(589b) Towards a Heterogeneous Mechanism for Elemental Mercury Capture by Carbon Surfaces in Combustion Flue Gas

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.