Energy & Fuels 2008, 22, 1239–1243
1239
Structural Study of Graphite Materials Prepared by HTT of Unburned Carbon Concentrates from Coal Combustion Fly Ashes Miguel Cabielles, Miguel A. Montes-Morán, and Ana B. Garcia* Instituto Nacional del Carbón (CSIC), Francisco Pintado Fe 26, 33011 OViedo, Spain ReceiVed October 10, 2007. ReVised Manuscript ReceiVed NoVember 23, 2007
Unburned carbon concentrates with different mineral matter contents were obtained from coal combustion fly ashes by an oil agglomeration procedure. The concentrates were then heated in the temperature interval 1800–2700 °C for the purpose of exploring their ability to graphitize. The influence of the treatment temperature and mineral matter of the unburned carbon on the structural characteristics of the materials prepared was studied. The interlayer spacing, d002, and crystallite sizes along the c-axis, Lc, and the a-axis, La, calculated from X-ray diffractometry (XRD) as well as the relative intensity of the Raman D-band, ID/It, were used to assess the degree of structural order of the materials. Graphite materials with structural characteristics comparable to those of other oil-derived synthetic graphites were prepared from the unburned carbon concentrates at temperatures g2400 °C. It was also observed that more-ordered materials were obtained from the unburned carbon concentrates with higher mineral matter content. The influence of the mineral matter on the graphitization of the unburned carbon concentrates is the result of two countereffects, thus limiting its extent. On the one hand, the lateral coalescence of the crystallites is preferentially promoted. Reasonably good linear correlations were attained between the mineral matter of the unburned carbon concentrate and the XRD parameter La of the materials. However, on the other, this coalescence also facilitates the flattening of the pores, thus decreasing the temperature at which their breakage occurs. As a consequence, from this point on, the structural evolution of the materials with increasing mineral matter is only noticeable by the slow vegetative growth of the crystallites along the a-axis.
Introduction Synthetic graphite is a highly valuable material with many applications.1–3 Currently, petroleum coke is used as the main precursor material in the manufacturing of synthetic graphite. Different factors, however, have prompted a research interest into other alternative precursors such as coals and coal-derived products.4–10 Graphite materials with structural characteristics * Corresponding author. E-mail:
[email protected]. (1) Pierson, H. O. Handbook of Carbon, Graphite, Diamond and Fullerenes; Noyes: Park Ridge, NJ, 1993; pp 87–121. (2) Olson, D. W. Graphite. U.S. Geological SurVey 2005 Minerals Yearbook; U.S. Department of Interior: Reston, VA, 2006; pp 33.1–33.9. (3) Inagaki, M. In Graphite and Precursors; Delhaés, P., Ed.; Gordon and Breach Science Publishers: Amsterdam, The Netherlands, 2001; pp 179– 198. (4) Oberlin, A.; Terriere, G. Carbon 1975, 13, 367–376. (5) Bustin, R. M.; Rouzaud, J. N.; Ross, J. V. Carbon 1995, 33, 679– 691. (6) Atria, J. V.; Rusinko, F., Jr.; Schobert, H. H. Energy Fuels 2002, 16, 1343–1347. (7) González, D.; Montes-Moran, M. A.; Suárez-Ruiz, I.; García, A. B. Energy Fuels 2004, 18, 365–370. (8) Seehra, M. S.; Pavlovic, A. S.; Babu, V. S.; Zondlo, J. W.; Stansberry, P. G.; Stiller, A. H. Carbon 1994, 32, 431–435. (9) Kawano, Y.; Fukuda, T.; Kawarada, T.; Mochida, I.; Korai, Y. Carbon 200, 38, 759–765. (10) Pappano, P. J.; Rusinko, F.; Schobert, H. H.; Struble, D. P. Carbon 2004, 42, 3007–3009. (11) Evans, E. L.; Jenkins, J. L.; Thomas, J. M. Carbon 1972, 10, 637– 642. (12) González, D.; Montes-Moran, M. A.; García, A. B. Energy Fuels 2005, 19, 263–269. (13) Oya, A.; Fukatsu, T.; Otani, S.; Marsh, H. Fuel 1983, 62, 502– 507. (14) Blanche, C.; Rouzaud, J. N.; Dumas, D. Extended Abstracts, 22nd Biennal Carbon Conference, 1995; pp 694–695.
