Thermodynamic analysis of poly-generation system for gas-biochar-heat-electricity based on supercritical water gasification of biomass waste

Biomass energy is renewable and abundant worldwide, providing a solution for the shortages of fossil fuels and the serious environmental pollution. The proposal for efficient utilization methods of biomass waste demands urgent attention. Supercritical water gasification (SCWG) process is a potential technology. In this work, a polygeneration system based on SCWG was developed to convert biomass waste to gas, biochar, heat, and electricity. Firstly, the mass, energy, and exergy flows were calculated under typical conditions. Subsequently, the impact of various operating parameters on the yield of hydrogen-rich syngas, generated electricity, and the thermodynamic performance of the system was investigated. The results demonstrated that exergy loss primarily occurred in the cooler, reactor, heat exchanger, and preheater, accounting for more than 90 % of the total exergy loss under different conditions. This loss arose from irreversible reactions, heat transfer, and heat dissipation. Higher temperatures, higher biomass concentrations, and greater amounts of preheated water positively affected hydrogen-rich gas production and supplied heat energy. Energy efficiency increased with the rising quantities of preheated water and biomass concentration, with the impact of biomass concentration being more significant. Conversely, the evaluated gasification temperature displayed an adverse effect on energy efficiency. The maximal exergy efficiency reached approximately 58.3 % at 550 degrees C, with a biomass concentration of 33 % and a preheated water mass flow rate of 900 kg h-1.