Abstract
Supercritical hydrothermal combustion, a new and promising homogeneous combustion technology with a wide range of application scenarios and broad development prospects, provides creative ideas and means for the enhanced degradation of organic wastes, hydrothermal spallation drilling, thermal recovery of heavy oil, etc. This technology is elaborated upon in five parts: (1) introducing the main devices including semi-batch reactor and continuous reactor to study the hydrothermal flame in accordance with research institutions, (2) presenting the research status of related numerical simulation from the angles of reaction kinetics and flow-reaction, (3) summarizing the characteristics of hydrothermal flame and combustion by five key parameters, (4) dividing up ignition process and explaining ignition mechanism from the perspectives of critical physical properties of water and heat transfer and mixing conditions, (5) discussing and forecasting its industrial applications including hydrothermal spallation drilling, the thermal recovery of heavy oil, the clean conversion and utilization of coal-based fuel, and the harmless treatment of pollutants. By and large, this paper analyzed in detail everything from experimental equipment to industrial applications, from combustion characteristics to ignition mechanisms, and from summary conclusions to prospect prediction. In the end, herein is summarized a couple of existing paramount scientific and technical obstacles in hydrothermal combustion. Further significant studies in the future should include excellent reactors, advanced monitoring techniques, and powerful computational fluid dynamics.
Highlights
Supercritical water (SCW) refers to the water at the conditions above the critical point (Tc = 374.15 ◦ C, Pc = 22.12 MPa)
Compared with the moderate temperature of supercritical water oxidation (SCWO) (450–650 ◦ C), the local temperature (>1000 ◦ C) reached by the flame improves the removal efficiency of organic matters (10–100 ms needed for completion) and recalcitrant organic matters, making it possible to design smaller and more effective reactors
If the hydrothermal flamesof arenitrogen not properly controlled in theincreasing reactor, thisdevelopment will lead to the acceleration of thermal wearthe andstudy corrosion and the production of nitrogen
Summary
Due to SCW’s special properties, like low viscosity, low dielectric constant and high diffusivity, the reactions between organic matters and oxidants in SCW, called supercritical water oxidation (SCWO), are thoroughly homogeneous and rapid Compared with the moderate temperature of SCWO (450–650 ◦ C), the local temperature (>1000 ◦ C) reached by the flame improves the removal efficiency of organic matters (10–100 ms needed for completion) and recalcitrant organic matters, making it possible to design smaller and more effective reactors The heat it releases can be used as the internal heat source of SCWO, reducing its dependence on preheating and avoiding corrosion and salt-deposition in the process towards the supercritical state. We hope to inject raw materials at room temperature to overcome salt-deposition-caused blockage and preheating-caused high-energy consumption in the SCWO treatment of recalcitrant pollutants. This review is beneficial for readers to understand the ignition mechanism and the factors affecting flame stability, as well as the design and optimization of HFR, promoting its industrial application
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