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Effect of halogens on mercury conversion in SCR catalysts
FUE L PR O CE SS I N G TE CH N O LO G Y 89 ( 20 0 8 ) 1 1 5 3–1 1 5 9
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Effect of halogens on mercury conversion in SCR catalysts Sandhya Eswaran⁎, Harvey G. Stenger Department of Chemical Engineering, Lehigh University, 111 Research Drive, Bethlehem, PA-18015, USA
AR TIC LE I N FO
ABS TR ACT
Article history:
The effect of halogen acids HCl, HBr and HI on mercury conversion was studied in a
Received 7 May 2007
laboratory-scale SCR reactor using simulated flue gases, and is presented here. Two types of
Received in revised form 23 April 2008
commercially available SCR catalysts, Honeycomb and Plate type catalysts, were used in
Accepted 14 May 2008
these studies. HBr and HI both had shown much stronger effects on mercury conversion than HCl. Both HBr and HI oxidized more than 85% of the gas phase mercury at a low
Keywords:
concentration of 2 ppm. The age of the catalyst and the type of catalyst also have an effect
Mercury
on mercury conversion. A larger extent of mercury oxidation was observed in the presence
Oxidation
of a Honeycomb catalyst than with the Plate catalyst.
Hydrogen chloride
© 2008 Elsevier B.V. All rights reserved.
Hydrogen bromide Hydrogen iodide SCR Catalyst
1.
Introduction
Coals contain trace amounts of mercury, with concentrations generally in the range of 0.02 to 1.5 ppm, perhaps typically 0.2 ppm by weight. The mercury concentration varies with coal rank, with lower rank coals such as sub-bituminous and lignite having higher concentrations of mercury. It is also observed that power plants burning lower rank coals emit greater proportions of their mercury as elemental (Hg0), rather than oxidized (Hg2+) compared to those burning bituminous coals. This is believed to be caused by the low concentrations of chlorine and high calcium contents of low rank coals compared to bituminous coals [1]. The range of mercury concentrations in coal can result in uncontrolled emissions of mercury in flue gas from 3–20 μg Hg/Nm3 flue gas, and typically in the 5–10 μg/Nm3 range [2]. Selective catalytic reduction (SCR) is used widely to reduce NOx emissions by more than 90%, through the reaction of NO with NH3. Field and pilot tests performed at power plants in Germany and the USA have indicated that the vanadia–titania catalyst used in SCRs might also catalyze the oxidation of mercury [3,4]. Oxidized mercury compounds are generally
reactive and water-soluble, therefore more effectively captured in air pollution control devices in power plants. Mercury measurements made in four full-scale SCR units were reported in EPRI technical reports [3,4]. All of the units showed increases in mercury oxidation caused by the SCR systems, but the extent of oxidation varied from 10–70%, with some correlation with chlorine content, and little correlation with operating conditions. These measurements and other data from EPAs Information Collection Request (ICR) suggest that SCR systems provide co-benefits for mercury oxidation and removal along with NOx emission control; however the predictability of the oxidation extent is unreliable. Laboratory and pilot-scale tests have been performed by several researchers using chlorine as the oxidizing species. Senior [5] has suggested two mechanisms for the interaction of mercury with chlorine on an SCR catalyst, the first whereby Hg0 and HCl bind to adjacent vanadium sites and a reaction takes place between the two bound species. By the other mechanism, Hg0 and HCl compete to bind on a vanadium site, and the bound HCl reacts with gaseous Hg0. Modeling studies by Senior [6] predict that Hg conversion is generally higher across Plate catalysts than across monolith
⁎ Corresponding author. Tel.: +1 908 333 2048. E-mail address: [email protected] (S. Eswaran). 0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2008.05.007
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catalysts for a given space velocity, temperature and HCl content in the flue gas. Plate catalysts were predicted to cause higher levels of mercury oxidation at both high (50 ppm) and at low levels of HCl concentrations (1.5 ppm), while monolith catalysts were predicted to show mercury conversion equivalent to Plate catalysts at low HCl concentrations. Experimental studies by Gale et al. [7] show that mercury oxidation increases with increase in HCl concentration, in Plate, Honeycomb and hybrid SCR catalysts. At 371 °C, mercury oxidation across the Honeycomb catalyst and the Plate catalyst were almost similar over the entire range of HCl tested (0 to 100 ppm), when NH3 was present in the flue gas. Lee et al. [8] used simulated gases with different sulfur and chlorine contents, in a bench-scale reactor. The base flue gas composition was 3.5% O2, 15% CO2, 5.3% H2O, 350 ppm NOx, 315 ppm NH3, 190 μg/m3 Hg and balance N2. Concentration of HCl was varied from 0 to 200 ppm and SO2 was varied from 280 to 2890 ppm to simulate combustion of different coals. Even at a low HCl concentration of 8 ppm, about 97% of Hg was oxidized. When no chlorine was present, no oxidation was observed. The high mercury concentrations used in Lee's work may make it difficult to compare to others where mercury concentrations are much lower and closer to coal combustion flue gas (10 to 20 μg/m3). Lee et al. [9] performed additional pilot-scale tests involving the combustion of three different bituminous Illinois coals and one sub-bituminous Powder River Basin (PRB) coal in a furnace. This generated different flue gases with varying chlorine and sulfur contents. The HCl concentrations that result from these coals are either very high—greater than 90 ppm – or very low – less than 8 ppm.
However, their results did confirm that chlorine strongly influences mercury oxidation across SCR catalysts. Lee et al. [10] presented the effect of HCl on Hg oxidation in an entrained-flow reactor containing a Honeycomb catalyst. The flue gas mixture included 7.1% O2, 3.5% CO2, 6.8% H2O, 5 ppm CO, 200 ppm NOx, 180 ppm NH3, 500 ppm SO2 and no fly ash. The inlet Hg concentration was in the range of 20–25 μg/m3 and the flue gas temperature was either at 350 or 400 °C. At zero ppm HCl, no Hg was oxidized. As the HCl concentration increased to 20 ppm, 86–91% Hg oxidation was observed. Mercury retention on the catalyst reduced from 45–50% at 0 ppm HCl to zero at 20 ppm HCl. With 20 ppm HCl in the flue gas, all the Hg is released either as Hg0 or Hg2+. The authors believe that in the absence of HCl sites may be present on the SCR catalyst surface which captures Hg0. In our previous work [11] we reported that 0 to 35 ppm HCl caused 20 to 67% of the inlet mercury to be oxidized in an SCR Honeycomb catalyst, operating at typical SCR conditions. While these prior studies have investigated the importance of chlorine in mercury oxidation in SCR systems, there are no studies that discuss or present results on other halogen acids. A few measurements of halogen content of coal have been reported [12,13], and although the concentration of fluorine, iodine and bromine are small, they are still present at many times the concentration of mercury. Thus if they are effective oxidants, they are present in great stoichiometric amounts. Interest in the use of bromine for mercury removal has been raised by the work of Sorbent Technologies, where mercury removal performance is enhanced, compared to activated carbon injection, by using brominated activated carbon. In their work mercury capture in the range of 70–98% was achieved
Fig. 1 – Experimental apparatus. Base flue gas composition: 3% O2, 12% CO2, 8% H2O, 76.8% N2, 409 ppm NO, 360 ppm NH3 and 12 μg/m3 Hg. 371–390 °C. Gas flow = 7 slpm.
