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Fragmentation Behavior of Pyrite and Calcite during High-Temperature Processing and Mathematical Simulation Li Yan,* Raj Gupta, and Terry Wall CRC for Black Coal Utilization, Department of Chemical Engineering, University of Newcastle, Callaghan, NSW 2308, Australia Received July 12, 2000. Revised Manuscript Received November 13, 2000
The fragmentation behavior of pure pyrite and calcite during coal combustion was examined in a laminar drop-tube furnace at 1300 °C. A scanning electron microscope (SEM) and a laserdiffraction particle sizer (Malvern) were used to analyze particle size variation from raw minerals to ash residues. Poisson distribution was used to simulate the particle size variation resulting from the fragmentation of mineral grains. The results for pyrite suggested that a large pyrite grain most likely generated four pieces of fragments, whereas a calcite was most likely expected to produce three segments due to fragmentation. The SEM images of calcite high-temperature residue indicated that the fragmentation of calcite could be more extensive at higher boiler flame temperatures. These simulated results can be used in a comprehensive model of ash formation in pulverized coal combustion.
Introduction Minerals in a pulverized coal are generally classified into included and excluded minerals, respectively, on the basis of their associations with coal carbon matrix. Fragmentation of excluded minerals into a number of smaller particles is important as it influences the particle size distribution (PSD) of fly ash, particularly for a coal with a significant fraction of excluded minerals. For instance, Helble et al.1 reported a North Dakota lignite with a significant fraction of excluded pyrite exhibited a much finer PSD of combustion ash compared to that of minerals in the parent coal. The ash particle size has been found to affect ash transport behavior to a great extent. Large ash particles tend to impact boiler heat-transfer surfaces by inertia, whereas fine ash particles tend to reach wall surfaces by thermophoresis or Brownian motion. For instance, a 60 µm ash particle was estimated to reach the deposit surface almost three times faster compared to a 30 µm particle primarily due to inertial effect.2 Figure 1 shows the effect of particle sizes on ash arrival velocity simulated in a test furnace.3 Therefore, understanding of the transformation behavior of excluded minerals in the course of coal combustion is important. Generally, mineral fragmentation is attributed to thermal shock due to rapid heating rate in the order of magnitude of 1000 °C/s and rapid gas release from * Corresponding author. Fax: +61-2-49218692. E-mail: YanL@ email.com. (1) Helble, J. J.; Srinivasachar, S.; Boni, A. A. Prog. Energy Combust. Sci. 1990, 16, 267-279. (2) Gupta, R. P.; Rezaei, H. R.; Gupta, S. K.; Wall, T. F. Final Report submitted to IHI Engineering Australia Pty Limited, CRC for Black Coal Utilization, Newcastle, 1999. (3) Yan, L. Ph.D. Thesis, Department of Chemical Engineering, the University of Newcastle, 2000.
Figure 1. Effect of particle size on ash arrival velocity on heat-transfer surfaces.
inside the particle. Thermal shock leads to sharp temperature gradients within a particle, resulting in the development of strong stresses within the particle. But the thermal shock alone may not sufficiently account for fragmentation. Rapid gas release (burst) may be another important factor, which leads to internal pressure build-up. A third mechanism is spalling as a result of chemical reaction involving a density change. The fragmentation is related to the thermal behavior of individual mineral species. Among the dominant mineral species occurring in coal, silicate minerals including quartz, illite, etc., were reported not to experience fragmentation on rapid heating to 2000 K.4 Experiments conducted by Srinivasachar et al.5 indicated no obvious change in PSD of illite after two seconds (4) Raask, E. J. Inst. Energy 1984, 57, 231-239.
