Effect of fluxing inorganic element-bearing acetate composite on the distribution of pyrolytic products from Highveld partially oxidized fine-coal reject and its beneficiated residue

Abstract

The role of mineral-matter in coal significantly influences pyrolysis, yet there is no consensus on the behavior of inherent-minerals and carboxylate salts during coal or fine-coal reject (FCR) pyrolysis. This study investigates the effects of carboxylate salts (Ca(C2H3O2)2, Mg(CH3COO)2·4H2O, and Fe(C2H3O2)2 composite (CSC) on FCR, demineralized FCR (FCRD), and demineralized > 1.9 g/cm3 FCR sink fraction (SI1.9D) in a Fischer-assay double retort set-up under the following pyrolysis conditions: 500 °C and heating rate of 5 °C/min for 3 hours in nitrogen. Analytical techniques identified kaolinite, quartz, dolomite, siderite and calcite, and characterized tar precursors, functional groups, carbon associations, and species evolution in FCR, FCR+CSC, FCRD+CSC, SI1.9D, SI1.9D+CSC and their chars. FCR+CSC exhibited the fastest decomposition rate due to extraneous-minerals and CSC interactions. FCRD+CSC and FCR+CSC achieved the highest carbon conversion, tar yield and gas yield, and the lowest char yield compared to SI1.9D and SI1.9D+CSC. The CSC addition increased the S proportion in FCRD+CSC and SI1.9D+CSC chars due to the H2S in-situ capturing to form nano-sulfides and S-bearing amorphous phases. Also, the SI1.9D+CSC blend released a higher CO2 via inherent nano-carbonates reacted with H3O+ from kaolinite/illite transformation, the HF/HCl aqueous solution and acetate decomposition. Tar yield, polycyclic aromatics, alkylated phenolics, and naphthalene compounds decreased due to hydrogen transfer by CSC, inhibiting secondary cracking of intermediate vapors. These results highlight the catalytic potential of carboxylate salts, dependent on inherent-minerals and dominating maceral groups during coal pyrolysis.