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using brominated powdered activated carbon (B-PAC) in slipstream and full-scale coal-fired power plants [14,15]. Nelson et al. [14] report that the injection of B-PAC in front of a boiler's existing particulate control equipment is a cost-effective method for removing mercury, with B-PAC costing $ 0.5/lb. Although bromine concentrations in coal are 50 to 100 times less than chlorine [12], it is possible that injecting bromine as HBr upstream of the SCR could improve mercury oxidation.
2.
Experimental procedure
The experimental apparatus used is described in detail in Eswaran and Stenger [11]. The flow diagram is shown in Fig. 1. Air, N2, CO2, NO in N2, and SO2 gas stream flow rates are individually controlled and fed through the steam pre-heater (SPH). Gases from the mercury permeation system, N2 and Hg, flow through the gas preheater (GPH). Water and dissolved acids are injected near the top of the SPH through 1/16 in. tubing using a Harvard Apparatus syringe pump. The syringe volume of 100 ml, gives enough liquid to run the system for approximately 4 h before the syringes need to be refilled. Refilling typically takes 5 to 10 min, which causes some short disturbances in the system, mostly an increase in the temperature of the gas stream leaving the SPH. The effects of these disturbances typically subside after approximately 10 min. At the bottom of the reactor, where the gases from the GPH and the SPH were mixed, a 5% mixture of NH3 in N2 was added to give a stoichiometric ratio of NH3 to NO of 0.90. The temperature of this mixing location was monitored and maintained above 350 °C by adjusting the temperatures of the GPH and SPH. This temperature prevented the formation of ammonium bisulfate. Exiting gases were analyzed for NO and O2, using a NOVA combustion gas analyzer, and Hg0, and HgT (total mercury) using a PSA mercury analyzer. The PSA mercury analyzer is based on the wet-chemistry method. It comprises a conditioning unit, a stream selector box and the mercury analyzer. The conditioning unit consists of two impingers—one impinger contains a solution of 10% KCl and 10% NaOH in distilled water and traps Hg2+ while allowing Hg0 to pass through it. The other impinger contains a solution of 2% SnCl2 and 20% NaOH in distilled water. SnCl2 helps to convert any oxidized mercury back to Hg0 so that the total Hg in the sample stream (HgT) is measured as Hg0. Further, moisture and acid gases get trapped in the two impingers, in order to protect the PSA mercury analyzer. The extent of oxidation (Hg2+) is calculated as the difference between HgT and Hg0. The stream selector box consists of a set of valves that control the flow of a specific sample line (HgT or Hg0) into the PSA mercury analyzer. The mercury analyzer is capable of measuring only one sample at a time. It works on the principle of atomic fluorescence. Two commercial vanadia–titania SCR catalysts were tested in this work and will be referred to as Plate and Honeycomb. The reactor was a 3.175 cm (1.25 in.) stainless steel schedule 40 pipe that was 56 cm (22 in.) long. For the Honeycomb catalyst, two 15 cm long sections were loaded into the reactor pipe. Each section had twelve 0.71 × 0.71 cm cells, giving 1022 cm2 external surface area in contact with the flowing gas. The recommendation of the Plate catalyst manufacturer was to load the same external surface area of catalyst as the Honeycomb catalyst. To achieve this, twenty-four, 2.5 × 7.6 cm strips of the Plate catalyst were loaded in the reactor. The strips were held in six sets of four using a stainless steel mesh. This gave a length of 45.7 cm of the Plate catalyst. The total gas flow rate was held at 7 slpm for all tests in this work. For the Honeycomb catalyst this gives a space velocity of 3600 l/h. More importantly, this flow rate gives a surface space velocity of 411 and 450 std cm3 of gas per hour per cm2 surface area of catalyst for the Honeycomb and the Plate catalysts, respectively. No fly ash was present in the flue gas.
3.
Results
3.1.
Achieving steady state
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Both the Honeycomb and the Plate catalysts, when first tested, showed significant intrinsic oxidative capacity. For example, running the fresh Honeycomb catalyst with a gas mixture of just air, N2, CO2, NO, NH3, H2O and mercury yielded approximately 65% mercury oxidation, of which about 50% of the converted mercury was retained on the catalyst. It was uncertain what caused this high intrinsic activity and retention, but both slowly diminished as the catalyst was conditioned for several days. For the Honeycomb catalyst, the intrinsic oxidation extent reduced to 25% with no retention after being conditioned for 14 days with gas containing 15 ppm HCl and 10 ppm H2SO4. The Plate catalyst was similarly conditioned before taking reportable data. Following this conditioning, the catalyst operated at a steady state, as long as the gas conditions were held constant. Changes in the gas composition can quickly affect the retention or desorption of mercury from the catalyst. Fig. 2 shows results of an extended test for the Honeycomb catalyst to demonstrate steady state behavior. This test was performed for a period of 31 h using flue gas composed of 3% O2, 12% CO2, 8% H2O, 76.8% N2, 409 ppm NO, 360 ppm NH3, 15 ppm HCl, 10 ppm H2SO4 and about 12 μg/m3 of Hg at 371 °C. Hg0 and Hg total reached steady values of about 3.5 and 10.5 μg/m3, respectively. Fluctuations in the data were caused while re-filling water syringes, and slight drifts in the measurements were caused by changing ambient conditions, or small flow changes in the PSA CEM. Keeping in mind that the concentrations measured are less than 1 ppbv, the consistency of the data is very good. Even after the initial 14 day period of catalyst conditioning, the exiting concentration of HgT can quickly change, even if the entering HgT is being held constant. This is caused by the sensitivity of the catalyst to retain or desorb mercury when the gas composition changes. For example, the catalyst was held overnight at constant mercury concentration but at lower
Fig. 2 – Steady state exit mercury concentrations from Honeycomb catalyst. Gas composition: 3% O2, 12% CO2, 8% H2O, 76.8% N2, 409 ppm NO, 360 ppm NH3, 15 ppm HCl, 10 ppm H2SO4 and 12 μg/m3 of Hg at 371 °C.
