Effects of various inorganic sodium salts present in Zhundong coal on the char characteristics

- Na Loaded on H-form coal retains in chars and inhibits smoothing of the surfaces.
- Na inhibits the graphitization, leading to loose char structure (by Raman).
- Reactivities of Na-containing chars were significantly higher than that of H-char.

This study aims to clarify the effects of inherent inorganic sodium salts present in Zhundong coal on the morphology, element composition, structures, and reactivities of chars from fast pyrolysis. The concentration of the main anions present in the raw coal determined by ion chromatography indicates that Na could exist in the forms of NaCl, Na2SO4, NaNO3, CH3COONa (NaAc), and NaHCO3, which were then loaded on acid-washed coal respectively by wet impregnation. The pH changes of slurries after the adding of NaAc and Na2CO3 illustrated that sodium carboxylate (-COONa) was generated. The characteristics of chars obtained by fast pyrolysis of the prepared coals in a drop tube furnace were determined. Scanning electron microscopy and energy dispersive X-ray fluorescence spectroscopy of chars revealed that loaded Na, especially -COONa, could be retained on the char surface and inhibit smoothing of the particle surfaces during pyrolysis. Raman intensity indicated that loose char structures formed in chars derived from sodium-loaded coals as a result of inhibiting the process of graphitization and increasing cross-linking density and the number of substitutional groups during fast pyrolysis. Specific reactivities of chars prepared at 900 °C increased in the order: H-char-900 < Na2SO4-char ∼ NaCl-char < NaNO3-char < NaAc-char < Na2CO3-char, which may be attributed to the combined effects of active Na, active sites, and loose char structures.

Catalytic effects of inorganic sodium salts present in Zhundong coal were investigated. The pH changes of slurries after immersion of acid-washed coal in Na2CO3 solution illustrated that -COONa was generated. Loaded -COONa could be retained on the surface and increase the O/C of char. Moreover, loose char structures was formed as a result of inhibiting the process of graphitization. Specific reactivities of Na2CO3-char was much higher than that of H-char-900, which may be attributed to the combined effects of active Na, active sites, and loose char structures.180