球形氧化铝负载锰基双金属氧化物催化剂催化臭氧深度氧化NO

在工业锅炉烟气处理领域,由于锅炉容量低,烟气温度往往无法满足传统选择性催化还原(SCR)所需温度窗口.工业锅炉烟气成分的复杂性也给氮氧化物治理带来了严峻考验.臭氧深度氧化NO结合湿法洗涤同时脱硫脱硝技术具有独特的应用优势.传统臭氧氧化技术中,NO被臭氧氧化为NO2,进而在脱硫塔中实现一体化脱硫脱硝.但由于NO2溶解度相对较低,需要在脱硫浆液中加入添加剂提高脱硝效率,造成运行成本增加.NO经臭氧深度氧化后,NO2进一步转化为溶解度高的N2O5,传统脱硫石膏浆液即可实现高效吸收N2O5,从而有效提高氮氧化物吸收效率.但由于N2O5生成反应速率低,深度氧化存在臭氧投入量大、反应时间长及臭氧残留多的缺点.臭氧耦合催化剂深度氧化NO可有效解决以上问题.首先,本文采用溶胶-凝胶法合成一系列单金属氧化物(Mn,Co,Ce,Fe,Cu,Cr)作为臭氧深度氧化NO的催化剂.结果发现锰氧化物表现出最高的催化活性,在70oC下,O3/NO摩尔比为2.0时经过0.12 s的反应时间催化剂即可实现80%以上的转化效率.但根据N2O5生成的总包反应(2NO+3O3=N2O5+3O2)可以看出,O3/NO摩尔比为1.5时即可实现N2O5的完全转化.由于催化臭氧氧化反应温度较低,中间产物在催化剂表面聚集,占据大量活性位,进而导致无法实现1.5摩尔比的高效转化.通过采用球形氧化铝作为载体,避免粉末状催化剂紧凑型布置,增加换热面积,可有效降低催化剂表面中间产物聚集;同时延长了气体与催化剂的接触时间,提高反应效率.在球形氧化铝载体上负载锰基双金属氧化物(Ce-Mn,Fe-M,Cr-Mn,Cu-Mn和Co-Mn),在初始NO浓度为410 mg/m3,反应温度100oC,O3/NO摩尔比1.5,催化反应时间0.12 s的工况下,催化剂最终实现95%(Fe-Mn)和88%(Ce-Mn)的转化效率,剩余NO和NO2的浓度分别低于20 mg/m3(Fe-Mn)和50 mg/m3(Ce-Mn),臭氧残留浓度低于25 mg/m3.同负载单一锰氧化物(83%转化率)相比,双金属氧化物进一步提高了N2O5生成效率.因此,臭氧耦合催化剂深度氧化NO结合湿法吸收在工业锅炉超低排放(NOx<50 mg/m3)领域具有广泛应用前景.通过XRD、氮气吸附、H2-TPR和XPS等手段研究了催化剂的晶体结构、孔结构参数、氧化还原性能和表面原子价态.催化臭氧深度氧化NO主要与催化剂对臭氧的分解性能和对NO的氧化性能有关.较大的比表面积和孔容有利于催化剂的吸附.氧空位有利于臭氧的吸附和分解.Mn4+和Mn3+的均衡分布既有利于NO的吸附氧化又有利于臭氧的吸附分解,最终提高了N2O5生成效率.

Promotional effect of spherical alumina loading with manganese-based bimetallic oxides on nitric-oxide deep oxidation by ozone

Nitric oxide (NO) deep oxidation to dinitrogen pentoxide (N2O5) by ozone together with wet scrub-bing has become a promising technology for nitrogen-oxide (NOx) removal in industrial boilers. Catalysts were introduced to enhance the N2O5 formation rate with less ozone injection and leakage. A series of monometallic catalysts (manganese, cobalt, cerium, iron, copper, and chromium) as pre-pared by the sol-gel method were tested. The manganese oxides achieved an almost 80%conver-sion efficiency at an ozone (O3)/NO molar ratio of 2.0 in 0.12 s. The crystalline structure and porous parameters were determined. The thermodynamic reaction threshold of NO conversion to N2O5 is oxidation with an O3/NO molar ratio of 1.5. Spherical alumina was selected as the support to achieve the threshold, which was believed to improve the catalytic activity by increasing the surface area and the gas-solid contact time. Based on the manganese oxides, cerium, iron, chromium, cop-per, and cobalt were introduced as promoters. Cerium and iron improved the deep-oxidation effi-ciency compared with manganese/spherical alumina, with less than 50 mg/m3 of outlet NO+nitro-gen oxide, and less than 25 mg/m3 of residual ozone at an O3/NO molar ratio of 1.5. The other three metal oxides inhibited catalytic activity. X-ray diffraction, nitrogen adsorption, hydrogen tempera-ture-programmed reduction, and X-ray photoelectron spectroscopy results indicate that the cata-lytic activity is affected by the synergistic action of NOx oxidation and ozone decomposition.