Electrostatic Precipitation of Powdered Activated Carbon and

Mar 4, 2011 - Department of Mechanical, Materials, and Aerospace Engineering, Illinois Institute of Technology, 10 West 32nd Street, E1 Room 243, Chic...
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Electrostatic Precipitation of Powdered Activated Carbon and Implications for Secondary Mercury Adsorption within Electrostatic Precipitators Vinit Prabhu,* Sangkyoung Lee, and Herek L. Clack Department of Mechanical, Materials, and Aerospace Engineering, Illinois Institute of Technology, 10 West 32nd Street, E1 Room 243, Chicago, Illinois 60616, United States ABSTRACT: The injection of powdered sorbents, such as activated carbon, for mercury emissions control at coal-fired power plants has primarily taken place upstream of electrostatic precipitators (ESPs), which far outnumber baghouses in the U.S. Although full-scale sorbent injection tests have demonstrated varying degrees of mercury removal efficiency, the actual behavior of powdered activated carbon (PAC) within an ESP has not been well-established, particularly as this behavior relates to adsorbing gas-phase mercury. In the present experimental investigation, results obtained in a lab-scale ESP indicate that the electrical properties of PAC may cause its collection in a full-scale ESP to be significantly different from that of the native fly ash. There appears to be potential for significant collection of PAC on the discharge electrode wires of an ESP. Because these wires are typically not rapped as frequently as collection electrodes in an ESP, over time, such behavior could potentially create a series of cylindrical PAC structures that contribute non-negligibly to the total mercury removal efficiency within the ESP. To test this potential, the present investigation also presents results from a mass transfer model, in which the PAC-coated discharge electrodes of an ESP are represented as a row of cylindrical mercury sinks, whose radii and ultimate adsorption capacity for mercury vary in time. Results from the mass transfer model show that, after extended periods of PAC injection, the PAC-covered discharge electrodes would continue to adsorb mercury from the flue gas after injection ceases. Over time, the collected PAC becomes saturated, producing a slow rise in the measured mercury concentration at the ESP outlet. This trend agrees with that occasionally observed after full-scale sorbent injection tests and suggests a secondary mechanism for mercury adsorption within ESPs. Such anomalous behavior may require a separate evaluation to assess its impact on ESP operations and maintenance.

’ INTRODUCTION Mercury emissions control for industrial combustion processes has been an area of growing interest and research for several decades. Following the early successes in reducing mercury emissions from municipal and medical waste incinerators (collectively MWIs), efforts to reduce anthropogenic mercury emissions have largely focused on coal-fired power plants (CFPPs). Collectively, CFPPs are the largest remaining uncontrolled industrial source of mercury emissions into the atmosphere. Although the United States Environmental Protection Agency (U.S. EPA) in 2005 issued the Clean Air Mercury Rule (CAMR) to address mercury emissions from CFPPs, a federal appeals court struck down the rule in 2008 on the grounds that it contravened the Clean Air Act Amendments by not mandating the use of maximum achievable control technology (MACT). Despite this action at the federal level, roughly half of all states in the U.S. have implemented or are considering implementing state-level mercury emissions limits for CFPPs that are more stringent than those that were originally called for in the CAMR.1 In addition, two events in February 2009 indicate growing momentum for mercury emissions controls. First, the U.S. EPA Office of the Inspector General abandoned its plans to appeal the 2008 court decision overturning the CAMR,2 which has been interpreted as an indication that U.S. EPA intends to join the states in pursuing more stringent mercury emissions targets than existed in the CAMR. Second, the United Nations Environment Programme (UNEP) agreed to pursue a global treaty designed to reduce mercury emissions worldwide.3 Thus, at state, r 2011 American Chemical Society

