Microanalytical Characterization of Slagging Deposits from a Pilot

570

Energy & Fuels 1999, 13, 570-578

Microanalytical Characterization of Slagging Deposits from a Pilot-Scale Combustor Huafeng Wang, Janice West, and John N. Harb* Department of Chemical Engineering and Advanced Combustion Research Center, Brigham Young University, Provo, Utah 84602 Received May 12, 1998. Revised Manuscript Received January 12, 1999

Detailed analyses of coal ash deposits were performed with the use of SEM, X-ray, and image analysis in order to characterize local deposit properties as a function of position. Such analyses are important for understanding and predicting the thermal and physical properties of ash deposits. The present study used measurements of deposit composition and morphology to identify four characteristic regions in the deposits: the initial region, adjacent region, middle region, and outer region. These regions were used as a basis for quantitative description of the transitions which occur from the particulate initial layer to the highly sintered outer layer. Detailed properties of each of these regions are presented and discussed. Understanding developed from this characterization provides a basis for the development and validation of predictive models of the deposition process.

Introduction One of the primary problems associated with the use of coal in utility boilers is the deposition of fly ash on heat transfer surfaces.1 Boiler efficiency and availability are directly related to the properties of deposits formed on those surfaces. In particular, deposit structure has an enormous effect on properties such as deposit strength and thermal conductivity. Hatt2 suggested a classification system for deposits based on their macroscopic appearance. More recently, Laursen and Frandsen3 identified five different classes of deposits based on their microscopic appearance or texture. Scanning electron microscopy was used by Ramer and Martello to characterize quantitatively the average morphological features of several deposits.4 Their measurements included porosity, particle contiguity, and mean particle and pore chord lengths. In addition to morphological data, composition data are critical to our understanding of the processes by which deposits form and mature. Composition data can be obtained at discrete points in a deposit with the use of scanning electron microscopy point count (SEMPC), a technique which combines X-ray and image analysis.5 A limitation of the SEMPC technique is that it does not provide information on the morphology of the deposit.3 (1) Benson, S. A.; Jones, M. L.; Harb, J. N. In Fundamentals of Coal Combustion for Clean and Efficient Use; Smoot, L. D., Ed.; Elsevier: New York, 1993; pp 299-373. (2) Hatt, R. M. Prog. Energy Combust. Sci. 1990, 16, 235-241. (3) Laursen, K.; Frandsen, F. J. Classification System for Ash Deposits Based on SEM Analyses: An Engineering Foundation Conference on Impact of Mineral Impurities in Solid Fuel Combustion, Kona, Hawaii, November 2-7, 1997. (4) Ramer, E. R.; Martello, D. V. In Applications of Advanced Technology to Ash-Related Problems in Boilers; Baxter, L., DeSoller, R., Eds.; Plenum Press: New York, 1996; pp 309-323. (5) Jones, M. L.; Kalmanovitch, D. P.; Steadman, E. N.; Zygarlicke, C. J.; Benson, S. A. In Advances in Coal Spectroscopy; Plenum Press: New York, 1991; pp 1-28.

The objective of the present study was to characterize the local properties of ash deposits by analyzing both the morphology and composition as a function of position in the deposit. Of particular interest was the relationship between the local deposit chemistry and morphology. Also of interest was the identification of transition regions where significant changes in deposit properties occur and the mechanisms by which those changes are effected. Such changes in the deposit properties have an important impact on the transfer of heat and the development of strength in ash deposits. Heat transfer and strength development must be understood and quantified in order to adequately predict deposition behavior. Therefore, the increased understanding developed from detailed deposit characterization provides an improved basis for the development of predictive models of the deposition process in a utility boiler. Experimental Section Deposit Samples. Two deposit samples collected at the Fireside Performance Test Facility (FPTF) of Combustion Engineering, Inc.’s ABB Power Plant Laboratories were used in this study. The FPTF (Figure 1) is a pilot-scale facility which can be operated at temperatures, heat fluxes, and residence times representative of those found in full-scale units.6 The panels and sacrificial probe in Figure 1 were designed to simulate both the material and temperature of steam/water tubes in the radiant section of a utility boiler. Both deposit samples used in this study were formed while firing the same U.S. Eastern Bituminous coal (Table 1) at a rate of 3.5 MBtu/ h. Sample A was formed on the cooled sacrificial probe and extracted and mounted without removal from the deposit substrate. In contrast, sample B was collected from panel 1 and removed mechanically from the cooled deposition panel. (6) Thornock, D. E.; Borio, R. W. Developing a Coal Quality Expert: Combustion and Fireside Performance Characterization Factors; Report prepared for CQ INC.; U.S. Department of Energy, DE-FC2290PC89663, Combustion Engineering, Inc., 1992.

