Directional Conversion of Volatiles from Low-Rank Coal to BTX-Rich Tar by Combined In Situ and Ex Situ Catalytic Pyrolyses

[Image: see text] Realizing the directional conversion of volatile matter (especially tar) from catalytic pyrolysis of low-rank coal (LRC) and reducing the consumption of catalysts have been considered great challenges for the classified utilization of LRC. In order to realize this aim, in this work, combined in situ and ex situ catalytic pyrolyses (IECPs) were first applied to the conversion of low-rank coal. A small amount of in situ ZSM-5 (5 wt %) was mixed with LRC to regulate the pyrolysis reaction, and then a large amount of ex situ ZSM-5 (100 wt %) was used to control the volatiles produced by pyrolysis. The IECPs (550 °C) of LRCs were investigated in a fixed-bed reactor. For three LRCs with different coalification degrees, IECPs could obviously reduce the complexity of the tar components. When ZSM-5 dosage was 5 wt % and the reaction temperature was 550 °C, the relative content of BTX in ZT, BS, and JY tar increased from 20.23, 15.86, and 14.59% to 84.79, 77.26, and 50.11 area %, respectively. The relative contents of aliphatic hydrocarbons with a complex composition and a low price decreased from 67.84, 34.47, and 33.89 area % to 3.02, 1.81, and 7.60 area %, respectively. The catalysis mechanism was explored by TG-FTIR spectroscopy, which revealed that ZSM-5 had a great influence on the migration of aliphatic hydrocarbon intermediates in IECP, such that large amounts of aliphatic hydrocarbons of complex composition produced by IECP tended to be converted to small molecular substances (gas) or aromatic hydrocarbons (tar). It will provide a new theoretical support for the staged utilization of LRC.