Performance comparison of black liquor gasification and oxidation in supercritical water from thermodynamic, environmental, and techno-economic perspectives

Abstract

The black liquor poses a great threat to the environment and it is also a kind of available biomass resources. By the supercritical water gasification (SCWG) and supercritical water oxidation (SCWO) technology, the black liquid with high water content can be converted into hydrogen-rich gas or heat energy. In this study, a pulp mill integrated with black liquor SCWG and SCWO subsystems, which have a processing capacity of 11.2 t/h of black liquor with a concentration of 15.2 wt%, is designed. The thermodynamic analysis, techno-economic analysis (TEA), and life cycle assessment on the system are conducted. The gasification characteristics and thermodynamic performance of the SCWG and SCWO systems under different working conditions are studied. The thermodynamic analysis shows that the highest energy efficiencies of SCWG and SCWO systems are 41.53 % and 19.89 %, respectively. The lowest processing cost of black liquor is 398.68 RMB/t, which is obtained in the SCWO system at 600 °C. At 800 °C, the minimum hydrogen production cost of SCWG system is 23.28 RMB/kg H2. The LCA results show that the environmental benefits of SCW system at high temperatures are significantly better than those of integrated black liquor combustion systems. Finally, a multi-attribute decision making results of the SCWG and SCWO systems show that the optimal reaction temperature is 800 °C.