Energy & Fuels 2000, 14, 727-733
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Mercury Capture by Fly Ash: Study of the Combustion of a High-Mercury Coal at a Utility Boiler Tanaporn Sakulpitakphon,† James C. Hower,* Alan S. Trimble, William H. Schram, and Gerald A. Thomas University of Kentucky Center for Applied Energy Research, 2540 Research Park Drive, Lexington, Kentucky 40511 Received January 11, 2000. Revised Manuscript Received March 22, 2000
Mercury capture on fly ash was studied at a coal-fired utility boiler burning a single-mine source high-Hg Appalachian coal. Other elements were also studied incidental to the Hg study. Fly ash was collected through a cross-section of the ash collection system, representing ash captured at different flue gas temperatures. In general, Hg concentration in the fly ash increases with an increase in the fly ash carbon and a decrease in flue gas temperature. The relationship between the latter two parameters and the Hg content was not as well defined as in previous studies where the petrographic forms of the carbon could be isolated.
Introduction Mercury in coal and in emissions from coal-fired boilers is a topic of continued interest in the United States. The U.S. Environmental Protection Agency, using its authority under section 114 of the Clean Air Act amendments, is undertaking a three-phase investigation of U.S. coal-fired utilities.1 Part I, completed on 4 January 1999, involved the collection of general information on utility steam generating units. In Part II, U.S. Environmental Protection Agency is requiring utilities to report the Hg and Cl content of every sixth shipment of coal, with a minimum of three sets of analyses per month. Part III involves the collection of speciated Hg emissions data from more than 80 units at 75 selected utility sources representing a variety of coal sources, firing systems, and pollution control schemes. Part III testing is to be conducted at the inlet and outlet of the last pollution control device. Fly ash carbons have proven to be efficient collectors of Hg which would otherwise be emitted to the atmosphere. Bench-scale studies by Hassett and Eylands,2 Miller et al.,3 and Gibb and Clarke4 have noted a relationship between Hg capture and gas temperature in a variety of fly ashes. From descriptions of fly ash samples used by Hassett and Eylands,2 it is obvious that their samples contained, at least, a mixture of inertinite and coked carbon. In previous studies at the Center for Applied Energy Research, Hower et al.5 studied fly ashes collected from an electrostatic precipitator at a * Corresponding author. Phone: 859-257-0261. Fax: 859-257-0360. E-mail:
[email protected]. † Also, University of Kentucky, Department of Geological Sciences, Lexington, KY 40506. (1) U.S. Environmental Protection Agency. 1999, . (2) Hassett, D. J.; Eylands, K. E. Fuel 1999, 243. (3) Miller, S. J.; Dunham, G. E.; Olson, E. S.; Brown, T. D. Mercury trace elements, and particulate matter, 1-4 December 1998, McLean, VA, 1998. (4) Gibb, W. H.; Clarke, F. Mercury, trace elements, and particulate matter, 1-4 December 1998, McLean, VA, 1998.
utility unit burning a blend of high-sulfur, predominantly western Kentucky coals. They noted a significant relationship (r2 ) 0.98) between Hg and fly ash carbon. The carbon was dominated by isotropic coke. Hower et al.6 examined Hg capture by fly ash from mechanical and baghouse hoppers in two identical units burning central Appalachian coal. Inlet gas temperature and outlet temperature to the mechanical separators were 364 °C and 172 °C, respectively. Mercury capture on the fly ash proved to be a function of the amount of carbon in the fly ash within each collection system and of the collection temperature of the individual systems, the cooler baghouse collection system having more Hg in the fly ash despite a lower carbon content. Hower et al.7 noted an increase in Hg capture related to the BETsurface area (surface area measurements after Taulbee et al.8 and Maroto-Valer et al.9) in the order inertinite < isotropic coke < anisotropic coke (fly ash petrography after Hower et al.10). In the current study, we have the opportunity to examine the behavior of a single-mine feed coal in a 100 MW wall-fired boiler with cool-side electrostatic precipitators as the final stage of fly ash collection. Singlesource coals are rare among utility boilers in the eastern United States, and, indeed, single-source coal is not the typical fuel source for the study boiler. The coal source, a mine supplying the Manchester coal from Clay County, Kentucky, was known from utility monitoring of Hg content of fuel shipments to have a high Hg (5) Hower, J. C.; Trimble, A. S.; Eble, C. F.; Palmer, C. A.; Kolker, A. Energy Sources 1999, 21, 511. (6) Hower, J. C.; Finkelman, R. B.; Rathbone, R. F.; Goodman, J. Energy Fuels, submitted. (7) Hower, J. C.; Maroto-Valer, M. M.; Taulbee, D. N.; Sakulpitakphon, T. Energy Fuels, submitted. (8) Maroto-Valer, M. M.; Taulbee, D. N.; Hower, J. C. Energy Fuels 1999, 13, 947-953. (9) Taulbee, D. N.; Maroto-Valer, M. M.; Hower, J. C. 1999 American Coal Ash Association Meeting, 1999, 1, 24/1-15. (10) Hower, J. C.; Rathbone, R. F.; Graham, U. M.; Groppo, J. G.; Brooks, S. M.; Robl, T. L.; Medina, S. S. International Coal Testing Conference, 10-12 May, 1995, 11th, Lexington, KY, 1995, 49.
