Energy & Fuels 2008, 22, 3029–3036
3029
Application of a Diode-Laser-Based Ultraviolet Absorption Sensor for in Situ Measurements of Atomic Mercury in Coal-Combustion Exhaust Jesse K. Magnuson,† Thomas N. Anderson,† Robert P. Lucht,*,† Udayasarathy A. Vijayasarathy,‡ Hyukjin Oh,‡ Kalyan Annamalai,‡ and Jerald A. Caton‡ School of Mechanical Engineering, Purdue UniVersity, West Lafayette, Indiana 47907-2088, and Department of Mechanical Engineering, Texas A&M UniVersity, College Station, Texas 77843-3123 ReceiVed May 19, 2008. ReVised Manuscript ReceiVed July 16, 2008
A diode-laser-based ultraviolet absorption sensor was successfully demonstrated for both in situ and extractive sampling atomic mercury measurements in a laboratory-scale 29.3 kWt (100 000 BTU/h) coal combustor and in situ measurements in a flow reactor at Texas A&M University. Laser sensor measurements were compared to measurements from a commercial mercury analyzer (CMA). A 375 nm single-mode laser and a 784 nm distributed feedback (DFB) laser are sum-frequency-mixed in a nonlinear β-barium borate crystal to generate a 254 nm beam. By tuning the frequency of the DFB laser, the ultraviolet beam frequency was tuned across the transition frequency of mercury at 253.7 nm. The tuning range was large enough that an off-resonant baseline was clearly visible on both sides of the Hg transition. No pretreatment is required for elemental mercury measurements, and the effects of broadband absorption can be effectively eliminated during data analysis. Extractive sampling was demonstrated to improve the detection limit of the sensor and demonstrate the feasibility of total mercury concentration measurements in the future through extractive sampling. Significant variation in the atomic mercury concentration of coal-combustion exhaust was observed over short time periods during our in situ measurements. The sensor detection limits for in situ and extractive sampling are 0.3 and 0.1 parts per billion over a 1 m path length, respectively.
Introduction According to the Environmental Protection Agency (EPA) website, over 40% of mercury emissions released in the United States originate from coal-burning power plants. This amounts to approximately 50 tons each year.1 Because of the effects of high levels of mercury exposure on humans and environmental health, the EPA has issued the Clean Air Mercury Rule. In view of this ruling and others like it, a need has arisen to continuously and accurately measure elemental and total mercury concentrations in coal combustion flue gas.2 Three forms of mercury are present in coal flue gas: elemental (Hg0), oxidized (Hg2+), and particulate bound. Particulate bound and oxidized mercury are efficiently removed from coal exhaust through air-cleaning technologies, such as fabric filters, electrostatic precipitators (ESPs), and wet flue gas desulfurization (FGD) systems.3-7 However, elemental mercury is not easily removed4 and has a lifetime of 1-2 years in the atmosphere, * To whom correspondence should be addressed. Telephone: 765-4945623. Fax: 765-494-0539. E-mail:
[email protected]. † Purdue University. ‡ Texas A&M University. (1) U.S. Environmental Protection Agency (EPA). www.epa.gov/ mercury, 2006. (2) Sondreal, E. A.; Benson, S. A.; Pavlish, J. H.; Ralston, N. V. C. Fuel Process. Technol. 2004, 85, 425–440. (3) O’Dowd, W. J.; Hargis, R. A.; Granite, E. J.; Pennline, H. W. Fuel Process. Technol. 2004, 85, 533–548. (4) Kellie, S.; Duan, Y. F.; Cao, Y.; Chu, P.; Mehta, A.; Carty, R.; Liu, K. L.; Pan, W. P.; Riley, J. T. Fuel Process. Technol. 2004, 85, 487–499. (5) Senior, C. L.; Helble, J. J.; Sarofim, A. F. Fuel Process. Technol. 2000, 65-66, 263–288. (6) Romero, C. E.; Li, Y.; Bilirgen, H.; Sarunac, N.; Levy, E. K. Fuel 2006, 85, 204–212.
