inherent mercury
TRANSCRIPT
PCA R&D SN2841a
Inherent Mercury Controls within the Portland Cement Kiln System—Model of
Mercury Behavior within the Manufacturing System
by Constance Senior, Christopher Montgomery and Adel Sarofim
©Portland Cement Association 2008 All rights reserved
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KEYWORDS Emissions, mercury, model ABSTRACT As government agencies pursue mercury air emission reductions for electric utilities and other industries, portland cement manufacturers might receive mercury emission limits in the future. Understanding the behavior of mercury within cement kilns will help operators of portland cement kilns to devise methods for accurate measurement of mercury emissions and for reducing mercury emissions, if regulations are imposed.
This project has been divided into two activities with the objectives to: 1) assemble and analyze mercury mass balance data from portland cement facilities, and 2) develop a model of the mercury behavior within the manufacturing system. Schreiber & Yonley Associates (SYA) conducted the investigation into mercury mass balance data, and Reaction Engineering International (REI) developed the model of mercury behavior in the portland cement kiln system, which is presented in this report.
The REI model provides a useful tool for planning strategies for mercury controls by changes in the removal of dust and changes in the kiln operation. It is also provides a means for planning measurement campaigns to take into account the long times needed to reach steady state as a result of internal mercury recycle. With the knowledge gained from this investigation, the portland cement industry would be better prepared to discuss the fate of mercury in the cement manufacturing process, most notably the percentage of mercury actually emitted from the process. This information may also help the industry to enhance the inherent mercury controls to reduce or eliminate mercury emissions or in the selection of other control mechanisms. REFERENCE Senior, Constance; Montgomery, Christopher, and Sarofim, Adel, Inherent Mercury Controls within the Portland Cement Kiln System—Model of Mercury Behavior within the Manufacturing System, SN2841a, Portland Cement Association, Skokie, Illinois, USA, 2007, 32 pages.
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EXECUTIVE SUMMARY As government agencies pursue mercury air emission reductions for electric utilities and other industries, portland cement manufacturers might receive mercury emission limits in the future. Understanding the behavior of mercury within cement kilns will help operators of portland cement kilns to devise methods for accurate measurement of mercury emissions and for reducing mercury emissions, if regulations are imposed.
If mercury control regulations are placed on portland cement manufacturers, compliance verification will be required. In the past, various methods have been utilized to ascertain regulatory compliance, including mass balances that assume 100 percent of an element entering a manufacturing process will be released in the stack exhausts. For some industrial systems, this assumption may be true, but with the unique nature of cement kilns, this may be incorrect.
The major pathways by which mercury leaves the cement kiln system are stack emissions and cement kiln dust (if the latter is removed from the kiln system). In some instances, mercury has been measured in the clinker, although often the mercury measured in clinker was at the method detection limit. If some fraction of mercury were in the clinker, the transient behavior observed in cement kilns and predicted by the model would not change qualitatively, although stack emissions would be somewhat reduced.
Removal of cement kiln dust from the particulate control device can be used to reduce stack emissions of mercury. For kilns that have an in-line mill, removal of cement kiln dust when the mill is off line will prevent mercury from building up within the kiln system and reduce the concentration of mercury in the stack gas over the long run. For long kilns, removal of a portion of the cement kiln dust can be used to manage mercury emissions. The transient model can be useful in projecting steady-state stack concentrations of mercury.
Some kilns have a bypass stream, in which a portion of the kiln exhaust bypasses the preheater. This bypass is used to prevent the build-up of chloride, alkali, or sulfur species in the kiln (and the product). Mercury introduced into the primary fuel, which vaporizes in the flame, will be found in the kiln exhaust gas. Cooling of the bypass stream and collection of the dust can be used as a means to remove a portion of the mercury entering the kiln. This method does not remove mercury introduced with the raw meal, but it does reduce the amount of mercury being recycled within the kiln.
Concentrations of mercury in the stack are sensitive to the temperature of the particulate control device. For kilns with in-line mills, when the raw mill is on line the flue gas passes through the mill, which results in a lower temperature for the flue gas and a higher surface area for mercury adsorption, since mercury can be adsorbed on the raw material in the mill. Control of the temperature in the particulate control device, even when the raw mill is off line can be used to reduce stack emissions of mercury, as long as the dust from the particulate control device is not recycled back into the kiln.
Recycle of cement kiln dust back into the kiln feed results in long times needed for the mercury concentration in the stack emissions to come to a steady-state condition. Care must be exercised in making mercury measurements at a cement kiln, particularly if the kiln has an in-line raw mill and/or if the concentration of mercury in the fuel or feed changes significantly over a short period of time. The model suggests that many days are required for the mercury concentrations in the stack to come to a steady state after changes in operation such as taking the raw mill off-line or recycling of cement kiln dust.
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REI developed a model that can predict the effect of inherent mercury control within portland cement kiln systems. The model is modular so as to enable one to analyze cement kiln systems with different configurations. Modules have been developed for sections of a kiln: preheater cyclones, raw mill, feed silo with different degrees of mixing, and ESP or baghouse. These modules can be assembled to model long kilns, preheater kilns, and precalciner kilns, with and without a bypass duct, operated in either direct-operation or interconnected modes. Transient material balances on each module take into account (i) the evolution of mercury either chemically bound or adsorbed on the bed solids or entrained dust; (ii) adsorption of mercury by the solids from the gas phase; and (iii) entrainment of solids from the bed.
The model was benchmarked with a comprehensive dataset on the dynamic behavior of mercury in a portland cement kiln reported by the German Research Institute of the Cement Industry. Transient measurements were reported on the mercury content of the off-gas concentration and the kiln meal, including the recycled dust from the ESP, for operation with and without the raw mill on line.
The REI model provides a useful tool for planning strategies for mercury controls by changes in the removal of dust and/or changes in temperature of the particulate control device. It is also provides a means for planning measurement campaigns to take into account the long times needed to reach steady state as a result of internal mercury recycle.
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TABLE OF CONTENTS Page Keywords ......................................................................................................................................... i Abstract ............................................................................................................................................ i Reference ......................................................................................................................................... i Executive Summary ........................................................................................................................ ii Table of Contents ........................................................................................................................... iv Introduction ..................................................................................................................................... 1
Mercury Inputs to the Kiln .......................................................................................................... 1 Mercury Emissions from the Kiln ............................................................................................... 5 Kiln Operations Affecting Mercury Behavior ............................................................................ 8
Model Description ........................................................................................................................ 14 Results ........................................................................................................................................... 18
Case Study for Model Verification ........................................................................................... 18 Verification of Model ............................................................................................................... 23
Summary of Key Findings ............................................................................................................ 24 Acknowledgements ....................................................................................................................... 25 References ..................................................................................................................................... 26
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Inherent Mercury Controls within the Portland Cement Kiln System
by Constance Senior, Christopher Montgomery, and Adel Sarofim1
INTRODUCTION As the United States Environmental Protection Agency (US EPA), Environment Canada, and state/provincial agencies pursue mercury air emission reductions for electric utilities and other industries, portland cement manufacturers might receive mercury emission limits in the future. If mercury control regulations are placed on portland cement manufacturers, compliance verification will be required. In the past, various methods have been utilized to ascertain regulatory compliance, including mass balances that assume 100 percent of an element entering a manufacturing process will be released in the stack exhausts. For some industrial systems, this assumption may be true, but with the unique nature of cement kilns, this may be incorrect.
