Mercury Emission Measurement at a CFB Plant

Abstract

In response to pending regulation to control mercury emissions in the United States and Canada, several projects have been conducted to perform accurate mass balances at pulverized coal (pc)-fired utilities. Part of the mercury mass balance always includes total gaseous mercury as well as a determination of the speciation of the mercury emissions and a concentration bound to the particulate matter. This information then becomes useful in applying mercury control strategies, since the elemental mercury has traditionally been difficult to control by most technologies. In this instance, oxidation technologies have proven most beneficial for increased capture.Despite many years of mercury measurement and control projects at pc-fired units, far less work has been done on circulating fluidized-bed (CFB) units, which are able to combust a variety of feedstocks, including cofiring coal with biomass. Indeed, these units have proven to be more problematic because it is very difficult to obtain a reliable mercury mass balance. These units tend to have very different temperature profiles than pc-fired utility boilers. The flexibility of CFB units also tends to be an issue when a mercury balance is determined, since the mercury inputs to the system come from the bed material and a variety of fuels, which can have quite variable chemistry, especially for mercury. In addition, as an integral part of the CFB operation, the system employs a feedback loop to circulate the bed material through the combustor and the solids collection system (the primary cyclone), thereby subjecting particulate-bound metals to higher temperatures again. Despite these issues, CFB boilers generally emit very little mercury and show good native capture.The Energy & Environmental Research Center is carrying out this project for Metso Power in order to characterize the fate of mercury across the unit at Rosebud Plant, an industrial user of CFB technology from Metso. Appropriate solids were collected, and flue gas samples were obtained using the Ontario Hydro method, mercury continuous emission monitors, and sorbent trap methods. In addition, chlorine and fluorine were determined for solids and in the flue gas stream.Results of this project have indicated a very good mercury mass balance for Rosebud Plant, indicating 105 ±19%, which is well within acceptable limits. The mercury flow through the system was shown to be primarily in with the coal and out with the flue gas, which falls outside of the norm for CFB boilers.

A Highly Resolved Mercury Emission Inventory of Chinese Coal-Fired Power Plants

“…The total capacity of CFB boilers has grown to approximately 110 GWe, and a CFB unit with the largest installed capacity of 600 MW has begun trial operation in Sichuan . In-furnace desulphurization by spraying calcium and a flue-gas electrostatic precipitator is generally applied in CFB boilers to control SO 2 and particulate matter (PM) pollution. ,, …”

“…24 In-furnace desulphurization by spraying calcium and a flue-gas electrostatic precipitator is generally applied in CFB boilers to control SO 2 and particulate matter (PM) pollution. 41,51,52 Different types of APCDs show different performances in the add-on synergistic removal of mercury from coal-fired flue gas. 15,32 ESP or FF is applied as a dust collector, and FF achieves more stable and higher performance regardless of coal type, resulting in greater mercury emission reduction.…”