Research progress and prospects of coal direct liquefaction solvents

In the past three years,China’s external dependence on oil has reached over 70%,seriously threatening national energy security. The steady development of direct coal liquefaction technology for producing the oil could compensate for the shortage of oil resources,and promote the clean and efficient utilization of coal in China. To gain an indepth understanding about the importance of solvent for direct coal liquefaction and the hydrogen transfer mechanism,the authors summarized and compared the development history of the direct coal liquefaction processes worldwide,with the special emphasis on the improvement of direct coal liquefaction process developed by Shenhua Group in terms of solvent. The important roles of solvent during direct coal liquefaction in both physical and chemical aspects,the effect of polar compounds on solvents,and the interaction between solvents on coal hydrogenation and liquefaction reactions were discussed. Solvents not only have the role of dissolving and dispersing coal,but also dissolve hydrogen,supply and transfer hydrogen,and stabilize and protect free radicals generated by coal pyrolysis,and promote the cleavage of bonds in coal macromolecules. The composition and molecular structure of solvents were obtained by qualitative and quantitative analysis by using a full twodimensional gas chromatographytimeofflight mass spectrometry/hydrogen flame ionization detector. Also,the evaluation methods of coal direct liquefaction solvents such as the characteristic factor method,infrared absorption spectroscopy and nuclear magnetic resonance method were discussed. The progress of research on the different hydrogen transfer mechanisms of coal direct liquefaction solvents,such as hydrogen shuttle mechanism,hydrogenolysis mechanism,pyrolysis radical mechanism and synergisticbihydrogen transfer mechanism,was highlighted and the effect of temperature and pressure on the hydrogen transfer mechanism of coal direct liquefaction was discussed. At present,the research on liquefaction solvents is not sufficient enough,and the investigation should focus on the composition of solvent and hydrogen transfer mechanism in the future,which is important for reducing the production cost of enterprises and improving the oil yield and conversion of target products.