Experimental evaluation of the effects of quench rate and quartz surface area on homogeneous mercury oxidation

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1 Proceedings of the Combustion Institute 31 (2007) Proceedings of the Combustion Institute Experimental evaluation of the effects of quench rate and quartz surface area on homogeneous mercury oxidation Andrew Fry a, *, Brydger Cauch a, Geoffrey D. Silcox a, JoAnn S. Lighty a, Constance L. Senior b a University of Utah, Department of Chemical Engineering, 50 S Central Campus Dr., Room 3290 MEB, Salt Lake City, UT , USA b Reaction Engineering International, 77 West 200 South, Suite 210, Salt Lake City, UT 84101, USA Abstract This paper presents a mercury oxidation data set suitable for validation of fundamental kinetic models of mercury chemistry and for mechanism development. Experimental facilities include a mercury reactor fitted with a 300-W, quartz-glass burner and a quartz reaction chamber. While operated with a temperature profile representative of a typical boiler, a residence time of 6 s was achieved. Participating reacting species (chlorine, mercury) were introduced through the burner to produce a radical pool representative of real combustion systems. Speciated mercury measurements were performed using a Tekran 2537A Analyzer coupled with a conditioning system. Homogeneous mercury reactions involving chlorine have been investigated under two different temperature profiles producing quench rates of 210 K/s and 440 K/s. The larger quench rate produced 52% greater total oxidation than the lower quench at chlorine concentrations of 200 ppm. The effect of reactor surface area on oxidation was also investigated. The quartz surfaces interacted with mercury only in the presence of chlorine and their overall effect was to weakly inhibit oxidation. The extent of oxidation was predicted using a detailed kinetic model. The model predicted the effects of quench rate and chlorine concentration shown in experimentation. Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Mercury oxidation; Mercury model; Mercury kinetics; Mercury emission 1. Introduction New regulation of mercury emissions from coal-fired utility boilers has promoted the investigation of fundamental mechanisms of mercury transformations under conditions representative * Corresponding author. Fax: addresses: afry@eng.utah.edu (A. Fry), senior@reaction-eng.com (C.L. Senior). of real combustion systems. Thermodynamic equilibrium calculations do not accurately represent mercury speciation in industrial combustors and detailed kinetic models are necessary [1]. A kinetic model that would accurately predict mercury speciation over a broad range of operating conditions should include homogeneous and heterogeneous mechanisms. Several detailed kinetic models have been developed [2 5] and most of these models include both homogeneous and heterogeneous mechanisms. Uncertainties exist about /$ - see front matter Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi: /j.proci

2 2856 A. Fry et al. / Proceedings of the Combustion Institute 31 (2007) the accuracy of the proposed mechanisms, the quality of the developed rate parameters and even the importance of homogeneous versus heterogeneous kinetic pathways. Systematic validation of the reaction mechanisms is necessary to fully understand the importance of the different pathways. Uncertainties in the accuracy of rate parameters can be eliminated by implementing more sophisticated calculations [6]. However, validating reaction sets and understanding the importance of homogeneous versus heterogeneous pathways requires quality experimental data [7]. Experimental data should encompass conditions realistic for the gas-phase in coal-fired boilers. Radicals must be generated to validate the reactions of interest and to provide valid initial conditions for modeling; this requires a flame environment. All relevant operating conditions should be measured and reported allowing the data to be useful for model validation. Finally mercury measurements are difficult to obtain and, due to the low mercury concentrations found in the flue gas from coal-fired boilers, analytical techniques must be sensitive to determine concentrations without interference from surfaces, water and other interfering species. In this study, the effects of quench rate and quartz surface area on mercury oxidation in the presence of chlorine were investigated. An experimental homogeneous mercury reactor was used which has been evaluated for accuracy and repeatability. All experimental conditions necessary for modeling have been reported and the homogeneous and heterogeneous contributions, in this case surface reactions, have been addressed. Hence, the resulting experimental data set is suitable for validation of mechanisms for the gasphase reactions of mercury and chlorine species. 