federal, and international levels, there is growing regulatory activity driving the need for reliable and cost-effective mercury emissions control technologies. The most thoroughly tested of these technologies has been the injection of powdered sorbents, most often injection of powdered activated carbon (PAC) in a process commonly referred to as activated carbon injection (ACI). In ACI, PAC is injected into the post-combustion flue gas upstream of the particulate control device, which in the U.S. is typically an electrostatic precipitator (ESP). The point of injection is often no more than a few seconds upstream of the ESP based on distance and the flue gas velocity. Once the flue gas enters the ESP, the sorbent is collected along with the fly ash on the ESP plate electrodes. Adsorption of the mercury during ACI can occur while the sorbent is in-flight or after the sorbent has collected on a surface. Early results from ACI testing effectively had no temporal resolution because of the limits of the measurement method, and then, as now, there were no spatially resolved measurements of mercury concentration or capture within ESPs. With neither temporally nor spatially resolved measurements of mercury capture within an ESP, early ACI results were assumed to reflect sorbent collected on the ESP collection electrodes. Our mass transfer analysis4 refuted this assumption, showing that mass transfer rates from the flue gas to Received: August 18, 2010 Revised: January 11, 2011 Published: March 04, 2011 1010

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Figure 1. Schematic representation of data taken during ACI field tests conducted at the We Energies Pleasant Prairie Power Plant.5,7

Figure 2. Ratio of mean convective heat (mass)-transfer coefficients for a cylinder and a flat plate as a function of the plate length and fluid velocity. As the length L of the plate increases, the diameter D of the cylinder increases to maintain equivalent surface areas (L = πD). Fluid is air at 300 K and 1 atm, with 20 < ReD < 2  105 and 60 < ReL < 3.15  106.

ESP collection electrodes, even under ideal conditions, were too slow to produce the observed mercury removal efficiencies across ESPs during full-scale ACI tests. With the advent of time-resolved mercury concentration measurements, transient responses could be observed in mercury concentrations at the ESP outlet during ACI. These outlet mercury concentrations often decrease rapidly at the start of ACI, with an equally rapid recovery to their initial values when sorbent injection stops. However, in some cases, the transient recovery is much slower, sometimes taking hours after sorbent injection has stopped for the ESP outlet mercury concentrations to return to values matching those upstream of the ESP and injection lances, which remain nominally constant during ACI. Figure 1 schematically depicts such behavior using data taken from ACI field tests7 conducted at the We Energies Pleasant Prairie Power Plant from 2001 to 2002. Such a slow recovery indicates that mercury continues to be removed from the flue gas after sorbent injection stops. As before, in the absence of internal mercury or carbon measurements from within an ESP, speculation has arisen that the slow recovery in the mercury concentration at the ESP outlet is the result of sorbent in the dust cake collected on the ESP collection electrodes. In general, adsorption of mercury is frequently attributed to sorbent-covered surfaces; indeed, lab-scale testing of mercury sorbents (e.g., by Scala et al.10) has shown nonnegligible transient behavior attributed to continuous adsorption by sorbent deposited on the walls of the experimental apparatus. However, there are several reasons why this explanation is unlikely to hold true within full-scale ESPs. First, the mass density of powdered sorbent in the flue gas and, therefore, on the ESP collection electrodes is typically only a few percentages of the fly ash mass density. This results in a deeply diluted concentration of sorbent in the dust cake, leaving it with a greatly diminished capacity for adsorbing mercury. As noted in our earlier mass transfer analysis,4 the far more idealistic assumption of the dust cake as a perfect mercury sink does not, under typical operating conditions, produce mass transfer rates needed to yield the observed reductions in the mercury concentration within an ESP. Second, the slow recovery in the mercury concentration at the ESP outlet occurs when sorbent injection stops. Any sorbent present in the

