Towards a Heterogeneous Mechanism for Elemental Mercury Capture by Carbon Surfaces in Combustion Flue Gas

Thursday, November 11, 2010: 9:00 AM
Grand Ballroom J (Marriott Downtown)
Constance L. Senior, Reaction Engineering International, Salt Lake City, UT, Erdem Sasmaz, Energy Resources Engineering, Stanford University, Stanford, CA and Jost O.L. Wendt, Chemical Engineering, ICSE, University of Utah, Salt Lake City, UT

ABSTRACT

Dedicated mercury control technologies can be applied to plants to remove mercury from the flue gas.1  Activated carbon injection has been the most widely tested mercury control technology currently for coal-fired power plants.  Powdered activated carbon can be injected upstream of a particulate control device (electrostatic precipitator or fabric filter) or a spray dryer.  In the simplest application, all the activated carbon is collected with the fly ash generated by the plant.  In some instances, a new fabric filter is added after the plant's existing particulate control device so that the activated carbon that is injected into the flue gas can be collected separately from the fly ash collected in the existing particulate control device.  This approach preserves the economic value of the fly ash by keeping it separate from activated carbon.

Olson et al. conducted fixed-bed experiments to determine the effects of flue gas species such as sulfur dioxide (SO2), hydrogen chloride (HCl), nitrogen oxide (NO) and nitrogen dioxide (NO2) on the elemental mercury (Hg0) capture by a commercially available activated carbon.2 They generated Hg breakthrough curves by combining these flue gas species one at a time with a “baseline” gas and investigated the individual effects of these species on Hg0 sorption.

In this study, breakthrough curves obtained by Olson et al.are modeled using a one-dimensional, transient model and kinetic parameters for each proposed reaction are estimated. Activation energies and pre-exponential factor of each reaction between individual flue gas species and carbon surface were calculated from other studies available in the literature. 

Mechanisms for heterogeneous Hg reactions with carbon surfaces in the presence of HCl and  SO2  gases in mixture simulated flue gas mixture have been investigated.   Breakthrough curves obtained in the case of HCl and SO2 alone match the experiments well. Furthermore, with the combination of SO2-HCl gases, the proposed reaction mechanisms are also found to predict the experimental breakthrough curves with a good accuracy.  More work has to be done to understand the interaction of NO and NO2 on carbon surfaces.

Rate constants found in this study strictly depend on the active site concentration on the carbon surface. Therefore, definition and calculation of site concentration will change the rate constants

REFERENCES

  1. Feeley, T.J. III; Jones, A.P. An Update on DOE/NETL's Mercury Control Technology Field Testing Program.  DOE National Energy Technology Laboratory, Pittsburgh, PA, July, 2008, http://www.netl.doe.gov/technologies/coalpower/ewr/mercury/pubs/Updated%20netl%20Hg%20program%20white%20paper%20FINAL%20July2008.pdf
  2. Miller, S.J.; Dunham, G. E.; Olson, E.S. Mercury Sorbent Development for Coal-Fired Boilers, Air Quality Conference, Dec 1-4, 1998.

 


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