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Energy & Fuels 2003, 17, 1522-1527
Evaluation of Combustion Characteristics of Chinese High-Ash Coals Rong Yan,*,†,‡ Chuguang Zheng,† Yuming Wang,§ and Yujian Zeng† National Laboratory of Coal Combustion, Huazhong University of Science & Technology, Wuhan 430074, P.R. China, Institute of Environmental Science & Engineering, Nanyang Technological University, Innovation Centre, Block 2, Unit 237, 18 Nanyang Drive, Singapore 637723, and School of Mechanical and Production Engineering, Division of Thermal and Fluids Engineering, Nanyang Technological University, 50, Nanyang Avenue, Singapore 639798 Received February 25, 2003. Revised Manuscript Received August 6, 2003
The combustion characteristics of four Chinese high-ash coals were evaluated by thermogravimetric and surface analysis. Changes in surface area and porosity during combustion were investigated with original (as-received) coals, coal samples before ignition, and chars after burnout. To understand the roles of various ash components, comparative experiments were carried out with as-received, de-ashed, and impregnated coals. SiO2, Al2O3, CaCO3, MgO, Na2CO3, TiO2, K2CO3, Fe2O3, FeS2, FeSO4(NH4)SO4‚6H2O, and NH4Fe(SO4)2‚12H2O at different concentrations were used as chemical additives impregnated into the de-ashed coals to investigate their influence on coal combustion. Most of these minerals inhibited coal ignitability; only K and Ca were found to promote coal ignition. Fe, Al, Na, and Ca were found to hinder both ignition and burnout of coal. Other ash components demonstrated variable effects on coal ignition and burnout with changing concentrations and species. A full understanding of the different effects of mineral matter is essential to achieve improved performance with high-ash coal applications.
1. Introduction China is one of the major producers and consumers of coal, with 80% of its electricity generation from coal combustion. While China has an abundance of low-grade coals, i.e., low calorific value, high moisture content, and/or particularly high ash content, because of the lack of facilities, technologies, and financing, only a small portion of these coals undergo processes to remove mineral matter. Burning high-ash coal for power generation is a common practice in China and is generally associated with operating problems such as slagging, fouling, corrosion, and bed agglomeration. In addition, coal utilization efficiency is low, and unacceptable environmental pollution, mainly in the form of particulates, is produced. To reduce and/or eliminate these problems, a fundamental understanding of the effects of mineral matter in high-ash coal on combustion characteristics is crucial to achieve efficient and safe usage. It will also benefit the development of highperformance technologies suitable for burning high-ash coals. The effects of mineral matter on combustion, gasification, and pollutant emissions have been evaluated previously.1-15 Kurose et al.1 studied the combustion characteristics of three high-ash coals with ash contents * Corresponding author. Tel: 65-67943244. Fax: 65-67921291. E-mail:
[email protected]. † Huazhong University of Science & Technology. ‡ Institute of Environmental Science & Engineering, Nanyang Technological University. § School of Mechanical and Production Engineering, Division of Thermal and Fluids Engineering, Nanyang Technological University.
