Catalytic upgrading of coal pyrolysis volatiles over metal-loaded HZSM-5 catalysts in a fluidized bed reactor

Coal tars produced by conventional low-temperature pyrolysis processes require further upgrading before being used as fuels or high valuable aromatic chemicals owing to a low yield of light aromatics and high oxygen contents. In this study, a set of metal-loaded HZSM-5 catalysts were evaluated in a fluidized bed reactor for improving tar quality via the in situ catalytic upgrading of the pyrolysis volatiles. The product yields of the three different phases, the gaseous composition, the char reactivity, and the chemical properties and composition of the coal tar were comprehensively studied. The use of metal-loaded catalysts increased the amounts of light aromatics while decreasing the amounts of aliphatics and oxygenated compounds. The products distribution was optimized from a complex mixture with different functional group compounds to a tar concentrated in aromatics after catalysis. The possible reaction pathways were depicted to illuminate the mechanism of catalytic reaction based on the variation of the volatile products distribution. Incorporation of metals promoted hydrogen transfer within the reaction system. Hydrogen species formed by dehydrogenation of chain hydrocarbons may stabilize the radicals generated from coal initial pyrolysis stages. Moreover, these species can efficiently promote hydrodeoxygenation of phenolic compounds to aromatic hydrocarbons by metal-acid catalysis. The present work effectively promoted the formation of light aromatics and further clarified the catalytic mechanism of pyrolysis volatiles over metal/HZSM-5 catalysts.