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The Oxidation Kinetics of Mercury in HgOHCl System.

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Dev. Chem. Eng. Mineral Process. 13(3/4), pp. 483-494, 2005.
The Oxidation Kinetics of Mercury in
Hg/O/H/Cl System
Y. Qiao and M.H. Xu*
State Key Laboratory of Coal Combustion, Huazhong University of
Science and Technology, Wuhan 430074, P.R.China
The potential for regulation of mercury emission from coal-fired boilers is a concern
for the electric utility industry. Field data show a wide variation in the paction of
mercury that is emitted as a vapor versus that retained in the solid products. The
reason for this variation is not well understood. Near the end of the flue gas path,
mercury exists as a combination of elemental vapor and HgClz vapor. The data show
that HgCl? is more likely to be removed porn the flue gas. The need to describe
accurately mercury reaction products and their concentration-time correlation
prompted investigation of mercury chemical kinetic mechanisms and their application
to real combustion systems. This paper uses chemical equilibrium analysis to study
the speciation of mercury in flue gases during coal combustion and gasification. The
paper presents a simple kinetic model of mercury oxidation in the Hg/O/H/Cl system.
The results porn the model calculation are in reasonable agreement with the
Mamani-Paco and Helble [l] experimental data.
The emission of mercury by coal-fired power plants has recently become a concern on
the part of the electric utility industry. Field studies show that coal-fired plants emit
anywhere from 5% to 95% of the mercury contained in their coal [l-31. The reasons
for this variability are poorly understood, but appear to involve the oxidation state of
the mercury, the properties of the mineral matter associated with the coal, and the
type of existing air pollution control equipment installed on the furnace. The heart of
the problem is that the fundamental pathways governing the fate of mercury in the
furnace environment are not known. However, these fimdamental processes will,
ultimately determine whether mercury is retained with the ash or emitted with the
stack gas.
* Author for correspondence (
Y. Qiao and M.H. Xu
Over the past decade, the U.S.DOE,EPRI, the U.S.EPA and others have devoted
considerable effort toward the study and development of control technologies for lowlevel mercury concentrations as found in coal-combustion flue gases. The recent
decision by the U.S. Environmental Protection Agency to control Hg emissions from
coal-fired power plants has led to an increased concentration of efforts on mercury.
Oxidized mercury from coal combustion is generally thought to be HgC12.Relative to
elemental mercury, HgC12is slightly less volatile at stack temperatures and below.
The speciation of mercury in a combustion stack affects its extent of capture in air
pollution control equipment. For example, water-soluble oxidized species, such as
HgC12, are more readily removed from flue gases in scrubber systems. Therefore,
mercury emissions could be reduced by increasing the fraction of mercury present as
HgC12 in stack gases. Because of its enhanced water solubility, oxidized mercury also
has a much shorter atmospheric lifetime. It has a residence time of just 5 to 14 days in
the atmosphere. The atmospheric insoluble elemental mercury remains in the
atmosphere for times of the order of one year, until it is removed by dry deposition or
wet deposition after oxidation to the water-soluble forms [4-61. The above mentioned
studies thus imply that if mercury is present in a combustion system, its oxidized form
would be preferred in the stack gases due to its higher solubility in scrubber systems,
and may also be preferred because of its shorter atmospheric lifetime.
Previous studies of the mercury oxidation in combustion systems have been
dominantly focused on chemical equilibrium calculations and experimental
measurements, although the conversion of the mercury is kinetically controlled and
has been studied by several investigators [7-111. The reaction controlling the mercury
conversion is that between elemental mercury and the chlorine atom:
Hg + C1+ M = HgCl+ M
Therefore, it is important to determine the chlorine radical concentration. Chlorine
radicals persist in super-equilibrium concentrations down to low temperatures, since
the chlorine recombination reactions are slow [12, 131. The problem is further
complicated by the reactions with radicals containing H and 0, which is the focus of
this work.
It should be recognized that practical combustion conditions would be subject to
kinetic control. Therefore, it is necessary to develop mercury chemical kinetics for
application to real combustion systems. Mercury chemical kinetic mechanisms are
needed to describe accurately mercury reaction products and their concentration-time
correlation. The present study begins with the Hg/O/WCl system. Although it is ideal,
rather than a real coal-fired plant system, it is significant starting point for further
The current calculation is based on a package of CHEMKIN-3.6 [7], developed
for the analysis of elementary gas-phase chemical kinetics. In our work, kinetic
information on the individual isolated reactions is quite different from other
experiments and literature data. The elementary reaction rate coefficients are the
crucial input required in the kinetic modeling of the formation and emission of toxic
metal compounds in a combustion process. The net production rate of each species
can be written as a summation of the rate-of-progress variables for all reactions
involving the species. The rate-of-progress variable for the reaction is given by the
The Oxidation Kinetics of Mercury in Hg/O/H/Cl System
difference of the forward and reverse reaction rates. Reverse rates are obtained from
the forward rates and the equilibrium constants (evaluated from thermodynamic data).