2. Experimental setup UV Detector Gas Solenoid Valve Natural Gas, Air & Certified Gas Standards Mass Flow Controllers 300-W Quartz Burner 47 mm Quartz Reaction Tube Fig. 1. Experimental apparatus. PS Analytical CavKit Mercury Calibration Gas Generator Emergency Blow-Off Nozzle Quartz Washer Thermcraft Heater Gas Analyzers Tekran Mercury Analyzer Sample Conditioning System The mercury reactor used in this study is shown in Fig. 1. A 50-mm OD 47-mm ID quartz reaction tube (132 cm in length) runs through the center of a high-temperature Thermcraft heater. The quartz tube extends 79 cm below the heater to the sampling point and is temperature-controlled using heat tape and insulation. This allows the temperature profile to be configured for different quench rates. The mercury reactor was fitted with a natural gas, premixed burner made of quartz glass. The design burner heat input was approximately 300- W, producing 6 SLPM of combustion gas. In these experiments all reactants were introduced through the burner and passed through the flame in order to provide initial conditions for model validation. Tekran 2537A mercury analyzer coupled with a sample conditioning system designed by Southern Research Institute (SRI) provided a measurement of total and elemental mercury in the furnace exhaust gas. In this system sample gas was pulled in two streams from the last section of the quartz reaction tube into a set of conditioning impingers. One stream was bubbled through a solution of stannous chloride to reduce oxidized species to elemental mercury and then contacted a solution of sodium hydroxide to remove acid gases. This stream represented the total mercury concentration in the reactor. The second stream was first treated with a solution of potassium chloride to remove oxidized mercury species and then was treated in a caustic solution for acid gas removal. This stream was representative of the elemental mercury concentration in the reactor. Oxidized mercury species were represented by the difference between total and elemental mercury concentrations. A chiller removed water from the sample gas and then each stream was intermittently sent to the analyzer. A mass balance has been closed for this system across a range of mercury concentration from 0 to 500 lg/m 3. The mercury recovery was within 13% for all data points and within 5% at concentrations used in this study. Using the temperature-controlled bottom section of the reactor, two temperature quench rates

3 A. Fry et al. / Proceedings of the Combustion Institute 31 (2007) were obtained for study. The measured temperature data are presented in Table 2. Plots of these temperature profiles are shown in Fig. 2. The high temperature region, occurring between 0 and 53 cm in Table 2, was generated by the Thermcraft furnace. The temperature drop that occurred across the bottom section of the quartz tube, resulted in a quench rate representative of industrial conditions. The low-temperature region, at approximately 5 s residence time, represented flue gas temperatures after an air preheater and through air pollution control devices. These low temperatures were necessary for oxidation reactions to occur. This experimental configuration resulted in a maximum Reynolds number of approximately 150, well within the limits for laminar flow. The diffusion time of mercury from the center of the Table 1 Experimental flue gas composition Species Measured Calculated Condition Hg 25 lg/m 3 25 lg/m 3 Dry, standard Chlorine as Cl ppmv Wet H 2 O 18.10% O % 0.46% Dry NO 33 ppmv Dry CO % 11.47% Dry CO 9 ppmv Dry Fig. 2. Reactor temporal temperature profiles. reactor to the wall was estimated to be 1 s. This analysis was performed using the characteristic time for diffusion in a cylindrical system of radius r. s ¼ r2 ð1þ 4 D The diffusion time is significantly shorter than the residence time of the reactor. In addition, when step changes in mercury concentration were introduced, measured values reflected the changes within the 150 s resolution of the analyzer. These data suggest that the reactor may be modeled as a plug flow reactor. Table 2 Reactor temperature data Distance from burner (cm) residence time (s) temperature (K) residence time (s) temperature (K)

4 2858 A. Fry et al. / Proceedings of the Combustion Institute 31 (2007) Quartz surface interactions For each of the temperature profiles detailed in Fig. 2, oxidation experiments were performed. Chlorine (as Cl 2 ) was introduced into the reactor through the burner using a certified chlorine calibration gas standard in air. Kinetic modeling of the post-flame chlorine species shows that the Cl 2 passing through the flame is converted to atomic chlorine and subsequently to HCl. The natural gas burner was fired at its nominal firing rate. Experimental gas composition is presented in Table 1. Gas measurements for species other than mercury were performed after the mercury conditioning system and analyzer as detailed in Fig. 1. Chlorine concentrations were calculated based on the chlorine gas standard concentration and calculated combustion product flows. Chlorine concentrations were verified using EPA Method 26. The first step in each experiment was to establish a baseline concentration of mercury in the reactor at 25 lg/m 3 (dry, standard) and verify that both elemental and total mercury measurements adequately represented that concentration. Chlorine