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Understanding Mercury Conversion in Selective Catalytic Reduction (SCR) Catalysts Sandhya Eswaran and Harvey G. Stenger* Department of Chemical Engineering, Lehigh University, 111 Research Drive, Bethlehem, Pennsylvania 18015 Received March 31, 2005. Revised Manuscript Received September 12, 2005
The effect of oxidizing agents on mercury oxidation and retention in a laboratory-scale selective catalytic reduction (SCR) system, using a commercial honeycomb vanadia-titania catalyst, is reported in this paper. Two oxidizing agents (HCl and H2SO4) are introduced into the feed gas, individually and in various combinations, at different temperatures and space velocities. The effect of these oxidizing agents on mercury oxidation, in the presence of individual flue gas constituents, is shown in detail. As the concentration of the individual oxidants increases in the simulated flue gas, the oxidation of mercury increases. At high concentrations of 35 ppm of HCl and 50 ppm of H2SO4, respectively, a maximum of 70% of the mercury is oxidized. Temperature is shown to have a positive effect on the extent of oxidation, as does the residence time.
Introduction Mercury (Hg) pollution is a major environmental concern, with serious long-term implications on human health. Coal-burning power plants are the single largest source (40%) of mercury pollution, resulting in uncontrolled emissions of ∼50 tons per year, at gas concentrations in the 5-10 µg/Nm3 (0.55-1.12 ppb by mole) range. The United States Environmental Protection Agency (USEPA) has stated regulations to control mercury emissions by December 2007. In addition to mercury, NOx emissions from coal-fired power plants are a major environmental concern, because they can lead to acid precipitation, smog, and ozone formation. Most coal-combustion utilities are fitted with low-NOx burners; however, stringent standards for NOx emissions have made the installation of selective catalytic reduction (SCR) units very essential. SCR technology has demonstrated a >90% reduction in NOx emissions since its first commercial application in the 1970s in Japan.1 Approximately 80-90 of the U.S. utilities are planning to install these units within the next five years.2,3 An SCR reactor consists of a fixed-bed metal oxide catalyst such as vanadium pentoxide (V2O5) supported on an open channel base, such as a titanium dioxide (TiO2) honeycomb monolith. Researchers have demonstrated that homogeneous mercury oxidation pathways exist4-6 and proposed heterogeneous interactions with fly ash surfaces where reactive species, catalysts, and active sites are available for heterogeneous Hg oxidation.7,8 Laboratory tests by Galbreath et al. have shown that, in a simple flue gas mixture that contains 8.5 mol * Author to whom correspondence should be addressed. Tel.: +1-610-758-4791. E-mail:
[email protected]. (1) Frey, H. C. Proc. Am. Power Conf. 1995, 57-II. (2) EPRI Technical Report No. 1005400, Electric Power Research Institute, Palo Alto, CA, 2002. (3) Schimmoller, B. K. Power Eng. 2000, (July), 45-48.
% O2, 30 ppmv NO, and ∼91.5 mol % N2, up to 60% of the elemental mercury (Hg0) was rapidly (200 °C. HCl and/or H2SO4 are injected, along with water, as required. Various concentrations of HCl (5, 15, and 35 ppm, by mole fraction in the final flue gas) and H2SO4 (5, 10, and 50 ppm) were chosen as the respective low, base, and high concentrations of the acids for various experiments. At SCR temperatures, H2SO4 rapidly decomposes to H2O and SO3. Therefore, the addition of H2SO4 may be considered to be equivalent to the addition of SO3. The two streams from the GPH and SPH combine and flow into a honeycomb SCR catalyst placed in the reactor. NH3, which was diluted at 5% in N2 and present in a NH3/NO molar ratio of 0.95, is injected separately into this gas stream just below the bottom of the catalyst to prevent the reaction of NH3 and SO3 at low temperatures. The preheaters are 18-in.-long, 1-in.-diameter stainless-steel tubes that are heated externally with ceramic heaters, whereas the reactor is a 22-in.-long, 1.5in.-diameter, schedule 40 stainless-steel pipe that is wrapped with a heating coil. The catalyst is a commercial honeycomb that is placed near the center of the reactor pipe. Gas flow is in the upward direction, and the gas temperature at the center of the catalyst is maintained at 343, 357, or 371 °C (( 2 °C). Samples are taken through a heated sample line from two ports: one is located before the inlet of the reactor, and the other is positioned near the exit of the reactor. Only one sample line is used at any given time. A continuous stream of mercury in nitrogen gas is generated by a permeation system and diluted in the GPH to maintain a concentration of 10-20 µg/m3 of mercury in the flue gas. The sample gas is continuously analyzed for both elemental and total mercury by a P S Analytical (PSA) mercury analyzer, which works on the principle of atomic fluorescence. NOx and O2 levels are continuously monitored using a NOVA portable flue gas analyzer, to ensure the SCR catalyst performance and the gas composition. These levels were analyzed from a separate stream of sample gas, without interfering with the mercury sampling.
