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Energy & Fuels 2003, 17, 1222-1232
Tracing the Origin of Unburned Carbon in Fly Ashes from Coal Blends Katia S. Milenkova,† Angeles G. Borrego,*,† Diego Alvarez,† Jorge Xiberta,‡ and Rosa Mene´ndez† Instituto Nacional del Carbo´ n, CSIC, Ap. 73, 33080 Oviedo, Spain, and Departamento de Energı´a, ETSIMO, Independencia 13, 33004 Oviedo, Spain Received February 25, 2003. Revised Manuscript Received July 8, 2003
This study shows the utility of optical microscopy to trace the origin of unburned carbon in fly ashes from coal blends. Coal blends currently burned in four power groups in Spain have been investigated. Mill samples were separated into size fractions, and petrographic analysis were performed to assess the extent of blend components segregation as a function of different coal grindability. Strong segregation was observed when coals from a wide rank interval were ground together. In this case, anthracite particles accumulated in the largest size fractions, high and medium volatile bituminous coals accumulated in the smallest ones, and semianthracites accumulated in the intermediate size fractions. No clear segregation trend was observed for anthracites ground together with an inertinite-rich semianthracite. Chars from single coals were prepared in a drop tube furnace operated at 1300 °C under 5% oxygen in a nitrogen atmosphere. Petrographic analysis of chars were based on the optical texture and development of porosity, which are features that are maintained even in extensively burned material. This information was used to identify the coal responsible for the unburned carbon particles in fly ashes. Vitriniteand inertinite-derived material from anthracite and semianthracite were the major components of unburned carbon; however, inertinite from high volatile bituminous coals and other nonassignable inertinites also were recorded. Results indicate a rather different combustibility for the different coals in the blends. High volatile bituminous coals burned almost to completion, whereas the semianthracite-derivedsand, particularly, the anthracite-derivedsparticles were responsible for most of the unburned material. This was true even for blends where anthracite was only a minor component of the feed blend.
Introduction More than 80% of the total coal produced in the world is burned in pulverized fuel power plants to produce electricity, resulting in the emission of both gaseous and particulate matter and the generation of ∼480 million tons of fly ashes per year as byproduct.1 Despite the ongoing development of new products that incorporate fly ash, a considerable amount of them are used in rather unspecific applications, such as landfill, road bed, or open cast restoration material,2 which are not subject to particularly stringent specifications. Fly ashes are still mainly used as an additive in the cement and concrete industry,3 for which the unburned carbon content must be kept to 150, 150-100, 100-75, 7545, 45-20, and 150 µm (vol %) 100-150 µm (vol %) 75-100 µm (vol %) 45-75 µm (vol %) 20-45 µm (vol %) 150 µm (vol %) 100-150 µm (vol %) 75-100 µm (vol %) 45-75 µm (vol %) 20-45 µm (vol %) 150 µm (vol %) 100-150 µm (vol %) 75-100 µm (vol %) 45-75 µm (vol %) 20-45 µm (vol %) 150 µm (vol %) 100-150 µm (vol %) 75-100 µm (vol %) 45-75 µm (vol %) 20-45 µm (vol %) 75 µm in size, and from this fraction, 10% was hardto-burn anthracite particles. A high grinding efficiency was also observed in blend M4, where only 10% of particles were >75 µm in size and from this fraction, only 1% corresponded to anthracite particles. The lowest grinding efficiency was observed in blend M3, of which 36% of the sample was >75 µm. Brief Petrographic Description of the Chars. The morphology and optical texture of the chars are dependent on the operating conditions, the extent of burning, and the plastic behavior of the coal-forming material. Overall, it is accepted that temperatures of ∼1300 °C yield chars that morphologically resemble those of full-scale power plants. The 5% oxygen concentration was selected to prevent excessive swelling that would have occurred under nitrogen and, at the same time, to ensure the persistence of enough carbonaceous material to study its optical