Transient Model for Behavior of Mercury in Portland Cement Kilns

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Ind. Eng. Chem. Res. 2010, 49, 1436–1443

Transient Model for Behavior of Mercury in Portland Cement Kilns† Constance Senior,* Christopher J. Montgomery,‡ and Adel Sarofim Reaction Engineering International, 77 West 200 South, Suite 210, Salt Lake City, Utah 84101

The U.S. Environmental Protection Agency has proposed to regulate emissions of air mercury from Portland cement kilns. 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 when regulations are imposed. A transient model for mercury behavior was developed and benchmarked with a comprehensive data set on the dynamic behavior of mercury in a Portland cement kiln reported by the German Research Institute of the Cement Industry. The model was able to reproduce key features of the transient behavior, including spikes in the exhaust gas concentration during the period when the plant switched from the raw mill on-line to the raw mill off-line; the high concentration of mercury in the electrostatic precipitator (ESP) dust; and the weekly transients resulting from operation over weekends with the raw mill on-line. Introduction In May 2009, the U.S. Environmental Protection Agency (EPA) proposed a new rule for the regulation of mercury emissions from Portland cement kilns, under the Clean Air Act (CAA).1 EPA surveyed existing emissions measurements from Portland cement kilns as part of the rulemaking process. They observed stack emissions of mercury from 12 to 3300 lb/106 tons of clinker. Clinker is the solid product from the kiln. EPA performed a maximum achievable control technology analysis (MACT) as directed by the CAA and proposed stack emission limits (on a 30-day rolling average) of 43 lb of Hg/106 tons of clinker (21.5 mg of Hg/metric ton) for existing sources and 14 lb of Hg/106 tons of clinker (7 mg/metric ton) for new sources. Thus, the range of mercury emissions is very large, and the proposed rule could result in substantial reductions in mercury emissions. Most modern Portland cement kilns use a “dry” design, which means that the raw material is injected into the system in a dry form. Figure 1 illustrates a dry “preheater” kiln system. Note that three preheaters are shown in the figure, but the number of preheaters varies among kiln systems; four or five preheaters can be used. This system is similar to a “precalciner” kiln. In the latter system, a portion of the fuel is burned in the duct or in a dedicated chamber between the preheater and the exit of the rotary kiln in order to calcine the limestone in the raw materials. In the case of either a preheater or precalciner kiln, the features of the system that affect the mercury behavior are the same. Dry kilns have mills to grind the raw material to a suitable size. These raw mills send the ground raw materials to a silo, and the material is injected from the silo to the preheater, where the solids are heated countercurrently by the exhaust flue gas from the rotary kiln. When the raw mill is off-line (that is, shut down), stored material from the silo continues to be injected † The research reported in this paper was conducted with the sponsorship of the Portland Cement Association. 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. * To whom correspondence should be addressed. Tel: +1 801 364 6925; fax: +1 801 364 6977; e-mail: [email protected]. ‡ Present address: URS Washington Division, 3604 Collins Ferry Rd., Morgantown, WV 26505.

into the preheater. When the raw mill is on-line, the gases at the exit of the preheater pass through the mill before going to the particulate control device; particulate matter is removed in the latter device, either using an electrostatic precipitator (ESP) or a fabric filter (FF). When the mill is off-line, the exhaust gases bypass the raw mill and go directly to the particulate control device. Figure 1 illustrates several other features of dry kilns. The particulate matter (sometimes called cement kiln dust or CKD) collected by the particulate control device is most often recycled back into the raw material silo. EPA surveyed 89 Portland cement kilns and concluded that 42% of kilns recycled all of the CKD back into the process; of the 58% that disposed of CKD, an average of 16.5% of the CKD was disposed of, with the rest being recycled back to the process.1 Some kilns withdraw a portion of the flue gas at the exit to the rotary kiln. This bypass stream is typically filtered to remove particulate matter and then exhausted through a stack. In kilns where high amounts of alkali sulfates form, a portion of the flue gas must be removed to prevent the build-up of chloride, alkali, or sulfur compounds in the kiln. Whether or not a kiln has a bypass depends on the composition of the raw materials. This bypass represents another potential source of emissions of mercury. Mercury enters the kiln system in the fuel and in the raw materials. Data in Figure 2 give the range of coal mercury contents in the United States for solid fuels burned in cement kilns. The data are from EPA’s Information Collection Request (ICR) for multiple fuel samples from every coal-fired power plant in the United States taken during the fourth quarter of 1999.2 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. These levels of mercury in fuels can be compared with recent measurements of mercury in limestone (the primary raw material entering a Portland cement kiln) published by U.S. EPA.1 The median mercury content in limestone in the set of limestone measurements from 89 cement plants was 0.02 µg/g. Raw materials typically have lower mercury concentrations than coal, and 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. According to EPA’s analysis of cement kilns, most of the mercury input to the

10.1021/ie901344b  2010 American Chemical Society Published on Web 12/14/2009

Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010

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Figure 1. Schematic of dry, preheater cement kiln flows.

