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您当前的位置: 气体分离设备商务网 → 行业书库 → 在线书籍 → 《气体爆炸手册(GAS EXPLOSION HANDBOOK )》


Combustion Properties of Fuel-Air Mixtures

 

The consequences of a gas explosion will depend strongly on the type of fuel and oxidiser and the fuel and oxidiser concentration. Through years of research and safety work in the industry, essential characteristic properties for characterising the reactivity and damage potential of various combustible substances have been established. These data are the back-bone of gas explosion safety work in the industry. An excellent source of information on such data is the US Bureau of Mines reports:

  • Coward and Jones (1952), Bureau of Mines Bulletin 502
  • Zabetakis (1965), Bureau of Mines Bulletin 627.
  • Kuchta (1985), Bureau of Mines Bulletin 680.

The book by Nabert and Schön (1963) is also a good information source.

The objective of this chapter is not to list up characteristic data for fuel-air mixtures, but to:

    i) define the terminology and

    ii) point out the importance of these properties, i.e. how these properties can be used to evaluate explosion hazards for a fuel.


4.1 Flammability Limits (LFL and UFL)

A premixed fuel-air mixture will only burn as long as the fuel concentration is between the upper and lower flammability limits, i.e. UFL and LFL.

The flammability limits are experimentally determined data. The flammability limits in air depend on initial temperature and pressure. Standard test conditions are 25°C and 1 atm.

Figure 4.1 shows the flammable range for some fuel-air mixtures.


Figure 4.1. Flammable range for fuel-air mixtures at 1 atm. and 25°C.


The wide flammable range of hydrogen tells us that it is easy to get a flammable cloud of hydrogen in air. For propane and methane, the flammable range is much narrower, but as discussed in Section 3.4, an ignition source may "sit and wait" until the cloud can be ignited and explode. If the UFL has been passed, one has to go through the flammable concentration in the dilution process. It is good practice to operate safely below the LFL.

As shown in Figure 4.2, the flammable range will widen when the initial temperature is increasing. Changes in initial pressure will for hydrocarbons in air not change the LFL significantly, but the UFL will increase.


Figure 4.2. The effect of temperature on LFL and UFL.

Flammability limits for fuel mixtures may be calculated by Le Chatelier's law:


where C1, C2 ....Ci [vol.%] is the proportion of each gas in the fuel mixture without air (Kuchta, 1985).

Von Niepenberg et al. (1978) used Le Chatelier's rule for predicting flammability limits for fuel mixtures containing inert gas. Hustad and Sønju (1988) found a good agreement between experiments and Le Chatelier's law for LFL at elevated temperature and pressure for fuel mixtures. It should be noted, however, that the formula does not work properly for H2 and for unsaturated hydrocarbons. It is also only valid if the components are chemically similar.


4.2 Explosion Limits

Same meaning as flammable limits, i.e. UEL = UFL and LEL = LFL. We recommend using the term flammability limit instead of explosion limit.


4.3 Stoichiometric Compositions

The Stoichiometric composition is defined as the composition where the amounts of fuel and oxygen (air) are in balance so that there is no excess of fuel or oxygen after the chemical reaction has been completed.

C3H8 + 5 (O2 + 3.76 N2) ® 3CO2 + 4H2O+ 5 (3.76 N2 )

Table 4.1. Stoichiometric concentration in air for various fuels.

Hydrogen

Ethylene

Propane

Methane

% fuel (vol)

30

6.5

4.0

9.5

(g/m3)

26.9

81.7

79.1

67.8

For practical purposes the Stoichiometric composition can be regarded as the composition giving the highest explosion pressure for a single component fuel. (Exceptions to this exist, e.g. acetylene-air).


4.4 Flash Points

The flash point of the fuel is the minimum temperature at which it gives off sufficient vapour to form a flammable mixture with air, near the surface of the liquid or within the vessel (in all locations) used. Operating at temperatures lower than the flash point of the liquid fuel will not lead to a flammable mixture being formed unless a mist cloud (e.g. due to splashing) is generated.


Figure 4.3. Flash point.


