2112
Energy & Fuels 2007, 21, 2112-2120
Characteristics of Fly Ashes from Full-Scale Coal-Fired Power Plants and Their Relationship to Mercury Adsorption Yongqi Lu,† Massoud Rostam-Abadi,*,†,‡ Ramsay Chang,*,§ Carl Richardson,| and Jennifer Paradis| Illinois State Geological SurVey, 615 East Peabody DriVe, Champaign, Illinois 61820, Department of CiVil and EnVironmental Engineering, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801, EPRI, 3412 HillView AVenue, Palo Alto, California 94303, and URS Corporation, 9400 Amberglen BouleVard, Austin, Texas 78729 ReceiVed March 22, 2007. ReVised Manuscript ReceiVed May 17, 2007
Nine fly ash samples were collected from the particulate collection devices (baghouse or electrostatic precipitator) of four full-scale pulverized coal (PC) utility boilers burning eastern bituminous coals (EB-PC ashes) and three cyclone utility boilers burning either Powder River Basin (PRB) coals or PRB blends (PRBCYC ashes). As-received fly ash samples were mechanically sieved to obtain six size fractions. Unburned carbon (UBC) content, mercury content, and Brunauer-Emmett-Teller (BET)-N2 surface areas of as-received fly ashes and their size fractions were measured. In addition, UBC particles were examined by scanning electron microscopy, high-resolution transmission microscopy, and thermogravimetry to obtain information on their surface morphology, structure, and oxidation reactivity. It was found that the UBC particles contained amorphous carbon, ribbon-shaped graphitic carbon, and highly ordered graphite structures. The mercury contents of the UBCs (Hg/UBC, in ppm) in raw ash samples were comparable to those of the UBC-enriched samples, indicating that mercury was mainly adsorbed on the UBC in fly ash. The UBC content decreased with a decreasing particle size range for all nine ashes. There was no correlation between the mercury and UBC contents of different size fractions of as-received ashes. The mercury content of the UBCs in each size fraction, however, generally increased with a decreasing particle size for the nine ashes. The mercury contents and surface areas of the UBCs in the PRB-CYC ashes were about 8 and 3 times higher than UBCs in the EB-PC ashes, respectively. It appeared that both the particle size and surface area of UBC could contribute to mercury capture. The particle size of the UBC in PRB-CYC ash and thus the external mass transfer was found to be the major factor impacting the mercury adsorption. Both the particle size and surface reactivity of the UBC in EB-PC ash, which generally had a lower carbon oxidation reactivity than the PRB-PC ashes, appeared to be important for the mercury adsorption.
Introduction Recent interpretations of data from several full-, pilot-, and bench-scale studies reveal that unburned carbon (UBC) in fly ash can both adsorb and oxidize mercury in coal combustion flue gases.1-6 These studies indicate that mercury capture and oxidation generally increase with an increasing concentration * To whom correspondence should be addressed. E-mail: massoud@ isgs.uiuc.edu (M.R.-A.) or
[email protected] (R.C.). † Illinois State Geological Survey. ‡ University of Illinois at Urbana Champaign. § Electric Power Research Institute (EPRI). | URS Corporation. (1) Rosenhoover, W. A.; Carty, R. Correlate Fly Ash Capture of Hg with Ash Carbon Content and Flue Gas Temperature. Technical Report for Illinois Clean Coal Institute, Carterville, IL, October 31, 1999. (2) Hassett, D. J.; Eylands, K. E. Mercury Capture on Coal Combustion Fly Ash. Fuel 1999, 78, 243-248. (3) Sakulpitakphon, T.; Hower, J. C.; Trimble, A. S.; Schram, W. H.; Thomas, G. A. Mercury Capture by Fly Ash: Study of the Combustion of a High-Mercury Coal at a Utility Boiler. Energy Fuels 2000, 14, 727733. (4) Gullett, B. K.; Ghorishi, B.; Jozewicz, W.; Ho, K. The Advantage of Illinois Coal for FGD Removal of Mercury. Technical Report for Illinois Clean Coal Institute, Carterville, IL, October 31, 2001. (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, 89165.
