Methane 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. 

NATURAL GAS Flammable gas consisting essentially of methane with very minor proportions of other gases. Flammable limits approximately 5-15%. Odourized for commercial distribution within the UK. 

Preeautions also have to be instituted to proteet against the inherent properties of the eylinder eontents, e.g. toxie, eorrosive, flammable (refer to Table 9.1). Most gases are denser than air eommon exeeptions inelude aeetylene, ammonia, helium, hydrogen and methane. Even these may on eseape be mueh eooler than ambient air and therefore slump initially. Eventually the gas will rise and aeeumulate at high levels unless ventilated. Hydrogen and aeetylene, whieh both have very wide flammable limits (Table 6.1), ean form explosive atmospheres in this way. 

The summary at the bottom of Table 5 indicates the relative agreement between the calculated data and that experimentally determined for this particular producer gas. It is suggested that the difference between calculated and determined data in this case may be due more to inaccuracies in the analysis of the produeer gas (particularly for methane) than to the fault of the mixture rule formula. This points up the fact that reliable gas analyses also are a necessary part of the calculated flammability limit data. 

Figure 14. Flammable limits for hydrogen, carbon monoxide, methane, with nitrogen, carbon dioxide and water vapor.

FIGURE 3-9. Effect of various gases on the flammability limits of methane-inert-gas-air mixtures at 25°C (77°F) and atmospheric pressure (Zabetakis 1965). 

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 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. 

Sankaran, R. and Im, H.G., Dynamic flammability limits of methane-air premixed flames with mixture composition fluctuations, Proc. Combust. Inst., 29, 77, 2002. 

Flammability limits for methane, hydrogen, and gasoline. (Courtesy of Air Products and Chemicals, Inc., Allentown, PA.)... 

Figure 6-5 Maximum pressure for methane combustion in a 20-L sphere. The flammability limits are defined at 1 psig maximum pressure. Data from C. V. Mashuga and D. A. Crowl, Process Safety Progress (1998), 17(3) 176-183 and J. M. Kuchta, Investigation of Fire and Explosion Accidents in the Chemical, Mining, and Fuel-Related Industries A Manual, US Bureau of Mines Report 680 (Washington, DC US Bureau of Mines, 1985).

Flammability limits for vapors are determined experimentally in a specially designed closed vessel apparatus (see Figure 6-14 on page 255). Vapor-air mixtures of known concentration are added and then ignited. The maximum explosion pressure is measured. This test is repeated with different concentrations to establish the range of flammability for the specific gas. Figure 6-5 shows the results for methane. 

A methane leak in a closed room is assumed to mix uniformly with air in the room. The room is (4 x 4 x 2.5)m high. Take the air density as 1.1 kg/m3 with an average molecular weight of 29 g/g mole. How many grams of methane must be added to make the room gases flammable The lower and upper flammability limits of methane are 5 and 15 % by volume respectively. 

Methane leaks from a tank in a 50 m3 sealed room. Its concentration is found to be 30 % by volume, as recorded by a combustible gas detector. The watchman runs to open the door of the room. The lighter mixture of the room gases flows out to the door at a steady rate of 50 g/s. The flammable limits are 5 and 15 % by volume for the methane in air. Assume a constant temperature at 25 °C and well-mixed conditions in the room. The mixture of the room gases can be approximated at a constant molecular weight and density of 25 g/mol and 1.05 kg/m3 respectively. After the door is opened, when will the mixture in the room become flammable ... 

For example, the lower flammability limit of methane in air at sea level is a concentration (by volume or partial pressure) of about 5%. The upper flammability limit is about 15% by volume or partial pressure. Heavier hydrocarbons tend to have lower LFLs. The LFL and UFL of some common hydrocarbons are given in Table B-2. 

An increase in temperature tends to widen the flammable range, reducing the LFL. For example, the LFL for methane in air is commonly quoted as 5%. As the temperature of methane increases to autoignition temperature, the LFL falls to around 3%. Stronger ignition sources can ignite leaner mixtures. Flammability limits also depend on the type of atmosphere. Flammability limits are much wider in oxygen, chlorine, and other oxidizers than in air (NFPA, 1997). 

Use laminar premixed free-flame calculations with a detailed reaction mechanism for hydrocarbon oxidation (e.g., GRI-Mech (GRIM30. mec)) to estimate the lean flammability limit for this gas composition in air, assuming that the mixture is flammable if the predicted flame speed is equal to or above 5 cm/s. For comparison, the lean flammability limits for methane and ethane are fuel-air equivalence ratios of 0.46 and 0.50, respectively. 

Figure 7.1-1. Compressed methane/air mixtures. Left, pressure dependence of the upper flammability limit v, 20°C X, 100°C o, 200°C right, ignition temperatures, methane/oxygen (mol/mol) X, stoichiometric methane/air (mol/mol) 0,1/1 v, 1/2 , 1/3.

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