Figure 2. Cumulative distribution of mercury in bituminous coal, petcoke, and tires fired at coal-fired power plants2 and limestone input to cement kilns.1

cement kilns was in the raw materials; the fraction of mercury input from fuel was 17% (median value) in this data set.1 Mercury is released into the kiln gases by vaporization from the fuel or the raw materials. Mercury in raw materials is released into the kiln gases in the preheater section of the kiln. Senior and Eddings3 reported on the release of mercury as a function of temperature from pulverized limestone samples. Limestone samples were observed to evolve mercury at temperatures between 200 and 700 °C, which corresponds to the preheater section of a Portland cement kiln,. Thus, in preheater or precalciner kilns, mercury in the raw materials will be released in the preheater section. In the high-temperature combustion environment of the flame in the rotary kiln (and in the secondary combustion chamber in precalciner kilns), mercury is vaporized from the fuel and enters the gas as elemental mercury.4 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. FFs or ESPs remove this absorbed mercury. Figure 3 shows data reported by Seo et al.5 of the

Figure 3. Mercury concentration in gas at two cement kilns, measured at fabric filter (FF) inlet and stack.5,6

mercury concentrations in the flue gas at two Korean cement kilns. Both kilns were preheater/precalciner kilns.6 Mercury concentrations were measured in the gas 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. At the two kilns, about 76% of the inlet mercury was removed across the fabric filter. The temperature in the particulate control device is affected by whether the raw mill is on-line or off-line. Furthermore, when the kiln exhaust gases pass through the raw mill, there is an increase in the contact time between solids and gas. Both of these factors affect how much mercury is adsorbed by solids and thus the mercury concentration in the stack. Scha¨fer and Hoenig7 carried out mercury measurement campaigns at two German precalciner cement kilns over a period of 3-4 weeks. The kilns burned coal and 50-60% secondary fuel. The authors state that only a small amount of mercury was introduced via the secondary fuel: the mercury entering the kiln from the fuel was a factor of 10 lower than the mercury entering the kiln from the raw material. A mercury continuous emission monitor (CEM) was used to measure gaseous mercury

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Figure 4. Mercury emissions with removal of ESP dust from Sunday to Sunday: Hg stack concentration and stack temperature are shown; shaded areas indicate periods with raw mill on-line.7

Figure 5. Mercury emissions without removal of ESP dust from Sunday to Sunday: Hg stack concentration and stack temperature are shown; shaded areas indicate periods with raw mill on-line.7

in the stack gases. Mercury was also measured in the fuel, raw materials, clinker, and dust from the ESP hopper. Figures 4 and 5 illustrate the effects of temperature and contact time from plant 1, sampled by Schaefer and Hoenig.7 These figures show the stack temperature and stack mercury concentration in the flue gas as a function of time for 1-week periods. The data in Figure 4 come from a week when the ESP dust was being sent out for disposal, while the data in Figure 5 come from a week when the ESP dust was being recycled back into the kiln. When the raw mill was off-line, the temperature in the ESP was 135 °C, while when the raw mill was on-line, the ESP temperature was 110 °C. When the raw mill was on-line, the mercury concentration in the stack was about 20 µg/m3, while when the raw mill was off-line, the mercury concentration in the stack gas was 35-40 µg/m3. The large decrease in stack Hg in going from 135 to 110 °C was due to both decreased temperature and increased contact time between gases and solids in the raw mill.

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. In many plants, the recycled dust is mixed into the blend silo with the raw mill product. This allows for even mixing and 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. Scha¨fer and Hoenig7 also noted a higher mercury concentration in the ESP dust when the raw mill was off-line. When the plant switched the raw mill off-line, Hg concentration in ESP dust increased from about 1.7 to about 2.6 µg/g. When the raw mill came on-line, the concentration of mercury in the ESP dust was observed to decrease from about 3.5 to about 2 µg/g.


