• 标题：Experimental study on the effects of electrode materials on coal breaking by plasma
• 关键词：Plasma,Electrode,Breakdown voltage,Coal
• 作者：Xiangliang Zhang, Baiquan Lin, Yanjun Li

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The extensively applied plasma technology boasts potential application prospects in the fifield of coal seam permeability enhancement. However, the effffect of electrode materials on coal breaking by plasma (CBP) has not been reported. For this reason, the effffects 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 diffffer under the action of difffferent electrodes. Specififically, 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 difffferent electrode materials correspond to difffferent modes of crack initiation on coal surface. The results of liquid N2 adsorption show that the aluminum electrode exerts the poorest effffect on improving micro-pores in coal, while the copper electrode achieves the best effffect. Furthermore, the effffects of electrode materials on the current and voltage waveforms at difffferent voltages were analyzed by using a Rogowski coil and a HV test rod, and the effffects 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 diffffer obviously; in contrast, at high voltages, the voltage and current waveforms become almost identical. At the same voltage, the deposited energies formed by difffferent electrode materials also diffffer due to the difffference 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 fifirst 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 effffects 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 difffferent. Among the four electrode materials, the brass electrode forms the brightest light and generates the greatest energy.