Energy Fuels 2010, 24, 4351–4356 Published on Web 07/23/2010
: DOI:10.1021/ef1004336
Oxidation of Mercury under Ultraviolet (UV) Irradiation L. Jia, R. Dureau, V. Ko, and E. J. Anthony* Natural Resources Canada/CanmetENERGY, 1 Haanel Drive, Ottawa, Ontario K1A 1M1, Canada Received April 6, 2010. Revised Manuscript Received June 15, 2010
Oxidation of mercury under ultraviolet (UV) irradiation was carried out in a reaction chamber using both simulated and real flue gases from pilot-scale combustors burning coal. The gases examined included SO2, CO, CO2, NO, CH4, O2, water vapor, alcohol vapor, and their mixtures. The temperature of the reaction chamber was 37.8 °C (100 °F) and 137.8 °C (280 °F). A 10 W low-pressure UV light bulb was placed in the center of the reactor to provide the irradiation, with the main UV irradiation band at 253.7 nm. In addition to the simulated flue gas tests, four slipstream tests were conducted with flue gas diverted from the vertical combustor of CanmetENERGY, burning Saskatchewan lignite, and also with the mini-circulating fluidized-bed combustor (CFBC) of CanmetENERGY, burning Powder River Basin coal. The results showed that oxidation of mercury by 253.7 nm UV irradiation occurred in both synthetic and real flue gas environments, with levels of up to 65.5% of the total mercury being oxidized. The temperature had a strong effect on the oxidation level, which decreased significantly when the temperature was raised to 137.8 °C. Methane also had a positive effect on Hg oxidation. It is hypothesized that this was due to the generation of free hydrogen atoms when CH4 collided with UV-excited Hg to produce free radicals and cause chain reactions. NO could reduce the effectiveness of UV irradiation on mercury oxidation, possibly through the removal of ozone and free oxygen atoms. It was observed experimentally that the residence time of the flue gas in the reaction had little impact on the Hg oxidation level, which implies that Hg oxidation is a fast photochemical process.
mercury control options for coal-fired power plants.5-7 Plants with fabric filters combined with flue gas desulfurization (FGD) wet scrubbers have the highest mercury removal rates, estimated at about 80%. Other utility emission control technologies are far less effective. Most can only remove between 4 and 30% mercury. Activated carbon injection is currently being tested in various-sized facilities for mercury control. Unfortunately, the C/Hg weight ratios range from 2000 to 15 000 or even higher.5 Other sorbents tested include zeolite8 and noble metals, such as gold, supported on a suitable substrate.9 Ultimately, all of these approaches depend upon the fact that it is better to remove Hg in either a solid or liquid form and treat and dispose of those materials than permit it to escape to the atmosphere. Several novel control technologies have been advanced on the basis of chemical, photochemical, or electrocatalytic methods, and CanmetENERGY has tried to develop methods using, for instance, Fenton reactions.10,11 Exploratory
1. Introduction Mercury is of major concern among air toxic substances because of its volatility, persistence, and potential for bioaccumulation and neurological health impacts.1 Anthropogenic emission sources primarily emit Hg in three forms:2 elemental, which is present in the gas phase as Hg0, gas-phase inorganic, HgII, and particulate, Hgp. Collectively, coal-burning electric utilities are identified as the largest distinct mercury source in the U.S.3 Thermal power stations are responsible for about 1/3 of the known anthropogenic mercury air emissions in the U.S. or slightly less than 50 tons/year. Although base metal smelting