Often in industrial applications, the elementary kinetics are not known. In some cases,
we have to modify the reaction order for the reaction so that the rate of reaction is
proportional to the concentration of a species raised to an arbitrary power (different
from its stoichiometric coefficient). The model also contains some three-body
reactions, the concentration of the effective third body must appear in the expression
for the rate-of-progress variable. Each reaction proceeds according to the mass
balance and the forward rate coefficients are in the modified Arrhenius form:
= ATBexp(-EIRT)
where the activation energy E, the temperature exponent p, and the pre-exponential
constant A, are parameters in the model formulation. The initial conditions required
are temperature, pressure and composition of the mixture, and also details of the
chemical reactions and the thermochemical properties.
Overview of Mercury Oxidation Behavior
Equilibrium calculations conducted for mercury at stack gas conditions indicate that it
is the oxidized form that is thermodynamically favored. Of the two possible oxidized
forms, there appears to be little experimental evidence for the existence of mercurous
compounds in coal combustion flue gases [8]. Mercury speciation in post-combustion
conditions thus requires an understanding of the partitioning between elemental
Hg (0) and Hg (+2) oxidation states only. Equilibrium calculations conducted for coal
combustion systems indicate that, in the absence of chlorine, elemental mercury vapor
is the dominant compound throughout the post-combustion environment. Sliger et al.
[ 161 reported that 50% equilibrium conversion of mercury to HgC12 occurred at about
950 K in the presence of 500 ppm HCl, and at 800 K in the presence of 50 ppm HCI.
Senior et al. [lo] reported that the 50% conversion point depends on coal chlorine
content. For a coal containing 1000 ppm chlorine, the conversion or breakpoint is
approximately 830 K; and for a coal containing 4000 ppm chlorine, it increases to
900 K. Other calculations show conversion points in the same general range of 800 K
to 900 K [17].
Mercury is of sufficiently high volatility that it is presumed to completely vaporize
in the flame, irrespective of its original form [ 181. At flame temperatures, equilibrium
considerations indicate that mercury will exist in the elemental state. As temperatures
fall, the favored equilibrium product shifts to HgC12. Figure 1 shows equilibrium
partitioning between Hg and HgClz for three Clz concentrations (50, 100, 300 ppm).
The crossover temperature between the elemental and oxidized forms increases from
750 K to 850K as the background C12 concentration goes from 50 to 300 ppm. This
crossover point is not influenced by mercury concentration, provided Clz is present in
excess which is the usually the case. Figure 2 shows that less than 10% of the mercury
is predicted to be present as HgO for 50 ppm Clz concentration. These trends are
consistent with literature values [ 18, 191.
Y. Qiao and M.H. Xu
Figure I . Equilibrium distribution of elemental and oxidized mercury
under diferent C12concentrations.
Kinetic Model for the Hg/O/H/Cl System
The experimental system chosen consists of mercury, oxygen, hydrogen and chlorine
[ 13. The external factors considered included temperature from 400 K to 1800 K, and
a constant pressure of 1.0 atm. The elemental composition is approximately 26%v/v
for water, 13%v/v COz, and 60%v/v Nz, a mercury concentration of 50 pg/Nm3,
corresponding to the flue gas equivalence ratio of a real coal-fired boiler. The main
reactions of the Hg/O/H/Cl system are tabulated in Table 1.
A mechanism was assembled using the reaction set 1 to 8 from Widmer et al. [20],
because these 8 important reactions have been used frequently since it was proposed
in 2000, along with the reactions 9 to 26 involving C1, Clz, HC1, C10, HOC1, from the
NIST database [21]. For a simple kinetic system, these species are shown not to be
negligible through a thermodynamics analysis. These species were also considered for
kinetic investigation of mercury oxidation by Sliger [ 161.
In the absence of rate data for the gas phase reactions of mercury, the Arrhenius
constant was estimated from a variety of sources. Nearly all the reactions involve
reaction between free radicals or between radicals and molecular species; therefore
the pre-exponential factors were all taken to be near the collision limit. For reactions
2, 5 and 6, the activation energies were taken initially as the reaction enthalpy,
although the activation energy for reaction 5 was then adjusted to bring the rate into
closer agreement with similar chlorine abstraction reactions involving HOC1.
