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Ash Formation from Excluded Minerals Including Consideration of Mineral-Mineral Associations† Yinghui Liu,*,‡ Rajendar Gupta,§ and Terry Wall‡ CooperatiVe Research Centre for Coal in Sustainable DeVelopment, Department of Chemical Engineering, UniVersity of Newcastle, EB Building, Callaghan, New South Wales 2308, Australia, and Chemical Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2E1 ReceiVed August 20, 2006. ReVised Manuscript ReceiVed NoVember 30, 2006
During pulverized coal combustion processes, mineral matter in coals transforms into ash, which impacts combustion systems in various ways. According to its association with the coal matrix, mineral grains can be classified as included and excluded mineral grains. Excluded minerals behave differently from included minerals. In this paper, a state-of-the-art microscopic technique QEMSCAN was used to study the transformation of excluded minerals with a consideration of mineral-mineral associations. The concept of mineral-mineral association for excluded minerals refers to a single excluded mineral grain consisting of several different mineral phases. Three size-graded Australian coals have been burned in a laboratory drop-tube furnace at a temperature of 1400 °C. Ash particles are collected by a cyclone located at the outlet of a quench probe. Both mineral matter in coals and ash after combustion were analyzed by QEMSCAN. Transformation from mineral grain to ash particles resulted in changes in morphology, which has implications on ash fusion. Results show that illite as well as ankerite and siderite change sperical shape after combustion, while other minerals, including quartz, kaolinite, and calcite, do not have significant changes in shape. Mineral association of quartz with kaolinite has been positively identified in excluded minerals in coals and derived ash particles. However, mineral association of carbonates with silicates is not a common occurrence in the limited number of coals investigated.
Introduction Pulverized coal-firing technology plays a vital role in electricity generation for the sustainable development of the society. During combustion, inorganic matter, most of which is mineral matter in higher rank coals, transforms into ash residues with wide variations in size, chemical composition, and morphology. The deposit of ash particles onto heat-transfer surfaces is a major operation problem to power-plant operators. The topic on deposition has been substantially reviewed previously.1-5 The deposit in areas within the furnace that are directly exposed to flame radiation, such as the furnace walls and pendent superheaters, is referred to as slagging, whereas fouling referred to the deposit in those areas not directly exposed to flame radiation, such as the more closely spaced tubes in the convection sections of the boiler. With a better understanding † Presented at the 2006 Sino-Australia Symposium on Advanced Coal Utilization Technology, July 12-14, 2006, Wuhan, China. * To whom correspondence should be addressed. E-mail: liu_yinghui@ msn.com. ‡ University of Newcastle. § University of Alberta. (1) Reid, W. T. Coal ashsIts effect on combustion systems. In Chemistry of Coal Utilization; M.A. Elliot, M. A., Ed.; Wiley: New York, 1981; pp 1389-1445. (2) Scott, D. H. Ash BehaVior during Combustion and Gasification; IEA Coal Research: London, U.K., 1999; p 38. (3) Raask, E. Mineral Impurities in Coal Combustion: BehaVior, Problems, and Remedial Measures; Hemisphere Publishing Corporation: Washington, D.C., 1985; p 484. (4) Couch, G. Understanding Slagging and Fouling During Pf Combustion; IEA Coal Research: London, U.K., 1994; p 118. (5) Bryers, R. W. Fireside slagging, fouling, and high-temperature corrosion of heat transfer surface due to impurities in steam-raising fuels. Prog. Energy Combust. Sci. 1996, 22, 29-120.