is the dominant point source for mercury emissions in Canada, electric utilities nevertheless also emit significant quantities of mercury, estimated to be about 2 tons/year in 2007.4 Several approaches have been examined to control mercury emissions, and a number of reviews exist on the subject of *To whom correspondence should be addressed. E-mail: banthony@ nrcan.gc.ca. (1) United States Environmental Protection Agency (U.S. EPA). A study of hazardous air pollutant emissions from electric utility steam generating units: Final report to Congress. EPA-453/R-98-004a; U.S. EPA Office of Air Quality Planning and Standards, U.S. Government Printing Office: Washington, D.C., 1998. (2) Prestbo, E. M.; Bloom, N. S.; Hall, B. Mercury speciation in coal combustion flue gas: Methodology, intercomparison, artifacts and atmospheric implications. Water, Air, Soil Pollut. 1995, 30, 145–158. (3) Johnson, J. Power plants to limit mercury. Chem. Eng. News 2001, 79 (1), 18–19. (4) Environment Canada. 2008 National Pollutant Release Inventory (NPRI) Reviewed Facility Data Release, 2008; http://www.ec.gc.ca/inrpnpri/default.asp?lang=en&n=8FAD0588-1. (5) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Status review of mercury control options for coal-fired power plants. Fuel Process. Technol. 2003, 82, 89–165. Published 2010 by the American Chemical Society
(6) O’Dowd, W. J.; Hargis, R. A.; Granite, E. J.; Pennline, H. W. Recent advances in mercury removal technology at the National Energy Technology Laboratories. Fuel Process. Technol. 2004, 85, 533–548. (7) Hower, J. C.; Senior, C. L.; Suuberg, E. M.; Hurt, R. E.; Wilcox, J. L.; Olson, E. S. Mercury capture by native fly ash carbons in coal-fired power plants. Prog. Energy Combust. Sci. 2010, 36, 510–529. (8) Morency, J. R.; Panagiotou, T.; Senior, C. L. Laboratory duct injection of a zeolite-based mercury sorbent. Proceedings of the 93rd Annual Meeting of the Air and Waste Management Association; Salt Lake City, UT, 2000; Paper AEIA 610. (9) Butz, J. R.; Turchi, C.; Broderick, T. E.; Albiston, J. Options for mercury removal from coal-fired flue gas streams: Pilot-scale research on activated carbon, alternative and regenerable sorbents. Proceedings of the 17th Annual Pittsburgh Coal Conference; Pittsburgh, PA, Sept 11-14, 2000; Paper 19b-3. (10) Lu, D.; Anthony, E. J.; Tan, Y.; Dureau, R.; Ko, V.; Douglas, M. A. Mercury removal from coal combustion by Fenton reactions; Part A: Bench-scale test. Fuel 2007, 86, 2789–2797.
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: DOI:10.1021/ef1004336
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Table 1. Quenching Cross-sections8 for Hg 6(3P1) to Hg 6(1S0) species
quenching cross-section (cm2)
HCl NO O2 CO CO2 H2O N2 Ar
37.0 10-16 24.7 10-16 13.9 10-16 4.1 10-16 2.5 10-16 1.0 10-16 0.4 10-16 0.04 10-16
Table 2. Synthetic Flue Gases Tested by Granite and Pennline12 composition A B C D E
16% CO2, 5% O2, 2000 ppm SO2, 300 ppb Hg, balance N2 16% CO2, 5% O2, 2000 ppm SO2, 500 ppm NO, 300 ppb Hg, balance N2 2% H2O, 300 ppb Hg, balance N2 1000 ppm NO, 300 ppb Hg, balance N2 13.9% O2, 300 ppb Hg, balance N2
studies12,13 have shown that mercury oxidation can be promoted by ultraviolet (UV) radiation alone or a nanosized TiO2 catalyst activated by UV radiation.14 The photochemical reaction of mercury with various gases in flue gas presents an attractive alternative to sorbent- or direct scrubber-based processes for mercury capture. The overall reaction between mercury and oxygen under the influence of 253.7 nm light is given by the overall global reaction 1 as suggested by Dickinson and Sherrill.15 Hg þ 2O2 þ 253:7 nm light f HgO þ O3 ð1Þ
Figure 1. Test reactor.
