THE COMTHERM ON-LINE TECHNICAL MANUAL
FUEL DATA
All common fuels are either naturally occurring fossil fuels or the result of the industrial and chemical processing of a fossil fuel.
The fuels are found in solid, liquid or gaseous form, depending on the geology and history of the environment in which they are found.
All of these fuels consist of a combination of hydrogen (H)and carbon (C); the two common exceptions being pure hydrogen
itself which obviously contains no carbon and carbon monoxide (CO) which contains no hydrogen.
Carbon and hydrogen are seldom burned in their pure forms.
Environmental considerations have led to a preference for the use of gaseous fuels for domestic and many
industrial applications, even for electricity generation.
Because of the plentiful supply of crude oil, liquid fuels have become predominant for large scale steam and hot
water applications, particularly when fuel gas supplies are limited.
Historically sold fuels, such as wood, peat and coal were used in most domestic and industrial processes
GASEOUS FUELS
The following Notes provide information with regard to the properties of fuel gases. However quick reference to fuel gas properties
can be made by examining the applicable Comtherm information tables.
For Component gases - see table CT-1a
For 1st Family gases - see table CT-1b
For 2nd Family gases - see table CT-1c
For 3rd Family gases - see table CT-1d
For Miscellaneous gases - see table CT-1e
Commercially available fuel gases are obviously marketed and sold on a heat value basis, for this reason the most
important factor used to classify and grade fuel gases is the heat value or calorific value.
The calorific value is sometimes specified by volume, sometimes by weight.
The modern accepted international units of measurement for the heat value of a unit volume of gas is Megajoules per cubic metre (MJ/Cu.M.).
Traditionally in the UK and USA calorific value was specified in terms of British thermal units per cubic foot (Btu.cf) and
in Europe and most other countries in terms of Kilocalories per cubic metre (Kcal/CM).
The older traditional units are far more practical and meaningful and are often preferred by practical fuel engineers.
The combustion of most gases results in the formation of water vapour, (see combustion chemistry) the latent heat of
vaporisation contained in this water cannot normally be recovered; for this reason the calorific value is
quoted as either the gross value (Hs) or the net value (Hi).
The gross value (Hs) assumes recovery of the latent heat of the water vapour, it is the total heat obtained from
combustion of a specified amount of fuel which is at N.T.P. from when combustion starts and the combustion products
are cooled to N.T.P. before the quantity of heat released is measured.
The net value (Hi) (available heat) which assumes that the value of latent heat of vaporisation in the water vapour cannot be utilised,
is the gross quantity of heat released within a combustion chamber minus both the dry flue gas loss and the moisture loss.
It is the quantity of heat left to do useful work, it is the gross heating value minus the latent heat of vaporization of the water vapour
formed by the combustion of hydrogen in the fuel (in a fuel with no H2, the net and gross heating value are the same).
In combustion applications, the products of combustion are seldom cooled to N.T.P., therefore, the term "available heat" is a much more useful one.
Apart from the calorific value the other major property of a fuel gas and the one that influences volume flow is the relative
density (d) (specific gravity), this is the weight of gas per unit volume compared with the weight of the same volume of
air at the same conditions of temperature and pressure.
The relative density (d) and the calorific value (HS) have been combined to produce a gas classification system referred to as the
Wobbe index; this is in fact an indication of equivalent heat value of gases when used at identical pressures and is
used by gas appliance and equipment manufacturers to specify the suitability of gas types to a specific appliance.
Gas appliances are specified as being suitable for use within a specific gas Wobbe group or range.
The Wobbe index is expressed as:
W = H ÷ √d
Wobbe index data is stated at a temperature of 15C and pressure of 1013mbar pressure.
Depending on whether the gross or net calorific value is used both gross and net Wobbe numbers can be stated. (Ws or Wi)
The Wobbe index is used to classify most commercially available fuel gases into three gas families (see EN437) as described in the following notes.
Fuel gases can be divided into five basic groups as follows:-
- Component gases
- 1st Family gases
- 2nd Family gases
- 3rd Family gases
- Miscellaneous fuel gases
COMPONENT GASES
All normal available fuel gases consist of two or more component gases; these are the basic chemical compounds found in
all manufactured and naturally available fuels.
They are not found naturally in their pure form; however knowledge of their properties is essential in order to understand
the characteristics of the commercially available fuel gas mixtures.
Basic information on the component gases is provided in Comtherm table CT-1a.