This project was divided into two activities, with the objective to: 1) assemble and analyze mercury mass balance data from portland cement facilities, and 2) develop a model of the mercury behavior within the manufacturing system. Schreiber & Yonley Associates (SYA) conducted the investigation into mercury mass balance data (Schreiber et al., 2005), and Reaction Engineering International (REI) has developed the mercury model, which is presented in this report. Mercury Inputs to the Kiln
When solid fuels or wastes are used as fuel in a portland cement kiln, mercury in the fuel enters the kiln system. Figure 1 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 (US EPA, Utility Air Toxics Website). The figure demonstrates the range of mercury concentrations in solids fuels. Bituminous coals (median value of 0.1 μg/g) typically contain more mercury than petcoke (0.05 μg/g median) or tires (0.04 μg/g median), although the range of fuel mercury content is broad.
1 Reaction Engineering International, 77 West 200 South, Suite 210, Salt Lake City, Utah, USA, 84101, (801) 364-6925.
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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 µg/g, but published measurements of the mercury content of limestone are few (Johansen and Hawkins, 2003; Senior and Eddings, 2006; Hills and Stevenson, 2006). More recently, Hills and Stevenson (2006) analyzed a total of 291 raw material samples from 57 cement plants in Canada and United States. Table 1 provides the statistical summaries for each material type from this study, presented in order of increasing mean values. Raw materials typically have lower mercury concentrations than coal, although the mass flow of raw material into a kiln is greater than the mass flow of fuel; thus, the raw material accounts for more of the mercury input than the fuel.
0%
20%
40%
60%
80%
100%
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
%Le
ss th
an v
alue
Petcoke
Tires
Subbituminous
Bituminous
Lignite
Figure 1. Distribution of mercury concentrations in solid fuels from ICR, Part 2 data for fourth quarter, 1999.
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Table 1. Summary of Mercury Concentrations in Kiln Feed Material (Hills and Stevenson 2006)
Number of Samples
Mean μg/g
Standard Deviation μg/g
Minimum μg/g
Maximum
μg/g Mill Scale 26 0.012 0.011 0.001 0.040
Slag1 10 0.012 0.016 0.002 0.054
Limestone 90 0.017 0.046 <0.0012 0.391
Sand 34 0.029 0.095 0.001 0.556
MI-3 Miscellaneous3 12 0.042 0.040 0.004 0.140
MI-2 Silica/Alumina 4 9 0.048 0.075 0.003 0.236
Bottom ash 12 0.048 0.106 0.003 0.382
Clay 28 0.052 0.071 0.002 0.270
Shale 17 0.057 0.101 0.002 0.436
Iron Ore 12 0.078 0.189 0.002 0.672
MI-1 Silica 5 6 0.095 0.191 0.007 0.483
Fly ash 16 0.205 0.240 0.002 0.685
Recycled CKD 19 1.530 5.590 0.005 24.560 1 Includes samples identified as slag, steel slag, copper slag, and iron slag. 2 To perform statistical calculations in category with one result of <0.001 (below the detection limit), a
value of 0.001 was used for that sample. 3 Includes samples identified as wood ash, bauxite, alumina, alumina dross, soil, sludge, marl, and lime. 4 Includes samples identified as sludge incineration ash, fly ash/bottom ash mix, pond ash, FCC,
catalyst fines, brick, volcanics, and cenospheres. 5 Includes miscellaneous samples identified as sandstone, diatomaceous earth, silica fume, foundry
sand, and sand blast grit.
Table 2 gives examples of the amounts of mercury entering kilns from different input
streams, using data from EPA’s database of emissions from hazardous waste combustors (US EPA, 2006), compiled as part of the process of setting maximum achievable control technology (MACT) standards for hazardous waste combustors. This database contains emissions data and feed data from both wet and dry process cement kilns that burn hazardous waste. All the mercury inputs have been converted by EPA to the equivalent of μg per dry standard cubic meters (dscm) at 7 percent O2 in the flue gas.
As Table 2 illustrates, the raw material entering the kiln contributes as much or more mercury to the kiln system as does the coal, when coal is the primary fuel. For example, in the set of plants shown in Table 2, the average ratio of the mercury content in the coal to the mercury content in the raw material was 0.29.
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Table 2. Mercury in Feed Streams to Dry Process and Wet Process Kilns, Expressed as μg/dscm at 7% O2 in the Flue Gas (U.S. EPA, 2006)
ID Hg in Feed, Avg (μg/dscm)
Number Hazardous
Waste Raw Mat’l Coal Total 3029C11 4.1 2.9 1.0 8.0 3031C1 -- -- -- 682.1 3031C2 -- -- -- 199.1 303C3 52.5 320.3 -- 377.5 303C7 27.6 1,220.9 8.7 1,257.2 303C9 8.6 1,075.0 -- 1,089.8
203 17.6 6.2 6.6 30.4 204 4.2 87.1 4.0 95.3 205 9.8 46.1 1.7 33.7 205 4.0 76.7 2.0 82.7 206 19.7 63.0 3.2 85.9 206 19.0 91.0 2.4 112.4 207 8.2 28.0 3.1 39.4 228 13.3 71.5 0.5 86.5 300 434.2 2,398.6 -- 2,839.3 302 17.1 2.1 -- 19.2 302 49.8 631.5 -- 681.3 318 456.8 -- -- 456.8 319 5.9 0.7 0.3 7.9 319 18.9 0.8 0.8 20.5 319 5.3 48.3 1.7 27.7 322 71.4 1.2 -- 72.5 323 -- 3.1 3.6 6.7 323 24.0 3.1 -- 27.0 323 112.8 3.4 -- 114.5 323 152.9 5.4 -- 158.2 403 13.3 71.5 0.5 86.5 404 27.8 8.8 5.9 42.5 404 4.4 40.9 2.0 48.7 404 86.9 8.0 7.5 98.4 473 456.8 -- -- 456.8 491 413.8 1,315.7 -- 995.8 491 434.2 2,398.6 -- 2,839.3 3030 456.8 -- -- 456.8 302A 17.1 2.1 -- 19.2 302A 49.8 631.5 -- 681.3 473A 456.8 -- -- 456.8
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Mercury Emissions from the Kiln
The Portland Cement Association compiled a database of cement kiln emissions (Richards, 2002), 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 2 shows a frequency distribution of the measured mercury emission. Mercury concentration is reported in μg/dscm at 7 percent 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 fell into the expected range for stack concentrations of mercury in coal-fired boilers (0.1 to 20 μg/dscm). There were also a significant number of measurements of mercury above 20 μg/dscm.