1 2HCl + O2 h H2O + Cl2 2
(R1)
Hg + Cl2 h HgCl2
(R2)
1 SO2 + O2 h SO3 2
(R3)
1 Hg + SO3 + O2 h HgSO4 2
(R4)
Results and Discussion
H2SO4 h H2O + SO3
(R5)
A catalyst preconditioning routine was followed prior to each test, where 4 lpm of gas was sent through the
In an SCR control system, the addition of NH3 makes the following reactions also important: (13) Lee, C.; Srivastava, R.; Ghorishi, S. B.; Karwowski, J.; Hastings, T. W.; Hirschi, J. Presented at the Combined Power Plant Air Pollutant Control Mega Symposium, Washington, DC, August 30-September 2, 2004; Paper No. 7.
(14) Bock, J.; Hocquel, M. J. T.; Unterberger, S.; Hein, K. R. G. Presented at the Combined Power Plant Air Pollutant Control Mega Symposium, Washington, DC, May 19-22, 2003. (15) http://www.epa.gov/ttn/atw/combust/utiltox/utoxpg.html. (16) Senior, C. L.; Sarofim, A.; Zeng, T.; Helbe, J. J.; Mamani-Paco, R. Fuel Process. Technol. 2000, 63, 197. (17) Lee, C. W.; Kilgore, J. D.; Ghorishi, S. B. In Proceedings of the 93rd Annual Meeting of the Air and Waste Management Association; 2002; p 1688.
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Figure 1. Schematic of the SCR apparatus. Base conditions were as follows: 74% N2, 12% CO2, 6% O2, 8% H2O, 1000 ppm SO2, 400 ppm NO, and NH3/NO ) 0.9; gas flow rate ) 10.5 or 18 lpm; temperature ) 357-371 °C. Table 1: SCR Flue Gas Conditions property flue gas composition O2 CO2 H2O N2 SO2 NO H2SO4 HCl fly ash mercury concentration SCR conditions temperature space velocity NH3/NO ratio
value 6% 12% 8% 74% 1000 ppm 400 ppm 10 ppm 15 ppm none 10-20 µg/m3 357 °C 4000 h-1 0.9
reactor, and the gas composition was maintained close to the experimental conditions, given in Table 1. The concentrations of HCl and H2SO4 were kept at a reduced value, and mercury was not included in the preconditioning period. The system was run overnight at a gas temperature of 371 °C in preconditioning mode. The following day, the flow rate of the gas stream was increased to the flow rate of the experiment and mercury was added to the flue gas. That day’s experiments were then conducted. An initial experiment with an empty reactor (no catalyst) was run to test the inertness of the reactor and the accuracy of the system’s analytical method. The results of this test are shown in Figure 2. From point “a” to point “b” in Figure 2, the gas is sampled from the inlet of the reactor, and at point “h”, the inlet concentration of mercury is again measured. Between points “b” and “h”, gases are turned on, as indicated in the figure. A slight increase in mercury concentrations between points “b” and “c” may be due to adjustments in the gas flow or minor disturbances in the experimental settings. The absence of any large or systematic variations in the total or elemental mercury concentration across the reactor indicates that the reactor itself is not adsorbing
Figure 2. Empty reactor experiment. Conditions were as follows: gas flow ) 18 lpm, space velocity ) 8000 h-1, gas temperature ) 357 °C.
