Study On The Behavior Of Iron Oxide Sorbents Removing H₂S

Coal as the backbone of national economic development, is the main source of energy and chemical raw materials in China. It is of great significance to utilize coal properly for the development of economy, society and the environment. Power generation is the primary process of using coal. Intergrated Gasification Combined Cycle (IGCC) and Fuel Cells (FC) are recognized as the most promising clean coal technology due to high-efficiency and low pollution. A strict restrict on the concentration of H2S in hot gas is required for these systems. For example, the level of H2S content in IGCC system must be less than 100 ppm, and in FC system it is required be to lppm or less. Hot gas desulfurization is crucial for IGCC and FC systems.At present, the main problem existing in hot gas desulfurization is the unsatisfied stability of the sorbent during cycles. In addition, the effects of the complex coal gas composition, especially the strong reducing atmosphere on sulfidation process are not very clear. All of these may limit the industrialization of hot gas desulfurization. This work focuses on studying the effects of the texture, as well as the reducing atmosphere on the performance of the sorbent. 太原理工大学硕士研究生学位论文Polystyrene (PS) microspheres with different size were synthesized by emulsion polymerization and soap-free emulsion polymerization. By using the as-synthesized PS as pore-forming materials, red-mud from steel factory as activate component, clay as binder, iron oxide sorbents with different texture were prepared for hot gas desulfurization. The prepared sorbents were evaluated in a fixed bed reactor in simulated coal gas atmosphere. The kinetic study of sulfidation and the effects reducing gas on desulfurization experiments were performed in thermogravimetric analyser. Other techniques, such as mercury porosimer, nitrogen adorsption, XRD, XPS, SEM, were also used to characterize sorbents. Some important results supported on large amount of experiments are as followes:1. The polystyrene microsphere with diameters less than 100nm could be synthesised by emulsion polymerization, whereas polystyrene microsphere with diamer rang from 200-800nm could be synthesised by soap-free emulsion polymerization. PS microsphere with diferent size can be used to prepare iron oxide sorbents with different texture.2. The activity and stability of the prepared sorbents were evaluated in a fixed bed reactor in simulated coal gas at the temperature of 500℃. The results show that sorbents with larger pores and smaller fractal dimension exhibited good performance with high sulfur capacity and stability. There is a correlation between sulfur capacity and the volume of pore with diameter larger than 200 nm. 3. Thermogravimetric analysis was used to investigate the sulfidation kinetics. The modified shrinking core model was suitable for charactering the microkinetics, and the modified grain model could be used to characterize the macrokinetics. The results show that the surface chemical reaction activation energy is 28.7kJ/mol and the diffusion activation energy isl6.9 kJ/mol. The macrokinetic studies show that, for these four sorbents with different texture, the activation energy of chemical reaction are almost same, whereas the diffusion activation energy are different, suggesting that the effects of the texture of the sorbents on the performance are because of the different diffusion resistance in sombents.4. The thermogravimetric experiments were carried to study the effects of hydrogen as well as the pre-reduction on sulfidation. It was indicated that H2 inhibited significiently the reaction between iron oxide and H2S. However, increasing the temperature could release this inhibition. Apparent kinetics studies show that when 20% H2 was present in the system,the reaction activation energy were larger than that when hydrogen was absent in the system.5. It was indicated that when the sorbent was pre-reduced, the next sulfidation reaction rate increased with the increase of temperature at the range from 300℃to 650℃. However, the activity of the sorbent without pre-reduction performed best around the temperature of 300℃. The activation energy of sulfidation reaction decreased with the reduction time, however, the diffusion activation energy increased when the reduction time too long. This is because the over-reduction to the sorbents could destroy the pore structure of the sorbents.