1990
Energy & Fuels 2009, 23, 1990–1997
Prediction of Chinese Coal Ash Fusion Temperatures in Ar and H2 Atmospheres Wen J. Song,† Li H. Tang,† Xue D. Zhu,† Yong Q. Wu,† Zi B. Zhu,*,† and Shuntarou Koyama‡ Engineering Research Center of Large Scale Reactor Engineering and Technology, East China UniVersity of Science and Technology, Shanghai 200237, People’s Republic of China, and Electric Powder DeVelopment Corporation Limited, Tokyo 167-0023, Japan ReceiVed NoVember 11, 2008. ReVised Manuscript ReceiVed January 21, 2009
The ash fusion temperatures (AFTs) of 21 typical Chinese coal ash samples and 60 synthetic ash samples were measured in Ar and H2 atmospheres. The computer software package FactSage was used to calculate the temperatures corresponding to different proportions of the liquid phase and predict the phase equilibria of synthetic ash samples. Empirical liquidus models were derived to correlate the AFTs under both Ar and H2 atmospheres of 60 synthetic ash samples, with their liquidus temperatures calculated by FactSage. These models were used to predict the AFTs of 21 Chinese coal ash samples in Ar and H2 atmospheres, and then the AFT differences between the atmospheres were analyzed. The results show that, for both atmospheres, there was an apparently linear correlation and good agreement between the AFTs of synthetic ash samples and the liquidus temperatures calculated by FactSage (R > 0.89, and σ < 30 °C). These models predict the AFTs of coal ash samples with a high level of accuracy (SE < 30 °C). Because the iron oxides in coal ash samples fused under a H2 atmosphere are reduced to metallic iron and lead to changes of mineral species and micromorphology, the AFTs in a H2 atmosphere are always higher than those with an Ar atmosphere.
Introduction The ash fusion temperature (AFT) is the most traditional parameter used to access the fusibility of coal ash in a combustion system and predict the flow properties of ash in practical gasification systems.1,2 For example, the initial deformation temperature, which is the temperature at which the rounding of the tip of an ash cone is noted, can give an indication to designers and operators of pulverized-fired (pf) furnaces that the coal ash has been melting and therefore has become sticky.3 Meanwhile, for all types of slagging entrainedflow gasifiers, the operating temperature should be above the AFT to enable continuous slag tapping.4 It is therefore important to be able to accurately predict the AFTs of coal ash. For the entrained-flow gasifier, the raw syngas consists mainly of strongly reducing gases (CO and H2). Particularly, in drycoal feed entrained gasifiers (Kopper-Totzek, Shell, CCP, Engle, and Plenflo gasifiers), the content of the strongly reducing gas in raw syngas can be over 90%.5 A number of researchers have studied the fusibility of coal ash in oxidizing, inert, and mildly reducing atmospheres.6,7 For example, Vassilev et al.8 discussed the influence of mineral and chemical composition of coal ash * To whom correspondence should be addressed. Telephone: +86-2164252309. Fax: +86-21-64253626. E-mail:
[email protected]. † East China University of Science and Technology. ‡ Electric Powder Development Corporation Ltd. (1) Wall, T. F.; Creelman, R. A.; Gupta, R. P.; Gupta, S. K.; Coin, C.; Lowe, A. Prog. Energy Combust. Sci. 1998, 24, 345–353. (2) Bryers, R. W. Prog. Energy Combust. Sci. 1996, 22, 29–120. (3) Gupta, S. K.; Wall, T. F.; Creelman, R. A.; Gupta, R. P. Fuel Process. Technol. 1998, 56, 33–43. (4) Hurst, H. J.; Novak, F.; Patterson, J. H. Fuel 1999, 78, 1831–1840. (5) Higman, C.; Burgt, M. V. Gasification; Elsevier: Burlington, MA, 2003; pp 109-127. (6) Gupta, S. K.; Gupta, R. P.; Bryant, G. W.; Wall, T. F. Fuel 1998, 77, 1195–1201.
