Hg and Se Capture and Fly Ash Carbons from ... - ACS Publications

Energy & Fuels 2007, 21, 59-70

59

Hg and Se Capture and Fly Ash Carbons from Combustion of Complex Pulverized Feed Blends Mainly of Anthracitic Coal Rank in Spanish Power Plants I. Sua´rez-Ruiz,*,† J. C. Hower,‡ and G. A. Thomas‡ Instituto Nacional del Carbo´ n (INCAR-CSIC), Ap.Co. 73, 33080-OViedo, Spain, and UniVersity of Kentucky, Center for Applied Energy Research, 2540 Research Park DriVe, Lexington, Kentucky 40511 ReceiVed July 31, 2006. ReVised Manuscript ReceiVed October 10, 2006

In this work, the petrology and chemistry of fly ashes produced in a Spanish power plant from the combustion of complex pulverized feed blends made up of anthracitic/meta-anthracitic coals, petroleum, and natural coke are investigated. Moreover, the original coals previous to blending as well as the feed blends were also characterized. The work is, thus, focused on the identification of the unburned carbon present in the resulting fly ashes after the combustion of these feed blends, on the establishment of a simple and comprehensive petrographic classification of the fly ash components, on the potential relationship between the amount and the type of fly ash carbons, and on their capacity for capturing Hg and Se as trace elements of environmental interest. It was found that the behavior of fly ash carbons derived from anthracitic coals follows relatively similar patterns to those established for the carbons from the combustion of bituminous coals. Fly ashes were sampled in eight hoppers from two electrostatic precipitator (ESP) rows. The characterization of the raw ashes and their five sieved fractions (from >150 to 45 µm), there being a positive relationship between the amount of these carbons, which are apparently little modified during the combustion process, in the medium-coarse fractions of the ashes and the Hg retention. According to the results obtained, further research on this type of fly ash could be highly productive.

Introduction Among the factors influencing the type of fly ashes generated in a power plant, the type and characteristics of the pulverized feed blends used in the combustion process is of major significance. Some work has been carried out regarding the petrology and chemistry of fly ashes from the combustion of bituminous1-14 and sub-bituminous coals.15-16 Regarding the * E-mail: [email protected]. † Instituto Nacional del Carbo ´ n (INCAR-CSIC). ‡ University of Kentucky. (1) Hower, J. C.; Thomas, G. A.; Clifford, D. S.; Eady, J. D.; Robertson, J. D.; Wong, A. S. Energy Sources 1996, 18, 107-118. (2) Hower, J. C.; Robertson, J. D.; Thomas, G. A.; Wong, A. S.; Schram, W. H.; Graham, U. M.; Rathbone, R. F.; Robl, T. L. Fuel 1996, 75 (4), 403-411. (3) Hower, J. C.; Rathbone, R. F.; Robertson, J. D.; Peterson, G.; Trimble, A. S. Fuel 1999, 78, 197-203. (4) Hower, J. C.; Finkelman, R. B.; Rathbone, R. F.; Goodman, J. Energy Fuels 2000, 14, 212-216. (5) Hower, J. C.; Maroto-Valer, M. M.; Taulbee, D. N.; Sakulpitakphon, T. Energy Fuels 2000, 14, 224-226. (6) Hower, J. C.; Robertson, J. D. Int. J. Coal Geol. 2004, 85, 359377. (7) Sakulpitaphon, T.; Hower, J. C.; Trimble, A. S.; Schram, W. H.; Thomas, G. A. Energy Fuels 2000, 14, 727-733. (8) Sakulpitaphon, T.; Hower, J. C.; Trimble, A. S.; Schram, W. H.; Thomas, G. A. Energy Fuels, 2003, 17, 1028-1033. (9) Mardon, S. M.; Hower, J. C. Int. J. Coal Geol. 2004, 59, 153-169.

