Relationship Between Mercury in Parent Coal and Its Solid Combustion Byproducts: A Single-Mine Coal

摘要

It is believed that coal-fired power plants are a major source of mercury emission. In response to this concern, the U. S. Environmental Protection Agency (EPA) issued regulations on March 15, 2005, outlining how it proposes to regulate mercury's emission from coal-fired power plants. It is estimated that power plants burn coal, which contains approximately 75 tons of mercury. About a third of this, i.e., 25 tons of mercury, is captured by flue gas desulfurization (FGD) and electrostatic precipitator (ESP) systems. However, with the new regulations in place, there will be an added emphasis to enhance the capture of mercury by FGD and ESP systems. This can pose a serious problem for the effective utilization of the scrubber materials and fly ashes. Before technologies can be developed to mitigate the concerns associated with potential re-emission of mercury from coal combustion byproducts, we need to understand how parent coal controls the mercury concentration in its solid combustion byproducts. To understand the mercury transfer mechanisms to solid combustion byproducts, we have adopted an approach where we are monitoring the mercury concentrations in the parent coal, fly ash, bottom ash, and scrubber material from a power plant which burns a high sulfur bituminous coal from a single-mine source. The samples are being collected weekly and are analyzed for their mercury content and behavior. Even though the coal came from a single mine source, we observed large variations in its mercury concentration, i.e., as high as three times the lowest value of ～ 60 μg/kg. Surprisingly, we did not observe any statistically significant correlation between the mercury content in the parent coal and its solid combustion byproducts, raising the possibility that the transfer mechanisms may be non-linear. The mercury analysis of sulfate-rich FGD scrubber material, which in our case is largely FGD gypsum, showed very large variations in the mercury concentration from week-to-week though these variations did not mirror one observed for the parent coal. The variations in mercury concentration were as high as 33% from one week to the next. If these variations hold for other power plants, then it could present additional hurdles for the effective utilization of scrubber materials. It is worth noting that we did not observe a statistically significant variation in mercury concentration in the FGD gypsum for pre- and post-SCR. Similar, though much less pronounced, variations in the mercury concentration were observed for the fly ash. We did not see a statistically significant correlation between LOI content of fly ash and its mercury content. However, it should be mentioned that LOI content for our fly ash samples was < 4 wt%. The data analysis from ESP units showed that the finer the fly ash particle the higher their mercury content.