on their fusibility in an oxidizing atmosphere (air), and Wall et al.9 applied thermomechanical analysis to describe the fusibility of blended coal ash in an inert atmosphere (100% Ar). Huffman et al.10 investigated the high-temperature behavior of coal ash in oxidizing and mildly reducing atmospheres (60% CO/40% CO2). However, to our knowledge, little research has been reported on the fusibility of coal ash in strongly reducing atmospheres. Thus, it is necessary to study the fusibility in strongly reducing atmospheres and the difference of fusibility between different atmospheres and to predict accurately the AFTs of coal ashes in different atmospheres. Many researchers have used different methods to test and predict the AFTs of coal ash.11-13 Some investigations have attempted to relate the AFT to the coal ash composition, and fairly detailed relations, both statistical and empirical, have been established.14,15 For example, Gray et al.16 carried out multiple and stepwise regression analysis to relate the AFT to the coal (7) Bryant, G. W.; Browning, G. J.; Gupta, S. K.; Lucas, J. A.; Gupta, R. P.; Wall, T. F. Energy Fuels 2000, 14, 326–335. (8) Vassilev, S. V.; Kitano, K.; Takeda, S.; Tsurue, T. Fuel Process. Technol. 1995, 45, 27–51. (9) Bryant, G. W.; Browning, G. J.; Emanuel, H.; Gupta, S. K.; Gupta, R. P.; Lucas, J. A.; Wall, T. F. Energy Fuels 2000, 14, 316–325. (10) Huffman, G. P.; Huggins, F. E.; Dunmyre, G. R. Fuel 1981, 60, 585–597. (11) Kahraman, H.; Bos, F.; Reifenstein, A.; Coin, C. D. A. Fuel 1998, 77, 1005–1011. (12) Kahraman, H.; Reifenstein, A. P.; Coin, C. D. A. Fuel 1999, 78, 1463–1471. (13) Yin, C. G.; Luo, Z. Y.; Ni, M. J.; Cen, K. F. Fuel 1998, 77, 1777– 1782. (14) Winegartner, B. C.; Rhodes, B. T. J. Eng. Power 1975, 97, 395– 401. (15) Lloyd, W. G.; Riley, J. T.; Zhon, S.; Risen, M. A.; Tibbitts, R. L. Energy Fuels 1993, 7, 490–494. (16) Gray, V. R. Fuel 1987, 66, 1230–1239.
10.1021/ef800974d CCC: $40.75 2009 American Chemical Society Published on Web 03/03/2009
Chinese Coal Ash Fusion Temperatures
Energy & Fuels, Vol. 23, 2009 1991
Table 1. Ash Composition for Coal Ash Samples composition (wt %) number
ash samples
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Shanxi Tongchuan Yunnan Xiaolongtan Shandong Zaozhuang Ningxia Lingwu Shanxi Gujiao Shanxi Gaoyang Shandong Yanzhou Neimenggu Neimenggu Houbulian Shanxi Xishan Shanxi Changyan Shanxi Shenmu Liaoning Shenyang Anhui Huaibei Shanxi Shenfu Shanxi Xiaoyi Shandong Huangxian Gansu Huating Shanxi Yangquan NeimengguDongsheng Shanxi Jincheng
a
SiO2 Al2O3 CaO Fe2O3 MgO A/Ba 29.34 30.28 34.92 20.54 25.21 28.81 22.97 22.90 33.80 35.11 36.29 45.95 50.12 45.21 47.54 50.34 54.12 48.77 59.47 56.67 42.44
14.35 18.61 16.50 31.32 28.13 26.78 34.76 36.61 29.08 32.01 33.84 28.02 23.87 30.36 31.55 26.38 24.17 29.79 21.76 24.01 38.83
34.53 34.13 37.76 28.19 18.65 19.13 17.77 27.58 16.27 12.09 7.51 14.77 9.48 18.80 15.72 8.55 10.64 6.59 6.71 7.98 7.92
16.73 13.21 8.77 16.54 27.21 23.57 22.77 10.17 17.04 17.23 14.66 7.61 10.23 3.76 7.00 11.45 6.21 11.47 7.08 8.52 7.84
5.03 3.77 2.04 3.41 0.79 1.71 1.73 2.75 3.81 3.56 7.70 3.66 6.30 1.87 1.61 3.28 4.85 3.37 6.98 2.83 2.97
0.78 0.96 1.06 1.08 1.14 1.25 1.36 1.46 1.69 2.04 2.34 2.84 2.84 3.09 3.25 3.30 3.60 3.67 3.81 4.17 4.34
A/B ) (SiO2 + Al2O3)/(CaO + Fe2O3 + MgO).