The EPA database of emissions from hazardous waste combustors previously mentioned contains emissions data from wet and dry process cement kilns that burn hazardous waste. Figure 3 summarizes the distribution of mercury emissions from this database, based on 45 different samples. The mercury emission for each sample represents an average of three measurements. In some cases, metals and/or organics were spiked into the feed for testing purposes, but spiked samples were not included in the analysis in Figure 3.
02468
101214161820
0.01-0.1
0.1-1 1-10 10-20 20-50 50-100
100-500
>500
Mercury, ug/DSCM
Num
ber o
f Tes
ts in
Ran
ge
Range ofCoal-fired boilers
Range of MWCs
Figure 2. Distribution of stack concentrations of mercury (in μg/dscm at 7% O2) for selected
kilns that do not burn hazardous waste (Richards, 2002). MWC refers to Municipal Waste Combustor.
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Recently, a more comprehensive analysis was carried out on the mercury emissions data obtained from stack emissions tests conducted since 1992 on cement kiln systems utilizing hazardous waste-derived fuels and summarized by Schreiber et al. (2005). These facilities collected mercury concentration data in other input and output streams as part of stack testing required under the Resource Conservation and Recovery Act (RCRA). The reported data were used to calculate system removal efficiencies (SREs) for mercury, based on the amount of metals emitted to the air compared to the amount entering the kiln from all feed streams.
Table 3 summarizes mercury mass balance closure and SREs from the analysis by Schreiber et al. There is an inherent recycle of a volatile metal such as mercury within the kiln (Owens et al., 2001; Schäfer and Hoenig, 2002), both due to recycle of mercury-containing cement kiln dust and to large temperature gradients (in wet process kilns); therefore, it may take a long time for mercury to reach steady state in a cement kiln. As noted by Schreiber et al., “the data are from relatively short-term tests that consisted of up to four one-hour runs at a particular kiln operating condition.” This makes it difficult to make accurate mass balance measurements of mercury, particularly if mercury is only spiked in the fuel for short periods of time, as has occurred during some mercury testing campaigns. This may explain the poor mercury mass balance closure shown in the table. Another possible explanation for the poor balance closure is that, for many of the datasets, the mercury content reported in the clinker and the raw meal were identical, reported to only one significant figure, suggesting that the values might have represented the detection limit. If so, the reported SREs would be artificially high. Measurements of Hg in cement (PCA, 1992) showed that of 105 measurements only 21 (one-fifth) were above the detection limit. Approximately half of these showed Hg concentrations of under 0.01 μg/g, and two-thirds, under 0.02 μg/g. These low values are consistent with the hypothesis that most of the mercury is emitted from the stack. For the rest of this report, the clinker concentration will be taken as being zero, but finite values can be introduced when reliable data become available.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 100 200 300 400 500 600 700 800
Hg emission, ug/dscm at 7% O2
%Le
ss T
han
Unspiked dry kilnsUnspiked wet kilns
Range of coal-fired power plant emissions
Range of MWC emissions
Figure 3. Distribution of stack concentrations of mercury (in μg/dscm at 7 percent O2) for
selected kilns that burn hazardous waste (U.S. EPA, 2006).
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Table 3. Mass Balance Closure and SRE Results from Cement Kilns Burning Hazardous Waste-Derived Fuels (Schreiber et al., 2005)
Facility/ kiln reference Kiln type
Test No.
Test condition
Run number
Fractional closure (Hg mass out/ Hg mass in) Mercury
SRE (%) A/3 Dry preheater or precalciner 1 Raw mill
off 1 0.836 35.5 2 0.910 14.4 3 0.260 85.8
Raw mill 4 0.105 92.1 on 5 0.065 95.4 6 0.119 92.2 7 0.109 92.8 B/1 Long wet 1 N/A 1 0.167 90.1 2 0.133 90.5 3 0.390 80.0 4 0.420 79.9 B/2 Long wet 1 N/A 1 0.255 90.1 2 0.199 91.9 3 0.197 92.6 4 0.281 84.6 5 0.350 84.0 6 0.270 91.6 C/1 Long wet 1 NA 1 2.500 -34.6 2 1.354 40.3 3 0.789 65.6 C/1 Long wet 2 N/A 1 0.640 68.8 2 0.625 66.5 3 0.738 56.0 D/3 Dry preheater/ 1 Raw mill 1 0.027 98.8 precalciner off 2 0.018 99.3 3 0.023 98.5 Raw mill 4 0.011 99.7 on 5 0.011 99.7 6 0.005 99.9 Averages All kilns 0.407 77.3
All wet kilns 0.582 71.1 All dry kilns 0.192 84.9 Dry kilns – raw mill on 0.346 72.0 Dry kilns – raw mill off 0.061 96.0
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Kiln Operations Affecting Mercury Behavior
Mercury is released into the kiln gases by vaporization from the fuel or the raw materials. Mercury release into the kiln depends on the design and operation of the kiln.
Mercury in raw materials is released into the kiln gases in the preheat section of a kiln. Senior and Eddings (2006) reported on the release of mercury as a function of temperature from pulverized limestone samples, obtained by two different methods: release from entrained powdered limestone in an externally heated furnace at constant temperature and release from a powdered sampled in a crucible, externally heated in a tube furnace to 700oC at a heating rate of 15oC/min.. The entrained flow furnace temperature was varied from 200oC to 540oC. There was good quantitative agreement between the two techniques. Thus, limestone samples can be expected to evolve mercury in the preheat section of a portland cement kiln, at temperatures between 200oC and 700oC. In preheater or precalciner kilns, mercury in the raw materials will be released in the preheater tower. In long kilns (wet or dry), mercury in the raw materials will be released within the kiln.
In the high-temperature combustion environment of the flame in the kiln and the secondary combustion chamber in precalciner kilns, mercury is vaporized from the fuel and enters the gas as elemental mercury (Senior et al., 2000).