comparable to those of commercially available synthetic graphites were obtained from anthracites at temperatures g2400 °C.4–7 In addition to the treatment temperature, the anthracite mineral matter4,6,7,11–13 and its microtexture4,14–16 also influence the graphitization process. The latter was related to the ability of the anthracite to graphitize, specifically when there is a preferential planar orientation of the polyaromatic basic structural units (BSUs), whereas the mineral matter was found to behave as a graphitization catalyst. In a previous work,12 anthracites with different mineral matter content and similar microtexture were graphitized. A progressive increase in the degree of the structural order of the graphite materials prepared with increasing mineral matter content of the anthracite was observed. Regarding the composition of the mineral matter, most of its inorganic elements (such as aluminum, calcium, iron, magnesium, manganese, silicon, and titanium) have been used as graphitization catalysts for various carbon materials.17,18 Fly ashes are the main solid wastes generated in coal combustion plants, their total production in USA19 and Europe (EU15)20 in 2005 amounting up to ∼135 × 106 t. These wastes need to be disposed of and/or reused. Coal fly ashes disposal in dumps leads to several environmental and economic problems. The reusing in the production of concrete and cement (15) Duber, S.; Rouzaud, J. N.; Beny, C.; Dumas, D. Extended Abstracts, 21st Biennal Carbon Conference, 1993; pp 316–317. (16) Suarez-Ruiz, I.; Garcia, A. B. Energy Fuels 2007, 21, 2935– 2941. (17) Marsh, H.; Warburton, A. F. J. Appl. Chem. 1970, 20, 133–142. (18) Oya, A.; Marsh, H. J. Mater. Sci. 1982, 17, 309–322. (19) American Coal Ash Association (ACAA), 2005 Coal Combustion Product (CCP): Production and Use Survey, Aurora, Sept 2006. (20) European Coal Combustion Products Association Production (ECOBA), 2005 Production and Utilization, Essen, Germany, 2006.
10.1021/ef700603t CCC: $40.75 2008 American Chemical Society Published on Web 01/10/2008
1240 Energy & Fuels, Vol. 22, No. 2, 2008
(the main route of coal fly ashes utilization) is limited by the amount of unburned carbon present what has increased as a consequence of the greater implementation of the NOx control systemsintheutilityindustrytomeetenvironmentalregulations.19–24 Therefore, an increase of the proportion of coal fly ashes utilization is linked to the removal of the unburned carbon. Although several coal beneficiation methodologies have been successfully employed for removing the unburned carbon from fly ashes,25–27 the low value of these coal combustion byproduct has hindered their implementation in the power plants. However, the utilization of the unburned carbon as precursor for valueadded products would offset the cost of the beneficiation process. The unburned carbon from coal combustion fly ashes shows carbon contents >90% and a high degree of turbostractic order as indicated by XRD and HRTEM fringe imaging.28 On the basis of this, the unburned carbon present in fly ashes appears as a potential precursor for the preparation of synthetic graphite. In this paper, fly ashes from three pulverized Spanish coal combustion plants were studied. Unburned carbon concentrates with different mineral matter contents were obtained from the raw coal fly ashes by an oil agglomeration procedure.29 The unburned carbon concentrates were heated in a furnace in the temperature interval 1800–2700 °C to explore their ability to graphitize. The influence of the treatment temperature and mineral matter of the unburned carbon on the structural characteristics of the materials prepared was studied. The interlayer spacing, d002, and crystallite sizes along the c-axis, Lc, and the a-axis, La, calculated from X-ray diffractometry (XRD) as well as the relative intensity of the Raman D-band, ID/It, are used in this study to assess the degree of structural order of the materials. Both XRD and Raman spectroscopy techniques have been used extensively in the characterization of carbon materials.30–33 Experimental Section Unburned Carbon Concentrates: Preparation and Characterization. Coal fly ashes were collected from three different pulverized coal combustion plants (labeled A-C) located in northwest Spain. These particular coal