was then introduced into the reactor at known concentrations to promote the oxidation of mercury. Figure 3 details a typical time-resolved experiment. As seen in the figure, upon 200 ppm chlorine injection, at high quench conditions, measured values of total and elemental mercury decreased to approximately 18.5 lg/m 3. The total mercury concentration then gradually increased back to the baseline concentration while the elemental mercury concentration further decreased to a steady value. This suggests that, initially, mercury was being adsorbed on the reactor surfaces. At the end of the experiment, once the chlorine flow was stopped, the measured value of total mercury concentration increased to above 25 lg/m 3 and then gradually returned to baseline, this represents desorption of mercury. This behavior suggests that chlorine is causing mercury to be attracted to quartz surfaces in the reactor. While these data suggest the interaction of mercury with quartz in the presence of chlorine, they do not demonstrate an impact on oxidation. To investigate the impact of quartz surface on oxidation, a bundle of thin-walled quartz tubes was inserted into the quench section of the reactor. The tube bundle was made of seven tubes with a 14 mm OD 12 mm ID and 60 cm in length, tacked together in a hexagonal configuration. These tubes increased the surface area from 1000 to 3000 cm 2 in the quench section while only slightly reducing the cross-sectional area and, therefore, the residence time by only 5%. Introduction of the quartz tube bundle increased the size of the adsorption curve and impacted its shape suggesting that the adsorption is occurring on the quartz. The result of this set of experiments on mercury oxidation is presented in Fig. 4. The Low Surface data points represent an average of ten experimental results with no tube bundle; the error bars show the standard deviation for this average. The High Surface data point is the average of three experiments performed with the tube bundle inserted, with a corresponding error bar. These data appear in Table 3. These data show that a threefold increase in surface area resulted in a 19% decrease in oxidation. In combustion systems a predominant termination step for radicals is the trapping of the radical on the reactor wall. It has been shown that the reaction of elemental mercury with atomic chlorine is the primary pathway of homogeneous oxidation of mercury with chlorine [8]. It follows that the reduction in mercury oxidation due to the addition of quartz surface can be explained by chlorine radical termination on those surfaces. Attraction of mercury to terminated chlorine radicals can also explain the adsorption peak demonstrated by the data in Fig. 3. Release of the terminated radicals and bound mercury after chlorine injection has ceased would cause a desorption peak of oxidized mercury as demonstrated. Fig. 3. Mercury adsorption on quartz glass with 200 ppmv chlorine. Fig. 4. Impact of quartz surface on oxidation.

5 Table 3 Mercury oxidation data Chlorine concentration (ppmv) A. Fry et al. / Proceedings of the Combustion Institute 31 (2007) average oxidation (%) standard deviation average oxidation (%) standard deviation High surface average oxidation (%) High surface standard deviation Using linear extrapolation on the surface data, oxidation with no surface effects can be predicted. For this experiment 79.4% oxidation would occur at a chlorine concentration of 200 ppmv with no surface interaction demonstrating the extents of oxidation are insensitive to surface area effects and are within the experimental uncertainties. 4. Modeling Niksa, Helble and co-workers [3,4] have studied the post-flame homogeneous reactions of mercury and have developed a detailed chemical kinetic mechanism to describe these reactions. The mechanism includes sub-models for Hg chemistry, Cl chemistry, NO x chemistry (including NO Cl) and SO x chemistry. Included in the mercury sub-model are the following eight reactions: Hg + Cl + M! HgCl + M ð2þ Hg + Cl 2! HgCl + Cl Hg + HCl! HgCl + H Hg + HOCl! HgCl + OH HgCl + Cl 2! HgCl 2 +Cl HgCl + Cl + M! HgCl 2 +M HgCl + HCl! HgCl 2 +H HgCl + HOCl! HgCl 2 +OH ð3þ ð4þ ð5þ ð6þ ð7þ ð8þ ð9þ This reaction mechanism was derived from previous work in the literature [8,9]. Reaction 1 is thought to be the major kinetic pathway and possibly a rate-limiting step for mercury oxidation. For this reason an accurate chlorine mechanism is a fundamental part of the mercury oxidation model. The chlorine mechanism used in this model was developed by Roesler [10,11] and consists of 29 reactions. Chemkin 4 was used as the computational platform for this investigation. A perfectly stirred reactor (PSR) was used to react natural gas with air to generate a representative initial radical pool and flue gas matrix for the mercury reactions. A plug flow reactor (PFR) was used to model the mercury reactor. The gas composition generated by the PSR was used as an inlet stream to the PFR. Mercury and chlorine were added to this inlet stream. For modeling purposes it was assumed that all of the chlorine entering the PFR was atomic as it passed through the flame. The high- and low-quench temperature profiles of the mercury reactor were each used as the temperature profile of the PFR in the model. Volumetric flow was set equal to the flow used in the mercury oxidation experiments and the diameter and length of the PFR were set equal to those of the mercury reactor. 