At the lower temperatures characteristic of particulate removal devices or the cold end of a long kiln, mercury in the kiln gas adsorbs on entrained solids. Fabric filter (FFs) or electrostatic precipitator (ESPs) remove this absorbed mercury. Figure 4 shows data reported by Seo et al. (2007) of the mercury concentrations in the flue gas at three Korean cement kilns. No information on the type of kiln was given. Mercury concentrations were measured by the Ontario Hydro method and are total mercury (gas and particulate-bound). Mercury was measured at the inlet to the fabric filter and at the stack. Inlet temperature was given as 328oC (622oF); outlet temperature was given as 133oC (271oF). At two of the kilns, about 76% of the inlet mercury was removed across the fabric filter and at the third kiln, 28% was removed.
The temperature of the particulate control device influences how much mercury is converted to the particulate phase and removed across the particulate control device, and consequently how much mercury is emitted in the stack gas. Figure 5 shows the effect of ESP inlet temperature on mercury stack emission from a German cement kiln (Schäfer and Hoenig, 2002). In this example, mercury concentration in the stack gas doubled when the ESP temperature increased from 128-135oC (262-274oF) to 150-160oC (301-319oF).
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Figure 4. Mercury concentration in gas at three cement kilns, measured at fabric filter (FF) inlet and stack (Seo et al., 2007).
0
10
20
30
40
50
60
Hg
in s
tack
, ug/
m3
128 135 142 150 160
Temperature before ESP, oC
Figure 5. Mercury concentration in stack gas as a function of ESP inlet temperature (Schäfer and Hoenig, 2002).
0
10
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conc
entra
tion,
ug/
m3
Kiln 1 Kiln 2 Kiln 3
FF InletStack
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For plants with dry kilns and inline raw mills, the raw mill operation affects the temperature in the ESP or FF. Wet kilns typically use a wet ball mill. In wet kilns, the kiln exhaust gas is not used in the mill, since the ball mill produces a wet slurry feed. In dry kilns with inline mills, the kiln exhaust gases pass through the mill and then into the particulate control device when the mill is on line. When the mill is off line, the kiln exhaust gases go directly into the particulate control device. The temperature in the particulate control device is affected by mill operation; furthermore, when the kiln exhaust gases pass through the mill, there is an increase in the contact time between solids and gas. Both of these factors affect how much mercury is adsorbed by the kiln dust and thus the mercury concentration in the stack. Figure 6 illustrates this point with data from two German cement kilns (Schäfer and Hoenig, 2002). When the raw mill was on line, the temperature in the ESP was 130oC, while when the raw mill was on line, the ESP temperature increased. When the raw mill was off line, mercury concentration in the stack increased. When the raw mill was off line, the increase in mercury concentration in the stack with a temperature increase of 20oC was on the order of 10%. The large increase in stack Hg in going from 130oC to 134-137oC was therefore due to both temperature and increased contact time between gases and solids in the raw mill.
0
10
20
30
40
50
60
Hg
in s
tack
, ug/
m3
130 137 152 168 130 134 147 165
Temperature before ESP, oC
Plant A Plant B
Raw mill online
Raw mill online
Raw mill offline
Raw mill offline
Figure 6. Mercury concentration in stack gas as a function of raw mill operation and ESP inlet temperature (Schäfer and Hoenig, 2002).
When the raw mill is on line, mercury is adsorbed on the raw material. Some of the dust
carried from the kiln or preheater to the raw mill also becomes mixed with the ground feed and incorporated into the kiln feed, providing recycle of mercury into the kiln system. Additional recycle of mercury occurs if the dust from the particulate control device (FF or ESP) is recycled back into the kiln. Recycle of dust into the kiln can be accomplished by one of several. In many plants, it is mixed into the blend silo with the raw mill product. This allows for even mixing and
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consistent quality of the kiln feed. In some plants, dust from the particulate control device can be fed directly to a feed bin at the top of the preheater tower and into the kiln from there.
Operators of portland cement plants in the U.S. were surveyed about operational practices related to operation of the raw mills, recycling of dust, and blending of feed materials. While not a large survey, the answers (summarized in Table 4) illustrate the trends and variation in operation. The nine kilns in the survey were all dry kilns. In normal operation, the raw mills were not off line for very long: from one day out of seven to as little as eight hours every five weeks. Most plants had systems to blend the raw material in the silos. Recycled dust was sometimes added to the blend silos and sometimes added directly to the feed bin.
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Table 4. Responses from Kiln Operators Regarding Raw Mill Operation and Dust Recycling Question 1. Typical cycles for
mill on/mill off? 2. Mixing of raw material in silo?
3. Recycled dust to raw meal silo or directly to kiln?
4. Kiln bypass?
Plant A Mill taken down 10 hrs/wk for maintenance. Average 5 hrs/wk process downtime
Raw mix blended in four silos with Claudius Peters system used for blending.
Kiln baghouse dust is blended with raw mix.
Bypass operated. All dust removed from system.
Plant B Mill taken down 10 hrs/wk for maintenance. Other idle time 2-6 hrs/day due to silo limitations.
4 kiln feed tanks fed by 5 raw meal silos. Material from the raw meal silos extracted from the bottom and blended by a recirculation system.
Dust is added to the kiln feed tanks.
Facility operates long dry kilns with no bypass.
Plant C Mill taken down roughly 10 hours/wk (routine maintenance and inventory control).
Raw mill has two blend silos. Material from first cascades to the second and then to the kiln, resulting in mixing.
Dust returned directly to mill. Modification being made to allow plant the option of direct return to the mill or a return to the blend silos. Plant has not started return to blend silo equipment as of yet.
Facility operates long dry kiln with no bypass.
Plant D 6 days on, 1 day off. Basically plug-flow. Some mixing, but very little.
Dust returned directly to kiln.
No bypass.
Plant E 21 hours/day and may experience 1-2 brief interruptions per day on average.
Mixing occurs in blend silos, but extent of mixing depends on clinker specification targets and equipment status.
Has option of returning dust to blend silos and also directly back to kiln. Facility returns most to blend silos.
No bypass.
Plant F Two inline mills with annual utilizations of 80% and 85%.
Three blend silos, each aerated with compressed air. Efficient mixing of raw meal improves process stability.
Dust returned to blend silos for mixing with raw meal.
No bypass.
Plant G Mill on line as much as possible, only down for maintenance.
Recirculation of feed in homogenizing silos; aeration in blending silo.
Baghouse dust added directly into a feed bin on top of preheater tower; not typically blended in silos.
Plant H Mill uptime 93% of kiln uptime. Mill down 8-10 hr every 5 weeks for maintenance.
Raw meal mixed in kiln feed silo. Dust mixed with raw meal in blend silo.
Dust recycled into blend silo.
Precalciner kiln with alkali bypass.
Plant I One day off line and six days on line for raw mills.
Mixing in feed silos. All dust recycled back into kiln feed silos.
Precalciner kiln with no bypass.