elemental mercury. Also, the total mercury concentration is approximately equal to the elemental mercury, implying that there are no homogeneous reactions in the reactor that may have an effect on mercury. Thus, any change in the mercury values observed in subsequent tests is due to the catalyst or the flue gas components in the presence of the catalyst. To understand the influence of each flue gas component on Hg oxidation and retention, a series of tests were conducted in which each component was added individually. Figures 3 and 4 show these effects. Throughout the tests, N2 is used as the base gas, and its flow rate is adjusted to maintain a constant space velocity of 8000 h-1. In Figure 3, from point “a” to point “b”, the inlet gas is sampled. At point “b”, the gas is sent through the catalyst and the sample is taken from the exit of the reactor. As the gas, which contains just N2 and Hg, flows through the catalyst (from point “b” to point “c” in Figure 3), 70%-55% of the elemental mercury is lost
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Figure 3. Stepwise addition of gases NH3, O2, and HCl. Conditions were as follows: total gas flow ) 18 lpm, space velocity ) 8000 h-1, gas temperature ) 357 °C.
Figure 4. Stepwise addition of gases SO2, O2, and H2SO4. Conditions were as follows: total gas flow ) 18 lpm, space velocity ) 8000 h-1, gas temperature ) 357 °C.
and is retained on the catalyst. The addition of NH3 at point “c” releases the retained elemental mercury from the catalyst, whereas the addition of O2 at point “e” further enhances the retention of mercury on the catalyst to again reach ∼55% at steady state. At point “g”, the addition of 10 ppm HCl solution causes a rapid desorption of mercury from the catalyst. It has been observed in other tests that the addition of water alone also causes a sudden desorption of mercury. As steady state is reached at point “h”, the exiting mercury
concentration is almost the same as the inlet concentration of mercury, with a slight difference between the total and elemental mercury values. This indicates that the presence of HCl in a N2-only environment inhibits mercury adsorption onto the catalyst and causes a slight amount of oxidation. Figure 4 shows the effect of sulfur oxides on mercury adsorption in the presence of an SCR catalyst. At point “b”, SO2 is added to the gas stream and the flue gas is sampled from the exit of the reactor. Approximately 76%
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Figure 5. Varying concentration of HCl. Conditions were as follows: gas flow ) 10.5 lpm, space velocity ) 4000 h-1, gas temperature ) 371 °C.
of the mercury is retained on the catalyst. At point “c”, O2 is added. This causes a brief desorption of mercury and a return to a mercury retention value of 60%. The addition of a 15 ppm solution of H2SO4 at point “e” causes a sudden desorption of mercury, with the peak for desorption of total mercury being considerably higher than that for elemental mercury. Mercury subsequently is retained on the catalyst and the amount of adsorbed mercury remains at a steady value (∼40%) at point “f”. This removal amount remains stable, even with the addition of SO2, from point “f” to point “g”. At point “h”, gas is again sampled from the inlet of the reactor, to ensure that the feed mercury concentration was constant. The adsorption of mercury in a pure N2 atmosphere or an atmosphere of N2 and O2 seems to be weak in nature (see point “c” in Figure 3), because the mercury is easily desorbed when a strong reducing agent, such as NH3, is added. However, in the presence of H2SO4, mercury seems to be strongly adsorbed on the catalyst (see point “f” in Figure 4). The reduction in concentration of both elemental and total mercury during the experiments makes it difficult to define mercury conversion. The loss of elemental mercury could be due to conversion to an oxidized form that is adsorbed, or the adsorption of elemental mercury, or a combination of both. For our reporting of the results, the difference between the inlet and outlet elemental mercury will be considered to be oxidized mercury. Figure 5 shows the effect of HCl concentration on mercury oxidation. The flue gas has a space velocity of 4000 h-1 and a temperature of 371 °C. No H2SO4 is present in the flue gas, and the rest of the gas composition remains as shown in Table 1. At point “a”, the inlet mercury concentration is measured and the average inlet concentration is shown as a horizontal dotted line. Vertical dotted lines noted as “b”, “c”, and “d” in Figure 5 denote the time when the injected HCl concentration
Figure 6. Effects of HCl on mercury oxidation.