Flash points for some fuels can be found in Table 4.2.

Table 4.2. Flash point for various fuels. (Kuchta, 1986).

Propane

JP4

Kerosene

Diesel fuel (60cet)

Flash Point (°C)

< -104

-18

52

40 to 50


4.5 Minimum Ignition Energy

The minimum ignition energy is a measure of required energy for a localised ignition source, like a spark, to successfully ignite a fuel-oxidiser mixture. As shown in Figure 4.4 the ignition energy depends on the fuel concentration. For most combustible fuels the minimum ignition energy is between 0.1 and 0.3 mJ in normal ambient air. However, hydrogen, acetylene and carbon disulphide have one order of magnitude lower minimum ignition energy. (Kuchta, 1985).


Figure 4.4. Ignition energy for methane in air at 1 atm. and 25°C.



4.6 Autoignition Temperature

When a flammable mixture is heated up to a certain temperature, the chemical reaction will start spontaneously. As shown in Figure 4.5 this critical temperature for fuel-oxidiser is called the (minimum) autoignition temperature, AIT. The precise definition is: the autoignition temperature is the lowest temperature of a hot wall adjacent to the fuel-air mixture which can lead to ignition.


Figure 4.5. Autoignition temperature, AIT.


For most pure hydrocarbon derivatives in air, the AIT lies between 540°C (methane) and 210°C (n-decane). For mixtures of hydrocarbons, the AIT lies between the AIT of the pure hydrocarbons as shown by Kong and Alfert, 1991 (see Figure 4.6).

Figure 4.6. Autoignition temperature of methane-propane mixture as found in a 1 litre ignition bomb (stoichiometric mixtures).


Table 4.3. Minimum autoignition temperature (AIT).

Hydrogen

Ethylene

Propane

Methane

AIT (°C)

520

520

450

540


4.7 Heat of Reaction

In combustion technology, we use heat of combustion as a measure of the chemically bound energy in the fuel. This property is usually given as energy per mass of fuel, i.e. J/kg fuel. Regarding gas explosions, the heat of combustion may be misleading. Some fuels can have low values for heat of combustion, but still have a high gas explosion potential. With respect to gas explosion hazards, the heat of reaction of the premixed fuel-air is a more relevant property characterising the energy content. This tells us how much energy per volume can be released in a gas explosion. One should note however, that the reaction rate depends also on other parameters, like the diffusivity of the fuel (e.g. hydrogen which is very diffusive). Table 4.4 lists heats of reaction and combustion for some fuels including VCM (vinyl chloride monomer).

Table 4.4. Heat of reaction and heat of combustion (Baker et al., 1983) for fuel-air mixtures.

VCM

Hydrogen

Methane

Propane

Ethylene

Heat of reaction [MJ/m3] per m3 Stoichiometric gas mixtures

3.7

3.2

3.4

3.7

3.9

Heat of combustion [MJ/kg] (Low value)

18.6

120

50

46

47


4.8 Adiabatic Flame Temperature

The adiabatic flame temperature is the (maximum) temperature obtained when a fuel oxidiser is burning at a constant pressure with no heat loss (to walls, equipment, etc.). This is also a parameter characterising the energy content of the mixture. Figure 4.7 shows the adiabatic flame temperature for methane/air as function of methane concentration. The maximum adiabatic flame temperature occurs close to the Stoichiometric composition (i.e. 9.5% methane in air). For most hydrocarbon-air mixtures this maximum value is the same as for methane, i.e. about 2000°C.


Figure 4.7. Adiabatic flame temperature for initial conditions 1 atm. and 25°C.



4.9 Constant Volume and Constant Pressure Combustion

When the premixed cloud burns, the temperature of the gas will increase. From the ideal gas law:


we know that increased temperature will cause increased pressure, p, or decreased density, r, (i.e. expansion) or a combination of both.

The parameters characterising the pressure increase and the expansion are data for constant volume combustion and constant pressure combustion, respectively. Figure 4.8 illustrates these two situations.

Figure 4.8. Constant volume and pressure combustion.