of 36%, 44%, and 53%; they found that as the ash content increases, the gas temperature decreases, and oxygen consumption and NOx formation slow, due to the large heat capacity of the ash and the covering of combustible matter, which inhibited combustibility with ash during char oxidation. Ko¨psel et al. reported on the catalytic effects of ash components in low-rank coal gasification2,3 and the influences of ash elements on NOx formation in char combustion.4 The different effects of mineral matter distribution (extraneous or inherent) on the nature of combustion-generated ash were evaluated.5,6 The roles of lime (CaO) and iron oxide (Fe2O3) on the formation of ash and deposits were investigated (1) Kurose, R.; Ikeda, M.; Makino, H. Fuel 2001, 80 (10), 14471455. (2) Ko¨psel, R.; Zabawski, H. Fuel 1990, 69 (3), 275-281. (3) Ko¨psel, R.; Zabawski, H. Fuel 1990, 69 (3), 282-288. (4) Ko¨psel, R.; Halang, S. Fuel 1997, 76 (4), 345-350. (5) Russell, N. V.; Me´ndez, L. B.; Wigley, F.; Williamson, J. Fuel 2002, 81 (5), 657-663. (6) Gupta, R. P. Fuel Energy Abstr. 1997, 38 (4), 209. (7) Russell, N. V.; Wigley, F.; Williamson, J. Fuel 2002, 81 (5), 673681. (8) Vuthaluru, H. B.; Zhang, D. K.; Linjewile, T. M. Fuel Process. Technol. 2000, 67 (3), 165-176. (9) Manzoori, A. R.; Agarwal, P. K. Fuel 1993, 72, 1069. (10) Dik, E. P. Fuel Energy Abstr. 1998, 39 (3), 223. (11) Yan, R.; Gauthier, D.; Flamant, G.; Badie, J. M. Fuel 1999, 78 (15), 1817-1829. (12) Querol, X.; Ferna`ndez-Turiel, J. L.; Lo`pez-Soler, A. Mineral. Mag. 1994, 58, 119. (13) Rink, K. K.; Kozinski, J. A.; Lighty, J. S. Combust. Flame 1995, 100, 121. (14) Rubiera, F.; Arenillas, A.; Pevida, C.; Garcı´a, R.; Pis, J. J.; Steel, K. M.; Patrick, J. W. Fuel Process. Technol. 2002, 79 (3), 273-279. (15) Yan, R.; Zeng, H. C.; Yu, Q. Energy Fuel 2002, 16 (5), 11601166.
10.1021/ef030040z CCC: $25.00 © 2003 American Chemical Society Published on Web 10/22/2003
Combustion Characteristics of Chinese High-Ash Coals
by burning a suite of high-ash coals,7 and the varying contents of sodium (Na) and sulfur (S) in several Australian low-rank coals were identified as responsible for ash buildup and defluidization behavior.8 Manzoori and Agarwal9 studied the effects of coal inorganic matter on the formation of agglomerates in circulating fluidbed combustion. The role of coal mineral matter in binding sulfur was also investigated.10,11 The mechanisms of mineral matter transformation and ash formation during combustion were explored previously.11-13 Chemical demineralization was found to greatly influence coal structure and reactivity,14 as well as heavy metal emissions.15 Although, so far, a number of studies have been conducted to evaluate the roles of mineral matter in combustion processes, the combustion characteristics of high-ash coals have not yet been fully explored. This paper outlines a systematic study on the combustion characteristics (ignition and burnout) of four Chinese high-ash coals using thermogravimetric analysis. Changes in surface area and coal porosity during the combustion process were evaluated by surface analysis of as-received coals, coal samples before ignition, and chars after burnout. Coals were de-ashed and impregnated with different types of ash components to facilitate exploring the effects of mineral matter, providing a clearer understanding of their roles in the high-ash coal combustion. 2. Experimental Section 2.1. Characterization of Coals. Four Chinese high-ash coals with ash contents ranging from 29% to 50% were studied: two anthracite coals (Jinzhushan and Laiyang) and two bituminous coals (Qingshan and Heshan). Proximate analysis of these coals was conducted using a LECO Mac500 thermobalance, and ultimate analysis was carried out using a LECO CHN600 elemental analyzer combined with a chemical method for sulfur measurement. The major mineral components of coals were characterized using X-ray fluorescence (XRF), and the concentration of trace elements was analyzed by the methods previously described.16 The surface areas of coals were measured by nitrogen adsorption using the Micromeritics BET analyzer model ASAP 2000. The concentrations of excluded and included mineral matter were determined following different chemical treatment methods as described in the literature.17 2.2. Sample Preparation. In addition to the four original (as-received) coals, several specific coal samples were prepared using thermal or chemical treatment. These samples also underwent further surface and thermogravimetric analyses. (1) Thermal treatment. As-received coal samples were placed in the LECO Mac500 thermobalance in an oxygen atmosphere and heated at 20 °C/min to the ignition point of the coal. The temperature was maintained for 5 min, after which further combustion was stopped by switching to a nitrogen atmosphere. The coal samples after this treatment were kept as “coal sample before ignition” for further analyses. The “chars after burnout” were prepared by heating the asreceived coal samples to the burnout temperature in an oxygen atmosphere and maintaining it until a constant weight of char was obtained. The four original coals and samples before ignition and after burnout underwent surface analysis to evaluate the changes in surface area and porosity during the course of combustion. (16) Yan, R.; Lu, X. H.; Zeng, H. C. Combust. Sci. Technol. 1999, 145 (1-6), 57-81. (17) Pires, M.; Teixeira, E. C. Fuel 1992, 71, 1093-1096.
