Paper # 070EN-0400
Topic: Environmental aspects of combustion
8th U. S. National Combustion Meeting
Organized by the Western States Section of the Combustion Institute
and hosted by the University of Utah
May 19-22, 2013
Mercury Oxidation by Halogens under Air-Fired
and Oxygen-Fired Conditions
Ignacio Preciado, Tessora Young, and Geoffrey Silcox
Department of Chemical Engineering, University of Utah
Salt Lake City, Utah 84112, USA
Gas-phase mercury oxidation by halogens under air-fired and oxygen-fired (27% O2 in CO2) conditions was studied in a
bench-scale, laminar, methane-fired, 300 W, quartz-lined reactor. The experiments provided data on the extents of
elemental mercury oxidation in the presence of various flue gas components including chlorine, bromine, SO2, and NO.
A wet conditioning system and a Tekran 2537A mercury analyzer were used to determine mercury speciation and
concentrations.
Under the same experimental conditions and for both halogens, oxy-firing produced modest increases in mercury
oxidation. Extents of oxidation at chlorine concentrations of 100 to 500 ppmv (as HCl) ranged from 6 to 21% for oxyfiring and from 4 to 15% for air-firing. Oxidation by bromine at concentrations of 10 to 50 ppmv (as HBr) ranged from
43 to 69% for oxygen-firing and from 15 to 46% for air-firing. Under both firing conditions, the addition of NO (below
250 ppmv) had little or no effect on mercury oxidation by chlorine or bromine; and the addition of SO2 (below 400
ppmv) had no impact on oxidation by chlorine.
1. Introduction
Oxy-fuel combustion is being considered as a promising path to CO2 capture and sequestration,
especially for coal-fired power plants since its cost can potentially be less than or comparable to that
of conventional air-fired combustion with amine-based CO2 capture (Okawa et al., 1997; Nsakala et
al., 2001).
The fate and analysis of mercury in oxy-fired systems have not been fully investigated. The fate of
mercury under oxycombustion conditions is important, not only because of emissions, but also
because of downstream corrosion. Trace amounts of mercury may lead to embrittlement and
cracking of aluminum heat exchangers that are used in the cryogenic separation and compression of
CO2 (Santos, 2009; Santos, 2010).
Limited and to some extent uncertain data on mercury speciation in oxycombustion systems have
been published. Some experimental data have indicated that elevated CO2 concentrations potentially
enhance mercury retention on fly ash, while high O2 concentrations may promote mercury
vaporization and emission (Qiu et al., 2007). Additional mercury oxidation has been observed in
oxycombustion as a result of mercury chlorination (Ueno et al., 2008).
Suriyawong et al., (2006) evaluated pulverized subbituminous coal under oxygen-carbon dioxide
mixtures (20% O2/80% CO2 and 25% O2/75% CO2) to estimate the effects of O2 - CO2 coal
combustion on submicron particle formation and mercury speciation. It was found that 10 to 20 % of
the mercury in the coal was oxidized and the extent of mercury oxidation was unaffected by
changing from air- to oxy-firing.
Zhuang et al., (2011) have evaluated different Hg measurement techniques under oxy-combustion
conditions. They found that for flue gas with high CO2 concentration, modifications to continuous
mercury monitors (CMM) may be needed to eliminate biases caused my mass flow controllers or
impinger solutions in the flue gas conditioning system. Varied impinger chemistry under oxycombustion conditions suggests Hg sampling using wet-chemistry methods, such as Ontario Hydro,
may bias measured gas-phase Hg concentrations due to the high concentrations of CO2.
The homogeneous oxidation of mercury has been extensively studied at the University of Utah under
air and oxy-firing conditions (Senior et al., 2009; Silcox et al., 2010, Van Otten at al., 2011).
Experiments were performed in a bench-scale, methane-fired, quartz-lined reactor, in which quench
rate and flue gas compositions (which included halogens such as chlorine and bromine) were varied.
Mercury measurements were performed using a Tekran 2537A analyzer coupled with a wetchemical speciating and conditioning system.
