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Mercury Emissions Control
 In-House R&D

The scrutiny of mercury (Hg) emissions from coal-fired utilities that began with the Clean Air Act Amendments of 1990 (CAAA) resulted in a determination by the U.S. EPA that such emissions should be regulated. A number of techniques for control of mercury emissions from power plants have been evaluated at various scales. One technique that received a great deal of attention by the EPA, utilities, and technology developers was dry sorbent injection upstream of an existing particulate control device.

The in-house, air toxics research effort at NETL consisted of two distinct efforts: the first was aimed at characterizing an existing pilot unit for distribution and fate of hazardous air pollutants, including mercury ; the second was examining sorbents and photochemical oxidation as means for mercury removal from flue gas at laboratory-scale.

The pilot unit, which mimics an actual pulverized coal-powered generation facility, is a 500-lb/hr coal combustion unit that includes a furnace, air preheater, ductwork, and a pulse-jet fabric filter. In the years immediately following the CAAA, tests on this unit evaluated methods then being developed for mercury sampling and speciation. Afterwards, a series of tests were conducted on this unit to initiate operation of a sorbent injection system and obtain results on mercury removal with dry sorbent injection and a pulse-jet baghouse. A low-sulfur (~1%), bituminous coal from the Evergreen mine was used for these tests. This type of coal is burned by utilities that do not have flue gas desulfurization systems. For these utilities, sorbent injection may be the most cost-effective option for control of mercury, depending upon the nature of the emissions regulations.

Following the successful shakedown of the sorbent injection system, testing was conducted to study the effects of sorbent-to-mercury ratio and baghouse temperature on mercury removals, using a commercial activated carbon. Results expanded the previous data base on mercury removal efficiency, mercury speciation, and material balances, and included tests on the effects of humidification and unburned carbon levels. Initial results were based on standardized, wet-chemical methods for measurement of mercury and its species in flue gas. However, alternative methods, including solid sorbent methods and use of continuous analyzers, were evaluated and used to a greater extent in subsequent testing to reduce the time and effort involved.

Recognizing the limitations of the configuration of the existing pilot unit, subsequent work involved the design, fabrication, and installation of systems that allowed an evaluation of sorbent injection options that had expanded applicability to full-scale utilities. These systems included a full-flow electrostatic precipitator (ESP) for particulate control and a slip-stream for injection of sorbent for mercury control in a particulate-free, coal-derived flue gas.

Simultaneously, a laboratory-scale packed-bed reactor system was used to screen sorbents for their capability to remove elemental mercury from various carrier gases. When the carrier gas was argon, an on-line atomic fluorescence spectrophotometer (AFS), used in a continuous mode, monitored the elemental mercury concentration in the reactor inlet and outlet streams of the packed-bed reactor. The mercury concentration in the reactor inlet gas and the reactor temperature were held constant during a test. For flue gas, the capacity was determined off-line by analyzing the spent sorbent with a cold vapor atomic absorption spectrophotometer (CVAAS). The capacities and breakthrough times of several commercially available activated carbons as well as novel sorbents were determined as a function of several parameters. The mechanisms of mercury removal by the sorbents were suggested by combining the results of the packed-bed testing with various analytical results.

Interestingly, photochemical reactions of mercury with various constituents in flue gas produced by burning coal could be an attractive alternative to dry sorbent or wet scrubber-based processes for mercury capture. The homogeneous gas phase photochemical oxidation of elemental mercury using 253.7 nm ultraviolet radiation has been extensively studied. The photochemistry of elemental mercury in simulated flue gases was examined using quartz flow reactors. Simulated flue gases at temperatures between 80oF - 350oF were irradiated with 253.7 nm ultraviolet light. A high level of mercury oxidation occurred at 80oF and 280oF.

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  • For further information on In-House R&D for Mercury Emissions Control, contact Evan Granite, the NETL In-House R&D Project Manager.
 
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