Energy & Fuels, Vol. 17, No. 6, 2003 1523
Figure 1. Typical DTA curve from thermogravimetric analysis of coal. (2) Chemical treatment. The four coals were first de-ashed as follows: 10 g of 0.1-0.5 mm coal fraction was boiled with 100 mL of 20% HF acid for 50 min at a temperature of 60 °C in a muffle furnace. After cooling, samples were washed with distilled water until elutes were neutralized, followed by drying under vacuum at 90 °C. Around 95 wt % of the ash components was removed. The de-ashed samples were further treated with chemical impregnation. SiO2, Al2O3, CaCO3, MgO, Na2CO3, K2CO3, TiO2, Fe2O3, FeS2, NH4Fe(SO4)2‚12H2O, and NH4FeSO4‚6H2O were added via solution immersion and/or mechanical mixing. With solution immersion, the de-ashed coal was immersed into solutions containing various ash components for 2 days under continuous stirring. The solution concentration was determined on the basis of the absorption ability of the coal. With mechanical mixing, the ash components were mixed with the de-ashed coals in a dry state, and the mixtures were then moistened. In both cases, the samples were subsequently dried under vacuum for 3 h at 90°C. The combustion characteristics of the as-received coals, de-ashed coals, and coals after chemical treatment was evaluated using thermogravimetric analysis. 2.3. Thermogravimetric Analysis. The coal samples were combusted in a thermobalance (LCT-2 Thermobalance, made in China) heated at 20 °C/min to 1000 °C to achieve a constant weight loss while using oxygen at 50 mL/min. Nitrogen was used at 100 mL/min as purge gas to protect the balance system. Three curves of thermogravimetry (TG), derivatives of TG (DTG), and differential thermal analysis (DTA), which indicate the weight loss, reaction rate, and heat change during combustion, were recorded simultaneously with respect to time (t) and/or temperature (T). A typical DTG curve is shown in Figure 1. T1 and T2 represent the ignition and burnout temperature points of coal, respectively. Two peaks of the DTG curve, indicating two major stages of combustion, are frequently observed in TG analysis of coal samples, which were identified to associate closely with the coal ignition and burnout properties. Two indices (M and N) were thus calculated from the DTG curve to evaluate the combustion characteristics of coal on the basis of the following semiempirical equations:19
M ) 47e-0.0052T1 + 4.6e-0.0044T1max + 0.0093e0.36W1max + 0.011e0.44G1 (1) N ) 0.55G2 + 0.0043T2max + 0.14τ98 + 0.72τ98′ + S
(2)
where T1max and T2max are the temperatures (in Kelvin) of the first and second peaks and W1max and W2max are the highest reaction rates (mg/min) of the first and second peaks, respec(18) Tomeczek, J.; Palugniok, H. Fuel 2002, 81, 1251-1258. (19) Liu, W. Z. Chinese Thermal Power Generation 1991, 6, 15-19.
