Methane autoignition temperature

For straight paraffinic hydrocarbons (i.e., methane, ethane, propane, etc.) the commonly accepted autoignition temperatures decrease as the paraffinic carbon atoms increase (e g., methane 540 °C (1004 °F) and octane 220 °C (428 °F)). 

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). 

Very large reserves of natural gas are believed to lie at depths of 4600-9200 meters (15,000-30,000 feet), called deep gas. Since methane remains stable up to its autoignition temperature of550°C (1022°F), it is found at depths where oil is not found, presumably because oil will be transformed in part to methane at lower temperatures. Deep gas is expensive to drill for, but the quantities are estimated to be very large. Technology has been developed to enhance recovery of deep gas when it is found. 

Figure 27 also includes the vapor pressure curve (expressed as volume-percent methane in air) and the minimum autoignition temperature (AIT) of methane in air at atmospheric pressure. The intersection of the vapor pressure and lower limit of flammability curves yields the value of 7 (Figure 20) for methane in air at atmospheric pressure (—187°C). As usual, T[ is pressure-dependent, as the methane concentration in a saturated vapor-air mixture is inversely proportional to the total pressure at a fixed temperature. 

Methane An alkane (saturated aliphatic hydrocarbon) with one carbon atom, CH4. A colorless, odorless, highly flammable gas (autoignition temperature, 537°C). Reacts with chlorine in light. Occurs as natural and coal gas. Can be obtained synthetically from a mixture of carbon monoxide and hydrogen from steam treatment of hot coal. Used in petrochemical synthesis, for manufacture of carbon black and chlorinated solvents, and as fuel. 

Auto Ignition This is the temperature at which materials burn without a source of ignition. Gasoline autoignites at 450°F. Asphalt autoignites at 320°F methane at 1000°F. The heavier the hydrocarbon, the lower the autoignition temperature. Iron sulfide (pyrophorric iron) burns at room temperature when it is dry. 

A detailed study of the slow reaction in very rich methane—air mixtures during the ignition delay at pressures of 58—110 atm [54] showed that autoignition occurs at T = 350 °C, well below the ignition temperature at low pressures. Increasing the pressure in an 88% CH4 +12% air mixture ([O2]o = 2.8% of [CH4] - - [O2]) from 58 to 110 atm at T = 406 °C caused a decrease in the ignition delay (Fig. 3.4), i.e., an increase in the conversion rate, which obeyed the power dependence... 

---