Oxidation of Mercury under Ultraviolet (UV) Irradiation

Abstract

Oxidation of mercury under ultraviolet (UV) irradiation was carried out in a reaction chamber using both simulated and real flue gases from pilot-scale combustors burning coal. The gases examined included SO2, CO, CO2, NO, CH4, O2, water vapor, alcohol vapor, and their mixtures. The temperature of the reaction chamber was 37.8 °C (100 °F) and 137.8 °C (280 °F). A 10 W low-pressure UV light bulb was placed in the center of the reactor to provide the irradiation, with the main UV irradiation band at 253.7 nm. In addition to the simulated flue gas tests, four slipstream tests were conducted with flue gas diverted from the vertical combustor of CanmetENERGY, burning Saskatchewan lignite, and also with the mini-circulating fluidized-bed combustor (CFBC) of CanmetENERGY, burning Powder River Basin coal. The results showed that oxidation of mercury by 253.7 nm UV irradiation occurred in both synthetic and real flue gas environments, with levels of up to 65.5% of the total mercury being oxidized. The temperature had a strong effect on the oxidation level, which decreased significantly when the temperature was raised to 137.8 °C. Methane also had a positive effect on Hg oxidation. It is hypothesized that this was due to the generation of free hydrogen atoms when CH4 collided with UV-excited Hg to produce free radicals and cause chain reactions. NO could reduce the effectiveness of UV irradiation on mercury oxidation, possibly through the removal of ozone and free oxygen atoms. It was observed experimentally that the residence time of the flue gas in the reaction had little impact on the Hg oxidation level, which implies that Hg oxidation is a fast photochemical process.