ABSTRACT
One of the most important concerns associated with the production, transport, storage, and utilization of fuels is how to protect against the risk of fire and explosion� There is an enormous amount of information that includes guidelines for ensuring safe operation and handling of fuels at all levels� These include numerous rules, regulations, codes, good practice, instructions, and so on, originating from all levels of government, industry, and professional and trade associations� Moreover, there are detailed reports in the open literature of case studies that may have involved huge life and property losses with important lessons to learn� These need to be studied and familiarized with by all those who work in the energy and fuel industry�
Fire is usually defined as uncontrolled combustion and flame propagation, while an explosion is the rapid release of pressure resulting from the fast physical or chemical energy being released� Generally, fire has been classified into the following four categories:
1� Fires in common combustible materials such as wood, textiles, paper, and so on� Such fires can be extinguished by quenching or cooling, commonly through the application of water�
2� Fires of flammable liquid fuels such as gasoline, oil, grease, and so on� Such fires can be put out by the application of foam, carbon dioxide, or dry chemicals�
3� Fires involving electrical equipment such as electric motors, generators, transformers, and switches, which require a nonconductive extinguishing agent such as carbon dioxide�
4� Fires in combustible metals such as magnesium and powdered aluminum, which require the use of foam or suitable dry chemicals�
Very lean or rich fuel mixtures with air attain on combustion only moderately high temperatures� The associated reaction rates are relatively very slow� There is only a fairly narrow band of fuel concentrations in mixtures around the stoichiometric value, where the reaction rate is sufficiently appreciable to permit continued combustion� Hence, there are two limits for unimpeded flame propagation: one on the fuel-lean (or weak) side known as the lean (or lower) flammability limit of the fuel in air and the other on the fuelrich side known as the rich (or higher) flammability limit� Mixtures between these values will be considered flammable� The value of the limit is usually quoted as the volumetric concentration of the fuel in the fuel-air mixture� Figure 9�1 shows a representation of the flammable and autoignition zones of fuel-oxidant mixtures�
Table 9�1 shows a listing of lean and rich flammability values for a number of common fuels in air at ambient temperature and pressure� The very wide flammable range of hydrogen is evident� Also, the value of the lean flammability limit of gasoline vapor in air is exceedingly low, making any leakages of its vapor into air very hazardous�
Knowledge of these limit values for any fuel under certain conditions is important from the point of safety to guard against the risk of fire and because of the need to ensure continued flame propagation within the mixture for effective fuel utilization� Excessive heat losses from the flame
reaction front to the surroundings are mainly responsible for producing a limit to flame propagation�
The values of the limits for any fuel will depend on the prevailing operating conditions� Generally, the lean limit widens linearly with temperature (e�g�, approximately 6�8% per 100°C)� The rich limit rises with temperature at a greater rate than that for the lean limit� Figure 9�2 shows the changes in the limits for hydrogen in air with changes in mixture temperature� A significant widening in the flammable mixture can be seen with raising temperature�
The flammable range widens increasingly with pressure, particularly at very high pressures� However, at very low pressures, especially those well below atmospheric pressure, the flammable mixture range may become narrowed significantly, as shown in Figure 9�3� At sufficiently low pressures, even a stoichiometric mixture cannot support combustions� This, for example, takes place with most fuels at sufficiently high altitude�
An increase in the level of turbulence within the mixture widens the flammable mixture range� The limits are widest for upward flame propagation when the hot expanded products tend to rise due to thermal buoyancy effects, aiding in propagating the flame further� For increased safety and to guard against the risk of a fire, the values of the limits obtained for upward flame propagation are usually quoted�
The limits of flame propagation within tubes generally narrow as the tube diameter is reduced� A sufficiently small tube diameter can ensure that no flame propagation takes place even for stoichiometric mixtures� This tendency is exploited in the functioning of flame traps where sufficiently narrow passages are employed to ensure extensive heat transfer from the flame to adjacent conductive surfaces so that the flame gets quenched and will not propagate further�
