Mercury Chemistry in Wet Flue Gas Desulfurization Process
Miloš Bogataj, Peter Glavič*
Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, SI2000, Maribor
Abstract
This work is aimed at a simulation of mercury speciation in FGD process. The chemical reaction
model is being developed to describe the aqueous mercury-sulfite/sulfate-chloride-carbonate
system. The model takes into account the simultaneous occurrence of a number of reaction steps
and coexistence of three-phases (gaseous, aqueous and solid). It is to be used to correlate pilotscale experimental results as well as to predict kinetics at critical experimental conditions, such
as low pH regions and the SO2 gas-aqueous interface, which are difficult to investigate
experimentally. The currently obtained results indicate some agreement between the simulation
and the experimental solution.
Keywords: mercury, wet flue gas desulfurization, modelling, reactions
1. Introduction
Mercury emissions from coal-fired generators are a major environmental concern due to the
toxicity and persistence of mercury that accumulates in our waterways (UNEP, 2013a; Pacyna
et al., 2010). New regulations to limit mercury emissions from coal-fired generators are being
enacted in countries around the globe (EU: COM 2005/0020, COM/2010/0723; EPA: MATS;
UNEP, 2013b).
During combustion, the mercury (Hg) in coal is volatilized and converted to elemental mercury
(Hg0) vapour in the high temperature regions of coal-fired boilers. As the flue gas is cooled, a
series of complex reactions begin to convert Hg0 to ionic mercury (Hg2+) compounds and/or Hg
compounds that are in a solid-phase or Hg that is adsorbed onto the surface of other particles.
This relative distribution has great influence on the behaviour of Hg in the atmosphere and its
deposition in the reactive environment (EPA, 2006). The understanding is reached of the
possibilities to achieve effective and stable removal of Hg0 in wet FGD with the use of air
*corresponding author
(added into absorbing slurry for the oxidation of SO2 in first place), however the mechanisms
and the kinetics is not developed to the extent needed yet. Beside the oxidation of Hg0 to Hg2+,
which is perfectly soluble in the solutions, the fate of Hg2+ in the suspension of solids (gypsum,
calcium sulphite, insoluble residues of calcite, chemical precipitates in trace level) has also not
been determined to extent needed to predict final equilibrium or steady state in continuous
operation. It is agreed that to fully address and solve the mercury emission threat an
understanding of flue gas desulfurization process (FGD) chemistry is crucial; unfortunately,
this is not a straight forward task. Nevertheless, a development of efficient and reliable
computer simulation models as tools for speciation of mercury might fill in the missing gaps in
the knowledge.
2. PHREEQC software