1524 Energy & Fuels, Vol. 17, No. 6, 2003
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Table 1. Analyses of Coal Samples Laiyang anthracite coal
Heshan bituminous coal
Qingshan bituminous coal
3.22 7.49 29.03 60.29
3.69 10.67 49.48 36.16
2.03 17.49 31.36 49.13
ultimate analysis (%, as received) carbon 58.60 62.78 hydrogen 2.01 1.94 nitrogen 1.70 1.21 sulfur 1.10 0.80 oxygen 0.94 1.02
60.34 2.04 1.53 0.93 0.96
56.52 2.82 1.61 0.82 0.95
mineral matter (%) SiO2 CaO MgO Fe2O3 Al2O3 TiO2 P2O5 Na2O K2O As (µg/g) Be (µg/g) Cd (µg/g) Co (µg/g) Cr (µg/g) Cu (µg/g) Ge (µg/g) Mn (µg/g) Ni (µg/g) Pb (µg/g) V (µg/g)
20.50 1.42 0.38 2.71 10.59 0.20 0.06 0.25 0.75 9.90 2.60 0.15 9.50 25.40 33.40 1.48 0.01 18.60 18.10 100.0
17.20 1.81 0.55 2.49 7.83 0.21 0.05 0.19 1.20 14.50 3.10 0.19 8.50 36.80 27.50 1.95 0.01 13.90 20.90 76.50
coals
Jinzhushan anthracite coal
proximate analysis (%) moisture 0.77 volatile matter 7.62 ash 34.88 fixed carbon 56.73
20.4 0.81 0.47 1.82 10.10 0.44 0.06 0.14 0.47 11.80 2.20 0.26 12.20 26.32 30.20 0.66 0.01 14.20 15.30 95.20
17.80 1.26 0.28 1.57 7.18 0.30 0.07 0.59 2.12 12.10 1.80 0.29 11.60 21.60 31.40 0.47 0.015 17.10 12.20 70.80
tively, as shown in Figure 1. G1 and G2 are the areas under the first and second peaks (i.e., the weight losses in different combustion stages, mg). τ98 and τ′98 are the burnout times (min) for coal and char, respectively, defined as the time when the weight loss reaches 98% of the total weight loss. S is the surface area of char. The indices, M and N, were found to reflect the ignition and burnout characteristics of Chinese coals, and they have been used in assisting the design of boiler furnaces.19 In general, the bigger M is, the higher the coal ignitability, while the smaller N is, the easier the coal burnout. In the ref 19, the levels of coal ignition and burnout are classified as the followings: easy in ignition for M e 2.5, difficult in ignition for M g 3.35, and intermediate levels for 2.5 < M < 3.35; similarly, easy in burnout for N e 2.5, difficult in burnout for N g 3.5, and intermediate levels for 2.5 < N < 3.5. In this paper, these indices (M and N) were used to compare and evaluate the combustion characteristics of the studied coals under varying impregnation conditions, whereas S was not considered in this study when calculating the burnout index, N. The char surface area (S) is sometimes adopted to describe the behavior of diffusion and/or gas-solid reactions occurring at the char surfaces during the burnout stage and is considered only for those cases where the combination of DTG results (the first 4 items in eq 2) is unable to distinguish the burnout properties of two or several coals.
3. Results and Discussions 3.1. Coal Characterization. The proximate and ultimate analyses, as well as the concentration of mineral matter of the four original coals, are given in Table 1. The ash content of the four coals is generally high (>29%), particularly for Heshan bituminous coal (up to 49.48%). The major mineral matter is Si, Al, Ca, Mg, Fe, Ti, Na, and K elements; thus they were all considered in the subsequent chemical impregnation of de-ashing coals. In the four Chinese high-ash coals, the
Table 2. Excluded and Included Mineral Matters in Chinese High-Ash Coals (%) coals
total
excluded
included
ratio of excluded in total minerals
Jinzhushan Laiyang Heshan Qingshan
34.99 32.17 52.68 35.43
17.39 12.05 40.47 29.71
17.60 20.12 12.21 5.72
49.5 37.5 76.8 83.9
Table 3. Combustion of High-Ash Coals in Thermobalance coals ignition temperature (°C) burnout temperature (°C) activation energy (kJ/mol) ratio of volatile to ash
Qingshan Jinzhushan Laiyang Heshan 385 607 50.5 0.56
466 657 53.3 0.22
461 692 77.3 0.26
449 592 83.9 0.22
trace elements As, Be, Co, Cr, Cu, Ni, and Pb have concentrations similar to those in other countries’ coals, except for Cd, Ge, and Mn. Lower concentrations of Cd and Ge have been found, while Mn has surprisingly high concentrations in Chinese coals.16 Although certain trace elements in coal might also influence the combustion characteristics, for example, through catalysis, this study focuses on the major mineral matters of coal, i.e., Si, Al, Ca, Mg, Fe, Ti, Na, and K. The concentrations of excluded and included mineral matter in the four coals are shown in Table 2. The total mineral matter of the four coals are all slightly higher than the ash contents as shown in Table 1, probably due to (1) different measurement methods, or (2) certain mineral matter undergoing further decomposition during ash measurement. Qingshan and Heshan coals contain mainly excluded mineral matter at 83.9% and 76.8% of the total, respectively. The