dust cake would immediately be masked from the flue gas by a growing layer of fly ash as ash collection continues. An alternative hypothesis for the slow recovery in ESP outlet mercury concentration, one which the present investigation pursues, is that PAC behaves differently from fly ash within an ESP. Specifically, anecdotal evidence obtained in the course of a separate concurrent research project indicated that, during electrostatic precipitation, PAC collects on both the wire-discharge electrodes as well as the collection electrodes of a bench-scale ESP. This discovery, which to our knowledge has not been previously reported, has several potential implications. First, the preferential collection of PAC on the discharge electrodes will prevent its dilution by fly ash, which occurs on the plate electrode, thereby enhancing its adsorption potential per unit surface area. Second, in the event of discharge electrode rapping, reentrained PAC is likely to exhibit the same preferential collection on discharge electrodes in downstream fields of the ESP, effectively repeating the process. The third implication of this finding lies in the potential for convective heat (mass)-transfer rates per unit surface area to be higher for cylindrical forms than for plane walls. Figure 2 confirms this by comparing values of the mean convective heat (mass)-transfer coefficient over a cylinder (cyl) and a flat plate (plate), both exposed to the same fluid flow. To facilitate the comparison over a range of Reynolds numbers, the cylinder and the flat plate maintain equivalent surface areas with an increasing Reynolds number for a given fluid velocity (i.e., the length of the plate and the diameter of the cylinder are related by Lplate = πDcyl). The results in Figure 2 reflect commonly used Nusselt (Sherwood) number correlations for a flat plate (NuL = 0.644ReL1/2Pr1/3 for laminar conditions and NuL = [0.6664Rex,c1/2 þ 0.037(ReL4/5 - Rex,c4/5)]Pr1/3 for mixed laminar and turbulent conditions6) and a cylinder (NuL = CReLmPr1/3 from Hilpert,9 where C and m are functions of ReD). Figure 2 shows that, in the absence of electric field effects (socalled “corona wind” or “ionic wind”), the geometry of a cylinder in a cross-flow is much more effective than a flat plate for heat (mass) transfer between the fluid and the solid surface. Cylindrical forms outperform flat plates even as the plate length L and cylinder diameter D increase (while maintaining equivalent surface areas, i.e., L = πD). Mean convective heat (mass)-transfer 1011

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Table 1. ESP Parameters Used in the Numerical Model gas (air) velocity (m/s)

1.22 (4 ft/s)

gas (air) temperature (C)

132.2

gas (air) pressure (kPa)

101.325

PAC particle density (g/cm3)

0.5

PAC particle diameter (μm) PAC equilibrium adsorption capacity

18 240 (at 4 ppbm Hg

for Hg (μg/g of PAC) inlet mercury (Hg) concentration

in the gas phase) 4

in the gas flow (ppbm) discharge electrode diameter (mm)

1

PAC injection rate (g/m3)

0.16 (10 lb/MMacf)

coefficients for cylinders surpass those for flat plates by approximately a factor of 1.5 for shorter plates and more than a factor of 2.5 for the longest plates and the highest gas velocities (3 and 5 m/s). The comparison of convective heat (mass)-transfer coefficients in Figure 2 lends qualified support to the position that significant adsorption of mercury within an ESP is more likely to occur on cylindrical surfaces (e.g., sorbent-covered discharge electrode wires) than on planar surfaces (e.g., ash-covered collection electrodes). This inference must be qualified because it is not known what effect the electric field has on mass transfer for electrode configurations typical of wire-plate ESPs. Ohadi and co-workers8 studied enhancements to heat transfer in response to a discharge corona. However, their electrode configuration was a wire-in-tube type with heat transfer measurements made only along the tube surface. Thus, these results cannot be used to infer enhancements to mass transfer to the wire electrodes in a wire-plate ESP. As a result, the present investigation is a combined experimental and numerical effort to verify this potential mechanism for secondary mercury adsorption within ESPs. First, lab-scale experiments measure the differential collection behavior of PAC relative to fly ash in an ESP. Using the experimental results to estimate PAC collection rates on ESP wire-discharge electrodes, a numerical model evaluates mass transfer rates for a gas flow over a single row of PAC-coated wire electrodes (Table 1). This scenario is designed to recreate the basic mass transfer behavior that might result from preferential PAC collection on and subsequent mercury capture by PAC-covered wire-discharge electrodes.

