Energy, exergy, environmental, and economic evaluation and conceptual design of supercritical water gasification of bark integrated with poly-generation

The conceptual design and optimization of supercritical water gasification (SCWG) for poly-generation are performed in the viewpoint of the thermodynamic analysis and economic assessment, which can promote the industrial application. Assuming a high concentration slurry feed and proposing complicated process design in the current research reduce the possibility of practical application, and also fails to ensure the efficient energy output. In this study, a SCWG process scheme was designed specifically for a pulping modification process, coupling with a multistage reactor configuration and gradient heat transfer configuration. The optimal process conditions were explored by investigating the effects of water diversion, feed concentration, and in-situ hydrogen separation rate (ISHS) on the energy output of the SCWG system, as well as analyzing the energy distribution and exergy losses. In order to maximize the energy output by making full use of waste heat, the exergy losses and energy efficiency of the SCWG poly-generation of hydrogen-electricity (PGHE) and poly-generation of hydrogen-heat (PGHH) systems were compared to investigate the suitable poly-generation process. The suitable SCWG poly-generation process scheme was verified to have low carbon emission and energy cost by evaluating the environmental impact of the system over its life cycle and its levelized cost of energy (LCOE). It was shown that increasing the feed concentration and ISHS rate, and reducing the amount of water diversion can improve the energy and exergy efficiencies. The optimum hydrogen production, energy efficiency and exergy efficiency of the SCWG system were 1651.44 kg/h, 47.58 % and 48.87 %, respectively, for a feed concentration of 50 wt%, an ISHS rate of 40 %, a preheat water ratio of 4, and a water diversion of 1. The exergy loss is mainly concentrated in the oxidation reactor and cooling unit, and the waste heat utilization coupled with ISHS can reduce significantly the exergy loss in the cooling unit and improve the energy efficiency and exergy efficiency of the system. PGHH has a greater impact on energy yield than PGHE, and at an ISHS rate of 40 %, the exergy loss of the system is reduced to 69800.48 kW, and the energy efficiency and exergy efficiency are increased to 73.34 % and 72.38 %, respectively. Meanwhile, under the condition of PGHH, coupling with an ISHS rate of 40 % and carbon capture and storage can minimize the environmental impact of the system, with a global warming potential of 5.03 kg CO₂-eq. The LCOE of SCWG system coupled with PGHH is as low as 0.696 CNY/kW·h. The completion of the study can accelerate the industrial application of the SCWG poly-generation project with conceptual design and theoretical guidance.