Study On The Reaction Behaviors Of Formation,Pyrolysis And Polycondensation Of Coal Hydroliquefaction Intermediate Products

In order to investigate the reaction behaviors of formation,pyrolysis and polycondensation of asphaltene during the liquefaction process,direct hydroliquefaction experiments were performed in an intermittent high-pressure stirred reactor using raw coal and homemade asphaltenes as raw materials.The changes of free radical concentration and carbon structure of related substances were analyzed by means of electron paramagnetic resonance and solid-state nuclear magnetic resonance.The main contents include:the effects of different coals and solvents on the reaction behavior of asphaltenes and the change of free radical concentration of asphaltenes with temperature,the effects of solvent:asphaltene,temperature and asphaltene species on the re-hydrogenation process of asphaltenes and the change of free radical concentration of re-hydrogenated asphaltenes with solvent:asphaltene and temperature,the evolution of carbon structure of coal and related products during the liquefaction process and the deduction of asphaltene reaction mechanism.The results show that the asphaltene yield varies greatly with temperature among different coals.Both Naomaohu coal and Bulianta coal increase first and then decrease,while Heishan coal continues to decrease,with maximum values of 34.13%,39.02%,and 33.44%at 350℃,350℃,and 290℃,respectively.The reaction behavior is affected by the molecular structure of the coals.Using different solvents,the asphaltene yield increases first and then decreases with temperature,with maximum values of 34.13%,47.96%,and 13.29%at 350℃,400℃,and 350℃,respectively.The reaction tendency is together determined by the solubility,hydrogen supply and component complexity of solvents.The change of free radical concentration of asphaltenes is consistent with the change of yield,and reaction behavior of asphaltenes during the liquefaction process is revealed from the perspective of unpaired electrons.After further using homemade asphaltenes as the liquefaction raw material,the maximum oil yield is 71.74%when solvent:asphaltene is 5:1.Increasing the amount of solvent can enhance the dispersion and encapsulation of free radical fragments,and inhibits its pyrolysis and polycondensation reactions.Increasing temperature can promote the pyrolysis of asphaltenes.At 3:1,the oil yield first rises and then decreases,reaching a maximum of 68.03%at 430℃.However,too high temperature can easily lead to the occurrence of poly condensation reaction and there is a competitive relationship with the pyrolysis reaction.The degree of difficulty in the pyrolysis reaction of asphaltenes from different coals is determined by the molecular structure,and the effect of temperature on asphaltenes with higher degree of condensation is more obvious.Increasing the content of active hydrogen can weaken the condensation reaction of asphaltenes.The free radical concentration of re-hydrogenated asphaltenes can characterize the degree of pyrolysis of asphaltenes,including ring opening of condensed aromatic rings,and can also reflect the stability of re-hydrogenated asphaltenes.The analysis of carbon structure of coal and related products shows that the aromaticity of raw coal,asphaltenes at 430℃,and liquefaction residues at 430℃ are 0.67,0.81,and 0.90,respectively,and that of asphaltene raw materials at 350℃ and re-hydrogenated asphaltenes at different temperatures are 0.49 and 0.84-0.87,respectively,which reflects that aliphatic carbon is easy to pyrolysis and hydrogenate during the liquefaction process,is the core part of the liquefaction reaction,and generated asphaltenes will increase the cracking degree with the increase of temperature.Asphaltenes are mainly formed by the combination of methyl carbon and aromatic rings in coal,and then continue to pyrolyze,generating free radical fragments containing aliphatic carbon,1-2 ring aromatic rings,and 3 rings or above aromatic rings.The first two parts are hydrogenated and converted into oil and gas products,the last two parts are poly condensed with each other to form the final asphaltenes,and a very small part is polycondensed to form the liquefaction residues.