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water and acid concentrations, which was done to extend the time on-stream of the syringe pump charge. The next morning the flow of water was increased to its base value, thus increasing the acid concentrations in the gas stream. This change resulted in a large desorption peak of mercury from the catalyst. This desorption is the removal of the mercury retained overnight when the acid concentrations were lower. This desorption typically takes 1 to 3 h to subside and for the existing HgT concentration to return to its inlet value.
3.2.
Effect of HCl
Previously we reported that 70% Hg oxidation occurs with 35 ppm HCl in an SCR system containing a Honeycomb vanadia–titania catalyst [11]. Because reviewers of that work questioned the results due to our inability to prove steady state operation, the effects of 5, 15 and 35 ppm HCl on Hg oxidation was repeated here using a new Honeycomb catalyst (Honeycomb). Fig. 3 shows the mercury versus time plot for the tests of 5 and 15 ppm of HCl added to the flue gas mixture containing 77% N2, 3% O2, 12% CO2, 8% H2O, 400 ppm NO, 360 ppm NH3 and approximately 20 μg/m3 Hg at 390 °C. In Fig. 3 at point ‘a’ HCl solution was injected to give 5 ppm HCl in the flue gas. Prior to point ‘a’, the catalyst was exposed to 5 ppm of HCl, but at a lower concentration of water (3%) for 3 days. The initial transient period from 1000 to 1100 h, is caused by the change to a higher water concentration (8%). In Fig. 3, after 2.5 h of 5 ppm HCl injection Hg0 and HgT concentrations reached steady values of 5 and 19.5 μg/m3 respectively. At point ‘b’, the syringe composition was changed to give a gas mixture of 15 ppm HCl. At steady state (1600 h), Hg0 concentration is at 2.6 μg/m3 and HgT remained at about 19.5 μg/ m3 causing the extent of oxidation to increase to 87%, from the 74% oxidation with 5 ppm HCl. In a separate test (not shown here), 35 ppm of HCl was injected into the flue gas after conditioning the catalyst appropriately, and 86% of the Hg was oxidized. Fig. 4 shows a summary of mercury oxidation results at different levels of HCl injection for the two Honeycomb catalysts tested, the new catalyst described above and the previously tested catalyst [11] labeled as ‘old Honeycomb catalyst’. The new Honeycomb catalyst was exposed to flue
Fig. 4 – Summary of HCl effect on Hg oxidation in three catalyst systems.
gases (on-stream) for 1,030 h before the HCl tests were started and the old Honeycomb catalyst had been on-stream for 3,300 h. These two tests (old and new) were with catalysts with identical preparations, sizes and conditions. Because the only difference in these two data sets is time on-stream, these results may suggest that the ability of the catalyst to oxidize mercury decreases with catalyst age. However, the history-ofconditions for the catalysts were not the same, so that another possible cause could be the types of conditions the catalyst had seen during its time on-stream. Fig. 4 also includes the extent of Hg oxidation by the presence of 5, 15 and 25 ppm of HCl for the Plate catalyst. The Plate catalyst was on-stream for approximately 1,450 h before the HCl experiments. With increasing HCl concentration, the extent of mercury oxidation reaches approximately 65% at 25 ppm HCl. Because the catalyst is of a different composition and from a different manufacturer, it is difficult to speculate why it's activity is less than a similar age Honeycomb catalyst. For all tests the NO reduction was between 80 and 90% suggesting that the catalysts were all run at appropriate space velocities to achieve their primary purpose of reducing NO. A feature of all three sets of data in Fig. 4 is that the oxidation extent of mercury reaches a constant value as the concentration of HCl is increased. If the reaction of mercury with HCl is greater than zero order in HCl, the conversion of mercury should reach 100% as the concentration of the acid increases. The most probable cause of the conversion not reaching 100% is a mass transfer limitation of mercury to the catalyst surface. Since the configuration of the Plate and Honeycomb catalysts inside the reactor are different, the differences in mass transfer rates may be the cause of the activity differences. We will address this possibility in the Discussion section of this paper.
3.3.
Fig. 3 – Addition of 5 and 15 ppm HCl to base flue gas for Honeycomb catalyst. Feed concentration of HgT =19.5 ng/l.
Effect of hydrogen iodide (HI)
Fig. 5 shows the results of injecting HI dissolved in water with the inlet gas. The gas phase concentration of HI was set at 2 ppm, by injecting an appropriate solution of HI in water into the SPH. The other gas phase components were set at their base conditions. In the absence of HI, the inlet HgT and Hg0
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Fig. 5 – Injection of 2 ppm HI to base flue gas. Gas temperature = 390 °C.
values were measured to be 12 and 10 μg/m3 respectively (sections ‘a’ to ‘b’ in Fig. 5). At point ‘b’ the syringe pump was changed from water to the HI−water solution and the exiting concentrations of HgT and Hg0 were measured. Immediately, the exiting HgT rose to 120 μg/m3 and Hg0 dropped to 4.0 μg/m3. In 6 h, the HgT concentration gradually decreased and reached 30 μg/m3 while Hg0 level reduced to 1.8 μg/m3. At this point we believed the elemental mercury had reached steady state, but the total mercury was going to continue to decline for a considerable time. These results indicate that HI reacts rapidly with adsorbed (retained) mercury, releasing it from the catalyst surface as oxidized mercury, most likely HgI2. The amount of desorption, and the height of the desorption peak are dependent on the catalyst history. Since this catalyst had been on-stream for almost 2 months, the prior history may have left a significant amount of mercury retained on the catalyst. The results also indicate that the mercury fed to the catalysts is rapidly oxidized. If we assume the inlet concentration of elemental mercury is 12 μg/m3, the extent of oxidation of the fed mercury is 85% (exit Hg0 was 1.8 μg/m3). Following the period shown in Fig. 5, the catalyst was run under base conditions (without acids present) for 5 days. At the end of 5 days, the exiting mercury concentrations reached steady state values of 15.5 μg/m3 (HgT) and 12.0 μg/m3 (Hg0). The 2 ppm HI test was then repeated. Fig. 6 shows the results of this repeat test. Similar to the first HI test, a large desorption peak occurred, although not as large as the first test (Fig. 5). This test was run for approximately 30 h. At the end of 30 h, the exiting concentrations of HgT and Hg0 were approximately 22 and 2 μg/m3. These results confirm that HI has a strong ability to oxidize mercury, has the ability to desorb mercury that has been retained, and has the effectiveness to remove retained mercury is persistent.
3.4.
