Experimental Study On Modified Improving-permeability Of Different Rank Coals With Activated Ammonium Persulfate Solution

Gas,also known as coal-bed methane,is an efficient and clean energy source and a greenhouse gas with high extraction and utilization value.However,due to the complex geological conditions,coal seam in China have the characteristics of low permeability,easy adsorption and difficult desorption.Therefore,it is important to improve the permeability of coal seam for safe exploitation of coalbed methane.In this paper,the self-made heterogeneous cobalt-based activator activated ammonium persulfate solution was used to modify the coal samples of different ranks,the surface wettability,morphology,pore structure,gas adsorption characteristics and the permeability enhancement of coals before and after modification were studied experimentally,and its microscopic mechanism was analyzed,the main conclusions are as follows:(1)According to the contact angle test,the contact angle of the modified coking coal reduced from 114.93°to 45.53°,indicating that the Co-NCP/APS system has a significant effect on improving coal wettability;The temperature has a great influence on the surface of oxidized coal of the Co-NCP/APS system,and the contact angle decreased with increasing temperature;It is observed by scanning electron microscopy that surface of the modified coal becomes loose and rough,a large number of new holes and cracks appear,and better for low-rank coal.(2)Analyze the pore structure of coal before and after modification by combining the mercury intrusion method and the low temperature nitrogen adsorption method.The mercury intrusion method is more accurate for the seepage pore test,and the low temperature nitrogen adsorption method is more accurate for the adsorption holes.The mercury pressure test results show that the permeation volume(medium pores and large pores)of the modified coal increased pore volume and pore volume ratio,the adsorption volume(micro pores and small pores)decreased pore volume and pore volume ratio,indicating that Co-NCP/APS system play the role of increasing pore,expanding pore and dredging pore;The porosity of modified long-flame coal,coking coal,and anthracite increased by 43.50%,35.50%,12.54%,and the pore volume increased by56.02%,45.54%,20.08%,indicating that the modified treatment can increase the passage of gas migration in coal and promote permeability in coal seams.The low-temperature nitrogen adsorption test results show that after the modification of Co-NCP/APS,the specific surface area of long-flame coal,coking coal and anthracite decreased by 78.40%,65.32%,59.19%,the micropore volume decreased by 78.43%,52.50%,46.15%,indicating that the adsorption and storage capacity decreased.According to fractal theory,the modification of Co-NCP/APS can reduce the complexity of the pore structure and the roughness of the coal surface,make the coal pore surface smoother,reduce the coal adsorption and gas storage capacity.(3)Through isothermal adsorption test and initial velocity of gas emission test,it can be known that the adsorption constants a of long-flame coal,coking coal,and anthracite decreased by 16.14%,10.19%,6.01%,the adsorption constant b decreased by 45.19%,40.03%,28.51%,the initial velocity of gas emission decreased by 49.19%,33.33%,12.55%,indicating that the modification treatment can significantly reduce the methanophilic capacity of coal.(4)According to the permeability test,it can be seen that the Co-NCP/APS system's permeability enhancement on coal is significantly better than that of water and APS solution(unactivated),and the permeability enhancement on low-rank coal is better.After modification,the average permeability increases of long-flame coal,coking coal and anthracite are 620.15%,541.02%,334.42%.(5)Through the FTIR detection and TOC detection of coal sample filtrates under different conditions,the micro-mechanism of chemical anti-reflection of Co-NCP/APS system can be analyzed:the activated ammonium persulfate system can generate a large number of strongly oxidizing sulfate radicals(SO₄^·-),which can effectively oxidize and dissolve organic small molecular substances in coal.On the one hand,it clears the small molecules occupied or blocked by organic molecules,plays the role of increasing pore,expanding pore and dredging pore;.On the other hand,sulfate radicals(SO₄^·-)can oxidize the coal surface in situ,increasing the content of oxygen-containing functional groups.By changing the physical and chemical properties of coal surface to increase the wettability of coal surface and reducing the methane-philic capacity of coal,achieve the purpose of increasing the desorption and improving the permeability of the coal.