Energy, exergy, environmental, and economic evaluation of a polygeneration system integrating supercritical water gasification of rice straw with supercritical hydrothermal synthesis

Ultrafine copper powder (UCP) is a fundamental functional material for the rapid development of high-tech industries. The large-scale production of high-performance UCP is constrained due to the high cost of chemical reducing agents. This paper proposed a polygeneration technology framework integrating rice straw supercritical water gasification (SCWG) with supercritical hydrothermal synthesis (SCHS) for H₂, electricity, and UCP production. The feasibility of the technology was evaluated from three dimensions including thermodynamic, environmental, and economic performance. The SCWG-SCHS oxidation system may be a superior scheme for material and energy flow and conversion because the chemical equilibrium ultimately achieved in the oxidation reactor can increase H₂ yield by 31 kg/h under system pressure of 25 MPa, a preheated water to slurry ratio of 2, a gasification reactor temperature of 600 °C, and a SCHS reactor temperature of 450 °C. The oxidation reactor (1453.86 kW, 34.02%), multiflow heat exchanger 1 (1004.01 kW, 23.49%), multiflow heat exchanger 2 (539.76 kW, 12.63%), and SCHS reactor (393.99 kW, 9.02%) were components with optimization potential due to their high exergy losses. The system's energy efficiency ((Formula presented)) and exergy efficiency ((Formula presented)) attained 47.74% and 38.93%, respectively, after optimizing the operating parameters. The global warming potential (GWP) was primarily caused by direct emissions for the system. The enrichment of CO₂ can reduce the GWP for the production of 1 kg of Cu to 3.75-5.38 kg CO₂-eq. Estimated economic indicators showed that the technology is potentially low-investment and high-return. The cost of producing 1 kg of Cu is 129.29 CNY, which is only 30.42-44.58% of traditional technology costs. This work may offer a new approach for the clean, efficient, and low-cost preparation of UCP.