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Fig. 6 – Injection of 2 ppm HI to base flue gas-repeat test.
and sample lines) was cleaned with a dilute solution of sodium hydroxide and then repeatedly flushed with water. This cleaning was done to remove any residual effects of the HI tests. The same catalyst was then reloaded and the HBr test was conducted. Fig. 7 shows the effect of adding 2 ppm HBr. Similar to the HI tests, the HBr was dissolved in the water feed and injected and vaporized in the SPH. From points ‘a’ to ‘b’ in Fig. 7, Hg concentrations from the exit of the reactor were measured with no HBr injection. Both Hg0 and HgT values are approximately 12 μg/m3. These values indicate that the reactor cleaning was effective in removing any residual HI from the system. At point ‘b’ the HBr–H2O solution was injected to give 2 ppm HBr in the gas stream. Similar to previous tests with HCl and HI, a peak of HgT is observed. The height of the peak was approximately 50 μg/m3. After 4 h, Hg0 and HgT reached steady values of 1.3 and 6.5 μg/m3 respectively. HBr injection was then stopped, and the feed was changed to the base condition gas for 12 h (overnight). The following day, injection of 2 ppm HBr was continued and mercury concentrations remained at approximately the same level as the previous day; Hg0 between 1.5 and 2.0 μg/m3
Effect of hydrogen bromide (HBr)
Prior to testing the effects of HBr, the catalyst was removed from the reactor and the reactor system (feed lines, reactor,
Fig. 7 – Injection of 2 ppm HBr to base flue gas. Gas temperature= 390 °C.
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and HgT concentration at about 6 ± 2 μg/m3. The data from this second day are shown in Fig. 8. Again the total mercury was below the inlet value, indicating that HBr was oxidizing and retaining mercury on the catalyst.
4.
Table 1 – Physical properties of mercury halides Compound HgCl2 HgBr2 HgI2
Molecular weight 271.5 360.4 454.4
Melting point (C)
Boiling point (C)
276 237 259
302 322 354
Discussion
The conversion versus HCl concentration curves shown in Fig. 4 offer several observations. Comparing the two Honeycomb catalysts, the effect of age is apparent, with the older catalyst (3,300 h) being less active than the younger catalyst (1,030 h). A second observation is that the extent of oxidation reaches a constant value, significantly below 100% conversion. This may suggest that mass transfer resistances are hindering the rate of reaction, since if mass transfer were not limiting, the conversion would be expected to reach 100% as HCl concentration increases. Modeling studies by Tronconi and Beretta [16] for the SCR DeNOx reaction show that for similar catalyst geometries as used in this work, both internal and external mass transfer hinder the NOx/NH3 reaction. Thus a similar analysis for the Hg/HCl reaction might also show the rate limitations of internal and external mass transfer. A third observation is that the Plate catalyst is less active than the Honeycomb catalyst. Again, this may be a result of mass transfer differences between the Plate and Honeycomb configuration. Tronconi and Beretta's work also showed that for similar area velocities, the Honeycomb catalyst is more effective due to better mass transfer rates than the Plate catalyst. The comparison of the effects of HBr, HI and HCl show that all three halogen acids can oxidize mercury, but their behaviors are slightly different. Unfortunately, our conclusions are clouded by the inability to operate at steady state for long periods of time. However, the inability to reach steady state easily and reproducibly is an important observation, and may help to explain mass balance inconsistencies in full-scale SCR measurements. Regardless of the extent to which steady state is achieved, the results show that HCl is the least effective oxidant, while HBr has the ability to oxidize and retain mercury, and HI oxidizes and releases mercury.
In an attempt to determine a reason for these differences, the boiling and melting temperatures for the mercury halides of these three acids are listed in Table 1. It is interesting to note that the melting point for HgBr2 is less than that for HgI2 and HgCl2. However, HgBr2 is less stable and less soluble in water than HgCl2. Therefore, HgBr2 can easily get adsorbed and form a stable complex on the SCR catalyst surface, which may suggest why HBr shows strong retention of oxidized mercury. While there is limited information on the bromine [17] and iodine [18] content of coal, the concentrations used in this work are low enough (2 ppm) that intentional addition of iodine or bromine to the coal or flue gas would not be difficult or expensive and may be a suitable way to enhance the oxidation of mercury in coal-fired power plants where an SCR is installed.
5.
Conclusion
The effect of HCl on Hg oxidation was studied on both a Honeycomb and a Plate catalyst. The effect of HCl on Hg0 oxidation is affected by the age of the catalyst and the concentration of HCl. Mass transfer resistances appear to be important in both the Honeycomb and Plate catalysts with the Plate catalyst having greater mass transfer limitations than the Honeycomb catalyst. Both HBr and HI are shown to be stronger mercury oxidants than HCl. At concentrations of 2 ppm, both HBr and HI oxidize more than 85% of the inlet elemental mercury. HBr causes a large amount of the oxidized mercury to be retained on the catalyst while HI desorbs previously retained mercury from the catalyst surface. In all experiments the ability to reach steady state for long periods of time were challenging, but provide insight into the potential mass balance errors for full-scale testing.
Acknowledgments The authors thank Mr. Paul Chu for his advice during this project and the Electric Power Research Institute (EPRI) for their financial support.
REFERENCES
Fig. 8 – Injection of 2 ppm HBr to base flue gas. Second day of testing.
[1] J.H. Pavlish, M.J. Holmes, S.A. Benson, C.R. Crocker, K.C. Galbreath, Application of sorbents for mercury control for utilities burning lignite coal, Fuel Process. Technol. 85 (6–7) (2004) 563–576. [2] K.C. Galbreath, C.J. Zygarlicke, Mercury transformations in coal combustion, Fuel Process. Technol. 65–66 (2002) 289–310.
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[3] EPRI Technical Report No. 1005400, 2002. Power plant Evaluation of the Effect of Selective Catalytic Reduction in Mercury. [4] EPRI Technical Report No. 1004000, 2002. Impacts of NOx Controls on Mercury Controllability. [5] C. Senior, Oxidation of mercury across SCR catalysts in coalfired power plants burning low rank fuels, DOE Quarterly Progress Report, 2004. [6] C.L. Senior, Oxidation of mercury across selective reduction catalysts in coal-fired power plants, J. Air Waste Manage. Assoc. 56 (2006) 23–31. [7] T.K. Gale, G.A. Blankenship, S.W. Hinton, J.W. Cannon, B.W. Lani, Hg oxidation compared for three different commercial SCR catalysts, EPA-DOE-A&WMA MEGA Symposium, Baltimore, M.D., 2006. [8] C.W. Lee, R.K. Srivastava, S.B. Ghorishi, T.W. Hastings, F.M. Stevens, Investigation of selective catalytic reduction impact on mercury speciation under simulated NOx emission control conditions, J. Air Waste Manage. Assoc. 54 (2004) 1560–1566. [9] C.W. Lee, R.K. Srivastava, B.S. Bhorishi, J. Karwowski, T.W. Hastings, J.C. Hirschi, Pilot-scale study of the effect of selective catalytic reduction catalyst on mercury speciation in Illinois and Powder River basin coal combustion flue gases, J. Air Waste Manage. Assoc. 56 (2006) 643–649. [10] S.J. Lee, C.W. Lee, S.D. Serre, Y. Zhao, J. Karwowski, T.W. Hastings, Study of mercury oxidation by SCR catalyst in an entrained-flow reactor under simulated PRB conditions, 5th International Conference on Air Quality, Virginia, 2005.