Further information on flame speed,
flame temperature, ignition temperatures and limits of combustion can be obtained in the relevant sections of these notes.
1st, 2nd & 3rd FAMILY GASES
NOTE - Appliances and burners for domestic and commercial applications are tested for suitability to operate within a certain
specified Wobbe range by testing on gases having light back and lift off characteristics consistent with gases
at the extreme ends of the specified Wobbe range.
These test gases are refered to as the light back and flame lift limit gases and are specially mixed at the various national appliance test stations. ( see EN437)
1st FAMILY
Before the introduction of natural gases into Western Europe in the 1960s virtually all distributed fuel gas was manufactured from coal,
using various types of production plant.
These gases usually contained small amounts of sulphur or hydrogen sulphide which had health and environmental consequences.
The gases also contained significant amounts of poisonous carbon monoxide.
This family of gases consist of gases manufactured originally from coal and in later times from oil products -
generally referred to as Towns gas, Coal gas or manufactured gas; hydrogen forms a major part of these gases.
The methods used in the production of the gases are difficult to control so the quality and consistency of the gases supplied can be problematic.
The big advantage of these gases when compared with natural gases is their relative ease of combustion, with wide
limits of inflammability and fast flame speeds.
Although historically a wider range of Wobbe index was available, modern standards restrict the
Wobbe range and specify that Gases in this family have a Wobbe range, refered to as Group A, of :- 22.4 to 24.8
Basic information and general data concerning some typical examples of the various varieties of towns gas (1st family gases) is
provided in Comtherm table CT-1b,
Further information on flame speed, flame temperature, ignition temperatures and
limits of combustion can be obtained in the relevant sections of these notes.
2nd FAMILY
This family contains all natural gases and has methane as the main constituent with a Wobbe index range of
39.1 to 54.7
There are two main groups of these natural gases so the family has been subdivided into two groups, L and H.
Group L include gases containing substantial volumes of non combustible (ballast) gas such as nitrogen and
carbon dioxide; gases in this group have a Wobbe index range from 39.1 to 44.8
Group H gases have high methane contents and very little ballast gas ( nitrogen, carbon dioxide etc.;) natural gases
distributed in the U.K. are classified in this group.
Gases in this group have a Wobbe index range of 45.7 to 54.7
A further group having a Wobbe range generally covering both group L and is specified as group E, gases in this
group have a Wobbe range of 40.9 to 54.7
Basic information and general data concerning the various varieties of natural gases (2nd family gases) is provided in
Comtherm table CT-1c,
Further information on flame speed, flame temperature, ignition temperatures and limits of
combustion can be obtained in the relevant sections of these notes.
3rd FAMILY
The gases referred to as L.P.G. (liquefied petroleum gas) form this family of gases. In general they are a product of the
oil refining industry and although gases (at N.T.P.) - they can be pressurised at normal temperature and
distributed in liquid form.
These gases contain mainly propane or butane.
The gauge pressures of the liquid gases at normal temperatures for propane and butane are approximately 7 bar and 2 bar respectively.
L.P.G's have relative densities greater than air (heavier than air); this fact must be taken into consideration for safety purposes.
Gases in this family have a Wobbe index range of 72.9 to 87.3
This family has been subdivided depending on whether the gas is propane, butane or a mixture of both.
Group B/P has a Wobbe range of 72.9 to 87.3
Group P has a Wobbe range of 72.9 to 76.3
Group B has a Wobbe range of 81.8 to 87.3
Basic information and general data concerning the various varieties of L.P.G. (3rd family gases) is provided in Comtherm table CT-1d,
Further information on flame speed, flame temperature, ignition temperatures and limits of combustion can be obtained in the
relevant sections of these notes.
It should be noted that both propane and butane can be mixed with air to produce gases with Wobbe indices equivalent to 1st and 2nd family gases.
MISCELLANEOUS FUEL GASES
Basic information and general data concerning the various miscellaneous fuel gases available is provided in Comtherm table CT-1e,
Further information on flame speed, flame temperature, ignition temperatures and limits of combustion can be obtained in the relevant sections of these notes.
Although not generally distributed to fuel gas consumers, the fuel engineer may come across these gases in various
parts of the world were local feedstocks, such as sugar cane etc. may be used to produce fuels.
LIQUID FUELS
THIS SECTION OF THESE NOTES IS YET TO BE COMPLETED
SOLID FUELS
THIS SECTION OF THESE NOTES IS YET TO BE COMPLETED
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