as compared to HgO with a lifetime of only a few days.2,8 Because elemental mercury is more difficult to remove and has a longer lifetime, it is the form of mercury of greatest concern. Several methods have been applied to measure mercury concentrations, both elemental and total, from coal-combustion exhaust. The three most widely used methods are wet chemistry, atomic absorption spectroscopy (AAS), and atomic fluorescence spectroscopy (AFS).9 All of these methods have been demonstrated to work effectively in some conditions,2,7 but there are several issues common to these methods that diode-laser-based sensors are able to overcome. These issues include the necessity of sampling, pretreatment of the sampled gas, and for some sensors, excessive amounts of sample time. The location or shape of the probe can create a bias in mercury concentration measurements. Mercury can adsorb to the sampling probe, line connectors, or lines themselves,10,11 and heterogeneous mercury reactions, catalyzed (7) Brown, T. D.; Smith, D. N.; Hargis, R. A., Jr.; O’Dowd, W. J. J. Air Waste Manage. Assoc. 1999, 49, 628–640. (8) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Fuel Process. Technol. 2003, 82, 89– 165. (9) Anderson, T. N.; Lucht, R. P. Development of a diode-laser-based ultraviolet absorption sensor for real-time, in situ measurements of atomic mercury. In Proceedings of the 22nd Annual International Pittsburgh Coal Conference, Pittsburgh, PA, September 12-15, 2005; Paper 17-2. (10) Granite, E. J. Obstacles in the development of mercury continuous emissions monitors. In Proceedings of the 20th Annual International Pittsburgh Coal Conference, Pittsburgh, Pennsylvania, September 15-19, 2003; Paper 30-1. (11) Laudal, D. L.; Thompson, J. S.; Pavlish, J. H.; Brickett, L. A.; Chu, P. Fuel Process. Technol. 2004, 85, 501–511.
10.1021/ef800372k CCC: $40.75 2008 American Chemical Society Published on Web 08/16/2008
3030 Energy & Fuels, Vol. 22, No. 5, 2008
Magnuson et al.
Figure 1. Schematic diagram of the diode-laser-based absorption sensor for atomic mercury and the experimental system configured for in situ measurements in the laboratory-scale coal combustor at Texas A&M University.
by the walls, can occur in sampling lines, producing a bias.12,13 After extraction, the sample is often pretreated to remove particulates and broadband absorbing species, such as SO2, NOx, and HCl.8 Failure to remove these interferents can result in skewed data or a damaged system.9,10 Some methods of monitoring mercury, especially those involving wet chemistry, tend to be slow, giving essentially no time resolution.11,14 Diode-laser-based sensors can overcome each of these issues.15-18 The sensors can be configured to acquire data in situ and remove the need for sampling. Diode-laser-based sensors eliminate the need for pretreatment by using a narrowbandwidth source with absorption spectroscopy. Using a wide laser-wavelength scan range, the off-resonant baseline can be established accurately on both sides of the mercury absorption spectral feature. The off-resonant baseline allows us to discriminate between mercury absorption and attenuation by other species or particles, thereby enabling in situ measurements even in media with significant broadband absorption or scattering. Also, diode-laser-based sensors can be used to make measurements very quickly, providing good time resolution. In this paper, we demonstrate data acquisition times of 10 s in the exhaust stream of laboratory-scale coal combustor. An in situ diode-laser-based continuous emissions monitor (CEM) was used to measure atomic mercury in the exhaust of a laboratory-scale 29.3 kWt (100 000 BTU/h) coal combustor located at Texas A&M University. Measurements were demonstrated both in situ and with extractive sampling. Extractive sampling was used to improve the detection limit and demonstrate the feasibility for total mercury measurements via extractive sampling. Concentration measurements were performed without sample pretreatment to remove broadband absorbing species, such as SO2 and NOx. Although broadband absorption was very strong, the effects of the broadband absorption were corrected and minimized in the data analysis, as will be discussed. Concentration measurements obtained using the (12) Ariya, P. A.; Khalizov, A.; Gidas, A. J. Phys. Chem. A 2002, 106, 7310–7320. (13) Hall, B.; Lindqvist, O.; Ljungstrom, E. EnViron. Sci. Technol. 