Thermodynamic analysis and optimization of supercritical water gasification and oxidation systems for papermaking black liquor

Pulp black liquor is a byproduct and an important biomass from the papermaking process. Supercritical water gasification and oxidation is an innovative method for treating high-moisture-content pulp black liquor. In supercritical water, pulp black liquor is converted into hydrogen-rich gas, thereby achieving green utilization of resources. In this study, the process of supercritical water oxidation and gasification of pulp black liquor is simulated by using Aspen Plus, and the energy and biomass conversion in the supercritical water reaction process is analyzed. The effect of various operating conditions, such as reaction temperature, feedstock concentration, reaction pressure, and oxidant dosage, on the gasification yield was simulated. It indicates that elevated temperature and reduced feedstock concentration facilitate the increase of hydrogen yield, whereas increased reaction pressure proves detrimental to hydrogen production rate. Furthermore, the system is optimized to enhance thermal efficiency by using three routes, including integration of organic Rankine cycle to recover residual heat, and adoption of supercritical CO₂ coupled cycle for power generation. This paper will provide a theoretical basis for the design and optimization of supercritical water gasification system for paper black liquor.