From: 	<Westlin.Peter@epamail.epa.gov>
To:	<Roy.Oommen@erg.com>
CC:	<Shrager.Brian@epamail.epa.gov>, <Segall.Robin@epamail.epa.gov>, <Mcalister.Gary@epamail.epa.gov>, <Parker.Barrett@epa.gov>, <Myers.Ron@epamail.epa.gov>, <Dewees.Jason@epamail.epa.gov>, <Grimley.William@epamail.epa.gov>, <agnew.rachel@epa.gov>, <driscoll.tom@epa.gov>, <Ryan.Jeff@epamail.epa.gov>, <Eddinger.Jim@epamail.epa.gov>, <Maxwell.Bill@epamail.epa.gov>
Date: 	8/10/2010 4:56 PM
Subject: 	Mercury control via carbon injection and PM capture


Roy:

As per your request, I have prepared this note to summarize some basic
points regarding the effectiveness of various PM control measures
associated with mercury control and the use of activated carbon
injection.  I hope that this strikes the right tone for your needs.

Activated carbon in the sorbent mix is usually in a powder form of
particle sizes ranging from about 15 um to about 90 um.  A common
practice for fossil fuel mercury control is to inject the activated
carbon sorbent into the flue gas downstream of the combustion process.
The sorbent mixes with the gas stream and the mercury in the gas will
adsorb on the carbon particles and flow downstream towards a PM control
device.  This is called in-flight capture.  The PM control device will
collect the sorbent particles with the adsorbed mercury, along with the
other PM emissions, and there are several types of PM control measures
that can apply.

The type of particulate control equipment is a key parameter defining
both the amount of sorbent that is required and the ultimate amount of
mercury removal.  There are three basic types of PM capture devices used
on fossil fuel-fired processes.  Perhaps the most common type of PM
control measure used at electric utilities is the electrostatic
precipitator or ESP.  The ESP provides an electric charge to particles
in a slow moving gas stream and collects those particles on grounded
plates from which the collected dust is periodically removed to hoppers.
The collection efficiency of ESP depends on particle size and energy
input (i.e., secondary electrical power).  The ESP can be highly
efficient (99+ percent) at collecting relatively large particles (>3
microns) but less so (97-98 percent) for submicron particles.

Fabric filters, a second type of PM control measure, remove particles
from the gas stream in much the same way a home vacuum cleaner works.
The exhaust gas stream passes through a filter, the collected particles
form a filter cake, and the collected dust is removed periodically to a
hopper.  FFs can provide very high PM control efficiencies (99.9+
percent) for virtually all particle sizes (sub micron to >10 microns).
The third type of PM control device is the high energy wet scrubber
commonly called the Venturi scrubber in which water droplets collide
with and collect particles in a high velocity gas stream.  Water
droplets are then removed mechanically.  The PM removal effectiveness of
a Venturi scrubber is also dependent on particle size and the energy
input (gas velocity and liquid flow rate).  A Venturi can remove 98 to
99 percent of large (>10 microns) particles but collection efficiency
can decrease to 95 percent or less for small and submicron particles.

Given the activated carbon particle sizes cited above (>15 microns), any
of the particulate control measures should be fairly effective at
collecting mercury adsorbed on the sorbent particles.  In addition to
the generally higher control efficiencies, the collection process
inherent to FFs provides an advantage over other types of control
devices.  That is, the FF provides a second opportunity for mercury to
adsorb on to carbon sorbent as exhaust gases pass through the filter
cake (i.e., with the collected sorbent particles) increasing the contact
between sorbent and mercury in the gas.  ESPs and Venturi scrubbers
offer much less opportunity for second contact between mercury and
sorbent once the PM is captured.  There can be a factor of ten
difference in carbon feed rates required for ESPs and Venturi scrubbers
as compared to fabric filters to achieve similar mercury removal rates.

I have cited a few references with much more detailed information about
activated carbon injection for mercury control and the comparisons
between different types of PM control devices.  I hope that this
information is helpful.  I have copied others probably more expert than
I on this note and would expect corrections, if needed.

http://www.icac.com/files/public/MEGA03_82_Hg.pdf
http://www.reaction-eng.com/downloads/Senior_AQIV.pdf
http://www.epa.gov/appcdwww/aptb/EPA600R03110.pdf
http://www.ccsd.biz/PSE_Handbook/5/6/5/index.cfm

Peter Westlin
Measurement Policy Group
OAQPS, SPPD
919-541-1058