of UBC in fly ash. Several studies have attempted to correlate mercury adsorption capacity and oxidation reactivity of UBC with forms of carbon,7-8 Brunauer-Emmett-Teller (BET)N2 surface area,6 and the mass ratio of UBC to Hg in flue gas (UBC/Hg).8 Petrographic analyses of UBCs derived from burning bituminous and anthracitic coals reveal a general trend toward higher mercury retention in anisotropic carbon, followed by isotropic carbon and inertinite.7,9 Mercury adsorption may further be associated with the presence of various surface functional groups (O, S, and Cl) of the UBC. A recent study indicates that oxygen functional groups of UBC might promote mercury adsorption.10 In contrast, other studies have suggested that oxygen functional groups have no impact on the perfor(6) Dunham, G. E.; DeWall, R. A.; Senior, C. L. Fixed-Bed Studies of the Interactions Between Mercury and Coal Combustion Fly Ash. Fuel Process. Technol. 2003, 82, 197-213. (7) Hower, J. C.; Maroto-Valer, M. M.; Taulbee, D. N.; Sakulpitakphon, T. Mercury Capture by Distinct Fly Ash Carbon Forms. Energy Fuels 2000, 14, 224-226. (8) Senior, C. L.; Johnson, S. A. Impact of Carbon-in-Ash on Mercury Removal Across Particulate Control Devices in Coal-Fired Power Plants. Energy Fuels 2005, 19, 859-863. (9) Suarez-Ruiz, I.; Hower, J. C.; Thomas, G. A. Hg and Se Capture and Fly Ash Carbons from Combustion of Complex Pulverized Feed Blends Mainly of Anthracitic Coal Rank in Spanish Power Plants. Energy Fuels 2007, 21, 59-70.
10.1021/ef070145s CCC: $37.00 © 2007 American Chemical Society Published on Web 06/22/2007
Fly Ashes Relationship to Mercury Adsorption
Energy & Fuels, Vol. 21, No. 4, 2007 2113
Table 1. Power Plant Sources of Collected Fly Ash Samples particulate collector fly ash
boiler type
FA1b
PC F-fired (Riley)/ PC T-fired (CE) PC F-fired (Riley) PC F-fired (Riley) PC cyclone B&W7 cyclone (partial OFAd) B&W7 cyclone (partial OFA) cyclone (OFA) cyclone (LNBe/OFA)
FA2 FA3 FA4 FA5 FA6-a FA6-b FA7 FA8
type
temperature (°F)
EB
baghouse
290-315
n/a
n/a
EB EB LSEBc 70% PRB plus 30% Bit PRB
ESP baghouse ESP ESP ESP
280-300 270-300 n/a n/a 284
n/a n/a 0.88 n/a n/a
n/a n/a 10.12 n/a n/a
PRB
ESP
284
0.051
6.63
PRB PRB plus tire
ESP ESP
292 292
0.057 0.057
7.12 7.12
coal
Hg in coal (ppm)
Hg in flue gas (µg N-1 m-3) at 3% O2a
a Equivalent Hg concentration in the flue gas calculated from the Hg content in coal. b A blend of ash collected from two PC boilers burning the same coal. c Low-sulfur eastern bituminous coal. d Overfire air. e Low NOx burner.
mance of activated carbon for mercury adsorption11 and, in some cases, even reduce physical adsorption of mercury.12 Mercury adsorption capacity of inorganic fractions appears to be very low compared to the UBC present in fly ash.13 However, magnetite in fly ash may play a role in enhancing the oxidation of elemental mercury.2,6,14 Mercury adsorption on unburned carbon in fly ash is a complex phenomenon that depends upon temperature, secondary flue gas components (including SOx, HCl, and NOx),14 UBC particle size,15 surface area,15 porosity, and surface chemistry. The amount and properties of native UBC in turn depend upon fuel selection (coal type) and combustion conditions. Previous studies mainly have sought to correlate the total mercury capture to a single property of UBC. In reality, however, it could be attributed to the effects of multiple properties of the UBC. The detailed mechanisms of the interactions of unburned carbon in fly ash with mercury in flue gas are not yet well-understood mainly because of the heterogeneous properties of the fly ash, the complex time-temperature history of the coal particles during the combustion process, and the presence of various gas species in the flue gas. Available data in the literature provide some insight into how some macroscopic properties of fly ash carbon impact mercury capture and oxidation in flue gas. However, little fundamental work has been performed to determine the effect of coal and boiler types on the performance of UBC in mercury capture and oxidation. In this study, fly ash samples collected from full-scale coalfired power plants were tested to gain some insight on the impacts of boiler and coal types as well as particle size, surface area, carbon structure, and morphology of UBC on the adsorp(10) Maroto-Valer, M. M.; Zhang, Y.; Granite, E. J.; Tang, Z.; Pennline, H. W. Effect of Porous Structure and Surface Functionality on the Mercury Capacity of a Fly Ash Carbon and Its Activated Sample. Fuel 2005, 84, 105-108. (11) Ghorishi, S. B.; Keeney, R. M.; Serre, S. D.; Gullett, B. K.; Jozewicz, W. S. Development of a Cl-Impregnated Activated Carbon for EntrainedFlow Capture of Elemental Mercury. EnViron. Sci. Technol. 