characteristics. Chars from the high-volatile bituminous coal A2 were mainly formed by cenospheric particles that had abundant secondary porosity within the walls and exhibited either isotropic or incipient anisotropic optical texture (see Figure 4a, b). A range of variation was observed in the appearance of inertinite-derived material, most of it maintaining its original shape and showing limited signs of thermal alteration (Figure 4c, e). Most of the chars from semianthracite (C2 and M4) showed rounded but not spherical external shapes and networklike structures (Figure 4f-h). The walls were formed by strongly anisotropic material with well-developed domains, indicating the passage through a metaplast stage where the aromatic lamellae that form the coal were able to grow and stack (Figure 4f). A large variety of inertinite-derived material was observed in M4 char where similar amounts of fused inertinitesboth isotropic (Figure 4g) and anisotropic (Figure 4h)swere recorded. The anisotropic, porous, inertinite-derived material in semianthracite chars showed smaller and more-elongated pores parallel to each other and more-angular shapes than the vitrinite-derived particles. Also, some inertinites remained
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Table 4. Petrographic Analysis of the Unburned Carbon in the Fly Ashesa PP1 classes
80% H1L8
20% H1L2
80% H1R8
20% H1R2
vitrinite in anthracite inertinite in anthracite anisotropic material from semianthracite unfused inertinite from semianthracite fused isotropic inertinite isolated unfused inertinite isotropic fragments anisotropic fragments Corg. (wt %)
56.3 3.9 27.0 1.4 3.9 1.1 4.3 2.1 4.70
40.2
66.6
43.1 2.7 8.3 2.7 2.0 1.0 4.68
24.3 1.2 3.7 1.5 1.7 1.0 6.82
36.0 0.2 45.7 1.4 7.6 1.6 3.3 4.2 6.53
a
PP2b H2L1
H2L2
H2R1
H2R2
56.4
45.3 0.5 43.4 2.5 2.1 0.2 2.1 3.9 3.46
70.6
68.6 0.3 29.2 1.1 0.8
29.3 3.6 7.6 1.0 0.8 1.3 4.31
20.5 0.3 7.2 0.3 0.8 0.3 8.97
8.36
H3 from PP3
H4 from PP4
97.4 2.0 0.7
55.5
1.13
30.6 1.9 7.0 0.5 2.1 2.4 9.80
All values in table given as volume percentages. b Collection/recovery in PP2 was ∼95% for all sides.
Figure 5. Typical view of unburned vitrinite from anthracites, showing a burning pattern parallel to the bedding plane. Top photographs are optical microscopy images taken under reflected light, oil immersion, with crossed polars and a 1 λ retarder plate. Bottom image is a SEM secondary electron image.
unfused and isotropic. The anthracites do not fuse under the operating conditions of pulverized fuel boilers, and only some of the inertinites still retain enough volatiles to develop devolatilization voids. The morphology of the particles was maintained after leaving the reactor, and the main change was the formation of small cracks parallel or subparallel to the bedding plane (Figure 4ik). The features described previously were sufficient to distinguish between vitrinite- and inertinite-derived material from the high volatile bituminous coals, the semianthracites, and the anthracites. The optical characteristics of the medium volatile bituminous coal chars (B1, B2, B4) would be intermediate between those of semianthracite and high volatile bituminous, and, given the low concentration of these components in the blends, it would be quite difficult to try to separate them. The same applies to several of the anthracites, because the
lower the rank of the anthracite, the more drastic the transformations in the reactor;27 thus, particles from lower- and higher-rank anthracites would only differ in the amount of cracks, which is an insufficient criterion to establish divisions.28 Fly Ash Characterization. The ash, carbon, and sulfur contents of the fly ashes are shown in Table 3. A broad range of variation was observed in the carbon content (1.13%-9.8%), with the lowest values corresponding to the boiler that was burning local anthracites (H3). PP1 and PP2 provided fly ash from both sides of the boiler. The carbon content indicated a certain (27) Me´ndez, L. B. Interactions of Minerals-Organic Matter in Anthracites Combustion in Pulverised Fuel Boilers (in Sp.). M.Sc. Thesis, University of Oviedo, Oveido, Spain, 1999. (28) Alonso, M. J. G.; Borrego, A. G.; Alvarez, D.; Parra, J. B.; Mene´ndez, R. J. Anal. Appl. Pyrolysis 2001, 887-909.
