Preparation Of Vanadia-titania SCR Catalyst For De-NO_x And Study Of Deactivation Causes And Regeneration Method Of Commercial Catalysts

Nitrogen oxides(NOx) has become one of the important factors that affect the national air quality. Its emission has contributed significantly to the severe environmental pollution in Chinese cities including acid rain, dust-haze and smog. Selective catalytic reduction of NO using NH3 as reductant is the most mature technologies for NOx abatement. Catalysts are critical to selective catalytic reduction of NOx. They are the main factor affecting economical efficiency and the NOx removal of the whole SCR system. The fuel composition is complex in our country. The substances(including alkali, alkaline earth metals and arsenic, etc.) in coal are easy to cause catalyst poisoning. Therefore, the study on the adaptability of SCR catalysts to the firing flue gas in our country is important and meaningful. V2O5-MoO3/TiO2 catalyst for selective catalytic reduction(SCR) denitrification has been prepared by using the distributed maceration method with TiO2-supports.The influences of TiO2 texture parameters and ration of active components on the catalytic properties of catalysts for De-NOx were studied. Based on the catalyst with excellent qualities, alkali oxides(K2O and Na2O) and alkaline earth oxide(CaO) were doped on the fresh catalyst by impregnation method to achieve the poisoned catalysts. The effect of alkali oxides and alkaline earth oxide on catalytic activity and physicochemical properties of catalyst was researched. The deactivation causes of commercial plate-type SCR catalysts which were used in coal-fired power plant at a relatively high temperature for a long time was investigated by means of characterizations via BET, SEM, XRF and Raman. And the comprehensive technology to regenerate the deactivated catalysts was proposed. The main conclusions are shown as follows:(1) The influences of Ti O2 texture parameters and ration of active components on the surface properties and SCR performance of V2O5-MoO3/TiO2 catalyst for De-NOx were studied. It was found that large specific surface area of anatase TiO2 is advantageous to SCR reaction over the catalysts. The most active catalysts were obtained with the V2O5/MoO3 mass ratio of 1.25:6. At the typical SCR reaction temperature(350-450?), more than 90% of NO conversion and lower oxidation rate of ammonia were obtained at the gas hourly space velocity(GHSV) of 10,000h-1 and NH3/NO molar ratio 1.0 over the catalysts.(2)The effects of the molar ratio of K/V, Na/V, Ca/V(0.3, 0.5, 1.0, and 2.0)on NO conversion of K2 O,Na2O, and CaO-doped catalysts were researched. It was found that with the doping of K, Na, and Ca, specific surface area, the acidity and oxidability of the catalysts decreased, which will cause the reduction of SCR reaction ability. The poisoning capacity from strong to weak is as followed: K2O>Na2O>Ca O. The main reason of the catalyst chemical poisoning by K and Na is that K or Na coordinates to Br?nsted acid sites on the catalysts' surface and neutralizes the acidity of them. However, Br?nsted acid sites are important for the absorption and activation of NH3 in SCR reaction. And yet the main reason of the catalyst deactivation by Ca is that CaO deposits on the catalysts' surface and blocks the active sites.(3) The commercial plate-type V2O5-MoO3/TiO2 SCR catalyst used about 30,000 h at 380 °C exhibited the lower NO conversion, the maximum NO conversion of the catalyst is only 50% at GHSV 10,000 h-1. For the deactivated catalyst, there were plenty of poison(including Al, Na, Mg, S, K and Ca) in deposit and then the amount and strength of active acid sites decreased markedly. Meanwhile, the specific surface area and pore volume decreased by 42.96% and 54.19% respectively owing to pores plugging and the surface sintering. The crystal form of anatase TiO2 did not change and MoO3 still kept a well dispersion in the catalyst, but the morphostructure of V2O5 on the catalyst surface gradually changed from oligomeric state to crystal. The content of V2O5 and MoO3 were decreased by 0.15 wt.% and 0.42 wt.% respectively in deactivated catalyst. The specific surface area and the De-NOx activity of the catalyst improved greatly(69.74 m2·g-1, 95% at 400?)after regeneration by comprehensive technology. Main reasons are as follows: the toxic elements can be washed away effectively after washing, and pore structure and active components of the catalyst recovered sharply after regeneration.