Mechanism of salt deposition, nucleation and separation in the supercritical water gasification of black liquor

Supercritical water gasification (SCWG) is a promising technology for the harmless treatment of black liquor, converting organic matter into hydrocarbon gases. However, salt precipitation remains a significant challenge, hindering the resource utilization of black liquor in SCWG. Herein, the deposition, nucleation, and separation of inorganic salts in supercritical water (SCW) were investigated in this study via experiments and molecular dynamics (MD). First, the deposition behaviors of Na₂CO₃, K₂CO₃, Na₂SO₄, K₂SO₄, NaCl, and KCl in SCW were explored in a rapid pressure-released reactor at temperatures ranging from 370 °C to 500 °C. This study found that the temperature of 400 °C was optimal for promoting salt deposition at the reactor bottom. Potassium salt was preferred to deposit at the bottom of the reactor, and sodium salt was prone to adhere to the inner surface of the reactor. Second, the MD study on these salts showed that the nucleation of salts was more intense at higher temperatures, and sodium salts were more likely to form tightly packed clusters than potassium. Finally, the salt separation performance in the SCWG of black liquor was evaluated in a continuous experimental system; the maximum recovery rates of CO₃^2–, Cl⁻, K⁺, Na⁺, and SO₄²⁻ were 69%, 57%, 53%, 40%, 27%, respectively. This study would lay a foundation for the salt recovery in the SCWG of hypersaline organic wastewater.