From: Westlin.Peter@epamail.epa.gov

Sent: 8/30/2005 9:47:16 AM

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

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