Characteristics of sodium sulfate deposition in hydrogen production from supercritical water gasification: A review

Supercritical water gasification (SCWG)having been developed for over 30 years is an efficient and clean hydrogen production technology. Biomass and coal are often used as good raw materials of SCWG, meanwhile achieving their clean conversion and utilization. However, salts such as sodium sulfate, generally exist in the raw materials and can poison the necessary catalyst during SCWG, so that they should be removed as much as possible before the raw materials are pumped into the catalytic reactor. Additionally, a large amount of inorganic salts will be generated simultaneously during the gasification reaction. As the solubility of various inorganic salts decreases sharply beyond the water critical temperature, salts in the supercritical water easily separate out and deposit on the walls of equipment and piping, resulting in a series of potential issues such as reduced heat-transfer efficiency, increased flow resistance and even the related-facilities blockage. In this paper, the experimental analysis and numerical simulation for the solubility and deposition characteristics of the sodium sulfate in supercritical water are systematically summarized; on this basis the corresponding deposition and plugging mechanisms are described and analysed. Additionally, this paper also introduces the potential technical methods being able of avoiding the equipment blockage caused by sodium sulphate deposition, and proposes the necessary subsequent research, which is of great significance for solving the disadvantageous effect of sodium sulfate deposition on the long-term stable operation of SCWG commercial plants.