ash composition. Kucukabyrak et al.17 established the relationship between chemical composition and AFTs for some Turkish lignites. However, although these approaches can give good results if applied to some coal ash samples with similar chemical composition, it is less effective when applied to samples from different sources and with a wide range of composition.18 Some researchers have applied thermodynamic considerations to AFT predications. Huggins et al.19 correlated the liquidus surfaces of the SiO2-Al2O3-XO (where X ) Fe, Ca, and K2) ternary systems with trends obtained for the AFT of coal ash. Hurst et al.20 and Wall et al.21 used SiO2-Al2O3-CaO and CaO-MgO-FeOn ternary equilibrium diagrams to predict the effect of CaO and Fe2O3 on the AFT of coal ash. However, these methods could only be applied to a limited range of conditions and were only able to qualitatively predict the AFTs of coal ashes. Predictions of AFTs have recently been undertaken with the aid of computer thermodynamic modeling of phase equilibria.22,23 Goni et al.24 evaluated slagging by the determination of AFT curves using MTDATA software and the NPLOX3 database for the main coal ash oxides. Jak et al.25 applied the thermodynamic computer package FactSage to predict AFTs and found that they do correlate with the liquidus temperatures. These approaches are based on phase equilibrium science rather than simple relationships with bulk composition. Analysis of coal ashes on a weight percent basis shows that they consist mainly of SiO2, Al2O3, Fe2O3, CaO, and MgO, so that the properties of coal ashes at high temperature might be (17) Kucukbayrak, S.; Ersoy, M. A.; Haykiri, A. H.; Guner, H.; Urkan, K. Fuel Sci. Technol. Int. 1993, 11, 1231–1249. (18) Seggiani, M. Fuel 1999, 78, 1121–1125. (19) Huggins, F. E.; Kosmack, D. A.; Huffman, G. P. Fuel 1981, 60, 577–584. (20) Hurst, H. J.; Novak, F.; Patterson, J. H. Energy Fuels 1996, 10, 1215–1219. (21) Bryant, G.; Bailey, C.; Wu, H. W.; McLennan, A.; Stanmore, B.; Wall, T. In The Impact of Mineral Impurities in Solid Fuel Combustion; Gupta, R. P., Wall, T. F., Baxter, L., Eds.; Kluwer Academic/Plenum Press: New York, 1999; pp 581-594. (22) Li, H. X.; Ninomiya, Y.; Dong, Z. B.; Zhang, M. X. Chin. J. Chem. Eng. 2006, 14, 784–789. (23) Dyk, J. C. V.; Melzer, S.; Sobiecki, A. Miner. Eng. 2006, 19, 1126– 1135. (24) Goni, Ch.; Helle, S.; Garcia, X.; Gordon, A.; Parra, R.; Kelm, U.; Jimenez, R.; Alfaro, G. Fuel 2003, 82, 2087–2095. (25) Jak, E. Fuel 2002, 81, 1655–1668.