5. Results Oxidation experiments were performed using the two temperature profiles detailed in Fig. 2 and chlorine concentrations ranging from 0 to 600 ppm. The data produced by these experiments are presented in Table 3. The model used as input the experimental conditions: composition and time temperature history. A comparison between experiments and the model predictions is presented in Fig. 5. Oxidation curves for the high and low quench rates yielded the expected results. In these experiments, the high-quench temperature profile resulted in significantly higher mercury conversion than the low-quench rate. This result can be attributed to longer residence times at low temperatures and higher concentrations of Cl radicals generated by the higher quench rate as discussed by Procaccini [12]. An elevated concentration of chlorine radicals was also demonstrated in the modeling results for high-quench conditions, driving the rate-limiting step in the reaction mechanism forward (Eq. (1)). These observations detail the potential impacts of chlorine radical wall termination.

6 2860 A. Fry et al. / Proceedings of the Combustion Institute 31 (2007) An existing model developed by Niksa, Helble and co-workers [3,4], using Chemkin 4 as the computational platform was used to predict the experimental data. The data and model predictions were in excellent agreement for these conditions. The effects of quench rate were accounted for by the various mechanisms within the model and demonstrated the importance of chlorine radicals. Fig. 5. Oxidation data and model results. 6. Conclusions The data generated by this experimental investigation showed the importance of quench rate on homogeneous mercury oxidation. A change in quench rate from 210 K/s to 440 K/s resulted in an increase in oxidation from 34% to 86% at a chlorine concentration of 300 ppm. In each of these quench situations the outlet temperature was the same. This is strong evidence that mercury was not in chemical equilibrium with the flue gas and its oxidation was kinetically controlled. Chlorine radical concentration is sensitive to temperature, and thus oxidation kinetics are also very dependant on quench rate. The experimental study demonstrated that quartz surfaces attracted mercury in the presence of chlorine. This attraction was likely due to the termination of chlorine radicals on the quartz surface and the subsequent attraction of mercury to the terminated radicals. Introduction of a quartz tube bundle into the reactor, which effectively increased the surface area by a factor of three, caused a decrease in oxidation of about 19% at a chlorine concentration of 200 ppm. This result supported the hypothesis that these surfaces remove chlorine radicals from the reaction gases and that quartz surfaces do not catalyze mercury oxidation reactions, but inhibit them. However, linear extrapolation to zero surface oxidation showed that this value was within experimental uncertainty. This suggests that these surface interactions can be neglected and that only homogeneous oxidation of mercury is represented by this data set. Acknowledgments This paper was prepared with the support of the US Department of Energy, under Award No. DE-FG26-03NT However, any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the DOE. Also a special thanks to Jost O.L. Wendt for his input and expertise and to Dana Overacker for many hours in the laboratory. References [1] J.D. Kilgroe, Report No. EPA-600/R , [2] J.R. Edwards, R.K. Srivastava, J.D. Kilgroe, Air Waste Manag. Assoc. 51 (2001) [3] S. Niksa, J.J. Helble, N. Fujiwara, Environ. Sci. Technol. 35 (18) (2001) [4] J. Qiu, R.O. Sterling, J.J. Helble, in: 28th International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, Fl, [5] M. Xu, Y. Qiao, C. Zheng, L. Li, J. Liu, Combust. Flame 132 (2003) [6] J. Wilcox, J. Robles, D.C.J. Marsden, P. Blowers, Environ. Sci. Technol. 37 (18) (2003) [7] C.L. Senior, D. Lignell, Z. Chen, B. Shiley, A. Sarofim, in: Air Quality IV Conference, Arlington, VA, [8] R.N. Sliger, J.C. Kramlich, N.M. Marinov, Fuel Proc. Technol. 65 (2000) [9] N.C. Widmer, J. West, J.A. Cole, in: 93rd Annual Air and Waste Management Association Conference, Salt Lake City, Ut., [10] J.F. Roesler, R.A. Yetter, F.L. Dryer, Combust. Sci. Technol. 85 (1992) [11] J.F. Roesler, R.A. Yetter, F.L. Dryer, Combust. Flame 100 (1995) [12] C. Procaccini, J.W. Bozzelli, J.P. Longwell, K.A. Smith, A.F. Sarofim, Environ. Sci. Technol. 34 (21) (2000) Comments Roman Weber, TU Clausthal, Germany. Is sulfur or bromine present in your system? For what reason and what would be their effect? Reply. Sulfur was not included in this experimental investigation. The effects of SO 2 have been investigated in a similar experimental apparatus ([4] in paper).