of deposit formation mechanism, it is possible to develop a predictive model.6,7 As an essential component of the mechanistic model, ash formation mechanisms and their modeling have obtained great attention. Chemical transformation of mineral matter to ash under low temperatures and low heating rates has been extensively studied. As a result, an understanding on phase changes for the majority of minerals in coals has been established.8,9 Chemical transformation of mineral matter in the flame also has been studied with pure minerals or coals containing minerals.10-16 Except chemical transformation, physical transformations, such as fragmentation, coalescence, evapo(6) Sarofim, A. F.; Helble, J. J. Mechanisms of ash and deposit formation. in Engineering Foundation Conference on the Impact of Ash Deposition on Coal Fired Plants; Taylor and Francis: Solihull, U.K., 1993. (7) Beer, J. M.; Sarofim, A. F.; Barta, L. E. From coal mineral matter properties to fly ash deposition tendencies: A modelling route. Proceedings of the Engineering Foundation Conference on Inorganic Transformation and Ash Deposition during Combustion, The American Society of Mechanical Engineers: Palm Coast, FL, 1991. (8) O’Gorman, J. V.; Walker, P. L., Jr. Thermal behaviour of mineral fractions separated from selected American coals. Fuel 1973, 52, 71-79. (9) Mitchell, R. S.; Gluskoter, H. J. Mineralogy of ash of some American coals: Variations with temperature and source. Fuel 1976, 55, 90-96. (10) ten Brink, H. M.; Eenkhoorn, S.; Hamburg, G. A fundamental investigation of the flame kinetics of coal pyrite. Fuel 1996, 75, 945-951. (11) ten Brink, H. M. E. S.; Weeda, M. The behaviour of coal mineral carbonates in a simulated coal flame. Fuel Process. Technol. 1996, 47, 233243. (12) Bailey, C. W., et al. Investigation of the high-temperature behavior of excluded siderite grains during pulverized fuel combustion. Energy Fuels 1998, 12, 464-469. (13) Srinivasachar, S.; Helble, J. J.; Boni, A. A. Mineral behaviour during coal combustion. 1. Pyrite transformations. Prog. Energy Combust. Sci. 1990, 16, 281-291. (14) Srinivasachar, S., et al. Mineral behavior during coal combustion. 2. Illite transformations. Prog. Energy Combust. Sci. 1990, 16, 293-302.
10.1021/ef060414z CCC: $37.00 © 2007 American Chemical Society Published on Web 01/24/2007
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ration, and subsequent condensation, also play an important role during ash formation.17-21 According to its association with the coal matrix, mineral grains can be classified as included and excluded mineral grains. Included minerals refer to those discrete mineral grains that are associated with organic matter. Excluded minerals are those discrete minerals liberated out of coal particles and thus have none or negligible association with organic matter in coal. An included mineral grain transforms differently from an excluded mineral grain because of differences in particle temperature, local atmosphere, and proximity to other included mineral grains.22 However, in the literature, the mineral-mineral association of excluded mineral grains and their transformation during combustion have not been explored. The concept of mineralmineral association for excluded minerals refers to a single mineral grain consisting of several different mineral phases. The objectives of this study aimed to identify mineral-mineral association in excluded mineral grains in a limited number of coals and to obtain an understanding of the impact of mineralmineral association on ash transformation. In this study, a stateof-the-art microscopic technique QEMSCAN has been used to demonstrate mineral-mineral association in excluded mineral grains in coal and ash from combustion. Experiment Section Three Australian black coals received from the CCSD coal bank represent a wide range of ash compositions. The coal samples were grounded and sieved to obtain narrow-sized coal samples in the size cut of 63-90 µm. The coals were subjected to analysis for ash content and composition, with results shown in Table 1. QEMSCAN analysis has been applied to characterize the excluded mineral grains in the coal samples and indicates that they are mainly in the form of silicates and carbonates. QEMSCAN is a state-of-the-art automated mineral analysis instrument capable of analyzing