Figure 1. Pseudo-ternary equilibrium phase diagrams for the system Si-Al-Ca-Fe-Mg-O: (a) Ar atmosphere and (b) H2 atmosphere.
modeled by a five system SiO2-Al2O3-Fe2O3-CaO-MgO synthetic ash. For example, both Hurst et al.26 and Vorres et al.27 used synthetic ashes to study the flow properties of coal ashes at high temperature. However, to our knowledge, little work has been published regarding the use of synthetic ashes to study the fusibility of coal ashes. Meanwhile, because the composition of synthetic ash samples can be easily controlled, the experimental conditions can be simplified and can exclude any interference by trace elements. Thus, we have measured the AFTs of 60 synthetic ash samples formed by mixtures of these five oxides (SiO2, Al2O3, CaO, Fe2O3, and MgO) and 21 Chinese coal ash samples, in both an inert atmosphere (100% Ar) and a strongly reducing atmosphere (100% H2). The computer software package FactSage was used to calculate the temperatures corresponding to different proportions of the liquid phase and predict the phase equilibria of synthetic ash samples. (26) Hurst, H. J.; Novak, F.; Patterson, J. H. Fuel 1999, 78, 439–444. (27) Vorres, K. S.; Greenberg, S.; Poeppel, R. In Mineral Matter and Ash in Coal; Vorres, K. S., Eds.; American Chemical Society: Washington, D.C., 1986; pp 109-116.
1992 Energy & Fuels, Vol. 23, 2009
Song et al. Table 2. Ash Composition for Synthetic Ash Samples
number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
SiO2 Al2O3 CaO Fe2O3 MgO A/B
30.00 12.00 40.00 15.00 3.00 0.72
30.00 12.00 15.00 40.00 3.00 0.72
33.57 13.43 15.00 35.00 3.00 0.72
33.57 13.43 45.00 5.00 3.00 0.89
33.57 13.43 35.00 15.00 3.00 0.89
35.71 14.28 15.00 15.00 20.00 1.00
31.20 20.80 30.00 15.00 3.00 1.08
37.14 14.86 40.00 5.00 3.00 1.08
37.14 14.86 15.00 30.00 3.00 1.08
37.14 14.86 30.00 15.00 3.00 1.08
37.86 15.14 15.00 15.00 17.00 1.13
40.71 16.29 25.00 15.00 3.00 1.33
46.52 10.48 20.00 20.00 3.00 1.33
40.71 16.29 15.00 25.00 3.00 1.33
34.20 22.80 25.00 15.00 3.00 1.33
number
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
SiO2 Al2O3 CaO Fe2O3 MgO A/B
40.71 16.29 35.00 5.00 3.00 1.33
43.57 17.43 15.00 15.00 9.00 1.56
40.62 21.38 30.00 5.00 3.00 1.63
37.20 24.80 20.00 15.00 3.00 1.63
44.29 17.71 15.00 20.00 3.00 1.63
40.29 21.71 18.00 17.00 3.00 1.63
44.29 17.71 30.00 5.00 3.00 1.63
45.00 18.00 15.00 15.00 7.00 1.70
42.59 22.41 15.00 15.00 5.00 1.86
46.43 18.57 15.00 15.00 5.00 1.86
36.55 30.45 15.00 15.00 3.00 2.03
47.85 19.16 25.00 5.00 3.00 2.03
43.90 23.10 15.00 15.00 3.00 2.03
52.11 14.89 15.00 15.00 3.00 2.03
52.11 14.89 10.00 20.00 3.00 2.03
number
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
SiO2 Al2O3 CaO Fe2O3 MgO A/B
47.86 19.14 15.00 15.00 3.00 2.03
50.25 16.75 15.00 15.00 3.00 2.03
54.82 12.18 15.00 15.00 3.00 2.03
