On the Z-shaped explosion limits of acetylene-oxygen mixtures

The explosion limits of the acetylene-oxygen (C₂H₂-O₂) mixture have been investigated and it is demonstrated, for the first time, that the C₂H₂-O₂ explosion exhibits a Z-shaped response in the pressure-temperature regime with three distinct limits from low to high pressures, which resembles the explosion limits of hydrogen-oxygen (H₂-O₂) mixtures. Kinetic analysis shows that the first limit is mainly controlled by the reaction of C₂H₂ (+ M) = H₂CC (+ M) and the subsequent pathways of H₂CC→HCO→H. The second and third limits are controlled by the H-addition reaction of C₂H₂ to form the C₂H₃ radical. The C₂H₃ radical is less reactive at intermediate pressures, and its pressure dependent formation, C₂H₂ + H (+ M) = C₂H₃ (+ M), results in the second limit. However, C₂H₃ radical is reactivated at high pressures through C₂H₂ + HO₂ = C₂H₃ + O₂, from which the HO₂ radical formed activates the HO₂→H₂O₂→OH chain branching channel, leading to the third limit. Further, we explored the explosion limits of the H₂-C₂H₂-O₂ mixtures, which shows that with increasing H₂ concentrations, the explosion limit curve rotates clockwise around a turnover point. Such rotation only occurs in the second and third limits, while the first limit basically coincides with that of the C₂H₂-O₂ system. The kinetic reasons responsible for the observed behavior are identified through the competing pathways of H₂ and C₂H₂. Furthermore, the effects of changing of equivalent ratio, dilutions of nitrogen and water, with and without surface reactions on the explosion limits are also discussed. The results of the present work are useful to better understand the explosion limits of C₂H₂-O₂ and H₂-C₂H₂-O₂mixtures and further study of more complex hydrocarbon fuels.