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Energy & Fuels, Vol. 17, No. 5, 2003 1229
Figure 6. Unburned anthracite particles in the fly ashes. Top photographs are OM images showing the vitrinite-inertinite association in the parent coal (reflected light, oil immersion, under crossed polars). Middle photographs are unburned anthracite particles that are likely derived from vitrinite-inertinite associations similar to those in the upper portion of the figure (reflected light, oil immersion, with crossed polars and a 1 λ retarder plate). Bottom photographs are SEM images illustrating the combustion pattern of compact anthracite particles.
asymmetry in the combustion performance, because the carbon content in samples from one side of the boiler was greater than the carbon content in samples from the other side, the differences being lower for PP1 (2%) than for PP2 (4%). No significant differences were observed between the amount of carbon in the samples from the first and the second row of hoppers in PP1, whereas the sulfur contents were greater in the fly ash from the second row. Petrographic analysis of the concentrated carbon allowed the parent coal to be identified, keeping the
uncertainty within reasonable limits. The classes established in Table 4 are a compromise between what can be readily distinguished with OM and what might be relevant for improving the understanding of coal blend performance. The anthracite particles underwent limited transformation, because of thermal stress in the boiler; therefore, parent macerals were identifiable and, thus, inertinite- and vitrinite-derived material in anthracite particles were distinguished. Most of the anthracite particles in fly ash burns following a combustion pattern through the structural planes (see Figure
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Figure 7. Typical view of unburned vitrinite-derived material from semianthracite, as observed via OM (upper photographs) and SEM (lower photographs).
5). This behavior, which is clearly illustrated by SEM, is characteristic of the combustion of ordered aromatic structures, because the edge carbons in the graphenic units are more reactive than carbons in the basal units.29 In addition, mineral matter linings are typically parallel to the bedding plane in the particles, thus increasing the surface accessibility to the reacting gas. Less common is the finding of compact anthracite particles that exhibit, as a single sign of burning, small holes on the surface (see Figure 6). These particles were observed by OM as including both inertinite and vitrinite (intimately associated) and were also identified in the parent coal (Figure 6). Contrary to vitrinite, whose chemical structure approaches a graphitelike structure by burial, the carbon-rich, cross-linked structure of inertinite, achieved as a consequence of early alteration in the depositional setting, has a limited ability to order during coalification and remains isotropic, even in high-rank anthracites. The inertinitevitrinite associations diminish the homogeneity of the particle and would hinder the progress of combustion through the structural planes. Data in Table 4 shows that anthracite-derived material was the major component of the fly ash, regardless of the proportion of anthracite in the feed blend, which indicates that it is more difficult to burn than any other accompanying material. Anthracites can be burned very efficiently, as shown by the fact that the unit that was burning only anthracites had the lowest carbon content (29) Marsh, H.; Kuo, K. In Introduction to Carbon Science; Marsh, H., Ed.; Butterworth: London, 1989; pp 107-151.
in ash but, when the boiler is adjusted to burn highervolatile coals, the anthracites had a tendency to accumulate in the fly ashes. Anthracites not only yield the highest amount of char (5%-7% higher than the semianthracites) but also yield the most refractory material, because they were only fed in amounts of 10%, 27%, and 12% in PP1, PP2, and PP4, respectively, but accounted for 53%-60%, 45%-71%, and 56% of the unburned material in the fly ashes. For each combustion group, it was found that the higher the anthracitederived material, the greater the amount of unburned carbon in the ash. The large size of some of the anthracite particles undoubtedly will be a factor that influences their poor combustibility; however, no general relationship could be found between the amount of large anthracite particles and the unburned carbon values and/or the amount of anthracite-derived unburned carbon. Semianthracite particles underwent drastic changes after entering the boiler. Vitrinite of this rank released volatiles and passed through a metaplast stage, with the subsequent reorganization of the remaining carbonaceous material. As a result, the particles swelled and developed vesicles and the char walls were formed from well-ordered anisotropic domains. Most of the semianthracite-derived particles observed in the fly ash had a single central void, more-rounded shapes, and smaller size anisotropy in the walls (see Figure 7) than those of the 1300 °C chars, which indicated that the heating rates and temperatures in the industrial boiler were higher than in the drop tube reactor. Some of the
Tracing the Origin of Unburned Carbon in Fly Ashes
Energy & Fuels, Vol. 17, No. 5, 2003 1231
Figure 8. Typical view of unburned anisotropic inertinite-derived material from semianthracite, as observed via OM (upper photographs) and SEM (lower photographs).
