The Properties Of CO Shift Catalyst In Hydrogen And Methane Generation System From Coke Oven Gas

Through CO shift reaction converting CO in COG into H2,the product gas will be composed mainly of H2 and CH4, which can be easily separated into H2 and CH4.Coke oven gas contains high concentration of tar and multiple forms of sulfur, which requires that the CO shift catalyst should be more active, more sulfur-tolerant and more anagotoxic.Based on the Co-Mo main catalyst, in this paper, the active component, additives and carriers were adjusted, which made the catalyst applicable to COG atmosphere.The effects of additives and carriers on catalytic performance were investigated in detail.Meanwhile, the techniques of XRD, TPR, TG were used to characterize the catalysts, so as to discuss the active sites and structure.More importantly, the influences of tar and high CH4 content on catalytic activity were studied.The main conclusions are made as follows:1)In the optimizing experiments, the Co-Mo-K/y-Al2O3 catalyst prepared by two-step impregnation exhibits the highest activity, when the atomic ratio of Co/Mo reaches 0.576 and the content of K2O reaches 7.0 wt.%～11.0 wt.%.2) At 200℃, the Co-Mo-K/y-Al2O3 catalytic activity increases with the atomic ratio of Co/Mo increasing at a specific range and decreases with the content of Mo increasing, indicating that Co-Mo-S structure model serving as the active sites of Co-Mo shift catalyst is more reasonable.3)CeO2 has a promoting effect on the low temperature and medium temperature catalytic activity, which may be due to valence variability and excellent oxygen-storage capacity of CeO2. However,the promotion of CeO2 is limited owing to the low loading,which suggests that CeO2 should coordinate with other additives to further increase the conversion of CO in COG.4) CeO2-K2O promoted Co-Mo-Ce-K/γ-Al2O3 catalyst has good selectivity, and the activity increases significantly. When the total mount of promoter CeO2-K2O reaches 9.0 wt-% and the mass ratio of CeO2/K2O reaches 0.5,the conversion of CO reaches 91.72% at 300℃. Moreover, the addition of CeO2 increases the water adsorb ability of catalyst, which makes the catalyst maintain a high CO conversion unser low steam/COG ratio.The contrast experiments show that there is a synergy effect between Ce and K, which has the catalytic activity increase obviously.5)XRD results demonstrate that CeO2 is in favor of Co dispersion and mainly produces electronic effect. TPR characterization results indicate that the addition of CeO2-K2O in Co-Mo-Ce-K/γ-Al2O3 catalyst decreases the reduction temperature of active components, and part of octahedrally coordinated Mo6+ transforms into tetrahedrally coordinated Mo6+ which has a close relationship with the catalytic activity. In addition, the reduction of octahedrally coordinated Mo6+ decreases the surface acidity of catalyst and makes for CO shift reaction.6) The thermal satbility of catalyst are greatly increased by substituting on of y-Al2O3 support with ZrO2-Al2O3 support, which can provent the sintering of active comoponent in case of the high content of O2 in the coke oven gas.7) The effects of factors on the thermal stability of catalyst can be ordered by Ce×K>K2O> Ce×Zr>ZrO2>CeO2>preparation method. The content of factors interacted with each other can be determined by the binary diagram so as to work out the optimum level:CeO2～1.0 wt.%, ZrO2～10.0 wt.%, K2O～6.0 wt.%.Impregnation-precipitation is prefered because the influence of preparation method on the thermal stability of catalyst is not significant. The CO conversion of catalyst prepared in accordance with the optimal experimental conditions is 93.76% at 300℃,and the content of CO in COG is 0.55%.The descent rate of CO conversion at 400℃is 4.06%, being 0.21% lower than the lowest descent rate.8)If the content of tar and benzene in the cock oven gas is higher, there will be a coking reaction on the catalyst. It can be substantiated by the exothermic peaks appeared after 700℃on the DTA curve. C6H6 concentration in the gas at the reactor inlet and outlet shows that the deposition of C6H6 on the Co-Mo-Ce-K/ZrO2-Al2O3 catalyst is less than that of commercial catalyst FB301, which may be interpreted by that the modification of ZrO2 on the carrier is not easy to benzene adsorbed on such catalysts. This indicates Co-Mo-Ce-K/ZrO2-Al2O3 catalyst can run longer under the coke oven gas atmosphere.