10.1021/ef000006+ CCC: $19.00 © 2000 American Chemical Society Published on Web 04/18/2000
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Sakulpitakphon et al.
Table 1. Moisture, Ash, Sulfur, Maceral Content, Microlithotype Content, and Vitrinite Maximum Reflectance and V-type Percentages for Feed Coal and Pulverized Coal Samplesa macerals and minerals (vol %)a sample 92634 92635
size 3A belt pulv 1
02737 92637
whole whole +100 100 × 200 200 × 325 325 × 500 -500 whole whole
wt (g)
32.74 167.56 80.78 63.23 129.4
% of total
moist.
ash
sulfur
vit
fus
sfs
mic
mac
exn
res
min
6.91 35.37 17.05 13.35 27.32
6.14 2.74 1.81 2.06 2.80 1.95 2.01 2.61 2.68
8.63 9.05 4.66 6.46 6.75 7.76 16.08 9.48 8.90
1.66 1.78 0.98 0.71 1.90 2.05 1.85 1.68 1.86
77.4 83.2 75.0 80.2 83.2 82.8 79.8 85.8 80.6
7.2 5.6 7.4 6.4 8.0 7.8 11.6 4.8 5.4
6.0 4.0 4.0 4.0 4.2 5.2 5.2 3.2 3.8
0.2 0.0 0.0 0.4 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
8.2 5.6 13.0 8.2 4.2 3.6 1.6 5.2 8.8
0.0 0.0 0.0
1.0 1.6 0.6 0.8 0.4 0.6 1.8 1.0 1.4
microlithotypes (vol. %) monomaceritesb sample 92634 92635
92636 92637
3A belt pulv 1
bimaceritesc
vitrinite reflectance (oil, 456 nm) trimaceritesd
V-types
size
Vt
Lp
In
Cl
Du
Vi
Dc
Cd
Vl
Cm
Rmax
s.d.
v6
v7
v8
v9
whole whole +100 100 × 200 200 × 325 325 × 500 -500 whole whole
31.6 50.4 21.6 36.0 44.0 57.6 72.7 53.6 42.2
0.2 0.4 1.0 0.6 0.0 0.2 0.4 0.6 1.2
6.4 7.2 1.4 2.6 6.2 8.6 15.8 5.0 5.6
10.4 10.4 13.0 11.2 12.6 8.6 3.0 10.2 13.2
2.2 0.2 4.4 2.0 1.6 1.8 0.4 1.0 1.0
5.4 3.4 3.6 6.8 5.2 5.8 2.2 3.2 3.2
33.2 22.0 44.4 31.2 25.2 14.6 2.8 20.2 26.4
4.0 1.8 3.2 3.6 0.6 0.6 0.4 0.6 2.0
2.4 0.6 4.4 2.8 1.0 1.0 0.0 1.8 1.2
4.2 3.6 3.0 3.2 3.6 1.2 2.2 3.8 4.0
0.82 0.77
0.05 0.06
8
20 60
76 30
4 2
0.80 0.80
0.05 0.06
2
40 46
58 44
2 8
a Vit - vitrinite, Fus - fusinite, Sfs - semifusinite, Mic - micrinite, Mac - macrinite, Exn - exinite, Res - resinite, Min - undifferentiated mineral matter in organic microlithotypes. b Vt - vitrite (>95% vitrinite (V); volume percent in all cases), Lp - liptite (>95% liptinite (L)), In - inertite (>95% inertinite (I)). c Cl - clarite (>95% V + L, each >5%), Du - durite (>85% L + I, each >5%), Vi - vitrinertite (V + I >95%, each >5%), Dc - duroclarite (V > L, I; each >5%), Cd - clarodurite (I > V, L; each >5%). d Vl - vitrinertoliptite (L > V, I; each >5%), Cm - carbominerite (20% < silicates or carbonates < 60% or 5% < sulfides < 20%).
content, making it an ideal candidate for a study of Hg capture. The source mine was also sampled. Through that portion of the study, not reported here, we found that much of the Hg is concentrated near the top of the coal bed in a thin, high-S lithotype with 0.52 ppm Hg. We must emphasize that this study represents a rather extreme case, not typical of the coal feed, or the coal combustion byproducts, over an extended period. Previous studies either focused on relative concentrates of individual carbon forms 7 or on mixed carbons without the analysis of Hg in size fractions, 6 thus missing the opportunity to examine a another layer in the complexity of the trace element distribution. In this study, we examine a complete array of ash collection devices across a single unit and, for selected devices, examine the chemistry and petrography of size fractions of fly ash. The primary purpose of the study was the examination of the Hg partitioning through the ash collection devices, but the distribution of other elements, with As used as an example, is also noted. Procedure A stockpile of coal from a single-mine source, the Manchester coal bed from west of the town of Manchester, Clay County, Kentucky, was burned in East Kentucky Power Cooperative’s Cooper Station unit 1 on 19-20 May 1999. The unit was operated under steady conditions through the period of the burn. Coal, both raw feed (3A belt) and pulverized (1, 2, and 3 pulverizers), and fly ash collection was done on 20 May 1999, at all available points. Fly ash was emptied from the hoppers on at least a daily basis; therefore, fly ash collected on the second day should have been strictly from the single-mine coal feed. One of the pulverized coals and selected fly ashes were wet screened at 100, 200, 325, and 500 mesh (150, 75, 38, and 25 µm, respectively) in order to further resolve the chemistry and petrography of the samples.
