Volatile potassium transfer behavior in carbon matrix and its effect on char gasification

Abstract

Coal catalytic gasification is one of the effective ways to produce natural gas from coal. However, high cost and low recovery efficiency of catalyst increase the cost of catalytic gasification, which has become a restrictive factor for large-scale promotion of coal catalytic gasification. Therefore, the study on the migration behavior of potassium-based catalysts in the gasification process is of great significance for catalysts recovery. To understand the volatilization behavior between potassium and carbon matrix, the experiments were carried out in high temperature stage microscope (HTSM) and fixed-bed reactor. The migration of volatile potassium between particles was analyzed by SEM-EDS, while the chemical fraction of potassium species was determined by AAS, and the surface elements of the samples was investigated by XPS. The results showed that potassium would volatile at 765-770℃, C/K≈6.2–9.5(wt%), which would be adsorbed by the other char particles, leading to an increasing of the content of soluble potassium and ion-exchanged potassium, promoting carbon conversion in a limited effect and a decreasing of hydroxyl functional groups on the char particles. Meanwhile, the potassium content was proportional to the migration distance, the migration distance was about 720–800 μm at maximum under HTSM. The migration of potassium at high temperature caused the formation of liquid phase around the surface of char particles, which would prevent the contact between the gasifier agent and char particles and increase mass transfer resistance, thus preventing the increase of particle carbon conversion. In addition, no obvious liquid phase formation occurred at 800 °C in SFA-K, indicating that the formation of potassium-containing liquid phase and the volatilization of potassium at 800 °C were directly related to the carbon in the char.