Effect of high-voltage thermal breakdown on pore characteristics of coal

Effect of high-voltage thermal breakdown on pore characteristics of coalZhu Chuanjie a,b,⇑,Lu Ximiao b ,Gao Zishan b ,Yan Fazhi b ,Guo Chang b ,Zhang Xiangliang ba State Key Laboratory of Coal Mine Disaster Dynamics and Control,Chongqing University,Chongqing 400044,China bSchool of Safety Engineering,China University of Mining and Technology,Xuzhou 221116,Chinaa r t i c l e i n f o Article history:Received 11January 2017Received in revised form 20March 2017Accepted 9April 2017Available online 10July 2017Keywords:High-voltage thermal breakdown Permeability enhancement Porosity FractureGas adsorptiona b s t r a c tHigh-voltage thermal breakdown has great potential application in permeability enhancement of coal seam.The characteristics of the breakdown channel,coal element,porosity and microscopic coal petrog-raphy of coal under high-voltage electric load were experimentally studied.The coal interior left apparent tracks due to electric current burning with high temperature.The percentage of C,O,Al,Fe,and Si had slightly decreased,while the content of element N increased obviously.Low-pressure nitrogen gas adsorption (LP-N 2GA)and mercury intrusion analysis showed that coal porosity increased.The increases of micropores and mesopores are beneficial to promotion of the ability of gas storage,and the increase of macropores could enhance the gas seepage and migration.The results of scanning electron microscope (SEM)show that there are many exogenous fractures in coal,which is also beneficial to gas seepage and migration.The results lay a theoretical foundation for application of high-voltage thermal breakdown in coal mines.Ó2017Published by Elsevier B.V.on behalf of China University of Mining &Technology.This is an openaccess article under the CC BY-NC-ND license (/licenses/by-nc-nd/4.0/).1.IntroductionIn china,the gas (coalbed methane)adopted in high and low rank coal account for more than two-thirds of the total resources,which has the characteristics of microporosity,low permeability rate,and high adsorption.The special nature of gas occurrence leads to the low efficiency of gas extraction,which not only has a great threat to coal mine’s safety,but also is bad for the utilization of gas and will pollute the environment [1].Meanwhile,the green-house effect of methane is 20times of CO 2.Therefore,to improve the effect of gas extraction is the fundamental measures to solve these problems [2].At present,the main way is to increase the per-meability of coal seams,that is to say,it can produce the fracture network through using some technical measures in order to pro-mote the gas desorption and flow in the coal.Now,the existing technologies of relieving pressure and increasing permeability effectively are mainly:mining protected layer,dense borehole drainage,high-pressure water jet cutting,deep-hole loose blasting and hydraulic fracturing,etc.[3–7].However,all these methods exist some technical problems that are difficult to overcome.In recent years,some scholars have put forward a technology called high-voltage electrical pulse (HVEP)to improve the perme-ability of coal seam,its basic principle is to use the liquid-electric effect of high-voltage,which can produce shock waves to make coal seam crack,then,the coal seam’s permeability will be enhanced.In fact,the technology was first applied to the reservoir block for secondary oil recovery [8].In 1975,the former Soviet scholar first applied the technology to the oil reservoir fracturing production,and succeed.Then the technology had been applied well in the former Soviet Union’s oil fields with a significant effect.The success rate of tests reached up to 75%in the individual wells and the average validity was 10months or so [9].The power of electric detonation can even reach a few hundred kilojoule and have great destructive power [10].In recent years,high-power electric pulse technology has also made great progress in mineral sorting [11–13].Meanwhile,some scholars have applied it to the food processing industry [14].Recently,coal mines have also tried to apply the technology to the development of coalbed methane on the ground or underground [15,16].However,due to the big differ-ence between coal and oil reservoir in occurrence conditions and mechanical characteristics,some basic