In the absence of SO2, preliminary results with oxy-firing conditions (air was replaced with a
mixture of 27 % oxygen, balance carbon dioxide) showed a dramatic effect in oxidation of mercury
by chlorine. At 400 ppm chlorine (as HCl equivalent), oxidation levels were around 80 % vs. 5 %
oxidation obtained under air-firing tests for the same chlorine concentration. Oxidation levels with
bromine at 25 and 50 ppm (as HBr equivalent) ranged from 80 to 95 % and were about the same for
oxy- and air-firing conditions. Subsequent work has not reproduced the dramatic effects seen with
chlorine.
When SO2 at 500 ppm was included in the oxy-fuel tests, the ability of chlorine and bromine to
oxidize mercury dropped dramatically. Mercury oxidation on the order of 40 -50 % were obtained
with bromine at 25 and 50 ppm (as HBr equivalent) and between 10 - 20% for chlorine at 400 ppm
(as HCl equivalent).
This work will study the fate of mercury oxidation by halogens in the gas phase under oxy-fuel and
air firing conditions with an optimized mercury sampling system in order to ensure unbiased
measurement of mercury speciation in oxy-combustion flue gas.
2. Methods
The reactor design used in this study is described in detail in Fry et al., (2007) and was developed to
study the effects of halogens such as chlorine and bromine, and species such as NO and SO2 on
homogeneous gas-phase mercury oxidation. The reaction chamber is a quartz cylinder, having a 47mm inner diameter and 135-cm length. The first 54 cm of the cylinder are enclosed by a Thermcraft
high-temperature heater and the remaining section of the quartz tube was heated with four,
individually controlled heating tapes. The later permitted the quench rate to be adjusted in order to
produce time-temperature profiles representative of industrial boilers.
A 300 W quartz burner was used to feed methane and oxidant into the reactor. All species were
introduced through the flame to create radical species representative of those seen in combustion
systems. The stoichiometric ratio and the total gas flow rate (gases were fed at 6 SLPM into the
burner) were the same for both, air and oxy fuel conditions. A mixture of 27% O2 and 73% CO2was
used in the oxy-firing combustion tests. The percentage of oxygen after combustion was monitored
with an oxygen analyzer and maintained at 2.0 - 2.2 % for all experiments.
2
Mercury is fed to the system using a mercury vapor permeation tube that is heated in an oil bath and
uses air as the carrier gas. The carrier gas is then diluted with the residual air or oxygen-carbon
dioxide mixture and sent to the burner. For oxy-fired experiments, carbon dioxide and oxygen were
regulated using mass flow controllers and blended before diluting the mercury stream.
Calibration gases in air were used as sources of chlorine and SO2 (6000 ppm Cl2 and 6000 ppm SO2)
and calibration gases in nitrogen were used as sources of bromine and NO (500 ppm Br2 and 6000
ppm NO). NO and SO2 were fed to the burner in select experiments involving chlorine or bromine.
The concentration of NO was measured at the reactor exit by a NOx analyzer, and the flow rate
through the burner was adjusted to give the desired concentration.
At the exit of the reactor, the flue gases (gas temperature around 350C) were split and sent to a wet
conditioning system. . One stream was fed to an acidic solution containing 4 wt% SnCl2 to reduce all
oxidized mercury to its elemental form. This gas stream represented the total amount of mercury fed
to the reactor. The other gas stream was fed to a solution containing 10 wt% KCl and 1 wt%
Na2S2O3 to capture oxidized mercury. This stream indicated the elemental mercury concentration
present in the system. Both sides then bubbled through 5% NaOH solutions to remove any acidic
species, with the exception of CO2. Flow designs ensured all solutions were being continually
refreshed within the quartz impingers. A Tekran mercury analyzer was used to measure the
concentration of elemental mercury. A four port sampler controlled which stream the Tekran
sampled. The difference between the total and elemental gas streams was the concentration of
mercury oxidized in the reaction chamber.