Previous studies on mercury emissions from cement kilns have not always demonstrated a good material balance closure, which has resulted in significant uncertainty about the fate of mercury. A comprehensive analysis was carried out on the mercury emissions data obtained from stack emissions tests conducted since 1992 on cement kiln systems utilizing hazardous wastederived fuels and summarized by Schreiber et al.8 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. The average mass balance closure for all kilns surveyed was 41%. As noted by Schreiber et al.,8 “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 spiked in the fuel only for short periods of time, as has occurred during some mercury testing campaigns; because of mercury recycle within the kiln, it takes a long time for the mercury concentration to reach steady state as described below. Another difficulty arises because measurement of mercury in the clinker, the principal solid output stream, is often below detection limits. Measurements of Hg in cement showed that of 105 measurements only 21 (one-fifth) were above the detection limit.9 Approximately half of these showed Hg concentrations of under 0.01 µg/g and two-thirds under 0.02 µg/g. Scha¨fer and Hoenig7 carried out mercury mass balance measurement campaigns at two German cement kilns. The mass balances around the plants 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 93% during the overall measurement campaign. Mlakar et al.10 made measurements around a dry, precalciner kiln that produced 2200 t of clinker/day. The kiln used coal and petcoke as primary fuels, while tires were burned in the precalciner. A fabric filter was used to remove dust, which was recycled back to the raw meal silo. Speciated mercury was measured in the process gas (at the stack, at the exit of the raw mill, and at the exit of the preheater) and in solid and liquid samples from the process. The mass balance, based on an average of all Hg emissions during the test period, was 55%. Two-thirds of the mercury leaving the plant was in the stack emissions. Closing the mass balance for mercury is difficult in Portland cement kilns, both because of recycling of mercury in the kilns, which leads to long transients with time constants of days, and because of 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 in dry, Portland cement kilns. 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 200 and 700 °C, while mercury in the fuel vaporizes in the flame. 3. Some of the mercury is adsorbed by the suspended solids, which are captured in the ESP or baghouse. In dry kilns with in-line mills, this can occur in the mill. In a preheater/precalciner

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kiln, this can also occur in the preheaters. In a long kiln, this can occur in the cold end of the kiln. 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 stack emissions will be reduced. 6. For plants with in-line mills, when the raw mill is offline, 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, all of the mercury will eventually be emitted from the stack under steady-state conditions. A steady-state model was developed11 to predict the distribution of trace metals, including mercury, in cement kilns. However, this model was not designed to model the transient behavior of mercury recycling in the kiln. Schreiber et al.12 created a transient mercury model for the Ash Grove Cement Durkee plant, using measurements from campaigns in 2001 and 2006. The kiln was a dry, preheater kiln. The model was based on hourly measurements of elemental and oxidized mercury concentrations in the stack. Curve fits of the data on stack emissions as a function of time provided transient information on the change in stack emissions when the raw mill was turned on or off. This model included separate curve fits for elemental and oxidized mercury. The behavior of mercury in cement kiln systems is complex, owing to the large internal recycle of this element. Steady-state conditions might be difficult to attain quickly, which affects strategies for collecting representative samples of stack emissions and also strategies for controlling mercury stack emissions. The goal of this study was to develop a transient model for mercury stack emissions from Portland cement kilns, which could be used by kiln operators to better describe and control mercury stack emissions. The major kiln processes were modeled as a reactor network, which allows for transient interaction among nodes. The model is based not on fitting data but on fundamental adsorption processes. Both elemental and oxidized mercury exist in the kiln environment. However, comprehensive data on mercury species within cement kiln process gas is not widely available. Thus, total mercury was modeled in this work. As more detailed data sets become available, the model can be extended to mercury species. Model Description The processes involved in the model are shown in Figure 6. 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 6. 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

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Figure 7. Schematic of preheater kiln with raw mill on-line, or interconnected operation.

Figure 6. Mercury modes and gas-solid exchange used in model.

in the solids with a concentration XC, adsorbed on the solid with a concentration XA, chemically bound in the dust with a concentration YC, or adsorbed on the dust with a concentration YA. The following simplifying assumptions will be made in formulating the model. 1. 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. 2. 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 particulate collection device. 3. The mercury in the feed is assumed to be chemically bound and to have a mass fraction of XC,0. 4. 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. There are no experimental measurements of Hg sorption on cement kiln dust or raw meal. Ghorishi et al.13 demonstrated that alkaline sorbents like hydrated lime could adsorb both Hg0 and HgCl2, although the capacities of hydrated lime for mercury were less than the capacities of commercial activated carbons in fixed-bed tests with simulated coal combustion gas. Sorption of mercury on activated carbon has previously been modeled by use of a Freundlich isotherm14 or a Langmuir isotherm.15 In this work, the assumption is made that an adsorption isotherm can be used for cement kiln dust and/or raw meal. 5. The rate of mercury interchange between mercury in the vapor and solids is assumed to be mass-transfer-controlled. Previous models for sorption of Hg on activated carbon particles in an entrained gas have been successful in modeling behavior observed in full-scale coal-fired power plants via a combination of an adsorption isotherm on the sorbent surface and masstransfer resistance to the particle surface.16 This approach was adopted for this work. Given these assumptions, the kiln model comprises a number of interconnected zones, as shown in Figures 7 and 8 for a

