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Energy & Fuels 2003, 17, 1028-1033
Arsenic and Mercury Partitioning in Fly Ash at a Kentucky Power Plant Tanaporn Sakulpitakphon,† James C. Hower,* Alan S. Trimble, William H. Schram, and Gerald A. Thomas Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, Kentucky 40511 Received January 9, 2003. Revised Manuscript Received April 30, 2003
Coal and fly ash samples were collected from a 500-MW unit at a Kentucky power plant, with the objective of studying the distribution of arsenic, mercury, and other trace elements in fly ash. The coal feed was low-sulfur, high volatile A bituminous central West Virginia coal. The plant produced a relatively low-carbon fly ash. In contrast to power plants with high-mercury feed coal, the fly ashes from the lower-mercury feed coal had low mercury values, generally not exceeding 0.01 ppm Hg. Mercury capture by fly ash varies with both the amount and type of carbon and the collection temperature; mercury capture is more efficient at lower temperatures. Arsenic in the feed coal and in the flue gas is of concern to the utility, because of the potential for catalyst poisoning in the selective catalytic reduction system (in the planning stage at the time of the sampling). Arsenic is captured in the fly ash, increasing in concentration in the moredistant (from the boiler) reaches of the electrostatic precipitator system.
Introduction Trace elements in coal are of interest for a variety of reasons. Several elements, including arsenic, lead, and mercury, have been considered to be potentially toxic components in emissions from power plants. In addition, the presence of certain elements, such as arsenic, can have implications in the operation of pollution-control devices. Recent findings by the U.S. Environmental Protection Agency (EPA) suggest that mercury emissions may be regulated in the coming years, possibly with draft regulations released in late 2003.1,2 Hower and coworkers3,4 and Sakulpitakphon et al.5 studied the capture of mercury by fly ash in Kentucky power plants. Overall, and corresponding with the observations of other researchers, they found that the mercury concentration increased as the temperature decreased and the amount of fly ash carbon increased. Hower et al.,6 in a study of fly ash carbons that had been concentrated by density gradient centrifugation, found that mercury concentration and surface area7 increased from iner* Author to whom correspondence should be addressed. E-mail:
[email protected]. † Now with Department of Geology, University of New Orleans, New Orleans, LA. (1) United States Environmental Protection Agency. http:// yosemite.epa.gov/opa/admpress.nsf/b1ab9f485b098972852562e7004dc686/ cd30963685856f30852569b5005ee740?OpenDocument (accessed Nov. 2002). (2) United States Environmental Protection Agency. http://www.epa.gov/ttn/atw/combust/utiltox/utoxpg.html (accessed Nov. 2002). (3) Hower, J. C.; Trimble, A. S.; Eble, C. F.; Palmer, C.; Kolker, A. Energy Sources 1999, 21, 511-525. (4) Hower, J. C.; Finkelman, R. B.; Rathbone, R. F.; Goodman, J. Energy Fuels 2000, 14, 212-216. (5) Sakulpitakphon, T.; Hower, J. C.; Trimble, A. S.; Schram, W. H.; Thomas, G. A. Energy Fuels 2000, 14, 727-733. (6) Hower, J. C.; Maroto-Valer, M. M.; Taulbee, D. N.; Sakulpitkakphon, T. Energy Fuels 2000, 14, 224-226.
tinite through isotropic coke to anisotropic coke. Of course, mercury is not going to be concentrated in the fly ash if it is not present in significant quantities in the feed coal. Aside from its potential to be a hazardous air pollutant if emitted to the atmosphere, arsenic, in the form of gaseous arsenic oxide (As2O3), is poisonous for selective catalytic reduction (SCR) catalyst; therefore, its use is detrimental to the operation of SCR units for NOx control.8 As2O3 solidifies on both active and nonactive sites in the vanadium-based catalysts that are used in SCRs, which reduces the activity of the catalyst.9 Both the mercury and arsenic concentrations in coal and the capture of the elements by fly ash were of interest to the utility. In this study, we investigate the partitioning of selected trace elements, with an emphasis on arsenic and mercury for a 500-MW unit that is burning a blend of central West Virginia coals. Speciation of arsenic in the flue gas, which is of interest for determination of the potential for catalyst poisoning, is beyond the scope of this study. Instead, we will focus on the concentration of the elements in the solid phase. Procedure Coal and fly ash were collected in March 2000 from a 500MW wall-fired unit that was burning a blend of high volatile A bituminous central Appalachian coal. The fly ash system (7) Maroto-Valer, M. M.; Taulbee, D. N.; Hower, J. C. Fuel 2001, 80, 795-800. (8) Nalbandian, H.; Carpenter, A. M. Prospects for Upgrading CoalFired Power Plants; Reference No. CCC/41; International Energy Agency Coal Research: London, 2000. (9) Soud, H. N.; Fukasawa, K. Developments in NOx Abatement and Control; Reference No. IEACR/89; International Energy Agency Coal Research: London, 1996.