two forms of mineral matter affect combustion differently, as discussed in previous papers.6,17 Excluded mineral matter is generally separated from organic particles, easier to wash off, and able to assist in the gas-phase mass transfer, thus enhancing coal ignition. On the other hand, included mineral matter is closely associated with organic coal particles, difficult to wash off, and hinders ignition due to its incombustibility. In a recent study, Tomeczek and Palugniok18 presented the different transformation mechanisms of excluded and included mineral matter during coal combustion, indicating the significance of distinguishing the two forms of mineral matter when the influence of ash components is evaluated. 3.2. Combustion Characteristics by Thermogravimetric Analysis. Around 15 mg of each asreceived coal were burnt in the thermobalance at 20 °C/min to 1000 °C. The DTG curves of Qingshan, Jinzhushan, and Laiyang have one small shoulder peak after a big main peak, while Heshan has a shoulder peak before the main peak. The results are summarized in Table 3. Two bituminous coals (Qingshan and Heshan) ignited at lower temperatures than the two anthracite coals, partly because of their higher volatile and excluded mineral matter contents. The burnout properties of the four coals were evaluated by burnout temperature together with combustion efficiency, showing decreasing tendencies of easier burnout coals as follows: Qingshan, Jinzhushan, Laiyang, and Heshan. Although the burnout temperature of Heshan is the lowest, its combustion efficiency is quite low (90%)
Combustion Characteristics of Chinese High-Ash Coals
Energy & Fuels, Vol. 17, No. 6, 2003 1525
Table 4. Results of Surface Analysis
coals
stage
surface area (m2/g)
Jinzhushan
original coal, So before ignition, Si after burnout, Sb original coal, So before ignition, Si after burnout, Sb original coal, So before ignition, Si after burnout, Sb original coal, So before ignition, Si after burnout, Sb
5.56 175.9 14.03 5.56 189.1 75.9 4.33 79.8 17.2 5.72 45.8 14.4
Laiyang Heshan Qingshan
pore volume (cm3/g)
ave. pore diameter (Å)
0.010 0.114 0.047 0.014 0.115 0.071 0.017 0.101 0.069 0.016 0.046 0.058
70.5 19.6 124.3 73.4 16.3 27.2 143.3 37.3 153.6 103.9 38.5 145.0
compared to the other three (>98%); i.e., there is still combustible carbon in the ash residue. Among the four high-ash Chinese coals, Qingshan has demonstrated the best performance, with good ignition and burnout properties. However, Heshan is found to have the most difficulty with burnout, although it has relatively good ignition performance. The two anthracites show intermediate characteristics. The combustion of as-received coals at six different ramping temperatures (2, 5, 10, 15, 20, and 30 °C/min) was also conducted to estimate the activation energy of the overall combustion reaction using the Kisinger method:
1 E ln r )AT R T2
(3)
where r is temperature ramp (K/min), E is the activation energy (joule), R is the universal constant, and T is the temperature (K) of the main combustion peak. Plotting 1/T with respect to ln r/T2 will give a straight line with E/R as the slope and A as the value of the intercept. In Table 3, Qingshan has the lowest activation energy, followed by Jinzhushan, Laiyang, and Heshan. Again, Heshan is the most difficult coal so far as combustion. The ratios of volatile and mineral matter contents of the four coals (see Table 3) also indicate that the high ash content of Heshan is the major cause of its poor burnout performance. Nevertheless, its high ratio of excluded mineral matter probably aids ignition. 3.3. Combustion Characteristics by Surface Analysis. Table 4 shows the results of surface analysis conducted on the four original (as-received) coals (So) and samples after thermal treatment: samples before ignition (Si) and after burnout (Sb). Among the four asreceived coals (So), Heshan demonstrates the smallest surface area and the biggest pore diameter and volume. Compared with the as-received coals (So), the Si surface areas increase dramatically for all four coals, whereas as the combustion further processes the surface area decreases again, as shown by Sb data. Nevertheless, the surface area of Sb is still higher than So. A similar tendency is found with the pore volume, except for the Heshan coal, whose Sb pore volume (0.058 cm3/g) is still higher than its Si (0.046 cm3/g). However, the change of average pore diameters is inverse: the smallest pore diameters are found with Si in the four coal cases. Figure 2 shows the adsorptive pore volume versus the pore distribution of Qingshan coal along with the combustion process (from So to Si and then Sb).
