Study On The Mechanism Of Oxidation Activity Enhancement Of Water-soaked Coal And Its Inhibition

China is a major coal production country.In the process of coal mining,the spontaneous combustion of coal is relatively serious.In the practice of coal production,it is found that the risk of spontaneous combustion of dried soaked coal increases,and the problem of spontaneous combustion of soaked coal has attracted more and more attention.Mine water in nature is weakly acidic or alkaline.The effect of immersion process under this condition on the transformation mechanism of coal active functional groups and the spontaneous combustion characteristics of coal is not clear.The paper conducts experimental research and theoretical analysis on the spontaneous combustion characteristics and physicochemical structure changes of soaked coal under different immersion conditions.The effect of water immersion on the oxidation characteristics of coal active groups was studied at the molecular level by using quantum chemistry theory,so as to reveal the enhancement mechanism of oxidation activity of soaked coal and carry out the research on oxidation inhibition of soaked coal.With the help of temperature-programmed experiments,the oxidation characteristics of soaked coals with different degrees of metamorphism,different p H conditions,and various immersion durations were studied.Results showed that the oxygen consumption and released gas concentration increased and the crossing point temperature decreased in the low-temperature oxidation process of soaked coals with different metamorphic degrees,indicating that the oxidation activity of soaked coal is enhanced.The apparent activation energy values of the two stages of slow oxidation and fast oxidation of soaked coal treated with different p H aqueous solutions（p H4,p H5,p H7,and p H8）were decreased.Through the experiments of different durations（30 d,90 d,180 d）,it is found that the longer the immersion durations,the more obvious the promotion effect on oxidation activity of soaked coal.The calculation method of oxidation kinetic parameters of long flame coal before and after coal immersion is optimized by multiple scanning rate method,so as to analyze the changing law of parameters more accurately.Results showed that the average E and ln A of soaked coal obtained by FWO,KAS,Friedman,and Kissinger kinetic methods are lower than those of raw coal,that is,soaked coal is more prone to spontaneous combustion.By comparing the kinetic parameter values and correlation coefficients of the four methods,it is concluded that the FWO method and KAS method are more accurate for calculating the kinetic parameters of the oxygen absorption stage and the thermal decomposition and combustion stage respectively.According to the Bagchi method analysis,the most probable mechanism functions of these two stages of long flame coal follow the Avrami-Erofeev equation（n=2）and the first-order chemical reaction respectively.The reaction mechanism functions of the coal did not change before and after immersion.The microscopic physical and chemical structure changes of coal before and after immersion were analyzed by using modern analysis and testing technology such as low-temperature liquid-nitrogen adsorption and infrared spectroscopy.Results showed that immersion effect significantly increased the average pore-size,and the fractal dimension D₁ of soaked coal increased and the D₂ decreased.Immersion process not only made the surface of mesopores and macropores rougher but also promoted the evolution of fine-mesopores to mesopores and macropores in coal.After immersion,some kaolinite in the coal is dissolved,the interlayer spacing is slightly increased,the degree of coalification is reduced,and the microcrystalline structure is loose.The total amount of aliphatic structure groups and oxygen-containing functional groups,especially the content of phenolic hydroxyl groups,in soaked coal is more than that in raw coal.The hydrogen-bonding force in coal molecules is weakened,and the degree of chain branching is increased.The changes in physicochemical structure of soaked coal increase the amount of oxygen transportation and its adsorption sites in coal,leading to an increased risk of spontaneous combustion.Given the importance of active functional groups participating in the reaction,the macromolecular structure model of long flame coal was constructed by using quantum chemical software Gaussian to reveal the immersion effect on the oxidation characteristics of coal functional groups at the molecular levels.Results showed that the interaction between coal and water molecules during immersion will affect the arrangement of extranuclear electrons,so that the bond length and bond order of active functional groups of coal molecule increases,the bond order decreases,and the NPA charge of its hydrogen atom increases significantly.The difference between the HOMO energy of coal and the LUMO energy of oxygen is reduced by 50%,which leads to the slow oxidation of coal and the generation of more active sites,thereby enhancing the oxidation activity of soaked coal.Based on the enhancement mechanism of oxidation activity of soaked coal,it is proposed to inhibit the spontaneous combustion of soaked coal by reducing the number of active free radicals and the production of active functional groups,and isolating oxygen and lowering the temperature.Research found that the mixture of tea polyphenols（TP）and ascorbic acid（VC）(m _TP:m _VC=2:1)can synergistically block the radical chain reaction of coal oxidation.The efficient controlled-release inhibitor formed by the combination of antioxidant mixture and thermosensitive hydrogels has a significant inhibitory effect on the low-and medium-temperature oxidation stage of soaked coals.The research results can provide theoretical and technical support for coal spontaneous combustion prevention and control.Figure[101]table[35]reference[193]...