43.90 23.10 25.00 5.00 3.00 2.03
46.06 20.94 15.00 15.00 3.00 2.03
40.20 26.80 15.00 15.00 3.00 2.03
16.62 55.38 15.00 15.00 3.00 2.18
27.00 45.00 15.00 15.00 3.00 2.18
45.21 23.79 15.00 15.00 3.00 2.18
49.29 19.71 15.00 15.00 1.00 2.22
49.64 19.86 15.00 15.00 0.50 2.27
49.50 22.50 10.00 15.00 3.00 2.57
47.17 24.83 10.00 15.00 3.00 2.57
43.20 28.80 15.00 10.00 3.00 2.57
51.43 20.57 20.00 5.00 3.00 2.57
number
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
SiO2 Al2O3 CaO Fe2O3 MgO A/B
51.43 20.57 10.00 15.00 3.00 2.57
39.27 32.73 15.00 10.00 3.00 2.57
51.43 20.57 15.00 10.00 3.00 2.57
47.17 24.83 20.00 5.00 3.00 2.57
47.17 24.83 15.00 10.00 3.00 2.57
49.50 22.50 15.00 10.00 3.00 2.57
52.94 24.06 5.00 15.00 3.00 3.35
55.00 22.00 15.00 5.00 3.00 3.35
55.00 22.00 5.00 15.00 3.00 3.35
50.45 26.55 5.00 15.00 3.00 3.35
55.00 22.00 10.00 10.00 3.00 3.35
57.14 22.86 2.00 15.00 3.00 4.00
57.86 23.14 15.00 1.00 3.00 4.26
58.57 23.43 10.00 5.00 3.00 4.55
60.17 21.83 7.00 10.00 1.00 4.55
Table 3. Species and Phases Considered in FactSage Version 5.5 Calculations gas phase
inert Ar
inert H2
liquid polynomial solution solid pure components
slag SiO2, Al2O3, Fe2O3, CaO, MgO, Al6Si2O13, CaAl2O4, Ca2Al2SiO7, Ca3A12O6, CaSiO3, Ca2SiO4, Ca3SiO5, Ca3SiO7, CaAl2Si2O8, Fe2Al4Si5O18, FeAl2O4, CaFe2O4, Ca2Fe2O5, CaFe2O7, CaFeSi2O6, Ca3Fe2Si3O12, CaMgSi2O6, MgSiO3, Mg2SiO4 spinel (SPIN), A monoxide (MeO_A), clinopyroxene (cPy), orthopyroxene (oPyr), A wollastonite (WOLLA), melilite, olivine (Oliv), Aa′Ca2SiO4, mullite (MulF), cordierite
slag SiO2, Al2O3, Fe2O3, Fe, FeO, CaO, MgO, Al6Si2O13, CaAl2O4, Ca2Al2SiO7,Ca3Al2O6, CaSiO3, Ca2SiO4, Ca3SiO5, Ca3SiO7, CaAl2Si2O8, Fe2Al4Si5O18, FeAl2O4, CaFe2O4, Ca2Fe2O5, CaFe2O7, CaFeSi2O6, Ca3Fe2Si3O12, CaMgSi2O6, MgSiO3, Mg2SiO4 wollastoite (WOLL), dicalcium silicate (CASI), corundum (CORU), iron spinel (FESP), olivine (Oliv), A monoxide (MeO_A), low clinopuroene (LcPy)
polynomial solid solutions
Empirical liquidus models were derived to correlate the AFTs under both Ar and H2 atmospheres of 60 synthetic ash samples, with their liquidus temperatures calculated by FactSage. These models were used to predict the AFTs of 21 Chinese coal ash samples in Ar and H2 atmospheres. In addition, a comparison of AFTs for the Chinese coal ash samples was carried out for both atmospheres, and X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS) were employed to study the ash behaviors. Experimental Section Coal Ash Samples. A total of 21 representative Chinese coal samples were used in the study. The ash samples were prepared in a muffle furnace at 815 °C for 24 h according to the Chinese standard GB/T 1574-1995. Chemical analysis of the samples was carried out using X-ray fluorescence (XRF). The chemical composition of the 21 coal ash samples is given in Table 1.
