Study On Bond Cleavage And Condensation Of Low And Middle Rank Coals During Direct Conversion

With increasing demand and shortage of petroleum resources,and dependence on external oil.The coal is considered as an important oil alternative energy due to the abundance of coal reserves.It is important to convert coal into oil and gas and chemicals for alleviating the shortage of oil.Although China’s direct coal conversion technology is ahead of foreign countries,there are still some problems such as low conversion,low oil yield,significantcoking,harsh reaction conditions and high energy consumption.The direct conversion process of coal starts from the bond cleavage of coal,and the condensation reactions accompany the whole direct conversion process.It is the key to understand the bond cleavage and the condensation reaction in coal direct conversion for solving those problems.According to the above analysis,four low and medium rank coals were used as research objects in this paper,and carried out a relevant study based on the bond cleavage and condensation reactions of low and medium rank coals in the direct conversion process,with the following details:The molecular structures of low and medium-rank coals were characterized by¹³C-NMR and elemental analysis,and the relationships between the structures of the four low-rank coals,bond cleavage and them were studied;the bond cleavage amounts and bond cleavage rates of the four coals were determined in the temperature range of 380-440 ^oC,and the kinetic parameters determined by the STLR-A and MTNLR methods were compared,and the significance of the kinetic parameters was considered in terms of chemical structure,and then a new method for modelling the bond cleavage kinetics is proposed;the evolution of the solid product structure during the liquefaction of Naomaohu coal is investigated;the differences in the structural condensation of Naomaohu liquefaction residues at different temperatures were analysed.1.Four low rank coals（demineralised material）from Naomaohu（NMH）,Zichang（ZC）,Zhundong（ZD）and Daliuta（DLT）were characterized by ¹³C-NMR,with deep analysis on the side chains,bridges between aromatic clusters,and the oxygen form.Bond cleavage of the coals were measured by reaction with an excessive amount of hydrogen donor solvent 9,10-dihydrophenanthrene（DHP）,and the gas and liquid products were quantified by GC and discussed from the viewpoint of molecular structure.About all the alkyl substituent in NMH,half in ZC and a quarter in ZD and DLT are in side chain.The bridge-substitute degree is13.0%for NMH and 25.3%for ZC.Almost all the bridges in NMH and half in ZD contain Ar–O structure.NMH contains the most oxygen（22.18wt%）and ZC contains the least oxygen（8.44 wt%）.The oxygen in ZD（16.93 wt%）is dominated by aryl ester group with some aldehyde/ketone and a little carboxyl group.The amount of bond cleaved follows the order of NMH>DLT?ZD>ZC at 380-420?C.More aryl-ether bond results in more large-size fragments,and the amount of large-size fragments follows the order of NMH>ZC>DLT>ZD.2.The bond cleavage in coal molecular structure is the primary step in pyrolysis and direct liquefaction.In this study,the bond cleavage amounts and rates of four low rank coals at 380-440 ^oC were determined by a hydrogen donor solvent DHP.A hybrid first order kinetics model was developed in this work to suit the complex and diverse properties of covalent bonds in coal,and the N_B,t of four low rank coals at 380-440 ^oC are fitted to determine the kinetics parameters.In specific,the hybrid model includes the single-temperature linear regression（STLR）and the multi-temperature nonlinear regression（MTNLR）.The STLR method yields the maximum amounts of cleavable bonds N_B,0 at several temperatures,which are then incorporated into the MTNLR to yield the kinetics parameters,E_aand A.The hybrid model significantly improves the fitting’s accuracy in comparison with the STLR and MTNLR models.The ln A-E_a relations in the three models for the four coals’bond cleavage kinetics follows similar compensation effect but differs from those for the mass change kinetics.3.To understand the main reactions in direct coal liquefaction and to find out the temperature at which condensation reactions become significant,the liquefaction of NMH^*,was studied at 380-460 ^oC in tetrahydronaphthalene（THN）.The main focuses are on the trends of products yield and evolution of solid residues’composition and structure.It was found that NMH^*is very active at 420 ^oC with a conversion of（29）90wt%and an oil-plus-water yield of 39 wt%in 40 min.The reaction,especially the condensation behaviors at temperatures lower than 420 ^oC are different from those at higher temperatures as indicated by the trends in products yield and in the residue’s morphology and properties.The residues obtained at 440-460 ^oC contain mesophase pitch spherules.The changes in residues’volatiles content,H/C and O/C ratios,FTIR and Raman spectra,and ESR parameters are slow at 380-420^oC but much fast at 440-460 ^oC.These different trends are clearly evidenced by plotting the residues’characterization parameters against the decreasing residue yield during the reaction.4.To understand the formation of residues and its relation with the vitrinite structure and the products yield in direct coal liquefaction（DCL）,a high vitrinite low-rank coal,NMH^*,was further studied at 380-460 ^oC in the presence of tetrahydronaphthalene and absence of a catalyst.The amount（Q_H）and rate of hydrogen donation during liquefaction were determined,and the structures of the coal and residue were characterised by ¹³C-NMR.The product yields and residue structure parameters obtained by elemental analysis ¹³C-NMR and ESR were correlated with the residue yield(Y_Res)and Q_H.It was found that the DCL can be roughly defined into two regions demarcated by 420 ^oC,with limited condensation at 380-400^oC and intensive condensation at 440-460 ^oC,for example.These two-region behaviors can be clearly evidenced by the different trends in products yield and the residues parameters,such as those relevant to the cleavage of bridge bonds(f_al,C_n,δ-C andδ-O)and those relevant to the residues condensation(f_ar,X_b,R_D and H/C),against Y_Res and Q_H.The condensation in the first 5 min has already reached a high level due to the extensively high radical fragments generation rates in the initial stage of the reaction higher than 440 ^oC.The condensation in the first 5 min has already reached a high level due to the extensively high radical fragments generation rates in the initial stage of the reaction at temperatures higher than 440 ^oC.The two-region behavior is important for optimization of DCL conditions and its processing scheme.