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Fate of Mercury in Cement Kilns
Paper #1203
C.L. Senior, A.F. Sarofim
Reaction Engineering International, Salt Lake City, UT
E. Eddings
Chemical and Fuels Engineering, University of Utah, Salt Lake City, UT
ABSTRACT
Mercury is one of a number of pollutants (like dioxins) that persist in the environment
and bioaccumulate in the food chain. Because of its toxicity and the potential for
bioaccumulation, mercury emissions to the environment are the subject of environmental
regulation. U.S. EPA estimates that 87% of the man-made emissions of mercury come
from point sources of combustion. There are currently emission limits on mercury from
certain categories of combustion sources, including cement kilns and incinerators burning
hazardous waste. Cement kilns that do not burn hazardous waste are not subject to these
emission standards. However, EPA is currently reviewing the need for emission
standards for mercury and other pollutants from cement kilns. In this paper, we review
the chemistry of mercury in the cement-making process and develop a simple model for
understanding the distribution of mercury among various streams in different types of
cement kilns.
INTRODUCTION
EPA, in its Mercury Study Report to Congress1 in 1997, put the amount of mercury
released into the atmosphere from human activities between 50 and 75 percent of the total
yearly release from all sources. According to this estimate, 158 tons of mercury was
emitted annually from all human activities in the U.S., most of this (87%) from
combustion point sources. Hazardous waste combustors, including Portland Cement
facilities that burn wastes, contributed 7.1 tons per year or 4.4% of the total.
Manufacturing sources contributed 10% to this estimate. The Portland Cement
manufacturing industry (excluding facilities burning hazardous waste) was estimated to
contribute 4.8 tons per year, or 3.1% of the total.
Only a fraction of Portland Cement kilns in the U.S. burn hazardous waste. In a survey
of cement plants in 20002, only 8 plants were identified as burning waste as a primary
fuel (Table 1). The capacity of these plants corresponds to about 5% of the total
manufacturing capacity in the U.S. However, a larger number of plants identified waste
as an alternate fuel, corresponding to about 50% of the total manufacturing capacity.
There are currently emission limits on mercury from certain categories of combustion
sources, including facilities burning hazardous waste. Table 2 summarizes the current
mercury emission standards for existing hazardous waste combustors in the U.S. and
Europe, compared with those for municipal waste combustors in the U.S. The EPA uses
a Maximum Achievable Control Technology (MACT) standard to specify control
technologies for specific pollutants. For mercury emissions from hazardous waste


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combustors, the MACT standard is feed rate control, based on input feed rates to the
combustor.
Table 1. Portland Cement Plants in the U.S. Using Waste as Fuel (Reference 2).

             Plants using waste as a primary fuel
                                                 Capacity
             Process                 Plants      tons/day 1000's tons/yr
             Dry                     1           2,130    680
             Wet                     7           11,654   3,889
             Total                   8           13,784   4,569
             Plants using waste as an alternate fuel
                                                  Capacity
             Process                Plants        tons/day    1000's tons/yr
             Dry                    7             13,498      4,295
             Dry(Preheater)         8             13,444      4,334
             Dry(Precalciner)       19            54,941      17,694
             Wet                    17            31,139      9,951
             Total                  51            113,022     36,274



 Table 2. Mercury emission standards for existing sources in µg per dry standard cubic meter
 in µg/dscm at 7% O2 (Reference 3).

    Hazardous        Hazardous          Hazardous            Hazardous           Large
      Waste         Waste Cement           Waste               Waste            Municipal
   Incinerators        Kilns            Lightweight          Combustors          Waste
                                         Aggregate            (Europe)         Combustors
                                           Kilns
       130                120               47                  130               80
Cement kilns that do not burn hazardous waste are not subject to these emission
standards. However, EPA is currently reviewing the need for emission standards for
mercury and other pollutants from cement kilns.
MERCURY BEHAVIOR IN COMBUSTION SYSTEMS
Coal is a common fuel used in cement kilns. Mercury is present in coal in low
concentrations, on the order of 0.1 µg/g. Mercury concentrations in petcoke and tires,
which are often burned in kilns, are lower.
Mercury is also present in certain wastes burned in cement kilns, in various
concentrations. In the high temperature combustion, all the mercury in the fuel is
vaporized as elemental mercury. Coal and liquid wastes are injected through the primary



