Study On The Evolution Of Physicochemical Structure And Mass Transfer Behavior Of Carbon Oxides During Low-temperature Oxidation Of Shenhua Non-caking Coal

With the rapid growth of China’s energy demand and the massive consumption of high-quality coal resources,the efficient,clean and safe utilization of low-rank coal has become the primary task to promote the energy revolution,achieve carbon peaking and carbon neutrality,and construct an energy super power.However,low-rank coal with well-developed pore structure and rich active structure is highly susceptible to spontaneous combustion and difficult to be cleanly utilized.The spontaneous combustion of coal is the result of physical-chemical interaction between coal and oxygen for a long time.The low-temperature oxidation of coal is the main causative agent of coal spontaneous combustion.By examining the evolution law of physical-chemical structure during the low-temperature oxidation of coal,exploring the macroscopic adsorption behavior and microscopic adsorption mechanism of carbon oxides,and constructing the influence mechanism of physical-chemical structure on the adsorption process of carbon oxides,it is beneficial to realize the accurate prediction and prevention of coal spontaneous combustion.Shenhua non-caking coal,which is prone to spontaneous combustion,was taken as the research object.Based on the kinetics of coal-oxygen complex reaction,the physical and chemical structural changes of Shenhua non-caking coal during low-temperature oxidation were investigated.Combined with molecular simulation and isothermal adsorption,this thesis revealed the physical adsorption properties of carbon oxides from the microscopic level,explored the mass transfer behavior of carbon oxides from the macroscopic level,and further constructed the influence mechanism of physical and chemical structure on the mass transfer behavior of carbon oxides.Firstly,the effect of oxidation degree on the coal-oxygen reaction characteristics was investigated by using simultaneous thermal analysis technique.With the deepening of the oxidation degree,the moisture and small molecule gas in the coal were gradually removed and the active structure was gradually consumed.The dehydration and degassing stage of the100℃ oxidized coal appeared to be significantly reduced,and the 150℃ oxidized coal was no longer oxygen absorbing and weight gaining.The removal of moisture and the consumption of active structure further led to the lower heat release of oxidized coal than the raw coal,and the stage of oxygen absorption and weight gain was the main stage of heat release decline.With the deepening of oxidation,the mechanism of coal oxygen reaction gradually changed from physical adsorption to chemical reaction.The consumption of easily oxidized functional groups led to a slight increase in the activation energy of the 50℃ oxidized coal compared with the raw coal in the oxygen uptake weight gain stage,but the activation energy of the 150℃ and200℃ oxidized coal decreased again as the oxidation degree deepened and the oxidation activity of the functional groups increased.Secondly,the evolution of the physical and chemical structure during low-temperature oxidation was explored.The pore structure evolution could be divided into two stages:pre-oxidation（30-100℃）and post-oxidation（100-200℃）.In the pre-oxidation stage,the micropore volume was sharply reduced by 60%,the specific surface area was sharply reduced by 80%,the main change of the pore structure was the collapse of micropores.In the post-oxidation stage,the pore volume decreased by 10%except for mesopores,the main changes of pore structure were the fusion of micropores and transition pores and the collapse of macropores.The active site of coal oxygen reaction was gradually shifted from methylene and methyl group to carboxyl group.The raw coal had many active sites and was dominated by the more active methylene.When the oxidation temperature≤150℃,the relative content of methylene decreased monotonically with the increase of oxidation temperature,while the relative content of carboxyl increased monotonically,prompting the active sites of 150℃oxidized coal shifted to mainly carboxyl groups.After that,the further increase of temperature prompted a large oxidation consumption of methylene and methylene,and the number of active sites was greatly reduced,but still dominated by carboxyl groups.Then,the microscopic adsorption mechanism and competitive adsorption of carbon oxides during physical adsorption was explored in combination with quantum chemical simulation software.Compared with O₂ and CO,the coal-CO₂interaction energy was larger,which makes CO₂ more easily adsorbed on the surface of Shenhua coal and the adsorption amount was larger.The adsorption sites of O₂,CO,and CO₂ on the surface of Shenhua non-caking coal during low temperature oxidation were hydroxyl,carbonyl carbon,and ester carbon,respectively.Under 35%CO₂atmosphere,S_CO2/O2 had been as high as 20-30,the preferential adsorption of CO₂ inhibited coal oxygen adsorption,and higher concentration of CO₂ did not significantly enhance the inhibition effect.The adsorption capacity of O₂ and CO was small,the increase of temperature led to the enhancement of the cage effect while intensifying the thermal movement,so the gas molecules were difficult to get rid of the coal,resulting in the diffusion coefficient increasing first and then decreasing with the increase of temperature.The CO₂ adsorption capacity was larger,there was no cage effect,the diffusion coefficient increased monotonically with increasing temperature.In CO₂-air multi-component atmosphere,CO₂ with higher interaction energy occupied the adsorption site and weakened the binding of O₂on the coal surface,the O₂ diffusion coefficient increased monotonically with the increase of CO₂ concentration.Finally,the isothermal adsorption method was used to examine the mass transfer characteristics of carbon oxides during low-temperature oxidation and to investigate the influence mechanism of physical and chemical structure on their mass transfer characteristics.In the low-temperature oxidation process,physical adsorption was the dominant adsorption mode for CO₂ and CO,its adsorption capacity decreased with increasing temperature and the diffusion coefficient increased with increasing temperature.But at 200℃,the diffusion resistance increased due to the influence of pore collapse,the diffusion coefficient showed a certain decrease.The dominant mode of O₂ adsorption during low temperature oxidation was chemisorption.During temperature increased from room temperature to 100℃,the amount of O₂ adsorbed and the diffusion coefficient increased with the increase of temperature under the combined effect of increased thermal motion and enhanced adsorption strength of functional groups.When the temperature increased to 150℃,the hydroxyl and methylene groups gradually changed to carboxyl groups with higher adsorption strength,the chemisorption dominance increased,and the O₂ adsorption amount further increased while the diffusion coefficient decreased.After the temperature increased to200℃,the decomposition of carboxyl groups and the large number of fatty side chains made the chemisorption difficult,the adsorption of coal oxygen was dominated by physical adsorption,the adsorption amount decreased while the diffusion coefficient increased abruptly.In summary,focusing on the evolution of the physical and chemical structure and the mass transfer characteristics of carbon oxides during the low-temperature oxidation of Shenhua non-caking coal,a mechanism for the evolution of the physical and chemical structure and its influence on the mass transfer characteristics of carbon oxides was constructed in this thesis,so as to provide theoretical support for the prevention and control of coal spontaneous combustion and promote the development and industrial application of easy spontaneous combustion coal mining,storage and transportation technology in China.