Thermodynamic analysis of the superiority of the direct mass transfer design in the supercritical water gasification system

Supercritical water gasification (SCWG) is a promising clean technology for coal utilization due to high chemical reactivity and no tar emission. However, the wall-type heat exchange in the SCWG system demands a large heating surface area and an extreme high-temperature heat source. This paper puts forward an optimized design, namely direct mass transfer, in which the final product gas is partially recycled back into the gasification reactor to supplement the gasification agent and provide sensible heat energy. A whole system simulation was established to analyze the mass flow and exergy efficiency in the reference design and the optimized design. The process analysis shows that the optimized design has higher exergy efficiency (85.6%) than that (72.5%) of the reference design. In the optimized system, direct mass transfer significantly alters the mass flow and exergy flow, with less influent water (33.3%), less heat transfer rate (30.5%), and less oxygen consumption (51.5%). Furthermore, sensitivity analysis shows that increasing circulation ratio, feedstock concentration, and preheating temperature can reduce oxygen consumption and improve gas output and efficiency. The study indicates that direct mass transfer has the potential superiority to solve the challenge of heat integration and facilitate the application of the industrial-scale SCWG system.