Study Of Deactivation, Regeneration And Recovery Of Vanadium-Titanium Based SCR Catalysts

Nitrogen oxides (NOX) are recognized as a major source of air pollution in the world. Selective catalytic reduction of NOX with ammonia (NH3-SCR) has been proved to be one of the most successful methods to remove NOx in the flue gas from stationary sources. Catalysts are critical to selective catalytic reduction of NOx and directly concerns to the performance and cost of the whole SCR system. Poisoning is inevitable for SCR catalysts in the running process. It is significant to study the catalyst deactivation under the actual operation condition. The construction of the SCR system will rapidly increased with the implementation of stricter NOX emission standards in our country. The activity of the catalyst will gradually decline during the running process and we have to deal with large quantities of spent SCR catalyst in the following years. Since the cost of the catalyst is a major part of the total cost in a SCR plant, it is necessary to consider the secondary utilization of the spent SCR catalyst, which can reduce the enterprises cost and environmental pollution. The regeneration of the catalysts is the best way to deal with the spent SCR catalyst, but still there are a large number of catalysts cannot or failed to be generated, therefore, we need to recover the useful metal oxides in the catalyst. The systemic studies were carried out on the V2O5/TiO2 catalyst' poisoning mechanism by KCl and SO2, as well as the regeneration and recovery of vanadium and tungsten of the spent industrial V2O5/TiO2 catalyst. The main results are shown as following:1. The SO2 in the simulated flue gas increases the deactivation of the in-house prepared V2O5/TiO2 catalysts which doped with KCl by impregnation, and the increased extent depends on the K/V molar ratio, At a K/V molar ratio of 0.1, K2S2O7 was formed and interacted with V2O5 to produce a eutectic; however, at a K/V ratio of 0.3, KCl may have transformed to K2SO4, the eutectic was not observed. The formation of V2O5-K2S2O7 eutectic decreased the BET surface area, which is not the main reason for the catalyst deactivation. This eutectic had no effect on the state of support TiO2, but decreased the amount of active vanadium species on the catalyst surface, which led to significantly decreased NH3 adsorption, particularly those on Br(?)nsted acid sites, and weakened the oxidation ability of the catalyst, thus significantly decreasing the SCR activity. In addition, the presence of H2O could restrains the formation of K2S2O7, which weakens the deactivation degree of the catalyst.2. The study of spent industrial V2O5/TiO2 catalyst indicates that the catalyst surface was covered with fly ash. The BET surface area and pore structure of the catalyst did not deteriorated seriously after blow away the fly ash deposited on the catalyst surface. The alkali metal potassium and alkali earth metal calcium was the main reason for the catalyst deactivation. Washing with sulfuric acid could remove more potassium compared to other regeneration methods and enhance the surface acid due to the deposition of SO42- on the catalyst surface, but the active components vanadium could dissolve in the solution. The remove of potassium was not effective with the sodium hydroxide solution washing method, and the active component vanadium and tungsten were both leached-out seriously. Besides, the some sodium from solution would deposit on the surface of regenerated catalyst, which led to the further deactivation due to the significant decrease of surface acid. Organic weak acid and complex agent, such as citric acid and EDTA, also used to regenerate the deactivated SCR catalyst, and the remove of potassium was not effective and the loss of vanadium and tungsten were quite serious as well. Water washing had good effect on the potassium removal with few losses of active components and the BET surface area of the catalysts partly recovered. The amounts of potassium and calcium removal were increased with the washing temperature increased from 30? to 60?, and the NH3 adsorption capacity and the reducibility of vanadium on the catalyst surface also increased. Water washing in 1 h at 30? could only partly removal of potassium and calcium on the catalyst surface and couldn't removal the potassium and calcium which interaction with surface vanadium species strongly. This part of potassium and calcium could removal by prolonging the washing time or increasing the washing temperature.3. For the vanadium leaching process from a spent industrial SCR catalyst by sulfuric acid at atmospheric pressure, the reduction of hydrodynamic boundary layer around the catalysts particles becomes minimal when the stirring speed is higher than 200 rpm; the vanadium recovery yield increases with increases in temperature, sulfuric acid concentration, and leaching time and decreases with an increase in solid to liquid ratio. The leaching data can not be described well by kinetic models commonly adopted for similar processes in the literature. The Avrami equation, originally developed for crystallization, is found to be most suitable. The leaching is controlled by diffusion in the solid with an activation energy of 5.90 kJ/mol. The leaching process of vanadium and tungsten from a spent industrial SCR catalyst by sodium hydroxide solution at atmospheric pressure showed the similar results. The leaching of vanadium and tungsten are both controlled by diffusion in the solid with activation energy of 16.91 kJ/mol and 19.95 kJ/mol, respectively.