Explosion characteristics of argon/nitrogen diluted natural gas-air mixtures

This work was initiated to address safety concerns related to natural gas (NG)-air mixtures. NG, being an alternative fuel for vehicles and a chemical feedstock in the manufacture of organic chemicals, has been widely used in the industrial process in the past decade. NG is a flammable, gaseous fuel and thus presents a fire and explosion hazard. In this study, an investigation of the explosion characteristics including flammability limits, maximum explosion pressure (p_max), maximum rate of pressure rise (dp/dt)_max, and laminar burning velocity (S_L) is carried out by systematically measuring the pressure histories in a standard 20-L spherical vessel. The dilution effects on the explosion characteristics are also explored through the addition of two diluents, i.e., argon (Ar) and nitrogen (N₂), into the NG-air mixture. The experimental results indicate that the flammability region ranges from 5.5% to 15% NG by volume at ambient conditions. It is found that reducing the initial pressure decreases the interval width of flammability limits. This is due to the distance between the molecules of the gas being shorter as the initial pressure increases and therefore, resulting in a higher probability of effective collision between molecules. Consequently, this effect in turn promotes the initial and subsequent chemical reactions. The results also show that increasing diluent ratio results in a narrower flammability region and that the effect of N₂ addition is more pronounced than Ar. The present results also confirm that the variation of p_max with the equivalence ratio of NG exhibits an inversely "U-shaped" behavior and p_max decreases with decreasing initial pressure. In general, the addition of Ar and N₂ would decrease the value of p_max. However, it is observed in this study that the value of p_max fluctuates when argon is added, especially for those mixtures with φ < 1. It is noted that increasing Ar dilution returns the mixture to an ignition condition and even reaches an optimal composition (i.e., close to the stoichiometric condition). On the other hand, the Ar dilution has a suppression effect on the explosion. These two competing effects with increasing Ar dilution therefore make p_max remain relatively constant within an interval of dilution percentage for fuel lean mixtures (e.g., with fuel concentration C_NG equal to 5% and 6%). Finally, results also indicate that S _L decreases with an increase of initial pressure and the rate of decrease of S_L is faster when the mixture is diluted with N ₂ compared to the effect of Ar. This can be explained by the fact that the density dominates over the retarding effect for S_L.