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In summary, mercury removal from the kiln largely occurs via stack emissions and the dust collected in the particulate control device, which may be recycled or be removed from the pyro-process. Preheater or precalciner kilns can also have a bypass, which removes a portion of the dust and kiln exhaust gas at the end of the kiln. The need for bypass is a function of the need to prevent chloride, alkali, or sulfur compounds from building up in the kiln product. Bypass gas will contain mercury that has evolved from the primary fuel in the kiln. Mercury can absorb on the bypass dust and be removed by the particulate control device on the bypass line. Both operation of the raw mill and management of dust from the kiln are important in determining mercury stack emissions from portland cement kilns.
Efforts are underway to develop models for the behavior of mercury in order to devise strategies for controlling mercury emissions from cement kilns. A better understanding of the evolution of mercury within the portland cement kiln will help in developing appropriate models for mercury in kilns and in making more accurate predictions of mercury partitioning and emissions from cement kilns.
Previous studies on mercury emissions from cement kilns have been reviewed in order to guide the development of models. Very few tests have a good material balance closure, which has resulted in significant uncertainty in the fate of mercury. Two reasons for the difficulty in closing material balances are
• the recycling of mercury in the kilns, which leads to long transients with time constants of days;
• the difficulty in measuring the low concentration of mercury in the solids feed and in the clinker.
The following generalizations appear to provide the most consistent picture of the fate of
mercury. 1. In most cases, the raw materials account for most of the mercury input to the
kilns, with the balance being provided by the fuel. 2. The mercury vaporizes from the raw material at temperatures between 200oC and
700oC, while mercury in the fuel vaporizes in the flame. 3. The mercury is adsorbed by the solids in the cold end of the kiln, which are
captured in the ESP or baghouse. 4. The mercury captured by the ESP or baghouse can be recycled to the pyro-
process with the dust. If so, a recycle loop is established that builds up the mercury levels in the gas and entrained solids until steady state is established.
5. If the ESP or FF temperature is reduced or if kiln gas is passed through the raw mill (when the mill is on line), the mercury emissions in the stack will be reduced due to the higher absorption by the solids. As the solids are recycled to the kiln, the levels of emissions will gradually return to the original values, in the absence of dust removal. However, if dust is removed, the amount of mercury removed with the dust will increase with decreases in ESP or baghouse temperature so that the net emissions will be reduced.
6. For plants with inline mills, when the raw mill is off line, there will be less mercury adsorbed on the entrained dust (which is subsequently captured in the particulate control device).
7. In the absence of disposal of cement kiln dust or the use of a bypass stack, the mercury will eventually be emitted from the stack.
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MODEL DESCRIPTION Steady-state models have been developed (Owens et al., 1994) that were used to predict the distribution of trace metals including mercury in cement kilns. A model previously developed by Reaction Engineering International was used to examine the effect of recycle on mercury emissions from cement kilns (Senior et al., 2005). However, these previous models were not capable of reproducing the transient behavior of mercury recycling in the kiln.
A new model has been developed that can reproduce the observations of the transient mercury emissions and behavior. The processes involved are shown in Fig. 7. There are five “modes” in which Hg can exist: adsorbed on raw meal, adsorbed on dust, gas phase, chemically bound on raw meal, and chemically bound on dust. There are six interchange routes between modes, two reversible and four irreversible, as shown in Figure 7. Each interchange route has an associated mass flow rate and mass fraction of Hg. The interchange mass fraction may or may not equal the mass fraction in the originating substance. Mercury is assumed to be in the gas phase with a mass concentration YG, chemically bound in the solids with a concentration XC, adsorbed on the solid with a concentration XA chemically bound in the dust with a concentration YC, adsorbed on the dust with a concentration YA.
The following simplifying assumptions will be made in formulating the model. • Material flows other than mercury are assumed to be in steady state. The flow rates of
gas and solid are specified along the length of the kiln. Default values are taken from a study carried out by REI on a four stage preheater/precalciner and a 13.5-foot diameter rotary kiln 225 feet long. The simulations also provide the gas and solid temperatures and major species concentration.
YG
Chemically bound in dust
Gas-phase
Chemically bound in solid
Adsorbed on solid
Adsorbed on dust
XC
XA
YC
YA
evaporation
evaporation
dust entrainment
Adsorption/desorption
Adsorption/desorption
dust entrainment
YC-G
YG
mC-G.
YG
Chemically bound in dust
Chemically bound in dust
Gas-phaseGas-phase
Chemically bound in solid
Chemically bound in solid
Adsorbed on solid
Adsorbed on solid
Adsorbed on dust
Adsorbed on dust
XC
XA
YC
YA
evaporation
evaporation
dust entrainment
Adsorption/desorption
Adsorption/desorption
dust entrainment
YC-G
YG
mC-G.
mC-G.
Figure 7. Mercury modes and gas-solid exchange used in model.
15
• Solids are entrained in the gas per unit length of kiln along the rotary kiln. The mass fraction of solids entrained by the gases from the preheater/precalciner is given by Fentrained . The value can be calculated from the rate of solid collection in the ESP.
• The mercury in the feed is assumed to be chemically bound and to have a mass fraction of XC,0.
• Mercury in the gas is adsorbed by the solids both in the bed and entrained in the gas. The mercury in the entrained solids is assumed to be in equilibrium with the gas: XA* = FA (Y, T). The equilibrium relationship, FA, between the vapor mass fraction Y and the solid mass XA, could be given by the Langmuir or Freundlich isotherm relationships
• The rate of mercury interchange between mercury in the vapor and solids is assumed to be mass-transfer controlled.
Given these assumptions, the kiln model comprises a number of interconnected zones, as
shown in Figures 8 and 9 for a preheater kiln when the raw mill is on line and off line, respectively.
16
Figure 8. Schematic of preheater kiln with raw mill on line, or interconnected operation.
Figure 9. Schematic of preheater kiln with raw mill off line, or direct operation.