Here, mercury serves as a sensitizer for the formation of ozone, and the ozone oxidizes mercury to form mercuric oxide. Photochemical formation of mercuric oxide can also have a significant impact on online UV-based methods for the measurement of mercury in flue gases. The quenching of fluorescent emissions by mercury in the 6(3P1) state is due to collisions with other gas atoms or molecules. There must be a transfer of energy from the photoexcited mercury to the other gas species. Mercury can return to the ground state after a quenching collision. The quenching efficiency or cross-sectional area is a function of the molecular or atomic size, shape, and reactivity of the collision partner. Granite and Pennline12 surveyed the literature and provided quenching cross-sections of several gases for the Hg 6(3P1) to Hg 6(1S0) transition (Table 1). It should be noted that the Hg line at l = 253.7 nm is the strongest resonance line of Hg, (3P1-1S0), i.e., appearing in both absorption and emission. Granite and Pennline initially performed experiments with a 6 W UV lamp shining from the side of a 1/4 in. outer diameter quartz reactor, which was maintained at temperatures in the range from 26.7 °C (80 °F) to 176.7 °C (350 °F).12 The gas
mixtures examined are given in Table 2. They found that from 0.8 to 71.6% Hg could be oxidized depending upon the gas mixture composition and temperature, and they also reported detecting HgSO4 on the inside wall of the quartz reactor tube. More recently, in a short communication, the same group reported some limited experiments with a larger system and a 36 W UV lamp on simulated flue gases, as prelude to an industrial test of the use of UV, in which they consistently report average Hg oxidation around 90% for a temperature range of “49-60 °C”.13 The current work expands the existing experimental envelope by testing other gas species that might be present in real flue gases, such as CH4 and CO, to study the effect of the gas composition on Hg oxidation under UV irradiation. Furthermore, because the initial experimental setup of Granite and Pennline12 can use only a very small fraction of the UV light emitted from the lamp, a different type of reactor was constructed to significantly improve UV availability. Finally, slipstreams of flue gas from the vertical combustor of CanmetENERGY and its mini-circulating fluidized-bed combustor (CFBC) were fed to the reactor to determine the Hg oxidization rate under “real” flue gas conditions.
(11) Lu, D.; Anthony, E. J.; Tan, Y.; Dureau, R.; Ko, V.; Douglas, M. A. Mercury removal from coal combustion by Fenton reactions; Part B: Pilot-scale tests. Fuel 2007, 86, 2798–2805. (12) Granite, E. J.; Pennline, H. W. Photochemical removal of mercury from flue gas. Ind. Eng. Chem. Res. 2002, 41, 5470–5476. (13) McLarnon, C. R.; Granite, E. J.; Pennline, H. W. The PCO process for photochemical removal of mercury from flue gas. Fuel Process. Technol. 2005, 87, 85–89. (14) Rodriguez, S.; Lee, T. G.; Furuuchi, M.; Biswas, P. Influence of SO2 particles on mercury removal using in situ generated TiO2 nanosize particles activated by UV irradiation. Proceedings of the 17th Annual Pittsburgh Coal Conference; Pittsburgh, PA, Sept 11-14, 2000; Paper 25-1. (15) Dickinson, R. G.; Sherrill, M. S. Formation of ozone by optically excited mercury vapor. Proc. Natl. Acad. Sci. U.S.A. 1926, 12, 175.
2. Experimental Section The test reactor is made of stainless steel (Figure 1). The internal length is 203 mm, and the internal diameter is 49 mm. A 10 W (electrical), low-pressure, UV lamp is located in the center of the reactor inside a quartz sleeve. Its output concentrates at the 253.7 nm band. The 10 W lamp can produce UV irradiation intensities of 40 mJ/cm2 at a gas flow rate of 1.43 L/min. The inside wall of the reactor was polished to enhance reflection of the UV light. The experimental setup is shown in Figure 2. 4352
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Figure 2. Experimental setup. Table 3. Experimental Results test number
gas composition
temperature (°C)
percentage of Hg oxidized (%)
percentage of Hg recovery (%)
1 13 2 4 3 5 6 7 8 11 9 12 10 14 15 16 17 18 19
