Advances in catalytic hydroconversion of low-rank coals

Low-rank coals (LRCs) are characterized by high chemical reactivity due to their low degree of coali-fication,high H/C atom ratio and high contents of bridge bonds,side chains as well as volatile matter.Catalytic hydroconversion (CHC) is one of the effective ways to obtain liquid fuels and chemicals from LRCs.However,high oxygen and moisture contents of LRCs,undesirably increasing the consumption of hydrogen and energy in CHC,is adverse to the formation of oil and results in difficulties to product separation.Meanwhile,CHC is usually conducted at high temperature under high pressure,and the product composition is complex with heavy fractions,leading to high cost and low return.Optimization of reaction system is a solution to improve the conversion rate of LRCs and the yield of liquid products,which is an urgent problem to be solved in the CHC of LRCs.Catalyst,a core technology in CHC of LRCs,plays an important role in activating hydrogen,promoting hydroconversion and removing heteroatoms,which directly determines the reaction system and product quality.Reaction condition,another component of the reaction system,can not only control the rate of covalent bond cleavage in coal,the generation and transference of active hydrogen,but also inhibit the polycondensation reaction,thereby affecting catalyst performance and product composition distribution.Additionally,pretreatment has an impact on the CHC characteristics of LRCs since it can change the physical structure and chemical reactivity of LRCs.This paper overviews the research progress of the effects of reaction conditions (temperature,solvent,atmosphere and pressure) and pretreatment methods (thermal pretreatment,swelling pretreatment,extraction pretreatment and hydrothermal pretreatment) on CHC or direct liquefaction of LRCs,as well as main catalysts used in LRCs hydroconversion.