1990, 24, 108–111. (14) Hall, B.; Schager, P.; Lindqvist, O. Water, Air, Soil Pollut. 1991, 56, 3–14. (15) Anderson, T. N.; Lucht, R. P.; Barron-Jimenez, R.; Hanna, S. F.; Caton, J. A.; Walther, T.; Roy, S.; Brown, M. S.; Gord, J. R.; Critchley, I.; Flamand, L. Appl. Opt. 2005, 44, 1491–1502. (16) Anderson, T. N.; Lucht, R. P.; Meyer, T. R.; Roy, S.; Gord, J. R. Opt. Lett. 2005, 30, 1321–1323. (17) Meyer, T. R.; Roy, S.; Anderson, T. N.; Lucht, R. P.; BarronJimenez, R.; Gord, J. R. Opt. Lett. 2005, 30, 3087–3089. (18) Allen, M. G. Meas. Sci. Technol. 1998, 9, 545–562.
diode-laser-based sensor were compared to results obtained using a commercial mercury analyzer (CMA). Experimental Section Laser System. The diode-laser-based mercury sensor is depicted schematically in Figure 1. Ultraviolet (UV) radiation at 253.7 nm is generated by sum-frequency mixing the outputs of a 375 nm single-mode diode laser (CrystaLaser) and a 784 nm distributed feedback (DFB) diode laser (Sacher Lasertechnik) in a β-barium borate (BBO) crystal. The lasers and optics for the atomic mercury sensor were mounted on an 18 × 18 in. aluminum breadboard to facilitate transport to the combustor. To optimize the amount of UV radiation generated in the sumfrequency-mixing process, the polarization of the 784 nm beam was rotated to vertical by a zero-order half-wave plate to match the vertical polarization of the 375 nm beam. The beams were overlapped on a dichroic mirror and focused into the BBO crystal. The BBO crystal was mounted on a rotary stage mounted on a translation stage, so that the crystal could be centered on the focal spot and rotated to maximize UV generation. The UV radiation generated in the BBO crystal was collimated using a fused silica lens. This lens was also mounted on a translation stage to optimize the collimation of the generated UV beam. The collimated UV radiation was reflected twice with mirrors coated to reflect 254 nm radiation, thus reducing the residual 784 and 375 nm beams. The UV beam was then split into two beams using a 70/30 beamsplitter. The 30% reflection served as the reference beam and was directed through a neutral density filter and narrowband interference filter (254 nm center wavelength, 10 nm fwhm) onto a solar-blind photomultiplier tube (PMT; Hamamatsu, Corp.). The optical density of the neutral density filter was selected to allow for the maximum number of photons to reach the PMT without saturating. The beam that was transmitted through the beamsplitter served as the signal beam and was directed through the test region and onto a second PMT after passing through similar optical filters. After successful generation of UV radiation, absorption spectra of atomic Hg were acquired. The wavelengths of both lasers were first tuned, so that the generated UV radiation was in resonance with the atomic Hg transition at 253.726 nm (vacuum). Center wavelengths were set to 375.1290 nm (vacuum) and 784.0183 nm (vacuum) for the single-mode and DFB lasers, respectively. To scan across the transition, the DFB laser wavelength was tuned by modulating the injection current with a triangle-wave function. Modulation was achieved by applying a 1.2 V (amplitude) trianglewave voltage ramp to an external input on the DFB laser controller. The frequency of the DFB laser was monitored during the scans using a spectrum analyzer (EXFO Burleigh, Inc.), with a free spectral range of 2 GHz. The voltage ramp is generated with a personal computer and an analog input/output card (National Instruments, Inc.), which is also used to simultaneously digitize and record the voltages from the PMTs. The PMT signals are measured across 10 kΩ resistors and then filtered with low-noise
Atomic Mercury in Coal-Combustion Exhaust
Figure 2. Coal combustor and test section for atomic mercury measurements at Texas A&M University. The coal is transported with primary air and fired with preheated secondary air.
