Effects of Chlorine Addition on Nitrogen Oxide Reduction and Mercury Oxidation over Selective Catalytic Reduction Catalysts

Abstract

The effect of chlorine on mercury oxidation and nitrogen oxides (NO x ) reduction over selective catalytic reduction (SCR) catalysts was investigated in this study. Commercial SCR catalysts achieved a high Hg 0 oxidation efficiency when Cl 2 was sprayed into the flue gas. Results indicated that an appropriate concentration of Cl 2 was found to promote NO x reduction and Hg 0 oxidation significantly. An optimal concentration of Cl 2 (25 ppm) was found to significantly promote NO x reduction and Hg 0 oxidation. Moreover, we studied the effects of Cl 2 on NO x reduction and Hg 0 oxidation over SCR catalysts under different concentrations of SO 2 . The SO 2 poisoning effect was decreased by Cl 2 when the SO 2 concentration was low (below 1500 ppm). However, sulfate gradually covered the catalyst surface over time during the reaction, which limited the impact of Cl 2 . Finally, different sulfur-poisoned catalysts were examined in the presence of Cl 2 . The NO x reduction and Hg 0 oxidation performances of sulfate-poisoned catalysts improved when Cl 2 was added to the flue gas. Mechanisms for NO x reduction and Hg 0 oxidation over fresh catalysts and sulfate-poisoned catalysts in the presence of Cl 2 were proposed in this study. The mechanism of Cl 2 -influenced NO x reduction was similar to that for the NH 3 -SCR process. With Cl 2 in the flue gas, the number of Brønsted active sites increased, which improved catalytic activity. Furthermore, Cl 2 reoxidized V 4+ –OH to V 5+ =O and caused the NH 3 -SCR process to operate continuously. The Langmuir–Hinshelwood mechanism was followed for Hg 0 oxidation by SCR catalysts when Cl 2 was in the flue gas. Cl 2 increased the number of Lewis active sites, and catalytic activity increased. Hg 0 adsorbed on the surface of the catalysts and was then oxidized to HgCl 2 . Adding Cl 2 to the flue gas increased the strength and number of acid sites on sulfate-poisoned catalysts.

In situ construction of halogen MOF bifunctional catalysts for synergistic removal of NO and Hg0

“…Vanadiumtitanium catalysts are widely employed due to their excellent catalytic performance. [3][4][5] However, these catalysts demonstrate poor activity at low temperatures (<200 °C), restricting their practical application in low-temperature flue gas NO x removal (low-temperature NH 3 -SCR). 6 Additionally, for Hg 0 , the SCR denitration catalyst struggles to efficiently remove the element while achieving NO removal, requiring HCl gas for Hg 0 oxidation.…”

Sulfur Dioxide Promoted Mercury Fast Deposition over a Selenite-Chloride-Induced Surface from Wet Flue Gas

“…11,12 Generally, SO 2 can inhibit the Hg 0 removal through competitive adsorption, such as consuming adsorbed oxygen and reducing active high valence metal sites, resulting in the surface sulfation. 13,14 For example, the Hg 0 oxidation efficiency of the V 2 O 5 -WO 3 /TiO 2 catalyst gradually decreased from ∼88 to ∼45% when the SO 2 concentration increased from 500 to 2500 ppm; 15 the Hg 0 adsorption efficiency of MnO 2 /CeO 2 -MnO 2 dropped from 89 to 65% with the addition of 500 ppm SO 2 , and it further decreased to 43% in the coexistence of SO 2 and H 2 O. 16 Although sulfur-based sorbents exhibited better SO 2 resistance than oxide-based sorbents, 11 the poisoning effect of SO 2 cannot be avoided because the concentration of SO 2 in flue gas is 4−5 orders of magnitude higher than the mercury concentration.…”

“…Among different mercury species in flue gases, gaseous elemental mercury (Hg 0 ) was regarded as one big challenge due to its insolubility and volatility, making it difficult to be removed by the current air pollutant control devices . Various technologies have been developed for gaseous mercury emission control in recent years. , With the assistance of catalytic oxidation or adsorption method, changing Hg 0 to oxidized mercury (Hg 2+ ) or particle-boned mercury (Hg p ) can realize the efficient capture of mercury in the wet-absorption unit or particle removal devices. ,, However, sulfur dioxide (SO 2 ) and water vapor (H 2 O), the important components in industrial flue gases, typically play adverse effects on Hg 0 removal, especially when they coexisted in the flue gas. , Generally, SO 2 can inhibit the Hg 0 removal through competitive adsorption, such as consuming adsorbed oxygen and reducing active high valence metal sites, resulting in the surface sulfation. , For example, the Hg 0 oxidation efficiency of the V 2 O 5 -WO 3 /TiO 2 catalyst gradually decreased from ∼88 to ∼45% when the SO 2 concentration increased from 500 to 2500 ppm; the Hg 0 adsorption efficiency of MnO 2 /CeO 2 -MnO 2 dropped from 89 to 65% with the addition of 500 ppm SO 2 , and it further decreased to 43% in the coexistence of SO 2 and H 2 O . Although sulfur-based sorbents exhibited better SO 2 resistance than oxide-based sorbents, the poisoning effect of SO 2 cannot be avoided because the concentration of SO 2 in flue gas is 4–5 orders of magnitude higher than the mercury concentration.…”