combustion plants are mainly fed with high-rank coals (anthracites). In a first step, unburned carbon concentrates denoted as CVP were obtained from the fly ashes by screening out the e80 µm fraction. Following an oil agglomeration methodolgy described previously,29 samples with higher unburned carbon content were obtained from the CVPs by using a waste vegetable oil. Samples prepared accordingly are (21) Manz, O. E. Fuel 1999, 78, 133–136. (22) Smith, I. M. Cement and Concrete-benefits and barriers in coal ash utilization, IEA Clean Coal Centre Reports, CCC/94, IEA, London, UK, Jan 2005. (23) Hall, M. L.; Livingston, W. R. J. Chem. Technol. Biotechnol. 2002, 77, 234–239. (24) Hower, J. C.; Robl, T. L.; Thomas, G. A. Fuel 1999, 78, 701–712. (25) Gray, M. L.; Champagne, K. J.; Soong, Y.; Killmeyer, R. P.; Maroto-Valer, M. M.; Andresen, J. M.; Ciocco, M. V.; Zandhuis, P. H. Fuel Process. Technol. 2002, 76, 11–21. (26) Soong, Y.; Schoffstall, M. R.; Gray, M. L.; Knoer, J. P.; Champagne, K. J.; Jones, R. J.; Fauth, D. J. Sep. Purif. Technol. 2002, 26, 177–184. (27) Gray, M. L.; Champagne, K. L.; Soong, Y.; Finseth, D. Fuel 2001, 80, 867–871. (28) Hurt, R. H.; Davis, K. A.; Yang, N. Y. C.; Headley, T. J.; Mitchell, G. D. Fuel 1995, 74, 1297–1306. (29) Valdés, A. F.; Garcia, A. B. Fuel 2006, 85, 607–614. (30) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martínez-Alonso, A.; Tascón, J. M. D. J. Mater. Chem. 1998, 8, 2875–2879. (31) Zerda, W.; Gruber, T. Rubber Chem. Technol. 2000, 73, 289–292. (32) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martínez-Alonso, A.; Tascón, J. M. D. Carbon 1994, 32, 1523. (33) Montes-Morán, M. A.; Young, R. J. Carbon 2002, 40, 845.
Cabielles et al. Table 1. Unburned Carbon (LOI Value) and Mineral Matter Contents of the Different Concentrates from A, B, and C Coal Combustion Fly Ashes concentrate
LOI (wt %, db)
mineral matter (wt %, db)
A/CVP A/CIQ1 B/CVP B/CIQ1 B/CIQ5 C/CVP C/CIQ1 C/CIQ3 C/CIQ5 C/IQS
54.64 87.36 20.93 78.35 68.02 10.41 76.36 67.03 62.06 97.43
49.83 12.74 80.50 22.97 32.15 90.01 23.80 33.10 38.10 2.50
named CIQ with a numerical suffix indicating the oil concentration employed (e.g., A/CIQ1 is the unburned carbon concentrate obtained from the CVP of the A combustion plant by agglomeration at an oil concentration of 1 wt %). In addition, an unburned carbon concentrate designed as C/IQS was prepared by demineralization of C/CVP sample after an acidic treatment with HCl/HF according to the ISO 602-1983 standard procedure for the determination of the mineral matter in coals. The unburned carbon content of the concentrates was calculated from the weight loss associated with firing the predried sample in an ashing furnace with air atmosphere for 2 h at 815 °C which is known as the loss of ignition (LOI). This is basically the standard ASTM D7348-07 procedure, and it was verified that the 2 h combustion achieved constant weight as required in that procedure. Each sample was analyzed at least twice. The LOI value is assumed to be the unburned carbon content in the fly ash. The unburned carbon (LOI values) and the mineral matter contents of the concentrates appear in Table 1. The mineral matter of the samples was determined by the above-mentioned ISO 602-1983 standard. High-Temperature Treatments. The HTT experiments of the A/CVP, A/CIQ1, B/CIQ1, B/CIQ5, C/CIQ1, C/CIQ3, C/CIQ5, and C/IQS unburned carbon concentrates were carried out at 1800, 2000, 2200, 2300, 2400, 2500, 2600, and 2700 °C in a graphite furnace for 1 h under an argon flow. The heating rate were 20 °C/min from room temperature to 2000 °C and then 10 °C/min from 2000 °C to the prescribed temperature. The carbon materials thus prepared were identified by the unburned carbon concentrate (precursor) designation and the treatment temperature, such as A/CVP/2400 or C/CIQ1/ 2600. X-ray Diffractometry (XRD) and Raman Spectroscopy. The diffractograms and Raman spectra of the carbon materials prepared were recorded as reported previously.12 The interlayer spacing, d002, was evaluated from the position of the (002) peak applying the Bragg equation. The crystallite sizes, Lc and La, were calculated from the (002) and (110) peaks, respectively, using the Scherrer formula, with values of K ) 0.9 for Lc and 1.84 for La.34 Typical standard errors of the XRD parameters are
