Micro Physicochemical Properties And Macro Thermal Effect Of Coal Influenced By Methane-containing Environment During Oxidation

In the mining of coal seams that are highly gassy and spontaneous combustion prone,the coexistence of methane and coal fire has been one of the characterizations of great severe accidents.The coal is immersed in methane-containing oxidation environment,especially around the press relief methane drainage boreholes and in the working face gob.However,there is a lack of systematic research in microphysicochemical properties and macro thermal effect of coal influenced by methane.Therefore,the study in this field is of great significance in the prevention of the coupling hazards of methane and coal fire.In this paper,a self-developed oxidation experiment system taking methane as the diluent to dilute dry air was employed to oxidize coal samples under methanecontaining environments.Besides,the same amount of nitrogen was also introduced as the diluent to make comparison to study the influence of methane on microphysicochemical properties and macro thermal effect of coal during oxidation.X-ray diffractometer was introduced to measure crystalline parameters of coal oxidized under different methane-containing oxidation environments.It was found that the height of coal crystallites,number of aromatic layers,diameter of coal crystallites,graphitization degree decline with the increasing methane concentration in oxidation environments.Besides,the stage characteristic was found in coal crystalline parameters.The reaction extent in the vertical dimension is greater than the radical direction when the methane is less than 15%.Although the graphitization degree grows,spatial ordering increases while inter-layer spacing drops with oxidation temperature at a constant methane concentration,only when the oxidation temperature exceeds a specific value,the diameter of coal crystallites begin to increase significantly.The role of methane on crystalline structure evolution was firstly revealed on the macromolecular structure level of coal.From the EPR experiments,micro chemical reaction mechanism involving methane in oxidation environments was revealed experimentally.Three stages were classified on the influence of methane on orbit-spin coupling: slow growing stage at low concentration(< 25%),fast increasing stage at intermediate concentration(25%~35%)and stabilization stage at high concentration(>35%).Methane and oxidation temperature show obvious stage characteristic: orbit-spin coupling and relaxation effect grow with oxidation temperature,especially when the temperature is over 150℃;although relaxation effect rises steadily,free radical drops fast with methane less than 35% in oxidation environment,the trend goes in opposite direction when the methane is greater than 35%.Free radicals were found not merely linked with oxygen and the existence of methane in oxidation environments could promote coal spontaneous combustion: compared with nitrogen,methane in oxidation environment can promote spin-orbit interaction,increase relaxation effect,raise free radical concentration and have promoting effect on coal spontaneous combustion at lower concentration less than 30% and vice versa.The development of surface elements and functional groups of coal during oxidation was revealed by XPS analysis.O element increases at first and decreases subsequently in methane-containing environments showing stage characteristic,while the existence of methane reduces the amount of C and relatively promotes the amount of O on the surface.The C-C and C-H bonds grows at the beginning and drops later with the growing methane concentration,while their relative amounts are increased by methane in comparison with nitrogen in oxidation environments.As to C-O bound,it is the main existence form of O,whereas C-O、C=O and O=C-O grow with the temperature.However,methane and nitrogen have little influence on C=O bond.C-O bond drops and then grows with the concentration of diluents,but when methane is less than 30%,the relative amount of C-O and C=O bounds are increased compared with taking the same amount of nitrogen as diluent,promoting coal-oxygen compounding.Acting mechanism of methane on surface elements and functional groups of coal during oxidation was further expounded from the boundary chemistry point of view.The influence of methane on kinetic mechanism of coal-oxygen reaction was illuminated and the development of coal-oxygen reaction complexity was gotten by analyzing the influence of methane on macro thermal effect of coal using TG-DSC experiments.Critical temperature and evaporation temperature were found mainly controlled by nitrogen and having little relationship with methane in oxidation environments.The existence of methane relatively shortened the distance between active temperature and evaporation temperature promoting the risk of coal spontaneous combustion relatively.The peak temperature falls slowly with oxygen and is slightly affected by methane and nitrogen in oxidation environments.In terms of coal-oxygen reaction heat,it would generate more heat taking methane as diluent than nitrogen in thermal decomposition and combustion stage,whereas in the range between 14.3% and 57.1% the existence of methane leads to more reaction heat compared with taking the same concentration of nitrogen as diluent.If the methane lies between 28.6% and 57.1%,the activation energy is reduced in fast oxidation stage and coal spontaneous combustion is enhanced compared with the same amount of nitrogen as diluent.