The Oxidation Kinetics of Mercury in Hg/O/H/Cl System
Figure 2. Equilibrium distribution of HgO with 50 ppm C12.
The rate chosen for reaction 2 is about three orders of magnitude slower than the
global rate derived by Sliger [22] from the data of Hall et al. [23]. However, it is
likely that in the Hall et al. experiments that the global rate measured involved the fast
chain propagation reactions 1 and 3, in addition to the initiation reaction 2, leading to
a global rate of mercury consumption much faster than reaction 2 alone.
For both reactions 2 and 6, the pre-exponential factor was taken as that presented
recently by Cosic and Fontijn [24] for the corresponding reactions of lead. The
activation energies were set equal to the reaction enthalpies. Although these rates
appear to be low compared with other reactions between metal atoms and HCl
[25-271, most of the latter were for highly exothermic reactions with stable products.
Reactions 3, 7 and 8, which are responsible for the formation of the stable HgClz
product, were all assigned pre-exponential factors matching the corresponding
reactions (2, 6 and 5) of mercury atom. These are near the collision limit in
accordance both with kinetic theory and with a large number of measured data from
numerous sources. For the exothermic reactions there was no particular reason for
selecting the activation energies, except to be consistent with measured data for
similar reactions involving similar species [28]. For reaction 7 which is approximately
20 kcal/mol endothermic, the assigned activation energy was slightly higher than the
reaction enthalpy. The rate constant for reaction 4 was taken as an average of the rate
constants for a number of exothermic atom-radical addition reactions.
Y. Qiao and M.H.Xu
Having considered all these possibilities, only reactions 1, 3, 7 and 8 proved to be
important for the present study. Once the rate constants for reactions 2 through 8 were
set, the Arrhenius parameters for reaction 1 were adjusted as the reaction mechanism
used to model the data of Widmer et al. [20].
Table 1. Kinetic data of chemical reactions in the Hg/O/H/Cl system [20, 211.
1. Hg+Cl+M=HgCl+M
2. Hg+Cl2=HgCl+Cl
3. HgCl+C12=HgC12+Cl
4. HgCl+Cl+M=HgC12+M
5. Hg+HOCl=HgCl+OH
6. Hg+HCl=HgCl+H
7. HgCl+HCl=HgC12+H
8. HgCl+HOCl=HgC12+0H
9. Cl+Cl+M=C12+M
10. H+Cl+M=HCl+M 2
1 1. HCl+H=Hz+Cl
12. H+C12=HCl+Cl
13. O+HCl=OH+Cl
14. OH+HCl=Cl+H20
15. O+C12=ClO+Cl
16. O+ClO=C1+02
17. Cl+HO2=HC1+02
18. Cl+H02=OH+C10
19. Cl+H202=HCl+HO2
20. ClO+H2=HOCl+H
21. H+HOCl=HCl+OH
22. Cl+HOCl=HCl+ClO
23. C12+OH=Cl+HOCl
24. O+HOCl=OH+ClO
25. OH+HOCl=H20+ClO
26. HOCl+M=OH+Cl+M
energy (E)
2 1,500
-1 800
constants (A)
The Oxidation Kinetics of Mercury in Hg/O/H/Cl System
Mercury Oxidation Experiments
In order to assess the reaction chemistry of mercury under conditions typically
encountered in coal combustion facilities, a bench-scale study of mercury oxidation in
the presence of different reaction species has been conducted by Mamani-Paco and
Helble [l]. In their experimental system, a methane-heled flat flame burner facility
was used to generate 800-1100 K post-combustion gases containing C12. Controlled
flows of mercury, generated from a permeation source, are injected into these gases at
the flame exit. The mercury-containing gas was cooled by passing through a 1.3 m
long insulated quartz reaction tube with cooling rates regulated through additional
electrical resistance heating surrounding the tube. Gases were sampled at one of four
equi-spaced sampling ports located along the tube, providing temperature and
residence time-resolved samples. Sampling and analysis by the Ontario Hydro
modification to EPA Method 29 and cold vapor atomic adsorption were used to assess
mercury speciation in each sample.
A bench-scale combustion system with controllable temperature profile was
constructed for the study of mercury transformations in the presence of chlorine
species under a controlled gas environment. The system consists of a multi-element
flat flame diffision burner, a ceramic-lined, insulated stainless steel gas-mixing
chamber into which mercury and chlorine species are introduced, and a horizontal
(13.5 cm outer diameter, 80 cm long) quartz tube reactor containing four sampling
ports along the bottom to permit sample extraction as a function of residence time.