Synthetic Ash Samples. A total of 60 synthetic ash samples, covering the range of composition of most of the Chinese coal ash samples, were prepared from Sinopharm Chemical Reagent Corp. laboratory-reagent silicon dioxide, aluminum oxide, ferric oxide, calcium oxide, and magnesium oxide. The compositions of the 60 synthetic ash samples are given in Table 2. Fusion Temperature Test. We performed the fusion temperature tests by following the Chinese standard procedures (GB/T 2191996) in a registered independent laboratory. This test involves heating a sample cone of specified geometry at a rate of 5 k/min in an Ar or a H2 atmosphere. The following temperatures are recorded for each sample, corresponding to specific shapes of the ash cones: initial deformation temperature (IDT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT). Characterization of Quenched Coal Ash Samples. Mineralogical and microstructural analyses of coal ash samples quenched in an Ar or a H2 atmosphere were determined by XRD (Rigabu D/max 2250)
Table 4. Average Ash Chemistry, Approximate Liquidus Temperatures, and Expected Mineralogy of Synthetic Ashes for Various Groups group
A/B
number of sample
I II
7% do not lie in the phase of these pseudo-ternary sections. In parts a and b of Figure 1, thick lines of the same color represent all compositions having a given liquidus temperature. Group I: A/B < 2. Group I synthetic ash samples contain very high levels of fluxing components (CaO + Fe2O3 + MgO > 35%) and only a little refractory components. As shown in parts a and b of Figure 1, the composition of the synthetic ash samples
A/B g 2
bias (°C)
standard error (°C)
bias (°C)
standard error (°C)
6.53 -0.74 -6.61 7.35
9.81 8.84 7.51 7.97
6.66 -1.49 10.38 -14.74
9.14 9.46 10.36 8.81
with A/B < 2 under Ar and H2 atmospheres lie in the anorthite (CaAl2Si2O8) and genhlenite (CaAl2SiO7) primary phase field of the pseudo-ternary diagrams SiO2-Al2O3-CaO/Fe2O3 and SiO2-Al2O3-CaO/Fe, respectively, and their liquidus temperatures are expected to be low. Group II: A/B g 2. Group II synthetic ash samples are characterized by the presence of reasonable levels of refractory components, so that the fluxing components CaO + Fe2O3 + MgO < 33%. Parts a and b of Figure 2 show that the composition of Group II ash samples have mullite (Al6Si2O13) as the first solid to precipitate and have high liquidus temperatures. Composition Ranges for Coal and Synthetic Ash Samples. The composition ranges for coal ash samples and synthetic ash samples are given in Table 5. As can be seen from Table 5,
Figure 6. Comparison of AFTs for 21 Chinese coal ash samples in Ar and H2 atmospheres.
Figure 7. XRD diffratograms of slowly cooled Shenfu and Xishan coal ash-fusion cones in Ar and H2 atmospheres.
1996 Energy & Fuels, Vol. 23, 2009
Song et al.
Figure 9. EDS spectrum of the spherical particle and nonspherical particle regions of SEM of coal ashes under a H2 atmosphere. Figure 8. SEM photomicrographs of slowly cooled Shenfu and Xishan coal ash-fusion cones in Ar and H2 atmospheres.
the range of content of the five oxides, SiO2/Al2O3, and the A/B ratio of synthetic ash samples covers those of the 21 Chinese coal ash samples. Correlation Analysis of Synthetic Ash. Linear regression analysis was used to compare correlations between the AFTs of synthetic ash samples and the temperatures with different liquid content calculated by FactSage. Generally, the reliability of the values calculated using the correlations is indicated by the correlation coefficient (R). The standard deviation, σ value, simply indicates how close the calculated value is to the experimental values. In our work, the R values are used to analyze the correlations between the four characteristic AFTs of synthetic ash samples and their temperatures with different liquid content. A R value of 0.7 is generally acceptable for predicting the AFTs of coal ash samples; a value of 0.8 is good; and a value of 0.9 or higher is excellent. Meanwhile, because the current standard ash fusion test, such as GB/T 219-1996, is somewhat subjective, being based on the operator’s judgment of the degree of deformation of the ash sample as the temperature rises at a specified rate in a controlled atmosphere, the repeatability limits for a single operator and apparatus is 30-50 °C and the repeatability limits for different operators and apparatus is 80-150 °C. Parts a and b of Figure 2 show the effect of the liquid content on the correlation coefficients between the AFTs and temperatures for different liquid content calculated by FactSage for synthetic ash samples with different A/B ratios. Figure 2a shows the increase in the four characteristic AFTs corresponding to R values of synthetic ash samples as the liquid contents increase under an Ar atmosphere. In a H2 atmosphere, the effect of the liquid content on the R values is similar to that for an Ar atmosphere (Figure 2b). As can be seen from parts a and b of Figure 2, when the liquid content increases to 100%, the corresponding R values for the four characteristic AFTs are almost the largest values for all of the samples in both Ar and H2 atmospheres, which indicates that there are significant correlations between the AFTs of synthetic ash samples and the liquidus temperature in Ar and H2 atmospheres. Parts a-d of Figure 3 present the AFTs of the synthetic ash samples with different A/B ratios as a function of the calculated liquidus temperatures in both Ar and H2 atmospheres. Good correlations between the AFTs and liquidus temperatures can
be clearly seen in Figure 3: the higher the liquidus temperature, the higher the AFT. Also, the AFTs of synthetic ash samples give R values that are always higher than 0.89, which indicates that there are good correlations between the AFTs and liquidus temperatures. Meanwhile, the σ values are always less than 30 °C, which indicates that the accuracy of the results is within experimental error. The AFTs of the synthetic ash samples with different A/B ratios were expressed as linear functions of the liquidus temperature. For example, the IDT of the coal ash sample with A/B < 2 (I) in a H2 atmosphere (H) is THFT (I) ) aHFT (I) + bHFT (I)TH liquidus (I)
(1)
The sum of squared differences between the measured and predicted AFT temperatures was minimized by adjusting the parameters a and b. The parameters obtained this way for this set of synthetic ash samples are given in Table 6. Prediction of AFTs of Coal Ash Samples. In our work, the liquidus models (eq 1), which have been developed by empirical correlations between the liquidus temperatures calculated by FactSage and AFTs derived using linear regression analysis, are used to predict the AFTs of 21 Chinese coal ash samples in Ar and H2 atmospheres. Parts a and b of Figure 4 show parity plots of the measured AFTs versus the predicted AFTs for 21 Chinese coal ash samples with different A/B ratios in an Ar atmosphere. All of the coal ash samples show deformation temperatures within experimental errors of (40 °C; therefore, the liquidus models can predict AFTs of Chinese coal ash samples in an Ar atmosphere. Parity plots for AFT prediction of the Chinese coal ash samples with different A/B ratios in a H2 atmosphere using these models (eq 1) are given in parts a and b of Figure 5. It is apparent that all of coal ash samples (apart from one) are observed to deform with experimental errors (40 °C, which indicates that the liquidus models provide a good prediction of the AFTs of Chinese coal ash samples in a H2 atmosphere. A summary of the bias and standard errors for each AFT is given in Table 7. Bias is calculated from the following equation: bias )
∑ AFTs(measured) -n AFTs(predicted)
(2)
Chinese Coal Ash Fusion Temperatures
Energy & Fuels, Vol. 23, 2009 1997
The standard error is calculated from the following: SE )
∑ σ(AFTs(measured)√-n AFTs(predicted))
(3)
where n is the number of data points and σ is the standard deviation. From Table 7, we see that the bias and standard error for the AFTs of each of the 21 Chinese coal ash samples are below 15 and 25 °C, respectively, which are lower than experimental errors; therefore, the liquidus models can predict the AFTs very well. Comparison of AFTs of Coal Ash Samples in Ar and H2 Atmospheres. The AFTs of a given coal ash sample are different in different atmospheres. For example, the AFTs of coal ash samples in mildly reducing atmospheres are lower than those of coal ash samples in oxidizing atmospheres. This is because most iron oxides in mildly reducing atmospheres are reduced to FeO, which can form a eutectic