2002, 36, 4454-4459. (12) Kwon, S.; Borguet, E.; Vidic, R. D. Impact of Surface Heterogeneity on Mercury Uptake by Carbonaceous Sorbents under UHV and Atmospheric Pressure. EnViron. Sci. Technol. 2002, 36, 4162-4169. (13) Hasset, D. J.; Eylands, K. E. Mercury Capture on Coal Combustion Fly Ash. Fuel 1999, 78, 243-248. (14) Norton, G. A.; Yang, H.; Brown, R. C.; Laudal, D. L.; Dunham, G. E.; Erjavec, J. Heterogeneous Oxidation of Mercury in Simulated Post Combustion Conditions. Fuel 2003, 82, 107-116. (15) Rostam-Abadi, M.; Lu, Y.; Funk, C.; Richardson, C.; Paradis, J.; Chang R. Properties of Unburned Carbons from Three Coal-Fired Power Plants and Their Relations to Mercury Capture. In Proceedings of the Air Quality V Conference, Arlington, VA, September 18-21, 2005.
tion of mercury by fly ash. Efforts were made to determine both the inter-relations between these properties and their separate effects on the mercury capture by UBC. A better understanding of relations between various properties of UBC and their roles in mercury capture could potentially reduce the cost associated with mercury emission control from coal-fired power plants by reducing the amount of mercury to be removed from the flue gas and enhancing the performance of wet flue gas desulfurization processes for mercury capture. Experimental Section Nine fly ash samples were collected from the particulate collection devices [electrostatic precipitator (ESP) or baghouse] of seven full-scale utility pulverized coal and cyclone boilers firing eastern bituminous, Powder River Basin (PRB) sub-bituminous, or blended coals. The sources of the fly ash samples are presented in Table 1. Four ashes (FA1-FA4) were generated from burning eastern bituminous coals in pulverized coal boilers and are classified as EB-PC ashes. The remaining five ashes were generated from burning either PRB coals (FA6-a, FA6-b, and FA7), a blend of 70% PRB plus 30% EB (FA5), or a blend of PRB with tires (FA8) in cyclone boilers. These samples were classified as PRB-CYC ashes. The FA1 and FA2 ashes were collected at the same power plant but from different boilers. The FA6-a and FA6-b ashes were collected at the same power plant but on different test dates. Ashes FA7 and FA8 were collected from burning their corresponding fuels (PRB and PRB plus tire, respectively) in the same boiler. The latter ash was generated while operating a low NOx burner. About 50 pounds of each of the as-received fly ash samples was subjected to a riffle-splitting process to obtain 400-1000 g of a representative sample for the subsequent sieving and characterization studies. Fly ash samples were then mechanically sieved to obtain +100 mesh (>150 µm), 100-200 mesh (75-150 µm), 200270 mesh (75-53 µm), 270-325 mesh (53-45 µm), and -325 mesh (150 µm fractions (>75% of the total UBC for all ashes). In comparison, large amounts of UBC in the EB-PC ashes were also distributed in the 75-150, 45-75, and 60% of the total UBC). The observed differences in the distribution of UBC in the PRB-CYC and EB-PC ashes could be attributed in part to the fact that the average particle size of coal burned in a cyclone boiler (100% < 6350 µm) is much larger than in a PC boiler (70% < 75 µm). In addition, subbituminous coals generally have higher combustion reactivity than bituminous coals,17 and thus for a comparable particle size fraction, sub-bituminous coals burn more efficiently than bituminous coals. UBC Morphology and Structure. The SEM images of a UBC particle and an inorganic particle from the FA4 ash are shown in Figure 5. The UBC particle (Figure 5a) has a porous structure. Some spherical inorganic particles are attached to the external surface, and some are penetrated inside the cavities and large pores of the UBC particle (Figure 5b). Three carbon structures, amorphous, ribbon-shaped graphitic, and highly ordered graphite, were observed in the UBC from the HRTEM analysis (Figure 6). The amorphous carbon form (Figure 6a) shows a flat surface and does not have a long-range crystalline order. However, a short-range order with varying carbon atomic distances and bond angles in the presence of dangling bonds and voids is present. The microstructure of the UBC in fly ash has been described as having isotropic and anisotropic carbon forms from petrographic analyses.9,19,20 Isotropic carbon has a more disordered microstructure than anisotropic carbon. Carbon materials with less ordered structures have higher surface areas than those with more ordered (18) Kulatos, I.; Hurt, R. H.; Subberg, E. M. Size Distribution of Unburned Carbon in Fly Ash and Its Implications. Fuel 2004, 83, 223230. (19) Baltrus, J. P.; Wells, A. W.; Fauth, D. J.; Diehl, J. R.; White, C. Characterization of Carbon Concentrates from Coal-Combustion Fly Ash. Energy Fuels 2001, 15, 455-463. (20) Hower, J. C.; Trimble, A. S.; Eble, C. F.; Palmer, C. A.; Kolker, A. Characterization of Fly Ash from Low-Sulfur and High-Sulfur Coal Sources: Partitioning of Carbon and Trace Elements with Particle Size. Energy Sources 1999, 21, 511-525.
Figure 5. SEM images of the UBC particles in FA4 ash.
structures. In addition, disordered carbon materials, such as coalbased activated carbons, have a larger number of active sites for adsorption than carbon materials with more ordered structures, such as graphitic materials. The surface area of the UBC is mainly attributed to the amorphous form of the carbon and thus is believed to be the carbon structure with the most activity for mercury adsorption. Ribbon-shaped graphitic carbon (Figure 6b) is highly curved with tangled bonds and voids. Stresses because of the nonuniform distribution of temperature inside a coal particle during combustion may cause the curvature of the graphite. The regular highly ordered graphite carbon (Figure 6c) is a layered material where sheets of hexagonally arranged carbon atoms are stacked and held together. Commercial graphite is prepared by processing a carbonaceous precursor, such as petroleum coke, at temperatures as high as 3000 °C. The high-temperature treatment develops a highly ordered, low-porosity structure in the carbon skeleton. Similarly, the graphite carbon in the UBC could be formed when the temperature inside the coal particle during the combustion rose high enough to transform a portion of the carbon skeleton into the graphitic structure. Several ash samples and their corresponding carbon-free ash (prepared by burning the ash samples in air at 750 °C), one commercial activated carbon, and one graphitic carbon were examined by X-ray diffraction (XRD) to identify and quantify the relative amounts of the different carbon forms in the UBCs. Interpretation of the XRD data did not provide any conclusive information regarding these parameters. This work is, however, continuing. The SEM instrument used in this study was equipped with an energy-dispersive X-ray fluorescence detector (EDX), which can produce compositional maps of the UBC particles. Typical images obtained for the UBC particles in FA3 are shown in
2116 Energy & Fuels, Vol. 21, No. 4, 2007
Lu et al.
Figure 8. ashes.
UBC contents in raw ashes and carbon-enriched
Figure 6. HRTEM images of the UBC in the 75-150 µm fraction of FA4 ash: (a) amorphous, (b) ribbon-shaped graphitic, and (c) graphitic.
Figure 7. Only the carbon, sulfur, silicon, and aluminum maps are shown. The images show that carbon and sulfur are dispersed across the UBC surface, whereas aluminum and silicon are concentrated in the large inorganic particles embedded in the carbon. UBC-Enriched Ashes. The enrichment of UBC with the gaspulsing incipient fluidized-bed process increased the UBC contents of ash samples between 4 and 322%, depending upon the size fraction tested (Figure 8). As was shown earlier, some inorganic particles were embedded in the skeletal structure of the UBC particles (Figure 5b). Separating these impurities from the UBC particles will be difficult even when employing a very efficient separation process. The largest increase in UBC contents was observed for the 75-150 µm size fractions. The reason for the lower enrichment in the >150 µm size fraction is because the UBC content initially present in this size fraction was relatively high (85%). During enrichment of the