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[11] S. Eswaran, H.G. Stenger, Understanding mercury conversion in SCR catalysts, En. Fuels. 19 (6) (2005) 2328–2334. [12] S.V. Vassilev, G.M. Eskenazy, C.G. Vassileva, Contents, modes of occurrence and behavior of chlorine and bromine in combustion wastes from coal-fired power stations, Fuel 79 (8) (2000) 923–937. [13] B. Wang, J.C. Jackson, C. Palmer, B. Zheng, R.B. Finkelman, Evaluation on determination of iodine in coal by energy dispersive X-ray fluorescence, Geochem. J. 39 (4) (2005) 391–394. [14] S. Nelson, R. Landreth, Q. Zhou, J. Miller, Accumulated powerplant mercury-removal experience with brominated PAC injection, Combined Power Plant Air Pollutant Control Mega Symposium, Washington, D.C., 2004. [15] M. McCoy, W. Rogers, R. Landreth, L. Brickett, Full-scale mercury sorbent injection testing at DTE Engery's St. Clair station, Combined Power Plant Air Pollutant Control Mega Symposium, Washington, D.C., 2004. [16] E. Tronconi, A. Beretta, The role of inter- and intra-phase mass transfer in the SCR-DeNOx reaction over catalysts of different shapes, Catal. Today 52 (1999) 249–258. [17] M.D. Durham, S. Sjostrom, A. O' Palko, R. Chang, Mercury control for PRB and PRB/Bituminous blends, POWER-GEN Intern. Conf., Las Vegas, NV, 2005. [18] http://www.det.csiro.au/science/e_e/Clean%20Air/pdf/ IODINE.pdf.
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Effect of halogens on mercury conversion in SCR catalysts Sandhya Eswaran⁎, Harvey G. Stenger Department of Chemical Engineering, Lehigh University, 111 Research Drive, Bethlehem, PA-18015, USA
AR TIC LE I N FO
ABS TR ACT
Article history:
The effect of halogen acids HCl, HBr and HI on mercury conversion was studied in a
Received 7 May 2007
laboratory-scale SCR reactor using simulated flue gases, and is presented here. Two types of
Received in revised form 23 April 2008
commercially available SCR catalysts, Honeycomb and Plate type catalysts, were used in
Accepted 14 May 2008
these studies. HBr and HI both had shown much stronger effects on mercury conversion than HCl. Both HBr and HI oxidized more than 85% of the gas phase mercury at a low
Keywords:
concentration of 2 ppm. The age of the catalyst and the type of catalyst also have an effect
Mercury
on mercury conversion. A larger extent of mercury oxidation was observed in the presence
Oxidation
of a Honeycomb catalyst than with the Plate catalyst.
Hydrogen chloride
© 2008 Elsevier B.V. All rights reserved.
Hydrogen bromide Hydrogen iodide SCR Catalyst
1.
Introduction
Coals contain trace amounts of mercury, with concentrations generally in the range of 0.02 to 1.5 ppm, perhaps typically 0.2 ppm by weight. The mercury concentration varies with coal rank, with lower rank coals such as sub-bituminous and lignite having higher concentrations of mercury. It is also observed that power plants burning lower rank coals emit greater proportions of their mercury as elemental (Hg0), rather than oxidized (Hg2+) compared to those burning bituminous coals. This is believed to be caused by the low concentrations of chlorine and high calcium contents of low rank coals compared to bituminous coals [1]. The range of mercury concentrations in coal can result in uncontrolled emissions of mercury in flue gas from 3–20 μg Hg/Nm3 flue gas, and typically in the 5–10 μg/Nm3 range [2]. Selective catalytic reduction (SCR) is used widely to reduce NOx emissions by more than 90%, through the reaction of NO with NH3. Field and pilot tests performed at power plants in Germany and the USA have indicated that the vanadia–titania catalyst used in SCRs might also catalyze the oxidation of mercury [3,4]. Oxidized mercury compounds are generally
reactive and water-soluble, therefore more effectively captured in air pollution control devices in power plants. Mercury measurements made in four full-scale SCR units were reported in EPRI technical reports [3,4]. All of the units showed increases in mercury oxidation caused by the SCR systems, but the extent of oxidation varied from 10–70%, with some correlation with chlorine content, and little correlation with operating conditions. These measurements and other data from EPAs Information Collection Request (ICR) suggest that SCR systems provide co-benefits for mercury oxidation and removal along with NOx emission control; however the predictability of the oxidation extent is unreliable. Laboratory and pilot-scale tests have been performed by several researchers using chlorine as the oxidizing species. Senior [5] has suggested two mechanisms for the interaction of mercury with chlorine on an SCR catalyst, the first whereby Hg0 and HCl bind to adjacent vanadium sites and a reaction takes place between the two bound species. By the other mechanism, Hg0 and HCl compete to bind on a vanadium site, and the bound HCl reacts with gaseous Hg0. Modeling studies by Senior [6] predict that Hg conversion is generally higher across Plate catalysts than across monolith
⁎ Corresponding author. Tel.: +1 908 333 2048. E-mail address: [email protected] (S. Eswaran). 0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2008.05.007
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catalysts for a given space velocity, temperature and HCl content in the flue gas. Plate catalysts were predicted to cause higher levels of mercury oxidation at both high (50 ppm) and at low levels of HCl concentrations (1.5 ppm), while monolith catalysts were predicted to show mercury conversion equivalent to Plate catalysts at low HCl concentrations. Experimental studies by Gale et al. [7] show that mercury oxidation increases with increase in HCl concentration, in Plate, Honeycomb and hybrid SCR catalysts. At 371 °C, mercury oxidation across the Honeycomb catalyst and the Plate catalyst were almost similar over the entire range of HCl tested (0 to 100 ppm), when NH3 was present in the flue gas. Lee et al. [8] used simulated gases with different sulfur and chlorine contents, in a bench-scale reactor. The base flue gas composition was 3.5% O2, 15% CO2, 5.3% H2O, 350 ppm NOx, 315 ppm NH3, 190 μg/m3 Hg and balance N2. Concentration of HCl was varied from 0 to 200 ppm and SO2 was varied from 280 to 2890 ppm to simulate combustion of different coals. Even at a low HCl concentration of 8 ppm, about 97% of Hg was oxidized. When no chlorine was present, no oxidation was observed. The high mercury concentrations used in Lee's work may make it difficult to compare to others where mercury concentrations are much lower and closer to coal combustion flue gas (10 to 20 μg/m3). Lee et al. [9] performed additional pilot-scale tests involving the combustion of three different bituminous Illinois coals and one sub-bituminous Powder River Basin (PRB) coal in a furnace. This generated different flue gases with varying chlorine and sulfur contents. The HCl concentrations that result from these coals are either very high—greater than 90 ppm – or very low – less than 8 ppm.
