An overview on issues for lignite direct liquefaction

Being a coal with the lowest degree of coalification,lignite is comprised of the basic aromatic structural u- nits. High H / C atom ratio,volatile content and vitrinite content of lignite favor the good liquefaction performance. On the other hand,high O / C atom ratio of lignite undesirably induces the generation of water during the hydrogenation, which is not conducive to the effective retention of potentially high value-added oxygen-containing compounds in the raw coal during the direct coal liquefaction. The direct liquefaction process of lignite in the presence of a hydrogen do- nor solvent leads to a series of reactions ( e. g. pyrolysis,hydrogenolysis,polycondensation and hydrogenation reac- tions). The macromolecular structural units of lignite contain a variety of oxygen-containing functional groups,such as phenolic,alcoholic,carboxylic,ether,etc. ,which exhibit the different chemical behaviors during the thermal transfor- mation process. Thermal cracking of oxygen radical fragments from the phenolic hydroxyl radicals in presence of the isufficient hydrogen radicals is inclined towards the crosslinking reactions. Also,these stable phenolic compounds unfa- vorably undergo the dehydroxylation at higher temperature that results in the formation of water. These two unsolicited processes heavily decrease the amount of oxygen contents which are expected to be present as phenolic compounds. This compels to explore the mechanism of retention and inhibition of polycondensation of oxygen-containing functional groups during the thermal transformation. However,contrary to this,at high temperature,the cleavage of aromatic ether bonds may become the valuable source of phenolic compounds. Therefore,for the investigation of the guided transfor- mation of phenolic compounds as the major target,it is essential to consider both the protection of the native phenols, and the trans-formation of ether bond and other functional groups into phenolic compounds. In this context,exploring the details about the minimization of hydrodeoxidation,which generates water or COx ,is beneficial to reduce the hydro- gen consumption and also to obtain the high value-added organic compounds containing oxygen functional groups. Suc- cinctly,it is presumed that this paper gives a sufficient information on the analysis of the transformation behavior of ox- ygen-containing organic compounds in lignite by the thermal hydrogenation. Through this review,the research ideas of guided transformation can be understood and the reasonable utilization pathways of high-value products on the basis of the characteristics of lignite are suggested.n