Hg Reactions in the Presence of Chlorine Species: Homogeneous Gas Phase and Heterogeneous Gas-Solid Phase

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1 Lee, TECHNICAL Hedrick, PAPER and Biswas ISSN J. Air & Waste Manage. Assoc. 52: Copyright 2002 Air & Waste Management Association Hg Reactions in the Presence of Chlorine Species: Homogeneous Gas Phase and Heterogeneous Gas-Solid Phase Tai Gyu Lee Department of Chemical Engineering, Yonsei University, Seoul, South Korea Elizabeth Hedrick National Exposure Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, Ohio Pratim Biswas Environmental Engineering Science Division, Department of Chemical and Civil Engineering, Washington University in St. Louis, Missouri ABSTRACT The kinetics of Hg chlorination (with HCl was studied using a flow reactor system with an online Hg analyzer, and speciation sampling using a set of impingers. Kinetic parameters, such as reaction order (α, overall rate constant (k, and activation energy (E a, were estimated based on the simple overall reaction pathway. The reaction order with respect to C Hg, k, and E a were found to be 1.55, exp( /t [(µg/m (s -1 ], and [kj/ mol], respectively. The effect of chlorine species (HCl, CH 2 on the in situ Hg capture method previously developed 28 was also investigated. The efficiency of capture of Hg by this in situ method was higher than 98% in the presence of chlorine species. Furthermore, under certain conditions, the presence of chlorine enhanced the removal of elemental Hg by additional gas-phase oxidation. INTRODUCTION Environmental engineers have paid considerable attention to Hg because of its high toxicity, tendency to bioaccumulate, and difficulties in its control. It is one of 11 IMPLICATIONS This paper reports the kinetic parameters such as reaction order, rate constant, and activation energy of the overall chlorination reaction of elemental Hg for conditions discussed in this study. A previously developed novel Hg capture method using in situ generated TiO 2 with UV irradiation in the presence of chlorine species (HCl, CH 2 is shown to be as effective as in the chlorine-free environment (>98% of elemental Hg removal efficiency. Furthermore, catalytically (TiO 2 enhanced production of Cl and results in an additional removal of elemental Hg by direct gas-phase oxidation. trace metals currently regulated by the U.S. Environmental Protection Agency (EPA, under Title III of the 1990 Clean Air Act Amendments. The major anthropogenic sources of Hg emissions are coal combustors and municipal waste incinerators. In the United States, total Hg emissions from all identified coal combustors (utility boilers and commercial/industrial boilers and waste incinerators (municipal, medical, and hazardous waste account for approximately 80% of the total emissions. 1 It is important to know the fate of Hg in combustion environments to effectively control emissions. 2-5 The presence of SO 2 and Cl in flue gas is known to affect speciation of Hg between the elemental and oxidized forms, with both SO 2 and Cl increasing the formation of the oxidized forms, such as HgSO 4 and Hg. 4-6 Once oxidized, Hg can be more effectively captured than the elemental form in conventional pollution control systems (e.g., wet scrubbers and particulate control devices. On one hand, Hg (II chloride has higher capture efficiency in scrubbers because it is watersoluble. On the other hand, Hg oxide is less volatile and may form particles. Thus, it can potentially be captured in particulate control devices. 7 Hence, the oxidized forms of Hg are preferred when generated by combustion facilities. 7,8 However, in the elemental form, an Hg atom has a 5d 10 6s 2 closed shell electronic structure, which is isoelectronic to He (1s This structure results in its unusual nonreactivity compared to other metals. Past studies have confirmed this by showing extremely slow 7,12 or no 13 oxidation in air at high temperatures. Other studies show that oxidation occurs only with strong oxidants, such as NO 2 and Few studies report on the kinetics of elemental Hg in high-temperature combustion systems. Even the field data measured at pilot- and full-scale systems are scarce. Most studies have been limited to interpretations of field data, 1316 Journal of the Air & Waste Management Association Volume 52 November 2002

