Reaction Behavior and Cost-Effectiveness of Halogen Radicals in Hg⁰ Removal: Performance, Kinetics, and Mechanism

Abstract

We used ultraviolet and heat (UV-heat) to costimulate HCl, HBr, and HI to yield Cl • , Br • , and I • for Hg 0 removal. Electron spin resonance (ESR) confirmed the radical formation. Cl • /Br • /I • were found to play the leading roles in Hg 0 removal, Br 2 /I 2 formed from the heat catalysis of HBr/HI also played key roles. The costs for removing 80% Hg 0 with UVheat/HCl, UV-heat/HBr, and UV-heat/HI were calculated to be 0.03, 2.4, and 36.9 USD/lb-Hg 0 , respectively. Rising heat catalysis temperature favored the Hg 0 removal. Declining flue gas residence time and adding NO and SO 2 impaired the Hg 0 removal by Cl • and Br • but did not include I • . Kinetics analyses indicated that UV increased the Hg 0 removal rate by 15.5 times for HCl, by 1.3 times for HBr, and by 0.2 time for HI. Radical quenching test with tertiary butanol (TBA) and CO 2 indicated that Cl • and Cl 2•− contributed almost 100% and <5% to Hg 0 removal, respectively. Density functional theory (DFT) calculation gives the reactivity order of oxidizing species toward Hg 0 , that is, Cl • > Br • > I • > I 2 > Br 2 > Cl 2 . The environmetnal implications were proposed finally.

Spontaneous adsorption–oxidation of gaseous elemental mercury via a conjugated unit –NH+˙–Cl*: creation and mechanisms

“…Compared with oxidized mercury (Hg 2+ ) and particulate mercury (Hg p ), elemental mercury (Hg 0 ) has received the most concern since it is difficult to remove using the current air pollution control devices. 4 In the past two decades, various demercuration technologies have been developed to reduce Hg 0 emissions. To date, activated carbon injection (ACI) is still regarded as the best commercially mature technology for mitigating Hg 0 emission from coal combustion processes.…”

“…However, the regeneration process of these sorbents needs high temperature and the performance of the regenerated sorbents is not so good, which makes them not suitable for continuous reuse and so they nally turn into hazardous waste, resulting in a high secondary disposal cost. On the other hand, the oxidation method is considered a good alternative for Hg 0 removal, mainly including catalytic oxidation [20][21][22][23] and advanced oxidation processes, 4,24,25 but an extra absorption or adsorption posttreatment procedure is needed, which requires extra energy and material consumption.…”

“…Compared with oxidized mercury (Hg 2+ ) and particulate mercury (Hg p ), elemental mercury (Hg 0 ) has received the most concern since it is difficult to remove using the current air pollution control devices. 4…”

Oxidation Absorption of Hg0 in the Gas Phase Using a Double Catalyzers–Double Oxidants Coactivation Technology

“…In the past ten years, multifarious wet free-radical AOTs, which mainly comprises photochemical oxidation, electrochemical oxidation, metal ion-activated peroxide oxidation, thermal activation peroxide oxidation, metal oxide-activated peroxide oxidation, and so forth have been developed widely for oxidizing Hg 0 in flue gas. − Among these potential candidates, these Fenton oxidation technique demonstrates a good development and industrial application prospect due to a simpler system and smaller device requirement. ,, However, the traditional Fenton oxidation technique has low yield in free radicals owing to a low Fe 2+ /Fe 3+ cycle rate, severe side reactions, and low utilization rate in free radicals, thereby causing low pollutant treatment capacity and high reagent consumption cost. ,− Thus, exploring a new, simple, and effective method to improve these shortcomings in the traditional Fenton activation system is highly desired in this area.…”

Disordered MoS2 Nanosheets with Widened Interlayer Spacing for Elemental Mercury Adsorption from Nonferrous Smelting Flue Gas

“…In general, mercury exists in three forms in NFS flue gas: elemental mercury (Hg 0 ), oxidized mercury (Hg 2+ ), and particle-bound mercury (Hg p ). − Hg 2+ and Hg p can be effectively removed by current air pollution control devices, whereas the insolubility and high volatility make Hg 0 difficult to remove. − The use of sorbents is considered as one of the most promising techniques for Hg 0 capture and recovery. , However, the concentration of SO 2 in NFS flue gas is generally about 6% . Owing to the competing adsorption, SO 2 decreases significantly the Hg 0 adsorption ability of various sorbents. , Metal sulfides are effective in Hg 0 adsorption and have a certain resistance to the poisoning of low concentration SO 2 .…”