Study On Low-temperature Selective Catalytic Reduction Of NOx In Flue Gas By CeO₂-Fe₂O₃ /ACF Catalyst

With the economic development, China's energy consumption has been increasing and nitrogen oxides(NOx) emissions have continued to grow, if not further control, NOx will cause serious pollution of atmospheric environment of our country. The most commonly and effective technology of NOx removing in flue gas used at home and abroad is the NH3-Selective Catalytic Reduction(SCR). We just selected SCR to reduce NOx in the flue gas into N₂ with NH3 as a reductan and under the influence of the catalyst in this study. The main component of SCR catalyst in the current project is V₂O₅-WO₃/TiO₂ series, and commercial SCR catalyst have good catalytic activity only at high temperature (300℃～400℃),with catalyst mainly deployed before the air preheater where it can meet the temperature requirements, but the location of the high ash and sulfur dioxide may easily cause catalyst poisoned. By placing low-temperature SCR device after the dust removal and desulfurization, it can be not only relatively easy for the existing boilers, but also ease the congestion and the poison of dust and SO₂, as well as avoid the flue gas preheat energy consumption and reduce the cost of denitrification. Catalyst is the core of SCR technology, and its catalytic performance is directly related to the effects of denitrification. Therefore, it's extremely urgent to research and develop a low-temperature SCR which can be arranged after the dust removal and desulfurization (temperature of flue gas is about 100℃～130℃). Active carbon fiber (ACF) is not only used as a good absorbent, but also a good catalyst because ACF has developed pore structure, so often it is also used as a catalyst carrier; Fe has a unique chemical properties, as well as a good catalyst components; the most important nature of oxidation cerium (CeO₂) is as a storage of oxygen, through the conversion of Ce⁴⁺ and Ce³⁺ to store and release oxygen in the conditions of oxidation and reduction; and with it NO converted to NO₂ can be promoted to improve the NH₃-selective reduction activity of NO. The above-mentioned three are widely used in catalytic areas of the NOx purification. But low-temperature SCR catalysts such as ACF load with CeO₂-Fe₂O₃ are still rare.In this study, the activated carbon fiber (ACF) was the support, and CeO₂ and CeO₂-Fe₂O₃ was loaded on it, with equal volume impregnation method,two series catalysts were prepared : different loading of CeO₂/ACF and different load ratio of CeO₂-Fe₂O₃/ACF. It was found that as the temperature increased from 80℃to 300℃, the 12%CeO₂/ACF catalyst showed good catalytic activity for SCR when single load; While mixed load, due to adding cocatalyst, the catalytic activity is significantly higher than single load with the same loading of CeO₂, especially in the lower temperature range(80℃～200℃), the activity is much better, 10%CeO₂-2% Fe₂O₃/ACF catalyst showed the best catalytic activity and stability. Therefore experiments of calcination temperature effction was took for the catalyst of 10%CeO₂-2% Fe₂O₃/ACF, meantime another three kinds of catalyst were prepared, at different calcination temperature which were 200℃, 300℃and 400℃, and it was found that 300℃was the best calcination temperature. The effect of the O₂ and NH₃ for the catalytic activity of 10%CeO₂-2% Fe₂O₃/ACF catalyst were respectively studied,from which we can learn the presence of O₂ and NH₃ do much influence to the NOx purification efficiency in SCR reaction,that is because only in the presence of O2 can NOx be better reduced to N₂. In the meantime,the catalytic activity and time relationship were also studied, and the result presented 10% CeO₂-2%Fe₂O₃/ACF had longer lifetime,and when surpassed thirteen hours it also showed much more improvement contract to previous research. In addition, a combination of various physicochemical techniques containing surface area (BET) measurement, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the physico-chemical properties of the catalysts. The reason of CeO₂-Fe₂O₃/ACF catalyst activity becomes larger might be that pore diameter got larger with the injection of cocatalyst Fe₂O₃, which changes the pore structure in the BET study.Cerium oxide's good dispersivity make it better loaded on ACF, so SEM showed the metal oxide particles distributed evenly on the carrier had best catalytic performance.And from CeO₂-Fe₂O₃/ACF XRD-diffraction we conclude that both CeO₂ and Fe₂O₃ are amorphous for 12%CeO₂/ACF and 10%CeO₂-2%Fe₂O₃/ACF were amorphous state, no obviously characteristic peaks were seen.