’ EXPERIMENTAL SECTION The lab-scale ESP used in the experiments consists of a hollow 1.27 cm thick, vertically oriented Plexiglas cylinder with an inner diameter of 12.7 cm and a height of 91.44 cm. Electrically grounded brass plates serve as the collection electrode and line the inner surface of the cylinder. A 1 mm diameter wire serves as the discharge electrode and stretches down the centerline of the ESP, kept taut by a weighting mechanism. A high-voltage power supply (UltraVolt High Voltage Power Supply, model 35-A12P4), connected to a direct-current (DC) power supply (Beckmann Industrial Dual DC Power Supply, model MPS60), charges the discharge electrode. Although this component configuration theoretically could provide higher voltages, it was observed that voltages on the discharge electrode higher than 19 kV caused electrical breakdown and arcing between the discharge and collection electrodes. Figure 3 shows a schematic of the setup. Tests involve determining the precipitation efficiency of PAC (Darco Hg-Norit Americas, mean diameter of 38 μm and standard deviation of 32 μm), bituminous and lignite fly ashes, and PAC-fly ash mixtures, at

Figure 3. Experimental setup. different voltages (5, 10, and 15 kV). At each voltage, a predetermined mass of powder (PAC or fly ash) is slowly poured through a coarse sieve and into the ESP. Within the ESP, a portion of the powder collects on the grounded collection electrodes, a portion collects on the discharge electrode, and the uncollected remainder exits the bottom of the ESP. After each trial, the mass of powder that passed uncollected through the ESP is captured and measured on an electronic balance (Mettler, model AE200). The masses of powder collected on the discharge and collection electrodes are separately and carefully removed with a brush, collected, and weighed. These three measured masses are then used to calculate a mass balance for each trial. Duplicate tests conducted using 20 g of PAC indicated measurement uncertainty of less than 7%, as shown in Figures 6 and 7.

’ ANALYTICAL METHOD The approach for the numerical model uses a flow geometry that simulates a gas flow between two plate electrodes of a wireplate ESP. Centered between the two plate (collection) electrodes is a single row of vertically oriented discharge electrode wires (Figure 4), which is typical of many ESPs in use in the U.S. and most ESPs associated with coal-fired power plants. A PAC coating continuously grows over the discharge wires as time 1012

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where dparticle is the mean PAC particle diameter and nparticle is the number of particles accumulated on the discharge electrode, which can be obtained from the mass of PAC collected on the electrode mcarbon ð5.3Þ nparticle ¼ Vparticle Fcarbon where Vparticle is the volume of a single PAC particle. The rate at which mercury is adsorbed by the PAC-coated discharge electrodes is given by

Figure 4. ESP geometry used in the numerical model.

progresses. A lumped capacitance mass transfer model treats the PAC-coated discharge electrodes as a sink for mercury in the flowing gas, with a capacity of 240 μg of Hg/g of carbon, similar to that assumed in the analysis contained in the final report of full-scale tests conducted at We Energies Pleasant Prairie Power Plant.7 While a mass transfer analysis alone fails to capture the complex chemistry known to be at the foundation of mercury transformation and adsorption by activated carbon, such an analysis represents an upper performance limit for conditions where reaction and/or adsorption kinetics are not rate-limiting. In the absence of a comprehensive mercury adsorption mechanism, such mass transfer analyses can prove useful in establishing performance limits and revealing trends. The model assumes that the PAC injected upstream of the ESP is collected equally on all 20 discharge electrodes. The total mass of carbon that accumulates on each electrode from time t = 0 to a given time t is given by ACI  t ð1Þ mcarbon ¼ 20 where ACI is the PAC injection rate in kg/s. Equation 2 represents the diameter of the PAC-coated discharge electrodes as a function of collected PAC mass. " #1=2 4mcarbon 2 d ¼ þ d0 ð2Þ πlFcarbon Incropera et al.6 provide a Nusselt number (Nu) correlation for an aligned configuration of a row of cylinders. The heat (mass)transfer analogy allows the Sherwood number to be replaced by the Nusselt number when the Prandtl number (Pr) is replaced by the Schmidt (Sc) number ð3Þ Sh ¼ 0:27Re0:63 Sc0:36 where Re is the Reynolds number. It should be noted that this correlation does not include the effects of the electric fields on the fluid motion. Equation 4 shows the relation between the convection mass transfer coefficient (hm) and the Sherwood number DHg ð4Þ hm ¼ Sh d where DHg is the binary diffusivity of mercury in air. The surface area A is given by A ¼ πdl

