Study On Spontaneous Combustion Characteristics Of Lignite Based On The Theory Of Equivalent Oxidation Exposure Time

Lignite has the characteristics of low metamorphism degree,high volatile matter and high water content,which is prone to spontaneous combustion in the packing state.The heat generated by coal oxidation is the basic and root of coal spontaneous combustion,and it is the premise to determine whether the numerical simulation can represent reality or not.However,most of the current mathematical models of coal spontaneous combustion do not consider the effect of aging effect on coal heat generation,resulting in the numerical simulation results that may not accurately reflect the actual coal heat production.Based on the present problem and the main research contents and achievements are as follows.Based on the open isothermal test,temperature rising curves of Baiyinhua lignite with different volumes under supercritical condition(ignition state)and subcritical condition(nonignition state)were measured.Based on Frank-Kammenetskii's self-ignition theory with considering the aging effect,the negative correlation between coal pile volume and critical selfignition temperature(CSIT)is analyzed.The characteristic parameter of Baiyinhua lignite by repeatedly drying/adsorbing water at different constant temperatures were obtained through water phase-transition experiment.The equivalent oxidation exposure time is a junction,combining the actual and ideal heat generation of coal-oxygen reaction,establishes the heat production model involving the aging effect.It is found that the drying and adsorption rates of moisture are not the same,but the drying curves(or adsorption curves)of different orders tend to be the same.Considering the equivalent relative humidity of coal,the relative humidity of air and the phase-transition time factor,a moisture phase-transition model is established.Based on the packing characteristics of coal and its boundary conditions in the open isothermal test,a three-dimensional physical model is established.The multi-fields coupling mathematical model of coal spontaneous combustion based on EOET theory is applied to open isothermal test.By comparing the experimental and numerical results,the empirical formula of decay power factor and temperature theory is determined.Compared with the Arrhenius model,it is found that the CSIT calculated by EOET model is within the error range of the experimental CSIT.The coupling process of different fields is analyzed.In addition,four energy terms including enthalpy change,energy from water phase-transition,oxidation heat and exchanged heat with oven are analyzed.The EOET model and Arrhenius model were used to simulate the raw coal and dry coal respectively.It was found that the presence of water had an important role in highlighting the effect of aging effect of heat generation of coal.Based on the “O-ring theory” of fracture distribution and accumulation form of residual coal in the goaf,a mathematical model of residual coal self-ignition in the high-wet goaf is established.Based on the moving mesh method,a comparison of applying EOET model and Arrhenius model certificate the practicability of the former,which is helpful to grasp the real law of spontaneous combustion of residual coal in goaf.Based on the EOET model,the effectiveness of different measures to prevent residual coal spontaneous combustion in goaf is analyzed and evaluated,which provides an important theoretical basis for the implementation of fire prevention measures in goaf.This thesis has 71 figures,13 tables and 208 references.