mineral-mineral associations in pulverized coals on a particle-by-particle basis.23,24 Information on mineral grain size also can be obtained from QEMSCAN. The detailed description on the QEMSCAN technique can be found elsewhere.24 Here, it only gives (15) Helble, J. J.; Srinivasachar, S.; Boni, A. A. Mechanisms of ash evolutionsA fundamental study. Part I: Low rank coals and the role of calcium. Proceedings of the Engineering Foundation Conference on Inorganic Transformation and Ash Deposition during Combustion, The American Society of Mechanical Engineers: Palm Coast, FL, 1991. (16) Helble, J. J.; Srinivasachar, S.; Boni, A. A. Mechanisms of ash evolutionsA fundamental study. Part II: Bituminous coals and the role of iron and potasium. Proceedings of the Engineering Foundation Conference on Inorganic Transformation and Ash Deposition during Combustion, The American Society of Mechanical Engineers: Palm Coast, FL, 1991. (17) Yan, L.; Gupta, R. P.; Wall, T. F. Fragmentation behaviour of pyrite and calcite during high-temperature processing and mathematical simulation. Energy Fuels 2001, 15, 389-394. (18) Raask, E. Creation, capture and coalescence of mineral species in coal flames. J. Inst. Energy 1984, 57, 231-239. (19) Flagan, R. C.; Taylor, D. D. Laboratory studies of submicron particles from coal combustion. Eighteenth International Symposium on Combustion, The Combustion Institution: Pittsburgh, PA, 1981. (20) Helble, J.; Neville, M.; Sarofim, A. F. Aggregate formation from vaporized ash during pulverized coal combustion. Twenty-first International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1986. (21) Neville, M., et al. Vaporization and condensation of mineral matter during pulverized coal combustion. Eighteenth International Symposium on Combustion, The Combustion Institution: Pittsburgh, PA, 1981. (22) Benson, S. A.; Jones, M. L.; Harb, J. N. Ash formation and depostion. In Fundamentals of Coal Combustion for Clean and Efficient Use; Smoot, D. L., Ed.; Elsevier: New York, 1993; pp 299-373. (23) Liu, Y., et al. Mineral-mineral associations in pulverized Australian coals. 5th Asian Pacific Conference on Combustion, Adelaide, Australia, 2005. (24) Liu, Y., et al. Mineral matter-organic matter association characterisation by QEMSCAN and applications in coal utilisation. Fuel 2005, 84, 1259-1267.
Liu et al. Table 1. Ash Contents and Compositions of the Three Coals Studied
ash content (wt, ad %) ash analysis (wt %) silicon as SiO2 aluminum as Al2O3 iron as Fe2O3 calcium as CaO magnesium as MgO sodium as Na2O potassium as K2O titanium as TiO2 manganese as Mn3O4 sulfur as SO3 phosphorus as P2O5
CRC272
CRC299
CRC306
12.3
20.2
19.4
48.9 27.2 7.8 7.7 0.65 0.09 0.38 2.0 0.07 3.1 1.4
54.0 28.5 12.0 1.3 0.88 0.09 0.2 1.6 0.22 1.3 0.07
44.7 17.6 15.4 11.6 2.5 0.27 0.97 0.85 0.16 4.3 0.82
a brief description on mineral-mineral association for an excluded mineral grain. QEMSCAN recognizes an excluded mineral grain from its high brightness in back-scattered electron images as shown in Figure 1a. Once an excluded mineral grain is located, it is scanned by a grid of points and the X-ray spectra emitted from each point are used to identify the elements present and thus classify the mineral species present. The image of the mineral grain is built-up point-by-point in this way, in which each pixel corresponds to a mineral species or phase in a region under the electron beam as shown in Figure 1b. Different colors have been used to present different mineral phases. QEMSCAN shows that the mineral grain in Figure 1b actually consists of quartz (presented in pink) and kaolinite (presented in green). By conventional ways, it is impossible to recognize whether this mineral grain consists of mineralmineral association or a single mineral phase. In such a unique operation strategy, mineral-mineral association in a single mineral grain has been determined. The sized coal samples were burned in air in the Astro model drop-tube furnace located at the Chemical Engineering department
Figure 1. Typical QEMSCAN image. (a) Mineral particle under SEM. (b) QEMSCAN image.
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Figure 2. Quartz (a) in coal and (b) in ash after combustion in DTF at 1400 °C. (a) Quartz in coal observed under QEMSCAN. (b) Quartz combustion residues observed under QEMSCAN.