Mercury was supplied to the system such that a target reactor concentration of
50 pg/m3 could be achieved by feeding a dilute nitrogen stream containing low
concentrations of Hgo vapor to the mixing chamber. The vapor itself was generated at
a controlled rate through the use of a mercury permeation device (VICI Metronics)
maintained in a constant temperature reservoir. The temperature of the heating bath
was measured with an Omega thermistor connected to an Instrunet data acquisition
system, in order to provide constant monitoring of temperature change. Polypropylene
spheres were placed on top of the heating medium to minimize heat loss through
evaporation and also provide insulation. The heating solution required about five
hours to reach a steady-state temperature.
Samples were extracted by sampling through a quartz probe inserted through one
of the four sampling ports. The quartz probe was positioned such that it sampled from
the centerline of the reactor in all cases. A thermocouple wire was attached to the
quartz probe through a thm capillary tube inserted adjacent to be the probe to monitor
sampling temperature throughout an experiment.
All gas samples extracted from one of the reactor ports were passed to an impinger
train for separating the mercury species according to oxidation state. The Ontario
Hydro modification of EPA Method 29 was used in all cases [29]. In the Ontario
Hydro method, three impingers containing a solution of 1N potassium chloride
dissolved in de-ionized water are used to remove oxidized mercury. These are
followed by two impingers containing a 10% Hz02 /5% HN03 solution, three
impingers containing a 4%Kh4n04 /10%H2S04 solution, and a final impinger
containing silica gel. Each impinger is a 500 ml capacity container that contains
100 ml of the appropriate solution at the start of an experiment, and each is set within
a watedice bath to maintain temperature throughout an experiment. Samples are
Y. Qiao and M.H. Xu
collected at a gas flow rate of 15 slpm for a duration of 60 minutes in most
experiments. The line from the sampling probe to the impinger entrance is heat traced
with heating tape to maintain the sample temperature above 473 K as per the Ontario
Hydro protocol. At the conclusion of each experiment, impingers are rinsed and
solutions stabilized according to Ontario Hydro procedures. All samples are then
analyzed for mercury using cold vapor atomic absorption (CVAA) spectroscopy at the
Environmental Research Institute of the University of Connecticut.
The composition of the flue gas was approximately, 26%v/v of water, 13%v/v
COz and 60%v/v Nz [l], corresponding to a flue gas equivalence ratio of one.
Therefore, the flue gas did not contain Ozin significant quantities.
Experimental Results
Previous studies of mercury oxidation by C12 in a simulated flue gas showed that
significant mercury oxidation (up to 70%) could occur under post-combustion
conditions corresponding to cooling rates of 200-300 Ws, residence time 1.5 s, and
molecular chlorine concentrations 50 to 150 ppm [23]. To W h e r assess the effect of
chlorine concentration and cooling rate on mercury oxidation, additional experiments
were conducted in the flat flame reactor mentioned above. Experiments were again
conducted at a total gas flow rate of 31 slpm and a mercury concentration of
50 pg/Nm3. Chlorine concentrations of 50, 100, and 300 ppm were examined.
As shown in Table 2, with a chlorine concentration of 50 ppm and a mixing
chamber temperature of 1353 K, 7-10% of the mercury was oxidized. No dependence
on sampling position was observed. At chlorine concentrations of 100 ppm and 300
ppm, then 3645% and 66-69% oxidation was observed, respectively; also with no
appreciable dependence on sampling location.
An experiment was also conducted with both HCl and ClZsimultaneously. This
experiment was conducted under standard flame conditions and utilized a mercury
concentration of 50pg/Nm3, a Clz concentration of 5Oppm, and a HC1 concentration of
lOOppm Gas sampling was conducted through sampling port 1 only. As indicated in
Table 2, 10% of the mercury was oxidized in this experiment. This value was
consistent with levels observed for oxidation with Clz alone and suggested no change
in the concentration of reactive species, presumably C1 atom, as a result of increased
total chlorine concentration associated with HCl addition.
Table 2. Results of mercury oxidation experiments with CI?.