2 by identifying Hg species emitted and proposing the possible reaction pathways. 4,5,12-19 In recent years, many researchers have reported mercuric chloride (Hg as a major oxidized form of Hg from coal combustors and/or waste incinerators, 4,5,12,13,15-17,19-23 even though it has never been measured directly. The Hg chlorination mechanism in coal combustion is a current source of debate and deserves further exploration. Kinetic study of Hg reaction with chlorine species is important in understanding the Hg transformation quantitatively, which can be used to control Hg emissions more effectively. Previous studies by Biswas and co-workers have shown that in situ generated agglomerates possess a higher metal capture efficiency than that of bulk-type sorbent particles and suppress the formation of submicrometer particles, resulting in lower emissions. This is due to the higher surface area provided by the large number of agglomerated particles. Among the various sorbent materials, titania is shown to be effective for Hg capture The control methodology involves injecting a sorbent precursor into the high-temperature regions of the combustor. Conditions are manipulated so that a highly agglomerated titania sorbent is formed. When the TiO 2 particles are exposed to UV irradiation (as is present in an electrostatic precipitator corona, the absorbed elemental Hg gets oxidized and firmly bonded on the activated titania sites In a related study, 30 we estimated the kinetic parameters associated with this control methodology. The effect of SO 2 gas, one of the major species in the coal combustor, on the Hg capture efficiency using in situ generated, UV-irradiated TiO 2 was also investigated. 28,29 Due to the occupation of active sites on the UV-irradiated TiO 2 surface by SO 2 rather than Hg 0, the Hg capture efficiency decreased as the concentration of SO 2 in the system increased. A higher capture efficiency (~99% was restored by simply increasing the titania precursor feed rate. In this study, effects of chlorine species on the homogeneous and heterogeneous Hg oxidation were discussed. First, kinetics of Hg chlorination in the simulated post-combustion environment ( ºC in the presence of HCl was studied. Hg oxidation reaction with HCl was carried out at various inlet Hg concentrations, inlet HCl concentrations, and reaction temperatures. The effect of UV irradiation on the gas phase reaction of Hg and HCl also was investigated. Second, in situ generated, UVirradiated TiO 2 sorbent particles were tested for their Hg capture efficiency in the presence of chlorine species (provided from the injection of HCl or CH 2. Figure 1. Schematic diagram of the experimental system to study the kinetics of gas phase reaction of HG in the presence of HCI. Hg in the presence of HCl. A glass column covered with heating tape (Thermolyne was used to control the temperature with a power controller (Type 45500, Thermolyne. The glass column reactor was 60 cm in length and 5 cm in diameter, and was made of borosilicate glass (No.7740, Pyrex. The alumina tube in a furnace, typically used to simulate the high-temperature environment, was absorbing a significant amount of HCl, which then reacted with elemental Hg. Due to the reaction between the absorbed HCl and Hg at the alumina tube wall, it was difficult to perform reliable experiments at elevated temperatures. Compressed air was used as the carrier gas and was passed through a high-efficiency particle arresting (HEPA filter and air cleaner (75-62 FT-IR purge gas generator, Balston Filter Products to ensure that it was particle-free. Hg vapor was introduced into the system by passing particle-free air at a precisely controlled flow rate (MKS