Role Of Solvents And Hydrogen Transfer Mechanism In Direct Coal Liquefaction

Direct coal liquefaction(DCL)is clean coal technology for converting coal to liquid fuels.Improving economy of DCL is the key to industrial operation.Reducing hydrogen consumption is one way to reduce costs of DCL.In DCL,the hydrogen atoms are mainly from solvent and H2,thus,understanding the structural positions in which coal react with H2 or hydrogen-donor solvent,i.e.hydrogen transfer mechanism,is very important.The aim of this paper is to clarify role of solvents and hydrogen transfer mechanism.The behavior of direct coal liquefaction in binary solvent was studied.Roles of solvent and Shenhua nanosized iron catalyst during heating-up stage of DCL were investigated.Hydrogen transfer mechanism in DCL was further clarified by using a deuterium tracer method.Main research works and results in this paper are as follows.(1)Non-hydrogen donor solvent such as decalin,1-methylnaphthalene,naphthalene,anthracene,phenanthrene,fluorene,pyrene and fluoranthene,was mixed with a hydrogen donor solvent,tetralin,and was used as solvents.The effect of these non-hydrogen donor solvents on coal conversion and product yield of the liquefaction was studied when the non-hydrgen donor solvent was added to tetralin.In the absence of a catalyst,the mixed solvents of tetralin with phenanthrene,pyrene,and fluoranthene showed favorable effects on coal conversion and oil yield compared with pure tetralin,regardless of whether the reactions were conducted in a H2 or N2 atmosphere,indicating the positive effect of phenanthrene,pyrene,and fluoranthene in DCL.The reasons of this phenomenon are mainly ascribed to the higher physical dissolving capacities of phenanthrene,pyrene,and fluoranthene to coal,and the higher hydrogen-donating abilities of 9,10-dihydrophenanthrene,4,5-dihydropyrene,and 1,2,3,10b-tetrahydrofluoranthene,produced by the reactions of phenanthrene,pyrene,and fluoranthene,respectively,with tetralin or H2.In the presence of Shenhua nanosized iron catalyst from Shenhua Group,the addition of phenanthrene,pyrene,or fluoranthene to tetralin was found to improve coal conversion and oil yield in a N2 atmosphere,whereas in a H2 atmosphere,almost the same coal conversion and oil yield were obtained for all types of the mixed solvents.(2)Tetralin,phenanthrene-tetralin,pyrene-tetralin,fluoranthene-tetralin and decalin-tetralin were selected as solvent,and the roles of solvent and Shenhua nanosized iron catalyst during heating-up stage of DCL were investigated.In the heating-up stage of DCL,the major role of solvent is to provide activated hydrogen for coal.High dissolving capability of the liquefaction solvent that has high enough hydrogen donor capacity is beneficial for improving liquefaction performance.The dominant function of Shenhua nanosized iron catalyst is to promote the activation of H2,which can improve coal liquefaction performance in the heating-up stage of DCL.(3)Hydrogen transfer mechanism in DCL was further clarified using D2 and l,2,3,4-d4-tetralin as tracer.In DCL,the a-radical of tetralin is an important intermediate in hydrogen transfer to and exchange with coal.The abstraction rate of hydrogen from tetralin by preasphaltene and asphaltene(PAA)radicals is higher than that from H2.Hydrogen atoms in tetralin and H2 could stabilize different types of PAA radicals.The hydrogen atoms both in tetralin and H2 could enter alkyl groups remote from aromatic rings in preference to aromatic rings and a to aromatic groups of PAA during heating-up stage of DCL.Both the kinetic isotope effect and electrophilic aromatic substitution reaction indicate the existence of ionic reaction process besides a radical reaction process in DCL.(4)The results from deuterium tracer experiments and liquefaction with mixed solvents indicated that the hydrogen transfer approach in DCL is closely related to reaction process and catalyst.In DCL,coal-derived radicals can directly abstract hydrogen atoms from H2.In DCL with catalyst,tetralin inhibits the reaction of coal macromolecule to oil and the transfer of H2.In the heating-up stage of DCL,the de/re-hydrogenation cycle of tetralin is non-existence,and about 80%of the hydrogen consumption is from solvent in the absence of catalyst,while the hydrogen consumption from H2 and solvent are basically equal in the presence of Shenhua nanosized iron catalyst.When the reaction time of DCL is 60 min,in the absence of catalyst,the de/re-hydrogenation cycle of tetralin exists,but is not a dominating approach,and about 65%of the hydrogen consumption is from solvent.In the presence of Shenhua nanosized iron catalyst,the de/re-hydrogenation cycle of tetralin is almost non-existence,and about 65%of the hydrogen consumption is from H2.