Figure 2. Pore distribution of Qingshan coal and the samples before ignition and after burnout. Table 5. Thermogravimetric Analysis of Original and De-ashed Coals coals
stage
Jinzhushan original de-ashed Laiyang original de-ashed Heshan original de-ashed Qingshan original de-ashed
T1 (°C) Tmax (°C) T2 (°C) 466 431 461 437 449 415 385 268
494 465 484 475 460 446 409 317
657 660 692 700 592 499 607 589
M
N
1.265 1.499 1.409 1.579 1.340 1.659 1.756 2.569
5.830 6.300 5.308 6.551 6.560 4.859 6.047 6.365
According to the “Fuzzy Pore Model”,20 the macropores and mesopores in a coal particle which contribute to the pore volume serve as pathways for O2 diffusion while those micropores account for most of the surface areas, i.e., the active sites for O2 adsorption and the subsequent reaction with carbon. The small surface areas found in Heshan are consistent with its high activation energy. In addition, its large pore size and high pore volume facilitate O2 diffusion, thus promising a good ignition performance. From So to Si, the release of water and volatiles increases the incremental pore volume of Si in the entire range of pore diameters examined (Figure 2). As the average pore diameters decrease sharply from So to Si (Table 4), more micropores are found in Si than in So, consistent with the increased surface area observed in the former. After burnout, in comparison to Si, the big difference is found in the pore distribution curve of Sb in the fine-pore range. The number of micropores decreases from Si to Sb, causing the surface area to decrease significantly and the average pore diameter to increase consistently. The four coals behave differently in terms of pore opening and closing phenomena because of their different natures. 3.4. Effects of Ash Components. De-ashed coals were prepared, and various mineral components were impregnated to investigate the variable effects of different ash components. 3.4.1. Combustion of De-ashed Coals. The results of thermogravimetric analysis of de-ashed coals are given in Table 5 and compared with those of the as-received coals. The ignitability of de-ashed coals is better than (20) Chen, H. Ph.D. Dissertation, Huazhong University of Science & Technology, China, 1994.
1526 Energy & Fuels, Vol. 17, No. 6, 2003
Figure 3. Effect of mineral matter on combustion characteristics of Heshan coal.
