Research On Microstructure Change Characteristics And Macroscopic Performance Of Coal Samples With Different Metamorphic Degrees During Spontaneous Combustion

Coal occupies an important position in China’s energy structure,but coal spontaneous combustion fire is one of the most important accidents in all kinds of mine disasters,so the problem of coal spontaneous combustion urgently needs to be solved,and to study the mechanism of coal spontaneous combustion and put forward scientific and effective prevention and control measures.This paper choosed three different metamorphic degree of coal sample to study of the changes of the microstructure in the process of coal spontaneous combustion and macro performance,with micro functional evolution explained macro exothermic characteristics and change law of quality,and the key reactive functional groups affecting the spontaneous combustion process of coal were determined.It had important scientific significance for the improvement and development of coal spontaneous combustion mechanism.Thermogravimetric and differential scanning calorimetry combined experiment was used to study the weight and heat variation characteristics of different coal samples during spontaneous combustion,and determined the characteristic temperature point and characteristic temperature stage of each coal sample.Studies show that the burning of coal samples present the lag situation with the increase of metamorphic grade,stripping weightless stage low metamorphic degree of coal weight loss is the largest,oxygen weight stage high metamorphic degree of coal increases the weight,the largest thermal decomposition heat of coal combustion phase items are oxidized spontaneous combustion heat release,the rapid combustion of coal samples,the quality quickly reduce;Low metamorphic degree of coal spontaneous combustion in the process of high total weight loss is greater than the metamorphic degree of coal,but the high degree of metamorphism of coal heating ability is stronger,more fully burning.Based on the kinetic calculation of spontaneous combustion process of coal samples with different metamorphic degrees,the difference of kinetic parameters of spontaneous combustion stage of each coal sample was analyzed.The results show that the weight loss stage of desorption,the dynamic equilibrium stage and the weight gain stage of oxygen absorption conform to the second-order reaction mechanism function of common reaction,the Dayan lignite in the thermal decomposition combustion stage conform to the two-dimensional diffusion mechanism function,and the Ren Jiazhuang gas coal and Lvtang anthracite conform to the three-dimensional diffusion（P-T）mechanism function;The kinetic parameter activation energy E value of each coal sample at each characteristic temperature stage increases gradually with the continuous oxidation spontaneous combustion process,and the pre-exponential factor A value is also larger in the thermal decomposition combustion stage;refers to the former factor value in pyrolysis combustion phase is larger;With the increase of coal sample metamorphism degree,E and A of the first three temperature stages have little change,but E and A of Lvtang anthracite are obviously higher in the thermal decomposition combustion stage.In situ Fourier transform infrared spectroscopy was used to study the distribution of microscopic functional groups of coal samples with different metamorphic degrees and analyze the changes of functional groups in the process of oxidation and spontaneous combustion of each coal sample.Research shows that:with the coal metamorphic degree deepening,hydroxyl type of functional group content decreased after increased first,fat hydrocarbon functional groups increase after decreases first,oxygen containing functional groups gradually reduce,aromatic hydrocarbon functional groups increases gradually,low metamorphic degree of coal contains more active functional groups,high metamorphic degree of coal system of aromatic condensation degree of stronger;Each functional group structure in the coal sample oxidized spontaneous combustion process is on the decline as a whole,the hydroxyl functional group decreases obviously at low temperature stage,the aliphatic hydrocarbon and oxygen-containing functional group decrease obviously at high temperature stage,the aromatic hydrocarbon functional group has little change,and the inflection point of the functional group content change in the process of spontaneous combustion of each coal sample shows a lag with the deterioration degree of coal sample.In the process of spontaneous combustion of coal,the hydroxyl functional groups in the weight loss stage of desorption decrease greatly,and the water continuously evaporates and absorbs heat,and the quality of coal sample decreases gradually;At the dynamic equilibrium stage,methyl and methylene at the deformation vibration sites of aliphatic hydrocarbon structures are consumed first,and react with hydroxyl to form carboxyl oxygen-containing functional groups,releasing a little heat;In the oxygen absorption and weight increasing stage,the aliphatic hydrocarbons at the stretching vibration sites are oxidized to form carbonyl and carboxyl groups in large quantities,and the amount of oxygen absorbed is larger than the amount of gas generated by oxidation.The mass of coal sample increases and the reaction releases more heat;In the thermal decomposition combustion stage,the side-chain fat structure,carbonyl group,carboxyl group and aromatic C=C double bond of benzene ring break a large number of bonds,and the oxidation reaction generates gases such as CO,CO₂ and CH₄,which rapidly reduces the mass of coal sample and releases a large amount of heat.