USE OF FLY ASH FRACTIONS FROM WESTERN COALS FOR MERCURY REMOVAL FROM FLUE GAS STREAMS

摘要

Public Service Company of Colorado (PSCo) is under contract to the DOE to investigate the use of activated carbon as a mercury control sorbent in utility flue gas. Most of this project is being conducted by ADA Technologies in a pilot-scale test facility located at the Comanche Station of PSCo. In the course of demonstrating the use of activated carbon for removal of gas-phase mercury from utility boilers, fly ash present in the Comanche 2 duct was found to adsorb mercury, removing a majority of the total mercury present in the flue gas supplied to the pilot facility. This affinity for mercury was thought to be unusual, and had not been reported at other power plants. As part of the project, ADA has also performed full-scale sampling to quantify mercury removal across particulate control equipment at other PSCo generating plants. Several were found to have very low emissions of mercury. In these low-emission plants, vapor-phase mercury is captured on the ash, which is subsequently collected in baghouses. Given these results, tests were conducted at the Comanche pilot to determine which PSCo fly ashes work well for mercury removal, and at what rates they must be injected. Results from these tests indicated that fly ash from two PSCo plants was particularly effective in the removal of mercury from flue gas. However, to achieve mercury removal rates greater than 80%, the fly ash was injected at ratios much higher than required of activated carbon for similar removal levels. To identify the fraction of the sorbent fly ash that was most effective in removing mercury, a sample was subjected to a size separation followed by analysis for mercury. The smallest size fraction was seen to capture a disproportionate share of mercury. Laboratory tests were run to quantify the ability of a size-separated fly ash fraction to capture mercury from a simulated flue gas.