was changed from 5 ppm, to 15 ppm, to 35 ppm, respectively. When 5 ppm HCl is added at point “b”, the mercury is considerably oxidized and the oxidation reaches a steady value of ∼45%. As the concentration of HCl is increased to 15 ppm and 35 ppm (points “c” and “d”), the oxidation of mercury is increased to 60% and 68%, respectively. Figure 6 shows the results of the runs in Figure 5, together with a second set of experiments at a higher space velocity (8000 h-1) but a lower gas temperature (357 °C). This second set of data also had 10 ppm of H2SO4, in addition to the HCl. The three competing effects of temperature, residence time, and a second oxidant make it difficult to predict in which direction the conversion will change. In our future work, we will try to develop a model that can separate these competing effects. Figure 7 shows the effect of H2SO4 on mercury oxidation. The flue gas has a space velocity of 4000 h-1 and a temperature of 371 °C. No HCl is present in the
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Figure 7. Varying concentration of H2SO4. Conditions were as follows: gas flow ) 10.5 lpm, space velocity ) 4000 h-1, gas temperature ) 371 °C.
Figure 8. Effects of H2SO4 on mercury oxidation.
flue gas, and the rest of the gas composition remains as listed in Table 1. At point “a” in Figure 7, the inlet mercury concentration is measured and the average inlet concentration is shown as a horizontal dotted line. Vertical dotted lines noted as “b”, “c”, and “d” in Figure 7 denote the time when 5, 15, and 50 ppm of H2SO4 were injected, respectively. The addition of 5 ppm of H2SO4 at point “b” causes a rapid spike in the mercury concentrations. This was caused by residual mercury that is retained in the catalyst from previous runs. With time, mercury gradually reaches a steady value of ∼5.5 µg/m3, which is equivalent to ∼50% oxidation. As the concentration of H2SO4 is increased in the gas, shown at points “c” and “d”, a maximum oxidation of mercury of ∼70% is observed. Figure 8 summarizes the results from Figure 7, and another set of experiments at a higher space velocity of
8000 h-1, a lower temperature (357 °C), and in the presence of 15 ppm HCl. A trend of increasing mercury oxidation is observed, with an increase in concentration of H2SO4. However, the three competing effects of temperature, concentration, and space velocity are difficult to separate. Also noteworthy is the fact that ∼12% mercury conversion is observed with zero concentrations of both HCl and H2SO4. This may be due to the intrinsic ability of the catalyst to oxidize and retain mercury from stored chlorine and sulfur from previous runs. Figure 9 shows the effect of mercury oxidation in a temperature range typical for SCR systems. The flue gas compositions for the various temperature experiments is as given in Table 1. At 371 °C, the extent of mercury oxidation is almost twice that observed at 343 °C. This implies an apparent activation energy of ∼18 kcal/mol.
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Figure 9. Effect of temperature, at a space velocity of 8000 h-1.
Conclusions A laboratory-scale selective catalytic reduction (SCR) system was designed, built, and tested, using a honeycomb vanadia-titania catalyst. The influence of oxidizing agents such as HCl and SO3 on Hg oxidation was investigated at different concentrations, temperatures, and space velocities. Mercury oxidation was determined to increase with oxidant concentration, residence time, and temperature. The experimental apparatus did not adsorb mercury, and the homogeneous oxidation of mercury was not observed. Under various gas compositions, mercury was observed to adsorb onto the catalyst surface.
At a space velocity of 4000 h-1, a maximum of 70% Hg oxidation was observed with either HCl or SO3 in the flue gas, at the maximum tested concentrations of 35 and 50 ppm, respectively. This maximum may be attributed to gas-to-solid mass-transfer limitations. In the presence of HCl, or SO3, or both, oxidized Hg was determined to adsorb strongly to the catalyst. Acknowledgment. The authors thank the Electric Power Research Institute (EPRI) for its financial support for this research work. Assistance in experimental analysis by Ms. Faaiza Rashid and Mr. Jonathan McMullen is gratefully acknowledged. EF050087F
