Practical Limitation of Mercury Speciation in Simulated Municipal Waste Incinerator Flue Gas

Abstract

Wet scrubbers on municipal waste incinerators can be effective at capturing mercury; however, their effectiveness decreases in systems where rapid thermal quenching of the hot furnace gases are employed. Since wet scrubbers are more effective at capturing soluble mercury species, such as mercuric chloride, than they are at removing elemental mercury, it is proposed that quench rate, or time at temperature, is a factor in the gas phase oxidation of mercury to mercuric chloride. The fate of mercury was studied at different temperatures in a simulated municipal solid waste incinerator exhaust gas. The composition was approximately 10% O2 and CO2, 8% H20 and balance N2, HCl was introduced at two concentrations: 300 and 3000ppmV, and elemental Hg was introduced at a target concentration of 3000 μ/m3. Mercury speciation was determined using EPA method 29. Results indicate that mercury speciation approaches equilibrium at 800°C when sufficient residence time is provided. At temperature below 800°C, conversion of mercury to the oxide is kinetically limited, and the data are fit very well by an Arrhenius correlation with an activation energy of 45.4 k3/mol. The impact of HCl concentration on mercury conversion to the chloride is at least as strong as the effect of temperature. At lower HCI concentrations, coversion is limited by thermodynamics at higher temperatures and by kinetics at lower temperatures.