ð5.1Þ

For small values of t, the PAC collected on the discharge electrode does not attain 100% coverage. Therefore, the surface area is assumed to be the projected area of all of the PAC particles collected on the electrode and is calculated as π ð5.2Þ A ¼ nparticle d 2particle if A e πd0 l 4

m_ Hg ¼ hm AFHg ðC¥ - Csurface Þ

ð6Þ

where C¥ is the mercury concentration in the gas flow upstream of the electrode and Cs is the mercury concentration at the surface of the PAC layer on the electrode. It should be noted that Cs at time t = 0 is 0. Equation 7 uses the mercury adsorption rate of equation 6 to express the incremental increase in the mass of mercury adsorbed over a time interval Δt mHgt ¼ mHgt - Δt þ m_ Hg Δt

ð7Þ

The surface concentration of mercury can be obtained as shown in eq 8 C¥ mHgt ð8Þ Csurface ¼ ðθHg - C mcarbon Þ where θHg-C is the assumed equilibrium adsorption capacity for mercury on PAC. The mean concentration of mercury in the gas downstream of one discharge electrode in the series is given by ðm_ in - m_ Hg Þ ð9Þ Cout ¼ C¥ m_ in where m_ in is the flow rate of the flue gas in the channel between the two collection electrodes. The concentration of mercury in the gas downstream of the ith electrode equals the concentration of mercury in the gas upstream of the (i þ 1)th electrode. Thus Couti ¼ C¥i þ 1

ð10Þ

The properties of the flue gas are assumed to be similar to air. The numerical model has as its domain a row of 20 electrodes, with simulations extending over a period of 16 h. As noted earlier, in evaluating convective mass transfer between the gas and the cylindrical PAC-covered discharge electrodes, the effect of the electric field on the fluid motion is neglected. Using the results obtained from the experimental portion of this investigation, the simulation assumes a PAC mass loading in the flow entering the computational domain of 0.16 g/m3 (10 lbs/MMacf) and that PAC no longer collects on each discharge electrode once the total collected mass reaches 85 g. Such a simplified conceptualization of the deposition process belies the possibility that PAC accumulated on the discharge electrodes may be displaced or ablated. This lost PAC mass may expose a fresh layer of PAC or may be replaced by additional PAC deposition. However, the present model does not attempt to account for such details of the deposition process.

’ RESULTS AND DISCUSSION Figure 5 presents experimental results that reveal the different behaviors exhibited by PAC and fly ash during their collection on the collection and discharge electrodes of the lab-scale ESP. 1013

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Figure 5. Collection patterns of PAC, bituminous, and lignite fly ashes in a lab-scale ESP.

Figure 7. Percentage of PAC collected as a function of mass of PAC processed through the lab-scale ESP.

Figure 6. PAC collected as a function of mass of PAC processed through the lab-scale ESP.

Electrical property differences between fly ashes and PAC likely play a role. While fly ash resistivity values typically range from 108 to 1013 Ω cm, Espinola et al.11 measured PAC resistivity values of 1.0 Ω cm or less. The precipitation of both the bituminous and lignite fly ashes follows conventional ESP operation, with both ashes collecting on the collection electrodes. Furthermore, within the experimental uncertainty of the results, both fly ashes collect exclusively on the collection electrodes, with no ash observed collected on the discharge electrode. By comparison to the fly ashes, Figure 5 shows that PAC collects on both the collection electrodes and the discharge electrode. The PAC shows a strong bias for collection on the discharge electrode compared to the collection electrode on the basis of the available surface area. With the discharge electrode charged to 15 kV, Figure 7 shows that 6% of an initial mass of 14 g of PAC was collected on the wire. Given that the surface area of the wiredischarge electrode is 2 orders of magnitude smaller than the collection electrodes, the collected mass per unit surface area of PAC on the wire (0.0313 g/cm2) is about 2 orders of magnitude larger than the values for collection of any of the three powders on the collection electrodes (0.000 823, 0.000 798, and 0.001 08 g/cm2 for PAC, lignite ash, and bituminous ash, respectively). In comparison to this lab-scale experiment, the surface area ratios within full-scale wire-plate ESPs are likely to be greater and, thus, potentially present greater potential for preferential collection of PAC on the discharge electrodes. Interestingly, this bias showed evidence of intensification as the mass of PAC processed by the ESP increased. The percent of PAC collected on the discharge electrode increased as the total mass of PAC processed by the