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kiln burner and any mercury contained in them is exposed to flame temperatures, which
will convert all the mercury to the gaseous, elemental form.
Mercury is also found in the raw materials that enter the kiln. The range of mercury in
limestone, the largest component of the raw material, is reported to be 0.005 to 0.45 ug/g,
but measurements are scanty.4
In high temperature combustion systems, mercury exits the flame region in the elemental
gaseous form (Hg0). Subsequently, mercury can be oxidized homogeneously, oxidized
heterogeneously or adsorbed by ash or activated carbon. Gas-phase thermodynamic
equilibrium calculations suggest that HgCl2 is the dominant oxidized species in flue gas
at temperatures below 900-1100 F. Equilibrium predictions of mercury speciation do not
agree with measured mercury speciation at the inlet to particulate control devices in coal-
fired power plants5 nor with the speciation of mercury in medical waste incinerators
utilizing a water quench after the combustion chamber, which suggests that mercury
species are not in equilibrium as the flue gas cools.
At moderate temperatures in a flue gas environment, mercury is thought to react with
chlorine atoms, which are formed by the interaction of HCl with free radicals. Kinetic
calculations have been carried out on simulated combustion flue gas containing
chlorinated compounds.5 These calculations showed that equilibrium was not achieved
for chlorinated compounds in a rapidly cooling gas with cooling rates typical of the
convection section of a coal-fired power plant waste-to-energy plant. Thus, it seems
reasonable to conclude that the oxidation of mercury via chlorinated compounds does not
reach equilibrium under conditions of rapid quenching.
In incinerators that use a water quench, cooling is even more rapid than in combustion
systems that generate steam. There is indirect evidence of the inability of wet scrubbers
on incinerators with water quench to capture mercury,6 presumably because in those
systems, the mercury is still predominantly in the elemental form at the scrubber.
Elemental mercury, as discussed below, is not removed by wet scrubbers, unlike oxidized
forms of mercury. Experiments carried out on the speciation of mercury in a reactor that
mimicked the flue gas composition and residence time of a typical incinerator6
demonstrated that even at high (3000 ppmv) levels of HCl in the flue gas, mercury was
not oxidized if the gas was quenched too rapidly.
Mercury can be also oxidized heterogeneously or adsorbed by fly ash or activated carbon.
Ash plays a role in both the adsorption of mercury and the oxidation of elemental
mercury in flue gas at temperatures characteristic of particulate control device. Unburned
carbon in ash has been suspected of adsorbing mercury in coal-fired power plants.
In a dry process kiln, the gas exits the kiln at much high temperatures such that mercury
will exit the kiln in the gaseous form. But in the preheater tower, mercury may be
adsorbed on the raw meal, enriching both the kiln feed and the CKD. As with wet
process kilns, concentrations of mercury in the kiln itself should be higher than predicted
from the mercury content of the input streams.




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Detailed measurements of mercury within Portand cement kilns have not been made.
However, the behavior of thallium has been studied. Thallium is reported by Sprung7 to
have the highest volatility of the trace metals that were assayed in cement kilns; the other
metals were Zn, Pb, As, Ni, Cr, Cd. Figure 1 shows the concentrations of thallium and
other metals measured in solids collected from various stages in a preheater kiln. Volatile
                                                                       metals (e.g.,
 Figure 1. Distribution of selected metals in stages of preheater      thallium) condense
      7
 kiln.                                                                 at lower
                                                                       temperatures on the
     10000
              Stage: I II      III IV
                                                                       solids than other
       9000                                                            metals. Since the
                                                                       solids are being
   Hourly quantity in feed material, g/h




