Experimental Study On Spontaneous Combustion Characteristics And Microstructure Of Typical Coal Samples In Huainan Coalfield

The Huainan coalfield is the largest and most well-integrated coalfield in southeastern China,with its main mining areas consisting of A,B,and C coal groups.Some of the main coal seams within these groups are known to be prone to spontaneous combustion,creating a serious fire hazard.Conducting basic experimental research on the spontaneous combustion characteristics of typical coal samples from the Huainan coalfield is therefore of great theoretical and practical significance in effectively preventing mine fires.In this article,we conducted a systematic analysis of the combustion characteristics of four types of coal samples（1 coal,8 coal,11-2 coal,and 13-1 coal）from Zhangji Coal Mine in the Huainan coalfield.This was achieved through thermogravimetric tests（TG）,Fourier transform infrared spectroscopy（FTIR）,and thermogravimetric-infrared spectroscopy（TG-FTIR）experiments.The design of experiments and main findings are as follows:（1）Seven characteristic temperature values were determined based on the TG-DTG curve of coal spontaneous combustion process,namely:dry cracking temperature T₁,active temperature T₂,increasing temperature T₃,maximum mass temperature T₄,ignition temperature T₅,maximum weight loss rate temperature T₆,and burnout temperature T₇.These temperatures correspond to four stages of the combustion process:slight weight loss stage（30℃～T₁）,slow weight gain stage（T₁～T₄）,rapid weight loss stage（T₄～T₇）,and burnout constant weight stage（T₇～950℃）.Our findings suggest that the spontaneous combustion characteristic temperature values of coal samples from different coal seams follow the order of 1 coal>8 coal>11-2 coal>13-1 coal.（2）Our study also examined the impact of various factors on the characteristic temperature of spontaneous combustion in coal samples.It was found that as the oxygen concentration increases,the characteristic temperature of spontaneous combustion decreases.Specifically,the maximum weight loss rate temperature T₆ decreases by70～80℃,and the corresponding maximum weight loss rate V_max increases by0.2%～0.4%/min.Furthermore,the higher the heating rate,the higher the characteristic temperature value of coal spontaneous combustion,and the smaller the maximum weight loss rate V_max.The particle size of coal samples is positively correlated with the characteristic temperature value of spontaneous combustion,meaning that smaller particle sizes result in smaller characteristic temperature values.（3）Coats-Redfern model was used to calculate nine reaction mechanism functions and combined with the results of two equal conversion methods,it was concluded that the reaction mechanism of four coal samples is the same.The most probable mechanism function of coal samples in the slow oxidation stage is the parabolic law,with the reaction mechanism being one-dimensional diffusion.In the pyrolysis stage,the most probable mechanism function is the Avrami-Erofeev equation,with the reaction mechanism being random nucleation and subsequent growth.In the coal combustion stage,the most probable mechanism function is the Jander equation,with the reaction mechanism being three-dimensional diffusion and n=2.Based on the activation energy results,the ranking of coal samples according to their degree of spontaneous combustion,from difficult to easy,is as follows:1 coal>8 coal>11-2 coal>13-1 coal.The activation energy of coal samples decreases with the particle size,and the smaller the particle size,the more prone the coal sample is to spontaneous combustion.（4）Through FTIR experiments,it was found that in four typical coal samples from the Huainan coalfield,aliphatic aromatic hydrocarbons mainly existed in the form of long chains with few side chains.The polysubstituted benzene ring type,the oxygen-containing functional groups in the 1330～1060cm^-1 region,and the relative content of phenolic hydroxyl groups near 3430cm^-1 follow the order of 13-1 coal>11-2 coal>8coal>1 coal,from high to low.The higher the content of these micro groups in the coal sample,the more prone the coal sample is to spontaneous combustion.（5）The gas phase products of coal sample pyrolysis were analyzed through TG-FTIR experiments.It was found that the main gas product concentrations of CH₄,CO₂,and H₂O follows the order of 13-1 coal>11-2 coal>8 coal>1 coal,and the temperature at which the maximum concentration appeared follows the order of 13-1 coal<11-2 coal<8 coal<1 coal.These findings confirm that the higher the polysubstituted benzene ring type,the oxygen-containing functional groups in the 1330～1060cm^-1 region,and the relative content of phenolic hydroxyl groups near 3430cm^-1 in coal samples,the more intense the coal reaction and the lower the reaction temperature.The different micro-groups present in coal samples lead to varying spontaneous combustion characteristics of typical coal samples in the Huainan coalfield.Based on the differences in micro-groups among typical coal samples,as well as TG experiments and the calculation of kinetic parameters,it is concluded that the spontaneous combustion of typical coal samples in the Huainan coalfield occurs in the following order from easy to difficult:13-1 coal>11-2 coal>8 coal>1 coal.This study on the spontaneous combustion characteristics of typical coal samples in the Huainan coalfield provides a theoretical foundation for establishing fire prevention and extinguishing mechanisms in mines.Figure[39]table[15]reference[101]...