Mass-Flo Controller, MKS Instruments, Inc. above liquid Hg contained in a gaswashing bottle. An additional inlet was installed to introduce the HCl gas (278 ppm-n 2, Wright Bothers into the system. An online Hg analyzer (UV-1201 Hg Analysis Unit, Shimadzu was connected at the end of the glass column reactor for real-time monitoring of elemental Hg concentration in the gas stream. Effect of Chlorine Species (HCl, CH 2 on the In Situ Hg Capture Figure 2 shows the experimental apparatus used to investigate the effect of chlorine species (HCl, CH 2 on the Hg capture by in situ generated TiO 2 in the presence of UV irradiation. Detailed descriptions of the experimental setup EXPERIMENTAL SETUP Gas Phase Hg Chlorination (Addition of HCl Figure 1 shows the experimental setup used to study the kinetics of homogeneous gas phase reaction of elemental Figure 2. Schematic diagram of the experimental system to study the effect of chlorine species on the in situ Hg capture. Volume 52 November 2002 Journal of the Air & Waste Management Association 1317

3 are provided elsewhere A flow reactor with real-time measurement of particle size distribution and composition analysis was used to study the capture of Hg by in situ generated sorbent particles with and without UV irradiation. An alumina tube located inside the furnace (1700C laboratory tube furnace, Lindberg was used for this study. The alumina reactor tube was cm long, with an inner diameter of 2.54 cm. A titania sorbent precursor, titanium (IV isopropoxide, 97% (Ti[OCH(CH 3 2 ] 4, Aldrich, was introduced into the system by bubbling argon (prepurified, 99.99%, Wright Brothers through the precursor solution contained in a bubbler (Midget, 30 ml, Ace Glass. The bubbler was placed in a water bath (No wide neck flask, 500 ml, Pyrex, the temperature of which was controlled by a power controller (TM106 heating mantle, 500 ml, Glas-Col. The tubing before (connected to the argon source and after (connected to the furnace entrance the bubbler was wrapped with heating tape to prevent any losses due to condensation of the vaporized titania precursor. A photochemical reaction cell was placed at the exit of the reactor tube and irradiated with UV light when necessary. The cell was 60 cm in length, 5 cm in diameter, and made of borosilicate glass (No.7740, Pyrex. The transmittance of the glass was 94% for 360 nm, 72% for 320 nm, and 30% for 300 nm. The UV lamp (Type XX-40, 80 W, Spectronics was 120 cm long, and the intensity at 365 nm was 1850 mw/cm 2, at a distance of 25 cm. A glass fiber filter (No.61663, Gelman Science was placed in a filter holder made from borosilicate, located downstream, to collect TiO 2 particles for further analysis (i.e., composition analysis, BET measurements, and kinetic study in a fixed-bed system. The online Hg analyzer and a series of sampling impingers were connected at the end of the glass column, to capture the Hg species in the gas phase, using a modified EPA Method 29 (CFR The main air and Ti precursor were introduced to the system before the furnace reactor tube to generate TiO 2 particles in situ. The Hg and HCl gases were introduced between the furnace and the photochemical reaction cell. This was to evaluate the Hg capture efficiency of in situ generated TiO 2 at conditions similar to that of the postcombustion chamber. Finally, methylene chloride (CH 2, Fisher was used instead of HCl to minimize adsorption onto the alumina reactor walls at elevated temperatures. The CH 2 was introduced into the system by bubbling argon through the liquid solution of dichloromethane contained in a bubbler. It was introduced into the system before the furnace reactor along with air, Hg, and Ti precursor. PROCEDURES AND MEASUREMENTS The experiments were conducted only after both the system and the online analyzer readings had stabilized. The online Hg analyzer was calibrated by using a modified EPA Method 29, 31 then measuring the total mass of Hg captured using a cold vapor atomic absorption (CVAA analyzer (HgModule, Thermo