Mn/Fe负载活性炭低温脱硝协同脱汞实验研究

Experimental study on low temperature NO reduction and Hg^(0)removal of activated carbon loaded by Mn/Fe oxides

采用等体积浸渍法制备了Mn/Fe负载椰壳活性炭碳基催化剂Mn-Fe/HAC。在固定床实验台上研究了反应温度、体积空速(GHSV)和烟气组分(O_(2)、CO、SO_(2)和Hg^(0))对其脱硝脱汞性能的影响,并结合N_(2)吸附-脱附、NH_(3)-TPD、H_(2)-TPR、Hg-TPD以及瞬态反应测试,分析其脱硝脱汞机理。结果表明,Mn/Fe负载可以明显促进碳基催化剂低温脱硝活性,添加Fe可提高催化剂表面的酸性位点数量和还原能力,促进催化剂活性,进一步拓宽其脱硝温度窗口;7Mn0.5Fe/HAC催化剂在160−220℃下的脱硝效率可达95%,且在100−220℃下Mn和Fe负载碳基催化剂脱汞效率基本稳定在100%。7Mn0.5Fe/HAC催化剂脱硝效率随着GHSV增加而逐渐降低,而脱汞效率则保持稳定。无O_(2)条件下Mn/Fe催化剂脱硝效率较低(约为50%),而当烟气中O_(2)含量大于6%,其脱硝效率可稳定在95%以上。Hg^(0)浓度对Mn/Fe负载碳基催化剂脱硝性能影响不大,CO有一定抑制作用,而高浓度SO_(2)抑制作用较为显著。Mn和Fe共负载可提高抗硫性,在含150μL/L SO_(2)模拟烟气下,7Mn0.5Fe/HAC催化剂在180℃的脱硝效率仍可稳定在80%以上。Mn/Fe负载碳基催化剂脱硝遵循E-R机理,即NH3先吸附于活性位点,再与气态NO反应,最终将NO还原为N_(2);而脱汞遵循L-H机理,即Hg^(0)先吸附于活性位点,形成吸附态Hg^(0),然后与活性氧以及吸附态NO_(2)与SO_(2)反应分别生成HgO、Hg(NO_(3))_(2)和HgSO_(4)。

Mn-Fe/HAC carbon-based catalysts was prepared by equivalent-volume impregnation with coconut shell activated carbon as carrier and Mn(NO_(3))_(2)and Fe(NO_(3))_(3)·9H_(2)O as active components.NO reduction and Hg^(0)removal of carbon-based catalysts was carried out in a fixed-bed reactor.The effects of reaction temperature,gas hourly space velocity(GHSV)and flue gas components(O_(2),CO,Hg^(0)and SO_(2))on NO reduction and Hg^(0)removal were analyzed.The mechanisms of NO reduction and Hg^(0)removal over carbon-based catalysts were discussed based on the results of N_(2)adsorption-desorption,NH_(3)-TPD,H_(2)-TPR,Hg-TPD and transient response experiment.The obtained results indicate that NO reduction over carbon-based catalyst at low temperature can be enhanced significantly by Mn/Fe load,and Fe addition can increase the number of acid sites and the reducing capacity,which can improve NO reduction activity and further widen its temperature window on NO reduction.NO removal efficiency of 7Mn0.5Fe/HAC can reach 95%at 160−220℃,and Hg^(0)removal efficiency of carbon-based catalysts modified by Fe/Mn oxides is basically stable at 100%at 100−220℃.NO removal efficiency decreases and Hg^(0)removal efficiency is almost stable with increasing GHSV.A low NO removal efficiency of about 50%was obtained in absence of O_(2),however,high NO removal efficiency of more than 95%was present in the presence of more than 6%O_(2)in flue gas.Hg^(0)concentration has little effect on NO reduction of carbon-based catalyst modified by Mn/Fe,CO has a certain inhibitory effect,while high concentration SO_(2)has a significant inhibitory effect,and Mn/Fe co-loaded carbon-based catalyst improves tolerance to SO_(2).The NO removal efficiency of 7Mn0.5Fe/HAC can reach more than 80%at 180℃,150μL/L SO_(2).Carbon-based catalyst by loaded Mn/Fe for NO reduction follows E-R mechanism,i.e.,NH3 first adsorbs on the active site,then reacts with gaseous NO,and finally reduces NO to N_(2).However,Hg^(0)removal follows L-H mechanism,i.e.,Hg^(0)is first absorbed on the active site and forms absorbed Hg^(0),then reacts with reactive oxygen species and absorbed NO_(2)and SO_(2)to form HgO,Hg(NO_(3))_(2)and HgSO_(4),respectively.