Conversion of sulfur-free black liquor into fuel gas by supercritical water gasification

Supercritical water gasification of weak sulfur-free black liquor (BL) was performed in a batch autoclave at temperatures between 430 degrees C and 470 degrees C, pressure between 24 and 27 MPa and residence time between 2 and 63 min. Results show that the gas produced was a mixture of mainly hydrogen, methane, and carbon dioxide. Maximum conversion was achieved at 470 degrees C and 60 min. Energy recovery (ER, ratio between the energy in the gas and in the initial BL) was 46%. Thirty-four percent of the carbon and 53% of the hydrogen initially present in BL were converted into gases. Nearly 15% of initial organic carbon remains in the liquid phase and consists mainly of phenolic compounds, which are stable under those conditions. A higher temperature is needed to convert all the organic carbon. Thermodynamic equilibrium should be reached at 700 degrees C leading to a complete conversion and a better efficiency. Sodium recovery is close to typical kraft recovery value and compatible with causticizing.