In-house R&D
In-House R&D

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

The in-house, air toxics research effort at NETL consists of two distinct efforts. The first is aimed at characterizing an existing pilot unit for distribution and fate of hazardous air pollutants, including mercury. The second is 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 (approximately 1 percent), 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 that might be enacted.

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 removal, using a commercial activated carbon. Recent results expanded the previous data base on mercury removal efficiency, mercury speciation, and material balances, and include tests on the effects of humidification and unburned carbon levels. The results to date have all been 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, have also been evaluated and will be used to a greater extent in the future to reduce the time and effort involved in the testing.

Recognizing the limitations of the configuration of the existing pilot unit, current work plans involve the design, fabrication, and installation of systems that will allow an evaluation of sorbent injection options that will have expanded applicability to full-scale utilities. These systems will include a full-flow electrostatic precipitator 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 is used to screen sorbents for their capability to remove elemental mercury from various carrier gases. When the carrier gas is argon, an on-line atomic fluorescence spectrophotometer, used in a continuous mode, monitors 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 are held constant during a test. For flue gas, the capacity is determined off-line by analyzing the spent sorbent with a cold vapor atomic absorption spectrophotometer. 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 are 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 80 oF to 350 oF were irradiated with 253.7 nm ultraviolet light. A high level of mercury oxidation occurred at 80 oF and 280 oF. The implications of photochemical oxidation of mercury with respect to direct ultraviolet irradiation of flue gas for mercury control are being determined.

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