Thermodynamic and environmental assessment of black liquor supercritical water gasification integrated online salt recovery polygeneration system

Conventional black liquor treatment has weaknesses in pollution. Supercritical water gasification (SCWG) technology makes it possible to utilize the energy of black liquor cleanly. In this work, an auto-thermal SCWG black liquor polygeneration system integrated with online salt recovery was proposed. The performance of salt recovery and gasification was evaluated. The results show that the increasing gasification temperature, black liquor concentration, and salt discharge concentration are favorable to hydrogen production and system efficiency. For salt recovery, the sulfur-containing substances in black liquor are all converted into Na₂S/NaHS and recovered in this system. Through exergy analysis, the major exergy destructions are caused by the oxidation unit, SCWG unit, and sub-critical heat transfer. Then, the discharge brine heat is utilized to heat oxygen and feed to reduce the exergy destruction of reaction and heat transfer, which improved the energy efficiency from 78.03% to 88.16%, and the exergy efficiency from 60.86% to 64.10%. After adding a discharge salt utilization unit, the system's hydrogen production is 58,438 Nm³/h and the steam supply is 70,455 kW. This work provides theoretical guidance for the design and establishment of a black liquor SCWG industry system.