preamplifiers (Stanford Research Systems) before digitization with the input/output card. A mode-hop-free tuning range of 86 GHz was obtained with the DFB laser. Such a large scanning range allows for accurate determination of the off-resonant baseline to simplify interpretation of the data and account for absorption by particulates and other interfering species, such as SO2 or O3. Both diode laser control and data acquisition are performed under computer control; the sensor can easily be incorporated into a mercury emissions monitor. For more detail concerning this diode-laser-based atomic mercury absorption sensor, see Anderson et al.19 Coal-Fired Boiler Burner. A schematic diagram of the coalfired combustor (29.3 kWt) at Texas A&M University is shown in Figure 2. The down-fired reactor 15 cm in diameter is constructed of eight modular refractory sections of inside dimension 152.40 mm (6 in.) made of a steel frame containing a 50.8 mm (2 in.) layer of insulation and 50.8 mm section of refractory. Coal was transported to the burner using primary air and fired with preheated secondary air in a swirl burner at the top of the combustor. Coal was supplied at rates of 97 g/min for Wyoming sub-bituminous, 123 g/min for Texas lignite, and 124 g/min for the coal/biomass blend (a mixture of 80% Texas lignite and 20% feedlot biomass). Because the heating values are lower for lignite and the blends, the differing flow rates of fuel were used to maintain similar thermal output. The total air flow rate was varied to change the combustion stoichiometry. More detailed information on the 30 kWt coal-fired boiler burner may be obtained from Frazzitta et al.20 and Annamalai et al.21 The coal exhaust flowed through 9 m of insulated duct work before reaching the test section. Measurements were acquired at this location in either the in situ mode or the extractive sampling mode. A thermocouple was inserted into the exhaust flow 1 m upstream from the test section to provide an accurate temperature for in situ mercury analysis, as shown in Figure 2. The exhaust gas was released into the atmosphere after flowing through this test section. To prevent the particulates and hot gas escaping into the atmosphere and to collect ash and unburnt combustibles, a water quench was sometimes used to cool the exhaust gases before they exited the combustor. Water was sprayed through the exhaust gases, formed a pool in the bottom of the combustor, and was drained through the water outlet. Although unnecessary for operation of the combustor, the water quench had a significant effect on the concentration of elemental mercury in the exhaust. The water quench also drastically affected the temperature of the exhaust. The temperature of the exhaust in the duct decreased 150 °C when the water quench was turned on. Because of this large (19) Anderson, T. N.; Magnuson, J. K.; Lucht, R. P. Appl. Phys. B: Laser Opt. 2007, 87, 341–353. (20) Frazzitta, S.; Annamalai, K.; Sweeten, J. J. Propul. Power 1999, 15, 181–186. (21) Annamalai, K.; Thien, B.; Sweeten, J. Fuel 2003, 82, 1183–1193.