Figure 8. Schematic of preheater kiln with raw mill off-line, or direct operation.

Figure 9. Control volume for preheater or rotary kiln node.

preheater kiln when the raw mill is on-line and off-line, respectively. The balance for Hg adsorbed on the solids in the nth module, shown schematically in Figure 9, is given by mS,n

dXA,n )m ˙ S,n-1XA,n-1 - m ˙ S,nXA,n - m ˙ e,nXA,n + dt - XA,n) (1) kAn(X* A,n

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) adsorbed mercury on the solids flowing into the module, (ii) adsorbed mercury on the solids flowing out of the module, (iii) adsorbed mercury on the solids entrained into the dust in the module, and (iv)


mass-transfer-controlled adsorption of mercury from the gas phase, where ) FA(YA,n, Tn) X* A,n

(2)

1 ˙ +m mS,n ) (m ˙ S,n-1)τres 2 S,n

(3)

From mass conservation: ˙ S,n-1 - m ˙ C-G,n - m ˙ e,n m ˙ S,n ) m

(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). The mass fraction of the solid that is in chemically bound form is therefore XC,n ) XC,0FC(Tn)

(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 ˙ C-G,n ) m ˙ s,n-1XC,0FC(Tn-1) m ˙ C-G,nYC-G,n ) m m ˙ s,nXC,0FC(Tn)

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Table 1. Key Inputs to the Model for the Conditions of Scha¨fer and Hoenig,7 Plant 1 clinker production (metric tons/day) clinker production (kg/h) fuel (kg) per clinker (kg) fuel (kg/h) air (kg) per fuel (kg) air (kg/h) coal ash content (%) ash flow (kg/h) (becomes dust) combustion products gas flow (kg/h) CKD (kg) per clinker (kg) total CKD (kg/h) CKD generated (kg/h) raw meal (kg) per clinker (kg) raw meal flow (kg/h) total gas outflow (kg/h) (into ESP) gas generated (kg/h) raw meal Hg (mass fraction) coal Hg (mass fraction) combustion products gas Hg (mass fraction)

3400 141 667 0.2a 28 333a 11.6a 328 667a 10.0a 2833a 354 167a 0.14a 19 833a 17 000a 1.52a 215 333a 410 833a 56 667a 3.0 × 10-8 1.0 × 10-7 b 8.0 × 10-9 a

a Values estimated from typical precalciner kiln operation.17 b Typical coal Hg concentration.

(6)

Similar equations can be set up for YG, YC, and YA. 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. Results Since there are a multiplicity of operational modes and kiln designs, the model must be tested against data from a specific kiln, in which there are 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 Comparison. Data from Plant 1 described in ref 7 were the focus of the comparison of the model with published results in the present study, because more results from this plant were reported in the paper. This plant had an output of 3400 t of clinker/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 for 3-6 hours/day; on the weekends, the raw mill was on-line. When the raw mill was off-line, the temperature in the ESP was 135 °C, but when the raw mill was on-line, the temperature was 110 °C. 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 20-22 µg/m3, whereas when the raw mill was offline, the mercury concentration in the stack gas rose as high as 43 µg/m3. Table 1 summarizes the inputs and assumptions used to model Plant 1. In instances where parameters were not provided by Schafer and Hoenig, values considered to be reasonable across the industry were used. Major assumptions were as follows: 0.2 kg of fuel/kg of clinker, 11.6 kg of air/kg of fuel, 0.14 kg of cement kiln dust/kg of clinker, and 1.52 kg of raw meal/kg of clinker. The values were adapted from Peray.17 The fuel was assumed to be a bituminous coal with 10 wt % ash. Figure 10 diagrams the modules and their connections used to model Plant 1. Dotted lines indicate flows during intercon-

Figure 10. Diagram of modular model setup for the conditions of Scha¨fer and Hoenig.

