Study On The Structure-Pyrolysis Characteristics Of Pretreated Shendong Coal And High Value-Added Utilization Of Aromatic Hydrocarbons

For a long time,China's energy consumption structure is dominated by coal,pyrolysis is an important way to realize coal efficient and clean conversion.However,the structure and pyrolysis mechanism of coal have not been fully understood,which restricts the conversion and application of coal.Therefore,it is of great significance to study the structure and pyrolysis characteristic of coal.Pretreatment can effectively adjust the composition and physicochemical properties of coal,can regulate the distribution of coal pyrolysis products,which is of helpful to understand and reveal the matching relationship between coal structure and pyrolysis behavior.There are many aromatics in coal pyrolysis tar,the preparation of high value-added chemicals from aromatics can realize the deep processing and refined development of coal tar.In this paper,the macromolecular structure of Shendong coal(SD)was analyzed by conventional analyses,FTIR,XRD,XPS,¹³C CP/MAS SSNMR,¹H MAS SSNMR,BET,Laser particle size analyzer and SEM.The content and composition of low molecular compounds in SD coal was studied using tetrahydrofuran solvent extraction,and the effect of low molecular compounds on the pyrolysis characteristics of SD coal was investigated by Thermal gravimetric analysis(TGA),Gray-King(G-K)and Pyrolysis-gas chromatography/mass spectrometry(Py-GC/MS)devices.Secondly,different pretreatment methods(thermal pretreatment,hydrothermal pretreatment,tetralin solvent pretreatment)were used to adjust the composition and physicochemical properties of SD coal and then affect its pyrolysis behavior.Finally,the conversion and utilization of aromatics,a key component of pyrolysis tar,was studied.Naphthalene was used as model compound to prepare high-value tetraline by selective catalytic hydrogenation.(1)The average ring number of SD coal is 1.988.-CH₂-content accounts for about 82%in aliphatic C-H region and the ratio of ether bond C-O,aromatic ether C-O and C=O is about2:1:11 in oxygen-containing functional groups region.Microcrystalline structure of SD coal by XRD shows that the interlayer distance,average crystallite stack height,crystallite diameter and layer numbers is 1.832(?),14.688(?),20.852(?)and 9.017,respectively.Nitrogen mostly in the form of pyridine and pyrrole,with a content of approximately81.01%,and organic sulfur is mainly thiophene and sulfoxide.The aromaticity of SD coal is 55.67%,the aliphatic carbon ratio is 29.97%,average methylene chain is 1.817,and alkyl chain branching is 29.836%.The molecular structure was mainly constructed by taking accurate ¹³C SSNMR experimental spectrum as a comparison standard with the aid of other characterization results.(2)The content of low molecular compounds in SD coal is 3.88%,and the content of oxygen-containing compounds in low molecular compounds is more than 60%.When using tetrahydrofuran to extract low molecular compounds,the weak non-covalent bonds in the coal would be destroyed,and the average pore size in the coal would increase,which would facilitate the escape of volatiles.After extraction,the content of unsaturated carboxyl groups in SD coal significantly reduced,pyrolysis activity decreased and the yield of tar decreased.The content of light components in pyrolysis volatile decreased.(3)SD coal was single-step and multi-step pyrolyzed by Py-GC/MS.The relative content was modified by multiplying the pyrolysis rate with the relative content,with the increase of single-step pyrolysis temperature,the composition content of SD coal pyrolysis volatile increases,which can better explain the content change of coal pyrolysis at different pyrolysis temperatures.O-containing structure in SD coal can be completely broken before700 ^oC.One possible pathway for the formation of mono-ring aromatics is the dehydroxylation of phenolics.Naphthalene may come from the ring-opening of multi-ring aromatics and the cyclization of mono-ring aromatics in the single-step pyrolysis process.p-cresol in the single-step pyrolysis products may be derived from isomerization of o-cresol or alkylation of phenol.(4)The pyrolysis mechanism of SD coal was investigated by combining the characteristics of TG,Py-GC/MS and G-K apparatus.The results showed that the order of coke yields are G-K(76.35 wt%)>TG(73.11 wt%)>Py(70.03 wt%),the increments of G-K coke yield caused by condensation reaction and secondary reaction are 3.08 wt%and 3.24 wt%,respectively.It is found that fast pyrolysis has stronger fracture ability for coal molecules and alkenes will be hydrogenated to alkanes during slow pyrolysis process.The secondary reactions concluded from this study involve the cleavage of alkane and esters,condensation of mono-ring aromatics with low carbon alkene,ring opening,isomerization of tri-ring aromatics,hydrogenation of aromatics and acids.(5)In the process of thermal pretreatment,the fat chain will break and escape,the aromatic ring side chain will fall off and the condensation of aromatic ring structures will occur.After thermal pretreatment,the content of low molecular compounds and carboxyl in coal decreases and the degree of coalification increases.The structure of coal sample(H₂-300-6)pretreated at 300 ^oC and 6 MPa H₂changed significantly and its tar yield was higher.The chemical formula of H₂-300-6 model was C₁₈₂H₁₅₆O₃₆N₂S.(6)Compared with thermal pretreatment coal(T-SD),hydrothermal pretreatment coal(HT-SD)has a higher volatile,hydrogen,aromatic ether C-O contents and tar yield,the content of phenols increased.XBP value of HT-SD decreases slightly,indicating that hydrothermal pretreatment can destroy part of aromatic ring structure in coal.The value of V_daf/Tar_yield was used to reflect the strength of the secondary reaction of coal samples.Pretreatment can weaken the secondary reaction for T-SD and HT-SD coals,especially for T-SD coal.(7)After tetralin solvent pretreatment,the net weight loss of W_200-600and W_600-800was increased,the yield of pyrolysis coke and gas were increased,the yield of pyrolysis water was decreased,the tar yields of THN-250 and THN-300 were increased.After tetralin solvent pretreatment,the content of aliphatic hydrocarbons in the pyrolysis volatiles decreased,the content of aromatic hydrocarbons and oxygen-containing compounds increased and the content of polycyclic aromatic hydrocarbons decreased significantly.(8)Oxidative 4wt%NiO-20wt%MoO3/Al2O3 is the best catalyst for the preparation of tetralin.The conversion of naphthalene and the selectivity of tetralin can reach 95.62%and99.75%respectively at 200 ^oC,8 h and 6 MPa.Compared with reduced and sulfureted4wt%NiO-20wt%MoO3/Al2O3 catalysts,oxidative 4wt%NiO-20wt%MoO3/Al2O3 has a well dispersed and uniform monolayer of the active metals,larger pore volume and size,and larger total acidity.NiO-MoO₃/Al₂O₃ has a synergistic effect between NiO activity and MoO₃ selectivity.