N2 N2 3% O2 in N2 3% O2 in N2 3% O2, 250 ppm CH4, balance N2 3% O2, 250 ppm CH4, balance N2 130 ppm CO in N2 130 ppm CO in N2 500 ppm NO in N2 500 ppm NO in N2 11.2% CO2 in N2 11.2% CO2 in N2 2000 ppm SO2 in N2 2000 ppm SO2 in N2 2.08% H2O vapor in N2 2.08% H2O vapor in N2 5.05% alcohol vapor in N2 5.05% alcohol vapor in N2 500 ppm NO, 2000 ppm SO2, 3% O2, 11.2% CO2, 0.78% alcohol vapor, balance N2 500 ppm NO, 2000 ppm SO2, 3% O2, 11.2% CO2, 0.78% alcohol vapor, balance N2 500 ppm NO, 2000 ppm SO2, 3% O2, 11.2% CO2, 1.78% water vapor, balance N2 500 ppm NO, 2000 ppm SO2, 3% O2, 11.2% CO2, 1.78% water vapor, balance N2 500 ppm NO, 2000 ppm SO2, 3% O2, 11.2% CO2, 250 ppm CH4, 1.78% water vapor, balance N2 500 ppm NO, 2000 ppm SO2, 3% O2, 11.2% CO2, 250 ppm CH4, 1.78% water vapor, balance N2
37.8 37.8 37.8 137.8 37.8 137.8 37.8 137.8 37.8 137.8 37.8 137.8 37.8 137.8 37.8 137.8 37.8 137.8 37.8
44.4 ( 7.7 45.8 ( 22.0 53.4 ( 2.6 35.2 ( 20.0 91.1 ( 2.2 27.3 ( 13.1 54.1 ( 4.5 7.8 ( 12.3 36.2 ( 10.7 20.6 ( 10.4 72.9 ( 3.3 66.5 ( 1.4 39.2 ( 4.8 63.2 ( 5.3 37.7 ( 5.4 7.4 ( 4.3 38.0 ( 1.4 35.6 ( 5.5 47.2 ( 3.8
93.2 93.6 86.1 86.3 97.2 89.0 98.0 94.2 96.0 90.7 88.1 99.0 97.7 90.8 99.5 87.1 86.5 97.6
137.8
18.8 ( 8.5
91.5
37.8
55.3 ( 3.7
103.2
137.8
7.2 ( 5.1
97.8
37.8
38.7 ( 14.1
91.6
137.8
4.5 ( 3.5
101.3
20 21 22 23 24
impinger contained a mixture of 40 mL of 5% KMnO4 and 10 mL of 50% H2SO4 to permit removal of oxidized and elemental Hg, respectively. The impingers were placed inside an ice bath. Water vapor and alcohol vapor were added to the synthetic flue gas by allowing N2 to bubble through a bubbler filled with deionized water or alcohol. A total of 3-4 separate samplings of Hg were performed for each test condition. The reactor was washed with KMnO4 solution after each test. The amount of Hg recovered in the washing was added to the Hg trapped by the impingers to calculate the Hg recovery.
All connections were made with Teflon tubing. Ultra-high-purity (UHP) gases were used to make the synthetic flue gas. The mass flow meters used have an accuracy of (1.0% full scale. Tests were conducted at two temperatures: 37.8 °C (100 °F) and 137.8 °C (280 °F). The temperature variation at any given temperature was (2 °C. Gas compositions tested are given in Table 3. The flow rate of synthetic flue gas was maintained at 1.43 L/min for all of these tests, producing an average residence time of 12.6 s. All runs lasted for 10 min. Two impingers were used: the first impinger contained 50 mL of 1 M KCl, and the second 4353
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Jia et al. Table 4. Slipstream Results
test number
25 26 27
28 29 30
temperature (°C)
average flue gas composition
percentage of Hg oxidized (%)
Hg emission rate (μg/min)
Flue Gas Slipstream Tests on the Vertical Combustor of CanmetENERGY, Burning Saskatchewan Lignite ∼50 35.5 13.9% CO2, 288 ppm SO2, 5.85% O2, 836 ppm NO, non-detectable amount of CO (balance N2) 14.7% CO2, 437 ppm SO2, 4.17% O2, 750 ppm NO, ∼50 65.5 non-detectable amount of CO (balance N2) direct sampling; flue gas composition is the 12.3 same as in test number 26 Flue Gas Slipstream Test on the Mini-CFBC of CanmetENERGY, Burning Powder River Basin Coal 56.7 42.0 12.4% CO2, 280 ppm SO2, 5.8% O2, 138 ppm NO, 520 ppm CO 57.1 41.6 12.4% CO2, 280 ppm SO2, 5.8% O2, 138 ppm NO, 520 ppm CO direct sampling; flue gas composition is the same as in test number 28 57.0 18.9
A Vici Metronics model 500 dual-chamber dynacalibrator was used as the elemental mercury source (mercury generator). The Hg0 flow rate was in the range of 0.15-0.2 μg/min. The amount of Hg0 vapor generated from the mercury generator was measured before and after each test run. A Tekran model 2600 cold vapor atomic fluorescence mercury analyzer was used to analyze the samples. In addition to the synthetic flue gas tests, two runs were performed using a slipstream of flue gas from the vertical combustor of CanmetENERGY, firing Saskatchewan lignite. The flue gas was drawn from a port in the duct after the baghouse. During these runs, the temperature of the reactor was about 50-60 °C. The duration of the two runs was 1 h and 20 min. The flue gas flow rate was 14.23 L/min for the first run and 13.84 L/min for the second run. A blank (without UV reactor in the sampling train) test was also conducted. The residence time for the flue gas in the UV reactor was 1.27-1.31 s.
