Effect of silicon oxide additive on the transformation characteristics of sodium and sulfur in Zhundong coal ash under atmospheric and elevated pressure

The effects of silicon oxide additive on the transformation characteristics of sodium and sulfur in coal ash under atmospheric and elevated pressure were investigated in this study. The results indicated that silicon oxide additive significantly inhibited the release of sodium under high pressure. The sodium content in ash with 4% of silicon oxide additive was 3.5% at 0.1 MPa, which was higher than that without additive. However, the sodium content increased to 5.4% without additive and 6.9% with 4% additive at 4 MPa, respectively. The sodium mainly existed in the forms of NaAlSiO₄ and NaAlSi₃O₈ at 0.1 MPa, and the content of NaAlSiO₄ increased with increasing additive dosage, which weakened the agglomeration of ash. The decomposition of low melting point mineral CaSO₄ was inhibited at 4 MPa, and the formation of Na₆Ca₂Al₆Si₆O₂₄(SO₄)₂ from NaAlSiO₄ and CaSO₄ was promoted significantly with increasing additive dosage. Furthermore, the inhibition mechanism of sodium and sulfur released from coal ash by silicon oxide under high pressure was proposed.