EXPERIMENTAL AND THEORETICAL INVESTIGATIONS OF HETEROGENEOUS MERCURY REACTIVITY IN COAL-FIRED FLUE GAS

Coal-fired power plants are the major source of worldwide mercury emissions. Mercury in coal is vaporized into the gas-phase through coal combustion processes. This elemental form can be oxidized by subsequent reactions with other gaseous components (homogeneous) and solid materials (heterogeneous) in coal-fired flue gases. While the oxidized form in is readily controlled by the interaction with fly ash and co-benefit of existing sulfur dioxide controls, elemental mercury is hardly controlled without the application of a specific control method. Therefore, it is important to understand mercury reaction mechanisms to predict the levels of mercury emissions from coal-fired power plants and determine the best applicable control technologies.

Experimental studies have been conducted using a bench-scale packed-bed system to further understand heterogeneous mercury reaction mechanisms. Methane is combusted in a tubular burner and mixed with elemental mercury and halogen (chlorine) to simulate the flue gas environment. Flue gas species (NOx, SOx, etc.) are introduced into the reactor to understand their effect on mercury oxidation/adsorption. Carbon-based sorbent materials (powder, fibers, functionalized graphite, etc.) before and after simulated flue gas exposure are analyzed using X-ray photoelectron and X-ray absorption spectroscopy characterization techniques.

Mercury speciation is conducted by direct measurement using a custom-built electron ionization quadrupole mass spectrometer. A benefit of employing a mass spectrometer is, unlike traditional impinger methods, the oxidized forms can be isolated and individually identified because it separates the products based on their mass-to-charge ratio. By directly measuring mercury species accurately, one can determine the actual extent of mercury oxidation in the flue gas, which will aid in developing mercury control technologies. Finally, theoretical calculations have been carried out based upon density functional theory. These calculations are performed in parallel to the surface characterization experiments to elucidate the surface adsorption/oxidation mechanism of mercury on carbon-based surfaces.