Study On The Ignition Mechanism Of Low Rank Coal

Low rank coal is a type of coal with a low degree of coalification and easy to spontaneously ignite.The NO_x,SO₂,soot,etc.released during combustion are very polluting to the environment.However,due to its abundant surface functional groups and complex spontaneous combustion,traditional models and mechanisms cannot fully explain its fire behavior,so this dissertation focuses on the ignition mechanism of low rank coal.In this dissertation,a thermogravimetry is used to study the effects of oxygen concentration,carrier gas flow,heating rate,and sample weight on the ignition temperature of 15 low rank coal,and optimize the optimal reaction conditions.Under the optimal reaction conditions,the effects of moisture,ash,volatile matter and CO₂ reactivity on the ignition temperature of low rank coal are studied,and the prediction formula of the ignition temperature of low rank coal is fitted to predict its spontaneous combustion tendency.At the same time,the surface heterogeneous ignition mechanism for low rank coal is proposed,and it is believed that low rank coal exhibit very obvious heterogeneous characteristics during ignition,that is,the trace active components on the surface of low rank coal show up during the heating process.Different from the behavior of particles,the ignition temperature of low rank coal mainly depends on the temperature of the active components on its surface rather than the temperature of the particles.Further research found that the active substances on the surface of low rank coal are some small molecular active functional groups,of which the highest content is hydroxyl,carboxyl and alkyl side chains.In this dissertation,two different low rank coal（BRXL,YJL52）are selected,heated at 100℃ temperature intervals during the pre-fire stage,and infrared measurement is performed to explore the changes in the content of main functional groups at each stage.Combining previous research in the literature with infrared analysis,it is found that the relevant reactions are as follows:phenolic hydroxyl groups are converted to alcoholic hydroxyl groups,alcoholic hydroxyl groups are further oxidized to carboxyl groups,and carboxyl groups are decarboxylated to alkyl side chains.The changes of phenolic hydroxyl and carboxyl groups on the surface of coal in the temperature range of 100℃ were determined by titration,which further revealed the main reactions that occurred at each temperature range.Using the reaction enthalpy of the above main reaction to discuss the actual exotherm in different temperature ranges,it is found that the temperature range of 200-300℃ is the key temperature stage leading to spontaneous combustion.In order to further study the reaction speed of each stage before the fire,the kinetics of the low rank coal before the fire stage is fitted.According to the fitted curve,the pre-fire stage is segmented for the second time,and verified by the type and content of gas released and the characteristics of the pore diameter change.It is found that the first stage mainly occurs in endothermic reactions that require a higher energy barrier,such as decomposition reactions.The coal-oxygen reaction in the initial stage of the second stage gradually begins to occur,and the coal-oxygen reaction in the subsequent stage occupies a dominant position,accumulating heat for the subsequent reaction stage.In the third stage,a large amount of coal-oxygen reaction occurs on the surface of coal particles,releasing a large amount of heat,and the activation energy shows typical first-order kinetic characteristics.In the initial stage of the fourth stage,the coal-oxygen reaction on the surface of coal particles continues.In the subsequent stage,a large amount of heat generated by the surface coal-oxygen reaction is transferred to the inside of the particles.At this moment,it is necessary to cross a higher energy barrier and absorb more heat to make the entire coal particles occur.on fire.In this dissertation,based on the physical and chemical properties of different types of low-rank coal and the coal oxygen reaction characteristics,the thermal behavior of low-rank coal was studied in a piece-wide scenario,and the spontaneous combustion process of low-rank coal was revealed in detail,which has important theoretical guiding significance for preventing spontaneous combustion of low rank coal and reducing its environmental impact.