(637a) Effects of Oxy-Fuel Firing On Gas-Phase Mercury Oxidation by Halogens

The extents of gas-phase mercury oxidation were measured in a quartz-lined, bench-scale reactor. The reactor is fired with methane, and for these experiments the combustion air was replaced with a mixture of 27% O₂ and 73% CO₂. Post-combustion quench rates of 210 and 440 K/s were used. Chlorine concentrations of 100 and 400 ppm (as HCl) and bromine concentrations of 25 and 50 ppm (as HBr) were tested. At all chlorine and bromine levels, the extent of mercury oxidation was 80 percent or higher. The effects of quench rate were negligible. Previous results obtained with chlorine, using air firing, showed levels of oxidation that were consistently less than 10 percent.

Possible reasons for the disagreement may be that the high CO₂ levels are interfering with the chemistry of the sample conditioning system or altering the kinetics of the gas-phase oxidation process. The later could be due to the higher efficiency of CO₂, relative to N₂, in the third-body recombination reaction, Hg + Cl + M = HgCl + M.

The high levels of mercury oxidation by chlorine under oxy-fuel combustion conditions have practical significant. Elemental mercury degrades the aluminum cooling equipment used in the cryogenic separation and compression of oxy-fuel flue gases.