that of the original samples, as shown by the lower ignition temperatures (T1) and higher M values. However, de-ashing causes the burnout to be a little more difficult for the three coals: Jinzhushan, Laiyang, and Qingshan, whose N values are all increased, whereas a smaller N is found only for Heshan coal, indicating the positive effects of de-ashing on its burnout. Again, it makes the previous assumption stronger; i.e., the mineral matter in Heshan is the main contributor of its poor combustion performance. A previous publication14 indicated that demineralized coal is more reactive in terms of ignition and burns out more easily than the original coal. The variable nature of the coals accounts for their burnout differences. Furthermore, the burning rates of de-ashed coal are lower than those of the asreceived coal, which is in accordance with previous publications.2,3 3.4.2. Effects of Various Additives. The influence of mineral matter on coal combustion characteristics was further investigated by adding various chemicals into the de-ashed coal samples, followed by thermogravimetric analysis. The M and N values of the chemically impregnated coals were used to evaluate the effects of different additives. The two ways of chemical impregnations (solution method and mechanical mixing) were compared for certain water-soluble chemicals, a slight increase in the coal combustion rate was observed with mechanical mixing. Heshan coal was selected for this study since the mineral matter of this coal is supposed to influence combustion the most significantly. The following 16 additives were used: (1) Si: 17.1% SiO2; (2) Al1: 4.9% Al2O3; (3) Al2: 10.6% Al2O3; (4) Al3: 15.6% Al2O3; (5) Ca1: 1.6% CaCO3; (6) Ca2: 2.4% CaCO3; (7) Ca3: 3.6% CaCO3; (8) Mg: 0.4% MgO; (9) Na: 10.1% Na2CO3; (10) Ti: 0.6% TiO2; (11) K1: 1.1% K2CO3; (12) K2: 1.3% K2CO3; (13) K3: 7.2% K2CO3; (14) Fe1: 3.3% Fe2O3; (15) Fe2: 13.7% FeSO4(NH4)SO4‚ 6H2O; (16) Fe3: 24.5% NH4Fe(SO4)2‚12H2O. The M and N values of the impregnated samples were obtained by thermogravimetric analysis; they are shown in Figure 3. The M and N data of as-received Heshan coal and its de-ashed form are indicated by two squares, whereas those of the impregnated coals are indicated with triangles. To facilitate the description, the figure is
Yan et al.
divided into four zones on the basis of the M and N data of the de-ashed coal. In Zone I, the additives in this zone, including Al1, Ca1, Ca3, Fe2, Fe3, and Na, make the ignition and burnout of the de-ashed coal more difficult. The asreceived coal is also in this zone, indicating that the mineral matter naturally occurring in the Heshan coal makes its ignition and burnout more difficult. In Zone II, the additives (K2 and K3) allow easier ignition, but a more difficult burnout, compared to the de-ashed coal. In Zone III, the additives allow an easy burnout, but more difficult ignition. Al2, Al3, Fe1, Mg, Si, and Ti are found in this zone. In Zone IV, Ca2 and K1 are the mineral matter allowing an easier ignition and burnout compared to the de-ashed coal. 3.4.3. Effects of Coal Types. The effects of various mineral matters are not always the same for different coals. For instance, SiO2 improves the burnout of Heshan coal slightly, but it significantly reduces the combustion rates of Jinzhushan and Laiyang coals. On the other hand, several types of mineral matters demonstrate the same effects on different coals, such as K tends to improve both ignition and burnout, while Si, Al, Fe, Mg, Na, and Ti tend to prolong ignition time. Both Si and Al hinder coal burnout, and TiO2 increases the combustion rate of coals. The effect of quartz (SiO2) and kaolinite (mostly SiO2 and Al2O3 after decomposition) in inhibiting the reaction by increasing ignition temperatures was reported previously;21,22 our findings also confirm this. 3.4.4. Effects of Concentrations and Species. Varying concentrations of the same mineral matter was also considered, such as Al1, Al2, and Al3 indicating the cases of adding, respectively, 4.9%, 10.6%, and 15.6% of Al2O3. Other cases can be found with Ca1, Ca2, and Ca3, and K1, K2, and K3. Potassium in three concentrations demonstrates almost the same effect: promoting ignition but hindering burnout. Al species in three concentrations all hinder ignition, while promoting burnout at high concentrations (Al2 and Al3) but hinder it at the lower (Al1). It is possible that an optimum concentration of the additives exists; for instance, Ca1 and Ca3 hinder the ignition and burnout of Heshan coal, whereas Ca2 promotes both. Different species of elements as additives were also studied; i.e., four types of chemicals used for iron, including Fe2O3, FeS2, FeSO4(NH4)SO4‚6H2O, and NH4Fe(SO4)2‚12H2O. The presence of pyrite in coal is believed to have a catalytic effect on the carbon-oxygen reaction and the ignition temperature is decreased, as found by several authors.21-23 Fe2O3 and Fe were the major products of iron-bearing minerals in early stages of coal conversion.24 In this study, the effects of adding FeS2 were studied only for Jinzhushan and Laiyang (21) Manzanares, P. L.; Garbett, E. S.; Spears, D. A.; Widdowson, M.; Richards, G. Ignition of Coal Particles. In Coal Science; Pajares, J. A., Tascon, J. M. D., Eds.; Elsevier: New York, 1995; pp 587-590. (22) Spears, D. A. Appl. Clay Sci. 2000, 16, 87-95. (23) McCollor, D. P.; Jones, M. L.; Benson, S. A.; Young, B. C. Promotion of Char Oxidation by Inorganic Constituents. In Proceedings of the 22nd International Symposium on Combustion; The Combustion Institute: Pittsburgh, 1988; pp 59-67. (24) Vuthaluru, H. B.; Eenkhoorn, S.; Hamburg, G.; Heere, P. G. T.; Kiel, J. H. A. Fuel Process. Technol. 1998, 56, 21-31.
