Energy Fuels 2010, 24, 182–189 Published on Web 08/11/2009
: DOI:10.1021/ef900537m
Effect of Coal Ash Composition on Ash Fusion Temperatures† 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, PR China, and §Electric Powder Development Corporation Ltd., Tokyo 167-0023, Japan Received May 26, 2009. Revised Manuscript Received July 12, 2009
The ash fusion temperatures (AFTs) of coal mineral matter at high temperature are important parameters for all gasifiers. Experiments have been conducted in which mixtures of selected coal ashes and SiO2, Al2O3, CaO, Fe2O3, and MgO were subjected to the standard test for ash fusibility. The computer software package FactSage has been used to calculate the liquidus temperatures of coal ash samples and the proportions of the various phases present as a function of temperature. The results show that the AFTs of coal ash samples first decrease with increasing CaO, Fe2O3, and MgO contents, then reach a minimum value, before increasing once more. However, for the effect of S/A ratio, its AFTs are always increased with increasing S/A ratios. The measured AFTs all show variations with mixture composition that correlated closely with liquidus temperatures for the appropriate pseudoternary phase diagrams. The liquidus and AFTs generally showed parallel compositional trends but are displaced from each other because of the influence of additional basic components in the coal ash. The liquidus temperatures of coal ash samples are always higher than its AFTs.
attempted to relate the AFT to the coal ash composition, and fairly detailed relations, both statistical and empirical, have been established.16-21 Some researchers studied the effect of some oxides on the AFTs of coal ash. For example, Huggins et al.22 used the ternary equilibrium phase diagrams to study the effects of Fe2O3, CaO, and K2CO3 on the AFTs of coal ash. Gray et al.23 studied the effects of acid and basic fluxes on the AFTs of coal ash. Vassilev et al.24 studied the influence of mineral and chemical composition of coal ashes on their fusibility. Song et al.25,26 applied the thermodynamic computer package FactSage to study the effect of CaO as pure compounds on the AFTs of coal ash. Wall et al. studied the thermomechanical analysis (TMA) fusibility of laboratory ash, combustion ash, and deposits formed from an Australian thermal coal. However, to the best of our knowledge, little work has been published regarding systematic research on the effect of coal ash composition on the AFT of coal ashes. In this work, we have measured the AFTs of 33 mixtures of coal ashes with SiO2, Al2O3, CaO, Fe2O3, and MgO additives. The computer software package FactSage has been used to calculate the liquidus temperatures of coal ash samples and the proportions of the various phases present as a function of
Introduction For all gasifiers, the ash fusion temperature (AFT) is an important variable for all gasifiers.1,2 For fluid-bed gasifiers, these properties govern the upper operating temperature at which agglomeration of the ash is initiated. For entrainedflow gasifiers, the operating temperature should be above the flow temperature (FT) of coal ash to enable continuous slag tapping.3 Thus, there is a need to study the AFTs of coal ashes. Many researchers have used different methods to test and predict the AFTs of coal ash.4-15 Some investigations have † Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. Telephone: þ86-2164252309. Fax: þ86-21-64253626. E-mail:
[email protected] and
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r 2009 American Chemical Society
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Table 1. Composition of Coal Ash Samples composition (wt. %) number
SiO2
Al2O3
1 2 3 4 5 6 7 8 9
52.73 49.95 47.18 44.40 41.63 38.85 36.08 33.30 27.75
35.03 33.18 31.34 29.50 27.65 25.81 23.97 22.12 18.44
CaO
Fe2O3
MgO
TiO2
Na2O
K2O
Shanxi shuangliu coal ash --- CaO mixtures 5.00 2.96 0.41 10.00 2.81 0.39 15.00 2.65 0.37 20.00 2.50 0.34 25.00 2.34 0.32 30.00 2.18 0.30 35.00 2.03 0.28 40.00 1.87 0.26 50.00 1.56 0.21
1.93 1.83 1.73 1.63 1.52 1.42 1.32 1.21 1.02
0.27 0.26 0.25 0.23 0.22 0.20 0.19 0.17 0.14
1.67 1.58 1.50 1.41 1.32 1.23 1.14 1.06 0.88
1 2 3 4 5 6 7 8
56.15 52.65 49.73 46.80 43.88 40.95 38.03 35.01
30.28 28.39 26.82 25.24 23.66 22.08 20.51 18.93
Henan yima coal ash --- Fe2O3 mixtures 4.91 4.02 1.45 4.60 10.00 1.36 4.34 15.00 1.28 4.09 20.00 1.21 3.83 25.00 1.13 3.58 30.00 1.05 3.32 35.00 0.98 3.07 40.00 0.90
1.29 1.21 1.14 1.07 1.01 0.94 0.87 0.81
1.87 1.75 1.65 1.56 1.46 1.36 1.26 1.17
0.59 0.55 0.52 0.49 0.46 0.43 0.40 0.37
1 2 3 4 5 6 7 8
53.93 53.16 52.06 50.97 49.87 48.77 47.68 46.58
30.76 30.32 29.69 29.07 28.44 27.82 27.19 26.57
Shanxi guojiawan coal ash --- MgO mixtures 5.68 4.48 1.52 5.60 4.42 3.00 5.49 4.33 5.00 5.37 4.24 7.00 5.26 4.15 9.00 5.14 4.06 11.00 5.03 3.97 13.00 4.91 3.88 15.00
1.29 1.27 1.25 1.22 1.19 1.17 1.14 1.12
1.65 1.63 1.60 1.56 1.53 1.49 1.46 1.43
0.67 0.59 0.58 0.56 0.56 0.54 0.53 0.52
1 2 3 4 5 6 7 8
27.17 30.70 33.90 36.82 39.50 42.73 45.63 48.25
16.98 16.17 15.41 14.72 14.11 13.35 12.68 12.06
Shandong yanzhou coal ash --- S/A mixtures 15.36 38.22 1.16 14.62 36.37 1.11 13.93 34.69 1.05 13.32 33.15 1.00 12.76 31.75 0.96 12.07 30.05 0.91 11.47 28.53 0.86 10.91 27.15 0.82
0.42 0.40 0.38 0.36 0.35 0.33 0.31 0.30
0.19 0.18 0.17 0.16 0.16 0.15 0.14 0.13
0.51 0.49 0.46 0.44 0.43 0.40 0.38 0.36
temperature. The relation between AFTs and pseudoternary phase equilibrium diagrams has been examined by preparing the mixtures of coal ashes with various oxides. The metallurgical microscopy has been used to analyze the effects of these oxides on the mineral.
the ash cones: initial deformation temperature (IDT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT). Thermodynamic Equilibrium Calculations. The thermodynamic software package FactSage is the fusion of two wellknown software packages in computational thermochemistry: Fact-Win and ChemSage.28 FactSage consists of a series of information, database, calculation, and manipulation modules that enable one to access and manipulate pure substances and solution databases. FactSage allows calculating and predicting multiphase equilibria, liquidus temperatures, and the proportions of the liquid and solid phases in a specified atmosphere for a multicomponent system. FactSage was used in this study to calculate the corresponding temperatures with a different proportion of liquid phase as well as the equilibrium product distributions for simplified coal ash systems. Phase formation data for these oxides and their combinations were selected from the FToxid database. Calculations were carried out between the solid temperature and liquidus temperature in Ar atmosphere at 1 atm pressure. The calculation method of FactSage is based on Gibbs’ energy minimization for each sample at a given temperature and composition range. Phases formed at concentrations below
Experimental Section Coal Ash Samples. Four 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). To study the effects of SiO2, Al2O3, CaO, Fe2O3, and MgO on the AFTs of coal ashes, we added the ash to proper Sinopharm Chemical Reagent Corp. laboratory regent silicon oxide, alumina oxide, ferric oxide, calcium oxide, and magnesium oxide. The chemical composition of the 33 mixtures of coal ashes with SiO2, Al2O3, CaO, Fe2O3, and MgO additives is given in Table 1. Fusion Temperature Test. We performed the fusion temperature tests by following the Chinese standard procedures (GB/T 219-1996) 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 atmosphere. The following temperatures are recorded for each sample, corresponding to specific shapes of
(28) Bale, C. W.; Chartrand, P.; Degterov, S. A.; Eriksson, G.; Hack, K.; Mahfoud, R. B.; Melancon, J.; Pelton, A. D.; Petersen, S. Calphad 2002, 26, 189–228. (29) Jak, E.; Degterov, S.; Hayes, P. C.; Pelton, A. D. Fuel 1998, 77, 77–84.
(27) Liu, Y.; Gupta, R.; Elliott, L.; Wall, T.; Fujimori, T. Fuel Process. Technol. 2007, 88, 1099–1107.
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Figure 1. Effect of CaO on ash fusion temperature and liquidus temperature.
