Homo- and Heterogeneous Mercury Oxidation in a Bench-Scale Flame-Based Flow Reactor

Abstract

An experimental study of hetero-and homogeneous mercury oxidation chemistry was conducted in a bench-scale flame-based flow reactor with a residence time of 2.9 s. Homogeneous mercury oxidation levels increased with an increasing HCl concentration, with oxidation increasing to 29% at HCl concentrations of 555 parts per million by volume (ppmv). The presence of SO 2 alone also led to Hg oxidation, with approximately 20% oxidation observed at SO 2 concentrations ranging from 100 to 900 ppmv. When both HCl and SO 2 were present, mercury oxidation was enhanced in the presence of SO 2 when the concentration of HCl was 200 ppmv and inhibited when concentration of HCl was 555 ppmv. To examine heterogeneous effects, iron oxide or montmorillonite particles were injected into the post-flame gases of the system. Mercury oxidation by HCl was enhanced at high iron oxide particle concentrations (>100 m α-Fe 2 O 3 2 /m 3 of flue gas) compared to the homogeneous system. When iron oxide particles were injected in the presence of 100À400 ppmv SO 2 without HCl, mercury oxidation was enhanced at particle loadings less than or equal to 1 m α-Fe 2 O 3 2 /m 3 of flue gas and decreased with an increasing particle loading. At a SO 2 concentration of 500 ppmv, mercury oxidation increased with an increasing particle concentration. Without HCl or SO 2 in the system, mercury oxidation in the presence of iron oxide particles alone was found to be negligible. To determine if the increased oxidation by HCl or SO 2 occurred solely because of an increased surface area, montmorillonite particles were injected, and under these conditions, no increase in the extent of mercury oxidation was observed, suggesting that iron oxide particle surfaces are an important contributor to the promotion of mercury oxidation in coal combustion systems.

Gas-Phase Oxidation of Mercury by Bromine and Chlorine in Flue Gas

“…Smith et al 5 obtained similar results in a different natural-gas-fired furnace: homogeneous oxidation of mercury was less than 30% up to HCl concentrations of 500 ppmv. In the experiments by Smith et al, the combustion gas was quenched from the flame temperature to about 25°C.…”

Analysis of Halogen-Mercury Reactions in Flue Gas

“…Laboratory studies show that increasing the chlorine content of combustion exhaust gases results in an increase in the amount of mercury in the exhaust gas in an oxidized form. 1,2,3,4 Utility boiler field-test data show that the chlorine content of the coal has some impact on the speciation of mercury at the inlet to the air pollution control device(s). 5,6 Comparatively little has been published about the effects of bromine on mercury speciation.…”

“…Smith et al 4 observed inhibition or enhancement of mercury oxidation by chlorine when SO 2 was added to a laboratory methane-fired furnace; the effect depended on the concentrations of SO 2 and HCl. There was little effect of SO 2 at 400 ppmv SO 2 , up to 550 ppmv HCl.…”

DFT and Experimental Study on the Mechanism of Elemental Mercury Capture in the Presence of HCl on α-Fe2O3(001)

“…Meng et al reported that with the increase in concentration (0–100 ppm) of HCl, the mercury removal ability of fly ash increased correspondingly . Smith et al also discovered in a bench-scale study that HCl promoted the heterogeneous reaction between HCl and Fe 2 O 3 …”