The simulation of mercury speciation in a FGD process was performed in PHREEQC.
PHREEQC is a computer program for simulating chemical reactions and transport processes in
natural or polluted water, in laboratory experiments, or in industrial processes. The program is
based on equilibrium chemistry of aqueous solutions interacting with minerals, gases, solid
solutions, exchangers, and sorption surfaces, which accounts for the original acronym – pHREdox-EQuilibrium, but the program has evolved to include the capability to model kinetic
reactions and 1D (one-dimensional) transport. Rate equations are completely user-specifiable
in the form of Basic statements. Kinetic and equilibrium reactants can be interconnected, for
example, by linking the number of surface sites to the amount of a kinetic reactant that is
consumed (or produced) in a model period (Parkhurst, Appelo, 2013).
3. Model
The reaction model used in this study comprises a set of chemical elements (Br, C, Ca, Cl, Fe,
H, Hg, Na, Mg, N, O, S) in their relevant oxidation states and a set of over 100 equilibrium
kinetic equations. In Table 1 we present those relevant to mercury species. Table 2 comprises
the reactions involving sulphite, sulphate, thiosulphate, dithionate, trithionate, tetrathionate and
pentathionate species. Alongside the reactions, corresponding equilibrium constants (Keq) used
in this work are presented.
Table 1: Equilibrium reactions involving mercury species.
Reaction
log(Keq)
Hg + H2O ↔ Hg(OH) + H
2+
+
–3,6
+
Hg2+ + 2H2O ↔ Hg(OH)2 + 2H+
–
Hg + Cl ↔ HgCl
2+
–6,2
+
6,7
HgCl+ + Cl– ↔ HgCl2
–
6,4
HgCl2 + Cl ↔ HgCl3
–
0,9
HgCl3– + Cl– ↔ HgCl42–
1,2
Hg + SO3 ↔ HgSO3
12,7
HgSO3 + SO32– ↔Hg(SO3)22–
11,4
Hg2++ HSO4– ↔ HgSO4 + H+
–0,6
2+
2+
Hg +
2–
CO32–
↔ HgCO3
11,1
Hg2+ + 2CO32– ↔ Hg(CO3)2 2–
14,5
Hg22+ ↔
1,94
Hg + Hg
2+
HgCl2 + SO32– ↔ ClHgSO3– + Cl–
–
15,0
–
–
ClHgSO3 + H2O ↔Hg + HSO4 + Cl + H
+
-0,1
Hg22+ + S2O32– ↔ HgS2O3
7,3
2Hg2+ + H2O ↔ Hg22+ +2H+ +0,5O2
-12,2
Hg2Cl2 ↔ Hg2
2+
+ 2Cl
–
-17,8
Hg2SO4 ↔ Hg22+ + SO42–
-6,11
HgO + 2H ↔ H2O + Hg
2,4
Hg + 2H+ + 0,5O2 ↔ H2O + Hg2+
14,2
+
2+
Table 2: Equilibrium reactions for sulphite, sulphate, thiosulphate, dithionate, trithionate, tetrathionate and
pentathionate species.
Reaction
log(Keq)
SO42– ↔ SO32– +0,5 O2
+
2–
2H + SO3
–46.6
↔ SO2 +H2O
9,1
2H+ + SO32– ↔ H2SO3
SO32–+
9,2
HSO3–
H ↔
+
7,2
2H+ + 2SO32– ↔ S2O32– + O2 + H2O
–40,3
2H+ + 2SO32– ↔ S2O42– + 0,5O2 + H2O
–25,2
+ 0,5O2 + 2H ↔ S2O6 + H2O
41,8
2SO32– + 1,5O2 + 2H+ ↔ S2O82– + H2O
70,7
–
–6.2
2SO32–
3SO32–
+
+ 6 H + 2e ↔
+
2–
S3O62–
+ 3H2O
4SO32– + 12H+ + 6e– ↔ S4O62– + 6H2O
5SO32–
–
+ 18H + 10e ↔ S5O6 + 9H2O
+
2–
–38,4
-99,4
In
addition
to
the
above
reactions,
a
kinetic
reaction
(1–2)
governing
the
dissolution/precipitation of calcite (Plummer et al., 1978) was used. The rate of dissolution is
given by:
Rd  k1  H    k2  CO 2 aq   k3  H 2 O 
444,0 