Clz concentration
Sample Port
~ g + ~ ( p p Hgo
~ (&
% Hg
The Oxidation Kinetics of Mercury in Hg/O/H/Cl System
Results and Discussion of Kinetic Modeling
In order to validate the reliability of the kinetic mechanisms, the kinetic modeling
results were compared with the thermodynamic equilibrium calculation. The
composition of the flue gas was approximately, 26%v/v of water, 13%v/v C02,
60%v/v Nz and a mercury concentration of 50 pglNm3. The external factors
considered included temperature from 400K to 1800K, and a constant pressure of
1.0 atm. Figure 3 shows that HgClz is the main product at the low temperature of
400-700 K. The results indicate that the mercury oxidation kinetics show some
agreement with thermodynamic equilibrium calculation, but still has some deviation.
This is perhaps because some estimated Arrhenius constants are not precise enough
and far away from real conditions, and needs to be improved in the future.
-0k i n e tic c a I c u l a tio n
Tern p e r a t u r e K
Figure 3. Comparison between kinetic modeling and equilibrium calculation.
M am ani-Paco
kinetic calculation
Figure 4. Kinetic calculation and experimental data.
Y. Qiao and M.H. Xu
Figure 5. HgCl, concentration-time correlation at 1353 K under drfferent
I 4
t 2
Figure 6. Kinetic modeling of experiments conducted with or without HCI.
The present calculations are based on a package of CHEMKIN-3.6 [3], developed
for the analysis of elementary gas-phase chemical kinetics. Figure 4 shows the
mercury oxidation kinetic calculation under three chlorine concentrations,
corresponding to experimental results tabulated in Table 2. As listed in Table 2, with
a chlorine concentration of 50 ppm and a mixing chamber temperature of 1353 K,
then 7-10% of the mercury was oxidized. No dependence on sampling position was
observed. At chlorine concentrations of 100 ppm and 300 ppm, then 3645% and 6669% oxidation was observed respectively; also with no appreciable dependence on
sampling location. In the current kinetic calculations, 13.3%, 25.0% and 66.6% of the
The Oxidation Kinetics of Mercwy in Hg/OH/Cl System
mercury oxidation was obtained at 50 ppm, 100 ppm and 300 ppm chlorine
concentrations respectively. Figure 5 shows the temporal concentration of HgC12
species under different C12 concentrations at temperature 1353 K. It can be seen that
mercury is oxidized to HgC12 in about one second under 50 ppm C12 at this
temperature. The trends are consistent with reports in the literature [30].
An experiment was also conducted by Mamani-Paco and Helble [ 13 with both HCl
and C12 simultaneously under standard flame conditions, and utilized a mercury
concentration of 50 pg/Nm3, Cl2 concentration of 50 ppm, and HC1 concentration of
100 p p n In their experiment, a mercury oxidization of about 10% was observed. This
value was consistent with levels observed for oxidation with C12 alone, and suggested
no change in the concentration of reactive species, presumably C1 atom, as a result of
increased total chlorine concentration associated with HCl addition. These trends are
consistent with reports in the literature [l].
Several studies have addressed the issue of mercury transformation kinetics. Studies
involving reaction with C12 are of principal interest, as it is expected to be the oxidant
of most importance in the combustion system. Mamani-Paco and Helble [ 11 noted that
HgCl is a relatively unstable intermediate. They further argued that this mechanism
can account for the super-equilibrium production of HgC12at a temperature of 1173 K
observed in both their data and the literature [17, 311. Therefore, the C1 atom
concentration is expected to be rate determined. However with the super-equilibrium
C1 atom concentration in the post-flame gases predicted by kinetic calculations,
Senior et al. [lo] pointed out that reaction to form HgC12 would be nearly
From the current investigation and previous studies, it is concluded that chlorine
species are the primary oxidants with mercury in coal combustion systems, with
molecular chlorine being more effective than HCl. However, the potential importance
of atomic chlorine suggests that reactions be examined under the rapid quench
associated with coal combustion conditions, in order to determine whether nonequilibrium species profiles affect the extent of mercury reaction.
This paper presents a simple kinetic model of mercury oxidation in the Hg/O/WCl
system. The lunetic modeling results are consistent with the thermodynamic
calculations. Meanwhile, the results from the kinetic model calculation are in
reasonable agreement with Mamani-Paco and Helble [ 11 experimental data. However,
some deviation still exists because of the aberration of some estimated Arrhenius
constants. Future efforts are acquired with the rate constants, both by experiments or
directly by calculation with quantum chemistry and other theoretic methods in order
to substrate the estimated value, thus improving the kinetic model of mercury
oxidation during combustion.
This work was sponsored by the National Key Basic Research and Development
Program, the Ministry of Science and Technology of China (2002CB2 11602). Partial
support from Natural Science Foundation of China (50325621) is also appreciated.
Y. Qiao and M.H.Xu
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