Study On Spontaneous Combustion Characteristics Of Pre-oxidized Coal With Different Temperatures And Oxygen Concentrations

There is a large amount of pre-oxidized coal in the goaf,enclosed fire area and coal field fire area.The spontaneous combustion characteristics of pre-oxidized coal differs greatly from that of raw coal,so the phenomenon of coal reignition occurs frequently in China.This paper used the method of temperature programming,oxidizes the raw coal at different oxidation temperatures and oxygen concentrations.Through theoretical analysis and experimental research,the similarities and differences of the microstructure characteristics,thermal mass loss characteristics and thermal effects of pre-oxidized coal and raw coal were analyzed.In addition,the influence of oxidation temperatures and oxygen concentrations on spontaneous combustion characteristics of pre-oxidized coal and the trend of the critical value of oxidation temperature with the change of oxygen concentration were determined.The research results have important guiding significance for the early-warning and prevention of the re-ignition fire of the goaf,fire zone unsealing process,and the prevention and control of coal field fire.The raw coal was oxidized to 100,200,300 and 400? in 5,9,13,18 and 21%oxygen volume fraction of oxygen and nitrogen mixture gas by means of programmed heating and gain pre-oxidized coal.According to the results of Fourier infrared spectroscopy experiments,the types of functional groups in pre-oxidized coal were similar to the raw coal,but the content is different.The content of the functional groups in the pre-oxidized coal varies with the oxidation temperatures,and the-OH and oxygen-containing functional groups show fluctuations;the aromatic hydrocarbon-CH gradually decreases,and the content of substituted aromatic hydrocarbons and aromatic ring-C=C-increases first and then decreases.Furthermore,with the increase of oxygen concentrations,the content of functional groups in pre-oxidized coal will also alter.The content of-OH first decreases and then increases.The aliphatic hydrocarbons in the pre-oxidized coal of 100? and 400? first increased and then decreased,while the oxidized coal of 200? and 300? showed a trend of gradual increase.The content of aromatic hydrocarbons increased first and then decreased.The oxygen-containing functional groups in the oxidized coal of 100?,200? and 300?showed a trend of first increase and then decrease,while that pre-oxidized coal of 400? was opposite.Through the comparison and analysis of TG and DTG curves of pre-oxidized coal and raw coal,it can be known that the thermal mass loss characteristics of pre-oxidized coal are very different from that of raw coal,and the oxidation temperature has a greater influence on the mass loss process of pre-oxidized coal.As the increasing of the oxidation temperature,the oxygen absorption mass gain peak of the pre-oxidized coal slow reaction stage is gradually not obvious,and most of the pre-oxidized coal shows a decrease in mass at this stage,which means that the pre-oxidized coal must start the oxidation reaction before the raw coal.The reaction and burnout capability,overall combustion performance,and combustion stability of the pre-oxidized coal in the later stage of combustion are inferior to those of the raw coal,but its reaction ability in the early stage of oxidation is stronger than that of the raw coal.Compared with raw coal,the process of pre-oxidized coal thermal mass loss reaction is advanced,and the oxidation reaction process takes shorter time and the reaction is more rapid.Using C80 micro-calorimeter and DSC differential scanning calorimeter to analyze the thermal effect of pre-oxidized coal oxidation reaction,it is found that THl and TH2 of pre-oxidized coal lag behind the raw coal,and the entire low-temperature oxidation process moves to a high temperature zone.Furthermore,the thermal release during the low-temperature oxidation stage of pre-oxidized coal is less than that of raw coal,and the increase in oxidation temperature will aggravate this phenomenon.The DSC curves of raw coal and 100? and 200? pre-oxidized coal have endothermic "step" due to the decomposition of macromolecular structures in the coal,while the 300? and 400? oxidized coal does not have this "step" And the exothermic thermal and maximum exothermic power of the entire oxidation reaction of pre-oxidized coal are less than that of raw coal.Combined with the TG-DSC and C80 experimental data,it is judged that the oxidation temperature of pre-oxidized coal has a critical value,and the spontaneous combustion risk of pre-oxidized coal below this temperature is greater than that of raw coal,otherwise it is lower than that of raw coal.The critical value of oxidation temperature is different because of the difference in oxygen concentration.As the oxygen concentration decreases,the critical value of oxidation temperature moves to the high temperature region.