Combustion Characteristics of Chinese High-Ash Coals
coals, it was found that this species improves the ignition of both coals. It is thought that the reaction FeS2 + O2 f Fe2O3 + Fe + SO2 favors the early ignition of coal. However, the other three Fe-containing species all hinder the ignition and burnout of Heshan coal significantly, especially for Fe3 (24.5% NH4Fe(SO4)2‚ 12H2O), which makes coal burnout extremely difficult. The variable effects observed from the different Febearing species are not clear yet. The performance improvement of Jinzhushan and Laiyang coals in the presence of FeS2 might be from the catalytic effects of the additive. However, different mechanisms are probably involved regarding the other three Fe-species with Heshan coal: for example, the decomposition of the additives [i.e., Fe3 (NH4Fe(SO4)2‚12H2O)] removes a large amount of heat from the release of water and NH3 and also changes the overall porosity of coal, inhibiting the reaction. 4. Conclusions Because of the complexity of different coal natures and ash components, the influence of mineral matter on combustion characteristics varies significantly with respect to types of coal and minerals, distribution, association, and concentrations of mineral matters. With carefully designed experiments, several general conclusions regarding the combustion characteristics of highash coals and the effects of mineral matter can be reached on the basis of this study: (1) Mineral matter is the main cause of the poor combustion performance of high-ash coals. An in-depth investigation of variable effects of different mineral matter is essential to understanding, on a fundamental basis, their role during the combustion of high-ash coals
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in order to achieve an efficiency-improved high-ash coal application. (2) Most of the mineral additives studied are located in Zones I and III (Figure 3), hindering the ignitability of Heshan coal. Only K (K1, K2, and K3) and Ca (Ca2), which are in Zones II and IV, promote coal ignition. Moreover, the chemical additives that hinder both ignition and burnout of coal include Fe (Fe2, Fe3), Al, Na, and Ca (Ca1 and Ca3). In particular, Fe3 strongly hinders the burnout of Heshan coal. (3) Ash constituents have demonstrated different effects on different coals, and the extent of their influence on coal ignition and burnout are not linear with respect to their concentrations. Particularly, only one Fe-bearing compound (FeS2) demonstrated catalytic effects in enhancing coal ignition, whereas other Fe species significantly inhibited both the ignition and burnout of Heshan coal. Further research is needed to understand better the roles of Fe-bearing species during combustion. (4) Detailed information on various effects of mineral matter can help interpret the combustion properties of the high-ash coals studied. For instance, the M and N of Laiyang is bigger and smaller, respectively, than those of Jinzhushan, indicating that Laiyang might perform better both in terms of ignition and burnout. From the mineral matter analysis (Table 1), the content of SiO2 and Al2O3 (the promoters causing burnout difficult) in Jinzhushan is higher than in Laiyang, while the content of K and Ca (the promoters of easy ignition) in Jinzhushan is lower than in Laiyang, which may account for the observed difference between them in terms of the combustion characteristics. EF030040Z