theories are not perfect.Additionally,the implementation effect of the technology is not obvious.In addition,the pressure of shock waves generated by the liquid-electric effect is generally low and its ability to damage the coal is limited.For this reason,some researchers later found that when the discharge time is less than 500ns,the liquid’s crit-ical breakdown strength is higher than the solid’s.So,if the solid mineral immersed in the liquid medium (usually water),the plasma channel of high-voltage breakdown will firstly pass/10.1016/j.ijmst.2017.06.0272095-2686/Ó2017Published by Elsevier B.V.on behalf of China University of Mining &Technology.This is an open access article under the CC BY-NC-ND license (/licenses/by-nc-nd/4.0/).⇑Corresponding author at:State Key Laboratory of Coal Mine Disaster Dynamicsand Control,Chongqing University,Chongqing 400044,China.E-mail address:anq021@ (C.Zhu).International Journal of Mining Science and Technology 27(2017)1051–1055Contents lists available at ScienceDirectInternational Journal of Mining Science and Technologyjournal homepage:www.else v i e r.c o m /l o c a t e /i j m stthrough the interior of the solid which will be broken down.This method requires a sharp discharge time which must be controlled within500ns.We propose a permeability enhancement method with high-voltage thermal breakdown(HVTB)based on air,the paper is mainly to study the influence of the technique on the micropore porosity of coal.2.Mechanism of high-voltage thermal breakdown on coalThe internal molecules will move violently and form conductive ions under high voltage.When the voltage rises to its breakdown limit,the ion movement will aggravate and the instantaneous cur-rent channel between the anode and cathode electrodes will form. Then,the coal will be punctured as a result.Due to the breakdown of the coal and the following huge electrical current,there will be melting or burning channels or even cracks as shown in Fig.1a of the thermal cracking of the main channel.At the same time,owing to the existence of conductive ions,the corrosion cracks or pores are also generated,which makes coal porosity increase.Because of the instantaneous huge current passing through the coal,a transient high temperature will be formed.As a result,the coal will expand and produce thermal stress.On one hand,It will make the coal form the tensile fractures,which leads to the coal broken to fragments of different sizes;on the other hand,the elec-trical current will also pass through the coal and produce expan-sion stress on the surface of the pore wall,making the pore structure and the pore size distribution change,as shown in Fig.1b and c.In addition,the condition for the breakdown of the coal is:E¼U=L>E0ð1Þwhere E is the breakdownfield strength of the coal;U is the voltage that applied to both sides of the coal;L is the electrode spacing on both sides of the coal;and E0is the critical breakdownfield strength of the coal.The condition for the changes of pore structure is:r>r0ð2Þwhere r is the thermal stress and r0is the limit stress of the pore. Under the condition of high voltage breakdown,the changes of pore in the coal is complicated.Fig.1b shows the stress of parallel or obliquefissures in Fig.1a.There are two stresses r x and r y of the thermal stress.The horizontal stress,r x,which makes the pore shrink and the pore reduce,is perpendicular to the surface of the pore wall.And the vertical stress r y acts on the pores in parallel causing the pore to expand and enlarging the pore’s size.In addi-tion,the thermal stress can also act on the vertical cracks in Fig.1a,changing the pore in both directions.In summary,high-voltage electric breakdown may produce dis-solution of pores or cracks,but also change the characteristics of the pore by the thermal stress of high-voltage electric thermal breakdown.3.Experimental device and materials3.1.Experimental principlesFig.2shows breakdownfield strength of different media with the voltage rise time of the front edge[17].At present,the most used method of crushing rock in high-power electric pulse,must immerse the rock into the water.Because the breakdown strength of water is higher than that of rock,the rock will be brokenfirst and the water will not be broken when the voltage rise time is less than500ns.Thus,the breakdown ionization channel only runs through