A laboratory study of Hg oxidation catalyzed by SCR
catalysts
Karin Madsen on 05.10.2010 at CHEC Annual Day
Anker Degn Jensen
Joakim Reimer Thøgersen
Flemming Frandsen
Outline
• Background
• Laboratory Study
• Conclusions
DTU Chemical Engineering
Technical University of Denmark
Background
• 1930 tons of mercury was emitted in 2005 (Pacyna, 2010)
– 45% comes from combustion of fossil fuels
• Existing air pollution control devices offers an economically feasible
option for mercury control
• Mercury species in flue gases from power plants
– Hg0 :
Volatile, difficult to capture
– Hg2+:
Water soluble, can be captured in a wet scrubber
– Hgp :
Can be captured in particulate control devices
• Mercury speciation is highly dependent on coal type/rank and operating
conditions
DTU Chemical Engineering
Technical University of Denmark
Mercury oxidation and the SCR
• Mercury is oxidized by halogens in the flue gas
• (Potential) net reaction
Hg0 + 2 HCl + 1/2 O2 = HgCl2 + H2O
• SCR catalysts for NOx-control also have
catalytic activity on the mercury oxidation
– The combination SCR + wet FGD can
offer >90% mercury capture
DTU Chemical Engineering
Technical University of Denmark
Laboratory Setup at Topsoe A/S
Air
Hg0 permeation
tube (T=50°C)
N2
H2O (T=17°C)
NO
SO2
Hg-analyser
HCl
Hg0
HgT
DTU Chemical Engineering
Technical University of Denmark
NH3
HgReduction
(T=750ºC)
SCR
Reactor
Mercury oxidation and Chlorine
DNX-catalyst, T=350ºC, NHSV=5800 h-1 ,
Hg0=40 µg/m3, NH3= 243 ppm, 1%H2O, 10%O2
Fraction Hg0 oxidized
1.00
0.90
0.80
0.70
0
2
4
6
HCl [ppm]
DTU Chemical Engineering
Technical University of Denmark
8
10
Mercury Oxidation and Ammonia
-1
DNX-catalyst, T=350ºC, NHSV=5800 h ,
0
Hg =40 µg/m3, HCl=1.6 ppm, H2O=1%, O2=10%, N2=balance
Fraction Hg 0 oxidized
1.00
0.90
0.80
0.70
0
50
100
150
NH3 [ppm]
DTU Chemical Engineering
Technical University of Denmark
200
250
Mercury Oxidation:
Laboratory versus Pilot-scale data
Laboratory data
Fraction Hg0 oxidized
1.00
Pilot-scale data (average)
0.75
0.50
0.25
0.00
0
50
HCl [ppm]
DTU Chemical Engineering
Technical University of Denmark
100
Conclusion:
Close Gap between Laboratory and PilotScale Data
• Difference in gas matrix
– Laboratory
Simulated flue gas containing Hg0, O2, H2O, NH3, HCl and N2
– Pilot
Real flue gas from subbituminous coal combustion including NO, SO2,
and fly ash
• Factor 10-20 difference in catalyst activity must be due to difference in
gas matrix
• Future work involves testing of more complex gas matrices including NO
and SO2
DTU Chemical Engineering
Technical University of Denmark