Propane flammability limits

Flammability limits for pure components and selected mixtures have been used to generate mixing rules. These apply to mixtures of methane, ethane, propane, butane. 

Propane is highly flammable and is considered a dangerous fire and expin risk. FI pt —104.4° ignition temp 468° flammable limits 2.4—9.5% crit temp 96.8°. It forms expl nitrocompds and nitrates... 

In contrast to the lean propane flame, the burning intensity of the lean limit methane flame increases for the leading point. Preferential diffusion supplies the tip of this flame with an additional amoxmt of the deficient methane. Combustion of leaner mixture leads to some extension of the flammability limits. This is accompanied by reduced laminar burning velocity, increased flame surface area (compare surface of limit methane... 

The effect of natural gravity on flammability limits has been known for a long time. The difference between flammability limits for downward and upward flame propagation was first observed by White [26], for hydrogen/air mixtures. Subsequently, similar effects were also found for other mixtures. For propane flames, the lean flammability limit for both downward and upward propagation was observed to be = 0.53. The rich limits were = 1.64 for downward and = 2.62 for upward propagation. Such wide gap between the flammability limits for rich mixtures is explained in... 

For propane flames, fhe quenching distance (for downward propagating flames) is limited by the distance between the walls of about 10 mm. In larger channels, the flame is quenched at the flammability limits. 

The creation of a steady flame hole was previously carried out by Fiou et al. [36]. In their experiments, a steady-annular premixed edge flame was formed by diluting the inner mixture below the flammability limit, for both methane/air and propane/air mixtures. They found that a stable flame hole was established when the outer mixture composition was near stoichiometry. Their focus, however, was on the premixed flame interaction, rather than on the edge-flame formation, extinction, or propagation. 

It is further found that the adiabatic flame temperature is approximately 1300 °C for mixtures involving inert diluents at the lower flammable limit concentration. The accuracy of this approximation is illustrated in Figure 4.19 for propane in air. This approximate relationship allows us to estimate the lower limit under a variety of conditions. Consider the resultant temperature due to combustion of a given mixture. The adiabatic flame temperature (7f ad), given by Equation (2.22) for a mixture of fuel (Xp), oxygen (Xo2) and inert diluent (Xd) originally at 7U, where all of the fuel is consumed, is... 

The behavior of flammability limits at elevated pressures can be explained somewhat satisfactorily. For simple hydrocarbons (ethane, propane,..., pentane),... 

Heavy-gas behavior is often dominant close to the point of release and in the near field. It is particularly important when considering large releases of pressurized or refrigerated flammable materials, for which the value of the lower flammable limit is low. Typical hydrocarbons that fall into this grouping are ethane, 2.9% by vol LFL propane, 2.1% by vol LFL and the butanes, 1.8% by vol LFL (Gas Processors Suppliers Association, 1972). Farther downwind, after additional mixing with air, the concentration of the flammable material is less important because it is then less than the lower flammable limit. The other hazard of heavy gases is asphyxiation of personnel who may inadvertently enter or be surrounded by the cloud. 

The behavior of flammability limits at elevated pressures can be explained somewhat satisfactorily. For simple hydrocarbons (ethane, propane,..., pentane), it appears that the rich limits extend almost linearly with increasing pressure at a rate of about 0.13 vol%/atm the lean limits, on the other hand, are at first extended slightly and thereafter narrowed as pressure is increased to 6 atm. In all, the lean limit appears not to be affected appreciably by the pressure. Figure 25 for natural gas in air shows the pressure effect for conditions above atmospheric. 

A gas mixture of methane, ethane, and pentane entering an adsorber has an upper flammability limit of 12.5% and a lower flammability limit of 2.85%. Given a methane concentration of 30%, calculate the concentrations of the other two components of the gas mixture. Flammability limits for methane, ethane, and propane at various concentrations are given in the following table. 

Table 4.12 shows a comparison of safety-relevant thermo-physical and combustion properties of hydrogen with those of methane, propane and gasoline [26]. The flammability limits are affected by temperature, as shown in Figures 4.9 and 4.10, so that a preheated mixture has considerably wider limits for coherent flames [27]. An increase in pressures up to lOkPa has only a small effect. Water vapor has a strongly inhibiting influence on the oxyhydrogen reaction. 

If the substance involved is not combustible, the pressure wave and the missiles will be the only effects of the explosion. This could happen if a steam boiler (water steam) exploded. If the substance is a fuel, however, as often happens in the process industry (for example, liquefied petroleum gas, such as ethylene or propane), the mixture of Uquid/gas released by the explosion will probably ignite, giving rise to a fireball of approximately hemispheric shape, initially at ground level. The effect of the thermal radiation in this first stage, which is usually only a couple of seconds, is very important. The whole mass of fuel can bum only at its periphery because there is no air inside the mass (the mixture is outside the flammability limits). 

At the lower limit of flammability there is more oxygen available than is required for stoichiometric combustion of the fuel. For example, the lower flammability limit for propane in air at 20 C is 2.2% by volume. 

Based on the information for propane in Table 15E5.1, produce a plot of heat released per m of mixture as a function of fuel concentration for propane-air mixtures at atmospheric pressure and 20°C. In the light of this, explain why fuels have an upper flammability limit. 

Answer To solve this problem, calculate and see if it falls within the flammable limits. Here, 4 2.2, which lies within the flammability limits. However, realize that there are many other factors to consider. For example, a homogenous mixture is unlikely to form, since the shed is not leakproof and propane is heavier than air. Similarly, air could leak in as propane leaks out, creating localized areas in which the mixture is not flammable. These types of calculations are best viewed as useful approximations and as starting points for further study and investigation. 

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