This paper presents a general method for the estimation of flammability envelopes for chemical mixtures containing gases, liquids, and/or solids based on chemical equilibrium. The impact of mixture initial temperature, the presence of diluents and elevated system pressures are implicitly accounted for. The performance of this method is tested against much of the experimental data reported in the literature for systems containing a wide range of chemicals from CHNO compounds to compounds containing sulfur, phosphorus, silicon and halogens. This method presents a very useful and accurate approach for assessing the flammability envelopes of mixtures where no experimental data is available, and to guide experimental flammability testing work, using Process Safety Office® SuperChems™ software.
Figure 1 summarizes thermochemical estimates of flame temperature for a mixture of methane and oxygen at 1 bar and 25 C. We observe from Figure 1 that both the lower and upper flammability limits occur at a temperature of around 1500 K. We also observe from Figure 1 that the lower flammability limit (LFL) and the upper flammability limit (UFL) do not change significantly over a 500 degrees window. The calculated LFL varies from 3% at 1000 K to 4.8% at 1500 K. The calculated UFL varies from 60% at 1000 K to 68% at 1500 K. At the reported literature flammability limits (LFL of 5% and UFL of 60%), the methane-oxygen system has a threshold theoretical flame temperature of 1500 K. At this temperature, the combustion reaction is able to generate enough heat to produce a self-sustaining (propagating) reaction. Many other systems of interest have a similar behavior. Table 1 summarizes estimates of calculated theoretical flame temperatures at both LFL and UFL limits for a variety of chemicals. Most calculated flame temperatures are between the range of 1000 and 1500 K. For most organic chemicals, the flammability limits in air can be approximately related the the stoichiometric limits in mole or volume percent:
The stoichiometric limit can be obtained by finding the composition in air that yields the highest theoretical or adiabatic flame temperature. For the combustion of methane in air, the above equations yield:
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