Thermodynamic analysis and optimization of auto-thermal supercritical water gasification polygeneration system of pig manure

A large amount of livestock manure is produced every year around the world, which requires harmless utilization to reduce environmental pollution and produce energy. As a promising and pollution-free technology in recent years, Supercritical water gasification (SCWG) can utilize pig manure with high moisture to produce hydrogen. In this paper, a novel process modeling an auto-thermal SCWG polygeneration system of pig manure was established with thermodynamic analysis, coupled with in-situ hydrogen separation and waste heat recovery method. Simultaneously, the environmental significance was analyzed through Life cycle assessment (LCA), and the GWP was 1.73 kgCO₂-eq/kgH₂ under optimal conditions with CCS. The results showed that the system with in-situ hydrogen separation would produce more H₂ to improve energy efficiency. Adding a waste heat recovery unit could reduce the exergy loss of 89.14% of the cooling unit, which had the largest exergy loss in the system. In this study, there were 5081Nm³/h of H₂ and 16377 kW of hot steam produced when the SCWG system was auto-thermal (620 ℃, 25 MPa, 650 t/d dry manure and 10 wt% reactor concentration), where the energy efficiency of the system could reach 79.85% that had a 3.6–35.64% increase to the previous SCWG systems. In addition, the optimal operating parameters of this polygeneration system were obtained, which the exergy efficiency could reach 54.25%. A lower complete gasification temperature and a larger in-situ H₂ separation ratio were conducive to achieving higher H₂ yield when the optimal ratio of preheated water to dry material was 8.