Under the Clean Air Mercury Rule (CAMR) issued by the U.S. Environmental Protection Agency in March 2005, the estimated 48 tons of mercury (Hg) emitted annually by coal-fired power stations nationwide must fall to 34 tons by 2010 and to 15 tons by 2018. A handful of states have already mandated shorter time frames, deeper reductions—or both—for their Hg emissions.
The U.S. Department of Energy's National Energy Technology Laboratory, in collaboration with EPRI and several prominent utilities, has been conducting a comprehensive field-testing program to evaluate available Hg control options. Test data from more than 40 full-scale flue gas cleaning systems for an assortment of Hg control technologies are already available at www.netl.doe.gov/technologies/coalpower/ewr/mercury/index.html.
The program also has tested the so-called "inherent Hg removal" of particulate collection devices (PCDs) and scrubbers. Finally, most utility companies in Midwest, North Central, Mid-Atlantic, and Southeast states have recorded their own internal Hg test data. All these data can help utilities assess the performance and costs of available Hg control options and their compatibility with various coals and gas cleaning system configurations.
With so much data available, we might even expect someone to have developed engineering models that can predict the Hg emissions from a particular plant burning coal with specific properties accurately enough for compliance planning. Certainly, many statistical methods have been used to sort out the underlying factors that affect Hg emissions. But none has yet been able to deliver the level of quantitative accuracy that utility executives need to ensure regulatory compliance. Reason: A bewildering array of factors determines whether Hg will be collected on flyash, sorbents, or an SO2 scrubber's slurry, or whether it will exit the stack.
Mercury species
To manage Hg emissions, one needs to know the chemical form or "speciation" of Hg as it enters various air pollution control devices in a flue gas cleaning system. Mercury is present in flue gas in three forms:
Elemental (Hg0) vapor in the flue gas
Oxidized components (Hg2+) in the flue gas
Attached to unburned carbon (UBC) or other components of flyash (Hg-P)
The speciation is critical because although Hg2+ is highly water-soluble and therefore easily retained in wet scrubber solutions, Hg0 is not soluble at all. Both Hg0 and Hg2+ vapors may become attached to flyash or sorbents to form Hg-P, and any kind of PCD collects essentially all the Hg-P. The problem is that the proportions of Hg0, Hg2+, and Hg-P often are very different for different fuels burned by plants with the same downstream cleanup configuration, and just as different for the same fuel at plants with different cleaning systems.
There are many instances in the testing literature of variations in Hg speciation in what appeared to be the same fuel in the same cleaning configuration on different days in the same test campaign. Since these variations have confounded statistical attempts to predict Hg emissions, they are worth exploring in greater detail.
Path of least resistance
The clearest perspective on Hg removal comes from recognizing Hg speciation as a marker for the various chemical reactions that transform mercury as it passes through a gas cleaning system (Figure 1). The starting point for this chemistry is very well defined because all the Hg fed into the furnace with the fuel will be present as Hg0 at the furnace exit. But after the flue gas is cooled by an economizer, the Hg0 starts to oxidize into Hg2+ via reactions in the flue gas, and does so at an even faster rate via reactions on suspended particles of UBC.
1. Mercury lifecycle. Hg speciation changes from pure Hg0 vapor at the furnace exit to changing mixtures of Hg0, Hg2+, and Hg-P, depending on the levels of Cl and unburned carbon, whether an SCR system is present, and many other cleaning conditions. Source: Niksa Energy Associates LLC
Hg0 oxidizes even faster along the catalyst in a selective catalytic reduction (SCR) system. By the time the flue gas leaves the air preheater, it is cool enough for Hg0 and Hg2+ to bind to UBC or to any sorbents injected to recover it as Hg-P in a downstream PCD. The mixture of Hg0 and Hg2+ leaving the PCD will be stripped of almost all its Hg2+ by a wet scrubber. Therefore, most of the Hg that exits the stack will be Hg0 vapor whenever a scrubber is present.
Hg chemistry is governed by changes in temperature through the cleaning system, the residence time of its various devices, and the concentrations of the participating species. But none of these parameters is the same for different gas cleaning systems, or even for the same system on different days. As a result, the otherwise confounding variations in Hg speciation measurements are due mostly to differences in the Hg chemical environment that naturally arise among utility gas cleaning systems.
Continue to this link:
http://www.powermag.com/environmental/Use-predictive-techniques-to-guide-your-mercury-compliance-strategy_214_p2.html
http://www.powermag.com/environmental/Use-predictive-techniques-to-guide-your-mercury-compliance-strategy_214_p3.html
http://www.powermag.com/environmental/Use-predictive-techniques-to-guide-your-mercury-compliance-strategy_214_p4.html
Also visit :
http://anto-hendarto.blogspot.com/2013/03/mercury-removal-technology.html
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