at the University of Newcastle in Australia. The schematic diagram of the drop-tube furnace as well as the feeder and sampling probe can be found elsewhere.25 The furnace was operated at a temperature of 1400 °C, and the residence time of coal combustion is estimated to be around 1 s. Coal was feed from the top of the furnace, and the ash particles were collected using a cyclone collector and a subsequent filter from the bottom of the furnace. The cyclone captured ash particles with sizes greater than 2 µm, and filter paper captured the finer particles. The ash from combustion was analyzed by QEMSCAN for mineral-mineral association. Transformation of silicates and carbonates, which are two major types of minerals present in the coals, has been studied. In the experiments, sized coal samples of size cut 63-90 µm were used and no actions were taken to removal included mineral grains out of the coal sample, so that some included mineral grains transform together with excluded mineral grains. QEMSCAN analysis on the mineral matter in coal indicates that the particle size of excluded mineral grains has a size range similar to coal particles, i.e., between 63 and 90 µm, while the particle size of included mineral grains is much finer. Further experiments carried out on transformation of included mineral grains confirmed that all of the ash particles formed from included mineral grains are smaller than 63 µm.25 The choice of the 63 µm cut off makes it possible to select ash particles derived from excluded mineral grains out of the whole ash population. However, this is relatively conservative because some excluded mineral grains can form ash particles finer than 63 µm by fragmentation and mass loss. Furthermore, ash particles from the chemical composition of iron oxides may derive from both pyrite and siderite. In the coals used in this study, iron is primarily in the form of siderite; thus, we assume that all of the iron oxides present in the ash after combustion derived from siderite rather than pyrite.
Results 1. Silicates. Silicate minerals present in coals generally consist of quartz, kaolinite, illite, muscovite, montmorillonite, feldspars, chlorite, etc. In the three coals studied, silicate minerals are mainly composed of quartz, kaolinite, and illite. Quartz images are shown in Figure 2a, most of which are of angular elliptical shapes. Images indicate that the association of quartz with kaolinite is a common occurrence, while the association of quartz with carbonates is in much less frequent circumstances. The first several quartz grains are pure excluded quartz, some of which have sharp angles. There is one particle in which quartz is embedded inside a siderite grain. It is common to observe quartz embedded within kaolinite. The association of quartz with kaolinite in a mineral entity is the most commonly found mineral association. The quartz ash residues collected in the cyclone shown in Figure 2b indicate that the excluded quartz residues preserve their original characteristic angular shapes and (25) Liu, Y. Analysis of mineral associations in pulverized coal and ash form combustion. In Chemical Engineering; Doctoral Thesis, The University of Newcastle: Callaghan, Australia, 2007; p 298.
that no spherical shape was found in quartz residues, although in some cases, sharp edges in the original quartz transformed into rounded tips. This indicates that quartz does not melt at the furnace temperature of 1400 °C in the laboratory drop-tube furnace. QEMSCAN images also demonstrated that some quartz grains are embedded in the mullite phase derived from kaolinite. At the current experiment temperature of 1400 °C, it is expected that neither quartz nor kaolinite melt. In regard to the genesis of such ash particles, they can only be formed from quartz associated with kaolinite in coal. Quartz is an erosive mineral to furnace tubes because of its high hardness and refractory property in the flame. Coarse quartz especially those in the excluded form give a higher risk to tube metal loss in power generation systems than quartz embedded in kaolinite and that in an included form. Thus, identification of pure excluded coarse quartz can provide a guideline for erosion prediction. Quartz associated with siderite as shown in Figure 2a leads to the formation of iron silicates, which have a low viscosity, high stickiness, and higher slagging potential than either pure quartz or pure siderite. In this study, only three coals have been used; with exploration of a large number of coals, the association of quartz with siderite is expected to be significant in some coals from specific geographic locations. Kaolinite shown in Figure 3a illustrates that kaolinite exist as pure minerals as well as an associated form with quartz. Under 1400 °C, the majority of kaolinite grains preserve their original size and shape. QEMSCAN images shown in Figure 3b indicate that kaolinite may form a honeycomb network structure after combustion but that the dominant form is a dense solid. The association of quartz with mullite derived from kaolinite is obvious in ash particles. Illite has a higher amount of potassium and higher ratio of SiO2 to Al2O3 than kaolinite. The presence of impurities, such as iron, also has been found in some illite grains. QEMSCAN demonstrated that illite intimately associated with kaolinite in coal CRC306 as shown in Figure 4a. At least, such information indicates the amount of potassium is not evenly distributed in one single mineral grain. The ash residue shown in Figure 4b indicates gas bubbles or holes present in ash particles. The phase diagram for the K2O-Al2O3-SiO2 system indicates that the eutectic (minimum melting point) for a mixture of oxides of K2O, Al2O3, and SiO2 under equilibrium condition is around 980 °C. During combustion, illite forms a molten slag and gases released from inside produced gas-filled bubbles, resulting in porous spherical or thick-walled cenosphere ash particles. 2. Carbonates. The major carbonates in the coals include siderite (FeCO3), calcite (CaCO3), dolomite (CaCO3‚xMgCO3), and ankerite (CaCO3‚xMgCO3‚yFeCO3). QEMSCAN analysis results indicate that coal CRC299 only contains siderite as
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Figure 3. Kaolinite (a) in coal and (b) in ash after combustion in DTF at 1400 °C. (a) Kaolinite in coal observed under QEMSCAN. (b) Kaolinite combustion residues observed under QEMSCAN.