       8000

       7000                                  Pb                        preheated before
                                                                       introduction into the
       6000
                                                                       kiln, the condensed
       5000
                                                                       metal will be
       4000                                                            recycled back into
       3000                             K2O x10                  -3    the kiln. The
       2000
                  Thallium                                             concentration of
       1000
                                      Cl x10                -3         volatile metals will
                                                                       build up in kiln over
          0
            0    500      1000   1500      2000 2500 3000
                                                                       time.
                                           Temperature, F
                                                                      The example of
                                                                      thallium suggests
that mercury will have similar behavior; the boiling point of mercury is considerably
lower than that of thallium.
In a wet process kiln, the gases cool to 400oF or less at the kiln exit. Mercury may be
oxidized and/or it may condense on the raw material. In the latter case, the mercury will
move along the kiln in the solid bed and then vaporize in mid-kiln after the solids dry and
begin calcining. This will result in the gaseous mercury concentration within the kiln to
build up to high levels, setting up a recycle loop for mercury within the kiln. As
discussed below, some of the mercury will adsorb on the cement kiln dust (CKD) and be
removed in the particulate control device and some will leave the kiln in gaseous form.
When CKD is reinjected into the kiln, as is commonly done, levels of mercury in the kiln
itself will be considerably higher than one would predict from looking at the content of
mercury in the fuel and raw materials.
MERCURY EMISSIONS FROM PORTLAND CEMENT
MANUFACTURE
Sources of mercury in cement kiln feed streams
Coal is a primary fuel for many cement kilns, whether or not they burn hazardous waste.
Coal contains very small amounts of mercury; typical values of mercury in bituminous
coals of economic importance in the U.S. are 0.05 to 0.25 µg/g coal (dry basis). When
translated into concentration in the flue gas, this corresponds to 4 to 20 µg/DSCM at 7%
O2.


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Figure 2 shows the cumulative distribution of mercury in solid fuels. These data come
from EPA’s Information Collection Request (ICR) and represent multiple fuel samples
from every coal-fired power plant in the US taken during the fourth quarter of 1999.8
The figure demonstrates the range of mercury concentrations in solids fuels.

 Figure 2. Distribution of mercury concentrations in solid fuels from ICR, Part 2 data
 for fourth quarter, 1999.8
                     100%
                                Petcoke

                     80%
                                                               Lignite
                                Tires
  %Less than value




                     60%                                 Bituminous


                                                 Subbituminous
                     40%




                     20%




                      0%
                            0       0.05   0.1    0.15   0.2     0.25        0.3   0.35   0.4   0.45   0.5
                                                          Hg content, ug/g

Cement kilns in the United States have been co-firing hazardous waste with other fuels
for 30 years. Most kilns that burn hazardous waste are wet process kilns. The reason for
this has more to do with economics than process, however. Wet process kilns require
more energy per ton of clinker than dry process kilns. Since fuel costs are an important
part of the cost of cement production, burning a “fuel” that generates income helps wet
process kilns reduce operating costs. Liquid hazardous wastes are most commonly fired
in cement kilns through the primary burner. Liquid wastes include the following:
                       •        Residues from industrial or commercial painting operations;
                       •        Metal-cleaning fluids and lubricants;
                       •        Electronic industry solvents;
                       •        Solvents from automotive aftermarket operations.
Some of these wastes contain trace metals, including mercury. Many of them also
contain significant amounts of chlorine, which can affect the chemistry of mercury in the
flue gas.
The kiln feed materials also contain measurable amounts of mercury as illustrated in
Table 3 using data from EPA’s database of emissions from hazardous waste combustors
(HWCs)9 as part of the process of setting maximum achievable control technology
(MACT) standards for HWCs. This database contains emissions data from 13 different


                                                                         5
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wet process cement kilns that burn hazardous waste. In some cases, metals and/or
organics were spiked into the feed for testing purposes. In Table 3, a subset of the data is
shown for samples taken without spiking of metals in the feed and presumably (from the
low percentage of mercury in the raw material) without recycle of CKD into the raw
material. All the mercury concentrations have been converted to the equivalent of
µg/DSCM at 7% O2 in the flue gas. The amount of mercury entering the kiln in the raw
material appears to be on the same order as that in the coal, for these particular kilns. The