Rot.Kiln n
Rot.Kiln n+1
Raw Mill
ESP
Silo
PH1
PH2
PH3
PH4
CKD
Fuel + Air
Rot.Kiln n
Rot.Kiln n+1
Raw Mill
ESP
Silo
PH1
PH2
PH3
PH4
CKD
Fuel + Air
Rot.Kiln n
Rot.Kiln n+1
ESP
Silo
PH1
PH2
PH3
PH4
CKD
Fuel + Air
Rot.Kiln n
Rot.Kiln n+1
ESP
Silo
PH1
PH2
PH3
PH4
CKD
Fuel + Air
17
The balance for Hg adsorbed on the solids in the nth module, shown schematically in Fig. 10, is given by:
)*( ,,,,,,1,1,,
, nAnAnnAnenAnSnAnSnA
nS XXkAXmXmXmdt
dXm −+−−= −− &&& (1)
The term on the left represents the rate of change in the mass of adsorbed mercury on the
bed solids within the module. The four terms on the right hand side represent : (i) the adsorbed mercury on the solids flowing into the module; (ii) the adsorbed mercury on the solids flowing out of the module; (iii) the adsorbed mercury on the solids entrained into the dust in the module; and (iv) the mass-transfer-controlled adsorption of mercury from the gas phase, where
),( ,*
, nnAAnA TYFX = (2)
resnSnSnS mmm τ)(21
1,,, −+= && (3)
From mass conservation: nenGCnSnS mmmm ,,1,, &&&& −−= −− (4)
The chemically bound mercury in the feed is assumed to be released in amounts that are determined solely by temperature T, with the fractional release given by FC (T), with FC determined from TGA experiments. The mass fraction of the solid that is in chemically bound form is therefore
)(0,, nCCnC TFXX = (5) The evaporation rate is the rate that mercury is released from the feed in a given node. Assuming that the mass fraction of the mercury vaporized YC-G,n is unity, this rate is given by
( ) )(0,,10,1,,,, nCCnsnCCnsnGCnGCnGC TFXmTFXmmYm &&&& −== −−−−− (6) Similar equations can be set up for ACG YYY ,, .
Under this setup each module has a constant temperature, mass flow, and residence time. The number of nodes required to model a specific plant configuration depends on the temperature profile in the pyro-process and the locations at which solids are added and removed.
18
Figure 10. Control volume for preheater or rotary kiln node. RESULTS Since there are a multiplicity of operational modes and kiln designs, we have chosen to illustrate the model with an example for which we have input and output mercury measurements as a function of time, and for which there was a good mercury mass balance. The model is flexible and modular; it can be reconfigured for other kiln systems in the future. Case Study for Model Verification Schäfer and Hoenig (2001) carried out mercury mass balance measurement campaigns at two German cement kilns. These mass balance measurements were carried out over a period of three to four weeks. The kilns burned coal and 50-60 percent secondary fuel. The authors state that only a small amount of mercury was introduced via the secondary fuel. The mass balances around the plant were carried out by measuring Hg continuously in the stack gas and by taking hundreds of solid samples. The authors state that the mercury mass balance was closed within 7 percent during the overall measurement campaign.
Plant 1 was the focus of this mercury model benchmarking exercise in the present study, because more results from this plant were reported in the paper. This plant had an output of 3400 metric tons of clinker per day. The mercury content of the raw materials varied from 0.03 to 0.06 μg/g over the testing period. The kiln was operated with the raw mill off line (direct operation) for three to six hours per day; on the weekends, the raw mill was on line (interconnected operation). When the raw mill was off line, the temperature in the ESP was 135oC (275oF), but when the raw mill was on line, the temperature was 110oC (230oF).
The temperature differences between operation with the raw mill on line versus with the raw mill off line contributed to differences in the concentration of mercury in the clean stack gas: when the raw mill was on line, the mercury in the stack gas was 20 to 25 μg/m3, whereas when the raw mill was off line, the mercury concentration in the stack gas rose to as high as 42 μg/m3.
mS,n XC.n XA,n
mG,n YG,n
mC,n YC,n
mA,n YA,n
nAne Xm ,,&1,1, ++ nAnA Ym&
1,1, ++ nGnG Ym&
nAnA Ym ,,&
nGnG Ym ,,&
nAnCnS XXm ,,,&
nCne Xm ,,&
nCnC Ym ,,& 1,1, ++ nCnC Ym&
nGCnGC Ym ,, −−&
mS,n XC.n XA,n
mG,n YG,n
mC,n YC,n
mA,n YA,n
nAne Xm ,,&1,1, ++ nAnA Ym&
1,1, ++ nGnG Ym&
nAnA Ym ,,&
nGnG Ym ,,&
nAnCnS XXm ,,,&
nCne Xm ,,&
nCnC Ym ,,& 1,1, ++ nCnC Ym&
nGCnGC Ym ,, −−&
19
Table 5 summarizes the inputs and assumptions used to model Plant 1 with the model that
REI developed, as described above. In instances where parameters were not provided by Schafer and Hoenig, values considered to be reasonable across the industry were used.
Table 5. Key Inputs to the Model for the Conditions of Schäfer and Hoenig (2001)
Figure 11 diagrams the modules and their connections used to model Plant 1. Dotted lines
indicate flows during interconnected operation. The notation “g&d” indicates flow of gas and dust, while “s” indicates flow of solids. See Table 6 for conditions in each of the modules.
clinker production (metric tons per day) 3,400clinker production (kg/hr) 141,667kg fuel per kg clinker 0.2fuel (kg/hr) 28,333kg air per kg fuel 11.6air (kg/hr) 328,667coal ash content 10.0%ash flow (kg/hr) (becomes dust) 2,833combustion products gas flow (kg/hr) 354,167kg CKD per kg clinker 0.14total CKD 19,833CKD generated (kg/hr) 17,000kg raw meal per kg clinker 1.52raw meal flow (kg/hr) 215,333total gas outflow (kg/hr) (into ESP) 410,833gas generated (kg/hr) 56,667raw meal Hg (mass fraction) 4.50E-08coal Hg (mass fraction) 1.00E-07combustion products gas Hg mass fraction 8.0E-09
20
PH1 PH2 PH3 PH4
ESPSilo
Kiln 1Kiln 2
gas to stack
Raw mill
clinker
combustion products g&d
g&d
g&dg&dg&d
g&d
s
s
sss
sg&
d
dust
s g&d
PH1PH1 PH2PH2 PH3PH3 PH4PH4
ESPESPSiloSilo
Kiln 1Kiln 1Kiln 2Kiln 2
gas to stack
Raw mill
clinker
combustion products g&d
g&d
g&dg&dg&d
g&d
s
s
sss
sg&
d
dust
s g&d
Figure 11. Diagram of modular model setup for the conditions of Schäfer and Hoenig (2001). Table 6. Characteristics of Modules used in the Model Module T (oC) Gas Residence
Time (s) Solids Residence Time (s)
Dust Generation (kg/s)
Gas Generation (kg/s)
Silo 20 n/a 0 0 Preheater 1 300 0.78 0.5 0 0 Preheater 2 500 0.59 0.5 0 0 Preheater 3 700 0.47 0.5 0 0 Preheater 4 850 0.41 0.5 0 0 Kiln 1 1100 2.0 1560 17000 56667 Kiln 2 1800 2.0 1680 0 0 ESP 110-135 10. n/a 0 0 Raw Mill 110-135 n/a n/a 0 0
Adsorption of gas-phase Hg onto the dust and solids was modeled following the experiments of Karatza et al. (1996), with the temperature dependence of the adsorbed mass fraction modified to better fit the data of Schäfer and Hoenig. Adsorption was assumed to be a fully equilibrated function of the gas-phase Hg concentration, the gas temperature, and the mass of solids and dust present.