Separation Products. Due to the tendency of Hg to adsorb, extra precaution was exercised. Before every experiment, the reactor was purged with particle-free air, maintaining the furnace at 1200 ºC, and blank measurements were conducted to demonstrate that no residual Hg remained in the system. The impingers (bottles + stoppers were acid cleaned and scrubbed using detergent and water (Powdered Precision Cleaner, Alconox, Inc.. The Teflon tubing was also changed prior to each experiment or acid cleaned. Also, the online Hg analyzer was cleaned using a weak acid solution before every experiment. Measurements were made after the system had stabilized, and at least three measurements were averaged for each run. Gas Phase Hg Chlorination (Addition of HCl Table 1 shows the experimental plan. First, the HCl concentration was varied from 5 to 120 ppm, keeping the inlet Hg concentration constant. The reactor temperature was maintained at 190 ºC. The flow rate of air through the Hg washing bottle was 50 cc/min; water bath temperature was 24 ºC. Next, the concentration of HCl was maintained constant (27.8 ppm, and the inlet Hg concentration was varied by changing the airflow rate through the Hg washing bottle (10, 20, 30, 40, and 50 cc/min, at 24 ºC. Finally, the reaction rate was measured at various temperatures (24, 140, 170, and 190 ºC, maintaining the Table 1. List of experimental conditions for the kinetic study of the homogeneous gas phase Hg reaction with chlorine species. Total Flow Rate in Airflow Rate through Hg Hg Feed Bottle Flow Rate of HCl Gas Temperature Set No. Reactor (cm 3 /min Feed Bottle (cm 3 /min Temperature (ºC (cm 3 /min (ºC Air + Hg + HCl , 50, 100, 200, , 140, 170, , 20, 30, 40, , 140, 170, 190 Analyzer type: Online Hg analyzer Journal of the Air & Waste Management Association Volume 52 November 2002

4 constant inlet concentrations of Hg and HCl. The flow rate of air through Hg bottle was 50 cc/min at a water bath temperature of 24 ºC; HCl concentration was 27.8 ppm. The flow rate of main air was fixed at 1.0 L/min, creating a residence time in the reactor of less than 1 min. Effect of Chlorine Species (HCl, CH 2 on the In Situ Hg Capture The base gas (air + Ti precursor + chlorine species flow rate was fixed at 1.0 L/min. The corresponding residence time in the furnace reactor was approximately 3 sec, at a set temperature of 1000 ºC. The residence time of the simulated flue gas under the UV lamps was less than 1 min (~56 sec. The temperature of a gas leaving the furnace reactor was cooled down to a room temperature (24 ºC. Also, the photochemical reactor temperature was maintained at 24 ºC. The average UV intensity measured inside the photochemical reaction cell was µw/ cm 2. The filter holder was first covered with aluminum foil and then with black cloth to ensure that the in situgenerated TiO 2 particles were irradiated only while passing through the photochemical reaction cell. Extreme care was taken to minimize the effect of TiO 2 particles deposited during the measurements inside the photochemical reaction cell. After each run, the glass column was washed with Alconox solution. Glass fiber filters were weighed before and after each run so that the total mass of TiO 2 participating in the heterogeneous reaction at a given residence time could be calculated. Experimental conditions are shown in Table 2. In the first set of experiments, the online analyzer was used. The effectiveness of in situ generated/uv-irradiated TiO 2 particles for Hg capture in the presence of various concentrations of HCl was measured. The HCl concentrations tested were 27.8, 55.6, 83.4, 111.2, 139.0, 166.8, and ppm. The flow rate of air through the Hg washing bottle was 20 cc/min, and the bottle temperature was maintained at 24 ºC. Both HCl and Hg were introduced to the system after the furnace reactor tube, while main air and Ti precursor were introduced before the furnace reactor tube. The argon flow rate through the Ti bubbler and the temperature of the water bath