Safety and Flammability

Natural gas and air in proper proportions will ignite and liberate heat. The liberated heat is absorbed primarily by the products of combustion and air. The sudden rise of temperature of the mixture gases also causes a sharp increase in pressure. Under a confined situation, this instantaneous pressure increase can result in an explosion. This is why it is important that everyone involved with the natural gas industry understand the flammability limits and the nature of combustion. 

Methane and air in the proper proportions do not react until some other source ignites the rapid process. Possible mechanisms of chemical reactions involved in the ignition process are given by Lewis and Von Elbe [79]. A number of oxygenated intermediates are formed along with free radicals like OH in the process of forming C02 and H20. 

The combustion process is initiated by an ignition source converting some number of methane molecules into free radicals. Free radicals are in turn converted to OH free radical. Possible oxygenated compounds include aldehyde, alcohol, carboxylic acid, and oxide. The hydroxyl free radical then reacts with methane and is regenerated. The successive (chain type) combustion reaction is impeded by destruction of the OH radicals. Solid surfaces often destroy the OH radicals before they can react with hydrocarbons. The same effect is exploited in a porous-bed flame arrestor. In general, the combustion rates are very fast and nearly measurable with a few exceptional situations where time scales can be expanded to microseconds (KT6 s). The 

There are two composition limits of flammability for air and a gaseous fuel under specified conditions [76]. The lower limit is the minimum concentration of combustible gas that will support combustion, while the higher limit is the maximum concentration. Table 5.1 shows the lower and higher limits for pure hydrocarbons in air at room temperature and atmospheric pressure (RTP) [76]. For methane in air, the flammability limit is 5—15 mol%. For ethane in air, the limits are 2.9—13.0 mol%. The limits become lower with increasing molecular weight. It also is interesting to note that the limits are the same for n-pentane and isopentane, and also for -butane and isobutane. 

Since natural gas is a mixture of various hydrocarbons, it is important to know how to estimate the flammability limits of the mixture gas. Furthermore, natural gas often contains hydrogen sulfide and its flammability limits are 4.3-45.5 mol%. 

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