A schematic of the experimental system is presented in Figure 1.
Figure 1. Experimental system for measuring elemental mercury.
3. Results and Discussion
Chlorine (as Cl2), bromine (as Br2), NO and SO2 were added through the burner. Before these were
added, the baseline mercury concentration was verified by a material balance. The mercury balance
3
considered total and elemental mercury, measured using the continuous emission monitor (CEM)
and sample conditioning system, at the beginning and end of each experiment. Figure 2 shows a
typical set of continuous measurements of mercury species as a function of time with chlorine
addition (ppm as HCl). The concentrations of total (HgT) and elemental (Hg0) mercury are
indicated; the mercury oxidation is evaluated for each condition and obtained by difference of the
total (HgT) and elemental (Hg0) mercury concentrations.
Effect of halogens on mercury oxidation
Figures 3 and 4 show the mean percentage of mercury oxidation for chlorine and bromine injection
under air- and oxy-firing conditions. Experiments were repeated to determine the range in the levels
of oxidation, and these are shown as error bars in the figures. Figures 3 and 4 show an increase in
mercury oxidation with increasing halogen concentration. Neither NO nor SO2 were added in these
experiments. When comparing average values, Hg oxidation by chlorine was 50% higher for oxyfiring compared to air-firing. Oxidation by bromine was 2 to 4 times higher for oxy-firing.
Sliger, Kramlich, and Marinov (2000) suggested that homogeneous Hg oxidation by halogens could
be explained by this simplified mechanism
Hg + X + M = HgX + M
(1)
HgX + X2 = HgX2 + X
(2)
where X = halogen and M = third body. The formation of HgX is the rate controlling step.
A possible explanation for higher oxidation under oxy-firing conditions is that the CO2 molecule is
more effective third body than N2. Carbon dioxide is more effective than N2 at removing energy
from the HgX transition state complex, facilitating the formation of oxidized Hg species.
As shown by Figures 3 and 4, bromine is a more powerful oxidant for mercury than chlorine, on an
equivalent molar basis. Similar findings have been reported in full-scale power-plant tests (Benson et
al., 2007 and Richardson et al., 2006). Bromine radical chemistry is very different from that of
chlorine. HCl is the dominant chlorine species for temperatures between 200 and 1000 C. With
bromine, HBr, Br2, and Br are all significant (Niksa et al., 2010). This drastically different halogen
chemistry could account for the increased homogeneous oxidation seen with bromine.
4
Hg fed in the system = 25 g/m3
Mercury Oxidation. Air-firing, chlorine
35
30
Mercury concentration (g/m3 )
Hg baseline
Hg baseline
25
20
HgT
100 ppm as HCl
200 ppm as HCl
500 ppm as HCl
Hg0
15
10
Hg2+ = HgTotal - Hg0
5
0
10:48:00
12:00:00
13:12:00
14:24:00
15:36:00
16:48:00
18:00:00
Time
Figure 2. Measured concentrations of total and elemental mercury for different chlorine
concentrations under air firing conditions
% Hg oxidation for air and oxy-firing tests
30
% Hg oxidation 25
20
15
10
5
0
0
100
200
300
400
500
600
Chlorine as HCl (ppm)
Air-firing
Oxy-firing
Figure 3. Mercury oxidation for different chlorine concentrations under air and oxy-firing conditions
5
% Hg oxidation for air and oxy-firing tests
80
Hg = 25 g/m 3
% Hg oxidation 70
2.0 % O2 (dry) in flue gas
60
50
40
30
20
10
0
0
10
20
30
40
50
60
Bromine as HBr (ppm)
Air-firing
Oxy-firing (27% O2 - 73% CO2)
Figure 4. Mercury oxidation for different bromine concentrations under air and oxy-firing conditions
Effect of NO and SO2 on mercury oxidation
Table 1 summarizes the results for mercury oxidation by chlorine and bromine when NO and SO2
were injected in the reactor. It can be observed that the addition of NO did not affect the Hg
oxidation by chlorine or bromine under both firing conditions. On the other hand, Hg oxidation by
chlorine remained basically unchanged when SO2 was injected under both, air and oxy firing
conditions.
