Solvents’ molecular structure analysis in direct liquefaction of Shenhua coal

In direct coal liquefaction,in addition to catalyst,the composition and structure of solvent are key factors that directly affect the rate of coal conversion and the oil yield in liquid product. After nearly ten years of stable operation,with no change in the catalyst and operating conditions,a slight difference was found between the oil yield of Shenhua coal direct liquefaction plant located in Ejin Horo Banner,Ordos and the oil yield results of the Shenhua Shanghai Research Institute pilot plant. Thus,the physicochemical structures of hydrogendonor solvents from the Shenhua Shanghai Research Institute(RS-S)and the Ordos Coal Direct Liquefaction Plant(RS-E),respectively,were investigated,such as functional groups and aromatic structures,etc. Qualitative and quantitative analyses have been performed using the infrared spectroscopy and simultaneous fluorescence spectroscopy,and the group selective twodimensional gas chromatographymass spectrometry/hydrogen flameionization detector method. Based on the above analysis,the C—H bond dissociation energy （BDE） of hydrogenated aromatics in solvent molecules and the factors influenced by the chemical environment were calculated using the density functional theory calculations. Results show that both the hydrogendonor solvents are dominated by unsaturated aromatics with substituents,and have the condensed aromatic rings,mainly the bicyclic,tricyclic,and tetracyclic aromatic systems. The content of partially hydrogenated aromatics is the highest in the analyzed two hydrogendonor solvents. The content of partially hydrogenated aromatics in RSS is 15.79% higher than that in RS-E,and the content of cycloalkanes is 17.11% lower than that in RS-E,indicating that the liquefied solvent of RS-E is overhydrogenated,and the hydrogen supplying capacity of RS-S is higher than that of RS-E. Different substituents can promote the dissociation of C—H bonds to some extent. When a substituent is located in the tetralin 1 position,the effect BDE,BDEC1—H value is strong,and the electronwithdrawing substituent has a more pronounced effect on BDEC—H of tetralin than electrondonating substituent. The BDE is linearly correlated with the spin density value of the C atom after the dissociation of the H atom. As the number of aromatic rings increases,the BDEC—H decreases,making it easier to break,resulting in polycyclic aromatic hydrocarbons that are more likely to be saturated by hydrogenation. In short,a good hydrogen donor solvent should be a mixture of methyl substituted,2-4 rings unsaturated aromatic hydrocarbons.