rock,which improves the efficiency of rock breaking.Fig.3a is the schematic diagram of current most used method. The coal isfirstly immersed in water,and then put the electrodes on both sides of the coal.After the application of high voltage, the coal will be crushed and water will be not.However,the coal is different from other solid materials,its conductivity is much bet-ter.For example,the breakdownfield strength of anthracite coal is only1.6–1.7kV/cm,which is much lower than that of air(18–18.3kV/cm)[18].Therefore,for the breakdown and permeability enhancement of coal,we can consider that it need not be soaked into water.For this reason,this paper presents a new method with high-voltage thermal breakdown based on air,and Fig.3b shows its schematic diagram.Another advantage of this method is that the coal itself has a certain volume and width,which causes that the breakdown of the air ionization channel is much higher than the coal(the breakdown of the channel of coal along the direction of the two electrodes)and ensures that the coal will be the broken firstly and the air will not be under the premise of having better conductivity and shorter breakdown of the path.3.2.Experimental setup and sampleAs shown in Fig.4,the experimental system of high-voltage thermal breakdown consists of AC power source,HV DC power source,HV switch,capacitor,and experimental cavity.Among them,the AC power source has an ordinary alternating current power supply with a voltage of220V,50Hz.The HV DC power source has a maximum voltage of50kV and a direct current sup-ply.The HV switch has an operating voltage of40kV and can with-stand an inrush current of140kA.The capacitor is8uF.In this experiment,the HV DC power source is connected to the capacitor, and then the two ends of the capacitor are connected to the work-ing electrode of the experimental cavity(no anode and cathode distinction).Then one end of the HV switch is connected to the working electrode,and the other end connected to the trigger elec-trode.At the same time,the experimental cavity must be grounded forsafety.Fig.1.Schematic of the influence mechanism of high-voltage thermalbreakdown.Fig.2.Relationship of breakdownfield strength and voltage rise time[17]. 1052 C.Zhu et al./International Journal of Mining Science and Technology27(2017)1051–1055The technology about high-voltage electric breakdown is mainly used in hard coal,especially the anthracite coal with a high gas content.Therefore,we selected anthracite coals as experimen-tal samples.The coal sample selected in the experiments were pro-cessed as cylinders (50mm diameter Â100mm length),as shown in Fig.5.Their industrial analysis parameters are shown in Table 1.4.Experimental results and analysis4.1.Characteristics of breakdown channel and changes of coal compositionThe basic principle of high-voltage electric breakdown is that the coal is put on high voltage and the coal will not be punctured until the voltage strength is higher than the breakdown strength of coal.As the instantaneous breakdown of coal,the internal will have a larger strength of electrical current that leads to a transient high temperature,which may alter the constituent of material sur-roundings,such as easily decomposable volatile at high tempera-tures and so on.These changes may have certain influence ongas adsorption,Fig.6shows coal surface and scanning electron microscopy (SEM)image of burning surface after high-voltage thermal breakdown.We can see that there are obvious burn marks in coal after cleavage,and other places also have slight burning signs.The SEM image shows that there are many burning particles on the coal surface.We have compared and analyzed the changes of the elements before and after the high-voltage electric breakdown.The results are shown in Fig.7.Fig.7b shows that there are some differences between the elements before and after the high-voltage electric breakdown.The percentage of C,O,Al,Fe,and Si slightly decreased after the breakdown,and element N increases ually,the coal elements,having more effect on gas adsorption,are mainly C,O,H,and other elements are usually less,such as Al,Fe and Si,which have little impact on the gas adsorption.It is shown that the constituent of element C and O decreased slightly but not obvi-ously.Therefore,from the changes of coal constituents point of view,the high-voltage electric breakdown will not affect the phys-ical adsorption of coal,and the change of pore and fracture may play a major role.4.2.Variation of coal poresUnder the action of high voltage,coal thermal breakdown occurs.The high-voltage current flowing through the coal will lead to changes in the constituent of the coal and affect the pore struc-tures of coal.We have analyzed the characteristics of coal pore dis-tribution by DFT and mercury intrusion analysis (MIP).The results are shown in Fig.8.From Fig.8a,it can be seen that the micropores and mesopores of treated coal sample have a substantial increase comparing with original coal sample.It is generally believed that the porosity whose pore size is more than 100nm is a favorable channel for gas migration and seepage.When the pore size is less than 100nm,the gas on the pore surface mainly exists in a state of adsorption.Therefore,the experimental analysis of micropores and mesopores shows that the area of adsorption of coal samples with electric pulse treatment increased.In order to investigate the distribution of macropores in the coal,we have also analyzed the coal samples after high voltage electric breakdown by mercury intrusion analysis,as shown in Fig.8b.As can be seen from Fig.8b,the number of macropores in coal also increased.Above all,after the destruction of coal by high-voltage electric breakdown,the micropores,mesopores and macropores have increased,which indicates that the storage ability of coal have been improved.At the same time,it is also beneficial to the gas seepage and migration in coal.In order to investigate the actual pore characteristics of coal,the coal petrography analysis was carried out by scanning electron microscopy.Fig.9a shows SEM image of original coal characteris-tics.It can be seen from Fig.9a that there are a few endogenetic(a) HVEP method (b ) HVTB method.Fig.3.Two different principlesof high-voltage electric breakdown.Fig.4.Schematic diagramof high-voltage thermal breakdown.Fig.5.Experimental coal samples.Table 1The industrial analysis parameters of coal sample (%).Water content Ash content Volatiles content Fixed carbon content4.375.4232.6157.6(a) Burning marks. (b ) SEM image after HVTB.Fig.6.Burning marks and SEM images of burning surface.C.Zhu et al./International Journal of Mining Science and Technology 27(2017)1051–10551053fractures and intergranular pores in the original coal.Fig.9b is a SEM image of 5000times about the surface of the two sides of the breakdown tunnel.It can be seen that the coal contains a large number of blowholes.But it is difficult to determine whether the hole is the secondary gasification hole,which generated by the high temperature process of coal breakdown current in the process of coal breakdown,because the process of coalification can also leave a similar hole due to the escape of volatile matter that gen-erates from the evolution of organic matter.At the same time,from the SEM images,it can be seen that the exogenetic fractures in coal are well developed,which is good for the gas seepage and migration.Generally,under the vertical pres-sure in the process of coalification,the volume of coal-forming materials shrinks evenly which generated the tension and resulted in cracks in the coal seam.Then,the endogenetic fractures ually,the fracture surface is smooth and the number of fractures in a range of 5cm is less than ten [19].In Fig.9a,there is only one endogenetic fracture in the SEM scanning area.However,the exo-genetic fractures often present in any part of coal.The orientations and shapes of these fractures are more irregular.Fig.9b shows that there are six fractures in the range of 100l m,which obviously exceeds the quantity of endogenetic fractures that anthracite coal should have.It can be concluded that these exogenetic fractures were mainly formed by the tensile stress,which produced in thetransient high temperature,during high voltage electricbreakdown.5.ConclusionsIn this paper,high-voltage electric breakdown device was used to study the characteristics of the breakdown channel and the coal composition,the porosity and the micro-coal petrography of the anthracite coal under the load of high voltage.The obtained con-clusions are as follows:(1)There are obvious burn marks in coal after cleavage,andother places also have slight burning marks.There are also some differences between the elements before and after the high-voltage electric breakdown.The percentage of C,O,Al,Fe,and Si is slightly decreased,while element N increases obviously.But the effect on the gas desorption and adsorption was not significant.