10.1021/ef030001n CCC: $25.00 © 2003 American Chemical Society Published on Web 06/05/2003
Arsenic and Mercury Partitioning in Fly Ash
Energy & Fuels, Vol. 17, No. 4, 2003 1029
Table 1. As-Received Moisture, Ash, Total Sulfur (St), and Sulfur Forms (Pyritic, Spy; Sulfate, Ssulf; Organic, Sorg) Contents and Heating Value (Moist, Mineral-Matter Free) for Feed and Pulverized Coala Sulfur content
Heating Value
sample type
sample
ash
moisture
St
Spy
Ssulf
Sorg
BTU/lb
MJ/kg
plant feed coal feeder A coal feeder B coal feeder C coal feeder D coal feeder E pulverizer
92679 92680 92681 92683 92684 92682 92685
10.96 11.25 12.52 10.10 9.93 10.96 13.74
4.20 5.22 3.94 4.98 4.05 4.30 2.88
0.81 0.72 0.72 0.73 0.71 0.72 0.70
0.14 0.12 0.13 0.11 0.11 0.13 0.13
0.01 0.02 0.01 0.01 0.01 0.02 95 vol % of macerals) or a vitrinite-poor trimacerite microlithotype, imply particularly tough (low Hardgrove grindability index) coals. Fly Ash. The fly ash composition is dominated by glass (Table 4), which is expected for a coal with the (10) Hower, J. C.; Rathbone, R. F.; Graham, U. M.; Groppo, J. G.; Brooks, S. M.; Robl, T. L.; Medina, S. S. Proceedings of the 11th International Coal Testing Conference, Lexington, KY, May 10-12, 1995; pp 49-54. (11) Bragg, L. J.; Oman, J. K.; Tewalt, S. J.; Oman, C. L.; Rega, N. H.; Washington, P. M.; Finkelman, R. B. Open-File Report 97-134, U.S. Geological Survey Coal Quality (COALQUAL) Database, Version 2.0 [CD-ROM]; U.S. Geological Survey: Reston, VA, 1998.
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Sakulpitakphon et al.
Table 3. Maceral Content (and Total, Undifferentiated Mineral Content) and Vitrinite Maximum Reflectance of Coal Feeder, Pulverized, and Screened Pulverized Coala type
sample
sizeb
plant feed coal feeder A coal feeder B coal feeder C coal feeder D coal feeder E pulverizer
92679 92680 92681 92682 92683 92684 92685
whole coal whole coal whole coal whole coal whole coal whole coal whole coal +100 100 × 200 200 × 325 325 × 500 -500
wt %
moisture
ash
sulfur
Vit
Fus
Sfs
Mic
Mac
Lip
Min
Rmax
s.d.
10.96 11.25 12.52 10.10 9.93 10.96 13.74 7.95 7.45 7.08 7.20 18.75
0.81 0.72 0.72 0.73 0.71 0.72 0.70 0.78 0.78 0.81 0.83 0.61
52.8 60.8 55.4 58.4 58.6 47.0 64.4 48.0 58.4 69.6 72.8 70.8
12.6 13.0.8 15.8 11.0 12.8 13.0 13.0 13.0 10.2 10.8 12.8 14.8
9.8 6.0 8.2 8.2 6.8 19.0 6.2 9.2 7.2 5.2 4.2 8.4
9.8 5.6 7.0 10.2 8.8 7.4 5.6 13.8 8.8 5.0 3.0 1.2
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
11.4 12.2 11.8 10.0 10.8 11.2 7.4 14.8 13.2 8.2 6.2 3.4
3.6 1.6 1.8 2.2 2.2 2.4 3.4 1.2 2.2 1.2 1.0 1.4
0.86 0.87 0.86 0.86 0.86 0.87 0.86
0.06 0.07 0.08 0.06 0.09 0.06 0.07
5.0 9.9 12.3 15.3 57.5
4.20 5.22 3.94 4.98 4.05 4.30 2.88 2.88 3.24 3.50 3.36 3.33
a Date of data collection: March 15, 2000. Note: Vit, vitrinite; Fus, fusinite; Sfs, semifusinite; Mic, micrinite; Mac, macrinite; Lip, exinite (sporinite and cutinite; resinite not detected in any samples); Min, mineral matter; Rmax, vitrinite maximum reflectance (50× oil-immersion objective, 546 nm filter); and s.d., standard deviation of reflectance. b Coal size is reported as the mesh size.