Results regarding the effects of SO2 on gas-phase mercury oxidation by bromine are not reported
due to inconsistent data. The inconsistencies are due to the aqueous-phase complexing of mercuric
ion by the bromide ion to form the stable species HgBr4-2. The complex resists reduction by stannous
chloride (Babi, Schaedlich and Schneeberger, 2002) and as a result no elemental mercury is
produced for analysis by the atomic fluorescent analyzer used in this study.
Figure 5 is a plot of the chlorine-SO2 data presented in Table 1 for the oxy-fuel tests. The red dotted
lines correspond to the range of Hg oxidation when feeding 200 ppm of chlorine without SO2 in the
system. Increasing the SO2 concentration from 0 to 400 ppmv had little or no effect on mercury
oxidation by chlorine, within experimental error.
6
Table 1. Range of mercury oxidation when adding SO2 and NO under air and oxy firing tests
Chlorine as HCl at 200 ppm
Species Air firing
Oxy firing
ppm
% Hg oxidation % Hg oxidation
(range)
(range)
SO2 - 0
4-8
6 - 11.5
SO2 - 100
5-9
6 - 10
SO2 - 200
4-8
5.5 - 9.5
SO2 - 400
4-8
5-9
NO - 0
NO - 25
SO2 - 50
SO2 - 100
SO2 - 250
4-8
4-8
4-8
3-7
3-6
Bromine as HBr at 30 ppm
Species Air firing
ppm
% Hg oxidation
(range)
NO - 0
30 - 39
NO - 50
29 - 38
NO - 100
30 - 40
NO - 250
31 - 43
-
% Hg oxidation for oxy-firing tests
20
% Hg oxidation Hg = 25 g/m 3
2.0 % O2 (dry) in flue gas
15
10
Range of % Hg oxidation
f or 200 ppm as HCl, without
SO2
5
0
0
100
200
300
400
500
SO2 (ppm)
Figure 5. Mercury oxidation by chlorine with the addition of SO2 under oxy-firing conditions
4. Conclusions
Homogeneous mercury oxidation by halogens under oxy-firing conditions has been shown to be
more effective than under air-firing conditions. The oxidation levels ranged from 6 to 21% for oxyfirings and 4 to 15% for air-firing at reactor chlorine levels of 100 to 500 ppmv (as HCl). The levels
ranged from 43 to 69% for oxy-firing and 15 to 46% for air-firing at reactor bromine levels of 10 to
50 ppmv (as HBr). Mercury oxidation by bromine was 2 to 4 times higher than by chlorine, with
about 10 times less halogen on a molar basis.
7
The addition of NO to the flame (up to 250 ppmv) had no impact on mercury oxidation by chlorine
or bromine. The addition of SO2 had no effect on mercury oxidation by chlorine at SO2
concentrations up to 400 ppmv.
Acknowledgements
This research was sponsored by the Clean Coal Technology Fund administered by the University of
Wyoming for the State of Wyoming.
References
Babi, D., Schaedlich, F. H., and Schneeberger, D.R. “Correction techniques for iodine and bromine
interferences in continuous flow aqueous mercury analysis”. Anal. Bioanal. Chem. 2002, 374: 1022
– 1027.
Benson, S. A.; Holmes, M. J.; McCollar, D. P.; Mackenzie, J. M.; Crocker, C. R.; Kong, L.;
Galbreath, K.; Dombrowski, K.; Richardson, C. “Large-scale mercury control testing for lignitefired utilities - oxidation systems for wet FGD”; Energy and Environmental Research Center: Grand
Forks, ND, March 2007; Final Report DOE NETL DE-FC26-03NT41991.
Fry, A.; Cauch, B.; Lighty, J. S.; Silcox, G. D.; Senior, C. L., “Experimental evaluation of the effects
of quench rate and quartz surface area on homogeneous mercury oxidation”. Proc. Combust. Inst.
