Experimental study on the effects of electrode materials on coal breaking by plasma

The extensively applied plasma technology boasts potential application prospects in the field of coal seam permeability enhancement. However, the effect of electrode materials on coal breaking by plasma (CBP) has not been reported. For this reason, the effects of four kinds of electrode materials, namely aluminum, copper, brass and stainless steel, on CBP were investigated by using a self-built CBP experimental system. The experimentally obtained statistics show that the sizes of coal fragments differ under the action of different electrodes. Specifically, the coal sample tends to be broken into small particles under the action of aluminum electrode, whereas the broken fragments generally have a diameter of larger than 15 mm under the action of brass electrode. The scanning electron microscopy results suggest that the different electrode materials correspond to different modes of crack initiation on coal surface. The results of liquid N2 adsorption show that the aluminum electrode exerts the poorest effect on improving micro-pores in coal, while the copper electrode achieves the best effect. Furthermore, the effects of electrode materials on the current and voltage waveforms at different voltages were analyzed by using a Rogowski coil and a HV test rod, and the effects of electrode materials on deposited energy generated by plasma in the coal were also discussed. The results indicate that at low voltages, the voltage and current waveforms under the action of four electrode materials differ obviously; in contrast, at high voltages, the voltage and current waveforms become almost identical. At the same voltage, the deposited energies formed by different electrode materials also differ due to the difference in breakdown voltages and current waveforms. The deposited energies formed by the aluminum electrode, the brass electrode and the stainless steel electrode inside the coal fall first and then rise with the increase of breakdown voltage, whereas that formed by the copper electrode goes up gradually with the increase of breakdown voltage. Based on the photograph of plasma discharge process shoot by a high-speed camera, the law of plasma propagation inside the coal was analyzed, and the effects of electrode materials on plasma development were discussed. A plasma streamer which is accompanied by dazzling white light and extremely high temperature is formed inside the coal at the moment of discharge, and it continuously expands outward as time passes by. The brightness degrees of plasma streamers formed under the action of electrode materials are different. Among the four electrode materials, the brass electrode forms the brightest light and generates the greatest energy.