Effect of solvent quality on covalent bonds cleavage behaviors of lignite during pyrolysis/liquefaction processes

Abstract

Effect of solvent quality on coal conversion and product distribution during lignite pyrolysis/liquefaction were investigated at the temperatures ranging from 180 to 460 °C, and the covalent bond cleavage behaviors in different solvents were comparatively elucidated. Results showed that at the temperatures below 280 °C, weak covalent bonds, such as methoxy and carboxyl groups, are cleaved to generate a small amount of gaseous products (mainly CO2) and liquefied oil. At the temperatures of 330–380 °C, ether bridges that connecting the aromatic clusters are cleaved and contributed significantly to the production of preasphaltene (PA). Meanwhile, oil yield and coal conversion increased to 27.50% and 64.58% in tetralin. At the temperatures of 430–460 °C, the cleavage of CH2 bridges, alkyl substituent groups and aromatic heterocyclic structures proceeded markedly, and the scission of CH2 bridges and some naphthenic CH2 bonds significantly contributed to oil production. Consequently, oil yield and coal conversion greatly increased to 43.90% and 84.13% in tetralin. Hydrogen donor solvent is conducive to the cleavage of strong covalent bonds, such as Car–O–Car and Car-Cal bonds, to realize the efficient liquefaction of coal.