Kinetic Reactivity And Molecular Transformation Mechanism Of Coal Char Lean-oxygen Combustion In The Front Of Coalfield Fires

Coal field fires are disorderly and uncontrollable combustion of coal.They are widely distributed and rapidly spread in Xinjiang,Inner Mongolia,Shanxi and Ningxia,burning a large number of coal resources and causing dozens of times of stagnant resources unable to be mined.At the same time,they cause great harm to the ecological environment,which has become an important disaster threatening the national energy strategy and sustainable development of the ecological environment Prime.The spread of coal fire is the result of the dynamic equilibrium development of thermodynamics in both space and time dimensions.The sustained oxygen poor combustion of the initial structure of colloidal semi coke,which is partially pyrolyzed in the front of the fire area,is the main source of heat to promote the development of the fire area,and its combustion kinetic reactivity is the key factor to determine the spread rate of coal fire.At present,the lack of research on the combustion state,combustion dynamic reactivity and molecular structure transformation mechanism of the fire front under the condition of limited oxygen supply leads to the inability to accurately grasp the development law of the fire front,and the recurrence of the fire combustion phenomenon,which seriously restricts the effectiveness and economy of the fire prevention and extinguishing engineering process.In this dissertation,the evolution of component structure,pore structure and oxygen diffusion mechanism,the formation mechanism of gas-liquid products and the characteristics of CO combustion in the process of lean combustion of coal char in the front of fire were studied by directional preparation of coal char samples and experimental means.On this basis,the combustion process was studied by experimental test and numerical calculation.The main innovative achievements are as follows:?1?Based on the TGA-DSC curve of pyrolysis and the theory of heat balance in the fire zone,it is determined that 400oC-60 min is the preparation condition of the initial coal char structure in the front of the fire zone.Through industrial element test,Fourier transform infrared diffuse reflectance test and X-ray diffraction test,the evolution law of components in the process of lean oxygen combustion of coal char in the front of fire area is studied from the aspects of element composition stratification plane,functional group distribution plane,industrial composition stratification plane and microcrystalline structure from small and large scale.The results show that the influence of oxygen concentration on the rate of volatile analysis is greater than that on fixed carbon combustion With the increase of combustion conversion,the O/C ratio first decreases and then increases,and the lower the oxygen concentration is,the larger the reduction range is,the smaller the increase range is,the H/C ratio continues to decrease,and the aromatic ring substituents gradually change to methyl based;the rapid oxygenation of C-H bond and C=C bond of aromatics starts at 500oC,and its oxidation rate is limited by the oxygen deficient concentration below 5%.In the early stage of devolatilization,the structure of carbon microcrystalline in coal char is mainly vertical stacking,while after dehydrogenation and condensation,it is mainly horizontal spreading.Compared with TLG coal char,the growth of YBS coal char carbon microcrystalline is more inclined to horizontal spreading.?2?The pore development mechanism and its influence on oxygen diffusion mechanism of coal char lean combustion in the front of fire area are expounded.Through the test method of CO₂ and N₂ adsorption and the quantitative analysis method of NLDFT model and bjh model,the pore structure characteristics of nano pore 0.4-300 nm are obtained.The results show that in the reaction process of raw coal?pyrolysis semicoke?combustion of coal char,the pore morphology of coal char is filled by irregular ink bottle pore pyrolysis to form a cylindrical pore,further oxidation pore expansion to form a slit The wedge-shaped hole will collapse and merge into the large hole.The pore development mechanism has successively experienced the formation stage of secondary micropores,the progressive expansion stage of mesopores and the free merging stage of macropores.During the pyrolysis process of weakly cohesive coal,it softens and melts to form colloid,and the flow and diffusion in the pores cause some mesopores to be blocked,and the mesopores develop to smaller size in reverse direction.The oxygen diffusion mechanism changed from Knudsen diffusion to transition diffusion.The carbon oxygen reaction interface was transferred from the inner surface of deep micropores to the inner surface of shallow mesopores and macropores.5%oxygen concentration was determined as the critical oxygen concentration limiting the development of pores under the condition of oxygen deficiency.?3?The formation mechanism of gas and liquid products and the CO combustion characteristics of coal char were studied.An integrated analysis device of coal char lean combustion