SCR is a selective catalytic reduction catalyst under the action of the reductant to nitrogen oxides to nitrogen, so as to achieve the purpose of denitrification flue gas. The SCR technology using ammonia (NH3) as a reductant is the most widely used fixed-source flue gas denitrification method. Common SCR denitrification system is usually arranged in the boiler economizer and air preheater, the reaction temperature is about 350 degrees, the use of vanadium-tungsten oxide loaded titanium dioxide as a catalyst, the denitration efficiency can reach more than 90%. However, the following shortcomings exist: (1) the catalyst is poisoned by the abrasion of dust and alkali / alkaline earth metal for a long time; (2) the boiler system is required to reserve a large space for arrangement between the economizer and the air preheater; (3) There is a certain amount of heat loss. Low temperature SCR technology is the technology of using NH3 to reduce NOx in flue gas to N2 and H2O under the condition of relatively high temperature, which has the advantages of low energy consumption, convenient system arrangement, long service life of catalyst, Low advantages, very industrial application prospects, is the current domestic and international research flue gas denitration technology.
1, low temperature SCR catalyst
The catalyst is the core of SCR technology. Among them, MMNOx / TiO2, MNOx-CeO2 / TiO2, MNOx / AI2O3 and CuO / Tio2 all exhibit good denitrification activity in the middle and low temperature range. The research shows that the catalyst with Mn-Ce oxide as active component has high catalytic activity and N2 selectivity, which is the focus of research on low-temperature SCR catalysts. In this paper, the existing research on low temperature SCR catalysts is summarized and analyzed from three aspects: the active component of the catalyst, the carrier of the catalyst and the modification of the catalyst.
1.1 active ingredients
The active component of the catalyst plays a crucial role in the adsorption and electron transfer of the reactants during the SCR reaction at low temperature, which directly determines whether the reaction proceeds smoothly and affects the catalytic activity and the selectivity of N2. Common low-temperature SCR catalyst active components mainly activated manganese oxide and two kinds of ceria.
1.1.1 active manganese oxide
MNOx lattice contains a large number of active oxygen, can effectively promote the SCR SCR denitration at low temperature. Common manganese oxides mainly MnO2, Mn2O3, M3O4 and Mn5O8, etc., their role in the SCR denitration reaction vary. Kapteijn other studies found that MnO2 catalyst has good low temperature activity, while Mn2O3 has a higher N2 selectivity. Manganese oxide catalytic activity of the order: MnO2> Mn5O8> Mn2O3> Mn3O4. The study found that, although pure MNOx high activity at low temperatures, but its N2 selectivity is poor, and susceptible to flue gas SO2 and H2O lead to catalyst poisoning. MNOx is usually combined with other oxides to prepare bimetallic or composite oxide catalysts to increase the catalyst activity and N2 selectivity and prolong the service life of the catalyst.
1.1.2 Ceria
CeO2 has good activity in low-temperature SCR reaction. When Ce is added catalytically, CeO2 can enhance the oxygen storage capacity of the catalyst and improve the activity of the catalyst. He Hong and other prepared by impregnation Ce / TiO2 catalyst and investigated the reaction performance. Wu Zhongbiao et al MNOx-CeO2 / TiO2 catalyst was prepared by adding Ce element to MNOx / TiO2 by sol-gel method. It was found that the addition of Ce can help increase the conversion rate of NO. Gu Tingting et al studied the activity of sulfated modified CeO2 catalyst. Previous studies have shown that, CeO2 has a strong surface acidity and oxygen storage capacity, can promote the activation of NH3 in the catalyst surface and adsorption.
1.2 Catalyst Carrier
The carrier is the key to the catalyst molding. The good catalyst carrier can not only promote the adsorption of the substrate, improve the catalytic activity, but also help the large-scale production and industrial application of the catalyst. Low temperature SCR catalyst carrier mainly titanium dioxide, alumina activated carbon, zeolite and so on.
1.2.1 Titanium dioxide
TiO2 is a common catalyst carrier that is not easily acidified, and can improve the activity of low temperature SCR catalytic reaction, N2 selectivity and sulfur resistance. TiO2 is usually anatase, rutile and brookite three crystal forms, of which anatase TiO2 is often used as a selective denitration catalyst carrier. Qi and other Mn, Cu, V, Fe and other transition metals were loaded on TiO2 to examine the activity of the catalyst, in which Mn was loaded on the TiO2 catalyst by impregnation method. WU Zhongbiao Mn / TiO2 catalysts were prepared by sol-gel method and modified with Fe, Cu, Zn, V and other transition metals. The results show that the catalyst activity can reach more than 95% at 150 ℃. Xu Wenqing and other prepared by impregnation Ce / TiO2 catalyst, between 275-400 degrees with excellent SCR activity, while the catalyst also has high water resistance, sulfur resistance.
1.2.2 activated carbon
Activated carbon has a high specific surface area and pore structure, is a common adsorbent, but also a good catalyst carrier, with rich sources, low prices, easy to regenerate and so on. Calvez et al. Studied the performance of catalysts supported on activated carbon by vanadium and found that the acid sites on the surface of active catalysts after vanadium loading increased to a certain extent, and the catalyst was more capable of adsorbing NH3 after the loading of V2O5. An, et al., Prepared the noble metal Pt-supported activated carbon catalyst at 170-210 degrees NO conversion over 90%, and the catalyst showed excellent water resistance, NO conversion in the presence of 4% H 2 O did not significantly change .
1.2.3 Zeolite Molecular Sieve
Zeolite has a certain SCR activity, but difficult to block the water vapor in the flue gas, easily lead to catalyst deactivation. RahkamaaTolonen other studies have concluded zeolite molecular sieve helps to promote the SCR catalyst reaction. It was found that the Fe-ZSM-5 catalyst prepared by co-impregnation and ion-exchange impregnation exhibited excellent activity during SCR reaction and some researchers proposed that co-impregnation method is the most effective preparation method.
1.2.4 Alumina
AI2O3 has higher thermal stability and is favorable for the adsorption and reduction of NOx. AI2O3 is an ideal low temperature SCR catalyst carrier. Jin Ruiben and other prepared by impregnation MNOx-CeO2 / AI2O3 catalyst and found that the catalyst has a larger specific surface area, pore volume and high concentration of hydroxyl, catalytic activity increases with increasing temperature at 160 degrees NO removal Rate reached 98%. Wang Xiaobo and other prepared by impregnation Zr-Mn-Fe / AI2O3 catalyst, the catalyst high stability, low temperature catalytic activity, at 180 degrees NO removal rate can reach about 98%, is a low temperature SCR catalyst, but the anti Sulfur performance is poor.
1.3 catalyst modification
Ion doping can effectively alleviate the phenomenon of catalyst sintering and promote the improvement of catalyst activity. Gu Tingting and other studies have shown that Ca doping on the activity of SCR catalyst has some inhibitory effect, but Ca doped at the same time inhibition of N2O greater inhibition, thus promoting the N2 selectivity of the catalyst, so Ca doping on the overall performance of the catalyst Good help. Phil et al. Found that appropriate doping of Se, Sb, Cu and S favors the improvement of the catalytic activity of V2O5 / TiO2. At 150-400 degrees Celsius, 2% Sb-doped V2O5 / TiO2 catalyst has the best activity with Sb- The miscellaneous catalysts have relatively better sulfur resistance in the presence of water. Choi and other studies have found that Cu doping under low temperature conditions can improve the zeolite catalyst activity as a carrier. Jiang Boqiong doped Fe and other Mn / Tio2 and found that the role of Fe catalyst, 90 degrees, the removal rate of NO can reach 90%.
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