ESP increased (Figure 6). When 20 g of PAC was added, 15% was collected on the discharge electrode, increasing to approximately 35% collection on the discharge electrode when 40 g of PAC was processed (Figure 7). The growth in the percentage of PAC collected on the discharge electrode eventually reverses, ultimately leading to “saturated” conditions, where the mass of collected PAC collected on the discharge electrode remains constant. These data strongly suggest that a substantial amount of PAC may collect on discharge electrodes within an ESP. The process of PAC layer growth on the discharge electrode likely represents a balance between the strength and stability of the electric field and corona discharge, as compared to gravitational and aerodynamic forces. Indeed, Espinola demonstrated11 that the resistivity of bulk carbonaceous powders generally decreases with increasing compaction pressure, suggesting that electrical properties of the PAC layer will vary with layer thickness. In the intervals between discharge electrode rapping events, the PAC layers can grow, offering increasing, PAC-enriched surface area for mercury adsorption even after sorbent injection has ended. Larger PAC layers become subject to higher aerodynamic and gravitational forces, potentially leading to natural ablation or collapse, followed by continued PAC deposition and renewed growth of the layer. Even periodic electrode rapping may not disrupt this mercury-capture mechanism entirely. PAC is known for the ease with which it is resuspended in the flue gas during rapping. Discharge electrode rapping would resuspend substantial amounts of PAC, which would again be subject to preferential collection on downstream discharge electrodes. Because this process repeats in downstream fields, the loss of fly ash from the flue gas (primarily to the collection electrodes) would lead to further enrichment of PAC depositing on the discharge electrodes and further enhancement of the mercury adsorption potential of these layers. The resuspension of PAC during electrode rapping would also provide beneficial redistribution, exposing relatively fresh sorbent previously held within the accumulated layers. Rapping frequency, electrode design, and ESP operating conditions may all affect this process. For example, unlike wire-discharge electrodes, PAC collected on rigid discharge electrodes (RDEs) would form a planar surface for mercury adsorption, one that presents none of the mass transfer advantages of the cylindrical forms illustrated in Figure 1. Figure 7 shows the fraction of carbon collected on the discharge and collection electrodes as well as the fraction of 1014

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series of PAC-covered wire electrodes in an ESP. The predicted mercury concentrations downstream of the coated wires showed behavior similar to that observed at the end of full-scale ACI testing, suggesting the existence of a secondary mercury capture mechanism within ESPs.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

Figure 8. Numerical mass transfer analysis results showing Hg concentrations over time downstream of the 5th, 10th, and 20th PACcovered discharge electrodes in an ESP.

PAC that remains uncollected and exits the ESP. It can be seen that a significant amount of PAC is collected on the discharge electrode. As the mass of PAC processed by the ESP increases, Figure 7 shows that the PAC collected on the discharge electrode eventually exceeds the amount that accumulates on the collection electrodes, further potential evidence of the much lower resistivity of PAC, as compared to fly ash. Figure 8 shows the mercury captured by the PAC collected on the 5th, 10th, and 20th discharge electrodes, as simulated by the numerical mass transfer model. As the total mass of carbon depositing on each electrode increases, the diameter of the PAC layer increases. Consequently, each electrode has an increased surface area and increased potential to absorb mercury from the flue gas. Because some mercury is removed from the flue gas at each electrode, the concentration of mercury in the flue gas passing the 20th electrode is significantly lower than that at the inlet (4 ppbm). However, this trend changes once the mass of PAC accumulated on each electrode reaches a saturation point, at which time the mass of PAC and its ultimate mercury adsorption capacity becomes constant. Because this fixed mass of PAC continues to adsorb mercury from the gas, its mercury adsorption capacity decreases, the concentration of mercury at the surface of the collected PAC increases, and the overall rate of mercury adsorption from the gas decreases. Consequently, Cout increases and the concentration at the outlet of the ESP begins to increase gradually with time. This behavior is similar to that schematically represented in Figure 1 and observed during other full-scale ACI tests, suggesting a link between the PAC collection on the discharge electrodes and the long recovery of the measured mercury concentration at the ESP outlet. This finding is significant because it contradicts the assertion that the long recovery period sometimes seen after ACI (Figure 1) is the result of mercury adsorption by PAC collected on the collection electrodes.

