure[/GLOSS] provides a measure of fuel volatility. Whereas an individual hydrocarbon would exhibit a single boiling point, commercial fuel blends boil over a range of temperature. The distillation process or characteristic of a fuel blend facilitates a broad indication of the volatilities of the component fuels.

[GLOSS]Flashpoint[/GLOSS] is the most widely used indicator of a liquid fuel’s flammability, often used for safety purposes, whereas the spontaneous ignition temperature indicates the minimum temperature to which the fuel must be heated in the presence of air to promote ignition spontaneously, i.e. in the absence of an ‘external’ source of ignition such as a spark.  

The [GLOSS]calorific value[/GLOSS] is the quantity of energy released as heat per unit mass of fuel burned under prescribed conditions.

5. An overview of petroleum fuels

Liquid fuel products derived from petroleum are generally categorised in a number of broad categories (Figure 1). Within each category, there are various subdivisions or ‘classes’ for specific applications.

Gasolines are colourless blends of volatile fractions, which boil within the temperature range of about 20-200 °C.  For overall average properties, gasolines are often approximated to octane.

The major application for gasoline is the spark-ignition reciprocating-piston engine used for transport, where the anti-knock rating (e.g. Research Octane Number (RON)) is very important as it governs the proportion of energy that can be extracted from the fuel in SI engines. ‘Unleaded’ gasoline is now widely used due to health concerns.

Kerosines are colourless blends of relatively involatile petroleum fractions, which boil between about 150-250 °C, and have a relative density of about 0.8. The average properties of kerosene and high-flash kerosene are very roughly equivalent to dodecane and tridecane respectively. Depending on fuel ‘cut’, applications include domestic heating, cookers, camping stoves, some heavy SI engine applications and most notably as aviation fuel.

Gas Oils are brownish-coloured petroleum fractions comprising distillates boiling between 180 – 360 °C, with relative density of about 0.84.  Net calorific value is typically in the region of 42.5 MJ/kg, viscosity does not exceed 6 cSt (at 37.8 °C) and the flashpoint minimum is 55°C. Primary uses are for high-speed diesel engines for transport and relatively small static installations, small heating applications, furnaces, food-processing and agricultural drying. They are sometimes dyed for brand identification.

Diesel fuels are darkish-brown petroleum fractions with relative density of about 0.87, net calorific value typically 41.9 MJ/kg, maximum viscosities of about 14 cSt (at 37.8 °C) and minimum flash-point of about 60°C. Applications include heavy, large engines employed in marine and stationary electricity generating installations, operating at low rotational speeds, and which are less reliant on fuel quality. Industrial heating, hot-water boilers and drying processes are others applications, and minimum temperature for handling is 10°C due to the relatively high pour point.

Residual Fuel Oils are brownish-black petroleum fractions with relative density typically 0.95. Net calorific values are typically 40 MJ/kg. Viscosity is the critical property for these fuels, with viscosities ranging from 30 up to 500 cSt (at 82.2°C) for some classes, hence necessitating preheating.  Minimum flashpoints are 66°C. High sulphur content – up to 3.5% – can be prohibitive in terms of corrosion. Applications include heating, and steam-raising in ships, industrial process heating and power generation.

A further example of a modern industrial fuel derived from petroleum is [GLOSS]Orimulsion®[/GLOSS] which is an emulsion of natural Venezuelan bitumen and water, which has been fired as a “liquid” fuel, for example, in power station boilers designed for oil firing.

Finally [GLOSS]Petroleum Coke[/GLOSS] must be listed as a petroleum-derived industrial fuel, which is used, for example, in the firing of cement kilns, fired as [GLOSS]PF[/GLOSS].

The subject of the firing of industrial fuels and hazards associated with storage and handling are briefly introduced to the reader in the following two sections.

6. Firing of Petroleum Fuels

Liquid fuels are invariably prepared for firing in an industrial [GLOSS]burner[/GLOSS] using the process known as [GLOSS]atomisation[/GLOSS], which effectively increases the surface area per unit volume, hence increasing the rate of reaction and/or evaporation by breaking the liquid down into small particles or droplets.

The effectiveness of the atomisation process becomes more and more important as fuel volatility decreases, and viscosity increases, and has a significant bearing on the optimisation of liquid-fuel utilisation processes in terms of efficiency and pollution minimisation.

During the latter quarter of the 20^th century, atomisation techniques developed considerably such that now a broad range of technologies are utilised for improved atomisation performance under varying operating conditions and for different applications.

7. Petroleum Hazards

The widespread storage and transportation of petroleum products, together with exploration and production of petroleum in ever more demanding environments, poses ever more complex risks and hazards associated with liquid fuels.

This requires careful management with integrated mitigation strategies. Particular examples of liquid fuel hazards include tanker spillages which induce significant risk to the environment (e.g. the Exxon-Valdes in 1989), fires and explosions ( e.g. the massive explosions and fires at Ufa (Siberia) in 1989) and fireballs (e.g. the Ladbroke Grove (London) train crash (1999)).

Sources

[1] Goodger E.M. ‘Hydrocarbon Fuels’, 1975

[2] Lefebvre A.H. ‘Atomisation and Sprays’, 1989

[3] Goodger E.M. Journal of Institute of Energy, 1997

[4] Kempe’s Engineers Year-Book, Ed. J Hall Stephens (2002)

[5] Combustion of Sprays of Liquid Fuels, Alan Williams (1976)

[6] Robert H. Perry-Don W.G, Perry’s Chemical Engineers’ Handbook, 1976

[7] Elsevier science publishing, Shell International Petroleum Company, The Petroleum Handbook, 1983.

Acknowledgements

The author would like to thank Peter Kay (Ricardo-Sponsored PhD Student) and Anthony Giles (EU/EPSRC sponsored PhD student) for their assistance.