fly ashes produced by the combustion of high-rank coals, anthracite/meta-anthracite, or complex coal blends, there is little information.17-19 Similarly, while it has been noticed that the retention of some trace elements are not apparently related to the presence and/or the amount and type of unburned carbons in fly ashes (for example, As8 and Se6,9 may be more related to the temperature at the ash collection point, particularly As), Hg shows a complex relationship with the amount of fly ash carbon and the flue gas (10) Mastalerz, M.; Hower, J. C.; Drobniak, A.; Mardon, S. M.; Lis, G. Int. J. Coal Geol. 2004, 59, 171-192. (11) Vassilev, S. V.; Menendez, R.; Alvarez, D.; Diaz-Somoano, M.; Martinez-Tarazona, R. Fuel 2003, 82, 1793-1811. (12) Vassilev, S. V.; Menendez, R.; Diaz-Somoano, M.; MartinezTarazona, R. Fuel 2004, 83, 585-603. (13) Vassilev, S. V.; Menendez, R.; Borrego, A. G.; Diaz-Somoano, M.; Martinez-Tarazona, R. Fuel 2004, 83, 1563-1583. (14) Pires, M.; Querol, X. Int. J. Coal Geol. 2004, 60, 57-72. (15) Querol, X.; Fernandez-Turiel, J. L.; Lopez-Soler, A. Fuel 1995, 7, 331-343. (16) Goodarzi, F. Int. J. Coal Geol. 2005, 61, 1-12. (17) Milenkova, K. S.; Borrego, A. G.; Alvarez, D.; Xiberta, J.; Menendez, R. Energy Fuels, 2003, 17, 1222-1232. (18) Milenkova, K. S.; Borrego, A. G.; Alvarez, D.; Xiberta, J.; Menendez, R. Fuel 2003, 82, 1883-1891. (19) Sua´rez-Ruiz, I.; Hower, J. C.; Thomas, G. A. Petrology and chemistry of fly ashes derived from the combustion of complex coal blends in Spanish power plants. Proceedings of International Coal Ash Technology Conference, AshTech 2006, Birmingham, UK, May 14-17, 2006; CD-rom.

10.1021/ef0603481 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/22/2006

60 Energy & Fuels, Vol. 21, No. 1, 2007

temperature at the collection point.4,6,7,9 Arsenic and Hg are actually representative of the end members in behavior, with As deposition seemingly related to temperature, whereas Hg depends on the carbon content and temperature. These are conclusions from various studies on fly ash carbons derived from the combustion of bituminous coals. However, the capacity of retention of some trace elements in fly ash carbons produced from the combustion of anthracitic feed blends is mostly unknown and only briefly mentioned in Sua´rez-Ruiz et al.19 Data reported on Hg capture by fly ashes produced from bituminous coals have demonstrated that the Hg concentration (and for some other trace elements) is also strongly related to the concentration of trace elements in the feed coals. The following has been established: (i) the Hg concentration shows a positive correlation with fly ash carbons for fly ashes collected within the same row of the electrostatic precipitator (ESP) and, therefore, the same temperature; (ii) there is an increase in Hg capture even at lower carbon levels with a decrease in the flue gas temperature; and (iii) there is a relationship between the fly ash carbon type, the BET surface area, and Hg capture,5 even though the correlation of Hg capture and the specific microscopical carbon forms never has been satisfactorily demonstrated. In the case of Se, its concentration seems to increase with the decrease in temperature of the flue gas (decrease in temperature at the collection point),6 although some opposite results are also reported in the literature.9 For years, some Spanish power plants have been using complex coal blends in combustion comprising domestic highrank anthracite coals and imported coals of various origins (of lower coal rank) to get their quality specifications. In this research, taking into account the complexity outlined above and the sparse information about fly ash carbons and their potential capacity for retention of some trace elements, the petrology and chemistry of the fly ashes from combustion of complex feed blends (primarily of anthracitic coal rank) were investigated. The characteristics of the raw coals previous to blending and those of the pulverized feed blends were also determined. This work was focused on three specific objectives: (i) to identify the different unburned carbons in fly ashes from the combustion of these complex feed blends; (ii) to classify the fly ash components by establishing a comprehensive and simple petrographic classification according to a few selected criteria; and (iii) to determine the potential relationship between the retention of Hg and Se and some specific carbon forms identified in the studied fly ashes. Sampling and Analytical Procedures For this research, two raw coals (SC-1A and SC-1B) prior to blending and two pulverized feed blends (PB-2A and PB-2B) were sampled at a Spanish power plant. The two feed blends were sampled over a time interval of about 12 h to ensure that the recovered fly ashes were derived from the fired blends. Fly ashes were sampled from eight hoppers of the electrostatic precipitators (ESPs) distributed in two different rows (A1, A2, B1, and B2 fly ashes from hoppers located in the first row and A3, A4, B3, and B4 fly ashes from hoppers in the second row, Figure 1). Hoppers located in row 1 represent the hot side of the ESP, and those located in row 2 are the cold side of the ESP, as shown in Figure 1. These two rows recover more than 90% of the total fly ashes generated. The coals, the pulverized feed blends, and fly ashes were microscopically and chemically characterized. Vitrinite reflectance measurements were taken on SC-1A and SC-1B coals and coals from the PB-2A and PB-2B feed blends in a MPV Leica microscope

Sua´ rez-Ruiz et al.

Figure 1. Scheme of the fly ash collection system in the two rows of the electrostatic precipitators.

following the procedures described in the ISO 7404/5 norm.20 The analysis of the petrographic composition in terms of percentages of maceral groups and coke was carried out according to the ISO 7404/3 norm.21 The same procedure (point counting analysis) was followed to obtain the percentages of microscopical components in the whole samples and subsamples of the fly ashes (after being wet screened), although, in this case, polarized light and a retarder plate incorporated into the microscope system was used during the analysis. Microphotographs of fly ash carbons were taken on an Zeiss Axioplan microsope using Leica software to capture and analyze images. Chemical conventional analysis (proximate, ultimate, and total sulfur analysis) on coals, feed blends, and fly ashes were conducted following the appropriate ASTM procedures. For the coals and feed blends, sulfur forms and calorific values were also analyzed. Volatile matter and moisture analysis was carried out in a LECO TGA-601, and total sulfur was determined in LECO-SC-432 equipment. Ultimate analysis (C, H, and N determinations) was performed on a LECO CHN-2000 apparatus. Major oxides and minor elements were analyzed from fusedglass discs of high-temperature ash (750 °C) on a Phillips PW2404 X-ray spectrometer (X-ray fluorescence) in accordance with techniques described by Hower and Bland.22 Se was analyzed on pressed pellets of the whole samples, and Hg also determined on the whole samples was analyzed with a LECO AMA254 advanced mercury analyzer, using an absorption spectrometer technique. Cl was obtained on the same apparatus on pressed whole samples (unfused pellets). The whole fly ashes from the hoppers of the first and second rows (A2, B2 and A4, B4 hoppers in Figure 1) were wet screened at 100, 200, 325, and 500 mesh (150, 75, 45, and 25 µm, respectively, according to the corresponding ASTM and ISO norms). The fractions obtained were also petrographically and chemically characterized following the procedures described above to determine the evolution of the main characteristics, particularly the content and type of unburned carbons in fly ashes according to the grain size. Finally, scanning electron microscopy (SEM) observations on the fly ash carbons were carried out on the size fractions of the fly ashes with the highest carbon content. The analysis was performed by using a Zeiss DSM 942 scanning electron microscope.

Results and Discussion Original Coals and Pulverized Feed Blends. The main chemical and petrographic characteristics of the raw coals (SC1A and SC-1B) and those of the pulverized feed blends (PB2A and PB-2B) are shown in Tables 1-5. Table 1 includes the proximate, ultimate, and calorific value data for the samples (20) International Organization for Standarization. Methods for the Petrographic Analysis of Bituminous Coal and Anthracite - Part 5: Methods Determining Microscopically the Reflectance of Vitrinite. ISO 7404-5.; Geneva, Switzerland, 1994, p 11. (21) International Organization for Standarization. Methods for the Petrographic Analysis of Bituminous Coal and Anthracite - Part 3: Methods Maceral Group. ISO 7404-3.; 1994, Geneva, Switzerland, 4 pp. (22) Hower, J. C.; Bland, A. E. Int. J. Coal Geol., 1989, 11, 205-226.

Hg and Se and Fly Ash Carbons from Power Plants

Energy & Fuels, Vol. 21, No. 1, 2007 61

Table 1. Results from Proximate and Ultimate Analysis and Calorific Values for the Raw Coals (SC-1A, SC-1B) before Blending and for the Pulverized (PB-2A, PB-2B) Feed Blendsa samples

type

SC-1A SC-1B PB-2A PB-2B

coal coal feed blend feed blend

moisture ash VM F.C. C H N S O Sp Ss Sorg calorific value (%, ar) (%, db) (%, db) (%, db) (%, db) (%, db) (%, db) (%, db) (%, db) (%, db) (%, db) (%, db) (MJ/kg) 5.96 5.54 1.63 1.40

28.11 12.59 25.27 24.03

9.10 10.20 10.40 10.41

56.83 71.67 62.70 64.16

60.59 74.55 66.58 67.93

2.36 3.28 2.41 2.37

0.85 1.78 1.13 1.22

0.94 1.54 1.09 1.12

7.15 6.26 3.52 3.33

0.44 0.23 0.54 0.49

0.02 150 150-75 75-45 45-25 150 150-75 75-45 45-25 150 150-75 75-45 45-25 150 150-75 75-45 45-25 150 150-75 75-45 45-25 150 150-75 75-45 45-25 150 150-75 75-45 45-25 150 150-75 75-45 45-25 150 150-75 75-45 45-25 150 150-75 75-45 45-25 150 150-75 75-45 45-25 150 150-75 75-45 45-25 45 µm), and therefore, they mainly appear concentrated, although in variable percentages (12.79-35.4% vol), in the coarser fractions (>45 µm) of the A2, B2, A4, and B4 fly ashes and in the fly ashes of the second ESP row (Table 7). (ii) Particles derived from the combustion of semi-anthracite and bituminous coal rank. They are fused particles and have developed porous and vesiculated structures (Figure 3a). The degree and size of vesiculation and porosity is variable,17

depending, among other factors, on the fired coal rank from which they are derived. Although in the present work, the porous and vesiculated structures, such as networks with a varied degree of development (Figure 3a), were the only carbons quantitatively recorded (Table 7), the anisotropic cenospheric structures, such as the structures described in the literature,17,25 can also be differentiated and included in this category. Table 7 shows the percentages of these types of carbons found in the whole fly ashes and in their size fractions. In the whole fly ashes, these components appear in low percentages (45 µm), particularly in the case of those of the second row fly ashes (Table 7). For all the fly ashes studied (including their fractions), these carbons are represented by fused structures similar to networks such as those described above, which show different degrees of development and different amounts and sizes of pores and vacuoles (Figure 3a). Carbons with a cenospheric structure derived from the combustion of the bituminous coal rank from the feed blends were also observed in some fly ash samples. However, they are very scarce (the amount of bituminous coals in the feed blends was also scarce, as is shown in Table 3), and so, they were not recorded during the quantitative analysis (Table 7). (iii) Particles derived from the combustion of inertinites. They can be fused particles, irregularly porous, and vesiculate, developing a structure like networks such as those shown in the work of Milenkova et al.,17,18 and/or they may appear as relatively massive and dense particles (Figure 3b) of low porosity. In this category, a subdivision of these two types of particles can be made depending on the research requirements. In all cases, these unburned carbons are anisotropic, some times strongly anisotropic, but this anisotropy developed by the majority of this type of particles (Figure 3b) is totally different



Figure 2. Microphotographs of fly ash components. (a, d, e) Scanning electron microscopy (backscattered electron mode). (b and c) Optical microscopy, pictures taken in oil immersion (50× objective), polarized light, and retarder plate. (a) Inorganic and organic components in fly ashes. (b and c) Unburned carbons, unfused and anisotropic particles derived from vitrinite of anthracite coal rank. (d) Fly ash carbon from anthracitic vitrinite. (e) Fly ash carbon from anthracitic vitrinite, detail of surface porosity. This porosity cannot be observed in pictures b and c.

from that exhibited by the other anisotropic fly ash carbons. In the fly ashes studied (Table 7), this category is mainly represented by relatively massive, dense, and irregular particles with some porosity (Figure 3b). These carbons were found in low percentages for the whole ashes (5.5% vol) and appear more concentrated in the fly ashes of the second ESP row (Table 7). As for the size fractions, the distribution of these particles follows a similar trend to that observed for the previous categories, appearing more concentrated in the coarser fractions of the fly ashes and, in addition, in higher amounts (9.6-15.2% vol) in the coarser fractions of the A4 and B4 ashes (Table 7).

percentages for all the whole fly ashes and in their corresponding size fractions (Table 7). In the category of isotropic unburned carbons, the following particles may be taken into consideration: (i) Particles derived from the combustion of inertinite. These carbons are totally isotropic, unfused (Figure 3c), or partially fused with variable porosity like the particles shown in the work of Milenkova et al.17 They also appear as massive and dense carbons (Figure 3c) and/or as particles retaining their original inertinitic structure,26-28 as shown in Figure 3c. Table 7 shows the distribution of the isotropic particles derived from the

(iv) Undifferentiated anisotropic fragments. They are small anisotropic carbons, smaller than 10-15 µm, which cannot be assigned to any of the previously described categories of carbons. Undifferentiated fragments were found in variable

(26) Hower, J. C.; Wild, G. W.; Graham, U. M. Petrographic characterization of high-carbon fly ash samples from Kentucky power stations. Proceedings of the 11th International Symposium on use and management of Coal Combustion By-Products (CCBs), American Coal Ash Association, Orlando, FL, Jan 15-19, 1995; pp 62/1-62/12.



Figure 3. Optical microscopy. Microphotographs of fly ash components (in oil immersion and 50× objective), polarized light, and retarder plate are shown. (a) Unburned carbon, fused and anisotropic particles derived from the vitrinite of semi-anthracite coal rank. The glassy material is indicated by the arrow. (b) Unburned carbon, partially fused and anisotropic particles derived from inertinite. (c) Unburned carbon, unfused and isotropic particles derived from inertinite. (d) Anisotropic natural coke and glassy material (arrow). (e) Anisotropic particle of pet coke. (f) Particle of unburned coal (in the center) and glassy material.

combustion of inertinite for the whole fly ashes and their corresponding size fractions. They were found in variable percentages, but lower than 11.0% vol, appearing slightly more concentrated in the coarser fractions of the A4 and B4 fly ashes (second ESP row) and following the same trend described for the other categories of unburned carbons. The majority of the particles identified in this category were mainly isotropic, unfused, massive carbons, a few of the carbons retaining their original inertinitic structure, and with a very scarce porosity. Thus, all were grouped into only one category, as described above. (ii) Undifferentiated isotropic fragments. They are small particles smaller than 10-15 µm, similar to those previously described, but in this category, the unburned carbons are totally (27) Hower, J. C.; Rathbone, R. F.; Graham, U. M.; Groppo, J. G.; Brooks, S. M.; Robl, T. L.; Medina, S. S. Proceedings of the 11th International Coal Testing Conference, Lexington, KY, May 10-12, 1995; pp 49-54. (28) Hower, J. C.; Mastalerz, M. Energy Fuels, 2001, 15, 1319-1321.

isotropic. Undifferentiated fragments were identified in variable amounts, although in percentages lower than 3.3% vol, for all the fly ashes studied (Table 7). Because in the present work isotropic unburned carbons different from those from inertinite particles (Table 7) were not found, these undifferentiated isotropic fragments probably could be integrated in the previously described carbons from inertinite. However, due to the uncertainty of identification and because these carbons encompass all the characteristics described for the undifferentiated fragments, they are kept as an individualized category. (iii) Isotropic particles from the combustion of vitrinite of low-rank coals. This category of unburned carbons was not found in any of the studied fly ashes because of the high rank of the coals (mainly anthracitic rank) in the PB-2A and PB-2B feed blends. However, this category has to be defined in this proposed classification as including all the unburned carbons that can be found in fly ashes from the combustion of low-rank coals or blends containing low-rank coals, because, as was


previously stated, this is an open classification. The unburned carbons to be included in this category are fused particles, vesiculate particles, and those with a porosity as shown in the work of Hower et al.1,2,26,27 and Milenkova et al.18 Isotropic cenospheres and porous structures similar to networks, such as those shown in the work of Milenkova et al.17,18 with a different degree of development, should be enclosed in this category. Other Organics. This is the second main category of particles identified in the organic fraction of fly ashes. It includes unburned carbons which are not strictly derived from coal combustion, as was the case of the previously described categories. Other organics incorporate unburned carbons derived from the combustion of (i) pet coke, (ii) natural coke, and (iii) unburned coal particles (Figure 3d-f). The “other organics” may be either anisotropic (if derived from coke, Figure 3d and e) or isotropic carbons (if they are unburned coal from a bituminous coal rank, Figure 3f). Pet or natural cokes show the typical characteristics, with various optical textures as can be seen in Figure 3d and e. Natural coke particles may contain fragments of inertinite.1,2,19 Finally, the unburned coal is composed of particles that appear as unmodified coaly components in the fly ashes (Figure 3f). In these particles, vitrinite and liptinite macerals (i.e., relatively unburned and unmodified) can still be identified.26-27 In the fly ashes studied, the “other organic particles” are generally very scarce (Table 7), representing only a small amount (45 µm) particularly in the case of those from A4 and B4 fly ashes (Table 7). The amount of unburned coal particles is negligible in all the analyzed fly ashes. Inorganic Components. In order to organize these components, the classifications of Hower and Mastalerz28 and Hower et al.25 were followed. As inorganic components, glassy material (composed of alumino-silicates with lesser amounts of Fe, Ca, and other elements26-27), that can be found also associated with particles of fly ash carbons, quartz, oxides, mullite, and spinels, were identified. However, in the “other mineral matter” subdivision, some inorganics were included that, because of their size and/or the microscope resolution or because they could not be clearly identified, were not assigned to any of the other categories of inorganic components. In the present fly ashes, the inorganic components are the predominant constituents for the whole fly ashes, and they are found in variable percentages for the different size fractions of the ashes (Table 7). Microscopic analysis also shows that the inorganic fraction of the fly ashes, which is predominant in the finest fractions (45 µm fractions of the fly ashes (Table 7). It can be established that the organic phase of the studied whole fly ashes is dominated by anisotropic unfused and fused carbons (Table 7) which are the main contributors to the carbon content obtained from ultimate analysis, as can be seen in Figure 4, followed by isotropic, mainly unfused carbons, and lesser amounts of the components comprising the other organics which are generally anisotropic coke particles. From one-third to onehalf of the total anisotropic carbons are represented by particles derived from the vitrinite of anthracite coals (Table 7, Figure


Figure 4. Relationship between the carbon content (from ultimate analysis) of the whole fly ashes and A2, A4, B2, and B4 sized fractions and the fly ash carbons determined from petrographic analysis.

Figure 5. Concentration of fly ash carbon forms as determined from petrographic analysis in the whole fly ashes. (a) Total anisotropic carbons. (b) Fly ash carbons from anthracitic vitrinite.

4), which was expected from the composition in terms of the class of coals (mainly anthracitic to meta-anthracitic coal rank) determined for both pulverized feed blends (Table 3). The whole fly ashes of the second ESP row are slightly enriched in anisotropic carbons, particularly in those derived from anthracitic vitrinite with respect to those of the first row (Table 7 and Figure 5a and b). Regarding the sized fractions of the A2, B2 and A4, B4 fly ashes, the amount of total unburned carbons is the highest in the coarser fractions (>45 µm, Table 7). Again, the main fly ash carbons contributing to the Carbon content in the coarser fractions of the fly ashes are particles from vitrinite of anthracitic coal rank, their proportion being the highest in the coarser fractions of the second row fly ashes (Figure 6a and b). The distribution and concentration of this type of unburned carbons in the different fly ashes is of special interest regarding the capture of the trace elements (i.e., Hg), as is discussed below. Relationship between the Fly Ash Carbons from HighRank Coals and the Hg, Se, and Cl Retention. Data related to the concentration of Hg, Se, and Cl in the whole fly ashes



Figure 6. Partitioning of fly ash carbons from anthracitic vitrinite in the whole A2, A4, B2, and B4 fly ashes and in their sized fractions. Table 8. Hg, Se, and Cl Concentration in the Whole Fly Ashes and Hg in the Sized Fractions (in micrometers) of A2, B2, A4, and B4 Ashesa samples-ESP rows

size fractions

Hg (µg‚g-1)

Se (µg‚g-1)

Cl (%)

A1-row 1 A2-row 1

whole whole >150 150-75 75-45 45-25 150 150-75 75-45 45-25 150 150-75 75-45 45-25 150 150-75 75-45 45-25

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