 Table 3. Mercury in feed streams to select wet process kilns expressed as µg/DSCM
 at 7% O2 in the flue gas (Reference 9).
                     Source
                       ID       Haz.       Raw
                     Number     Waste      Mat’l      Coal      Total
                       203      17.6        6.2       6.6        30.4
                       302      17.1        2.1                  19.2
                      302A      17.1        2.1                  19.2
                       319      18.9        0.8        0.8       20.5
                       319       5.9        0.7        0.3        7.9
                       322      71.4        1.2                  72.5
                       323                  3.1        3.6        6.7
                       323       152.9      5.4                 158.2
                       323        24.0      3.1                  27.0
                       323       112.8      3.4                 114.5
                       404        86.9      8.0        7.5       98.4
                       404        27.8      8.8        5.9       42.5
                       473       456.8                          456.8
hazardous waste typically has more mercury than either the coal or the raw material and
there is a very wide range of mercury in the hazardous waste.


Emissions of mercury from cement kilns
In order to get an idea of the range of mercury emissions from cement kilns, we will look
at two sets of data, one for kilns burning hazardous waste and one for kilns that do not
burn hazardous waste.
The Portland Cement Association compiled a database of cement kiln emissions,10
including emissions of mercury for kilns that do not burn hazardous wastes. The mercury
measurements are from 35 different sampling reports with a total of 50 measurements.
Various types of kilns, fuels, and particulate control devices are represented in the
database. Figure 3 shows a frequency distribution of the measured mercury emission.
Mercury concentration is reported in µg per dry standard cubic meter (DSCM) at 7% O2.
Statistical analysis of the data did not reveal significant differences between the
emissions as a function of type of process, but the mean mercury emission was higher for
kilns with fabric filters as compared to those with electrostatic precipitators. One group
of measurements falls into the expected range for coal-fired boilers; this might not be


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                                                                                             DRAFT

  surprising since many of these kilns burn coal as the primary fuel. What is surprising,
  however, is the number of measurements above 50 µg/DSCM.


                                   Figure 3. Distribution of stack concentrations of mercury (in µg/DSCM at 7%
                                   O2) for selected kilns that do not burn hazardous waste (Reference 10).


                                                                                    Range of                       Range
                                                                                    Coal-fired boilers             of MWCs
                                       Number of Tests in Range




                                                                  20
                                                                  18
                                                                  16
                                                                  14
                                                                  12
                                                                  10
                                                                   8
                                                                   6
                                                                   4
                                                                   2
                                                                   0
                                                                       0.01- 0.1-1 1-10 10-20 20-50 50-             100- >500
                                                                        0.1                         100             500
                                                                                             Mercury, ug/DSCM


Figure 4. Distribution of stack concentrations of mercury
                                                                                                                 The EPA database of
(in µg/DSCM at 7% O2) for selected kilns that burn
                                                                                                                 emissions from hazardous
hazardous waste (Reference 9).
                                                                                                                 waste combustors
                                                                                                                 (HWCs)10 previously
                                                                  Range of                           Range       mentioned contains
                                                                  Coal-fired boilers                 of MWCs     emissions data from 13
                                                                                                                 different wet process
   Number of Tests in Range




                              18                                                                                 cement kilns that burn
                              16                                                                                 hazardous waste. In some
                              14
                                                                                                                 cases, metals and/or
                              12
                              10                                                                                 organics were spiked into
                               8                                                                                 the feed for testing
                               6                                                                                 purposes. Figure 4
                               4                                                                                 summarizes the distribution
                               2                                                                                 of mercury emissions based
                               0                                                                                 on 60 different samples.
                                   0.01- 0.1-1 1-10                           10-      20-    50-    100- >500   The mercury emission for
                                    0.1                                       20       50     100    500         each sample represents an
                                                                        Mercury, ug/DSCM                         average of three
                                                                                                                 measurements.