Table 4. Fly Ash Petrographya row
1 2 3 4
1 2 3 4
glass
mullite
spinel
quartz
isotropic coke
anisotropic coke
inertinite
3.9 2.3
2.7 1.3
0.6 0.2
av. s.d.
91.1 4.1
0.1 0.1
1.3 0.3
Economizer Side 0.3 0.1
av. s.d. av. s.d. av. s.d. av. s.d.
94.3 1.5 94.9 1.1 94.4 4.6 90.8 0.8
0.2 0.2 0.1 0.1 0.0 0.0 0.0 0.0
1.0 0.2 0.4 0.2 1.7 1.3 3.6 0.2
ESP side A 0.2 0.2 0.0 0.0 0.1 0.1 0.0 0.0
2.7 0.5 2.5 0.5 2.6 2.2 3.7 0.1
1.6 0.8 1.8 0.5 1.2 1.0 1.9 0.5
0.2 0.1 0.5 0.2 0.0 0.0 0.0 0.0
av. s.d. av. s.d. av. s.d. av. s.d.
88.0 0.6 81.7 1.4 88.6 2.2 95.8 2.3
0.2 0.2 0.1 0.1 0.1 0.1 0.0 0.0
2.5 1.0 1.9 0.4 1.5 0.3 1.2 0.8
ESP side B 0.8 0.5 0.6 0.2 1.3 2.3 0.0 0.0
3.6 0.4 7.4 1.6 5.2 1.4 2.0 0.5
4.6 1.5 7.6 1.6 3.3 1.9 0.6 0.3
0.5 0.3 0.7 0.3 0.3 0.3 0.1 0.1
a Date of data collection: March 15, 2000. Average data value is represented by “av.”, and the standard deviation of the data is represented by “s.d.”.
mineral content that is dominated by clay minerals. The low spinel content reflects the low pyrite content of the feed coal. Examination of sized fractions of the fly ash, not shown, demonstrated that fly ash carbon was concentrated in the coarser fractions. Fly ash carbon, particularly for the March 2000 sampling, is remarkably asymmetric (Table 5), with the B-side hoppers having a larger amount of carbon. Hower et al.12 previously noted the same trend for the September 1999 sampling. In the latter case, however, there was a more pronounced asymmetry across each row. Still, for the March 2000 sampling, the second and third rows on the B side do have larger amounts of carbon on the right-hand (higher-bin-number) side. Detailed sampling, consisting of samples of multiple ESP hoppers on each of multiple rows, at other power plants has demonstrated that asymmetry in the distribution of carbon, and other fly ash entities, is common. A single fly ash sample of a plant, although arguably representative of the potential ash product, does not provide the picture of the variation in fly ash quality present in an (12) Hower, J. C.; Trimble, A. S.; Eble, C. F. Fuel Process. Technol. 2001, 73, 37-58.
array of ESP hoppers, which is a variation that could potentially be exploited with enhanced utilization schemes. Each row of the ESP collects ∼80% of the fly ash in the flue gas. The “80% rule” is a common assumption in the utility industry. Actual collection efficiency will vary according to many variables, such as combustion conditions and ash amount and type. The contributions of the later rows are, therefore, not as significant as the first row. This is an important consideration, with respect to the capture of trace elements on the fly ash. Although zinc, nickel, and arsenic are present in greater amounts in the fourth row, compared to that in the first row, simply comparing the absolute concentrations of the elements without any consideration of the declining ash collection in back rows of the ESP array is deceptive. Using arsenic as an example and applying the aforementioned “80% capture rule-of-thumb”, Figure 2 demonstrates that the cumulative amount of arsenic captured does not differ significantly from the feed coal arsenic content: 45 ppm (ash basis), which is the average of all coal samples collected (plant feed, five coal feeders, pulverized coal). The fly ash arsenic content
Arsenic and Mercury Partitioning in Fly Ash
Energy & Fuels, Vol. 17, No. 4, 2003 1031
Table 5. Fly Ash Chemical Compositiona side
sample
C
H
N
S
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
P2O5
TiO2
SO3
economizer 2 economizer 4
92686 92687 av. s.d.
3.95 3.12 3.54 0.42
0.01 0.03 0.02 0.01
