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Energy & Fuels 2000, 14, 224-226
Mercury Capture by Distinct Fly Ash Carbon Forms James C. Hower,* M. Mercedes Maroto-Valer,† Darrell N. Taulbee, and Tanaporn Sakulpitakphon‡ University of Kentucky Center for Applied Energy Research, 2540 Research Park Drive, Lexington, Kentucky 40511 Received September 7, 1999. Revised Manuscript Received October 14, 1999
Carbon was separated from the fly ash from a Kentucky power plant using density gradient centrifugation and a lithium heterolpolytungstate high-density media. Relative concentrations of inertinite (up to 77% vol), isotropic carbon (up to 77% vol), and anisotropic carbon (up to 76% vol) were isolated from the original fly ash. Mercury concentration was lowest in the parent fly ash (which contains non-carbon components); followed by inertinite, isotropic coke, mixed isotropic-anisotropic coke fraction, and, with the highest concentration, the anisotropic coke concentrate. The latter order corresponds to the increase in BET surface area of the fly ash carbons. Previous studies have demonstrated the capture of mercury by fly ash carbon. This study confirms prior work demonstrating the varying role of carbon types in the capture, implying that variability in the carbon forms influences the amount of mercury retained on the fly ash.
Introduction The amount of mercury in coal and in emissions from coal-fired boilers is a topic of current interest in the United States. The U.S. Environmental Protection Agency, using its authority under Section 114 of the Clean Air Act amendments, is currently undertaking a three-phase investigation of U.S. coal-fired utilities.1 Part I, completed on January 4, 1999, involved the collection of general information on utility steam generating units. In Part II, U.S. EPA 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 75 selected utility sources. In the latter part of the study, testing is conducted at the inlet and outlet of the last pollution control device. A number of plants have been targeted for study, including two in Kentucky. Neither Kentucky plant burns a coal blend solely from Kentucky or from the eastern United States. Fly ash carbons have proven to be a good collector of Hg which would otherwise be emitted to the atmosphere. In studies conducted elsewhere, Hassett and Eylands2 and Miller et al.3 noted a relationship between Hg capture and gas temperature in their laboratory studies of a variety of fly ashes. Gibb and Clarke4 noted * Corresponding author. E-mail:
[email protected]. † Current address: The Energy Institute, 405 Academic Affairs Building, The Pennsylvania State University, University Park, PA 16802. † Also at Department of Geological Sciences, University of Kentucky, Lexington, KY 40506. (1) U.S. Environmental Protection Agency. 1999, . (2) Hassett, D. J.; Eylands, K. E. Ash Utilization Symposium, 2022 October 1997, Lexington, KY, 1997, p 264. (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.
an increase in Hg capture with an increase in carbon content and a decrease in flue gas temperature in a 1 MW experimental combustor. In previous studies at the Center for Applied Energy Research, Hower et al.5 studied fly ashes collected from an electrostatic precipitator at a utility unit burning a blend of high-sulfur, predominantly western Kentucky coals. Each fly ash was sized at 100, 200, 325, and 500 mesh. Insufficient material was available for analysis of the +100 mesh fly ash. The other four size fractions from each of two collections were analyzed for Hg. In this case, there was a significant relationship, r2 ) 0.98, between Hg and carbon content (Figure 1). The fly ash carbons were dominated by isotropic coke forms (see Hower et al.6 for discussion of fundamentals of fly ash petrology). However, in another set of sized fly ashes, we did not observe a strong relationship between Hg and C, perhaps due to variations in proportions of carbon forms among the fly ash size fractions (Sevier fly ashes discussed in Hower et al.7). Hower et al.8 examined Hg capture by fly ash from two identical units burning central Appalachian coal. Samples were collected from mechanical and baghouse hoppers. The mechanical fly ash from each unit had a higher amount of carbon than the baghouse hoppers from the same unit: 14.8% vs 6.8% for unit 1 and 5.7% vs 5.0% for unit 2. Inlet gas temperature to the mechanical separators was 364 °C and outlet temperature was 172 °C. The inlet temperature to the baghouse and (5) Hower, J. C.; Trimble, A. S.; Eble, C. F.; Palmer, C. A.; Kolker, A. Energy Sources 1999, 21, 511. (6) 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, p 49. (7) Hower, J. C.; Robl, T. L.; Rathbone, R. F.; Groppo, J. G.; Graham, U. M.; Taulbee, D. N. Australian Coal Science Conference, 7th, 2-4 December 1996, Gippsland, Victoria, Australia, 1996, p 347. (8) Hower, J. C.; Finkelman, R. B.; Rathbone, R. F.; Goodman, J. Energy Fuels 2000, in press.
10.1021/ef990192n CCC: $19.00 © 2000 American Chemical Society Published on Web 12/04/1999
Mercury Capture by Distinct Fly Ash Carbon Forms
Energy & Fuels, Vol. 14, No. 1, 2000 225 Table 1. Basic Petrographic Components of Fly Ash (after Hower et al.)6,a • Inorganic neoformed - glass • 70 to>90% of most FA - mullite - spinel • Inorganic coal-derived - quartz
• Organic neoformed - isotropic coke - anisotropic coke • Organic coal (or fuel)-derived - inertinite - petroleum coke
a Carbon forms encountered in this study are emphasized in bold type. Not all fly ash components were encountered in this study.
for the fly ash is a blend of Eastern Kentucky, low- to medium-sulfur high volatile B and A bituminous coals. Figure 1. Hg vs carbon (ultimate analysis) for sized fly ashes from a unit burning high-sulfur coal (from Hower et al.5).
Figure 2. Relationship of Hg vs C (ultimate analysis) from two types of collection systems. Note that the mechanical separation system operates at a higher flue gas temperature than the baghouse separation (from Hower et al.).8
temperature gradient across the baghouse system is not known, although it is known that the inlet temperature would have been lower than the mechanical outlet temperature. Mercury capture on the fly ash did prove to be a function of the amount of carbon in the fly ash, high-C unit 1 fly ash having a greater amount of Hg than lower-carbon fly ashes from unit 2 for the same type of collection system (Figure 2). The baghouse ashes, with less C than the mechanical ashes, have significantly more Hg than the mechanical ashes. Hower et al.8 attributed this to the lower collection temperature in the baghouse, allowing more Hg to condense out of the vapor phase. The increase in Hg between baghouse hoppers within the same unit may be a function of the further decrease in temperature across the series of five rows of hoppers. Delineation of the Hg variation versus the proportions of the types of fly ash carbons was less definitive. The authors did note a slight increase in Hg with an increase in the ratio of isotropic coke/all carbons, but no statistical significance could be attributed to the increase. In the current study, we are examining the Hg content of a series of fly ash carbon concentrates. Each concentrate has been examined petrographically and for BET surface area, allowing comparisons of carbon form and surface area with the Hg content. The fuel source
Procedure About 12 kg of fly ash was collected from the mechanical separators of the 70 MW unit 3 at East Kentucky Power’s Dale Station. Carbon enrichment was accomplished through an initial screening at 140 mesh (106 µm). The +140 mesh fraction was triboelectrostatically separated to obtain a carbonenriched fraction which was then processed via density gradient centrifugation (DGC) using a lithium heteropolytungstate high-density media. Preliminary DGC runs were conducted to determine the relative concentrations of the different carbon forms across the density spectrum. Large-scale DGC separations were conducted with density cut points determined by the carbon form concentrations delineated in the preliminary runs. Overall, 17 fractions from