The introduction of an inert diluent such as nitrogen, carbon dioxide, steam, or helium, whether with the fuel or the air, will narrow the limits to an extent that depends on the fuel, diluent present, and its concentration as displayed in Figure 9�4� Hence, the introduction of diluents to fires in sufficient concentration will quench the flames; this is employed in some firefighting measures� On the other hand, the introduction of additional oxygen can widen the limits considerably, especially the value of the rich limit� This tendency makes the burning of fuels in oxygenated air potentially much more hazardous than when only air is used (Figures 9�3 and 9�5)�
It has been found, especially for the lean limit values of the most common fuels, that limit mixtures correspond approximately to those mixtures that on combustion produce the same value of flame temperature (~1580 K)� Accordingly, this fact can be employed to estimate approximately the values of the limits for various fuels and conditions� This is also employed in
apparatus for evaluating how close an atmosphere containing some fuel vapor may be flammable�
The values of the limits for a wide range of fuels and conditions are usually available in the open literature such as fuel and safety handbooks� For mixtures of different fuels, the limits in air can be estimated from the corresponding values of the components of the fuel mixture on their own using the simple Le Chatelier’s rule� This is based on the assumption that the mixture of limiting mixtures will be a limiting mixture itself (e�g�, the mixing of a limit mixture for methane in air of 5�0 by volume with a corresponding lean limit mixture of 3�12% for ethane in any proportion will result in a limiting mixture also)� On this basis, Le Chatelier’s rule for the flammability of fuel mixtures in air becomes:
�0 L
y L
y L
y L
= + + +…
where y is the fractional volumetric concentration of a fuel component in the fuel mixture, L is the corresponding flammability limit value of the same fuel component, and Lmix is the flammability limit of the fuel mixture�
Example: What are the lean and rich limits in air of a fuel mixture made up of 90% methane and 10% hydrogen by volume? Take the lean and rich limits for methane to be 5�0% and 15�0%, respectively, and the corresponding values for hydrogen to be 4�0% and 75�0%�
Answer: For the lean limit:
1 9
5 1
4 2 5
� �
� �
�= + = 0 0
0 0 0
0 0 0
that is, Lmix,L is 4�78%� For the rich limit:
1 9
15 1
75 6 133
� �
� �
�= + = 0 0
0 0 0
0 0 0
that is, Lmix,R is 16�3� It is to be noted that Le Chatelier’s rule is acknowledged to have been
based on the assumption that the mixing of limiting mixtures results in a limiting mixture� This implies that the mixing does not bring about any chemical interaction between the fuel mixture components or
components of their products� On this basis, the formula can be derived as follows:
L Y
Y Y
Y Y L
L Y Y Y
L
=
+
= −( ) = +( ) =
1 1
/
Σ Σ Σ
1 1
1 1
1 1
+ −
+ −
+…
Y L
Y Lf f1
But 1
Hence 1
:
:
Y Y Y Y
L
Y
L
+ + + +… =
= + 2
3L
Y
L f
+ +…
The assumption of a fixed combustion flame temperature for limiting mixtures tends to be restrictive in comparison and relies on a nonmeasured calculated value that usually brings about a poorer agreement with experimental results� Such an assumption has been employed when there is no information about the values of the individual limits of the components or when the individual values are available but for different initial conditions�
It is well known that some fuels such as unsaturated hydrocarbon-rich mixtures do undergo some decomposition before combustion within the flame, resulting in an effective composition that is different from that initially considered� This is also seen with fuels that undergo prominently cool flame production when partial oxidation products such as aldehydes are formed� Accordingly, the simple constant limiting flame temperature approach would not work well with excessively fuel-rich mixtures requiring more elaborate approximate approaches�
The flammability limit of a fuel-diluent mixture can be estimated in terms of the corresponding limit of the pure fuel on the basis of the following relationship:
1�0 L
y
L a( y )
f d= + 1−
where a is a constant with a very small value that depends on the fuel and diluent system being considered� For example, to obtain an approximate value for the lean limit of a fuel-nitrogen mixture, the value of a can be taken to be approximately zero (Figure 9�6)�
The following are some basic recommendations for the protection of a fuel installation against fire and explosion hazards:
1� Search and remove any nonrequired or uncontrolled presence of fuels such as through fuel spills, combustible gas leakages, and so on�
2� Keep any fuel separated from air and other oxidants� 3� Keep the size of any fuel and air that may become mixed down to
an absolute minimum so that the energy release on ignition will be manageably small�
4� Keep any resulting fuel and air mixture well outside the corresponding lean and rich flammability limits�
5� Search for and remove any likely sources for ignition of any such mixtures�
6� Restrict and confine as far as possible any fire spread that may develop�
7� Separate and isolate the combustion region once fire takes place� 8� Cool the burning fuel�
9� Reduce and cut off heat transfer to the reacting system� 10� Slow down the reaction rate of the combustion system through the
addition of auxiliary materials such as diluents�
Figure 9�7 shows an example of some of the measures that may be employed to reduce the hazards in the storage of a liquid fuel in a tank installation� These include the provision of external water spray to the heavily insulated vessel with a vapor release valve system to avoid the build up of pressure due to the boiling of liquid fuel� Measures are also provided to avoid the pooling of burning fuel beneath the vessel�
The term “flash point” is widely used to characterize the relative tendency of liquid fuels to produce a propagating flashing flame over the liquid fuel surface in the presence of an external flame nearby� The value of the flash point corresponds to the minimum temperature of the liquid fuel at which a flame flashing occurs� A high value of the flash point of a fuel indicates a lower fire hazard than another of a lower point� The determination of the flash point takes place under a specified set of standard conditions and using a standard apparatus (Figure 9�8)� Liquid fuels with flash points less than 60°C are considered to be highly flammable� Table 9�2 shows a listing of the flashpoint values for a range of liquid fuels� The corresponding values for the minimum autoignition temperature are also shown� It can be seen that for any fuel the autoignition temperature, which is predominantly chemically controlled, relates to the minimum temperature for the autoignition of the homogeneous mixture of the fuel vapor and air� On the other hand, the value of the flash point, which is significantly lower than that of autoignition, is controlled mainly by physical factors such as the volatility and diffusional characteristics of the liquid fuel�
The presence of diluents such as carbon dioxide and nitrogen in the atmosphere reduces the flame speed and narrows the flammable mixture range� The corresponding maximum safe concentrations of oxygen in fuel-diluent mixtures that will not support flame propagation under atmospheric conditions are shown in Table 9�3� It can be seen that carbon dioxide is more effective in narrowing the flame speed range than nitrogen�
Figure 9�9 schematically shows typical changes in the extent of the flammable mixture zone for a typical fuel with mixture temperature when ignited by an external energy source such as an electric spark or a pilot flame� It can
be seen that with the increase in temperature, this zone is widened, especially for the rich flammability limit boundary� Owing to excessive heating of the mixture, conditions may reach those for autoignition of the fuel-air mixtures in the absence of an external source for ignition� Expectedly, the region most prone to autoignition mixture is that around the stoichiometric value� It can also be seen that at sufficiently low temperatures, the fuel-air mixture reaches saturation conditions beyond which a fuel mist is formed� The temperature of the boundary for the beginning of mist formation varies
with the concentration of the fuel and its type� The flash point of the fuel would correspond to the lowest temperature at which a combustible fuel vapor-air mixture is formed�
As outlined earlier, the ignition and flash point temperatures are quite different� The ignition temperature corresponds to the autoignition of a homogenous fuel-air mixture in the absence of an external source of ignition, and it is primarily a function of the chemical properties of the fuel� On the other hand, the flash point temperature corresponds to the lowest temperature when a sufficient fuel vapor-air mixture is formed and can be ignited by a pilot jet flame� Its value then depends more on the physical and less on the chemical properties of the fuel�
Figure 9�10 indicates a schematic representation of the three main ingredients for having a fuel fire� The breaking of this triangle through removal of one or more of these ingredients will lead to the extinguishing of the fire� Figures 9�11 and 9�12 show two typical examples of fire and explosion in two installations of fuel transport and processing facilities, respectively�
Autoignition: the acceleration of oxidation reaction rates due to the energy release in excess of the prevailing losses, without the aid of deliberate external ignition energy sources�
Autoignition temperature: the lowest temperature at which a combustible material ignites in air without an external spark or a flame�
Compression ignition: the ignition of a fuel-air mixture at a high temperature and pressure that are generated by rapid compression such as in diesel engines�
Deflagration: subsonic flame or oxidation reaction front propagation in a combustion system�
Detonation: the self-propagation of a combustion reaction front or a flame that propagates supersonically and is associated with the formation of shock waves that intensify the reaction process and its rate�
Fire: uncontrolled combustion and flame propagation� Flammability limits: the fuel-air mixture that can just support flame
propagation from an ignition source� The lean or lower limit relates to the leanest mixture in fuel that is flammable, while the rich or higher limit relates to the richest mixture in fuel that is still combustible�
Flash point: the lowest temperature at which vapors from a liquid fuel will ignite on the application of a small jet flame within a standard apparatus under specified test conditions� A high flash point fuel indicates a lower fire hazard than another of a lower point�
Ignition energy: the minimum amount of energy required to ensure ignition will take place within the fuel-air mixture� The amount will vary with the type of fuel and operating conditions�
Preignition: the undesirable, premature, often uncontrolled ignition of a fuel-air mixture in a combustion device before proper passage of the intended spark�
Quenching distance: the largest distance under a certain specified condition when a flame cannot propagate due to excessive heat loss such as quenching through narrow gaps or within a boundary layer�
Spontaneous combustion: ignition of combustible material following slow oxidation without the application of high temperature from an external energy source�
Some air leaked into a reservoir containing a gaseous fuel mixture at atmospheric temperature and pressure� The volumetric composition of the final contents was found to be
4 5 CH 2 1 C H 3 H 15 O and 75 4 N4 2 6 2 2 2� % , � % , � % , � % , � %0 0
Determine:
1� The composition of the original fuel-gas mixture, assuming it was oxygen free but contained some nitrogen�
2� The equivalence ratio of the contents of the reservoir� 3� Establish whether the contents of the reservoir are flammable at
atmospheric temperature and pressure or not�
You may assume the original fuel mixture follows Le Chatelier’s rule for both the lean and the rich limits of the fuel� Take the following lean and rich flammability limit values for CH4: 5�0 and 15%, C2H6: 3�0 and 12�4%, H2: 4�0 and 75% by volume, respectively� Consider N2 to be inert�
Answer: Consider 100 mol of a mixture with a composition of
4 5 CH 2 1 C H 3 H 15 O and 75 4 N4 2 6 2 2 2� % , � % , � % , � % , � % �0 0
Since no oxygen was reported to have been in the fuel, use its concentration to find how much air there is % air = 15�0/0�21 = 71�43% is the amount of air and the remainder 28�57% is the fuel� Hence, % N2 in fuel = 75�4 − 71�43 × 0�79 = 75�4 − 56�43 = 18�97%�
The percentage composition of the fuel:
% �
� � � � � %
%
CH 4 5 1
4 5 2 1 3 18 97 15 75
C H
=
×
+ + + =
=
00 0
2 1 28 5
7 35
H 3 1
28 5 1 5
N2
� �
� %
% �
� � %
%
0 00
0 00 0 02
× =
=
× =
= =
18 97 28 5
66 4 � �
� %0
n n
actual ratio on volume ba= = 71 43 28 5
2 500 � �
� ( sis)
Find the corresponding stoichiometric air to fuel ratio� Per one mole of fuel mixture:
0 0 0 0 0 0� � � �1575 CH 735 C H 1 5 H 664 N (O 7
4 2 6 2 2 2+ + + + +a 9
21 N ) CO
H O N
→ +
+
b
d f
Carbon balance: b = 0�1575 + 2 × 0�0735 = 0�3045 Hydrogen balance: d = 0�1575 × 2 + 0�0735 × 3 + 0�105 = 0�6405 Nitrogen balance: f = 0�664 + 79/21 a = 0�664 + 3�76 a Oxygen balance: a = b + d/2 = 0�3045 + 0�6405/2 = 0�6248
n n
6248 21
2 975= = 0
0 �
� �
The mixture is richer in fuel than in stoichiometric air since [(na/nf)stoich > (na/ nf)]� Hence, find the rich flammability limit for fuel and check that the actual air to fuel ratio is less, that is, richer in fuel for safety:
1 1575 15
735 12 4
1 5 75
1783 mix
� � � �
� �
00 0 0 0 0 0 0
L = + + =
56 1 nitrogen
L
L
n n
=
=
=
� %
43 9 56 1
� �
�=
>
n n
n nfuel mix
>
n n
The mixture is thus potentially flammable since its fuel to air ratio does not exceed than that for the rich limit� Its equivalence ratio is merely = 2�975/2�5 = 1�19�
1� Define what is meant by (a) the flammability limits and (b) the quenching distance of a fuel in air� Indicate whether the flammable mixture range of a fuel will widen or not with (i) increasing mixture temperature, (ii) increasing pressure, (iii) addition of oxygen, (iv) addition of hydrogen, (v) addition of carbon dioxide, (vi) direction of flame propagation in relation to gravity, (vii) addition of steam, (viii) size of retaining vessel or tube, and (ix) flow velocity�
2� It is suggested that the fire hazards within a potentially flammable fuel-air mixture can be reduced by the following actions� Indicate whether each of these statements is “true,” “false,” or “maybe”:
a� Avoid heat transfer away from the mixture to the surroundings b� Promote stirring and currents within the mixture c� Avoid catalytic surfaces d� Provide additional air e� Provide additional fuel
(Answers: F, T, T, MB, MB) 3� If information is available about the main chemical and physical
properties of a certain liquid hydrocarbon solvent, list the main properties you would consider to assess the potential of fire and explosion hazards associated with its storage, transport, and use both in industrial and domestic settings�
4� Estimate the lower flammability limit in air at atmospheric temperature and pressure of a mixture of natural gas and a synthesis gas in the volumetric proportion of 2:1, respectively� You may assume Le Chatelier formula for fuel mixtures to apply� Take the lower flammability limits by volume in air, under normal temperature and pressure for the following fuels to be
CH 5 C H 3 1 C H 2 4 CO 12 5 and H4 2 6 3 8− − − −� %, � %, � %, � %,0 0 2 4 by volume�
− � %0
Assume the natural gas to be of the following volumetric composition:
CH 87 C H 6 2 and C H 6 84 2 6 3 8− − −� %, � %, � %0
The synthesis gas is made up of H2: 67�8% and CO: 32�2% by volume� Indicate very briefly how you would proceed to estimate the
approximate value of the flammability of the fuel mixture at a temperature of around 100°C� (Answer: 4�69%)
5� Show whether the fuel-air mixture with the following composition by volume is likely to be combustible or not under ambient conditions:
CH 1 7 H 2 C H 1 5 O 19 9 and N 744 2 3 8 2 2: � %, : %, : � %, : � %, : �9%
You may take the flammability limit values by volume in air for the pure fuels at ambient temperature to be the following: CH4: 5�0%, H2: 4%, and C3H8: 3�2%� (Answer: 3�97%)
6� A vessel containing a fuel-gas mixture at ambient temperature and pressure had some air leaking into it� A sample of the contents showed the following composition by volume: CH4: 25�0%, C2H6: 5�0%, H2: 10�0%, CO: 7�0%, O2: 11�1%, and N2: 41�9%�
Determine: a� Whether the original fuel contained some nitrogen or not know-
ing that no oxygen was originally present in the fuel b� The volumetric composition of the original fuel c� The fuel to air mass ratio as analyzed d� The rich flammability limit of the fuel e� The relative additional mass of air that needs to leak into the ves-
sel to make the contents just flammable You may assume that Le Chatelier’s mixing rule applies, and
the values of the rich flammability limits on volume basis at atmospheric temperature and pressure are the following: CH4: 15%, H2: 75%, C2H6: 15�0%, and CO: 74%� (Answer: 4�44)
7� A stoichiometric binary fuel mixture made up of 30% propane and 70% methane by volume in air is to be rendered incombustible at ambient condition by the addition of nitrogen gas� Estimate the minimum amount of nitrogen needed� Take the lower and higher flammability limits to be 5% and 15% for methane and 2�37% and 9�5% by volume for propane, respectively� Assume aN2 to be zero� (Answer: CH4: 0�376, C3H8: 0�161, N2: 0�463)
8� In an industrial plant, hydrogen gas is produced as a by-product� Assuming that it was decided to utilize the fuel for the production of power using a spark-ignition engine with cogeneration, indicate very briefly some of the key measures that need to be taken in the design and operation of such an arrangement to guard against the risk of an explosion�
9� Explain briefly the difference between the terms “flash point” and “ignition temperature” for any common fuel� Explain why the corresponding listed values of these terms for methane and n-heptane show very large differences�
10� List very briefly four procedures that ought to be considered to safeguard a propane tank during a filling operation against the risk of fire and explosion�
11� Briefly outline the basis for the view that the use of hydrogen as a fuel requires extra care to protect against the risk of explosion�
Great care is needed to protect operators, the public, and installations against the risk of fuel fires and explosions� This is to be aided by knowledge of key relevant properties such as flammability limits, autoignition temperatures, and flash
point values� Information about the changes in the values of these limits with changes in the prevailing mixture conditions is often available in the open literature for a wide range of fuels, including accounting for the presence of diluents�
