Energy & Fuels 2007, 21, 1915-1923
1915
Relationship between Textural Properties, Fly Ash Carbons, and Hg Capture in Fly Ashes Derived from the Combustion of Anthracitic Pulverized Feed Blends Isabel Sua´rez-Ruiz* and Jose´ B. Parra Instituto Nacional del Carbo´ n (CSIC), Ap.Co., 73, 33080-OViedo, Spain ReceiVed December 28, 2006. ReVised Manuscript ReceiVed March 8, 2007
In this work, the textural properties of a series of whole anthracitic-derived fly ashes sampled in eight hoppers from the electrostatic precipitators and their sized fractions (from >150 to 25% db). This trend (48) Standard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, ASTM C 618-05. American Society for Testing and Materials, Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 2006; 04.02, pp 326-328. (49) Hower, J. C.; Wild, G. W.; Graham, U. M. Proceedings: Petrographic Characterization of High-Carbon Fly Ash Samples from Kentucky Power Stations. 11th International Symposium on Use and Management of Coal Combustion By-Products (CCBs), Orlando, FL, Jan. 15-19, 1995; American Coal Ash Association 62/1-62/12. (50) Hower, J. C.; Rathbone, R. F.; Graham, U. M.; Groppo, J. G.; Brooks, S. M.; Robl, T. L.; Medina, S. S. 11th International Coal Testing Conference, Lexington, KY, May 10-12, 1995; pp 49-54.
Energy & Fuels, Vol. 21, No. 4, 2007 1917
is not followed by the A2 and B2, 75-45 µm fractions because their carbon contents are lower than 25%. These ashes are from hoppers located in the first ESP row (Figure 2a), the hot side of the ESP system, and it is known that there is a relationship between the variation trend in the carbon content of the fly ashes and the decrease in temperature of the flue gas in the ESP system.6,8,22 For the same size fractions, the A4 and B4 ashes show a lower density and therefore a higher carbon content because they were recovered from the cold side of the system (Figure 2a). The whole fly ashes show high and variable density values (Figure 2b) in accordance with their inorganic content (Table 2). These values also differ for fly ashes taken from hoppers located in the same ESP row (Figure 2). N2 77 K Adsorption Isotherms. The adsorption isotherms obtained, exhibiting a similar trend for all ash samples, are Type II according to the Brunauer-Deming-Deming-Teller classification,51 which agrees with the data reported for other fly ashes from the combustion of lower-rank coals.29-31-34 The results of the total pore volume, VTOT (for total fly ash), are shown in Figure 3. The very low values obtained (3.9 m2/g). The lowest surface area values ( 150 µm A4 and B4 sized fractions
The predominant carbons22 in these fly ashes (Figure 1a), which are apparently only slightly modified after combustion, probably retain to a certain extent some of the textural characteristics of the parent material (anthracitic vitrinite). Some data found in the literature reported N2 BET surface area values for anthracites and meta-anthracites of around 7.0 and 49.0 m2/ g,52-53 respectively. Other unpublished data reported values of 5.0-8.0 m2/g for anthracites. For instance, the result obtained for one parent feed blend (PB-2B) of these ashes was 10 m2/g coal (14 m2/g carbon). This low surface area, therefore, might justify the low BET surface areas found for the corresponding derived unburned carbons which undergo little modification (Figure 1a). The carbons that are totally or partially modified (Figure 1b-d) during the combustion of other feed blend components do not seem to increase their surface areas, probably because they are scarce in these fly ashes (Table 1). This would partially justify the different textural properties found between the ashes from the combustion of anthracitic coal blends and those of the fly ashes from bituminous coals, even though both are class F. Relationships between the BET Surface Area, the Type of Fly Ash Carbons and Hg Retention. Although the BET surface area is strongly influenced by the total amount of unburned carbons (Figure 5a) in the fly ashes, according to Maroto-Valer et al.,31 this parameter is also conditioned by the specific type of carbons (anisotropic and isotropic) as demonstrated by the fly ashes from the combustion of bituminous coals. The relationship shown in Figure 5b confirms the influence of the anisotropic carbons on the surface area for anthracite-derived fly ashes. Taking into account that the anisotropic carbons in these ashes are different than the anisotropic carbons found in ashes from bituminous coals, the variation in surface areas must also depend on the subtype of anisotropic carbons. However, in this work, it was not possible to clearly establish this assumption with the different anisotropic carbon subtypes found in these ashes. This was mainly because the relationships between the various subtypes of anisotropic carbons (Table 1) and the surface areas (Table 2) were found to be acceptable in all cases and similar to that shown in Figure 5c. As for the ability of the fly ash carbons to capture Hg in combustion processes, it was demonstrated that concentrates of anisotropic coke (anisotropic carbons) with greater surface areas than those of isotropic coke (isotropic carbons) and inertinite particles25,31 from the combustion of bituminous coals adsorbed (52) Krevelen van, D. W. Coal, 3rd rev. ed.; Elsevier: New York, 1993; p 979. (53) Parra, J. B. Textura de Carbones. In Exploracion, EValuacio´ n y Explotacio´ n del Metano de las Capas de Carbon; Zapatero Rodriguez, M. A., Pandas Ferna´ndez, F., Loredo Pe´rez, J., Eds.; Instituto Geolo´gico y Minero de Espan˜a (MEC): Madrid, Spain, 2002; Serie: Recursos Minerales, Vol. 2, pp 19-28. (54) Hill, R.; Sarkar, S. L.; Rathbone, R. F.; Hower, J. C. Cem. Concr. 1997, 27, 193-204.
the greatest amount of Hg from the flue gas stream. In this work, the relationship between the surface area and Hg concentration (Figure 6) is significant for samples with carbon contents higher than 25% (db) because they have the highest anisotropic carbon content, especially in those from anthracitic vitrinite. Thus, the Hg concentration in these fly ashes is apparently related to the anisotropic carbons from anthracitic vitrinite (they may reach 35% volume of the total ash components), as is suggested by the relationship between the BET surface areas and Hg capture for this type of carbon (Figure 5c). Despite the acceptable relationships between surface areas, the increase in carbon content, and the anisotropic carbon content, the correlation coefficient between the surface areas and Hg retention for these fly ashes is slightly lower (Figure 6) and similar to that previously reported between the Hg retention and the carbon content in these samples (r ) 0.82),22 probably induced by the different characteristics found for the finest ash fractions (
