Experimental study on oxidative improving-permeability of coal samples with activated ammonium persulfate solution

In order to improve the effect of ammonium persulfate (APS) aqueous solution on the oxidative im-proving- permeability of coal samples,the self-made heterogeneous cobalt-based activator (Co-NCP) was used to activate am- monium persulfate to oxidize and modify three kinds of coal samples with different metamorphic grades,including long- flame coal,coking coal and anthracite coal. The influences of Co-NCP / APS oxidative system on the contact angle θ (wettability),pore structure,gas adsorption constant,initial velocity of gas emission and permeability of coal samples were systematically investigated. The experimental results show that the treatment with Co-NCP / APS system can sig- nificantly decrease the contact angle θ of coal surface and the adsorption capacity of methane. After treated by Co- NCP / APS system,the adsorption constant a of long-flame coal,coking coal and anthracite coal decreases by 16. 15% , 10. 21% ,and 6. 03% ,the adsorption constant b decreases by 44. 97% ,41. 10% ,and 28. 32% and the initial velocity of gas emission decreases by 49. 19% ,33. 33% ,and 12. 55% ,respectively. Meanwhile,the pore structure of the coal samples can be changed by Co-NCP / APS treatment. After modification, the porosity of coal samples increases, in which the proportion of pore volume of large and medium pores increases,and that of small and micro pores decreases. The porosity of long-flame coal,coking coal and anthracite coal increases by 43. 50% ,35. 50% and 12. 54% accord- ingly,and the pore volume increases by 56. 02% ,45. 54% ,and 20. 08% respectively. For the activated ammonium persulfate system,a large amount of sulfate radicals (SO4·- ) with strong oxidative ability are in-situ generated,which can oxidize and corrode the surface of coal samples with different metamorphic grades. The organic small molecular phase are partially oxidized and dissolved out of the pores of coal samples. The pores occupied or blocked by small molecules are dredged,consequently improving the pore connectivity. In the meantime,a large number of new cracks and holes are created,demonstrating that the oxidative modification with Co-NCP / APS system plays the role of “ in- creasing pore”,“expanding pore”,and “dredging pore”. Moreover,for the coal samples with lower metamorphism de- gree,the improvement effect of permeability is more significant. After treated by Co-NCP / APS system,the average in- creases of permeability are 620. 15% ,541. 02% ,and 334. 42% for long-flame coal,coking coal and anthracite coal, respectively. As a result,the heterogeneous cobalt-based activator (Co-NCP) can activate ammonium persulfate solu- tion to oxidize and modify coal samples with different metamorphic degrees. This oxidative modification can reduce the coal’s affinity for methane,improve the pore connectivity of coal samples and the fluidity of gas in coal body,which thus significantly decreases the gas storage capacity of coal body. This study provides the theoretical basis and experi- mental support for the application of ammonium persulfate solution in improving permeability of coal seam.