Study On Spontaneous Combustion Characteristics And Inhibition Pathway Of Thermal Invaded Coal Based On The Generation And Oxidation Of Active Sites

Thermal invasion process of coal is also named as low temperature pyrolysis process of coal.Thermal invasion of coal is widely existed in the process of coal fire propagation,magma erosion,sealed fire zone and torrefaction of low rank coal.After thermal invasion,it is found that the coal spontaneous combustion risk is significantly enhanced after a long period of low temperature pyrolysis.Related research explained this phenomenon mainly from the aspect of moisture content and pore structure,but coal under the heat will not only leads to the change of physical structure,but also results in the decomposition of oxygen-containing functional groups associated with the production of active sites.However,the decomposition process during the thermal invasion and its effect on coal oxidation is rarely studied.Therefore,in this paper,thermal invaded coal was taken as the research object,and the phase of thermal decomposition and oxidation of coal samples was separated.A systematic study was carried out on the generation and oxidation process of active sites of thermal invaded coal,so as to reveal the spontaneous combustion mechanism of thermal invaded coal and to put forward efficient inhibiting methods.With the help of modern analytical testing methods and classical programmed heating experiments,the physicochemical structure and spontaneous combustion characteristics of coal samples before and after thermal invasion were first studied.Results showed that thermal invasion process of coal will lead to the reduction of the pore volume and specific surface area,in addition to the thermal decomposition of oxygen-containing functional groups.Low temperature pyrolysis of coal had an important effect on the low temperature oxidation process.The oxidation of active sites produced by the decomposition of oxygen-containing functional groups is the main reason for the higher gaseous production in the low temperature oxidation process compared to the pyrolysis process,besides,it was also the reason why the spontaneous combustion risk of thermal invaded coal is significantly higher than that of raw coal.Then,the oxidation characteristics of active sites of thermal invaded coal were analyzed by pyrolysis-isothermal oxidation experiments.Results showed that active sites could be stably existed and accumulated under the inert gas conditions.Once in contact with oxygen,even at room temperature,oxidation reaction will be happen,heat and a large number of CO and CO₂ will be given off.In addition,there are also a small number of primary active sites existed in the original coal samples,and the oxidation of the active sites of thermal invaded coal should include the oxidation of the primary and secondary active sites.On this basis,the viewpoint of room temperature oxidation of active sites is proposed,and it is considered that room temperature oxidation of active sites is the initial heat source of self-heating and uncontrolled spontaneous combustion of the thermal invaded coal.Considering the importance of active sites,the generation law and kinetic characteristics of the active sites in thermal invaded coal were analyzed based on the isothermal pyrolysis of coal samples.Low temperature pyrolysis of coal samples under different conditions was analyzed experimentally.The evolution of active sites in the thermal decomposition of oxygen-containing functional groups was deduced by the generation of gaseous products during the pyrolysis process.By comparing the gaseous production in the pyrolysis process with that in the pyrolysis-isothermal oxidation process under multi-factor conditions,it was found that the production curves of CO and CO₂ in the two reaction processes were in line with the complex exponential decay function.Equal conversion method was then used to solve the generation kinetics of two gases according to the gaseous generation law during pyrolysis process.Apparent kinetic parameters during low temperature pyrolysis of coal samples were also obtained by Coats-Redfern method,the apparent activation energies of thermal decomposition of the four coal samples were 39.33 k J/mol,45.79k J/mol,70.60 k J/mol and 126.65 k J/mol,respectively.The dynamic results of gas generation showed that the process of CO₂ production by thermal decomposition of carboxyl functional groups is dominant in the production of active sites.Furthermore,based on the important effect of the decomposition of carboxyl groups on the generation of active sites,the effect of the morphology of different carboxyl functional groups on the production of active sites was further studied.Alkali metal element was introduced into the coal by ion-exchanging method,and the influence of carboxylate structures on the active sites was analyzed.Results showed that compared with carboxylic acid structure,the activation energy of carboxylate structure during thermal decomposition is lower,which can significantly enhanced the production of active sites,thus leading to strong oxidative exothermic of coal during the oxidation process at room temperature.The alkali treated WL coal sample(40 g)could be self-heating for 14 ^oC in 12 min.Therefore,the presence of carboxylate structures in coal significantly increased the spontaneous combustion risk of thermal invaded coal,and acid washing method can transforms the carboxylate structures into carboxylates,thus inhibiting the generation of active sites and the coal oxidation.To further reveal the nature of active sites and the spontaneous combustion mechanism of thermal-invaded coal,the evolution of functional groups and free radicals in the pyrolysis and oxidation process was analyzed by in-situ FTIR and in-situ ESR.The study showed that the active sites generated by the thermal decomposition of oxygen-containing functional groups were the alkyl free radicals,which can be oxidized into oxygen-containing functional groups under certain conditions.Therefore,the conversion relationship between oxygen-containing functional groups and the free radical active sites is established.Due to the stability,the alkyl structures need to undergo dehydrogenation activation to convert to the alkyl radicals.The oxidation process of free radicals was analyzed by means of quantum chemistry,and the oxidation process of alkyl free radicals was proved to be a zero-order non-barrier reaction.The spontaneous oxidation and heat release phenomenon of the active sites is manifested as the oxidation and heat release of coal at room temperature.On this basis,the mechanism of the oxidation of active sites at room temperature was analyzed,and the mechanism of coal spontaneous combustion of the thermal invaded coal was also expounded.Finally,based on the spontaneous combustion mechanism of thermal invaded coal,the inhibition pathways of active sites were analyzed.It was proposed to prevent thermal invaded coal from reducing the active sites generation and decreasing the active sites concentration.According to the room temperature oxidation characteristics of active sites,the evaluation method and the main quantitative evaluation index of the inhibition effect of the thermal invaded coal were established.The inhibition effect of four selected active sites inhibitors was compared as follows:acid inhibitor HCl>metal chelator CA>carbon center free radical capture agent HP-136>oxygen free radical inhibitor IR1010.Results showed that the inhibition effect of active sites production process was better than that of reducing the concentration of existing active sites.