Study on the catalytic pyrolysis of coal volatiles over hematite for the production of light tar

•A hematite catalyst was used for pyrolysis tar upgrading.•Hematite A and B decreased the pitch content by 7.59 and 8.88 % respectively.•Catalytic pyrolysis of coal enhanced the content of Fe2+ in hematite.•Fe and lattice oxygen were the decisive factors in the bond cleavage of heavy tar. In this paper, to investigate the catalytic pyrolysis behavior of coal (XJ) volatiles over two hematite (HA and HB), a double-layered fixed-bed reaction apparatus, which was easy to separate coal and catalysts was employed to measure the pyrolysis tar yield, and the composition of pyrolysis tar and gas was investigated by GC-MS and GC. The results indicated that both hematite A and hematite B reduced the tar yield and caused an increase in CH4, CO2 and H2 output. Analysis of the chemical composition of tar indicated that XJ had a high content of pitch (46.39 %). At the same time, the addition of HA and HB decreased the pitch content by 7.59 and 8.88 %, respectively. Then, the change in the hematite chemical composition and physical structure after catalytic pyrolysis was characterized by XRD, TEM, XPS, SEM and N2 adsorption-desorption. Furthermore, a catalytic mechanism was proposed to illustrate the transformation of pyrolysis tar over hematite. Fe atoms and lattice oxygen are the decisive factors in the bond cleavage of pyrolysis tar. Fe atoms can break the CC and CH bonds of aliphatic hydrocarbons, while lattice oxygen can promote CO bond cleavage and produce CO2.