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 degrees C to 900 degrees C. The H2O-soluble sodium in Na2SO4-chars remained unchanged below 700 degrees 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.