However, their results did confirm that chlorine strongly influences mercury oxidation across SCR catalysts. Lee et al. [10] presented the effect of HCl on Hg oxidation in an entrained-flow reactor containing a Honeycomb catalyst. The flue gas mixture included 7.1% O2, 3.5% CO2, 6.8% H2O, 5 ppm CO, 200 ppm NOx, 180 ppm NH3, 500 ppm SO2 and no fly ash. The inlet Hg concentration was in the range of 20–25 μg/m3 and the flue gas temperature was either at 350 or 400 °C. At zero ppm HCl, no Hg was oxidized. As the HCl concentration increased to 20 ppm, 86–91% Hg oxidation was observed. Mercury retention on the catalyst reduced from 45–50% at 0 ppm HCl to zero at 20 ppm HCl. With 20 ppm HCl in the flue gas, all the Hg is released either as Hg0 or Hg2+. The authors believe that in the absence of HCl sites may be present on the SCR catalyst surface which captures Hg0. In our previous work [11] we reported that 0 to 35 ppm HCl caused 20 to 67% of the inlet mercury to be oxidized in an SCR Honeycomb catalyst, operating at typical SCR conditions. While these prior studies have investigated the importance of chlorine in mercury oxidation in SCR systems, there are no studies that discuss or present results on other halogen acids. A few measurements of halogen content of coal have been reported [12,13], and although the concentration of fluorine, iodine and bromine are small, they are still present at many times the concentration of mercury. Thus if they are effective oxidants, they are present in great stoichiometric amounts. Interest in the use of bromine for mercury removal has been raised by the work of Sorbent Technologies, where mercury removal performance is enhanced, compared to activated carbon injection, by using brominated activated carbon. In their work mercury capture in the range of 70–98% was achieved
Fig. 1 – Experimental apparatus. Base flue gas composition: 3% O2, 12% CO2, 8% H2O, 76.8% N2, 409 ppm NO, 360 ppm NH3 and 12 μg/m3 Hg. 371–390 °C. Gas flow = 7 slpm.
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using brominated powdered activated carbon (B-PAC) in slipstream and full-scale coal-fired power plants [14,15]. Nelson et al. [14] report that the injection of B-PAC in front of a boiler's existing particulate control equipment is a cost-effective method for removing mercury, with B-PAC costing $ 0.5/lb. Although bromine concentrations in coal are 50 to 100 times less than chlorine [12], it is possible that injecting bromine as HBr upstream of the SCR could improve mercury oxidation.
2.
Experimental procedure
The experimental apparatus used is described in detail in Eswaran and Stenger [11]. The flow diagram is shown in Fig. 1. Air, N2, CO2, NO in N2, and SO2 gas stream flow rates are individually controlled and fed through the steam pre-heater (SPH). Gases from the mercury permeation system, N2 and Hg, flow through the gas preheater (GPH). Water and dissolved acids are injected near the top of the SPH through 1/16 in. tubing using a Harvard Apparatus syringe pump. The syringe volume of 100 ml, gives enough liquid to run the system for approximately 4 h before the syringes need to be refilled. Refilling typically takes 5 to 10 min, which causes some short disturbances in the system, mostly an increase in the temperature of the gas stream leaving the SPH. The effects of these disturbances typically subside after approximately 10 min. At the bottom of the reactor, where the gases from the GPH and the SPH were mixed, a 5% mixture of NH3 in N2 was added to give a stoichiometric ratio of NH3 to NO of 0.90. The temperature of this mixing location was monitored and maintained above 350 °C by adjusting the temperatures of the GPH and SPH. This temperature prevented the formation of ammonium bisulfate. Exiting gases were analyzed for NO and O2, using a NOVA combustion gas analyzer, and Hg0, and HgT (total mercury) using a PSA mercury analyzer. The PSA mercury analyzer is based on the wet-chemistry method. It comprises a conditioning unit, a stream selector box and the mercury analyzer. The conditioning unit consists of two impingers—one impinger contains a solution of 10% KCl and 10% NaOH in distilled water and traps Hg2+ while allowing Hg0 to pass through it. The other impinger contains a solution of 2% SnCl2 and 20% NaOH in distilled water. SnCl2 helps to convert any oxidized mercury back to Hg0 so that the total Hg in the sample stream (HgT) is measured as Hg0. Further, moisture and acid gases get trapped in the two impingers, in order to protect the PSA mercury analyzer. The extent of oxidation (Hg2+) is calculated as the difference between HgT and Hg0. The stream selector box consists of a set of valves that control the flow of a specific sample line (HgT or Hg0) into the PSA mercury analyzer. The mercury analyzer is capable of measuring only one sample at a time. It works on the principle of atomic fluorescence. Two commercial vanadia–titania SCR catalysts were tested in this work and will be referred to as Plate and Honeycomb. The reactor was a 3.175 cm (1.25 in.) stainless steel schedule 40 pipe that was 56 cm (22 in.) long. For the Honeycomb catalyst, two 15 cm long sections were loaded into the reactor pipe. Each section had twelve 0.71 × 0.71 cm cells, giving 1022 cm2 external surface area in contact with the flowing gas. The recommendation of the Plate catalyst manufacturer was to load the same external surface area of catalyst as the Honeycomb catalyst. To achieve this, twenty-four, 2.5 × 7.6 cm strips of the Plate catalyst were loaded in the reactor. The strips were held in six sets of four using a stainless steel mesh. This gave a length of 45.7 cm of the Plate catalyst. The total gas flow rate was held at 7 slpm for all tests in this work. For the Honeycomb catalyst this gives a space velocity of 3600 l/h. More importantly, this flow rate gives a surface space velocity of 411 and 450 std cm3 of gas per hour per cm2 surface area of catalyst for the Honeycomb and the Plate catalysts, respectively. No fly ash was present in the flue gas.
3.
Results
3.1.