nected operation. The notation “g&d” indicates flow of gas and dust, while “s” indicates flow of solids. See Table 2 for conditions in each of the modules. Adsorption of gas-phase Hg onto the dust and solids was modeled by use of a Langmuir isotherm, following the experiments of Karatza et al.15 on activated carbon, with the temperature dependence of the adsorbed mass fraction modified to better fit the data of Scha¨fer and Hoenig.7 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 1800 °C. 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. The raw mill was off-line for 3-6 h every day, according to Schaefer and Hoenig. Thus, the kiln system must have at least 0.25 day (6 h) of storage in the feed silo. The model assumed a mixing chamber residence time of 0.2 day (4.8 h) and 1 day of storage in the feed silo. The former was modeled as a perfectly stirred reactor (PSR), while the latter was modeled as a plug-flow reactor. Various PSR residence time and plug-flow

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Table 2. Characteristics of Modules Used in the Model module

T (°C)

gas residence time (s)

solids residence time (s)

dust generation (kg/s)

gas generation (kg/s)

silo preheater 1 preheater 2 preheater 3 preheater 4 kiln 1 kiln 2 ESP raw mill

20 300 500 700 850 1100 1800 110-135 110-135

n/a 0.78 0.59 0.47 0.41 2.0 2.0 10 n/a

0.5 0.5 0.5 0.5 1560 1680 n/a n/a

0 0 0 0 0 17 000 0 0 0

0 0 0 0 0 56 667 0 0 0

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 Scha¨fer and Hoenig data. Given the operating characteristics of the plant, these assumptions were reasonable. 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 were 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 4 and 5 showed data as reported by Scha¨fer 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. Comparison of Model to Measurements. Figure 11 shows the model’s predictions of mercury concentration in the stack gas as a function of time, together with the assumed stack temperature. Changes in stack temperature correspond to changes in kiln operation: high temperature means that the raw mill was off-line, and low temperature means that the raw mill was on-line. In the predictions shown in Figure 11, the dust was assumed to be recycled until a steady state was reached.

The model assumed that the raw mill was off-line for 6 h/day during the week and on-line during the weekend. These results should be compared with the data shown in Figure 5, corresponding to the condition when ESP dust was recycled. The model correctly reproduces the temporal behavior observed in Figure 5, including a gradual increase in the peak mercury concentration (raw mill off-line) from the beginning of the week to the end of the week. The peak Hg concentrations increased as the week went along in Figure 5, because the concentration of Hg in the cement kiln dust was increasing with time. This is because, during the previous week, the kiln was operated without recycle of cement kiln dust. During the week shown in Figure 5, Hg was building up in the cement kiln dust, which affected the Hg concentrations in the stack gas. The model predicts higher peak stack gas emissions than observed during testing. The mercury content of the raw materials varied from 0.03 to 0.06 µg/g over the testing period,7 and the model input was assumed to be 0.03 µg/g. Uncertainties in the mercury concentration in the raw materials would result in uncertainties in the predicted stack emissions. Figure 12 shows the predicted mass fraction of mercury in the ESP solids as a function of time (corresponding to the time period in Figure 5, with recycle of dust). The concentration of mercury in the dust was predicted to be about 0.55 µg/g when the raw mill was on-line but increased to 0.8-0.9 µg/g when the raw mill was off-line. The concentration of mercury in the dust dropped back to the previous level when the raw mill came on-line again. This is the same behavior observed by Scha¨fer and Hoenig in concentration of mercury in the ESP dust, although the model predictions were lower than the reported concentrations. The dust recycle was discontinued for a week (middle third of Figure 13), and then dust recycle was started again (right third of Figure 13). Without recycle of the dust, the spikes in stack Hg concentration were less pronounced (as was observed by Scha¨fer and Hoenig7). When dust recycle was turned off,

Figure 11. Predicted mercury concentration in the stack gas as a function of time with recycle of ESP dust, using plant parameters from ref 7.

Figure 12. Predicted mercury concentration in the ESP dust as a function of time with recycle of ESP dust, using plant parameters from ref 7.


