| Coal has played an important role in the development of modern industry.Making full use of coal resources and realizing coal transformation with high efficiency and low pollution is still an important subject for the development of C hina’s coal industry.Alkali metals play an important role in the pyrolysis of coal.In this paper,experiments and Reax FF simulations are used to study the mechanism of alkali metal Na’s influence on coal pyrolysis.Firstly,this paper constructed reasonable macromolecular structures of the three raw coals and the three acid-washed coals through elemental and industrial analysis,13 C NMR,XPS and FTIR experimental characterization results of YK coal,XX coal,JX coal and their acid-washed coals,respectively.Thermogravimetric experiments and thermogravimetric-mass spectrometry-Fourier infrared experiments were performed on YK acid-washed coal,XX acid-washed coal,JX acid-washed coal and their three Na-loaded acid-washed coals at different heating rates.Through comparative analysis,it is concluded that with the increase of heating rate,the maximum weight loss temperature during pyrolysis tends to increase,so does the maximum weight loss rate.However,the final weight loss rate has no obvious effect.The addition of Na increases the weight loss rate of the three kinds of acid-washed coals at the final reaction temperature,and decreases the maximum weight loss rate.The average activation energy of YK acid-washed coal,XX acid-washed coal,and JX acid-washed coal are 38.31 k J/mol,31.76 k J/mol,and 34.55 k J/mol,respectively.The addition of Na reduced the average activation energy of the above three acid-washed coal samples to 27.54 k J/mol,20.53 k J/mol and 30.46 k J/mol respectively.Based on the revised Wiser model,a bituminous coal macromolecule model and a sodium-containing bituminous coal macromolecule model were constructed,and the pyrolysis was simulated by Reax FF method.With the increase of pyrolysis temperature,the yield of char increases first and then decreases.The change trend of tar production is opposite to that of char,and the production of pyrolysis gas increases monotonically.Coal pyrolysis at low temperature mainly takes place one reaction,producing tar free radical fragments and small molecule gas.The secondary reaction of tar fragments at high temperature is obvious.During high temperature pyrolysis,C and H in coal gradually migrate to char and tar,while oxygen-containing functional groups are more active,and O gradually migrate to pyrolysis gas.In the secondary reaction stage,O is the most active,H is the next,and C is the most stable.The lower the heating rate,the more complex chemical reactions will occur in the pyrolysis process,especially to facilitate secondary reactions.In the simulation of coal pyrolysis by the Reax FF method with Na addition,the transition temperature of typical bituminous coal pyrolysis is advanced from 2000 K to 1800 K,which reduces the tar content in the pyrolysis process and increases the char content to some extent.As the interconnecting node of tar precursors,Na inhibits the diffusion of fatty precursors and aromatic precursors in tar.However,it promotes the further combination of macromolecular substances and the pyrolysis of small molecular gases,which is beneficial to the tendency of the secondary reaction to develop to low-temperature.After loading Na,the number and types of molecular fragments generated during coal pyrolysis are significantly higher than those of typical bituminous coal.The number of active sites increase,and the activation energy of weightlessness decreases from 39.69 k J/mol to 31.40 k J/mol.The catalytic effect of Na on the secondary reaction of coal pyrolysis is more obvious.At high temperatures,most Na comes from the interaction between char and volatiles.Free radicals replace Na in CM-Na,making Na exist alone to form Na atoms or collide with other free radicals and combine to form inorganic form of Na.Na promotes the yield of H2,NH3,H2 S and CO,and inhibits the yield of CH4 at high temperature. |
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