Thermal Depolymerization Of Low Rank Coals And Structural Characteristics Of Their Soluble Organic Species

There exist abundant low-rank coal resources in China,therefore,it is essential to study their thermal depolymerization under mild conditions,and the composition and structural characterists of soluble organic species in low-rank coals for their clean and value-add utilization.In this paper,the thermal dissolution depolymerization behaviors of six low-rank coals,including Baiyinhua lignite?BL?and Xinghe lignite?XL?,Buliangou subbituminous coal?BS?,Zhaoxian subbituminous coal?ZS?,Hefeng subbituminous coal?HS?and Naomaohu subbituminous coal?NS?,was investigated in a batch autoclave.Then the composition of thermal souble portions?SPs?was qualitatively and quantificationally characterized using Fourier transform infrared?FTIR?spectrometer,gas chromatograph/mass spectrometer?GC/MS?,and direct analyze in real time direct analysis time-of-flight mass spectrometer?DART/TOF-MS?to reveal the composition and structural characteristics of the soluble organic species in the low-rank coals investigated at molecular level.NS and BS were firstly extracted with isometric carbon disulfide/acetone mixed solvent under ultrasonic radiation at room temperature affording extracts and residues,then the extraction residues were sequentially thermal dissolved in cyclohexane and methanol at 210³30 ? obtaining souble portions?SPs?and thermal dissolution residues.The yields of the extracts from NS and BS are 10.64% and 8.00% respectively indicating there are abundant free small molecular compounds in the coal macromolecular network structure of the two lignites.The SPs yields of ERNS increase with temperature increasing,but the SPs yields of ER_BS increase with the increase of temperature below 300 ? then decrease rapidly with the increase of temperature above 300 ?,which may be due to the polymerization reaction of the SPs from the thermal dissolution of ER_BS above 300 ?.The SPs yields of ER_BS were significantly lower than those of ERNS,which could be attributed to the lower coal rank and higher oxygen content of NS resulting in that more oxygen bridge bonds in NS are easily cracked during thermal dissolution.GC/MS analyses show that the compounds in the extracts of NS and BS are mainly composed of benzene and naphthalene homologues with different alkyl side chains.The carbon numbers of the n-alkane in the cyclohexane SPs?CSP?of ERNS and ER_BS distribute in 11-25 and 10-28,respectively,and the relative content?RC?of the n-alkanes in CSPNS,300 are more than than those in CSP_BS,300.Nevertheless,the aromatic ring number and the RC of the arenes in CSP_BS,300 are more than those in CSPNS,300.The compounds in the methanol SPS?MSPs?of ERNS ? ER_BS are dominated with arenes and phenols,and the RC of phenols increases significantly with the increase of temperature.It was speculated that phenols in the SPs mainly resulted from the cleavage of aryl ether bonds in ArCH2OArOCH3 structure unit.TG analyses show that the weight losses of ERs are higher than those of their raw coals.It indicates that the swelling effect of the mixed solvents during extraction resulted in the incompact coal macromolecular framework of the two subbituminous coals,which is favorable for the release of volatile matter.The weight losses of thermal dissolution residues are obvious lower than those of raw coals because abundant volatile matter has been dissolved out during thermal dissolution.DART/TOF-MS analyses results show that the molecular weight of the soluble organic species in both NS and BS distributes in 100⁵00 Da.Six low-rank coals were sequentially thermal dissolved in cyclohexane and methanol at 300 ?.The results indicate that the SPs yields of lignites is obviously higher than those of subbituminous coals except for NS.GC/MS analyses show that the compounds in CSPs are dominated with hydrocarbons,and the relative contents of n-alkanes in the CSPs of subbituminous coals are higher than those of lignites.The compounds in the MSPs are composed of abundant arenes and phenols,and a few of oxygen-containing compounds inculding alcohols,ketones,and esters,etc.The RC of phenols in the MSPs of lignites are significantly higher than those of subbituminous coals.The arenes in the MSPs include benzene,naphthalene,acenaphthene,fluorene,anthracene,phenanthrene,fluoranthene,pyrene,chrysene,benzo[k]fluoranthene,benzo [ghi] perylene and their alkyl substitutes.A series of esters were dectected in the MSPs of subbituminous coals while none of them were identified in the MSPs of lignites.The amounts of n-alkanes,PAHs and phenols in the SPs from the six low-rank coals were quantitatively analyzed using external standard method.The amounts of n-alkanes in the SPs from BL,XL,ZS,BS,HS and NS are 394.85,241.70,673.44,589.29,7152.05 and 3155.74 mg/g,respectively,and the amounts of PAHs in the SPs from BL,XL,ZS,BS,HS and NS are 47.63,78.82,79.23,217.51,342.01 and 418.55 mg/g,respectively,indicating that the amount of soluble n-alkanes and PAHs increase with the increase of coal rank.The amount of phenols in the SPs from BL,XL,ZS,BS,HS and NS are 14372.91,8265.68,3260.43,5908.20,10292.86 and 19766.98 mg/g,respectively.The high amounts of soluble phenols in BL and NS are in accordance with their high oxygen content.