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available. The model could be improved if there were more laboratory-scale data on mercury sorption by cement kiln dust and raw materials and if there were more published data on actual measurements (particularly of speciated mercury measurements). Acknowledgment We are grateful to Professor Jost Wendt (University of Utah) for discussion and advice. The research reported in this paper was conducted with the sponsorship of the Portland Cement Association (PCA Project 04-07). Literature Cited Figure 13. Predicted stack emissions of mercury with and without recycle of ESP dust.

there was a shift in the baseline concentration of Hg in the stack gas (that is, in the average concentration). When dust recycle recommenced, Hg began to build up in the cement kiln dust, which affected the Hg concentrations in the stack gas. This illustrates that it can take longer than 1 week to reach a steady state regarding mercury emissions, after major changes in operation, such as dust recycle. 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. Conclusions A model was developed and benchmarked with a comprehensive data set 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 model was able to reproduce the following features of the data: (1) spikes in the exhaust gas concentration during the 3-6-h period during weekdays when the plant switched from the raw mill on-line to off-line (the actual values vary depending on the day of the week); (2) the increase in mercury concentration in the ESP dust when the raw mill was off-line; and (3) the weekly transients resulting from the operation over weekends with the raw mill on-line. These transients persisted over days. 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. The model, therefore, provides a useful tool for planning strategies for decreasing mercury emissions in the gas by increases in the removal of dust from, and decreasing the temperature of, the pollution control device. It is also provides a means for planning measurements to take into account the long times needed to reach steady state as a result of internal mercury recycle. Further work is needed to adapt the model to prediction of mercury species in the gas, as more data become

(1) U.S. Environmental Protection Agency. National emission standards for hazardous air pollutants from the Portland cement manufacturing industry: Proposed rule. Federal Register, Vol. 74 (No. 86), May 6, 2009; pp 21136-21192. (2) U.S. Environmental Protection Agency Air Toxics Website, Utility Toxics HAP Study, http://www.epa.gov/ttn/atw/combust/utiltox/utoxpg. html#DA4 (Accessed June 29, 2009). (3) Senior, C. L.; Eddings, E. EVolution of Mercury from Limestone; R&D Serial No. 2949; Portland Cement Association: Skokie, IL, 2006. (4) 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. (5) Seo, Y. S.; Park, K.-S.; Lee, J.-H. Emission characteristics of mercury from waste combustion facilities. Presented at 25th Annual Conference on Incineration and Thermal Treatment Technologies, Savannah, GA, May 1519, 2006. (6) Seo, Y. S., Yonsei University. Personal communication, November 2009. (7) Scha¨fer, S.; Hoenig, V. Operational factors affecting the mercury emissions from rotary kilns in the cement industry. Zement-Kalk-Gips 2001, 54, 591–601. (8) Schreiber, R. J.; Kellett, C. D.; Joshi, N. Inherent Mercury Controls within the Portland Cement Kiln System; R&D Serial No. 2841; Portland Cement Association: Skokie, IL, 2005. (9) An Analysis of Selected Trace Metals in Cement Kiln Dust; SP109T; Portland Cement Association: Skokie, IL, 1992. (10) Mlakar, T. L.; Horvat, M.; Vuk, T.; Strgarsˇek; Kotnik, J.; Tratnik, K.; Fajon, V. The role of fuels in mercury mass flows and cycling processes in the process of cement clinker production. Presented at the 6th Mercury Experts Conference, Ljubljana, Slovenia, April 22-24, 2009. (11) 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. (12) Schreiber, R. J.; Streitman, F.; Kellett, C. D. Development of a quantitative, predictive mercury model for a cement kiln. Presented at 12th Electric Utilities Environmental Conference, Phoenix, AZ, January 31Februrary 4, 2009. (13) Ghorishi, S. B.; Sedman, C. B. Low concentration mercury sorption mechanisms and control by calcium-based sorbents; Application in coalfired processes. J. Air Waste Manage. Assoc. 1999, 49, 694–704. (14) Meserole, F. B.; Chang, R.; Carey, T. R.; Machac, J.; Richardson, C. F. Modeling mercury removal by sorbent injection. J. Air Waste Manage. Assoc. 1998, 48, 1191–1198. (15) 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, PA, 1996; pp 2439-2445. (16) Cremer, M.; Senior, C.; Chiodo, A.; Wang, D.; Valentine, J. CFD modeling of activated carbon injection for mercury control in coal-fired power plants. Presented at the DOE-EPRI-U.S. EPA-A&WMA Combined Power Plant Air Pollutant Control Symposium: The Mega Symposium, Washington, DC, August 26-28, 2004. (17) Peray, K. E. Cement Manufacturer’s Handbook; Chemical Publishing Co., Inc.: New York, 1979.

ReceiVed for reView August 27, 2009 ReVised manuscript receiVed November 19, 2009 Accepted November 29, 2009 IE901344B

Transient Model for Behavior of Mercury in Portland Cement Kilns

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