containing the Ti bubbler were maintained at 250 cc/min and 85 ºC, respectively. The feed rate measured by weighting the filter before and after the run was found to be µg/sec. In the second set of experiments (Tables 2 and 3, a CVAA analyzer was used instead of the online Hg analyzer to identify the partitioning of the oxidized Hg between the gas phase and the particulate phase, since the online Hg analyzer can only provide information on the total amount of elemental Hg oxidized. First, the amount of Hg captured by the TiO 2 particles was measured by extraction of the Hg species from the filter. At the end of each run, filters were folded and placed immediately in a bottle filled with 10 ml of deionized water to extract any Hg. The filters were then sonicated and the liquid was analyzed by CVAA. The residue was digested in a 0.4% KMnO 4 /10% H 2 SO 4 solution and analyzed by CVAA. An Hg module (Thermo Separation Products, Riviera Beach, FL was used. It must be kept in mind that only Hg 2+ in solution is determined. For example, the filter digestion was not highly oxidative and would only solubilize Hg 2+ species such as HgO. Next, a procedure similar to EPA Method 29 was used to capture oxidized and elemental Hg in the gas phase. The first sampling impinger contained a 0.1% HNO 3 solution and was used to trap the oxidized forms of Hg. 32,33 A set of three other impingers containing 0.4% KMnO 4 /10% H 2 SO 4 were used to trap elemental Hg in the gas phase. 31 The impinger solutions were analyzed to determine the Table 2. List of experimental conditions for the kinetic study of the heterogeneous TiO 2 - Hg reaction with chlorine species. Argon Flow Argon Flow Airflow Rate Rate through Rate through Total Flow through Hg Ti Precursor Flow Rate of CH 2 Photochemical Rate in Reactor Feed Bottle Bubbler HCl Gas Bubbler Reaction Set No. (cm 3 /min (cm 3 /min (cm 3 /min (cm 3 /min (cm 3 /min Temperature (ºC Analyzer Type 1. Air + Hg + HCl ± UV Online Hg 2. Air + Hg + HCl + TiO ± UV analyzer 3. Air + Hg + HCl ± UV Impingers 4. Air + Hg + TiO + UV (sampling + 5. Air + Hg + HCl + TiO ± UV CVAA 6. Air + Hg + CH 2 + UV analyzer 7. Air + Hg + CH Cl + TiO + UV (analysis Furnace temperature: 1000 ºC; water bath temperature of the Hg washing bottle: 24 ºC; water bath temperature of the Ti precursor bubbler: 85 ºC; water bath temperature of the CH 2 bubbler: 24 ºC Volume 52 November 2002 Journal of the Air & Waste Management Association 1319

5 elemental Hg concentration and oxidized Hg species concentration in the gas phase, using CVAA. The HCl inlet concentration was fixed at 55.6 ppm; other conditions remained the same as for the first set. In the final set of experiments, CH 2 was used instead of HCl and introduced to the system before the furnace reactor tube along with air, Ti precursor, and Hg. This was due to the fact that CH 2 was not being absorbed as much as HCl through the alumina reactor wall. While the previous sets of experiments (using HCl were carried out under conditions simulating the post-combustion region (<150 ºC, in this set, the in situ Hg capture method was tested throughout the whole combustion system (from primary combustion chamber to the electrostatic precipitator. The argon flow rate through the bubbler containing dichloromethane (CH 2 was 20 cc/min. The water bath temperature was 24 ºC. The theoretically calculated feed rate of CH 2 was mol/min, and the inlet concentration was ~ µg/m 3. RESULTS AND DISCUSSION Gas Phase Hg Chlorination (Addition of HCl For the chlorination of Hg in flue gases, Hg is generally considered to be the major product. The simplest form of the overall pathway for the chlorination of elemental Hg is as follows: Hg + HCl oxidized products (HgCl and/or Hg (1 The direct reaction of Hg with HCl is known to be too slow at temperatures below 300 or 400 ºC, having no obvious reaction pathway and probably involving many elemental steps. 