What are the main petroleum fuel
products?
A Combustion File downloaded from the IFRF Online Combustion Handbook
ISSN 1607-9116
Combustion File No:
233
Version No:
1
Date:
28-04-2003
Author(s):
Phil Bowen
Source(s):
See Combustion File
Referee(s):
Peter Roberts
Sub-Editor
Peter Roberts
Status:
Published
Sponsor:
University of Wales – Cardiff
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 Petroleum Distillate Fuels, essentially “light” fuels, and
a range of Petroleum Residual Fuels, 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
petroleum refinery 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 crude petroleum (crude oil) are produced by
fractional distillation. Desulphurisation, hydrogenation, cracking, 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 reforming, alkylation and isomerisation) - (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 (LPG). Butane may to some
extent be added to motor gasoline.
The light distillates 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 tops or naphtha.
The heavier, higher boiling-point fractions in this range are the feedstocks for
reforming processes lighting, heating and jet engine kerosene.
Heavier distillates are used as gas oil and diesel fuel and also for blending with residual
products in the preparation of furnace fuels.
The residue is used for the manufacture of lubricating oils, waxes, bitumen, and
feedstocks for vacuum distillation and cracking units, and as residual fuel oil.
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:
No.
Property
(i)
Relative Density (formerly specific
gravity)
(ii)
Pour Point
(iii)
Shear viscosity
(iv)
Vapour pressure
(v)
Distillation
(vi)
Flashpoint
(vii)
Spontaneous Ignition temperature
(viii)
Calorific Value
Characterisation
Specifying properties of the matter
Indication of mass transfer
characteristics
Indication of reactivity and energy
characteristics
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 relative density 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 viscosity a liquid is a well-defined measure of its internal resistance to flow, and
decreases with temperature, whereas the Pour Point is used to characterise the
freezing characteristics of fuels.
Vapour pressure 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.
Flashpoint 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 calorific value 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, hotwater 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 Orimulsion®
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 Petroleum Coke must be listed as a petroleum-derived industrial fuel, which is
used, for example, in the firing of cement kilns, fired as PF.
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 burner using the process
known as atomisation, 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 20th 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)).
Glossary terms
Alkylation – A reaction in which a straight-chain or branched-chain hydrocarbon group,
which is called a radical or alkyl group, is united with either an aromatic molecule or a
branched-chain hydrocarbon.
Atomisation - In relation to the firing of liquid fuels, atomisation is the process of
transforming the fuel flow in the burner into a stream of droplets thus increasing the
surface to volume ratio to aid evaporation in the root of the flame
Burner - Device in which a fuel stream and a comburent stream are mixed and brought
into contact with an ignition source to produce a flame
Calorific value – The quantity of energy released as heat when a unit of fuel is
completely combusted
Cracking – A process whereby the large molecules of the heavier oils are converted into
smaller molecules. When this is brought about by heat alone, the process is known as
thermal cracking.
Crude oil – A liquid fossil fuel, also known as crude petroleum, that occurs naturally in
sedimentary rocks and consists of a mixture of hydrocarbons and hydrocarbon derivates
ranging from methane to heavy bitumen.
Crude petroleum – A liquid fossil fuel, also known as crude oil, that occurs naturally in
sedimentary rocks and consists of a mixture of hydrocarbons and hydrocarbon derivates
ranging from methane to heavy bitumen.
Diesel fuel - A general term covering oils used as fuel in Diesel and other compression
ignition engines.
Distillate – The liquid obtained by condensing the vapour given off by a boiling liquid
Distillation -A fractionation process based on the difference in boiling point of the
various constituents of the mixture to be fractionated.
Flashpoint – The lowest temperature under closely specified conditions at which a
combustible material will give off sufficient vapour to form an inflammable mixture with
air in a standardized vessel.
Gasoline – Light petroleum product with boiling point range between 20 and 200 °C, used
as a fuel for spark ignition engines.
Gas oil - A distillate of crude petroleum, intermediate in character between kerosene
and light lubricating oils, used as a heating oil or as a fuel for compression ignition
engines.
Hydrogenation – The filling of the free places in unsaturated structures by hydrogen
atoms.
Isomerisation – The conversion of a compound into its isomer.
Kerosine – Petroleum product with boiling point range between 150 and 250 °C.
LPG – Acronym for Liquefied Petroleum Gas, a very high calorific value gaseous fuel
composed primarily of a mixture of butane and propane, sold and transported in liquefied
form and evaporated prior to firing.
Naptha - Straight-run gasoline fractions, from the distillation of crude oil, boiling below
kerosene
Orimulsion® - The brand name given to a fossil fuel produced from natural bitumen
mixed with water.
Petroleum Coke - A petroleum refining by-product of a special form of thermal cracking
process; frequently used as an industrial fuel. See also PET Coke.
Petroleum distillate fuels - Relatively light liquid fuels obtained from the fractional
distillation of crude petroleum
Petroleum refinery - An installation where crude petroleum is refined into a range of
petroleum products
Petroleum residual fuels - Heavy products from distillation and other petroleum
refining processes which may be fired as a liquid fuel
Pour Point – The temperature below which an oil tends to solidify and will no longer flow
freely.
PF - Acronym - Pulverised Fuel
Reforming - The operation of modifying the structure of the molecules of straight-run
gasoline fractions in order to improve anti-knock quality. It can be achieved thermally
(thermal reforming) or with the aid of a catalyst (catalytic reforming).
Relative density – The mass of sample occupying unit volume at a specified temperature.
Residue - The bottom product from the atmospheric pressure distillation of crude
petroleum
Residual fuel oil - Fuel oil consisting mainly of long, short and cracked residue as
opposed to distillate fuel oil.
Straight-run - A term applied to a product of petroleum made by distillation without
chemical conversion.
Tops - The lightest gasoline fractions obtained when distilling crude oils.
Vacuum distillation - Distillation of a liquid under reduced pressure aimed at keeping
the temperature sufficiently low as to prevent cracking
Vapour pressure –The lowest pressure at which a liquid, contained in a closed vessel at
the given temperature, can remain in the liquid state without evaporation.
Viscosity – Fluid viscosity is the resistance offered by a real fluid to its continuous
deformation when subjected to a shear stress. The unit of viscosity (absolute viscosity)
in the SI system is the pascal-second (Pa.s).
Keywords
boilers; crude; coke; distillation; emulsions; fossil; fuel; furnaces gas; gasoline; heating;
industrial; kerosene; liquid; marine; oil; orimulsion; petroleum; power; residual; residue;
technology;
Related Combustion Files
62 – What are industrial fuels? - Open Domain
186 - What is petroleum? - Members' Domain
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.
File Placing
[Fuels and Comburents]; [Introduction to Fuels]; [Basics]
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