Adsorption of elemental mercury on pulverized coal flyash

Mercury has long been identified as a hazard to human health and the environment. Two sources of mercury in water and soil are municipal waste incinerators and coal-fired power plants.; One method used to remove elemental mercury from flue gas involves the injection of large quantities of pulverized activated carbon. The purpose of this project was to determine whether the unburned carbon that remains in coal flyash could be used as an inexpensive and effective replacement for activated carbon. Bench-scale tests were conducted at conditions representative of those found in the flue gas trains of coal-fired power plants and municipal waste incinerators. Two types of data were obtained: equilibrium data suitable for obtaining adsorption isotherms and breakthrough data suitable for obtaining adsorption kinetics. Temperatures were varied from 121 to 177°C and gas phase mercury concentrations ranged from 0.04 to 11 mg Hg/m3. Four pulverized-coal flyashes, with carbon contents ranging from 2 to 36% by weight, were examined. One commercially prepared activated carbon was also studied.; The equilibrium results indicate that the flyashes can adsorb up to 600 ppmw of mercury and that the Langmuir isotherm describes the equilibrium data. The amount of mercury adsorbed increased with increasing carbon content. Temperature had a tremendous impact on the adsorbed-phase mercury concentration. Mercury capture was also found to decrease with decreasing gas phase mercury concentrations.; Two mathematical models were developed to simulate the removal of elemental mercury in flue gas ducts and in baghouses. Simulation results indicate that a negligible amount of mercury can be adsorbed by a dilute suspension of flyash. Less than 5% of the vapor-phase mercury is removed when an injection rate of 100 g of ash per cubic meter of gas is used at a temperature of 121°C at an initial vapor-phase mercury concentration of 1 mg Hg/m3. The best option for controlling mercury emissions using flyash is to inject the ash in pulses prior to a baghouse. Pulsed injection allows the carbon in the ash to adsorb mercury throughout the entire baghouse collection cycle. Model predictions show that injecting a 32 wt.-percent-carbon flyash at a rate of 1200 g/m3 of gas prior to the baghouse, for the first minute of a 60-min baghouse cycle, will result in the exit concentration staying 50% below an inlet concentration of 1 mg/m3.; Mercury removal using coal flyash is recommended when a pulsed injection system is used in combination with a baghouse. Flyash is not recommended for utilities that use electrostatic precipitators since only a small amount of mercury is removed under these conditions.