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                                          DRAFT

Table 4 compares the two datasets. The non-hazardous waste dataset represents all types
of kilns and the hazardous waste dataset, only wet process kilns. However, there was no
statistically significant difference noted in the former dataset among various kiln types.
The dataset of hazardous-waste burning kilns has a mean and median that are about twice
that of the dataset from the kilns that don’t burn hazardous waste. Since the compositions
of wastes vary, no absolute comparison can be made. However, we can get an idea of the
range of mercury emissions from cement kilns, whether or not they fire hazardous
wastes.
                                                  There is an inherent recycle of a volatile
 Table 4. Mercury emissions from                  metal such as mercury within the kiln,
 cement kilns burning hazardous waste9            both due to recycle of mercury-containing
 and not burning hazardous waste10 (in            cement kiln dust and to large temperature
 µg/DSCM at 7% O2).                               gradients (in wet process kilns), it may
                                                  take a long time for mercury to reach
                                                  steady state in a cement kiln. This makes
  Kiln type              Wet        All           it difficult to make accurate mass balance
                         HW non-HW                measurements of mercury, particularly
  Median                 17.4       8.3           when mercury is only spiked in the feed
                                                  for short periods of time or where mercury
  Mean                   66.4      28.0
                                                  concentrations are changing rapidly in the
  Standard deviation 201.2         62.7           feed stream. To illustrate the difficulties
  Minimum                0.2        0.0           in closing the mercury mass balance, we
  Maximum              1237.7 385.6               compare the mercury in the feed streams
                                                  to the mercury in the stack for selected
                                                  samples from the EPA HWC database.9
We have excluded any measurements in which mercury was spiked into the feed, with
the understanding that it can take a long time to come to steady state when metals are
spiked into the feed. The data on mercury in the raw material appear to include both
CKD and raw meal in some cases, which suggests that CKD was being recycled for those
cases. We have adjusted the concentration of mercury in the raw material in those cases
either to reflect the mercury level in the absence of recycle (if those data are available for
that kiln) or the mercury level in typical raw material. Figure 5 shows the ratio of
mercury in the stack emission to mercury in the feed. There was generally a significant
amount of mercury in the stack, relative to the feed concentration. In many cases, the
amount of mercury in the stack was greater than the amount of mercury in the feed. As
discussed above, this illustrates the inherent difficulty of obtaining a steady state
measurement of mercury in cement kilns.




                                              8
                                                                DRAFT

 Figure 5. Ratio of mercury in stack emission to mercury in feed stream for selected wet
 process kilns with and without recycle of cement kiln dust (Reference 9).

                                             8
                                                                                    No Recycle
                                             7
                                                                                    Recycle
                                             6
                  Number of Tests in Range
                                             5

                                             4

                                             3

                                             2

                                             1

                                             0
                                                 0-0.1   0.1-0.5 0.5-1      1-2     2-5       5-10
                                                         Hg in stack emission/Hg in Feed



The distribution of mercury in a dry process, preheater kiln is illustrated in Figure 6.
These data are taken from a large study of trace metals in many different kilns.11 In this
particular kiln, mercury was also spiked into the feed, but the amount of the spike has
been removed from the mass balance because of the uncertainty of the time needed to


 Figure 6. Mercury flow rates in a dry process, preheater kiln; all flows in lb/hr
 (Reference 11).
              Main Stack: 0.0046

  Feed:
  0.0399
                                                    Bypass Stack: 0.0021   Cooler Stack: 0.0003
                 Preheater
                                                             Bypass Dust: 0.0009


                                                              Kiln             Cooler       Clinker:
                                                                                            0.0101

                                                                                    Liquid Waste:
    Inputs:   0.0419 lb/hr
                                                              Tires:                0.0003
    Outputs: 0.0179 lb/hr                                     0.00007
                                                                              Coal:
    Mass Balance Closure:                                                     0.0016
    43%




