Effect Of Low Molecular Compounds On The Formation Of Light Aromatic Hydrocarbons During Coal Pyrolysis/Catalytic Pyrolysis

Benzene,toluene,ethylbenzene,xylene,and naphthalene（collectively called BTEXN）are important chemical raw materials.Due to the resource distribution characteristics of"rich coal","lean oil",and"less gas"in China,obtaining these light aromatic hydrocarbons from petrochemicals is very restricted.Therefore,obtaining these high value-added light aromatic hydrocarbons from coal pyrolysis tar can alleviate the inconvenience caused by the lack of petroleum resources to a certain extent.And it can also reduce the environmental pollution caused by the release of light aromatic hydrocarbons from coal chemical industries.In order to explore the formation of BTEXN during coal pyrolysis and catalytic pyrolysis,n-hexane,methanol,methylene chloride,and cyclohexanone were used to extract low molecular compounds（LMCs）from Shengli lignite（SL）,Pingshuo gas coal（PS）and Hexi coking coal（HX）in this paper.Py-GC/MS was used to analyze the effect of different components in LMCs on BTEXN formation.The coal and residue structures were analyzed by TG and ¹³C NMR.The results will provide a theoretical basis for the formation of BTEXN during coal pyrolysis and catalytic pyrolysis.The main conclusions of this paper are summarized as follows:（1）The GC×GC/MS analysis results show that the n-hexane extracts are mainly aliphatic compounds,the methanol extract are mainly oxygen-containing aliphatic compounds,and chloromethane extract are mainly aromatic compounds.The composition of LMCs in coals varies with the coal rank.The LMCs in SL are mostly aliphatic hydrocarbons,alcohols,and esters,while LMCs in PS are main aliphatics,benzene series and aromatics with 2-3 rings.The amounts of LMCs in HX are less,and most of them are aromatic compounds;（2）Py-GC/MS was used to investigate the effect of LMCs on the formation of BTEXN during coal pyrolysis.The results show that the BTEXN from coal pyrolysis tar are mainly derived from coal macromolecular structure rather than LMCs,but LMCs in coal such as benzene series,naphthenes,and polycyclic aromatic hydrocarbons can also form a certain amount of BTEXN during pyrolysis.After n-hexane extraction,the total BTEXN yields from SL and PS residues pyrolysis are reduced by 29%and 18%,respectively.It shows that the aliphatics in LMCs can act as hydrogen donors to stabilize aromatic radical fragments from coal structure pyrolysis,promoting the formation of BTEXN.Moreover,the phenols yields from SL and PS residues pyrolysis are decresed by35%and 18%,respectively,indicating that the oxygen-containing compounds in LMCs can promote the formation of phenols;（3）USY zeolite was used to further explore the role of LMCs in coals with different rank on the formation of BTEXN during catalytic pyrolysis.It is found that the BTEXN from coal catalytic pyrolysis are mainly derived from the macromolecular structure rather than LMCs.However,oxygen-containing and aromatic compounds in LMCs will also produce a certain amount of BTEXN over USY zeolite,the BTEXN yield from LMCs catalytic pyrolysis is lower than that from coal macromolecular structure.Aliphatic and oxygen-containing compounds in LMCs can promote the the formation of BTEXN during catalytic pyrolysis.After extraction,the swelling makes the amount of alkylbenzenes,phenols,and 2-3 ring PAHs be increased,which can be catalytically cracked to produce BTEXN.As a result,the sum yield of BTEXN from residue and extract catalytic pyrolysis is higher than that from raw coal;（4）The evolutions of LMCs were explored during pyrolysis and catalytic pyrolysis at different temperatures.Resutlts show that LMCs undergo cracking reaction at 700 ^oC,the yields of aliphatics,phenols,monocyclic aromatics and PAHs reach the maximum.At 800 ^oC,LMCs mainly undergo polycondensation.During catalytic pyrolysis,the amount of benzene series and PAHs are increased as the temperature increses,while the amount of aliphatic compounds is significantly decreased.It indicates that aliphatic compounds can promote the catalytic cracking of aromatic compounds in LMCs to generate benzene series and 2-3 ring PAHs.