| Coal pyrolysis refers to the complex process of a series of physical and chemical changes that occur when the coal is continuously heated to a higher temperature in an inert atmosphere.In order to further study the mechanism of coal pyrolysis,on the basis of experience and reference macromolecular model of coal that have been reported in literature,the periodic structure model of coal,the hydrogenation model and the model compounds model were constructed by using the Materials Studio(MS 7.0).At last,the coal pyrolysis process at different temperatures was investigated by ADF(Amsterdam Density Function)based on ReaxFF(Reactive Force Field).The results show that:(1)Industrial analysis,elemental analysis,thermogravimetric analysis,infrared spectrum analysis,Py-GC/MS analysis and fixed bed pyrolysis experiment results show that Shenfu subbituminous coal has many oxygen-comtaining functional groups,and temperature is the main factor affecting the pyrolysis of Shenfu coal,which contains many kinds of compounds,complex composition with chain hydrocarbons and cyclic compounds.The coke yield,the yields of tar and gas composition were affected by temperature,between the temperature 600 and 800,the second reaction occurred obviously in the pyrolysis process,the tar and gas yield increased and the coke yield decreased.(2)The construction of coal periodic structure model and molecular mechanics(MM),molecular dynamics(MD)and annealing kinetics optimization simulation of their structures were carried out by Materials Studio 7.0 software,the optimized coal three-dimensional molecular structure model of cell size was 45.8? × 45.8 ? × 45.8 ?,after hydrogenation of the three-dimensional molecular the structure model of unit cell size was 46.1 ? ×46.1? × 46.1 ?.The density of minimum energy geometries of Shenfu coal and hydrogenation model in the periodic boundary conditions were 1.328 g·cm-3 and 1.299 g·cm-3 respectively,which is in the reasonable density range of subbituminous coal(1.24 to 1.60 g·cm-3).(3)Through the energy analysis of two kinds of coal structure model,the results show that in the macromolecular skeleton,van der Waals energy and the bond stretching energy play an important role in the process of structural optimization.For the periodic structure of Shenfu coal,the results show that van der Waals energy and Coulomb energy can play a stabilizing role in the aggregation and optimization of the whole model.(4)The pyrolysis simulation of the model was carried out based on the ReaxFF force field using ADF software,and the simulated temperature is from 1400 to 2600 K.Through the analysis of the product file: the temperature greatly affects the pyrolysis process and product distribution,with the increase of temperature,the type and number of pyrolysis products gradually increased;bridge bond is most likely to break in coal,and free radicals such as OH,H and CH3 plays an important role in coal pyrolysis process;dehydroxylation and decarboxylation reaction are easy to happen in the early stage,the sequence of gas generation was water,formaldehyde,carbon dioxide,ethylene,hydrogen,acetylene and methane.In addition,the bond length,bond order and Mulliken charge distribution analysis showed that strong electronegativity of heteroatom has a major impact on the stability of macromolecular skeleton on the coal.(5)The pyrolysis process of small molecules from coal was simulated by molecular dynamics method,the formation mechanism of four small molecules such as methane,hydrogen,ethylene and water were analyzed,and analyzed the influence of the number of bond length and charge structure model for bond breaking.The results show that ethylene is mainly from hydrocarbon cracking and Methane is a combination of methyl and active hydrogen;Water is mainly from the removing of free radical molecules in the unstable structure and the oxygen atom is from the unstable five membered ring structure;Hydrogen is mainly formed by the collision of hydrogen free radical with active hydrogen in the fragments. |
PDF Full Text Request