Ignition energy, fuels

Two properties of gases and vapors that may determine when an ignition can occur are the minimum ignition energy (MIL) and the antoignition temperature (AIT). These are discussed in Section 4.1.2 above. The MIL is a function of the pressure, temperature, and composition of a fuel-oxidant mixture. 

Optimum Mixture A specific mixture of fuel and oxidant that yields the most rapid comhustion at a specific measured quantity or that yields the lowest value of the minimum ignition energy or that produces the maximum deflagration pressure. The optimum mixture is not always the same for each comhustion property that is measured. 

Hydrogen has a wider range of flammability limits and a lower ignition energy than other fuels. 

The violence of an explosion is influenced by the initial pressure or pressure of the system in which the explosion takes place. Figure 7-57 illustrates this point for propane and a constant ignition energy source. For low pressure below aunospheric, the explosion reactions are reduced until they will not propagate through the fuel-air mixture [54]. 

It has been reported by Lewis et al. [1] that the equivalence ratio where the minimum ignition energy has a minimum is dependent on the fuel property for hydrocarbon fuel and air mixtures, and that it moves to the rich side as the molecular weight of the fuel increases. This equivalence ratio dependency has been explained by the preferential diffusion effect. 

Appendix H lists minimum ignition energies of many fuels for the stoichiometric condition at a pressure of 1 atm. The Blanc data in this appendix are taken from Fig. 7.6. It is remarkable that the minima of the energy curves for the various compounds occur at nearly identical values. 

FIGURE 7.6 Minimum ignition energy of fuel-air mixtures as a function of stoichiometry (from Blanc etal. [14]). 

Table 1. Minimum spark ignition energy data for fuels in air at 1 atm pressure...

Quenching distances can be obtained from the data presented in Table HI and the correlation given as Fig. 7.5. It is interesting to note that the most stable fuels have the least minimum ignition energy in the region of 0.2mj. 

TABLE HI Minimum Spark Ignition Energy Data for Fuels in Air at 1 atm Pressure... 

Fuel-air mixtures at or around stoichiometric concentration have the lowest autoignition temperature, lowest minimum ignition energies, and highest burning velocities. 

When heat is supplied to a gaseous mixture of oxidizer and fuel components, i. e., a premixed gas, an exothermic reaction occurs and the temperature increases. The reaction may conhnue and proceed into the unreacted portion of the mixture even after the source of the heat is removed. The amount of heat that has to be supplied to the mixture to achieve this is defined as the ignition energy. If, however, the reac-hon terminates after removal of the heat source, ignition of the mixture has failed. This is because the heat generated in the combustion zone is not sufficient to heat the unreacted portion of the mixture from the initial temperature to the ignihon temperature. 

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