Transformation and catalytic effects of sodium during coal pyrolysis

The abundant sodium in Zhundong coal can act as an excellent catalyst for its thermal conversion. A horizontal fixed-bed reactor was selected to minimize the release of Na. Chemical form of Na in the coal was simplified by using acid-washed coal loaded with H₂O-soluble and NH₄Ac-soluble sodium. Most Na was retained in the char after pyrolysis of the coals at 500°C to 900°C. The H₂O-soluble sodium in Na₂SO₄-chars remained unchanged below 700°C and then partly transformed to insoluble sodium; transformation of NH₄Ac-soluble sodium to the H₂O-soluble form dominated in the Na-chars. SEM-EDX analysis showed that the Na-char possessed a smoother surface than the Na₂SO₄-char. The Na/C ratio in the Na₂SO₄-char increased rapidly with temperature. Raman spectra of the chars showed that loaded Na₂SO₄ had little effect on its structure, but loaded NH₄Ac-soluble sodium increased the amounts of polyaromatic (≥6) rings and active sites. Reactivity of the char was measured using TGA. Association analysis of these results revealed that insoluble sodium in the char showed excellent catalytic ability, but Na accumulated during oxidation did not. Both the char structure and the amount and occurrence modes of Na had strong effects on the char reactivity.