25.8 51.9 58.6
25.8 5.91 8.14
Table 5. Analysis of UHP-Grade Nitrogen atmospheric pressure analytical results chemical ionization (APCI) for the particular N2 bottle (ppmv) specification (ppmv) total hydrocarbons oxygen water
0.5 4 3.5
0.01 0.95 0.4
all tests because impurity levels may vary from bottle to bottle. The effect of other reaction species is then seen despite the impurities in the nitrogen stream, and it is possible to determine if the specific gas will promote or impede the Hg oxidation in the presence of UV irradiation. Another caveat is due to the fact that the water present in the synthetic gases is significantly lower than might be expected in normal coal combustion, where ranges of the order of 10-20% moisture content can be expected for most fuels. Thus, at standard combustion conditions, the moisture content in the flue gas from combustion of Saskatchewan lignite (3.97% H and 21.64% moisture) will be 12.4%, while for the combustion of the Powder River coal, the water levels will be about 10%, assuming an inherent water content of 2% for a dried fuel. Unfortunately, this is a necessary impact of the fact that, in these types of experiments, water addition must be limited to avoid condensation, and the implications of this fact will be discussed later. 3.3. Runs with a Single Gas in Nitrogen. The oxidation of Hg with UV irradiation may be described by the global eq 1 along with the following expressions: Hg þ H2 O þ 253:7 nm light ¼ HgO þ H2 ð2Þ
3. Results and Discussion 3.1. Blank Baseline Runs. The first set of runs was a blank test with N2 carrying elemental mercury passing through the reactor and the sampling impingers with the UV light turned off at 37.8 °C (100 °F). Sampling at the mercury generator outlet showed that the Hg0 flow rate was 0.166 ( 0.0038 μg/ min. The reactor outlet Hg0 was found to be 0.167 ( 0.050 μg/min. The results showed that, without UV irradiation, Hg0 passes through the reactor with negligible system deposition. Blank tests conducted at 137.8 °C (280 °F) showed similar results (average inlet Hg0, 0.194 ( 0.005 μg/min; average outlet Hg0, 0.185 ( 0.004 μg/min). 3.2. Blank Runs with N2 Only and UV Irradiation. The experimental results with UV irradiation are shown in Tables 3 and 4. Mercury recovery was between 86.1 and 103.2%. As shown in Table 3, with N2 only (tests 1 and 13) at 37.8 °C, the Hg oxidation level was from 44.4 ( 7.7 to 45.8 ( 22.0%. This result raised the question as to how Hg oxidation occurs, because the nitrogen gas used was of UHP grade; the specifications for which are given in Table 5. The analysis indicated that, although nitrogen was of UHP, it still contained some oxygen, water vapor, and hydrocarbons. Because the Hg concentration in the synthetic flue gas was in the range of 87-116 ppbv, the UHP-grade nitrogen evidently contains sufficient impurities to oxidize Hg. The impurities in the UHP-grade nitrogen made the assessment of the effect of different gases on Hg oxidation difficult. One solution might be to obtain absolutely pure nitrogen, but unfortunately there is no commercial supplier of such a gas. The only way the test results can be made strictly comparable is to use the same bottle of nitrogen for
Hg þ CO2 þ 253:7 nm light ¼ HgO þ CO
ð3Þ
Hg þ SO2 þ O2 þ 253:7 nm light ¼ HgSO4
ð4Þ
Results for the single gases in nitrogen runs are shown in Table 3. The 250 ppm CH4 in N2 (tests 3 and 5) has 3% O2 added in the mixture because the CH4 gas mixture contains no oxygen. As expected on the basis of the previous work,12 for all but SO2 addition, higher temperatures produced lower Hg oxidation levels. If the results with N2 are used as the basis for comparison, then at 37.8 °C, the O2, CH4, and O2 mixture and CO and CO2 individually enhanced the oxidation of Hg. Other gases, such as NO, water vapor, and alcohol vapor, actually reduced oxidation. Interestingly, despite more effective UV irradiation achieved in the present work, Granite and Pennline12 reported higher Hg oxidation levels, as they also do in the later study.13 For instance, they 4354