Coal and fly ash analyses were performed at the Center for Applied Energy Research (CAER) following ASTM methods. Major oxides were determined on a Phillips AXS X-ray fluorescence unit. Minor and trace elements were determined by a variety of inductively and directly coupled plasma techniques. Mercury was analyzed at East Kentucky Power Cooperative’s laboratory on a LECO AMA254 Advanced Mercury Analyzer, an adsorption spectrophotometer system. Petrographic analyses of the fly ash were performed on epoxy-bound polished pellets using oil-immersion objectives at a final magnification of 625×. Sudan Black is added to the epoxy in order to better resolve the fly ash particles by the elimination of subsurface reflections. Categories identified follow the nomenclature outlined by Hower et al. 10
Results and Discussion Coal. As noted above, the coal was from a single mine. The coal is high volatile A bituminous rank, with a vitrinite maximum reflectance (oil immersion optics, 546 nm) of 0.82%, and has a moderate sulfur content (Table 1). Sizing of the product of pulverizer 1 showed that the coal feed, at least from that pulverizer, had less than 7 wt % exceeding 100 mesh (150 µm). The petrographic difference between the size fractions is illustrated subtly by the maceral percentages, note the general increase in vitrinite and decrease in exinite from the +100 mesh to the 325 × 500 mesh fractions. Differences are more striking when the microlithotype percentages are examined (microlithotypes defined in the caption to Table 1). The coarser fractions have a greater abundance of trimacerite microlithotypes, indicative of harder-to-grind coal particles, than the fine fractions. The -500 mesh fraction, in particular, is enriched in the relatively brittle monomacerites: vitrite and inertite. Overall, the size and microlithotype composition of the pulverized coal is a factor in the amount of carbon in the fly ash; finer, more vitrinite-rich coal
Combustion of a High-Mercury Coal
Energy & Fuels, Vol. 14, No. 3, 2000 729
Table 2. Major Oxides and Trace Elements for Feed Coal and Pulverized Coal Samples Major Oxides sample 92634 92635
9236 9237
size 3A belt pulv 1
+100 100 × 200 200 × 325 325 × 500 -500
% of total
ash
6.91 35.37 17.05 13.35 27.32
8.63 9.05 4.66 6.46 6.75 7.76 16.08 9.48 8.90
pulv 2 pulv3
sulfur Cl (dry) MgO Na2O Fe2O3 TiO2 1.66 1.78 0.98 0.71 1.90 2.05 1.85 1.68 1.87
0.30 0.20 0.22 0.17 0.18 0.24 0.13 0.27
SiO2
CaO K2O P2O5 Al2O3 SO3
0.14 0.24
1.41 0.97
19.97 20.02
1.24 1.16
41.48 1.04 2.08 42.22 1.11 2.15
0.12 0.08
21.45 0.71 21.85 0.55
0.12 0.16
1.01 1.40
17.87 20.99
1.24 1.17
45.85 1.04 2.29 40.80 1.19 2.07
0.07 0.07
22.53 0.58 20.61 0.73
Trace Elements sample 92634 92635
3A belt pulv 1
9236 9237
pulv 2 pulv 3
size
Cr
Mo
As
Cd
Pb
Se
Ni
Co
Mn
Cu
V
Hg (ppm)
+100 100 × 200 200 × 325 325 × 500 -500
246 208 347 213 285 315 220 226 260
91 78 96 87 102 102 90 86 93
892 760 784 725 989 1090 924 839 914
15 13 15 23 18 22 15 15 16
312 271 328 645 343 417 375 312 315
349 301 360 304 409 409 356 338 377
249 210 376 276 325 331 173 223 273
112 97 192 145 151 152 78 103 126
371 321 199 229 386 436 316 334 373
177 146 279 261 280 316 461 168 173
322 265 410 314 307 303 315 314 307
0.201 0.214 0.077 0.088 0.266 0.303 0.290 0.237 0.270
Figure 1. Map of the ash collection system. Electrostatic precipitators 7 and 8 could not be sampled. Figure 3. Size distribution of -500 mesh (25 µm) fly ash, expressed as volume percent passing the diameter in microns (as the log of the diameter).
Figure 4. Map of Hg distribution in whole fly ash samples. Figure 2. Cumulative carbon percent for the sized fly ashes.
generally leads to more efficient combustion. Central Appalachian, including Eastern Kentucky, coals, in general, have higher percentages of petrographically complex, harder-to-grind trimacerite microlithotypes than coals from other coal fields, leading to increased carbon in fly ash compared to the combustion of coals from outside of the region. Sulfur, in part as pyrite, is relatively, enriched in the -200 mesh fractions. The pyrite is dominantly in the form of
