Thermodynamic and environmental analysis of auto-thermal supercritical water co-gasification polygeneration system of sewage sludge with coal

An auto-thermal supercritical water (SCW) co-gasification polygeneration system has been developed to treat sewage sludge (SS) with coal addition for H2 and heat production. Thermodynamic analysis based on Gibbs free energy minimization and life cycle environmental impact assessment of the system was performed under different conditions. The sensitivity analysis results show that H2 yield increased first and then decreased with the increase of preheated water when the proportion of dry coal to dry SS was less than 1.75 but decreased monotonically when the proportion was over 2. It is necessary to add 21% dry coal of raw SS amount when the system was energy self-sufficient to treat SS directly for avoiding pretreatment energy consumption. The largest exergy destruction was in the SCW gasification reactor which could be over 11500 kW for a 10 t/h processing requirement under 650 degrees C. The energy and exergy efficiency of the SS SCW co-gasification system can be improved effectively by adding coal, which can reach 79.5% and 47.9% under the preheated water ratio of 1 and the dry coal ratio of 3, respectively. Under this condition, 347.1 kg/h H2 and 20.5 t/h steam could be produced. The global warming potential (GWP) of the system was only 1.10-2.5 kg CO2-eq by the life cycle assessment considering carbon capture and storage (CCS) at a dry coal ratio of 3 based on the natural enrichment of CO2-rich gas by the SCW gasification technology.