Experimental study on low temperature NO reduction and Hg⁰ removal of activated carbon loaded by Mn/Fe oxides

Mn-Fe/HAC carbon-based catalysts was prepared by equivalent-volume impregnation with coconut shell activated carbon as carrier and Mn(NO₃)₂ and Fe(NO₃)₃·9H₂O as active components. NO reduction and Hg⁰ 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 (O₂, CO, Hg⁰ and SO₂) on NO reduction and Hg⁰ removal were analyzed. The mechanisms of NO reduction and Hg⁰ removal over carbon-based catalysts were discussed based on the results of N₂ adsorption-desorption, NH₃-TPD, H₂-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 Hg⁰ removal efficiency of carbon-based catalysts modified by Fe/Mn oxides is basically stable at 100% at 100−220 ℃. NO removal efficiency decreases and Hg⁰ removal efficiency is almost stable with increasing GHSV. A low NO removal efficiency of about 50% was obtained in absence of O₂, however, high NO removal efficiency of more than 95% was present in the presence of more than 6% O₂ in flue gas. Hg⁰ concentration has little effect on NO reduction of carbon-based catalyst modified by Mn/Fe, CO has a certain inhibitory effect, while high concentration SO₂ has a significant inhibitory effect, and Mn/Fe co-loaded carbon-based catalyst improves tolerance to SO₂. The NO removal efficiency of 7Mn0.5Fe/HAC can reach more than 80% at 180 ℃, 150 μL/L SO₂. 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 N₂. However, Hg⁰ removal follows L-H mechanism, i.e., Hg⁰ is first absorbed on the active site and forms absorbed Hg⁰, then reacts with reactive oxygen species and absorbed NO₂ and SO₂ to form HgO, Hg(NO₃)₂ and HgSO₄, respectively.