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reported oxidation levels of 25.4 ( 5.7% for 2% water vapor in N2 at 137.8 °C, which is significantly higher than the results obtained here in test 16 (7.4 ( 4.3%). Granite and Pennline12 also reported test results for 1000 ppm NO in N2 at 137.8 °C, which showed a Hg oxidation level of only 0.8%, effectively the same as the baseline test. In the current test series, 500 ppm NO in N2 at the same temperature (test 11) permitted an average Hg oxidation level of 20.6 ( 10.4%, which indicates that NO reduces the level of Hg oxidation, but unless there is a pronounced concentration effect, this result is still significantly different from the earlier tests. The best result obtained here was with 250 ppm CH4 and 3% O2 (test 3). The Hg oxidation level was 91.1 ( 2.2% at 37.8 °C. It is postulated that, when excited mercury atoms collide with CH4 molecules, H atoms may be produced and, in turn, induce further reactions. Hg 6ð1 S 0 Þ þ 253:7 nm light f Hg 6ð3 P 1 Þ
ð5Þ
Hg 6ð3 P 1 Þ þ CH4 f Hg 6ð1 S 0 Þ þ CH3 þ H
ð6Þ
H þ O2 f OH þ O
ð7Þ
Hg þ O f HgO
ð8Þ
Table 6. Analysis of Coal (%, Dry Basis)
C H N S O ash Cl (ppm)
Powder River Basin coal
Saskatchewan lignite
70.76 4.85 1.05 0.48 15.61 7.25
61.30 3.97 1.00 0.60 17.82 15.31 24
the synthetic flue gas used by Granite and Pennline contained no CO. All of the earlier results showed that NO had very strong effects on Hg oxidation, presumably because it is a strong reducing agent and perhaps also because NO can reduce ozone or O through reactions: ð9Þ NO þ O3 f NO2 þ O2 NO2 þ O3 f NO þ 2O2
ð10Þ
overall 2O3 f 3O2
ð11Þ
As shown in Table 1, NO has the second largest quenching cross-section among the gas species that may be present in flue gas. Furthermore, NO in N2 results (tests 8 and 11) showed that there was no improvement of Hg oxidation, and Noyes17 had determined that mercury photosensitizes the conversion of NO to N2O3. The current results on the NO effect were similar to those obtained by Granite and Pennline.12 3.5. Slipstream Tests. Two tests were conducted with a flue gas slipstream from the vertical combustor of CanmetENERGY. The flue gas was extracted from a port on the flue gas duct downstream of the baghouse. The vertical combustor used a pulverized coal burner, fired with Saskatchewan lignite (coal analysis in Table 6). The coal firing rate was about 34.5 kg/h. The gas flow rate of the slipstream was much higher than for the synthetic flue gas tests; thus, the resulting residence time was 1.27-1.31 s. The mercury oxidation levels were 35.5% from the first test and 65.5% from the second test. Direct sampling of the flue gas right after the second test showed about 12.3% oxidized mercury in the flue gas. The split of elemental/oxidized mercury was comparable to previous values measured from this facility firing the same coal. The different mercury oxidation levels from the two UV tests cannot be easily explained in terms of the average flue gas composition (Table 4). However, during the first slipstream run, the total Hg emissions rate was only 25.8 μg/min. The total Hg emissions rates measured from the second UV test and the direct sampling of the flue gas were 51.9 and 58.6 μg/min, respectively. Historical values for total Hg emissions rates from the same burner firing the same coal were between 50 and 60 μg/min. Therefore, one possible explanation for the differences might be that the vertical combustor was not running in steady-state mode during the first UV test run, and this caused fluctuations in the flue gas composition and affected mercury oxidation. It is clear that more tests are required. Nonetheless, the slipstream test results indicated that UV irradiation was effective in promoting the oxidation of mercury in a real flue gas environment. The residence time for the slipstream
It was hoped that other hydrocarbons might behave in the same way. The easiest hydrocarbon to add to the synthetic flue gas was alcohol. However, tests with up to 5.6% alcohol vapor in the synthetic gas stream failed to produce an improvement in the Hg oxidation level. Interestingly, CO2 showed strong positive effects on Hg oxidation even at higher temperatures. At least 2/3 of the Hg could be oxidized by UV irradiation when 11.2% CO2 was present in the N2 stream. 3.4. Synthetic Flue Gas Tests. UV Hg oxidation with synthetic flue gases was also explored. Hg oxidation levels were in the range from 38.7 ( 14.1 to 55.3 ( 3.7% at 37.8 °C. When the temperature was raised to 137.8 °C, the Hg oxidation level fell to below 20%. The general trend for the temperature effect is evidently in agreement with previous work. For a reaction temperature higher than about 150 °C, virtually no Hg oxidation could be observed.11 The addition of 250 ppm CH4 in the synthetic flue gas did not improve the Hg oxidation performance under these conditions. These results also suggest lower Hg oxidation levels than seen in the work by Granite and Pennline.12,13 For a synthetic flue gas containing