The kiln was represented as two modules with fixed temperatures of 1100 and 1800oC. Four preheaters were assumed. Their residence times were derived from a typical preheater volume and the flows of gas and solids. Typical temperatures and residence times were also assumed for the kiln modules. The model was insensitive to the choice of parameters for the kiln and silo modules as will be described below.
21
The silo was modeled as a perfectly stirred reactor (PSR) for operation with the raw mill on
line, in which the raw material and recycled dust were mixed, followed by a delay, which was modeled as a plug flow reactor. Various PSR residence time and plug-flow durations were investigated. It was found that a 0.2 day PSR residence time with a 1.0 day plug-flow delay gave the best agreement with the Schäfer and Hoenig data.
The raw mill, when on line, was treated as an extension of the ESP, having the same temperature and inflow gas Hg concentration, but having additional solids on which Hg might adsorb.
The temperatures were high enough and the residence times long enough in the kiln and preheater modules that all the chemically bound Hg from the feed entered the gas phase and negligible adsorption onto solids or dust occurred in the kiln or preheater modules. This was consistent with the findings of essentially no Hg in clinker cited earlier. As a result, the system dynamics as predicted by the model were completely dominated by adsorption of Hg onto dust in the ESP and by the recycle of mercury-enriched dust through the silo, if dust was recycled. This insensitivity to the dynamics of the kiln and preheater modules allowed the simplification of placing all the gas and dust generation from the feed (somewhat arbitrarily) in the first kiln module.
Figures 12 and 13 show data as reported by Schäfer and Hoenig for weekly operation of Plant 1, with and without removal of the dust, respectively. From the dates on the x-axis, the week in which dust was removed from the ESP immediately preceded the week without dust removal. The authors did not mention what type of operation occurred before the week with dust removal.
Figure 12 gives the temperature and mercury concentration in the stack from Sunday to Sunday. During this period, the raw mill was off line for periods during the first three days of the week; the stack gas temperature gives an indication of when the raw mill was off line. Spikes in the mercury concentration in the stack gas (up to 35 μg/m3) were not always observed when the raw mill was off line.
Figure 13 gives the temperature and mercury concentration in the stack during a week in which the dust was not removed from the ESP, but recycled into the feed silo. The cycling of the raw mill from on line to off line resulted in spikes in the mercury concentration in the stack gas (up to 42 μg/m3). The spikes in the stack gas mercury concentration appeared to be more sensitive to temperature changes in the stack gas when the raw mill was off line versus on line.
Schäfer and Hoenig also noted an increase in mercury concentration in the ESP dust when the plant switched from on-line to off-line operation of the raw mill, from about 2 μg/g to about 3.5 μg/g. When the raw mill came back on line, the concentration of mercury in the ESP dust dropped back down to the previous level. The authors of the study suggested it would be more effective for mercury emissions control to remove of dust from the ESP when the raw mill was off line, because of higher concentrations of mercury.
22
Figure 13. Mercury emissions without removal of dust from Sunday to Sunday: Hg stack concentration (Hg-Reingas-Konzentration); stack temperature Reingas-Temperatur); and raw mill on line (Verbunddetrieb). (Source: Schäfer and Hoenig, 2001)
Figure 12. Mercury emissions with removal of dust, from Sunday to Sunday: Hg stack
concentration (Hg-Reingas-Konzentration); stack temperature (Reingas-Temperatur); and raw mill on line Verbunddetrieb). (Source: Schäfer and Hoenig, 2001)
23
Verification of Model Figure 14 shows the model’s predictions of mercury concentration as a function of time. In the model for Plant 1, the dust was assumed to be recycled until a steady state was reached (left third of Figure 14). The dust recycle was discontinued for a week (middle third of Figure 14), and then dust recycle was started again (right third of Figure 14).
Figure 14. Predicted mercury concentration in the stack gas as a function of time.
The model assumed that the raw mill was off line for six hours per day during the week, and on line during the weekend. Figure 14 shows that the model responded to changes in stack temperature when the raw mill was off line. Without recycle of the dust, the spikes in stack Hg concentration were less pronounced (as was observed by Schäfer and Hoenig).
The different curves in Figure 14 represent the use of different lengths of residence time in the raw meal silo. Longer residence in the silo tended to dampen the concentrations of mercury in the stack gas.
When dust recycle was turned off, there was a shift in the baseline concentration of Hg in the stack gas (that is, in the average concentration). This illustrates that it can take longer than one week to reach a steady state regarding mercury emissions, after major changes in operation, such as dust recycle.
Figure 15 shows the predicted mass fraction of mercury in the ESP solids as a function of time. In the first week shown (with recycle of dust), the concentration of mercury in the dust was 1.0-1.5 μg/g when the raw mill was on line, but increased to 2.5-4.5 μg/g when the raw mill was off line. The concentration of mercury in the dust dropped back to the previous level when
0
10
20
30
40
50
60
70
80
890 892 894 896 898 900 902 904 906 908 910
time (days)
Hg
gas
emis
sion
s (u
g/m
^3)
1.0 day PSR0.2 day PSR0.2 day PSR, retuned
W
recycle on
recycle onrecycle off
MM FSa0
10
20
30
40
50
60
70
80
890 892 894 896 898 900 902 904 906 908 910
time (days)
Hg
gas
emis
sion
s (u
g/m
^3)
1.0 day PSR0.2 day PSR0.2 day PSR, retuned
W
recycle on
recycle onrecycle off
MM FSa
24
the raw mill came on line again. This is the same behavior observed by Schäfer and Hoenig in measurements of the mercury concentration in the ESP dust.
Figure 15. Predicted mercury concentration in the ESP dust as a function of time. In summary, emissions of mercury from cement kiln stacks are sensitive to the temperature
in the particulate control device, which changes if the raw mill is on line as opposed to off line. The solids residence time in the silo and the recycle of dust from the particulate control device to the silo also affect the stack concentrations of Hg. SUMMARY OF KEY FINDINGS
The model developed by REI was benchmarked with a comprehensive dataset on the dynamic behavior of mercury in a portland cement kiln reported by the German Research Institute of the Cement Industry. Transient measurements were reported on the mercury content of the off-gas concentration and the kiln meal, including the recycled dust from the ESP, for direct and interconnected operation. The REI model was able to reproduce the following features of the data:
• The spikes in the exhaust gas concentration from 20 μg/m3 to 35 μg/m3 during the three to six-hour period during weekdays when the plant switched from the raw mill on line to the raw mill off line (the actual values vary depending on the day of the week). The temperature of the exhaust gas typically increased from 110°C (230°F) to 135°C (275°F) during the switch from on line to off line.