34 Theoretical and experimental investigations suggest the following pathways to Hg : 13,19,22,23,35 or Pathway 1 Hg + Hg (2 Pathway 2 Hg + Cl HgCl (3 HgCl + Cl Hg (4 HgCl + HCl Hg + H (5 In pathway 1, 13 the formation of in the gas phase may be kinetically limited. Senior et al. 4 calculated that only about 1% of the chlorine is converted to at the air pollution control device inlet, whose temperature is within the range of those tested in this study ( ºC. Also, Sliger et al. 22 showed a slow conversion of HCl to at temperatures below 400 ºC. However, Senior et al. 4 suggested that could also be catalytically generated even at relatively low temperatures (such as in this work in the presence of particulate matter, according to the following Deacon process: 2HCl + 1/2 O 2 + H 2 O (6 In pathway 2, 19,22 the source of the Cl-atom is free radical attack on HCl: HCl + OH Cl + H 2 O (7 which is thermodynamically favored at higher temperatures. Due to the different pathways, a kinetic study of Hg chlorination is difficult, especially since the elemental steps are not well quantified. In this paper, the overall reaction rate is established, and the kinetic parameters were estimated using the rate expression for eq 1. Based on eq 1, the overall rate of the gas phase Hg chlorination in the presence of HCl can be written as follows: dchg dt α β = k CHg CHCl where C Hg is the gas phase elemental Hg concentration, C HCl is the gas phase HCl concentration, and k is the overall reaction rate constant. With respect to the concentrations of Hg and HCl, α and β are the reaction orders, respectively. At the range of temperatures and C HCl tested in this study, the Hg reaction rate remains constant, regardless of the HCl concentration, indicating practically zero order with respect to C HCl. As discussed by Niksa et al. in their recent report, 23 for a high HCl/Hg ratio, the extent of the Hg oxidation reaches a saturation limit, and Hg concentration is low enough to limit its own oxidation. The reaction rate was more sensitive to the reaction temperature than any other variables. Consequently, HCl can be assumed to be in excess relative to elemental Hg, and eq 8 becomes: dchg α = k CHg (9 dt where k = k C HClβ. By taking the natural log on both sides, we have: dchg ln( = ln k + α ln CHg (10 dt Therefore, (-dc Hg /dt versus C Hg on a log-log plot would yield α (reaction order with respect to C Hg as the slope. Rearranging and integrating eq 10 from 0 to t and from C Hg,o to C Hg gives the following equation: [ ] = The overall rate constant, k can be expressed using activation energy as follows: (8 1 1 α 1 α CHg CHg, o k t (11 α Journal of the Air & Waste Management Association Volume 52 November 2002

6 Ea k = A exp RT (12 where A is a pre-exponential factor and R is the gas constant. Therefore, once the value of α is found, results obtained from experiments performed at various reaction temperatures can be used to estimate the activation energy, E a, and the overall rate constant, k, as a function of temperature. Figure 3 is a plot of ln(-dc Hg /dt versus lnc Hg. The value of the slope obtained from the curve fit implies that α is ~1.55. Similarly, the activation energy and the overall rate constant were estimated by using eqs 11 and 12 (Figure 4. The values of E a and k are [kj/mol] and exp( /t[(µg/m (s -1 ], respectively. It should be noted that the kinetic parameters presented above were to assist the design of specific Hg control technology (utilizing in situ generated TiO 2 + UV in the high temperature conditions similar to the ones described in this study (i.e., coal combustors and/or waste incinerators. For other specific conditions, one can still follow the procedures that are presented and discussed in this study to evaluate the corresponding kinetic information. However, it is obvious that a kinetic study of an elementary mechanism that would directly handle variable gas composition and reaction conditions must be carried out. Effect of Chlorine Species (HCl, CH 2 on In Situ Hg Capture In this section, the effect of chlorine species on the Hg capture by in situ generated TiO 2 was investigated