no NO, these workers reported that the Hg oxidation level was 71.6 ( 30.1% at 137.8 °C and 67.8 ( 28.8% at 26.7 °C. These results appear to be opposite to the trend that Hg oxidation levels are higher at lower temperatures; however, the experimental errors were quite significant, making a firm conclusion difficult to draw. One possible reason for the decrease of the Hg oxidation level at higher temperatures is that the thermal decomposition rate of ozone becomes appreciable at temperatures above 100 °C.16,18 When the synthetic flue gas contained 1000 ppm NO, Granite and Pennline12 reported that the Hg oxidation level dropped to 26.8 ( 11.7%. These results are comparable to the results presented here given that experimental errors in both studies are quite significant. It should also be noted that
(17) Noyes, W. A. The photochemical reaction between nitric oxide and mercury vapor. J. Am. Chem. Soc. 1931, 53, 514. (18) Horvath, M.; Bilitzky, L.; Huttner, J. Ozone; Elsevier: New York, 1985.
(16) Maron, S. H.; Lando, J. B. Fundamentals of Physical Chemistry; Macmillan: New York, 1974.
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tests was 10 times shorter than for the synthetic flue gas experiments. However, Hg oxidation levels from the two sets of experiments were roughly comparable. This was an indication that Hg oxidation under UV irradiation was fast and residence time has little effect. Two more tests were performed using the mini-CFBC of CanmetENERGY, burning Powder River Basin coal. Table 6 gives the coal analysis. The two runs produced very similar oxidation levels for Hg, 41.6 and 42.0%. The baseline measurement showed that oxidized mercury accounted for 18.9%. Granite and Pennline12 found that HgSO4 was formed and deposited on the inside walls of the reactor in their tests. Because the total amount of mercury injected during each synthetic flue gas run conducted in this series of tests was only 1.5-2 μg and the internal surface area of the reactor was far greater than that used in Granite and Pennline’s work, no deposit of Hg on the surface could be collected. Therefore, it was impossible to confirm their findings of HgSO4 collection on surfaces. The temperature had a very strong effect on Hg oxidation under UV irradiation. As reported in the literature, a decrease in the rate with an increasing temperature has been observed for many photochemical reactions.16,18 Unfortunately, it was not possible in the current work to achieve mercury oxidation levels greater than 20% at 137.8 °C (280 °F) with synthetic flue gas containing all of the gas species typically expected in real flue gas. This result is different from that of Granite and Pennline,12 who reported a mercury oxidation level of 71.6% at the same temperature (albeit the synthetic flue gas used contained no NO), which was actually higher than the levels that they obtained at 26.7 °C (80 °F).
An industrial demonstration (2 MW mobile slipstream test unit) of this technology was scheduled for 2007, to be carried out by the Powerspan Corporation at the Rush Island plant of AmerenUE in Jefferson County, MO, but unfortunately the tests did not go ahead.19 4. Conclusions Oxidation of mercury in the presence of 253.7 nm UV irradiation is effective in synthetic and real flue gas environments. This work indicates that up to 65.5% of the total mercury can be oxidized. Furthermore, the temperature has a strong effect on the UV oxidation of mercury as the oxidation level decreased significantly when the temperature was raised from 37.8 to 137.8 °C. NO can reduce the effectiveness of UV irradiation on mercury oxidation, possibly through the removal of ozone and free oxygen atoms. It is also clear that there are some significant differences between results achieved in this study and results produced by earlier workers12,13 and that further work will be needed to determine why these differences occur. Equally, the question of whether other hydrocarbons would perform better than CH4, given that CH4 has the strongest H bond of any hydrocarbon, needs to be answered. Finally, because NO with its large quenching cross-section has been shown to reduce oxidation, a similar question must be raised for HCl, which has an even larger quenching cross-section. Thus, it would be helpful to know if this UV technique can be used successfully with high-Cl Canadian coal (0.3%) and other similar coals. (19) Granite, E. J. Private communication. U.S. Department of Energy (DOE), May 2008.
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