The staged gas-phase catalytic pyrolysis of low-rank coal is a promising strategy for achieving high value utilization. The directional conversion of low-rank coal by rapid catalytic cracking suffers from the problems of low product yield and selectivity, difficult flexible regulation and catalysts deactivation. The catalytic reforming characteristics of coal rapid pyrolysis volatiles over solid base catalyst (calcium aluminate, AlCa), acid catalyst (HZSM-5, Z5), and acid-base composite catalyst (AlCa-Z5) were investigated using the falling bed reactor and Py-GCMS instrument. In addition, the coal catalytic pyrolysis reaction system was simplified to reveal their acid/base catalytic pyrolysis mechanism by separating coal into chemically similar group components (saturates, aromatics, light resins, heavy resins, asphaltenes, carbenes). Furthermore, the composition and properties of coke deposits on various catalysts were systematically compared and evaluated using NH₄/CO₂-TPD, XPS, Raman spectroscopy, and temperature programmed oxidation (TPO). The findings indicate that AlCa and Z5 markedly increased olefins and aromatics in tar, respectively, and the AlCa-Z5 elevated olefins, aromatics and phenols to over 70 %. The product distribution from catalytic pyrolysis can be flexibly regulated by adjusting the acid-base properties of composite catalysts. AlCa formed less catalytic coke than Z5 (48.76 vs. 60.09 %), although the coke graphitization and particle size were greater owing to its exceptional dehydrogenation properties. Specifically, the graphitization and particle size of coke formed on AlCa-Z5 were remarkably reduced. The C-C/C-C structures (>80 %) dominate in coke, exhibiting more O-C-O on AlCa and more C-O on Z5. AlCa had little effect on the composition of the catalytic pyrolysis products of saturates, whereas Z5 significantly promoted the formation of aromatics and olefins. AlCa significantly increased the yield of phenolic compounds during the catalytic pyrolysis of aromatic fractions. Compared with non-catalytic pyrolysis, the heteroatom-containing compounds content in the products of resins catalyzed by Z5 was significantly reduced. The research results support the advancement and industrialization of coal hierarchical pyrolysis gas-phase catalytic cracking technology.

Coal,Catalytic pyrolysis,Acid-base composite catalyst,Coke,Regulatory mechanism