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Elemental Mercury Capture by Activated Carbon in a Flow Reactor
Shannon D. Serre
Brian K. Gullett
U.S. Environmental Protection Agency
National Risk Management Research Laboratory
Air Pollution Prevention and Control Division
Research Triangle Park, North Carolina
2003 ACERC Conference
February 20, 2003
Mercury from Coal-Fired Power Plants• EPA has decided to regulate the emission of mercury from coal-
fired boilers– Announce regulations by Dec. 15, 2003– 48 tons emitted from U.S. CFPP in 1999
• Present as – Ionic or Oxidized (HgO, HgCl2)– Particulate (Hgp)– Elemental (Hg0)
• One Hg0 control method includes injection of activated carbon into a gas stream with removal by the particulate control device
Approach
• Bench-scale Hg research has been done in fixed-bed reactors• Most coal-fired utilities have ESPs
– Dispersed-phase capture– Reactivity is more important than capacity
• Mercury flow reactor is used to simulate in-flight capture of Hg0 over a short residence time
• Examine– Particle size, residence time, temperature on capture– Effect of flue gas components: NOx, SOx, H2O– Increasing reactivity of carbon
• In-flight capture
– Duct/ESP• Seconds
• Reactivity
Flow Reactor Fixed-Bed Reactor
0
20
40
60
80
100
0 1000 2000 3000 4000 5000 6000
Carbon to Mercury Ratio
Hg
Rem
oval
(%
)
• Packed-bed capture
– Baghouse/FF• Minutes/hours/days
• Breakthrough/capacity
0
50
100
150
200
250
0 20 40 60 80 100 120 140
Time (min)
Hg
Upt
ake
(ug/
g ca
rbon
)
Hg Flow Reactor
Hg0/N2
Buck Hg Analyzer
N2
Fluidized Bed Feeder
Lindberg3-Zone Furnaces
Carbon Trap
Exhaust
Carbon Trap
SP 1
SP 2
SP 3
Filter Reducing Furnace
Pump
CH4
Air
Dilution N2
NOx
Air/H2O
SO2
SO2 /O2 Analyzer
Nafion Drier
Hg0/N2
Carbon Properties
Darco FGD Calgon WPL
Base Material Lignite Bituminous Coal
Surface Area(m2/g)
515 990
Size (m) 4-8, 8-16,16-24, 24-44
5-25
AR Moisture (%) 4 13
0
10
20
30
40
0 2000 4000 6000 8000 10000 12000
Carbon to Mercury Ratio
Hg
Rem
oval
(%)
>4-8 um>8-16 um>16-24 um>24-44 um
Effect of Particle Size
•FGD•100 °C•86 ppb Hg0
•N2 carrier
Effect of Residence Time
•WPL •150 °C •124 ppb Hg0
0
0.01
0.02
0.03
0.04
0.05
0.06
Hg
Upt
ake
(g/
g C
arbo
n/s)
23 100 125 150 200 250
Effect of Temperature
•FGD •SP2 •44 ppb Hg0
Temperature (°C)
Effect of Vapor-Phase Moisture
•WPL
•150 °C
•124 ppb Hg0
Effect of Sulfur Dioxide
0
20
40
60
80
100
0 1000 2000 3000 4000 5000 6000 7000
Carbon to Mercury Ratio
Hg
Rem
oval
(%)
Nitrogen
w/500 ppm SO2
•WPL •100 °C •124 ppb Hg0
0
10
20
30
40
50
0 2000 4000 6000 8000 10000 12000
Carbon to Mercury Ratio
Hg
Rem
oval
(%)
Nitrogen
w/200 ppm NO
Effect of Nitric Oxide
•FGD •100 °C • 86 ppb Hg0
0
20
40
60
80
100
0 2000 4000 6000 8000 10000 12000
Carbon to Mercury Ratio
Hg
Rem
oval
(%)
Nitrogen
w/22 ppm NO2
w/90 ppm NO2
Effect of Nitrogen Dioxide
•FGD•100 °C•86 ppb Hg0
0
10
20
30
40
50
0 3000 6000 9000 12000
Carbon to Mercury Ratio
Hg
Rem
oval
(%)
Nitrogen
NO2 and SO2
Effect of Sulfur and Nitrogen Dioxide
•FGD•100 °C•86 ppb Hg0
Add 22 ppm NO2 and 500 ppm SO2
0
20
40
60
80
100
0 2000 4000 6000 8000 10000 12000
Carbon to Mercury Ratio
Hg
Rem
oval
(%)
Nitrogen
Flue Gas
WPL in Flue Gas
•WPL•100 °C•86 ppb Hg0
7% O2, 6.8% H2O, 200 ppm NOx, and 500 ppm SO2
0
20
40
60
80
100
0 4000 8000 12000 16000
Carbon to Mercury Ratio
Hg
Rem
oval
(%) 16% Moisture
4% Moisture
0% Moisture
Effect of Carbon Moisture Content
•FGD •100 °C •86 ppb Hg0
•N2 carrier
What is going on???• Hg0 is not soluble in water• Evaporation of water from the carbon surface
– Cooling the carbon, higher capture at a lower temperature
• Maximum evaporative cooling effect ~40 C – Tests with dry carbon (WPL and FGD) at 50 C show minimal
removal at a C:Hg of 10K:1
• Formation of new C-O functional groups through weathering of the carbon during hydration?– Boehm titrations did not reveal an increase in C-O functional
groups
0
10
20
30
40
50
Hg
Rem
oval
(%
)
0% Water
13% Water
No SO2500 ppm SO2
WPL in Flue Gas
•WPL •150 °C •124 ppb Hg0
•C:Hg=3100:1
Methane flue gas doped with 200 ppm NOx, with and without SO2
0
2
4
6
8
10
Hg
Rem
oval
(%
)
3% Water
16% Water
No SO2500 ppm SO2
FGD in Flue Gas
•FGD •100 °C•86 ppb Hg0
•C:Hg=10k:1
Methane flue gas doped with 200 ppm NOx, with and without SO2
0
20
40
60
80
100
753 1330 1830 2870 4425 5100
Hg
Rem
oval
(%
)
Virgin FGDFGD-Cl
Chlorine Impregnated FGD
•FGD•100 °C•86 ppb Hg0
• N2 Carrier
Carbon to Mercury Ratio
FGD washed with 0.05N HCl
0
20
40
60
80
100
1379 2148 3588 5187 6493
Hg
Rem
oval
(%
)
N2Flue Gas
Chlorine Impregnated FGD
•FGD•100 °C•86 ppb Hg0
Carbon to Mercury Ratio
• A vertical flow reactor was used to examine the removal of Hg0 using activated carbon
• Higher Hg0 removal with decrease in particle size
• Slightly higher Hg0 removal with increase in residence time
• Higher Hg0 removal with decreasing temperature
• The addition of vapor-phase moisture resulted in a drop in Hg0 removal compared to tests in dry N2
• Sulfur dioxide competed for or poisoned the active sites for Hg0 adsorption thereby reducing Hg0 removal
Summary
• Nitric oxide reduced Hg0 removal by competing for the active sites
• Nitrogen dioxide oxidized the Hg0 and increased removal
• Tests in nitrogen and flue gas revealed that Hg0 removal correlates with carbon moisture content
– Increasing the moisture content increased the reactivity
– Removing free moisture resulted in a less reactive carbon
• Chlorine impregnated FGD
– >80% removal in flue gas tests at C:Hg ratio of 6000:1
Summary