                                                                     9
                                                      DRAFT

reach steady state. The mass balance closure is 43%.
MODELING MERCURY IN CEMENT KILNS
Models have been developed12 that can be used to predict the distribution of trace metals
including mercury in cement kilns. In this work, a model previously developed by
Reaction Engineering International was used to examine the effect of recycle on mercury
emissions from cement kilns.
The model for the long, wet kilns is illustrated in Figure 7. Table 5 gives the assumptions
used in the model. As shown previously, recycle of kiln dust back into the kiln has an
impact on mercury emissions. Figure 8 illustrates the predicted stack emissions with and
without recycle of cement kiln dust. Stack emissions are higher with recycle, which is
the behavior that has been observed (Figure 5).

 Figure 7. Long, wet kiln process model for mercury.
 Main Stack
                                                                                                                 Coo
                                                                                                                 Vent


                   M18           A        M9
                                                                                                       M15
                                                                                             C
                                                                  M6   M5              M13
                           M19                       Long Kiln
                                                                                                 M14
                                                                            M10        M11             M12
              M1                     M2              M3           M4
              Raw Meal                                                                                 Clinker

                                                       M21
                                               Kiln fuel




                         Table 5. Assumptions in long kiln process model.
                         Dust entrainment in kiln, g CKD/g
                         clinker                                                   8.0%
                         Hg vaporization/recycle in kiln                          90.0%
                         Dust entrainment in clinker cooler                       10.0%
                         Collection efficiency, main ESP                          95.0%
                         Collection efficiency, cooler ESP                        30.0%
                         CKD recycle                                               0.0%
                         Hg concentration in fuel, mg/kg                            0.10
                         Hg concentration in limestone, mg/kg                       0.10
                         Fuel usage, kg fuel/kg clinker                             0.20
                         Limestone usage, kg/kg clinker                             1.52




                                                             10
                                                                         DRAFT

The model for the long, wet kilns is illustrated in Figure 8. Table 6 gives the assumptions
used in the model. As shown previously, recycle of kiln dust back into the kiln has an
impact on mercury emissions.
In precalciner kilns, the preheater/precalciner can be bypassed. A portion of the flue gas
is often recycled through a separate stack. The bypass stream does not pass through the
preheater, where mercury can condense on the raw material and be recycled into the kiln.
Figure 9 illustrates the predicted emissions from the main stack and bypass stacks. The
percentage of flue gas that exits through the bypass stack has an influence on stack
concentrations of mercury. Bypass shifts mercury emissions from the main stack to the
bypass stack.

 Figure 8. Precalciner kiln process model for mercury.
                                                                                                                       Bypass Stack


                                                                         M7                           M17
                                                                                           B


                                                                                               M16
     Main Stack                                      Preheater/Precalciner
                                                                                                                                                          Cooler
                                                                                                                                                          Vent


                           M18         A        M9                  M8
                                                                              M20                                                               M15
                                                                                                                                      C
                                                                                                 M6         M5                M13
                                 M19                                          Kiln
                                                                                                                                          M14
                                                                                                                 M10          M11               M12
                  M1                       M2                                 M3                 M4
                  Inputs                                                                                                                        Clinker
                                                           M22                       M21
                                                           Precalciner fuel          Kiln fuel




                                       Table 6. Assumptions in precalciner process model.
                                       Dust entrainment in kiln, g CKD/g
                                       clinker                                                                               14.0%
                                       Hg vaporization/recycle in kiln                                                       70.0%
                                       Dust entrainment in clinker cooler                                                    10.0%
                                       Kiln bypass                                                                           30.0%
                                       Carryover from preheater to APCD                                                      60.0%
                                       Collection efficiency, main ESP                                                       30.0%
                                       Collection efficiency, bypass ESP                                                     30.0%
                                       Collection efficiency, cooler ESP                                                     30.0%
                                       Hg concentration in fuel, mg/kg                                                       20.0%
                                       Hg concentration in limestone, mg/kg                                                    0.26
                                       Fuel usage, kg fuel/kg clinker                                                          0.11
                                       Fraction of fuel burned in kiln                                                            1
                                       Limestone usage, kg/kg clinker                                                          1.52


                                                                              11
                                                          DRAFT


     Figure 9. Effect of bypass ratio on the emissions of mercury from main and bypass
     stack on precalciner kiln.