10.1021/ef980119h CCC: $18.00 © 1999 American Chemical Society Published on Web 03/02/1999

Slagging Deposits from a Pilot-Scale Combustor

Energy & Fuels, Vol. 13, No. 3, 1999 571

Figure 2. Backscattered electron (BSE) image of a deposit.

Figure 1. Fireside Performance Test Facility (FPTF) at ABB Combustion Engineering (adapted from ref 6). Table 1. Coal Analysis moisture volatile matter fixed carbon

Proximate Analysis, wt % 1.2 ash 26.0 HHV, Btu/lb 63.2

moisture hydrogen carbon sulfur

Ultimate Analysis, wt % 1.2 nitrogen 4.6 oxygen (diff) 76.5 ash 0.8

SiO2 Al2O3 Fe2O3 CaO MgO

Ash Composition, wt % 51.5 Na2O 33.4 K2O 5.3 TiO2 1.9 P2O5 0.7 SO3

Ash Fusibility Temperature, °F initial deformation -2700 hemispherical softening +2700 fluid

9.6 13587

1.1 6.2 9.2

0.7 1.7 1.4 0.3 1.4 +2700 +2700

The two samples were very similar in appearance, with the exception of a small highly fused region at the outer edge of sample B which was not present in sample A. The samples were cast in epoxy, cross-sectioned, polished, and carbon-coated prior to analysis. The deposits were approximately 2 cm thick. Analytical Procedure. Scanning electron microscopy (SEM), X-ray analysis, and image analysis were used to characterize the two slagging deposit samples. Analysis was performed on a JEOL JSM-840A scanning electron microscope equipped with an ISIS microanalytical system. The procedure used was an adaptation of the scanning electron microscopy point count (SEMPC) technique.5 X-ray analysis was performed at points on a predefined grid (32 × 26) for each field analyzed to obtain the elemental compositions of the deposit at those points. Since the deposits are porous, image analysis was used to distinguish between grid points on the deposit and those on the epoxy based on the brightness of the backscattered electron image at the grid point. Analysis was only performed at grid points on the deposit. Use of image analysis to distinguish between grid points on the deposit and those on the epoxy proved to be more efficient than checking the X-ray count rate at each point for the samples analyzed. The backscattered electron (BSE) image of each field was also collected and saved for later visual and image analysis. A typical analysis of one deposit sample included 9-55 fields in each of 11-13 discrete regions of the deposit, from the initial layer to the outer surface of the deposit. The total number of analysis points was approximately 25 000 for each sample. The

results were grouped into four representative regions, according to their morphology and compositions, as discussed below. Morphology. The stored images of the two-dimensional cross-sections of the deposits were used to characterize the deposit morphology. The images themselves provided a qualitative view of the morphology and a means for qualitative comparison of the different regions of the deposit. In addition, image processing was used to provide a more quantitative characterization of the deposit morphology. Because of the sharp contrast between the “bright” deposit and the dark mounting medium, a single threshold value of brightness was used to distinguish the deposit from the epoxy on a pixel-bypixel basis. The bright deposit pixels were then grouped into clusters based on connectivity; pixels adjacent or connected to other pixels in the deposit cross-section were grouped into the same cluster. The cluster size was the total area of the pixels which formed the cluster, as determined from the image magnification and the number of pixels in the cluster. This area was converted to an equivalent diameter (circle) for convenience in presentation of the data. Most of the deposit sample analysis in this study was performed at 300×. However, both the innermost layer of the deposit and the fly ash sample consisted of small discrete particles (in cross-section) and were analyzed at a higher magnification of 600×. In all cases, only clusters with an equivalent diameter >0.54 µm were considered. This minimum feature size corresponded to a minimum of three connected pixels necessary to form a cluster at 300× and 11 connected pixels at 600×. In the analyzed deposits, the number of small clusters exceeded by several orders of magnitude the number of large clusters. However, the large clusters made up a significant fraction of the total analyzed deposit cross-sectional area. Therefore, to provide an indication of the relative importance of the different cluster sizes in the deposit, the cluster distribution was expressed in terms of area fraction. In other words, the distribution represents the fraction of the deposit area that is made up of clusters of a given size. Composition. Qualitative information on deposit composition was available from the BSE images since the brightness of a particular point in the image is related to the elemental composition at that point. Points with heavy elements such as iron are bright white, aluminosilicates and silicates are various shades of gray, and the epoxy medium is black (see Figure 2). The images were used to make a qualitative comparison of the composition in the different regions of the deposit. X-ray analysis of the deposit samples produced large quantities of raw data consisting of the elemental compositions at analysis points spread throughout the deposit cross-section. A useful method of processing these data is to identify and classify each point into a finite number of composition groups. Some of the points have compositions which are readily associated with well-defined species such as iron oxide and silica. However, most of the points are associated with amorphous regions of the deposit, which have a wide composition distribution. The task of classifying these amorphous compositions is complicated as there are no well-defined

572 Energy & Fuels, Vol. 13, No. 3, 1999

Figure 3. BSE micrograph of the initial layer of sample A (600×).

