Study On The Pyrolysis And Oxy-fuel Combustion Of Zhundong Coal Using Reactive Molecular Dynamics Simulations

The process of integrated coal pyrolysis and combustion provide a way for the efficient and clean use of low-rank coal.This technology involves the processes of coal pyrolysis,gas and char combustion.For pyrolysis,coal pyrolysis reaction especially the secondary reaction mechanism remains unclear,because traditional experimental methods are difficult to detect the free radicals and intermediates during pyrolysis process.For combustion,oxy-fuel combustion technology is considered as a promising method for CO₂ capture.It is important to understand the chemical effect mechanisms of CO₂ and H₂O on the combustion of gas and char.In this paper,the reactive molecular dynamics simulation method（ReaxFF MD）is used to study the pyrolysis and combustion of coal at molecular level.The main research contents are as follows:Firstly,four Zhundong coal molecular models were built based on the ultimate analysis,solid-state ¹³C NMR spectroscopy and X-ray photoelectron spectroscopy of coal samples.The pyrolysis process of Hefeng coal was studied using the reactive molecular dynamics simulations（ReaxFF-MD）.The results show that the secondary reactions of tar occur at high temperatures,contributing to an increase of char yield and production of large amount of H₂and CO.The secondary reactions of tar need more energy compared to the initial thermal cracking of coal.The secondary reaction mechanisms of tar were revealed by analyzing the evolution of pyrolysis species at at high temperature.Ethenone（C₂H₂O）is found as one of the main intermediates from thermal cracking of tar.There are two pathways of the secondary reaction of tar leading to char formation:（1）one tar fragment reacts with another leading to the generation of char;（2）the radicals from tar decomposition attack the char leading to the growth of char fragment accompanied by the release of H₂ and CO.Zhundong coal contains high amount of alkali and alkaline earth metals（AAEM）,which are largely responsible for the problems of slagging,fouling and corrosion during the direct combustion of Zhundong coal.On the other hand,these AAEM species can act as effective catalysts for coal pyrolysis process.The nascent tar systems are constructed by extracting the tar radical fragments from coal primary pyrolysis products.The effect of Ca on the secondary reactions of tar was studied using ReaxFF.Results show that very little amounts of gas-Ca and atomic Ca are observed at low temperatures,Ca is mainly involved in a repeated bond-breaking and bond-forming process between tar and coke.Ca species only promotes the polymerization of tar at the low temperatures.While at high temperatures,a large amount of Ca is released in the form of atom,which will recombine with tar radicals and thus promoting the polymerization of tar.In the meanwhile,the atomic Ca will also attack the tar fragments and enhance the cracking of tar.The activation energies for the polymerization and cracking of tar are determined to be 26.6 and 20.3 kcal/mol in the absence of Ca,compared to 19.7and 20.1 kcal/mol in the presence of Ca.The role of Ca in reducing the activation energy for tar polymerization is much more significant than that for tar cracking reactions.In addition,the effect of cooling rate on the reaction of coal pyrolysis volatiles was also studied using ReaxFF.Results reveal that only the agglomeration of tar fragments is observed in rapid cooling.While,the reactions of tar in slow cooling condition undergoes two stages:agglomeration of tar fragments resulting in the formation of coke compounds,and subsequently reactions of the coke with tar fragments leading to the growth of coke weight.The activation energy for the secondary reactions of nascent tar is determined by fixed-temperature simulations and is found to be 24.4 kcal/mol,which is in good agreement with recent experimental results.Then,CH₄ oxidation during oxy-fuel combustion was studied using ReaxFF.70%concentration of CO₂ inhibited the reaction rate of CH₄ at low temperatures,but advanced CH₄ oxidation rate at high temperatures because of CO₂ reactivity expressed by CO₂+H→CO+OH.30%concentration of H₂O promoted the oxidation rate of CH₄ even at low temperatures.Reacting with O and H radicals through H₂O+O→OH+OH and H₂O+H→H₂+OH dominate the effect of H₂O.The presence of 30%H₂O promoted the conversion rate of CO₂ at relatively low temperatures by providing more H radicals for CO₂+H→CO+OH,therefore enhanced CO production.However,an inhibition effect of H₂O on CO₂conversion rate was found at high temperatures.H₂O+H→H₂+OH tended to occur at high temperatures,which would compete with CO₂+H→CO+OH and thus the production of CO was suppressed.The effect of H₂O reactivity on CH₄ oxidation during oxy-steam combustion was also studied.High concentration of H₂O inhibited the oxidation rate of CH₄at low temperatures due to the third body efficiency,but advanced the oxidation of CH₄because of H₂O reactivity at high temperatures.CH₃ and CH₂O were found to be important intermediates.The effect of H₂O promoted the consumption of hydrocarbon radicals,leading to intermediates CH₂O generation.CO+OH→CO₂+H is the main reaction,leading CO consuming,which is promoted in O₂/H₂O combustion due to the presence of a large amount of OH radicals.The effect of H₂O were less pronounced with increasing O₂ concentration.Finally,the combustion of char in O₂/CO₂ and O₂/H₂O atmospheres were studied using ReaxFF.Resutls show that the conversion of coal char was inhibited at low temperatures due to the lower the diffusion of O₂ in O₂/CO₂ and O₂/H₂O atmospheres.However,the gasification reaction of CO₂ and H₂O were significant at high temperatures,promoting the conversion rate of coal char.The gasification reaction of high concentration of CO₂ promoted the formation of CO,while the gasification reaction of high concentration of H₂O promoted the formation of H₂ and CO₂,but inhibited the formation of CO.The activation energies of coal char reaction in O₂/N₂,O₂/CO₂ and O₂/H₂O atmospheres were 203,246 and 241 kJ/mol,respectively.The high concentration of CO₂ and H₂O increased the activation energy of coal char reaction.By analyzing the dynamic evolution of the molecular structure of coal char,the effect mechanism of CO₂ and H₂O on coal char reaction were revealed.The gasification reaction of CO₂ and H₂O were less pronounced with increasing O₂ concentration.