using HCl and CH 2. Depending on the type of chlorine species used, the location at which the Hg and chlorine species were injected was changed. When HCl was used, only air and Ti precursor were injected into the system before the furnace reactor (1000 ºC and Hg and HCl were injected between the furnace reactor and the photochemical Figure 3. Plot of In(-dC Hg /dt vs. InC Hg (T = 190 ºC, C HCI = 27.8 ppm. Figure 4. Plot of In{(1/1-α(C Hg 1-α -C Hg,o 1-α } vs. 1/T (C Hg = 5.5 ± 1.0 µg/ m 3, C HCI = 55.6 ppm. reaction cell (maintained at 24 ºC. However, later, when CH 2 was used instead of HCl as chlorine species, all the flue gas constituents were injected into the system right before the furnace reactor tube (1000 ºC. First, the online Hg analyzer was used to measure the total amount of Hg oxidized in both gas phase and on the surface of TiO 2 with and without UV irradiation. Table 3 shows that elemental Hg is removed by both gas phase chlorination and titania particles. Also, the Hg removal efficiencies of the UV-irradiated TiO 2 particles at various HCl concentrations are shown. The results indicate that in the presence of HCl, the total amount of elemental Hg removed increases. This was due to the fact that the elemental Hg was not only captured by in situ generated/ UV-irradiated TiO 2 particles, but also oxidized by chlorine species (Cl in the gas phase as was also suggested by Sliger et al. 22 Therefore, as the HCl concentration increased, elemental Hg removal efficiency increased. The enhancement of elemental Hg removal by HCl in the presence of titania particles can be explained similarly to the oxidation mechanism proposed by Senior et al. 4 The is catalytically generated by the interaction of HCl with the in situ generated particles (Deacon process, eq 6. Once formed, the rapidly reacts with the Hg (eq 2, particularly if Hg were already adsorbed on the particle surface. In the presence of UV irradiation, Cl-atom can also be generated by TiO 2 -OH, then reacts with absorbed elemental Hg (eq 7. Meanwhile, the direct gas-phase oxidation of elemental Hg by Cl and (generated from HCl can be best explained using the pathway 2 (eqs 3 5. Table 4 shows the results of the CVAA analysis on the Hg collected using a modified EPA Method 29. It should be noted that the Hg capture efficiency shown in Table 4 is defined as the ratio of the amount of Hg captured only on the filter to the total amount of Hg measured from both filter and impingers. It does not account for the Volume 52 November 2002 Journal of the Air & Waste Management Association 1321

7 Table 3. Hg 0 removal efficiency by in situ Hg capture method in the presence of HCl. System Elemental Hg Removal Efficiency (% Air + Hg + HCl* Air + Hg + HCl + UV* Air + Hg + HCl + Ti* Air + Hg + HCl + Ti + UV* Air + Hg + HCl + Ti + UV Initial HCl inlet concentration (ppm Elemental Hg removal efficiency (% Airflow rate through the Hg washing bottle: 20 cc/min; water bath temperature of the Hg washing bottle: 24 ºC (C Hg,o = 6.5 ± 0.5 µg/m 3 ; airflow rate through the Ti precursor bubbler: 250 cc/min; water bath temperature of the Ti precursor bubbler: 85 ºC; *HCl concentration: 55.6 ppm amount of elemental Hg removed by the gas-phase oxidation, which explains the lower Hg capture efficiency in the presence of HCl. The results of sets 5 and 7 show that Hg capture efficiency is greater than 98%. Despite the high inlet concentration of CH 2, it shows high Hg capture efficiency (>85% using in situ Hg capture method. Interestingly, the results of sets 1, 2, and 6 suggest the formation of submicrometer particles from chlorinated Hg in the gas phase. CONCLUSIONS First, the gas phase Hg transformation in the presence of HCl was investigated. The rate of Hg oxidation was highly dependent on the reaction temperature. The chlorination rate was zero order with respect to HCl concentration at the conditions used in this study. Kinetic parameters, such as the reaction order with respect to Hg concentration (C Hg, overall rate constant (k and activation energy (E a, were estimated. They are 1.55, [kj/mol], and exp( /t [(µg/m (s -1 ], respectively. Table 4. Results of CVAA analysis on the mass of Hg captured. Mass of Hg Mass of Hg Captured on the Captured in the Hg Capture Set Filter (µg Impingers (µg Efficiency (% 1. Air + Hg + HCl* Air + Hg + HCl + UV* Air + Hg + Ti + UV * Air + Hg + HCl + Ti* Air + Hg + HCl + Ti + UV * Air + Hg + CH Cl + UV ** Air + Hg + CH Cl + Ti + UV ** *Air + Ti precursor and Hg + HCl were injected into the system before (1000 ºC and after (24 ºC the furnace reactor, respectively; **Air + Ti + Hg + CH Cl were injected 2 2 into the system before the furnace reactor (1000 ºC Next, the effect of chlorine species (HCl, CH 2 on the in situ Hg capture was investigated. The in situ generated/uvirradiated TiO 2 was very effective at capturing elemental Hg even in the presence of chlorine species. The results indicate that the total removal of the elemental Hg is enhanced by the presence of chlorine species, due to the additional removal by the gas phase Hg transformation to the oxidized forms. In a full-scale system, oxidized forms of Hg can readily be removed in conventional control devices, such as wet scrubbers, due to their high solubility. In situ generated/uv-irradiated TiO 2 was demonstrated to effectively capture elemental Hg in a simulated combustion flue gas system. Furthermore, utilizing a corona of the ESP as a UV source, the novel gas phase sorbent (Ti precursor injection technique can be readily applicable to full-scale coal combustors/waste incinerators for the control of Hg emissions. ACKNOWLEDGMENTS The work was performed at the University of Cincinnati by the authors, and was funded by a contract from the Ohio Coal Development Office, Columbus, OH. Elizabeth Hedrick acknowledges support from the NERL Internal Grants Program, EPA. NOMENCLATURE α = Reaction order with respect to gas phase Hg concentration β = Reaction order with respect to HCl concentration E a = Activation energy (kj/mol k = Overall rate constant (= kc HClβ C Hg = Gas phase Hg concentration (µg/m 3 t = Reaction time (sec C HCl = Gas phase HCl concentration (ppm k = Overall rate constant A = Pre-exponential factor REFERENCES 1. Biswas, P. Hg Measurement and Its Control: What We Know, Have Learned, and Need to Further Investigate; J. Air & Waste Manage. Assoc. 1999, 49, Wu, C.Y.; Biswas, P. An Equilibrium Analysis to Determine the Speciation of Metals in an Incinerator; Combust. & Flame 1993, 93, Galbreath, K.C.; Zygarlicke, C.J. Hg Speciation in Coal Combustion and Gasification Flue Gases; Environ. Sci. Technol. 1996, 30, Senior, C.L.; Bool, L.E., III; Huffman, G.P.; Huggins, F.E.; Shah, N.; Sarofim, A.; Olmez, F.I.; Zeng, T. A Fundamental Study of Hg Partitioning in Coal-Fired Power Plant Flue Gas. In Proceedings of the Annual Meeting of A&WMA; A&WMA: Toronto, Canada, 1997; Paper 97-WP72B Senior, C.L.; Morency, J.R.; Huffman, G.P.; Huggins, F.E.; Shah, N.; Peterson, T.; Shadman, F.; Wu, B. Interaction between Vapor-Phase Hg and Coal Fly Ash under Simulated Utility Power Plant Flue Gas Conditions. In Proceedings of the Annual Meeting of A&WMA; A&WMA: San Diego, CA, 1988; Paper 98-RA79B Journal of the Air & Waste Management Association Volume 52 November 2002

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Presented at the Fifth International Congress on Toxic Combustion Byproducts, Dayton, OH, About the Authors Dr. Tai Gyu Lee (corresponding author is an assistant professor in the Department of Chemical Engineering at Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul , South Korea. Dr. Pratim Biswas is the Jens Professor and director of the Environmental Engineering Science Program at Washington University in St. Louis, Box 1180, Department of Chemical & Civil Engineering, Washington University in St. Louis, St. Louis, MO Elizabeth Hedrick is a researcher at EPA s National Exposure Research Laboratory, Cincinnati, OH Volume 52 November 2002 Journal of the Air & Waste Management Association 1323