7 A. Fry et al. / Proceedings of the Combustion Institute 31 (2007) Bromine was also excluded from this investigation. Experimental data on homogeneous bromine mercury reactions are limited although computational studies suggest that the reaction Hg + Br + M = HgBr + M is a likely route to mercury oxidation ([13] in paper). Bromine has also been added to full-scale coal-fired utility boilers, and has been shown to affect mercury speciation ([14] in paper). To determine the homogeneous and heterogeneous contributions to oxidation, well-controlled laboratory experiments in a particle-free combustion system are necessary. d Keith Schofield, University of California at Santa Barbara, USA. As you may know, I am among the group of believers that mercury s combustion chemistry is totally heterogeneous. Consequently, I am most interested in your work, which is very commendable utilizing quartz materials. Nevertheless, there remain several disturbing aspects in your interpretation. Firstly, as you indicate a homogenous model depends crucially on the initial reaction of Hg + Cl + M. Recently, the rate constant for this has been most carefully re-measured by well-recognized kineticists [1]. They now find it to be about 200-fold slower than the value used in your analysis. Accepting such a value totally eliminates any role for homogeneous kinetics. Moreover, the levels of Cl and Cl 2 in combustion systems still is very questionable as it is generally accepted that flame chlorine is at least 99% HCl. This further imposes rather severe concentration constraints on the mechanism that requires quite significant concentration of chlorine for initiation. Consequently, the model fails for low chlorine coals such as lignites. This selective nature implies another model is needed in such cases. As a result, your results become extremely interesting but rather than resolving a question they appear to have produced a further problem in analyzing the behavior of mercury in such a system. Reference [1] D.L. Donohoue, D. Bauer, A.J. Hynes, J. Phys. Chem. A. 109 (2005) Reply. Experimental determination of the rate constant for the reaction of mercury with atomic chlorine is difficult; published values conflict as detailed in the paper by Donohoue. Conditions that may cause errors in measurement have been detailed in other studies ([15,16] in paper). Quantum mechanical methods may be more suitable for rate parameter development and have been used to determine the rate parameters for several reactions of mercury with chlorine species ([17] in paper). When these calculated rate parameters are implemented into the model, observed mercury oxidation levels are similar to predictions presented here. Model results support the assertion that HCl is the predominant post-flame chlorine species. When 300 ppmv of atomic Cl is introduced before the flame, a concentration of 4 ppm is predicted in the quench section and only ppb concentrations persist to the sample location. Molecular chlorine concentration increases to about 2 ppm by the sample location with the remainder of the chlorine present as HCl. Even while Cl concentrations are extremely low they are significantly greater than the mercury concentration and sufficient for oxidation of mercury. In practical combustion systems heterogeneous mercury oxidation is important and in some cases predominant. Detailed kinetic models suitable for predicting mercury behavior in these systems must include both homogeneous and heterogeneous pathways. This study focuses on the validation of fundamental homogeneous mechanisms of mercury oxidation. It has been demonstrated that quartz reactor surfaces do not catalytically participate. Within the measurement uncertainty of the experimental apparatus, a data set for validation of homogeneous mechanisms of mercury oxidation has been provided.