mixture with the other oxides.16,34 Figure 6 shows a comparison of the AFTs of 21 Chinese coal ash samples in both Ar and H2 atmospheres. From the four graphs, we find that the AFTs of coal ash samples in a H2 atmosphere are always higher than those in an Ar atmosphere. Parts a-d of Figure 7 depict the X-ray diffractograms of two slowly cooled Chinese coal samples (Shenfu and Xishan samples) from ash-fusion cones in Ar and H2 atmospheres. The species of mineral matters and the main forms of iron clearly changes between the Ar and H2 atmospheres. For example, the characteristic peaks of minerals in a H2 atmosphere (parts c and d of Figure 7) are clearly less than that those in an Ar atmosphere (parts a and b of Figure 7), which indicates that the amount of mineral matter in a H2 atmosphere is less than in an Ar atmosphere. Meanwhile, the main form of iron in a H2 atmosphere is metallic iron. Figure 8 shows the SEM analysis for micromorphologies of two slowly cooled Chinese coal samples (Shenfu and Xishan samples) from ash-fusion cones in Ar and H2 atmospheres. It can be seen that, in an Ar atmosphere, the micromorphologies of the coal ash samples show irregular net structures (parts a (28) Bale, C. W.; Chartrand, P.; Degterov, S. A.; Eriksson, G.; Hack, K.; Mahfoud, R. B.; Melancon, J.; Pelton, A. D.; Petersen, S. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2002, 26, 189–228. (29) Jak, E.; Degterov, S.; Hayes, P. C.; Pelton, A. D. Fuel 1998, 77, 77–84. (30) Schobert, H. H.; Streeter, R. C.; Diehl, E. K. Fuel 1985, 64, 1611– 1617. (31) Seggiani, M.; Pannocchia, G. Ind. Eng. Chem. Res. 2003, 42, 4919– 4926. (32) Folgueras, M. B.; Diaz, R. M.; Xiberta, J.; Garcia, M. P.; Pis, J. J. Energy Fuels 2005, 19, 2562–2570.
and b of Figure 8). However, in a H2 atmosphere, the micromorphologies show bracket structures, which may make the physical structure of each coal ash samples firmer and not easy to deform in a H2 atmosphere. This may be one of the reasons why the AFTs are higher than those of coal ash samples in a H2 atmosphere. Also, from parts c and d of Figure 8, we can see that there are lots of spherical particles formed in a H2 atmosphere. The EDS analysis of a typical particle of Shenfu and Xishan coal ash samples in a H2 atmosphere (I and II sites in parts c and d of Figure 8) shows that the particles are composed of metallic iron (parts b and d of Figure 9). With regard to the plate site of Shenfu and Xishan coal ash samples in a H2 atmosphere (III and IV sites in parts c and d of Figure 8), they contain predominantly Si, Al, Ca, K, Na, and O but no Fe (parts a and c of Figure 9). The EDS analysis indicates that the iron oxides in the H2 atmosphere are reduced to metallic iron. This phenomenon coincides with the analysis results of XRD, which may be the important reason that leads to an increase of the AFTs of the coal ash sample in the H2 atmosphere. Conclusions In our work, empirical liquidus models have been derived to relate the AFTs under both Ar and H2 atmospheres of 60 synthetic ash samples, with their liquidus temperature calculated by FactSage. These models have been used to predict the AFTs of 21 Chinese coal ash samples in Ar and H2 atmospheres, and then the AFT differences between atmospheres were analyzed. In both Ar and H2 atmospheres, there is an apparently linear correlation and good agreement between the AFTs of synthetic ash samples and the liquidus temperatures calculated by FactSage (R > 0.89, and σ < 30 °C). These models predict the AFTs of coal ash samples with a good level of accuracy (SE < 30 °C). Because the iron oxides in coal ash samples in a H2 atmosphere are reduced to metallic iron and lead to mineral species and micromorphology changes, the AFTs of the Chinese coal ash samples under a H2 atmosphere are always higher than those under an Ar atmosphere. Acknowledgment. The authors acknowledge the financial support provided by the National Basic Research Program of China (20576040). EF800974D (33) Gupta, S. K.; Gupta, R. P.; Bryant, G. W.; Juniper, L.; Wall, T. F. In The Impact of Mineral Impurities in Solid Fuel Combustion; Gupta, R. P., Wall, T. F., Baxter, L., Eds.; Kluwer Academic/Plenum Press: New York, 1999; pp 155-169. (34) Folkedahl, B. C.; Schobert, H. H. Energy Fuels 2005, 19, 208– 215.
