Preparation Of Plate V₂O₅/TiO₂ Catalysts And Their Performances For Selective Catalytic Reduction Of NO By NH₃

Coal is the primary energy in China, mainly for power generation and heating. Emission of dust, SO2 and NOx due to coal combustion is the largest source of air pollution. Technologies of the electrostatic precipitator and desulfurization were widely used during "Eleventh-Five Plan", which almost resolved pollution of dust and SO2. However, NOx control is just a start. In recent years, NOx emission increases significantly with rapid growth in coal combustion and reached more than 10 million tons in 2010. Therefore, control of NOX emission has been as a highlight of the "Twelfth-Five Plan".Selective catalytic reduction (SCR) of NO by NH3 is the major technology for NOx abatement from flue gas. Currently, the most widely used catalyst for this progress is honeycomb V2O5/TiO2 based one, which accounts for more than 75% in the market. More than 7 production lines have been or are being built in China. Because the honeycomb catalyst is difficult to prepare and poor in dust-resistance, plate V2O5/TiO2-based catalyst has been developed and produced by Babcock-Hitachi (Japan) and Argillon (Germany). There is no domestic production reported. Because plate shaped catalysts are not only with high NO conversion, but also more suitable for high-dust flue gas, a characteristics of boiler in China, preparation of plate V2O5/TiO2-based catalysts was studied in this paper to develop a catalyst with high efficiency, high strength, low cost and independent intellectual property rights.The formula and preparation technology of V2O5-WO3/TiO2 catalysts are firstly studied based on the characteristics of domestic raw materials, such as TiO2 and binders. Then the molding process of plate catalysts was investigated, and effects of flue gas conditions and operating conditions on catalyst performance were studied. The main conclusions are as follows:(1) The homemade ultra-fine titanium was not suitable for the support of V2O5/TiO2 catalyst while the nano-grade T1O2 is appropriate. NO removal rate of nano-grade TiO2 derived catalyst is higher than 85%, similar to that of imported TiO2 derived catalyst. Nanometer TiO2 was conducive to its interaction with V2O5 due to its larger surface area, which make more V2O5 in the form of V+4 and thus lead to a higher NO removal rate. The lower surface area of ultra-fine titanium causes more V2O5 in the form of V+5, which leads to a lower NO removal rate.(2) Addition of ammonia when preparing pug reduces oxidation of SO2 as well as NO removal rate which reason is still not clear. The suitable calcination temperature of catalysts ranged form 400℃to 600℃. Both SCR activity and SO2 oxidation rate increase with increasing V2O5 loadings. To keep a relatively low SO2 conversion, V2O5 loading is recommended in a range of 1-2%. WO3 mainly inhibits oxidation of SO2 and its content of 5% is enough to ensure an acceptable SO2 oxidation rate. The species of binders, organic or inorganic, had little effect on catalytic activities.(3) The higher content of H2O in flue gas, the lower SO2 oxidation and NO removal rate. When the content of O2 increased, NO removal rate decreased slightly, and SO2 oxidation rate changed little. Within the space velocity (1000-7015 h-1) this work studied, the active sites were adequate.(4) With increasing the temperature from 300℃to 450℃, NO removal rate did not change much, but the SO2 oxidation rate increased gradually; NO removal rate increased with the increasing NH3/NO mole ratio from 1.0 to 1.1, and changed little with the further increasing NH3/NO mole ratio. The best NH3/NO mole ratio may probably be related to the active sites of the catalyst. The effect of SO2 on NO removal rate was related to NH3/NO ratio:NO removal rate was inhibited by SO2 when NH3/NO ratio was small; while the inhibiting effect disappeared when NH3/NO ratio was large enough. Sulfured by SO2 in the flue gas, the catalytic activities of the catalysts were improved. NO removal rate had a gradual stabilization, along with the stability of surface properties.(5) Glass fiber significantly improved the strength of the catalyst preventing dropout of the catalyst.In order to ensure catalytic activity and mechanical strength of the catalyst, the optimum addition amount of inorganic binder was about 20%. The appropriate aging time of the pug for the plate catalysts was 5 days to ensure uniform distribution of each component. Mechanical strength of the catalyst was high enough when the molding pressure was not less than 8 MPa. Adding moisture into the drying process made the plate catalysts chalked seriously. The ordinary drying was suitable for the plate catalysts, which was different from honeycomb catalysts.(6) The formula of the self-made plate catalyst was (wt.%):65.45% TiO2, 1.55% V2O5,8% WO3,5% glass fiber,20% bentonite,5% HPMC(based on the calcined catalyst); The pug aged for 5 days was molded at 8 MPa, followed by drying at 110℃for 2 h, and calcination at 450℃for 5 h. The overall thickness of the self-made plate catalyst was about 0.7 mm, with 2 h' abrasion rate 0.48%, which was the same as the industrial catalysts.The NO removal rate of the self-made plate catalyst was similar to that of the industrial catalysts (81%), but SO2 oxidation rate of the self-made plate catalyst was significantly lower than that of the industrial catalysts'(1.4% vs. 5%).