Natural gas versus methane: Ignition kinetics and detonation limit behavior in small tubes

Abstract

Detonation properties of methane and natural gas were studied using ZND simulations and detonation limit experiments. The experiments were performed in two tube sizes, 32 mm and 6.4 mm in inner diameter over a range of initial pressures (between 3.4 kPa and 35 kPa) and with stoichiometric fuel-oxygen compositions. The fuels considered are high purity methane, high purity methane with dopant ozone, a real natural gas, and a family of natural gas surrogates. The natural gas surrogates were developed based on North American natural gas composition variations and designed to capture the expected mean and variance of fundamental combustion properties of natural gases. All natural gases tested in this work show a substantially smaller detonation induction length (about 40%) and lower detonation pressure limit (30% in terms of limiting pressure) than high purity methane. The ozonated methane at 3000 PPMv of ozone doping performed similarly to the natural gases. Overall, the results suggest that in methane-based mixtures, a smaller induction length correlates with a more predictable detonation behavior as evidenced by a lower detonation pressure limit. As such, natural gases are expected to have a wider operating range when used as a fuel for detonation-based engines. As importantly, induction length calculation results reveal that the variability in detonation and combustion behaviors resultant from composition variability is expected to be similar between natural gases and commercial methane. Finally, the results suggest that for safety-related studies, neat methane is a poor surrogate for studying natural gas explosions.