Figure 4. Illite (a) in coal and (b) in ash after combustion in DTF at 1400 °C. (a) Illite in coal observed under QEMSCAN. (b) Illite combustion residues observed under QEMSCAN.
Figure 5. Siderite mineral (a) in coal and (b) in ash after combustion in DTF at 1400 °C. (a) Siderite in coal observed under QEMSCAN. (b) Siderite combustion residues observed under QEMSCAN. Table 2. Carbonates in the Three Coals Studied, with × Designating the Occurrence of the Mineral
siderite calcite dolomite ankerite
CRC299
CRC272
CRC306
×
× ×
× × × ×
carbonate, coal CRC272 contains both siderite and calcite, and coal CRC306 contains all four forms of carbonates as shown in Table 2. Siderite is mainly in the excluded mineral form in the three coals studied. Figure 5 illustrates results obtained from coal CRC299. Because there is no pyrite in coal CRC299, we can consider all of the iron oxides found in the ash residue derived from siderite. As shown in Figure 5a, siderite in coal is usually in angular shapes with sharp angles. The average size of excluded siderite is close to the size of coal particles, i.e., in the range of 63-90 µm. The iron-bearing ash residues after combustion shown in Figure 5b demonstrated the dependence of morphology on the particle size. For coarse particles, siderite ash is generally in irregular shapes, while fine particles tend to
be spherical in shape. In the fine size range, some irregularshaped particles can also be seen. From both siderite in raw coal and their combustion residue, there is no evidence indicating that siderite associates with other carbonates or silicates. In this sense, the behavior of pure siderite can be used to predict their transformation during combustion. Siderite as a major iron-bearing mineral found in Australian coals has been extensively investigated.26 In the transformation of siderite in high-temperature flames, two major factors are involved: the thermal effect and the local atmosphere effect. Under thermal shock, siderite decomposes to FeO, which reacts with oxygen that diffused the grain inward to Fe3O4, which has a melting temperature of 1590 °C. Excluded siderite may also fragment. Studies on siderite by Raask indicated that siderite disintegrated extensively into 0.1-1.0 µm FeO particles upon rapid heating as a result of the rapid gas evolution.3 In the literature, the absence of fragmentation for siderite has also been reported. Studies on siderite carried out by ten Brink11 in a (26) Bailey, C. W. High temperature transformations of siderite and the performance of a PF fired plant. In Chemical Engineering; The University of Newcastle: Callaghan, Australia, 1999; p 229.
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Figure 6. Calcite mineral (a) in coal and (b) in ash after combustion in DTF at 1400 °C. (a) Calcite in coal observed under QEMSCAN. (b) Calcite combustion residues observed under QEMSCAN.
Figure 7. (a) Dolomite in coal, (b) ankerite in coal, and (c) ankerite in ash after combustion in DTF at 1400 °C. (a) Dolomite in coal observed under QEMSCAN. (b) Ankerite in coal observed under QEMSCAN. (c) Ankerite combustion residues observed under QEMSCAN.
combustion furnace demonstrated that none of the siderite samples tested showed fragmentation. The research work carried out in the same laboratory of the authors on the siderite obtained from coal CRC299 indicated that there was little evidence for the fragmentation of siderite to form fines (