inertinites in the semianthracite still retained certain plastic properties and were able to fuse. They typically developed network structures with elongated voids subparallel to each other (see Figure 8). Because all occurrences between anisotropic networks and anisotropic cenospheres were found and sometimes the differences between vitrinite-derived and anisotropic porous inertinite-derived material from semianthracite were subtle, it was decided to group them in a single class. Unfused inertinites associated with the material described previously were counted separately. The anisotropic porous semianthracite-derived material was the second major contributor to the unburned material in the fly ashes (see Table 4). Most of this material was derived from vitrinite in M1 and M2, although, as observed in the chars, a significant portion of M4 inertinite yielded either isotropic or anisotropic porous char. Even in the group that was burning an inertiniterich semianthracite (PP4), the amount of unfused inertinite from semianthracite was rather small, which indicated that, at this high rank, most of the components still retained plastic properties. The semianthracite porous material preferentially concentrated in the second row of hoppers in PP1 (H1L2 and H1R2), as opposed to the massive anthracite particles that accumulated in the first row (H1L8 and H1R8). As shown by the petrographic study of the chars, vitrinite from the high-volatile bituminous coals yielded isotropic to incipient anisotropic cenospheric particles. No particles with these characteristics were identified in the fly ashes from PP1 and PP2, which indicated that this material was more reactive and burned away. Some
isotropic inertinites that exhibited porosity development (see Figure 9a) could be attributed to the high-volatile bituminous coals in PP1 and PP2, whereas in PP4, isotropic particles with elongated pores and thin walls are supposed to be derived from some of the inertinite in the semianthracite. Figure 9b shows the aspect of some isolated inertinites that remained unchanged and that could be derived from any of the feed coals. Conclusions This study has shown the ability of optical microscopy (OM) to trace the origin of unburned carbon in fly ashes. Vitrinite-derived material and inertinite-derived material from different coals can be readily distinguished, provided that the differences in rank between the coals are sufficiently large. The procedure is particularly useful for identifying the origin of unburned carbon in ash from coal blends. When coals ranging in rank from high-volatile bituminous to anthracite were ground together, the anthracites had a tendency to concentrate in the largest size fractions and the high- and medium-volatile bituminous coals had a tendency to concentrate in the smallest size fractions. Only a slight trend toward segregation was observed for the vitrinite-rich semianthracites that concentrated in the intermediate size fractions. The behavior of an inertinite-rich semianthracite was different, because it segregated into the smallest size fractions. No clear trend to segregation was determined for the accompanying anthracites. In blends that contained anthracites and lowerranked coals, anthracites proved to be rather refractory,
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Figure 9. (Top) OM images of (a) porous, isotropic, inertinite-derived unburned material probably generated from the highvolatile bituminous coal and (b) unfused inertinite of uncertain provenance. (Bottom) SEM images of isolated unfused inertinites.
because fly ashes were significantly enriched in anthracite-derived material. Size segregation would contribute to the poor combustibility of anthracite particles in the blend. Most of the semianthracite-derived material recorded in the fly ash showed intense porosity development and anisotropic domains, which indicated passage through a metaplast stage. In the blends that contained high-volatile bituminous coals, they burned almost to completion. OM has also shown the ability to detect differences in the boiler performance. A certain asymmetry has been observed in both PP1 and PP2, because the carbon
contents of the fly ashes from each side of the boiler, and also the petrographic composition of the unburned material, were similar to each other but different from those on the other side. Acknowledgment. Financial support through the European Project No. ECSC-PR071 is gratefully acknowledged. K.S.M. thanks the Spanish Agency for International Cooperation for a pre-doctoral fellowship. The Power Plants at Narcea, Soto de Ribera, and Lada are thanked for the kind donation of the samples. EF0300388