Achieving steady state
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Both the Honeycomb and the Plate catalysts, when first tested, showed significant intrinsic oxidative capacity. For example, running the fresh Honeycomb catalyst with a gas mixture of just air, N2, CO2, NO, NH3, H2O and mercury yielded approximately 65% mercury oxidation, of which about 50% of the converted mercury was retained on the catalyst. It was uncertain what caused this high intrinsic activity and retention, but both slowly diminished as the catalyst was conditioned for several days. For the Honeycomb catalyst, the intrinsic oxidation extent reduced to 25% with no retention after being conditioned for 14 days with gas containing 15 ppm HCl and 10 ppm H2SO4. The Plate catalyst was similarly conditioned before taking reportable data. Following this conditioning, the catalyst operated at a steady state, as long as the gas conditions were held constant. Changes in the gas composition can quickly affect the retention or desorption of mercury from the catalyst. Fig. 2 shows results of an extended test for the Honeycomb catalyst to demonstrate steady state behavior. This test was performed for a period of 31 h using flue gas composed of 3% O2, 12% CO2, 8% H2O, 76.8% N2, 409 ppm NO, 360 ppm NH3, 15 ppm HCl, 10 ppm H2SO4 and about 12 μg/m3 of Hg at 371 °C. Hg0 and Hg total reached steady values of about 3.5 and 10.5 μg/m3, respectively. Fluctuations in the data were caused while re-filling water syringes, and slight drifts in the measurements were caused by changing ambient conditions, or small flow changes in the PSA CEM. Keeping in mind that the concentrations measured are less than 1 ppbv, the consistency of the data is very good. Even after the initial 14 day period of catalyst conditioning, the exiting concentration of HgT can quickly change, even if the entering HgT is being held constant. This is caused by the sensitivity of the catalyst to retain or desorb mercury when the gas composition changes. For example, the catalyst was held overnight at constant mercury concentration but at lower
Fig. 2 – Steady state exit mercury concentrations from Honeycomb catalyst. Gas composition: 3% O2, 12% CO2, 8% H2O, 76.8% N2, 409 ppm NO, 360 ppm NH3, 15 ppm HCl, 10 ppm H2SO4 and 12 μg/m3 of Hg at 371 °C.
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water and acid concentrations, which was done to extend the time on-stream of the syringe pump charge. The next morning the flow of water was increased to its base value, thus increasing the acid concentrations in the gas stream. This change resulted in a large desorption peak of mercury from the catalyst. This desorption is the removal of the mercury retained overnight when the acid concentrations were lower. This desorption typically takes 1 to 3 h to subside and for the existing HgT concentration to return to its inlet value.
3.2.
Effect of HCl
Previously we reported that 70% Hg oxidation occurs with 35 ppm HCl in an SCR system containing a Honeycomb vanadia–titania catalyst [11]. Because reviewers of that work questioned the results due to our inability to prove steady state operation, the effects of 5, 15 and 35 ppm HCl on Hg oxidation was repeated here using a new Honeycomb catalyst (Honeycomb). Fig. 3 shows the mercury versus time plot for the tests of 5 and 15 ppm of HCl added to the flue gas mixture containing 77% N2, 3% O2, 12% CO2, 8% H2O, 400 ppm NO, 360 ppm NH3 and approximately 20 μg/m3 Hg at 390 °C. In Fig. 3 at point ‘a’ HCl solution was injected to give 5 ppm HCl in the flue gas. Prior to point ‘a’, the catalyst was exposed to 5 ppm of HCl, but at a lower concentration of water (3%) for 3 days. The initial transient period from 1000 to 1100 h, is caused by the change to a higher water concentration (8%). In Fig. 3, after 2.5 h of 5 ppm HCl injection Hg0 and HgT concentrations reached steady values of 5 and 19.5 μg/m3 respectively. At point ‘b’, the syringe composition was changed to give a gas mixture of 15 ppm HCl. At steady state (1600 h), Hg0 concentration is at 2.6 μg/m3 and HgT remained at about 19.5 μg/ m3 causing the extent of oxidation to increase to 87%, from the 74% oxidation with 5 ppm HCl. In a separate test (not shown here), 35 ppm of HCl was injected into the flue gas after conditioning the catalyst appropriately, and 86% of the Hg was oxidized. Fig. 4 shows a summary of mercury oxidation results at different levels of HCl injection for the two Honeycomb catalysts tested, the new catalyst described above and the previously tested catalyst [11] labeled as ‘old Honeycomb catalyst’. The new Honeycomb catalyst was exposed to flue
Fig. 4 – Summary of HCl effect on Hg oxidation in three catalyst systems.
gases (on-stream) for 1,030 h before the HCl tests were started and the old Honeycomb catalyst had been on-stream for 3,300 h. These two tests (old and new) were with catalysts with identical preparations, sizes and conditions. Because the only difference in these two data sets is time on-stream, these results may suggest that the ability of the catalyst to oxidize mercury decreases with catalyst age. However, the history-ofconditions for the catalysts were not the same, so that another possible cause could be the types of conditions the catalyst had seen during its time on-stream. Fig. 4 also includes the extent of Hg oxidation by the presence of 5, 15 and 25 ppm of HCl for the Plate catalyst. The Plate catalyst was on-stream for approximately 1,450 h before the HCl experiments. With increasing HCl concentration, the extent of mercury oxidation reaches approximately 65% at 25 ppm HCl. Because the catalyst is of a different composition and from a different manufacturer, it is difficult to speculate why it's activity is less than a similar age Honeycomb catalyst. For all tests the NO reduction was between 80 and 90% suggesting that the catalysts were all run at appropriate space velocities to achieve their primary purpose of reducing NO. A feature of all three sets of data in Fig. 4 is that the oxidation extent of mercury reaches a constant value as the concentration of HCl is increased. If the reaction of mercury with HCl is greater than zero order in HCl, the conversion of mercury should reach 100% as the concentration of the acid increases. The most probable cause of the conversion not reaching 100% is a mass transfer limitation of mercury to the catalyst surface. Since the configuration of the Plate and Honeycomb catalysts inside the reactor are different, the differences in mass transfer rates may be the cause of the activity differences. We will address this possibility in the Discussion section of this paper.
3.3.
Fig. 3 – Addition of 5 and 15 ppm HCl to base flue gas for Honeycomb catalyst. Feed concentration of HgT =19.5 ng/l.