                                    0.30

                                                    Main Stack
                                    0.25            Emissions

                                    0.20
                                             Bypass Stack
                      Hg/Hg Input



                                    0.15     Emissions

                                    0.10



                                    0.05                    Bypass ESP Dust

                                    0.00
                                        0%     5%   10%    15%   20%   25%   30%   35%

                                                           Bypass


CONCLUSIONS
There are currently emission limits on mercury from certain categories of combustion
sources, including cement kilns that burn hazardous waste. Cement kilns that do not burn
hazardous waste are not subject to these emission standards. However, EPA is currently
reviewing the need for emission standards for mercury and other pollutants from cement
kilns.
Mercury enters a cement kiln in the coal, in the raw material (kiln feed) and in hazardous
wastes (if they are burned). Mercury leaves the kiln in the clinker, CKD or via stack
emissions. The distribution of mercury within a cement kiln is difficult to measure
quantitatively because of the difficulty in reaching a steady state. There is an inherent
recycle of mercury between the hot and cold end of a kiln, and also from the reinjection
of mercury-containing CKD into the kiln. Models can be used to predict the distribution
of mercury in a cement kiln. In the future, such models might be used in conjunction
with control technology to minimize emissions of mercury from cement kilns.
REFERENCES
1     Keating, M.H., et al. Mercury Study Report to Congress, Volume I: Executive
      Summary, EPA-452/R-97-003, December 1997.
2.    Portland Cement Association, U.S. and Canadian Portland Cement Industry: Plant
      Information Summary, Data as of December 31, 2000 (Portland Cement Association,
      2001).
3.    U.S. Environmental Protection Agency, Hazardous Waste Combustion Frequently
      Asked Questions, http://www.epa.gov/hwcmact/faqs.html (March 26, 2002).



                                                            12
                                        DRAFT

4.   Johansen, V.C, Hawkins, G. J., Mercury Speciation in Cement Kilns: A Literature
     Review, PCA R&D Serial 2567 (Portland Cement Association, 2003).
5.   Senior, C.L., Sarofim, A.F., Zeng, T., Helble, J.J., Mamani-Paco, R., “Gas-phase
     transformations of mercury in coal-fired power plants,” Fuel Process. Technol.,
     2000, 63, 197-213.
6.   Gaspar, J.A., Widmer, N.C., Cole, J.A., Seeker, W.R., “Study of Mercury Speciation
     in a Simulated Municipal Waste Incinerator Flue Gas,” paper presented at the
     International Conference on Icineration and Thermal Treatment Technologies,
     Oakland, California, May 12-16, 1997.
7. Sprung, S., Technological Problems in Pyroprocessing Cement Clinker: Cause and
    Solution. (Beton-Verlag, 1985).
8. EPA Air Toxics Website - Utility Toxics HAP Study,
     http://www.epa.gov/ttn/atw/combust/utiltox/utoxpg.html#DA4
9.   U.S. Environmental Protection Agency, Hazardous Waste Combustion: NODA
     Documents, http://www.epa.gov/epaoswer/hazwaste/combust/comwsite/cmb-
     noda.htm, December 19, 2002.
10. Richards, J. “Compilation of Cement Industry Air Emissions Data for 1989 to 1996”,
    SP125, Portland Cement Association, Skokie, Illinois. .
11. Portland Cement Association, “An Analysis of Selected Trace Metals in Cement
    Kiln Dust,” SP109T, Skokie, Illinois, 1992.
12. Owens, W.D., Sarofim, A.F., Pershing, D.W. “The use of recycle for enhanced
    volatile metal capture,” Fuel Process. Technol., 1994, 39, 337-356.




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