Experimental study on enhancing methane explosion characteristics by Al and KMnO₄ powders under large pipelines

As an alternative to hydraulic fracturing, in-situ methane explosive fracturing technology holds great promise in exploiting shale gas and the difficulties of achieving shale fracturing lie in how to generate sufficient explosion overpressure, thus creating fractures and increasing the permeability of reservoirs. One of the efficient approaches to enhance explosion overpressure is to select optimum combustion improvers and initial parameters. In this work, the improved effects of aluminum (Al) and potassium permanganate (KMnO₄) powders on explosion characteristics of CH₄-O₂-N₂ mixtures were tested using a 500 L pipeline explosion setup. Meanwhile, the influences of intrinsic properties of KMnO₄ including particle diameter, specific surface area and loading amount on explosion characteristics were explored, and the parameters were optimized. Eventually, the underlying improved mechanisms of Al and KMnO₄ were discussed. Results indicate that methane rapidly enters the detonation stage after being ignited in high concentration of oxygen with the fastest pressure rise time of 0.5 ms. When both Al and pristine KMnO₄ are sprayed, the explosion overpressure has been enhanced to 9.29 MPa with a pressure rise rate of 9.255 GPa/s. After KMnO₄ has been ground into smaller particles with a diameter of 20 μm, 100 g KMnO₄ can generate an explosion overpressure of 30.64 MPa with a pressure rise rate of 20.78 GPa/s. Regardless of pristine or ground KMnO₄, it can enhance the explosion overpressure but cannot enhance the detonation speed of the methane. When Al and the pristine KMnO₄ are separately sprayed into methane, the average detonation speed for the case of Al has been enhanced to 1.0 km/s whereas that for the case of KMnO₄ almost remains unchanged.