Research progress of supercritical water hydrogen production from biomass

Green, safe, and reliable clean energy is ever increasing under the "double carbon" goal. Among them, the waste biomass can be converted into green fuel, particularly with the application of many thermochemical and biochemical technologies. Supercritical Water Gasification (SCWG) can be a promising potential to convert the organic matter in the biomass into hydrogen using supercritical water as a medium. The feedstock can be used as a resource for biomass and the waste. SCWG process shares the fast reaction speed, excellent hydrogen selectivity, and fewer by-products, compared with traditional hydrogen production. In addition, water as the reactant in the SCWG process can avoid high energy consumption during drying, and thus reduce the cost. Previous systematic analysis has been made on the SCWG influence factors. In this review, the special physical and chemical properties of supercritical water were introduced to expound the main components (such as cellulose, hemicellulose, and lignin biomass) in the SCWG process reaction, and the experimental device, reaction temperature, residence time, and pressure in the influence factors. It was found that the batch reactor was suitable for the phase behavior and reaction mechanism, due to the simple structure and strong applicability of raw materials. By contrast, the continuous reaction device was used to more accurately control the experimental parameters, and then to realize the continuous commercial production, thus suitable for the parameter research. The hydrogen yield was improved to increase the heating rate of the device during operation, the reaction temperature, and residence time, but to reduce the feed concentration within a certain range. However, there was the complicated influence of pressure on the hydrogen yield. The solvent cage was often used under high pressure, leading to the reduction of the decomposition reaction rate unsuitable for the production of hydrogen. It was necessary to select the appropriate pressure, according to the actual situation. The homogeneous and heterogeneous catalysts were utilized in the SCWG. Specifically, the homogeneous catalyst performed a better catalytic effect on the water gas conversion was reaction, but the strong corrosion was caused the equipment to clog. The heterogeneous catalyst presented high catalytic activity, easy recovery, and excellent stability, more suitable for large-scale SCWG production. At the same time, there were some influences of the acidity of the metal catalyst in the catalytic process. The strong acidity of the catalyst accelerated the formation of carbon deposition, resulting in the catalyst deactivation. Appropriate secondary metals were added, such as Ce, La, and Ru. The performance of the catalyst was accelerated to increase the service life of catalyst for the better hydrogen selectivity. Future research can be focused on the equipment with corrosion resistance and salt deposition resistance, or constantly optimizing operating parameters, while the deactivation mechanism of catalysts, even to optimize the number of catalysts, and the catalysts with high activity and reusable. The existing technical barriers and development prospects of SCWG were analyzed to combine with the current technical development of SCWG. The finding can also provide theoretical guidance for the biomass SCWG in the future.