Study On The Molecular Structure And Pyroysis Behaviors Of Low Metamorphic Coals

Low-metamorphic coal is a high-quality raw material for pyrolysis and direct liquefaction.Its conversion under mild conditions is an important way for its efficient and clean utilization.The carbon content of low-metamorphic coal is close,but the molecular structure is not similar.During pyrolysis and direct liquefaction,the first step is the cleavage of chemical bonds.Due to the different structure of low-metamorphic coal,the products of pyrolysis and liquefaction are quite different.According to the above analysis,four representative low-metamorphic coals were selected as the research objects in this paper.By characterizing the molecular structure and behaviors of bond cleavage,the relationship between structure,bond cleavage and products was analyzed.Through the liquefaction experiment of NMH coal,the effects of reaction temperature and DHP dosage on the hydrogen supply,free radicals and product distribution in the liquefaction process were studied.The main conclusions are the followings:(1)The bond cleavage reaction of low-metamorphic coal can be described by first-order kinetics.The amount of cleavable covalent bonds is kept between 4.0-9.8 mmol/g.The bond cleavage occurs on weak covalent bonds in the coal,in addition to the ?-position and-CH2-CO-O-CH2-of the aromatic carbon substituent,the bond cleavage occurs more in other positions of the organic structure,and the amount of bond cleavage within 6 min accounted for more than 60%.The average aromatic cluster of low-metamorphic coal is 2-3 rings.The oxygen structure is mainly carbonyl oxygen and carboxyl oxygen and most of the carboxyl structure(such as Ar-CO-O-CH2-,-CO-O-Ar)enters the liquid or solid product during the hydrogenation.During the pyrolysis process,the size of free radical fragments and bond cleavage determine the oil and gas yield.The pyrolysis tar of low-metamorphic coal is mainly composed of heavy components and the pyrolysis products cannot reflect the basic structural characteristics of raw coal.(2)The amount of hydrogen donor solvent and the reaction temperature are the main factors affecting the coal liquefaction process.The number of stable radicals is 4 orders of magnitude smaller than that of stable radicals under the same pyrolysis conditions and they show the opposite trend with the increase of reaction temperature.The amount of active radicals changes in the same way as the yield of coal liquefaction products and both increase significantly as the reaction temperature increases.When hydrogen supply is insufficient,the polycondensation reaction gradually increases the amount of stable radicals,which reduces the aliphaticity in residual char and increases the degree of aromatic ring condensation.When the hydrogen supply is excessive,the radicals are completely quenched,the amount of stable radicals decreases rapidly and gradually approaches zero,but the structurere of sidual char is similar to that of the raw coal.The degree of aromatic ring substitution and the length of the side chain are slightly reduced due to bond cleavage.