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

We used ultraviolet and heat (UV-heat) to costimulate HCl, HBr, and HI to yield Cl-center dot, Br-center dot, and I-center dot for Hg-0 removal. Electron spin resonance (ESR) confirmed the radical formation. Cl-center dot/Br-center dot/I-center dot 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 UV-heat/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 SO2 impaired the Hg-0 removal by Cl-center dot and Br-center dot but did not include I-center dot. 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 CO2 indicated that Cl-center dot and Cl-2(center dot-) 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-center dot > Br-center dot > I-center dot > I-2 > Br-2 > Cl-2. The environmetnal implications were proposed finally.

Automated summary

Ultraviolet and heat combined with HCl, HBr, and HI were used to remove Hg-0, with Cl·/Br·/I· and Br2/I2 playing key roles. Costs varied depending on the compounds used, while increasing heat catalysis temperature favored removal efficiency.