2007, 31, 2855–2861.
Niksa, S.; Padak, B.; Krishnakumar, B.; Naik, C. V. “Process chemistry of Br addition to utility flue
gas for Hg emissions control”. Energy & Fuels 2010, 24, 1020–1029.
Nsakala, N.Y., Marion, J., Bozzuto, C., Liljedahl, G., Palkes, M., Vogel, D., Gupta, J.C., Guha, M.,
Johnson, H., Plasynski, S., 2001. “Engineering feasibility of CO2 capture using existing US coalfired power plant”. 1st National Conference on Carbon Sequence. May 15–17, 2001, Washington,
D.C.
Okawa, M., Kimura, N., Kiga, T., Takano, S., Arni, K., Kato, M., “Trial design for a CO2 recovery
power plant by burning pulverized coal in O2/CO2”. Energy Convers. Manage. 38. 1997.
Qiu, J., Hao, L., Wen, C., Zeng, H., Wu, H., “Particulate matter, mercury and trace metals emissions
in oxy-coal combustion”. In Proceedings of the 233rd ACS National Meeting: Chicago, Illinois,
March 25–29, 2007, Paper No. 82 in the ACS Fuels Divison.
Richardson, C. F.; Dombrowski, K.; Chang, R. “Mercury control evaluation of halogen injection into
coal-fired furnaces”. Proceedings of the Electric Utilities Environmental Conference; Tucson, AZ,
Jan 23_25, 2006.
Santos, S., “Impact of mercury on oxy-coal combustion technology for power generation with CO2
capture”. Position paper, IEA Greenhouse Gas R&D Programme, Stoke Orchard,Cheltenham,
UK..,2008 also presented at IEA-CHG International Oxy-Combustion Network 3rd Workshop,
Yokohama, Japan, 5-8th March, 2009
Santos, S., “Challenges in understanding the fate of mercury during oxyfuel combustion”. MEC7
Workshop, June 16–18: DLCS, Strathclyde University, Glasgow, Scotland. 2010
8
Senior, C. L., Van Otten, B., Buitrago, P. A., Silcox, G. D, “Gas-phase mercury oxidation by
bromine: Effects of impinger-based mercury”, Air Quality VII, Arlington, Virginia, October 26-29,
2009.
Silcox, G. S., Buitrago. P. A., Senior, C. L., Van Otten, B., “Effects of oxy-firing on gas-phase
mercury oxidation by halogens”, 2010 AIChE Annual Meeting, Salt Lake City, Utah, November 712, 2010.
Silcox, G. S., P Buitrago. P. A., Senior, C. L., Van Otten, B., “Gas-phase mercury oxidation by
halogens: Effects of bromine and chlorine”, Air and Waste Management Association 103rd Annual
Conference, Calgary, Alberta, Canada, June 22-25, 2010.
Sliger, R. N, Kramlich, J. C, and Marinov, N. M., “Towards the development of a chemical kinetic
model for the homogeneous oxidation of mercury by chlorine species”. Fuels Processing
technology, 2000, 65 – 66, 423 – 438.
Suriyawong, A., Gamble, M., Lee, M., Axelbaum, R., Biswas, P. “Submicrometer particle formation
and mercury speciation under O2- CO2 coal combustion”. Energy & Fuels 2006, 20, 2357-2363.
Ueno, S.-I., Sato, N., Kamata, H., Yamada, T., “The behavior of mercury in the flue gas of oxy-coal
combustion”. Sekitan Kagaku Kaigi Happyo Ronbunshu 45. 2008.
Van Otten, B., Buitrago, P. A., Senior, C. L., Silcox, G. S. “Gas-phase oxidation of mercury by
bromine and chlorine in flue gas”. Energy & Fuels, 2011, 25, 3530-3536
Zhuang, Y., Pavlish, J., Lentz, N. and Hamre, L. “Mercury measurement and control in a CO2enriched flue gas”. International Journal of Greenhouse gas Control. 5S (2011) S136–S142
9