• The high concentration of mercury in the ESP dust of 2 to 3.5 μg/g for a raw meal mercury concentration of 0.045 μg/g.
0.0E+00
5.0E-07
1.0E-06
1.5E-06
2.0E-06
2.5E-06
3.0E-06
3.5E-06
4.0E-06
4.5E-06
5.0E-06
890 892 894 896 898 900 902 904 906 908 910
time (days)
adso
rbed
mas
s fr
ac o
n ES
P so
lids
1.0 day PSR0.2 day PSR0.2 day PSR , retuned
M Frecycle on
recycle on
recycle off
WSa0.0E+00
5.0E-07
1.0E-06
1.5E-06
2.0E-06
2.5E-06
3.0E-06
3.5E-06
4.0E-06
4.5E-06
5.0E-06
890 892 894 896 898 900 902 904 906 908 910
time (days)
adso
rbed
mas
s fr
ac o
n ES
P so
lids
1.0 day PSR0.2 day PSR0.2 day PSR , retuned
M Frecycle on
recycle on
recycle off
WSa
25
• The increase in mercury concentration in the ESP dust when the raw mill was off line. • The weekly transients resulting from the operation over weekends with the raw mill on
line. These transients persisted over days. • The change in the exhaust mercury content with changes in the temperature of the ESP.
In the absence of dust recycle the mercury concentration dropped with the decrease in temperature, but slowly returned to its original concentration as the mercury was recycled to the kiln in the dust.
The REI model, therefore, provides a useful tool for planning strategies for mercury controls
by changes in the removal of dust from and changes in temperature of the pollution control device. It is also provides a means for planning experiments to take into account the long times needed to reach steady state as a result of internal mercury recycle.
The transient model has demonstrated mercury emissions behavior that qualitatively reproduced a study from the literature in which mercury inputs and emissions were measured for several weeks at a cement kiln. The major findings can be summarized as follows:
• The major pathways by which mercury leaves the cement kiln system are stack emissions and cement kiln dust (if the latter is removed from the kiln system). In some instances, mercury has been measured in the clinker (see Schreiber et al., 2005), although often the mercury measured in clinker was at the method detection limit. If some fraction of mercury were in the clinker, the transient behavior observed in cement kilns and predicted by the model would not change qualitatively, although stack emissions would be somewhat reduced.
• Concentrations of mercury in the stack are sensitive to the temperature of the particulate control device and whether the raw mill is on line (i.e., whether the flue gases pass through the raw mill before exiting).
• Recycle of cement kiln dust back into the kiln feed results in long times needed for the mercury concentration in the stack emissions to come to a steady-state condition, which has implications for making mercury measurements in the flue gas.
With the knowledge gained from this investigation, the portland cement industry would be
better prepared to discuss the fate of mercury in the cement manufacturing process, most notably the percentage of mercury actually emitted from the process. This information may also help the industry to enhance the inherent mercury controls to reduce or eliminate mercury emissions or in the selection of other control technologies. ACKNOWLEDGEMENTS The authors are grateful to Jost Wendt (Reaction Engineering International) for discussion and advice. The research reported in this paper (PCA R&D SN2841a) was conducted by Reaction Engineering International with the sponsorship of the Portland Cement Association (PCA project Index No. 04-07). The contents of this report reflect the views of the authors, who are responsible for the facts and accuracy of the data presented. The contents do not necessarily reflect the views of the Portland Cement Association.
26
REFERENCES Hills and Stevenson, Mercury and Lead Content in Raw Materials, SN2888, Portland Cement Association, Skokie, Illinois, USA, 45 pages, 2006. Johansen, Vagn C., Hawkins, Garth, J., Mercury Speciation in Cement Kilns: A Literature Review, SN2567, Portland Cement Association, Skokie, Illinois, USA, 2003, 16 pages.
Karatza, D., Lancia, A., Musmarra, D., Pepe, F., Adsorption of Metallic Mercury on Activated Carbon. Twenty-Sixth Symposium (International) on Combustion. The Combustion Institute, Pittsburgh, Pennsylvania, USA, 1996, pages 2439-2445.
Miller, F.M., Young, G.L., von Seebach, M., Formation and Techniques for Control of Sulfur Dioxide and Other Sulfur Compounds in Portland Cement Kiln Systems, SN2460, Portland Cement Association, Skokie, Illinois, USA, 2001, 56 pages. 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. Portland Cement Association, An Analysis of Selected Trace Metals in Cement Kiln Dust, SP109T, Skokie, Illinois, USA, 1992. 52 pages. Richards, J. Compilation of Cement Industry Air Emissions Data for 1989 to 1996, SP125, Portland Cement Association, Skokie, Illinois, USA, 2002, 112 pages. Schäfer, S. and Hoenig, V., “Operational factors affecting the mercury emissions from rotary kilns in the cement industry,” Zement-Kalk-Gips, 2001, 54, 591-601. Schäfer, S. and Hoenig, V., "Effect of kiln operation on the behavior of mercury in the clinker burning process," presented at the VDZ Congress, Düsseldorf, Germany, September 23-27, 2002. Schreiber, Robert J., Kellett, Charles D., Joshi, Nalin., Inherent mercury Controls within the Portland Cement Kiln System, SN2841, Portland Cement Association, Skokie, Illinois, USA, 2005, 24 pages. Senior, C.L., Sarofim, A.F., Zeng, T., Helble, J.J., and Mamani-Paco, R., "Gas-Phase Transformations of Mercury in Coal-Fired Power Plants," Fuel. Proc. Technol. 2000, 63(2-3), 197-213. Senior, C.L., Sarofim, A.F., Eddings, E., "Fate of Mercury in Cement Kilns," Proceedings of the Air & Waste Management Association 98th Annual Conference and Exhibition, Minneapolis, Minnesota, USA, June 21-24, 2005. Senior, C.L and Eddings, E., Evolution of Mercury from Limestone, SN2949, Portland Cement Association, Skokie, Illinois, USA, 2006, 23 pages.
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Seo, Y.-C., Lee, S.-H., Kim, J.-H., Jurng, J., Lee, S.H., Park, K.-S., “Emission Characteristics of Mercury from Coal Power Plants and Other Combustion Facilities with the Efforts to Manage Korea in Mercury,” Presented at the 4th International Mercury Emissions from Coal Workshop, Tokyo, Japan, June 12-15, 2007. US EPA Air Toxics Website - Utility Toxics HAP Study, http://www.epa.gov/ttn/atw/combust/utiltox/utoxpg.html#DA4 U.S. EPA, Source Data for Hazardous Waste Combustors, February 22, 2006, http://www.epa.gov/epaoswer/hazwaste/combust/finalmact/sumshtxls/cmntkiln/ck-hg-07-05.xls (accessed 8/15/07).