Energy & Fuels, Vol. 22, No. 5, 2008 3031 variation, scans were taken as the system began to reach steady state, which was 90-110 °C with the water quench and 220-240 °C without the water quench. In Situ Sampling. The configuration for in situ measurements with the diode-laser sensor is shown in Figure 1. The signal beam was aligned with the fused silica windows to pass through the exhaust gases twice by reflecting off a 254 nm, 0° mirror. Then, via a 45° mirror, the returning beam was directed through a neutral density filter, a narrowband interference filter (centered at 254 nm), and onto the solar-blind signal PMT. Because of the large amount of ash in the exhaust, air was continuously blown on the fused silica windows that provided optical access through the exhaust duct. The air purge is shown in Figure 1 on either side of the exhaust duct. Over the course of several hours of experiments, ash built up on the windows despite the air purge. To reduce etalon effects and fluctuations in PMT response, air scans were performed at the beginning and end of each of the coal combustion cases shown in Table 1. Air scans were laser frequency scans where no mercury was present in the test section; the use of the air scans in the data analysis procedure is described in the Results and Discussion. For the in situ case, we stopped the flow of coal to the burner and waited until the mercury signal disappeared completely before we acquired the air scans. Extractive-Sampling-Based Absorption Cell. To increase the absorption path length and to eliminate particulate broadband attenuation, we built an optically accessible quartz absorption cell. The cell was 2.4 cm in diameter and 105.4 cm long, and the fused silica windows at the ends of the cell are set at 10° to eliminate etalon effects. A schematic of this system configuration is shown in Figure 3. Exhaust was extracted from a stainless-steel sampling probe in the center of the exhaust duct at the test section. We used this same location and probe to draw samples for a commercial mercury analyzer. After being drawn through a paper filter, the exhaust was pulled through the absorption cell and out into the atmosphere with a vacuum pump at a rate of 1 standard cubic foot per hour (SCFH). The flow rate was measured with a rotameter located after the absorption cell. The sampling probe was connected to the absorption cell with R-3603 Tygon tubing to reduce adsorption of mercury onto the walls of the lines. The pressure in the absorption cell was verified to be at atmospheric pressure (100 kPa). The system was configured so that the UV beam passed once through the absorption cell and was collected by the signal PMT. This configuration allowed us to easily acquire air scans by disconnecting the sampling line from the probe. Flow Reactor. A schematic diagram of the flow reactor system is shown in Figure 4. Simulated flue gas for the flow reactor system was generated using a gas mixing system. The mixing system incorporated three standard gas cylinders: 50% oxygen and 50% nitrogen, 50 ppm HCl with nitrogen balance, and nitrogen. Flow controllers (MKS type 1179A, MKS Instruments) maintained species concentrations in the simulated flue gas. The flow controllers are accurate to within 1% of the full-scale reading, according to MKS Instruments. For data reported in this paper, only nitrogen was used. The balance nitrogen flow was accurate within (0.01 L/min, and the mercury generator flow within (0.002 L/min. The mercury generator was used to add elemental Hg to a 0.2 L/min N2 flow, which was diluted by a N2 flow to produce a total flow rate of 1.1 L/min for all cases, except air scans, where only nitrogen was in the flow reactor and the flow controller would only allow up to 1.0 L/min. The mercury generator included a certified VICI Metronics elemental mercury permeation tube in a heated quartz tube. The quartz tube could be heated to 100 °C to produce 1746 ng/min ( 2% of elemental mercury. If the temperature of the permeation tube is changed by 1 °C, the elemental mercury production rate changes by 10%, increasing with increasing temperature. A temperature controller (BT-15 Series, HTS/Amptek) and a flexible electric heating tape were used to maintain the temperature. The controller monitors the temperature nitrogen carrier gas flow at either end of the mercury generator. The
3032 Energy & Fuels, Vol. 22, No. 5, 2008
Magnuson et al.
Table 1. Summary of Results for Mercury Concentrations in Parts Per Billion (ppb), Measured with the Diode-Laser-Based Absorption Sensor and a Commercial Mercury Analyzera
Wyoming sub-bituminous Texas lignite 80% Texas lignite 20% feedlot biomass a
in situ in situ absorption cell mercury analyzer absorption cell mercury analyzer
Φ ) 1.0
5% excess
10% excess
water quench
water quench
water quench
on
off
on
off