 0,98 
T 

where  is the ionic activity of species and k1  10
1737,0 

 -1,1 
T 

k3  10
(1)
,
2177,0 

 2,84 
T 

k1  10
, and
are the rate constants.
The overall rate (forward rate minus backward rate) for calcite is given by (2):

 PIA  3



K
 Rd 1  10 calcite 


2
Rcalcite





(2)
where PIA is the ion activity product and K is the equilibrium constant.
4. Simulation and results
The simulation setup is given as follows. Flue gas composed of carbon dioxide, sulphur dioxide,
oxygen, nitrogen, water vapour, and mercury in traces, its detailed composition is given in Table
3, is in contact with water, which is saturated with calcite. The initial concentration of mercury
was set to 5 μg/L and concentrations of sodium and chlorine ions to 0,14 mol/L . The initial pH
was set to 2,4, pe to 15 and temperature of the aqueous solution to 50 °C. The goal was to
inspect the distribution of mercury species ones the equilibrium was reached.
Table 3: Flue gas composition.
Component
x (%)
CO2
16,5
SO2
0,125
O2
7
N2
63,875
H2O
12,5
Several different simulation runs were performed. During these, some of the parameters
namely, equilibrium constants and reaction rate constants were varied to achieve maximal
agreement among the results of the simulation and experimental results. The results obtained
after 800 iterations indicate the pH of the solution is 4,6. The change in pH as a function of time
is given depicted in Figure 1. The increase in pH is a consequence of calcite dissolution and
formation of gypsum. Despite many changes in the parameters, a considerable discrepancy
between the pH of the experimental solution (pH = 5,6) and the simulated one is noticeable.
On the other hand, the changes in molalities of various species in the solution exhibit minute
dynamic behaviour as their values remain practically constant during the simulation. An
example for HgCl2 and HgCl+ is shown in Figure 2.
Figure 1: Change in pH as a function of time.
The simulation results regarding molalities of various species are given in Table 4. Note that
only the species with molalities greater than 10–6 mol/kg are presented. The results indicate that
mercury is predominantly present in the form of HgCl2 and HgCl+. Their existence in the
experimental solution is for now a matter of speculation as they should be confirmed by
analytical procedures. Additionally, the simulation predicts a complete absence of sulphite
species (SO32– ), although the species should be present in the range of 1∙10–3 mol/kg.
Figure 2: Change in molalities of HgCl2 and HgCl+ species as a function of time.
Table 4: Distribution of species.
Species
H
+
b(mol/kg)
2,20∙10
–02
Species
CaCl
b(mol/kg)
+
6,77∙10-01
OH–
2,08∙10–06
CaCl2
4,44∙10-02
H2O
5,55∙10+04
Cl–
1,11∙10+02
CO2
5,62∙10+01
NaCl
9,05∙10–01
CaHCO3+
1,20∙10+01
HCl
4,64∙10–04
HCO3–
2,99∙10+00
HgCl2
2,52∙10–06
NaHCO3
1,24∙10–01
HgCl3–
1,66∙10–06
CaCO3
7,50∙10–03
Na+
6,31∙10+01
CO3–2
4,80∙10–05
NaSO4–
5,54∙10+01
CaSO4
7,13∙10+01
O2
4,55∙10–04
Ca+2
1,33∙10+01
SO4–2
1,29∙10+03
CaNO3+
6,82∙10-01
HSO4–
8,53∙10–01
5. Conclusions
In this work a simulation of mercury speciation in FGD process was performed. The reaction system is
highly complex involving many species. The results show some agreement between the experimental
solution and the simulation results, however, the model still does not represents the actual reaction
system well enough. Knowledge and experiences obtained clearly point to expanding the model by
thiosulphite radical reaction mechanism, which is believed to play a crucial role in the given system.
6. Symbols and abbreviations
b
k
K
pe
PIA
R
x
γ
molality
rate constant
equilibrium constant
redox potential
ion activity product
reaction rate
molar fraction
ion activity
7. References
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Transport. UNEP Chemicals Branch, Geneva, Switzerland, available at:
http://www.unep.org/PDF/PressReleases/GlobalMercuryAssessment2013.pdf .
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P., (2010), Global emission of mercury to the atmosphere from anthropogenic sources in 2005 and
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http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=COM:2005:0020:FIN:EN:PDF.
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http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=COM:2005:0020:FIN:EN:PDF.
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20Mercury_booklet_English.pdf.
7) EPA, (2006), Control of mercury emissions from coal-fired electric utility boilers, available at:
http://www.epa.gov/ttnatw01/utility/hgwhitepaperfinal.pdf.
8) Parkhurst, D.L, and Appelo, C.A.J, (2013), Description of input and examples for PHREEQC version
3—A computer program for speciation, batch-reaction, one-dimensional transport, and inverse
geochemical calculations: U.S. Geological Survey Techniques and Methods, book 6, chap. A43, 497 p.,
available only at http://pubs.usgs.gov/tm/06/a43.
9) Plummer, L.N., Wigley, T.M.L., and Parkhurst, D.L., (1978), The kinetics of calcite dissolution in
CO2-water systems at 5 to 60 C and 0.0 to 1.0 atm CO2, American Journal of Science 278, p. 179-216.