Some data on pressure ratios and expansion ratios can be found in Table 4.5.

Table 4.5. Pressure, P, (absolute) and volume ratio (V/V0) for Stoichiometric fuel-air mixture at initial conditions 25° C and 1 atm (1.013 bar) (Baker et al. 1983).

Hydrogen

Ethylene

Propane

Methane

P (bar)

8.15

9.51

9.44

8.94

V/V0

6.89

8.06

7.98

7.72

It should be noted that pressure for constant volume combustion is not the maximum pressure that can be obtained in a gas explosion. Dynamic effects, such as pre-compression can cause much higher local explosion pressures: The energy released in the early part of an explosion may precompress the still unburned gas, which then upon burning will reach a higher pressure than if it were at its initial pressure.

The pressure for constant volume combustion is the pressure that may be obtained in closed vessels when the burning rate is low.


4.10 Laminar Flame Speed

The laminar flame speed is an experimentally determined property characterising the propagation velocity of the flame normal to the flame front into the reactants under laminar flow conditions.

Table 4.6 gives some data for laminar flame speed.

Table 4.6. Laminar flame speed for Stoichiometric composition (Harris, 1983).

Hydrogen

Ethylene

Propane

Methane

S (m/s)

28.0

6.5

4.0

3.5

For hydrocarbon-air mixtures one may say that the higher the laminar flame speed, the more reactive is the mixture. This means that the flame can propagate fast through a cloud and thereby cause flame acceleration and pressure build-up.


4.11 Pressure Build-up Potential

At present no single property exists which can be used for characterisation of the pressure build-up potential of a fuel-air mixture. The characteristic properties already discussed, gives only some indication. The pressure build-up in gas explosions depends strongly on the geometry where the explosion occurs. The individual fuels may behave somewhat differently depending on the conditions, but the relative fuel ranking shown in Figure 4.9 is expected to hold in most situations.



Figure 4.9. Comparison of explosion pressure for various Stoichiometric fuel-air mixtures in a 10 m wedge-shaped vessel (Bjørkhaug 1988b).



 

4.12 Other Atmospheres than Air

In process equipment or vessels, gas explosions may occur with other oxidisers than air. The oxidisers can be oxygen enriched air, pure oxygen, chlorine, NO or NO2.

When the oxygen concentration increases from the 21% oxygen as in air, the explosion hazard will increase. The minimum ignition energy for methane is reduced from 0.3 mJ in air to 0.003 mJ in oxygen. (Kuchta, 1985). Chlorine is also a strong oxidiser and can lead to serious explosions. Some of the flammability limits in air, oxygen and chlorine are given in Table 4.7. In oxygen and chlorine, the flammable range is much wider than in air.

Table 4.7. Flammable concentration range in air (Dokter 1985).

% Hydrogen

% Methane

% VC (VCM)

Air

4.0 - 75.6

4.0 - 16.0

3.5 - 15.4

Oxygen

3.9 - 95.8

5.0 - 61.9

4.0 - 67

Chlorine

3.5 - 89

5.5 - 63

9.0 - 49.2

By adding inert gases, such as nitrogen, N2, or carbon dioxide, CO2, the explosion hazard can be reduced. Figure 4.10 shows the flammability for hydrogen and methane versus inert gas / flammable gas ratio. As we can see from this figure, the ratio inert gas / flammable gas has to be fairly large for the gas to be outside the flammable range.



Figure 4.10. Flammability limits as function of the ratio of inert gas to flammable gas.
Table 4.8. Inerting requirements to prevent flame propagation in fuel-air with N, CO, Halon 1211 and 1301 at 25°C and 1 atm. (Kuchta, 1985).


Minimum inerting concentration (% vol.)

N2

CO2

Halon 1211

Halon 1301

Methane

36

23

4.0 (5.3)

2.0 (4.7)

Propane

42

28

5.9

6.5

Ethylene

48

39

9.6

11.0

Hydrogen

71

57

27

20.0

Halons are more effective than N2 and CO2 for inerting. However, since halon has a negative impact on the environment, its use will probably be limited in the future.

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