products at the front of coal fire was built.The gas-phase products,liquid-phase tar distribution and combustion characteristics of YBS,TLG coal char pyrolysis and lean combustion were accurately prepared,separated,collected and compared.The results showed that the content of aliphatic compounds in light tar decreased,the content of aliphatic compounds in heavy tar increased,and the average number of aromatic compounds increased.The effect of coal type difference on the distribution of tar components is mainly reflected in the light fraction,but less in the heavy fraction.The lean oxygen combustion process of tar is divided into tar evaporation stage and tar condensation combustion stage.The ignition temperature of tar separated from lean oxygen combustion is relatively low,which improves the ignition and reburning capacity of residual coal char containing tar,and enhances the combustion intensity and combustion stability.The precipitation mechanism of gaseous products includes hydrogen rich structure in the early stage of combustion,oxidation and pyrolysis of oxygen-containing functional groups,condensation and oxidation of aromatic lamellae in the middle and late stage of combustion to form small molecular hydrocarbons,H₂,CO and CO₂.High oxygen concentration in the early stage of combustion limits the precipitation of CH₄ and H₂ and promotes the release of CO₂.?4?Based on the change of the dynamic control model,the internal mechanism of the evolution of the combustion rate of coal char in lean oxygen is clarified.It is pointed out that the optimal dynamic control model of coal char lean oxygen combustion is NDM?RPM?SCM at the temperature scale,while at the particle size scale,the combustion dynamic control mechanism changes from chemical reaction control to chemical reaction internal diffusion control,and the critical temperature of the control mechanism changes with the increase of the average particle size of coal char to the low temperature zone.The combustion reaction rate constant is less sensitive to the size change of coal char in the low temperature chemical reaction control area,but higher in the chemical reaction diffusion control area.The internal diffusion resistance increases first and then decreases with the increase of the thickness of the ash layer.When the thickness of the ash layer reaches half of the coal char particle radius?x=0.875?,the internal diffusion resistance is the largest.The higher the combustion temperature is,the more obvious the internal diffusion is,and the more obvious the influence of the size of coal char on the combustion rate is.?5?The oxidation pyrolysis parallel competitive reaction mechanism of lean oxygen combustion was studied under oxygen concentration and coal char particle size gradient.Combined with tga-dtg experimental results and kinetic analysis method,the four-step lumped reaction mechanism of coal char lean combustion was determined,the contribution of each step reaction to kinetic rate and mass evolution was reduced,and the influence of oxygen concentration and coal char particle size was analyzed.The results show that the main reaction types of coal char lean combustion have experienced the evolution process of devolatilization to semi coke?direct oxidation to ash?semi coke polycondensation?coke oxidation to ash on the temperature scale,and the main solid components have successively changed into the initial active structure?semi coke?coke?ash.It is proposed that the lean oxygen concentration can guide the direct oxidation of coal char active structure by limiting the reaction intensity of direct oxidation The characteristics of coal char combustion stage change to high temperature zone,and the intensity and concentration of combustion decrease.However,the main control reaction of coal char combustion loss changes from direct oxidation of active structure of coal char to slow oxidation of coke at 3150?m by the formation of radial temperature and oxygen concentration gradient inside the coal char,and the combustion reactivity decreases.?6?The mechanism of structure transformation and the influence of oxygen concentration on the combustion of coal char were revealed on the molecular scale.The macromolecular structure model C₁₈₄H₁₂₄N₂O₁₆S of TLG400 long flame coal char was constructed based on the experimental results of structural components of coal char and the molecular structure model of hatcher sub bituminous coal.The combustion reaction of TLG400 under the conditions of poor oxygen,normal oxygen and rich oxygen was simulated based on the molecular reaction dynamics of ReaxFF force field.The results show that the apparent activation energy value of TLG400 coal char in poor oxygen combustion is 194.49 kJ/mol.The single benzene ring structure in the coal char has a bottleneck effect on the structural oxidation rate.The key intermediate for the oxidation and shedding of aromatic carbon atoms is the unstable hydroxyl complex and oxygen-containing seven membered heterocycle in the lean oxygen combustion,and the unstable meso or para semi quinone structure in the high oxygen combustion.There are 121 figures,55 tables and 177 references in this dissertation.