10.1021/ef000157c CCC: $20.00 © 2001 American Chemical Society Published on Web 01/26/2001
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residence time of high temperature (1500 K) processing in a drop-tube furnace. More recently, ten Brink et al.6,7 found that pure siderite and ankerite grains did not fragment in a laboratory facility (1350-1600 °C) similar to boiler burners. Under the same conditions (13501600 °C), pyrite and calcite indeed undergo fragmentation.7,8 Excellent SEM images were obtained by Srinivasachar et al.9 clearly showing fissures and cracks within decomposed pyrite (pyrrhotite) in the first stage of the pyrite transformation. However, a few semiquantitative descriptions in the literature are not consistent. Ten Brink et al.7 suggested one calcite particle of 60 µm broke up into fragments approximately 10 times smaller in a laboratory facility in which the environment of coal burner was simulated. This may be suspicious because the small fragments were observed to adhere to large silicate particles. There is little chance for excluded mineral grains to impact and adhere to silicate particles. On the other hand, Raask8 indicated that 50 µm calcite particles fragmented into segments of 10∼20 µm in size on rapid heating in a Leitz heating microscope (up to 2000 K at a heating rate 1000 K/s). In this work, experiments have been carried out to examine the changes in PSD of pure minerals of pyrite and calcite during high-temperature processing in a drop-tube furnace (DTF). For a better quantitative description of fragmentation behavior of these reactive mineral species, the Poisson distribution is used to simulate the variation of PSD. Experiments and Results Experimental Apparatus and Operating Conditions. Experiments of high-temperature processing were conducted in a drop-tube furnace (Astro 1000A graphite). A schematic diagram of the drop-tube furnace is shown in Figure 2. The recrystallized alumina core is 5 cm in its inner diameter and 54 cm in length. The central hot zone within the core is 25 cm. Sample particles were fed into the drop-tube furnace through a semi-fluidized bed by air entrainment. The feeding rate of sample was kept around 2-3 g/h. Compressed air was used as the primary air and secondary air. The total flow rate of the primary air plus secondary air was approximately 1700 mL/min and was held constant during experiments. The temperature of alumina tube was held constant at 1300 °C. The measured gas temperature inside the alumina tube approximated 1300 °C in the central point along the vertical direction, but was 100-200 °C below 1300 °C on the inlet point and outlet point. The residence time of particles in the hot zone of the furnace was estimated around 2 s under typical operating conditions. Combustion residues were collected at the bottom of the furnace with a collection probe that was fixed in position during all experiments. Air was used as the quench gas and wall gas for the collection probe. Solid spherical particles were assumed in the analysis of size distributions. The primary particle sizer used was the Malvern MasterSizer/E, which is based on the principle of laser (5) Srinivasachar, S.; Helble, J. J.; Boni, A. A.; Shan, N.; Huffman, G. P.; Huggins, F. E. Prog. Energy Combust. Sci. 1990, 16, 293-302. (6) Ten Brink, H. M.; Eenkhoor, S.; Weeda, M. The Impact of Ash Deposition on Coal Fired Plants, Solihull, England,; Taylor & Francis: 1993; pp 373-383. (7) Ten Brink, H. M.; Eenkhorn, S.; Weeda, M. Fuel Process. Technol. 1996, 47, 233-243. (8) Raask, E. Mineral Impurities in Coal Combustion: Behavior, Problems and Remedial Measures; Hemisphere Publication Corporation: Bristol, PA, 1985. (9) Srinivasachar, S.; Helble, J. J.; Boni, A. A. Prog. Energy Combust. Sci. 1990, 16, 281-292.
Yan et al. ensemble light scattering.12 When a particle scatters light, the measured light energy on the detector has a peak at a favored scattering angle which is related to its diameter. Large particles have peak energy in small angles of scatter and vice versa. The output signals from the detector are processed by a computer to provide size information. The SEM imaging method was used to obtain particle size information when the Malvern sizer was not suitable in the case of calcite product (CaO). In the SEM method,13 particles were well mixed with epoxy resin prior to cross-sectioning. The solidified pellets were sectioned and polished using ethanol. Then the pellet was placed into a SEM for image acquisition. The images of particle cross-sections were processed using an image analytical software to generate the particle size distribution. The size distribution based on area-equivalent circular diameter was taken as an approximate particle volumetric size distribution without stereological correction, because the correction effect on particle size distribution was generally small3 and the correction did not influence the comparison of size distribution in this study. Mineral Samples Studied. Two raw samples of pyrite and calcite were purchased from Geological Specimen Supplies (NSW 2074, Australia). These samples were selected to be representative of excluded minerals in pulverized coals as they came