’ CONCLUSIONS Complementary experiments and numerical modeling have shown that, unlike fly ash, PAC can collect significantly on the discharge electrodes of an ESP. Using this finding, a numerical model was developed that simulated mass transfer of gas-phase mercury to a series of adsorbent cylinders, representative of a

’ ACKNOWLEDGMENT This work was funded by the National Science Foundation Grant BES-0607292. The authors thank Dr. Shannon Serre at the U.S. EPA National Risk Management Research Laboratory for supplying fly ash samples. ’ NOMENCLATURE A = surface area of PAC-coated electrode ACI = PAC injection rate (kg/s) C¥ = Hg concentration in gas flow entering the control volume around the electrode Cs = Hg concentration at the surface of the electrode d = diameter of carbon-coated electrode d0 = initial diameter of the electrode DHg = binary diffusivity of Hg in air l = electrode length mcarbon = total mass of carbon that accumulates on each electrode m_ Hg = rate of mass transfer of Hg into PAC m_ in = flow rate of the flue gas in the ESP Fcarbon = density of PAC Re = Reynolds number Sc = Schmidt number Sh = Sherwood number θHg-C = PAC equilibrium adsorption capacity for mercury in carbon Vparticle = volume of PAC particle ’ REFERENCES (1) Becker, W. States’ programs to control mercury from coal-fired power plants. Proceedings of the Virginia Mercury Symposium; Newport News, VA, Nov 29, 2007. (2) U.S. Court of Appeals for the D.C. Circuit. http://pacer.cadc. uscourts.gov/docs/common/opinions/200802/05-1097a.pdf. (3) United Nations Environment Programme. http://www.chem. unep.ch/mercury/default.htm. (4) Clack, H. L. Mass transfer within electrostatic precipitators: Trace gas adsorption by sorbent-covered plate electrodes. J. Air Waste Manage. Assoc. 2006, 56, 759–766. (5) Cremer, M.; Senior, C.; Chiodo, A.; Wang, D.; Valentine, J. CFD modeling of activated carbon injection for Mercury control in coal-fired power plants. Proceedings of the Electric Power 2005; Chicago, IL, April 5-7, 2005. (6) Incropera, F. P.; Dewitt, D. P.; Bergman, T. L.; Lavine, A.S. Fundamentals of Heat and Mass Transfer, 5th ed.; Wiley: New York, 2005. (7) ADA Environmental Solutions. Pleasant Prairie Power Plant Unit 2. Sorbent Injection into a Cold-Side ESP for Mercury Control, Final Report; U.S. Department of Energy: Washington, D.C., May 2003; Cooperative Agreement DE-FC26-00NT41005. (8) Ohadi, M. M.; Nelson, D. A.; Zia, S. Heat transfer enhancement of laminar and turbulent pipe flow via corona discharge. Int. J. Heat Mass Transfer 1991, 4/5, 1175–1187. (9) Hilpert, R. Forsch. Geb. Ingenieurwes. 1933, 4, 215. 1015

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(10) Scala, F.; Chirone, R.; Lancia, A. In-duct removal of mercury from coal-fired power plant flue gas by activated carbon: Assessment of entrained flow versus wall surface contributions. Env. Eng. Sci. 2008, 25, 1423–1428. (11) Espinola, A.; Miguel, P. M.; Salles, M. R.; Pinto, A. R. Electrical properties of carbons—Resistance of powder materials. Carbon 1986, 24, 337.

’ NOTE ADDED AFTER ASAP PUBLICATION The version of equation 5.2 published March 4, 2011 is incorrect. The correct version published March 17, 2011.

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