Study On The Pyrolysis Process And Free Radical Evolution Of Ningdong Low Rank Coal Based On Reaction Molecular Dynamics Method

As an important primary energy source in China,the efficient and clean utilization of coal has become the focus of research.Pyrolysis of coal is an inevitable step for coal conversion such as gasification and liquefaction,and is also an important content in coal research.This paper takes Ningdong low-order Jinfeng(JF)coal as the research object,understands the molecular structure of coal based on various characterization methods,and constructs a macromolecular structure model of JF coal.The first-order coal samples were studied,and based on computational chemistry and molecular dynamics simulations,combined with free radical experimental analysis of pyrolysis products,the molecular structure evolution and free radical reaction mechanism under pyrolysis conditions were discussed in depth.The conclusion are as follows:(1)JF coal is a low-order bituminous coal,based on XPS,FTIR,13C NMR and other characterization methods:ether oxygen(CO)accounts for the largest proportion,56.59%,followed by carbonyl(C=O),carboxyl(COO-)minimum;the ratio of pyridine nitrogen and pyrrole nitrogen is about 1:1;the ratio of methyl,methylene and methine is about 2:5:4;the ratio of bridge carbon to weekly carbon is 0.315.According to the structural parameters of JF coal,a molecular model is constructed.In the model,the aromatic rings are dominated by 2 rings and 3 rings,and the fatty carbon atoms are mainly methyl,methylene and quaternary carbon.Finally,the average molecular formula of the JF coal macromolecular model is C199H120O30N2.The final reasonable model structure is obtained through annealing optimization,vibration analysis and density functional calculation.(2)ReaxFF MD pyrolysis simulation study of coal based on ADF software,found that the heating rate is too fast and is not conducive to the progress of pyrolysis reaction and the formation of pyrolysis products.The generation of small molecular fragments helps to generate the main gas products.The pyrolysis products are mainly gas and light tar,and there are fewer other products.H2 and CH4 are notably generated until greater than 2500 K,and the number of CO,CO2,H2O,and H2 molecules gradually increases with the pyrolysis simulation.(3)Using the EPR experiment to study the free radical concentration in the pyrolysis experiment of JF coal,the result is available,the JF coal round factor g=2.00296.In the pyrolysis temperature range of 350?-450? the free radical concentration of the pyrolysis solid product rises with the increase of residence time and then tends to be gentle,and the higher the temperature,the faster the rise;at higher temperatures,the free radical concentration first After the rise,it decreased a little,and the higher the pyrolysis temperature,the faster the decline rate.The free radical concentration of JF coal reaches a maximum of 34.78×1018/g at a temperature of 650? and a residence time of 3 minutes.(4)Carry out rapid pyrolysis simulation research on JF coal through ReaxFF MD to explore the change of free radical content during the simulation process.The simulation found that the higher the free radical content with the simulation of the pyrolysis reaction;the more complete the pyrolysis reaction with increasing temperature,the higher the free radical content generated by the coal particles.Combining EPR experiment and ReaxFF MD simulation conclusion for comparative analysis,the change trend of free radical content in coal pyrolysis process is basically consistent.Investigate the change law of oil and gas yield obtained by the JF coal pyrolysis experiment and simulation.The experiment is consistent with the simulation conclusion.The oil and gas yield increases with time and then tends to stabilize.According to factors such as time and energy consumption,the best pyrolysis condition of JF coal is 650?,the residence time is 5 min;the best simulated pyrolysis condition is 3000 K,and the pyrolysis duration is 200 ps.(5)Analyze the results of rapid coal pyrolysis simulation,and explore the formation mechanism and evolution path of free radicals in coal pyrolysis behavior by tracking the generation paths of various free radicals.Multiple evolution paths of·OH and·CH3 were obtained.The generation of-OH was dominated by thermal cracking of oxygen-containing functional groups.The generation of CH3 was dominated by methylene hydrogen absorption and methane bond breaking.