from natural deposits. The raw samples were crushed with a mortar mill and sifted into a number of size ranges using laboratory test sieves. Considering the combination of the DTF feeder performance and the size range of excluded mineral grains usually occurring in pulverized coal, the sieved fractions in the nominal size range of +45-63 µm were used in the experiments. The measured particle size distributions for the samples and the resulting ashes (residues) after hightemperature processing are reported in the following section. Experimental Results. The SEM technique was used to examine the variation of size and morphology of calcite in the high-temperature processing. Figure 3 compares a SEM image of calcite particles in crystal shape with sharp angles to that of the resulting residues. It is evident that fragmentation occurred during the high-temperature processing. Fissures and cracks are obvious on most of particle cross-sections. This may suggest a more severe fragmentation at higher temperatures (flame temperatures in furnace can be higher than 1500 °C). The sharp shape of calcite residue suggests no melting occurs during the high-temperature processing. The extent of fragmentation, however, is not as extensive as reported by ten Brink et al.7 A small fraction of calcite particles appear not to fragment, whereas a small fraction of calcite particles undergo comparatively extensive fragmentation. This indicates the stochastic nature of the fragmentation process. Figure 4 shows the comparison of size distribution between calcite and its high-temperature residue. Both curves were obtained with SEM image analysis. The volume-median diameter d50 (corresponding to 50% cumulative volume undersize) decreases from 44 µm for the raw material to 34 µm for the residue, indicating a certain extent of fragmentation. Both the sized pyrite sample and its high-temperature residue were examined by a Malvern sizer. The particle size distributions of both samples are shown in Figure 5. It is clear that pyrite fragmentation does occur during the high-temperature processing, with the volume-median diameter d50 changing from 65 µm for raw pyrite to 40 µm for its residue. SEM images obtained by Srinivasachar et al.9 clearly reveal that fissures occur on cross-sections of completely decomposed pyrite (pyrrhotite), which facilitate the possibility of fragmentation. (10) Tukey, J. W. Ann. Math. Stat. 1949, 20, 523-539. (11) Ten Brink, H. M.; Eenkhoorn, S.; Hamburg, G. Fuel 1996, 75, 945-951. (12) Malvern Instruments Ltd., MasterSizer/EsUser Manual, 1991. (13) Wu, H. W. Ph.D. Thesis, Department of Chemical Engineering, the University of Newcastle, 2000.
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Figure 2. System schematic diagram of the drop-tube furnace used.
Simulation of PSD Variation Based on Poisson Distribution The stochastic nature of the fragmentation of minerals during combustion requires a proper probability method to be employed to simulate the PSD changes. Analogous to an approach used by Tukey10 to describe molecular chain fragmentation of a mass of rubber due to oxidation with oxygen, a simple two-step simulation of excluded mineral transformation is used to account for physical fragmentation and chemical reactions. The first step is to assume that excluded mineral grains undergo physical fragmentation without chemical reactions. Then in the second step, each piece of the resulting fragments from the first step experiences chemical reactions (decomposition and/or oxidation) to become complete oxide products without further fragmentation. In fact, mineral physical fragmentation
takes place simultaneously with chemical reactions. This disengagement is wholly for mathematical treatment. Considering fragmentation first or chemical reactions first is not essential. Physical Fragmentation. Suppose there are a known number (N) of mineral grains prior to hightemperature processing. If the average number of fragments produced per original grain is estimated by some means, then the probability that an original mineral grain fragments into b + 1 segments (Pb) may be given by the Poisson distribution:
Pb )
e-λ‚λb (b ) 0, 1, 2, ...) b!
(1)
where the Poisson parameter λ is the average net growth number of particles per parent particle after
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Figure 4. Comparison of particle size distribution of calcite with its residue as measured by SEM.
Figure 5. Comparison of particle size distribution of pyrite with its residue as measured by Malvern sizer. Figure 3. (a) SEM image of calcite (the white bar size is 200 µm). (b) SEM image of calcite high-temperature residue (the white bar size is 200 µm).
fragmentation (i.e., the number of fragments per parent particle minus one). The possible number of original mineral grains, Nb, each of which breaks into b + 1 fragments, is estimated by -λ
mineral type
molecular formula
mass change factor
siderite calcite dolomite ankerite pyrite pyrrhotite gypsum
FeCO3 CaCO3 (Ca Mg) (CO3)2 (Ca, Fe, Mg)CO3 FeS2 Fe0.877S CaSO4‚2H2O
0.69 0.56 0.52 0.60 0.67 0.77 0.40
into account by a “size-change factor” which is defined by
b
e ‚λ (b ) 0, 1, 2, ...) Nb ) N‚Pb ) N‚ b!
Table 1. Mass-Change Factors for Some Major Minerals during Transformation
(2)
3
Cd ) In eqs (1) and (2), the only unknown variable is the Poisson parameter λ, which is related to specific mineral species and is fitted with experiment data. The offspring particles from a parent particle are assumed to be the same in size and composition. Chemical Reactions. The next step is to deal with chemical reactions of individual offspring particles generated from physical fragmentation. Even without fragmentation, one original mineral grain undergoes possible chemical reactions in the high-temperature processing, giving rise to mass and density changes. Under the assumption of solid particle structure, the reaction effect on size variation can be taken
x
Fmin Fox
Cm‚
(3)
Where, Cd ) size-change factor due to chemical reactions (without fragmentation); Cm ) mass-change factor due to reactions; Fmin ) mineral density; Fox ) oxide product density after reactions. The size of final product particle, if no further fragmentation occurs, is simply obtained by multiplying the original size with the size-change factor Cd. The mass-change factor Cm in eq 3 is specific to each mineral species. For example, Cm values are 0.56 and 0.67 for calcite and pyrite, respectively, according to respective stoichiometric calculations. Table 1 gives the values of Cm for major fragmenting mineral species commonly found in coal.
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Discussions
Figure 6. Particle size distributions of raw calcite and its high-temperature “ash”, compared to the fitted Poisson distribution.
Figure 7. Particle size distributions of raw pyrite and its high-temperature “ash”, compared to the fitted Poisson distribution.
Fitting the Poisson Parameter with Experimental Data. In the above approach, the only unknown variable is the Poisson parameter λ, which is assumed to be specific to individual mineral species alone. The measured size distributions of pyrite, calcite, and their corresponding “ashes” (residues after the high-temperature processing) were used to fit their characteristic Poisson parameters by minimizing the root-meansquare (RMS) deviation of the cumulative wt % distribution of the ashes. Figure 6 shows the comparison of size distributions of calcite, its high-temperature “ash”, and fitted by Poisson parameters of 1, 2, and 3, respectively. Poisson parameter 2, which means that a calcite particle most likely breaks on average into 3 offspring smaller particles, gives the best fit of the calcite ash size distribution. It is noted that the extent of calcite fragmentation obtained in this experiment is much less than that reported by ten Brink et al.7 The fitted size distributions of pyrite “ash” with Poisson parameters of 2, 3, and 4 are compared to experimental data in Figure 7. The best fitted result of Poisson parameter 3 means that a pyrite particle most likely fragments into 4 offspring particles, which is consistent with images of pyrite decomposition/transformation reported in previous studies.8,9
Extensive calcite fragmentation was reported previously indicating that calcite particles of 60 µm broke into fragments 10 times smaller.7 However, our experimental data showed significantly less fragmentation as discussed above. Although temperatures in the previous study7 (1350-1600 °C, close to that in power plant boilers) are higher than that used in this work (1300 °C), the temperature difference is not expected to result in significant discrepancy in particle fragmentaion. The residence time of particles subject to high-temperature processing in this work is similar to that in the previous study (∼2 s as in the boilers). It was noticed that minerals were obtained with a sink-float technique in the previous study, which might not exactly separate excluded minerals from included minerals. The images analyzed for calcite fragmentation in that study illustrated small calcium particles covering surfaces of large silicate particles. In fact, excluded minerals have little chance to collide with other particles during transformation in a furnace. Hollow cenospherical particles have been observed with SEM for pyrite oxidized products.11 Raask suggested that the formation of cenospherical particles was associated with carbon or silica.8 As excluded pyrite contains little carbon or silica, the ash particle structure is assumed to be a solid sphere in the current simulation. In addition, the Poisson parameters used may not only be related to individual mineral species, but also related to original particle size as well. It is possible that larger particles may undergo more extensive fragmentation than smaller particles due to difference in gas diffusion resistance within particles or higher temperature gradients inside particles. Unfortunately, the size effect was not experimentally investigated in this work due to a limitation in the particle feeding system of the DTF. If the fragmentation degree of large particles (>50 µm) is significantly different from small particles (
