An Investigation Of The Behaviour Of Free Radical Reactions And Their Distribution During Coal Pyrolysis

Coal pyrolysis is regulated by free radicals,and it is important to understand the reaction behaviour of free radicals and the distribution of free radicals during coal pyrolysis from the perspective of free radicals.Various mechanisms of coal pyrolysis have been proposed in the literature,but all of them basically follow the law of free radical reactions,in which the first step of the reaction is the breaking of covalent bonds,which is related to the covalent bond energy,and the distribution of covalent bonds in coal is closely related to the degree of coal metamorphism.By measuring the concentration of free radicals produced during pyrolysis the number of broken bonds in the coal can be revealed.Combined with the amount of volatiles produced due to covalent bond breakage,the size of free radical fragments after the thermal breakage of covalent bonds down can be obtained,which in turn reveals the reactivity of volatile fragments and predicts its subsequent reaction law,thus providing theoretical guidance for regulating the coal pyrolysis reaction.In theory,if the coal covalent bonds break to form volatiles,then there is one free radical on the solid coke and one in the volatiles,i.e.a 1:1 ratio of free radicals on the solid coke and in the volatiles,but in practice,volatiles may be formed by breaking multiple covalent bonds during coal pyrolysis,and subsequently covalent bonds on the solid coke and volatiles will break again,each for ming new free radicals.The intertwining of these reactions makes it impossible to experimentally deter mine the proportion of free radicals generated by coal pyrolysis on the solid coke and in the volatiles,as well as the pattern of change in that proportion and in the molecular weight of the free radicals as the reaction proceeds.To address this challenge,this thesis investigates this from a combination of experiments and simulations.Experimental methods such as thermal balance,miniature glass tube reaction system,electron paramagnetic resonance analyser,13 C NMR and elemental analysis were adopted to analyse the free radical reactions and weight loss behaviour of five representative coal pyrolysis processes.The concentrations of reactive radicals and stable radicals during pyrolysis were quantitatively analysed,and the bonding structure models of the five coals were obtained.On this basis,the distribution patterns of free radicals during the pyrolysis of these five types of coals were analysed by using ReaxFF MD computational simulations,and the escape patterns of free radical volatiles and molecular evolution patterns during coal pyrolysis were obtained from the molecular structure level,and the research content provided theoretical guidance for product regulation during coal pyrolysis.It was found that the pyrolysis process of these five types of coals was mainly regulated by reactive radicals.The concentration of stable radicals is less than 1‰ of the reactive radicals,but the stable radicals can reflect the structural characteristics of the coal to a certain extent and can be a class of parameters for the analysis of coal structure.Bond structure models can provide a good description of the structural information of coals.In these five types of coal bonding structure models,the structural parameters fatty carbon content and aromatic carbon content all have good linear relationships with H/C.and the stable free radicals have a certain relationship with the aromatic carbon content.The information obtained on the chemical bond concentrations in the five coals reveals that the chemical covalent bond concentrations vary between a few mmol/g and tens of mmol/g,and also that there is a relationship between their concentrations and H/C.The pyrolysis process of these five coals was simulated using a combination of ReaxFF MD.The software analysis obtained the evolution pattern of free radical content with time during the pyrolysis process,but the simulation results were not fully consistent with the coal rank.For the free radical distribution behaviour,it was found that there is a certain competition between the stabilised and coking reactions of free radicals during pyrolysis,and this competition deter mines the distribution of pyrolysis products.This also provides some theoretical support for the regulation of the pyrolysis products from the perspective of free radicals.