Review of Thermochemical Conversion of Agriculture and Forestry Waste for Hydrogen Production in Supercritical Water-Reaction Systems and Applications

Supercritical hydrothermal chemical hydrogen production technology converts biomass organic components in agroforestry solid waste into hydrogen energy. It is an efficient, economical and clean way to make use of energy in agroforestry solid waste. This paper carries out an in-depth analysis on this technology mainly from the perspective of reaction systems. According to studies on various types of reaction systems, it is concluded that it is easy to implement control over reaction temperature, reaction pressure, and reaction time with batch reactor. However, considering the difficulty in scaling up the production and recovering the thermal energy, it is not practical to use this type of reactor in industrial applications. Boasting advantages of good reaction performance and high economic benefits, continuous reaction system is the only way to usher supercritical hydrothermal chemical hydrogen production with agroforestry solid waste from laboratory to industrial applications. The reactor in continuous reaction systems is mainly made of Hastelloy or Inconel alloy resistant to high temperature, high pressure and corrosion. The combination of constant current pumps and back pressure valve is used for internal pressure control of the system. Contact resistive heating with embedded thermocouples is adopted for heating of supercritical water to achieve a rapid and controllable temperature rise of the system. The formation of coke tar is significantly reduced as the reaction material is sent into the hot supercritical water directly. Supercritical hydrothermal chemical hydrogen production technology enables conversion of biomass in various agroforestry solid wastes into hydrogen, but with significant difference in reaction characteristics depending on the waste type. For lignocellulose biomass waste, high cellulose content is favorable to reaction process. As to livestock manure waste, the conversion of pollutants such as heavy metals must be considered. Further studies on this topic will focus on corrosion prevention of reactor, blockage and salt deposition in reactor, online slag removal of reaction system, and modeling design, economic balance and coupling to other systems in industrial architecture. © 2023 Xi'an Jiaotong University. All rights reserved.