The Study Of Catalytic Depolymerization On Yunnan Lignite And Its Catalytic Mechanism

Coal is widely distributed and the most abundant fossil fuel in the word.As one kind of carbon-hydrogen resources,coal contains a complexity of chemical compositions and molecular structure and thermal conversion is the main way for its utilization.In the conventional ways,the entire conversion processes e.g.combustion or gasification are arranged in the same reaction space where the various structures of coal are handled equally.This not only lead to a waste of high-value components in coal,but also result in a huge energy consumption as well as carbon emissions.As one of the important technologies,coal pyrolysis can make full use of the hydrogen-rich parts and aromatic components by the coal structure,which can obtain high added-value liquid products-coal tar.However,the conventional coal pyrolysis has a numbers of problems such as low tar yield,high heavy components in tar.Meanwhile the conventional catalytic coal pyrolysis cannot fully solve these problems.Based on this,our research group proposed a new coal catalytic pyrolysis:coal catalytic depolymerization.By a novel catalyst preparation and adding process,the catalyst could be added into the matrix structure of coal directly,which can promote the cleavage of higher energy covalent bonds.As a result,the tar yield and high added-value components in tar could be both improved.In this work,Yunnan lignite was chosen as experimental subject and the effect of catalytic depolymerization were achieved and evaluated with addition of a little amount of catalysts.Combined with solvent extraction of the coal in mild conditions,the mechanism of coal catalytic depolymerization was discussed.Firstly,the effect of concentration and type of catalysts on catalytic depolymerization were investigated.Five optimal catalysts Co0.013,Zn0.014,CoFe(1:3),CoZn(1:1)and ZnFe(1:10)were found.Compared with raw coal,the tar yield by adding the above catalysts increased from 10.86%to 13.01%,11.83%,13.65%and 13.88%,respectively.The analysis of depolymerization products showed that the hydrogen were more migrated to tar.The analysis of tar indicated that the yield of aliphatic hydrocarbons and two-ring aromatics were increased significantly.Secondly,the acetone,hexane and methanol was used as solvents to extract YN lignite at mild conditions.The results suggested that the matrix structure of coal was not destroyed obviously.The extracts of three solvents were all important parts for tar or tar formation reactions.The results of catalytic depolymerization of residues displayed that the catalysts had a close matching relation with the functional groups in coal structures.Finally,through summarizing the information and conclusions obtained from the experiments,the catalysts were related with covalent bonds of coal,and then the mechanism of coal catalytic depolymerization was proposed.The optimum catalysts could promote the breakdown of higher energy covalent bonds directionally at different extent,and more high value-added polycyclic aromatic hydrocarbons were obtained.This research can provide fundamental data for catalyst screening and depolymerization mechanism exploration in the future,meanwhile this research also has a certain of guidance significance to low rank coal utilization in a cascade way by its characteristics.