(2)Coal porosity of different size (micropore,mesopore andmacropore)all increased after high-voltage electric break-down.The increase of micropores and mesopores is benefi-cial to gas storage,and the increase of macropores could enhance gas seepage andmigration.Fig.7.Energy spectrum analysis of coal and rock components (EDS).(a) DFT results (b ) MIP results.Fig.8.Effect of high-voltagethermal breakdown on coal porosity.Fig.9.Effect of high voltage electric breakdown on coal petrography:(a)original coal sample;(b)coal sample treated by HVTB (5000times);(c)coal sample treated by HVTB(3000times);(d)coal sample treated by HVTB (800times).1054 C.Zhu et al./International Journal of Mining Science and Technology 27(2017)1051–1055(3)SEM images show that in the process of the high-voltageelectric breakdown,the transient high temperature gener-ated the tensile stress which urged the coal to produce exo-genetic fractures.AcknowledgementsFinancial supports for this work,provided by the Scientific Research Foundation of the State Key Lab.of Coal Mine Disaster Dynamics and Control(No.2011DA105287-FW201509),the National Natural Science Foundation of China(No.51474211) and the Innovation Team of CUMT(No.2014QN001),are gratefully acknowledged.References[1]Olajossy A.Some parameters of coal methane system that cause very slowrelease of methane from virgin coal beds(CBM).Int J Min Sci Technol2017;27(2):321–6.[2]Huang BX,Cheng QY,Chen SL.Phenomenon of methane driven caused byhydraulic fracturing in methane-bearing coal seams.Int J Min Sci Technol 2016;26(5):919–27.[3]Yuan L.Theory and practice of integrated pillarless coal production andmethane extraction in multiseams of low permeability.Chinese Eng Sci 2009;11(5):72–80.[4]Zhou HX,Cheng YP,Liu HY.Permeability improvement technology of arraycrossing boreholes and its application in outburst coal seam.J China Coal Soc 2011;36(9):1515–8.[5]Lin BQ,Meng FW,Zhang HB.Regional gas control based on drilling-slotting-extracting integration technology.J China Coal Soc2011;36(1):75–9.[6]Guo DY,Li PF,Shan ZY,An X.Drilling parameters of deep-hole cumulativeblasting to improve coal seam permeability in gas drainage.J Univ Sci Technol Beijing2013;35(1):16–20.[7]Lin BQ,Li YF,Zhai C,Qiang B,Wen YY.Pressure relief and permeability-increasing technology based on high pressure pulsating hydraulic fracturing and its application.J Min Safety Eng2011;28(3):452–5.[8]Luo SH,Lu GF.Study and application of formation stimulation technique usingdown hole electric discharge.Oil Drilling&Prod Technol1995;04:84–7. [9]Zhu JP.Low frequency pulse wave enhanced oil production technology.Foreign Oilfield Eng1995;01:55–6.[10]Sun YH,Sun GS,Yan P.High-power electrical impulse oil recovery technologyand its application.Drilling and Prod Technol2002;25(5):61–3.[11]Wang E,Shi F,Manlapig E.Pre-weakening of mineral ores by high voltagepulses.Miner Eng2011;24(5):455–62.[12]Wang E,Shi F,Manlapig E.Factors affecting electrical comminutionperformance.Miner Eng2012;34(7):48–54.[13]Shi F,Zuo W,Manlapig E.Characterisation of pre-weakening effect on ores byhigh voltage electrical pulses based on single-particle tests.Miner Eng2013;s50–51(9):69–76.[14]Sarkis JR,Boussetta N,Tessaro IC,Marczak LDF,Vorobiev E.Application ofpulsed electricfields and high voltage electrical discharges for oil extraction from sesame seeds.J Food Eng2015;153:20–7.[15]Liu YH,Liu Q,Liu CY.Study on CBM blocking removal experiment.ChinaCoalbed Methane2009;6(4):34–7.[16]Qin Y,Qiu AC,Zhang YM.Experiment and discovery on permeability improvedtechnology of coal reservoir based on repeated strong pulse waves of high energy accumulation.Coal Sci Technol2014;42(6):1–7.[17]Yutkin LA.Electrohydraulic effect.English translation:U.S.Department ofCommerce,Office of Technical Services.Document62–15184MCL1207/1-2, 955.[18]Yan FZ,Lin BQ,Zhu CJ,Guo C,Zhou Y,Liu ing high-voltage electricalpulses to crush coal in an air environment:an experimental study.Powder Technol2016;298:50–6.[19]Yang Q,Li ST.Coal Geology.Beijing:Geological Publishing House;1979.C.Zhu et al./International Journal of Mining Science and Technology27(2017)1051–10551055。