Figure 4. BSE micrograph of the adjacent layer of sample A (300×).

Wang et al.

Figure 5. BSE micrograph of the middle region of sample A (300×).

Figure 6. BSE micrograph of the outer region of sample A (300×).

criteria. To accomplish this task, an algorithm recently developed by Slater et al.7,8 to identify and group analysis points of similar composition was used to define composition groups specifically for the samples in this study. Identification and classification of analysis points into a finite number of compositions permitted investigation of the composition changes through the deposit. The spatial resolution of the data was therefore very important. The role, if any, of the equilibrium chemistry during deposit growth was also of significant interest.

Results and Discussion Characterization of deposit morphology and compositions led to the identification of four distinct regions in the deposits, namely the (1) initial, (2) adjacent, (3) middle, and (4) outer regions (Figures 3-6). Data from the two deposit samples characterized in this study were similar except for some differences in the outer region as discussed below. Therefore, the changes in morphology and composition in the deposit are mainly illustrated for sample A. Information on the outer region of sample B has also been included as needed. The approximate thicknesses of the four regions were (1) 0.1 mm (initial layer), (2) 1 mm (adjacent layer), (3) 4 mm (middle layer), and (4) 13 mm (outer layer). Morphology. A BSE image of the initial layer of sample A is shown in Figure 3. The initial layer was characterized by the presence of small spherical par(7) Slater, P. N.; Abbott, M. B.; Harb, J. N. Algebraic Interpretation of Composition Phase Classification Criteria for CCSEM; Symposium on Ash Chemistry: Phase Relationships in Ashes and Slags, ACS Division of Fuel Chemistry Preprints, Spring 1996 National Meeting, New Orleans, LA, March 24-28, 1996. (8) Slater, P. N. Ph.D. Dissertation, Department of Chemical Engineering, Brigham Young University, Provo, UT, August 1998.

Figure 7. Cluster size distribution for the initial, adjacent, middle, and outer regions of the deposit and for the fly ash (sample A).

ticles and no evidence of particle sintering. A quantitative measure of the small particle sizes in this layer is given in Figure 7. The majority of the deposit crosssection was composed of clusters which corresponded to single particle cross-sections with equivalent diameters of 1-2 µm. This cluster size distribution is smaller than the fly ash size distribution, as measured by the same technique (see Figure 7). The observed morphology of the initial layer is consistent with the preferential deposition of small particles during the initial stage of deposition.1 The transition between the initial layer and the adjacent layer was very pronounced. Inspection of the images showed a sharp increase in cluster size and decrease in the number of round clusters (Figure 4). The distribution of cluster sizes shifted noticeably relative to the distribution found in the initial layer, as shown



Table 2. Bulk Oxide Compositions of the Initial, Adjacent, Middle, and Outer Regions of Sample A initial region adjacent region middle region outer region

Na2O

MgO

Al2O3

SiO2

P2O5

SO3

ClO

K2O

CaO

TiO2

Fe2O3

BaO

0.3 0.0 0.0 0.0

0.6 0.2 0.4 0.2

34.7 32.9 31.8 31.8

50.8 56.4 56.9 56.5

0.1 0.1 0.0 0.0

1.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

3.3 1.6 1.7 1.7

1.5 1.6 1.4 1.5

1.6 1.5 1.5 1.5

6.2 5.6 6.3 6.6

0.1 0.2 0.1 0.2

in Figure 7. The absence of small spherical particles was likely due to the agglomeration of these particles with other particles present in this region. Figure 5 shows a representative BSE image of the middle region of sample A. Cluster sizes in this region were larger than those in the adjacent layer, indicative of sintering. This shift in the cluster size distribution is apparent in Figure 7. Note that the cluster size bin which contained the greatest fraction of the deposit area increased from 15 µm in the adjacent layer to 68 µm in the middle region. The outer region of deposit was characterized by its highly sintered nature in both samples. Figure 6 shows a representative BSE image of the outer region of sample A. The size distribution of clusters for this region is also shown in Figure 7. The outer region had a larger mean cluster size than the middle region, which was the result of additional sintering. In the outer region, the largest clusters occupied almost the entire microscope field (150 µm in equivalent diameter). Thus, the largest cluster size in Figure 7 is limited by the microscope field size, and clusters in the outer region may have actually been larger. Composition. The bulk composition of each of the four deposit regions was determined from the SEMPC data and is shown in Table 2 for sample A. It was found that the only significant change in the bulk composition (K2O, SiO2) occurred between the initial and adjacent layers of the deposit. In contrast, the bulk compositions were relatively constant, within experimental error, for the other regions of the deposit. This change in composition was most likely due to the preferential deposition of small particles, whose composition was different from that of the bulk, during the initial stage of deposition.1 Qualitative information on deposit compositions in the four regions is available from the BSE images (Figures 3-6) in which the brightness of a particular point is proportional to the atomic number of the species at that point. Figure 3 shows that in the initial layer individual particles were relatively homogeneous but differed from one another in composition (brightness). Heterogeneity within individual clusters was observed in the adjacent region as shown in Figure 4, in which high-Si (dark) and high-Fe species (white) are labeled and the predominant medium gray color corresponds to aluminosilicate phases. Despite the increased sintering in the middle region, the extent of mixing was not complete on a local level, as shown in Figure 5. The outer region of the deposit was highly sintered (Figure 6); in particular, the presence of a smaller amount of discrete high-Si and high-Fe species inside the clusters was indicative of a greater degree of sintering and mixing. A significant amount of mullite crystals was also present in this region. Mullite is expected to precipitate from the aluminosilicate amorphous phase as the deposit approaches equilibrium. The possible role of equilibrium in the outer region is of interest and discussed in a later section of this paper.

The compositions at the specified analysis points throughout the deposit were classified into 32 groups using the method of Slater et al.7,8 Besides well-defined species such as quartz, iron oxide, alumina, mullite, etc., a series of phases or composition classifications characteristic of these deposits was developed. In the process of performing the classification, two key composition parameters became evident for the deposits investigated in this study: the Si/Al molar ratio and the mole percent of Fe. The complete list of composition classes and their criteria is given in Table 3. In these composition classes, the amorphous Fe-Al-Si classes are distinguished as high-Fe-Al-Si classes (8-20% Fe), medium-Fe-AlSi classes (3-8% Fe), and low-Fe-Al-Si classes (03% Fe) according to iron content. Further classification of each of these was performed based on the Si/Al ratio from class A (0.6 < Si/Al < 0.95) to class G (3.2 < Si/Al < 5.0). The amount (frequency %) present in each class in the different regions of the deposit is shown in Table 4 for sample A. Examination of Table 4 shows a wide range of composition classes present in the initial layer of the deposit, with a substantial fraction of medium- and lowFe-Al-Si classes having Si/Al ratios ranging from 0.6 to 1.7 (class A to class D). There was also a significant amount of K-Al-silicates present in this layer. The composition in the adjacent layer was significantly different, as shown in Figure 8 for nine composition classes. There was a sharp decrease in the number of points identified as belonging to the K-Al-silicate, medium-Fe-Al-Si B, and medium-Fe-Al-Si C classes, accompanied by a significant increase in the amount belonging to the low-Fe-Al-Si A, low-Fe-Al-Si B, lowFe-Al-Si C, low-Fe-Al-Si D, and medium-Fe-Al-Si D classes. Part of these compositional differences can be attributed to a change in the fraction of the impacting fly ash particles which adhere to the surface, as reflected in the different elemental composition of the adjacent layer (see Table 2). As the initial layer increases in thickness during deposition, the temperature of the surface increases due to the insulating effect of this layer. The physical nature of the surface onto which particles are impacting also changes. As a result, the tendency of particles to adhere to the surface increases. The nature of the deposit changes from the initial layer (dominated by small particles) to a deposit consisting of particles which arrive principally by inertial impaction and which more closely approximate the bulk composition of the ash. The data in Figure 8 also provide evidence for mixing of potassium aluminosilicate species with other species in the adjacent layer. In general, this mixing tends to lower the iron concentration and increase the Si/Al ratio and potassium content of the species with which the K-Al-silicates mix, consistent with the trends shown in Figure 8. Additional mixing was evident in the middle region of the deposit as shown in Table 4. The mixing or


Wang et al.

Table 3. List of Composition Classes and Defining Criteriaa 1. Quartz criterion 1: criterion 2: criterion 3: criterion 4: criterion 5: criterion 6: criterion 7:

Si g 80.0 Al e 5.0 Na e 5.0 K e 5.0 Ca e 5.0 Fe e 5.0 S e 5.0

1a. Quartz criterion 1: Si g 90.0 2. Fe2O3/FeCO3 criterion 1: Fe g 80.0 criterion 2: Si e 10.0 criterion 3: Al e 5.0 criterion 4: S e 5.0 3. Alumina Al2O3 criterion 1: Al g 85.0 criterion 2: Ca e 5.0 criterion 3: Si e 10.0 criterion 4: Mg e 5.0 criterion 5: Fe e 5.0 4. Ca-Al-Si criterion 1: 1.0Al + 1.0Si + 1.0Ca g 80.0 criterion 2: Ca g 5.0 criterion 3: Si g 20.0 criterion 4: Al g 20.0 criterion 5: Na e 5.0 criterion 6: S e 5.0 criterion 7: K e 5.0 5. K-Al-Si criterion 1: 1.0Al + 1.0Si + 1.0K g 80.0 criterion 2: K g 5.0 criterion 3: Al g 10.0 criterion 4: Si g 10.0 criterion 5: Ca < K criterion 6: Fe < K 6. Mullite criterion 1: criterion 2: criterion 3: criterion 4: criterion 5: a

1.0Al + 1.0Si g 85.0 Al/Si e 4.0 Al/Si g 2.3 Ca e 5.0 Fe e 5.0

6. Mullite criterion 6: S e 5.0 criterion 7: Ti e 5.0 criterion 8: Mg e 5.0 criterion 9: Na e 5.0 criterion 10: K e 5.0 7. High Fe criterion 1: Fe g35.0 8. Medium Fe criterion 1: Fe g 20.0 criterion 2: Fe e 35.0 9. High Fe-Al-Si A criterion 1: Si/Al e 0.95 criterion 2: Si/Al g 0.6 criterion 3: Fe g 8.0 criterion 4: Fe e 20.0 10. High Fe-Al-Si B criterion 1: Si/Al e 1.05 criterion 2: Si/Al g 0.9 criterion 3: Fe g 8.0 criterion 4: Fe e 20.0 11. High Fe-Al-Si C criterion 1: Si/Al g 1.05 criterion 2: Si/Al e 1.3 criterion 3: Fe g 8.0 criterion 4: Fe e 20.0 12. High Fe-Al-Si D criterion 1: Si/Al e 1.7 criterion 2: Si/Al g 1.3 criterion 3: Fe g 8.0 criterion 4: Fe e 20.0 13. High Fe-Al-Si E criterion 1: Si/Al e 2.2 criterion 2: Si/Al g 1.7 criterion 3: Fe g 8.0 criterion 4: Fe e 20.0 14. High Fe-Al-Si F criterion 1: Si/Al e 3.2 criterion 2: Si/Al g 2.2 criterion 3: Fe g 8.0 criterion 4: Fe e 20.0

15. High Fe-Al-Si G criterion 1: Si/Al g 3.2 criterion 2: Si/Al e 5.0 criterion 3: Fe g 8.0 criterion 4: Fe e 20.0 16. Medium Fe-Al-Si A criterion 1: Si/Al e 0.95 criterion 2: Si/Al g 0.6 criterion 3: Fe g 3.0 criterion 4: Fe e 8.0 17. Medium Fe-Al-Si B criterion 1: Si/Al e 1.05 criterion 2: Si/Al g 0.9 criterion 3: Fe g 3.0 criterion 4: Fe e 8.0 18. Medium Fe-Al-Si C criterion 1: Si/Al g 1.05 criterion 2: Si/Al e 1.3 criterion 3: Fe g 3.0 criterion 4: Fe e 8.0 19. Medium Fe-Al-Si D criterion 1: Si/Al e 1.7 criterion 2: Si/Al g 1.3 criterion 3: Fe g 3.0 criterion 4: Fe e 8.0 20. Medium Fe-Al-Si E criterion 1: Si/Al e 2.2 criterion 2: Si/Al g 1.7 criterion 3: Fe g 3.0 criterion 4: Fe e 8.0 21. Medium Fe-Al-Si F criterion 1: Si/Al e 3.2 criterion 2: Si/Al g 2.2 criterion 3: Fe g 3.0 criterion 4: Fe e 8.0

23. Low Fe-Al-Si A criterion 1: Si/Al e 0.95 criterion 2: Si/Al g 0.6 criterion 3: Fe e 3.0 24. Low Fe-Al-Si B criterion 1: Si/Al e 1.05 criterion 2: Si/Al g 0.9 criterion 3: Fe e 3.0 25. Low Fe-Al-Si C criterion 1: Si/Al g 1.05 criterion 2: Si/Al e 1.3 criterion 3: Fe e 3.0 26. Low Fe-Al-Si D criterion 1: Si/Al e 1.7 criterion 2: Si/Al g 1.3 criterion 3: Fe e 3.0 27. Low Fe-Al-Si E criterion 1: Si/Al e 2.2 criterion 2: Si/Al g 1.7 criterion 3: Fe e 3.0 28. Low Fe-Al-Si F criterion 1: Si/Al e 3.2 criterion 2: Si/Al g 2.2 criterion 3: Fe e 3.0 29. Low Fe-Al-Si G criterion 1: Si/Al g 3.2 criterion 2: Si/Al e 5.0 criterion 3: Fe e 3.0 30. High Si criterion 1: Si g 70.0 31. High P criterion 1: P g 50.0 32. Unclassified

22. Medium Fe-Al-Si G criterion 1: Si/Al g 3.2 criterion 2: Si/Al e 5.0 criterion 3: Fe g 3.0 criterion 4: Fe e 8.0

The quantities Na, etc., denote the element mole percentage on an oxygen-free basis.

assimilation of iron-containing species in this region was evidenced by a decrease in the amounts of low Fe-AlSi and by a significant increase in the amount of medium Fe-Al-Si (Table 4). This mixing was made possible by higher deposit temperatures in this region of the deposit relative to those of the inner or adjacent layers, due to the insulating effect of the inner layers. The higher temperatures decreased the viscosity of deposit and led to increased sintering, as manifested in an increased amount of medium-Fe-Al-Si phases at the expense of both high-iron- and low-iron-containing phases. Table 4 also shows a significant shift in the Si/ Al ratio of the medium-Fe-Al-Si classes and a decrease in the amount of phases containing high fractions of silicon (quartz and high Si). These compositional changes provide evidence of Si assimilation in the middle region. The trends continued in the outer region of the deposit to further increase the Si/Al ratio of the medium-FeAl-Si phases and decrease the amount of silicon-rich species. Assimilation of silicon-rich species was also strongly affected by deposit temperature. Since an

increase in the Si content of an Fe-Al-Si phase at constant temperature increases its viscosity, it was expected that the assimilation of silicon-rich species would require higher temperatures than the assimilation of iron-rich species. Two interesting observations concerning the outer layers of the deposit can be made from Table 4. First, significant amounts of mullite were found in the outer region of the deposit, in contrast to only trace amounts of mullite present in the middle region. As deposit temperatures increase, the deposit is more likely to approach equilibrium. The movement toward equilibrium will cause crystallization of mullite if the temperature is below the liquidus temperature, provided that the viscosity is sufficiently low that crystals may form. Thus, the presence of mullite may indicate equilibrium behavior in the outer region of the deposit. The significant shift in the Si/Al ratio of the medium-Fe-Al-Si classes observed in the outer region of the deposit (Table 4) was partially due to the crystallization of mullite, which left a residual amorphous phase with an in-



Table 4. Measured Compositions for Deposit Sample Aa class/index 1 quartz 2 Fe2O3/FeCO3 3 alumina Al2O3 4 Ca-Al-silicate 5 K-Al-silicate 6 mullite 7 high Fe 8 medium Fe

initial adjacent adjacent middle middle middle middle middle outer outer outer outer outer 1 2 3 4 5 6 7 8 9 10 11 12 13 3.9 0.6 0.1 2.3 19.4 0.1 1.6 0.4

8.7 0.3 0.0 4.7 4.7 0.0 1.3 0.7

5.8 0.8 0.0 3.8 1.6 0.0 2.2 1.6

1.9 0.0 0.0 0.6 1.7 0.2 1.1 1.5

2.8 0.1 0.0 0.7 1.5 0.2 1.6 1.3

1.3 0.0 0.0 0.0 0.0 0.6 0.8 0.5

0.9 0.0 0.0 0.1 0.1 0.1 0.5 0.8

6.0 0.2 0.0 0.0 0.0 0.1 0.3 0.4

0.2 0.3 0.0 0.0 0.0 2.1 0.4 0.7

0.5 0.3 0.0 0.0 0.0 2.6 0.9 1.5

0.1 0.2 0.0 0.0 0.0 1.0 0.4 0.8

0.0 0.2 0.0 0.0 0.0 1.5 0.4 1.0

0.0 0.5 0.0 0.0 0.1 0.7 0.6 0.7

1.3 1.2 1.6 0.3 0.0 0.0 0.0

1.6 0.7 1.8 0.3 0.0 0.2 0.0

1.2 0.6 1.2 0.8 0.7 0.0 0.0

1.5 0.4 0.9 0.6 0.6 0.3 0.2

1.9 0.2 1.0 0.8 0.4 0.9 0.7

0.5 0.5 0.4 0.7 0.2 0.5 0.0

0.8 0.6 0.6 0.9 0.5 0.2 0.1

0.2 0.1 0.4 0.6 0.3 0.3 0.2

0.4 0.1 0.4 0.6 0.9 4.2 0.3

0.6 0.3 0.7 1.1 2.1 10.7 0.9

0.3 0.0 0.3 0.7 0.8 2.6 0.3

0.3 0.2 0.4 0.3 0.3 1.1 0.0

0.8 0.3 0.5 0.7 0.5 0.9 0.0

16 medium Fe-Al-Si A 17 medium Fe-Al-Si B 18 medium Fe-Al-Si C 19 medium Fe-Al-Si D 20 medium Fe-Al-Si E 21 medium Fe-Al-Si F 22 medium Fe-Al-Si G

4.1 11.4 17.2 1.6 0.7 0.2 0.2

5.2 5.8 9.3 3.4 1.2 0.7 0.0

11.3 9.5 13.0 6.5 1.9 2.0 0.9

7.1 6.3 16.2 16.6 7.1 3.8 1.5

11.1 5.9 16.0 14.3 7.1 5.4 1.7

8.9 5.2 15.7 27.4 15.3 11.7 6.0

8.3 6.6 16.0 21.6 11.9 7.7 3.1

12.8 6.3 10.2 14.2 10.9 12.7 4.2

2.4 2.1 8.7 15.0 14.0 30.8 0.7

5.9 2.2 6.7 11.4 11.6 19.1 1.9

3.2 1.9 7.0 16.7 16.1 34.1 0.1

1.2 1.0 5.2 12.7 15.8 38.9 0.2

1.5 2.1 10.3 23.2 20.5 19.6 0.0

23 low Fe-Al-Si A 24 low Fe-Al-Si B 25 low Fe-Al-Si C 26 low Fe-Al-Si D 27 low Fe-Al-Si E 28 low Fe-Al-Si F 29 low Fe-Al-Si G

6.5 12.4 6.7 1.2 0.4 1.1 1.0

11.4 15.1 12.1 3.0 1.6 1.8 1.3

7.0 9.5 8.5 3.3 1.5 1.7 1.1

3.4 2.0 5.3 5.1 4.5 4.6 2.6

1.8 1.1 2.6 4.5 3.8 3.8 4.8

0.6 0.1 0.3 0.5 0.5 0.4 0.0

2.5 1.4 3.4 3.5 2.5 2.4 1.4

2.8 3.0 3.8 2.5 1.4 1.5 1.2

5.6 1.5 3.0 1.6 0.7 0.2 0.0

2.6 0.7 1.3 0.9 0.2 0.2 0.1

6.2 1.1 1.9 0.8 0.1 0.0 0.0

7.8 2.1 3.4 1.9 0.5 0.1 0.0

5.0 2.3 5.0 2.1 0.5 0.0 0.0

2.6 0.0 0.1

2.7 0.2 0.2

2.0 0.0 0.2

0.9 0.0 1.7

0.9 0.0 1.3

0.4 0.0 1.3

0.9 0.1 0.8

1.1 0.0 2.2

0.1 0.0 3.1

0.6 4.0 8.6

0.0 0.0 3.3

0.0 0.0 3.6

0.0 0.1 1.5

9 high Fe-Al-Si A 10 high Fe-Al-Si B 11 high Fe-Al-Si C 12 high Fe-Al-Si D 13 high Fe-Al-Si E 14 high Fe-Al-Si F 15 high Fe-Al-Si G

30 high Si 31 high P unclassified a

In frequency percent.

Figure 9. Distribution of analysis points according to Si/Al ratio for medium Fe-Al-Si composition classes in the outer region of both samples. Figure 8. Major composition classes in the initial and adjacent layers.

creased Si/Al ratio. Second, medium-Fe-Al-Si classes accounted for approximately 70-80% of the total phases measured in the outer region for both of the deposits examined, and a significant difference between the two samples was observed in this region (Figure 9). Sample A contained a substantial amount of medium-Fe-AlSi F class (Si/Al ) 2.2-3.2), while sample B included significant amounts of both medium-Fe-Al-Si F class (Si/Al ) 2.2-3.2) and medium-Fe-Al-Si G class (Si/Al ) 3.2-5.0). It is believed that the higher Si/Al ratio observed in sample B was the result of exposure to higher temperatures. Higher temperatures would be

required to overcome the viscous forces in order to increase the fraction of Si in the amorphous Fe-Al-Si species. Figure 10 provides ternary histograms of the adjacent, middle, and outer regions of sample A which are used to further illustrate the assimilation of iron and silicon. In these histograms, the height of the bars represents the relative amounts (frequency %) of each composition present. The predominant compositions in the adjacent layer correspond to kaolinite-derived phases present in the fly ash. In contrast, the middle region shows evidence of increased mixing and assimilation of ironrich and silicon-rich species. The movement of Si-rich and Fe-rich species away from the Si and Fe vertexes is evident in the histogram for this region, resulting in


Wang et al.

Figure 11. Fe-Al-Si equilibrium diagram showing crystalline mullite (C), bulk (B), and amorphous (A) compositions (adapted from ref 12).

Figure 10. Ternary histograms for the adjacent, middle, and outer regions of sample A. The axes show normalized mole percent, and the bar height is frequency percent.

an increase in the Si and Fe content of the Fe-Al-Si species that make up most of the deposit. The increase in the Fe content of the Fe-Al-Si species is shown by the movement of the Fe-Al-Si species slightly toward the Fe vertex, while the increase in Si content is shown by the movement of the Fe-Al-Si species toward the Si vertex. Additional assimilation of Si-rich species in the outer region is also evident in Figure 10, leading to increased homogeneity of the deposit. The gap in the ternary histogram of the outer region between the medium-Fe-Al-Si A (Si/Al ≈ 2/3) composition and the mullite (Si/Al ≈ 1/3) composition is indicative of mullite crystallization in the outer region. Equilibrium. One of the purposes of this study was to characterize the extent to which equilibrium could be used to represent adequately the measured compositions in the deposits. Equilibrium phase diagrams have been used in the literature to describe ash deposits.9 Figure 11 provides a ternary phase diagram for the FeAl-Si system of interest to the present study. Almost all of the compositions measured locally in the outer region of the deposit fall into the mullite primary field. When crystals are precipitated, the composition of the remaining amorphous phase moves away from the

crystal composition along the projection of the straight line connecting the crystal composition and the bulk composition. For example, the bulk composition (B) in the mullite primary crystallization region (labeled in Figure 11) will precipitate mullite crystals (C) and the composition of the remaining amorphous phase will lie on the line CBA between points B and A, depending on the system temperature. If a deposit system is in equilibrium, the isothermal lines in Figure 11 show the temperature at which the amorphous and crystalline phases will coexist. The relative amounts of these phases are given by the inverse lever rule. The applicability of using equilibrium to approximate the local phase distribution in the deposit was evaluated as part of the present study. In doing this, equilibrium was assumed to apply locally at each cluster rather than globally for the average composition of the deposit. From a modeling perspective, use of equilibrium to describe the deposit represents a significant simplification. Otherwise, it would be necessary to model the sintering behavior explicitly in order to predict the local properties of the deposit. Equilibrium is not expected to apply to the innermost regions of the deposit, which are relatively cool and unsintered. In contrast, the outer region of the deposit is at the highest temperature and is most likely at equilibrium. The outer region will be considered first since it is unlikely that equilibrium is reached in other regions if it is not achieved in the outer region. Several different observations were used to assess whether the outer layer was at equilibrium. While none of these observations alone provides definitive proof, the combination of the observations provides a good indication of the state of the deposit. The observations are detailed in the paragraphs which follow. The first observation regarding the outer region was the presence of mullite crystals. Crystals are clearly present in micrographs of the deposit and were identified as mullite based on composition. Mullite crystals might be expected under equilibrium conditions, although equilibrium is not required for crystallization to occur. In fact, poorly formed crystals such as those



Figure 12. Mullite weight percentage predicted by an equilibrium code10 for the outer region of sample A.

present in the deposit are an indication that equilibrium was not present. The second observation was that the amount of mullite measured by SEMPC was significantly less than the amount expected under equilibrium conditions. Equilibrium calculations10 were used to predict the equilibrium amount in order to account for the influence of components, such as potassium, which are not included in the ternary phase diagrams. Calculations were performed for a range of assumed temperatures from 1300 to 1700 °C. The results, shown in Figure 12, indicate that significant amounts of mullite are expected at temperatures lower than 1500 °C under equilibrium conditions. The calculations also indicate that no mullite crystals are expected at temperatures greater than 1700 °C. This temperature compares well with the liquidus temperature of 1720 °C estimated from the ternary diagram (Figure 11) using the normalized composition of the outer layer from Table 2. Deposit temperatures were estimated with the use of a deposition model together with a comprehensive coal combustion code.11 These simulations indicated that temperatures in the outer layer of the deposit were less than 1300 °C, well below the liquidus temperature. Consequently, one would expect significant amounts of mullite in the deposit (Figure 12). In contrast, the amount of mullite measured in the outer region by SEMPC was less than 3% (see Table 4). In making this comparison, there was concern that the SEMPC measurement underestimated the amount of mullite present in the deposit. It is possible for the excitation volume associated with the electron beam at a particular analysis point to include a portion of the glassy matrix in addition to the mullite crystal. Under such conditions, the measured composition would be a mixture of the mullite composition and that of the glass matrix. In recognizing this possibility, however, we note that manual placement of the beam on a mullite crystal yielded the expected crystal composition. Therefore, the mixed compositions would only be expected for small (9) Kalmanovitch, D. P.; Sanyal, A. J. Inst. Energy 1986, 59, 2023. (10) Harb, J. N.; Munson, C. L.; Richards, G. H. Energy Fuels 1993, 7, 208-214. (11) Wang, H. Ph.D. Dissertation, Department of Chemical Engineering, Brigham Young University, Provo, UT, December 1998. (12) Messina, C. G. Phase Diagrams for Ceramists; American Ceramic Society: Columbus, OH, 1986.

Figure 13. Ternary diagrams of the outer regions of sample A and sample B (composition scales in wt %).

poorly formed crystals and/or for situations where the beam hits the edge of a crystal. Poorly formed crystals are themselves evidence of nonequilibrium crystallization, and edge effects are not expected to be important. A relatively dense analysis grid (

Microanalytical Characterization of Slagging Deposits from a Pilot

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