Effect of hydrogen iodide (HI)
Fig. 5 shows the results of injecting HI dissolved in water with the inlet gas. The gas phase concentration of HI was set at 2 ppm, by injecting an appropriate solution of HI in water into the SPH. The other gas phase components were set at their base conditions. In the absence of HI, the inlet HgT and Hg0
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Fig. 5 – Injection of 2 ppm HI to base flue gas. Gas temperature = 390 °C.
values were measured to be 12 and 10 μg/m3 respectively (sections ‘a’ to ‘b’ in Fig. 5). At point ‘b’ the syringe pump was changed from water to the HI−water solution and the exiting concentrations of HgT and Hg0 were measured. Immediately, the exiting HgT rose to 120 μg/m3 and Hg0 dropped to 4.0 μg/m3. In 6 h, the HgT concentration gradually decreased and reached 30 μg/m3 while Hg0 level reduced to 1.8 μg/m3. At this point we believed the elemental mercury had reached steady state, but the total mercury was going to continue to decline for a considerable time. These results indicate that HI reacts rapidly with adsorbed (retained) mercury, releasing it from the catalyst surface as oxidized mercury, most likely HgI2. The amount of desorption, and the height of the desorption peak are dependent on the catalyst history. Since this catalyst had been on-stream for almost 2 months, the prior history may have left a significant amount of mercury retained on the catalyst. The results also indicate that the mercury fed to the catalysts is rapidly oxidized. If we assume the inlet concentration of elemental mercury is 12 μg/m3, the extent of oxidation of the fed mercury is 85% (exit Hg0 was 1.8 μg/m3). Following the period shown in Fig. 5, the catalyst was run under base conditions (without acids present) for 5 days. At the end of 5 days, the exiting mercury concentrations reached steady state values of 15.5 μg/m3 (HgT) and 12.0 μg/m3 (Hg0). The 2 ppm HI test was then repeated. Fig. 6 shows the results of this repeat test. Similar to the first HI test, a large desorption peak occurred, although not as large as the first test (Fig. 5). This test was run for approximately 30 h. At the end of 30 h, the exiting concentrations of HgT and Hg0 were approximately 22 and 2 μg/m3. These results confirm that HI has a strong ability to oxidize mercury, has the ability to desorb mercury that has been retained, and has the effectiveness to remove retained mercury is persistent.
3.4.
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Fig. 6 – Injection of 2 ppm HI to base flue gas-repeat test.
and sample lines) was cleaned with a dilute solution of sodium hydroxide and then repeatedly flushed with water. This cleaning was done to remove any residual effects of the HI tests. The same catalyst was then reloaded and the HBr test was conducted. Fig. 7 shows the effect of adding 2 ppm HBr. Similar to the HI tests, the HBr was dissolved in the water feed and injected and vaporized in the SPH. From points ‘a’ to ‘b’ in Fig. 7, Hg concentrations from the exit of the reactor were measured with no HBr injection. Both Hg0 and HgT values are approximately 12 μg/m3. These values indicate that the reactor cleaning was effective in removing any residual HI from the system. At point ‘b’ the HBr–H2O solution was injected to give 2 ppm HBr in the gas stream. Similar to previous tests with HCl and HI, a peak of HgT is observed. The height of the peak was approximately 50 μg/m3. After 4 h, Hg0 and HgT reached steady values of 1.3 and 6.5 μg/m3 respectively. HBr injection was then stopped, and the feed was changed to the base condition gas for 12 h (overnight). The following day, injection of 2 ppm HBr was continued and mercury concentrations remained at approximately the same level as the previous day; Hg0 between 1.5 and 2.0 μg/m3
Effect of hydrogen bromide (HBr)
Prior to testing the effects of HBr, the catalyst was removed from the reactor and the reactor system (feed lines, reactor,
Fig. 7 – Injection of 2 ppm HBr to base flue gas. Gas temperature= 390 °C.
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and HgT concentration at about 6 ± 2 μg/m3. The data from this second day are shown in Fig. 8. Again the total mercury was below the inlet value, indicating that HBr was oxidizing and retaining mercury on the catalyst.
4.
Table 1 – Physical properties of mercury halides Compound HgCl2 HgBr2 HgI2
Molecular weight 271.5 360.4 454.4
Melting point (C)
Boiling point (C)
276 237 259
302 322 354
Discussion
The conversion versus HCl concentration curves shown in Fig. 4 offer several observations. Comparing the two Honeycomb catalysts, the effect of age is apparent, with the older catalyst (3,300 h) being less active than the younger catalyst (1,030 h). A second observation is that the extent of oxidation reaches a constant value, significantly below 100% conversion. This may suggest that mass transfer resistances are hindering the rate of reaction, since if mass transfer were not limiting, the conversion would be expected to reach 100% as HCl concentration increases. Modeling studies by Tronconi and Beretta [16] for the SCR DeNOx reaction show that for similar catalyst geometries as used in this work, both internal and external mass transfer hinder the NOx/NH3 reaction. Thus a similar analysis for the Hg/HCl reaction might also show the rate limitations of internal and external mass transfer. A third observation is that the Plate catalyst is less active than the Honeycomb catalyst. Again, this may be a result of mass transfer differences between the Plate and Honeycomb configuration. Tronconi and Beretta's work also showed that for similar area velocities, the Honeycomb catalyst is more effective due to better mass transfer rates than the Plate catalyst. The comparison of the effects of HBr, HI and HCl show that all three halogen acids can oxidize mercury, but their behaviors are slightly different. Unfortunately, our conclusions are clouded by the inability to operate at steady state for long periods of time. However, the inability to reach steady state easily and reproducibly is an important observation, and may help to explain mass balance inconsistencies in full-scale SCR measurements. Regardless of the extent to which steady state is achieved, the results show that HCl is the least effective oxidant, while HBr has the ability to oxidize and retain mercury, and HI oxidizes and releases mercury.
In an attempt to determine a reason for these differences, the boiling and melting temperatures for the mercury halides of these three acids are listed in Table 1. It is interesting to note that the melting point for HgBr2 is less than that for HgI2 and HgCl2. However, HgBr2 is less stable and less soluble in water than HgCl2. Therefore, HgBr2 can easily get adsorbed and form a stable complex on the SCR catalyst surface, which may suggest why HBr shows strong retention of oxidized mercury. While there is limited information on the bromine [17] and iodine [18] content of coal, the concentrations used in this work are low enough (2 ppm) that intentional addition of iodine or bromine to the coal or flue gas would not be difficult or expensive and may be a suitable way to enhance the oxidation of mercury in coal-fired power plants where an SCR is installed.
5.
Conclusion
The effect of HCl on Hg oxidation was studied on both a Honeycomb and a Plate catalyst. The effect of HCl on Hg0 oxidation is affected by the age of the catalyst and the concentration of HCl. Mass transfer resistances appear to be important in both the Honeycomb and Plate catalysts with the Plate catalyst having greater mass transfer limitations than the Honeycomb catalyst. Both HBr and HI are shown to be stronger mercury oxidants than HCl. At concentrations of 2 ppm, both HBr and HI oxidize more than 85% of the inlet elemental mercury. HBr causes a large amount of the oxidized mercury to be retained on the catalyst while HI desorbs previously retained mercury from the catalyst surface. In all experiments the ability to reach steady state for long periods of time were challenging, but provide insight into the potential mass balance errors for full-scale testing.
Acknowledgments The authors thank Mr. Paul Chu for his advice during this project and the Electric Power Research Institute (EPRI) for their financial support.
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Fig. 8 – Injection of 2 ppm HBr to base flue gas. Second day of testing.
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