0.01 wt % were ignored. Because of the complexity of the thermodynamic models (quasi-chemical, sublattice) which represents the interaction of the components for phase formation, the convergence of the algorithms is slow and sensitive. In this study, we used the method of Jak et al.,28 which permits the approximation to start from lower-order subsystems, then finally reach the real or complete system. Once the phases have been determined, a total mass balance verifies the consistency of the system.
Figure 2. Calculated liqudius temperatures in the SiO2-Al2O3CaO-Fe2O3 system on the pseudoternary section with a SiO2/ Al2O3 weight ratio of 2.56.
Effect of CaO Content. CaO is a common additive that is used to decrease the AFTs of coal ash.30 In our experiments, CaO contents between 5% and 50% have been added to cover the range of CaO contents of most Chinese coal ash samples. Figure 1 shows the plots of fusion temperatures against CaO content for Shanxi shuangliu coal ash samples. Fusion temperatures of coal ash samples drop as CaO content increases until the CaO content reaches 35%; at higher CaO content, the fusion temperatures of coal ash samples increase quickly. Also shown in Figure 1 is a curve representing the change in liquidus temperature with CaO. It can be seen that the experimental AFT curves would closely parallel the liquidus temperatures. Figure 2 illustrates a pseudoternary section construction for the SiO2-Al2O3-CaO-Fe2O3 system that can be used to visually express liquidus temperatures of synthetic slag samples with a SiO2/Al2O3 (S/A) ratio of 2.56 as a function of CaO content. In Figure 2, the lines of the same color represent all compositions having a given liquidus temperature. The red point indicates that the SiO2-Al2O3-CaOFe2O3 system composition varies with changes of CaO content. As shown in Figure 2, the liquidus temperature of the sample with a CaO content of 5% is predicted to be above 1400 °C; however, the sample with a CaO content of 35% is in the low melting temperature composition region with a liquidus temperature below 1300 °C. This trend is similar to that seen in the AFTs of the coal ash samples with the change in CaO content. To illustrate in detail the crystalline minerals and their relative contents, the phase assemblage of synthetic slag samples for three different CaO content levels of 5%, 20%, 35%, and 50% as a function of temperature was calculated
by FactSage (Figure 3). Observations indicate that the subliquidus phase changes from high-melting mullite into low-melting gehlenite as the CaO content is increased from 5% to 35%. When the CaO content is further increased to 50%, the subliquidus phase changes to high-melting alpha again, which may account for the fact that the AFTs of coal ash samples with CaO contents of 5%, 20%, 35%, and 45% first decrease and then increase as the CaO content is increased. In the course of our study, when the furnace temperature was above the FT of the sample, the sample was slowly cooled (because rapid cooling has a deleterious effect on furnace life) and was then used to observe the microstructure and crystallized particles by using a DMM-300 metallurgical microscope. The maximum objective magnification is 100, and the minimum image field is 117 90 (μm). For the micrographs of the cooled coal ash samples, if the color is black and connected, the samples are said to be in-molten siliceous liquid slag phase. Meanwhile, the white and red colored discrete-like particles are the crystallized particles. Figure 4a-d presents micrographs of cooled coal ash samples with CaO contents of 5%, 20%, 25%, and 45%. It can be seen that the crystallized phase consists mainly of white crystalline particles. For most of the coal ash samples, when the temperatures reached FT, most of the particles had melted and dissolved, thus forming a siliceous liquid slag phase (black section in Figure 4). As a result, the white particles were crystallized out of the melt. According to the results calculated by FactSage (Figure 3), we can deduce that the white crystalline particles seen in Figure 4a,b were most probably composed of a mixture of leucite and mullite, while the white crystalline particles seen in Figure 4c were most likely composed of gehlenite. Furthermore, the particle size of crystalline particles from the synthetic slag with a CaO content of 35% is seen to be larger than that of the samples with CaO contents of 5%, 20%, and 50%. Effect of Fe2O3 Content. In coal, iron is predominantly in the form ferric iron in oxidizing and inert atmospheres31 and
(30) Seggiani, M.; Pannocchia, G. Ind. Eng. Chem. Res. 2003, 42, 4919–4926.
(31) Huffman, G. P.; Huggins, F. E.; Dunmyre, G. R. Fuel 1981, 60, 585–597.
Results and Discussion
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Figure 3. Phase assemblage-temperature curves for SiO2-Al2O3-CaO-Fe2O3-MgO-TiO2-Na2O-K2O with different CaO contents.
Figure 5. Effect of Fe2O3 on ash fusion temperature and liquidus temperature.
slag samples with different Fe2O3 contents. In Figure 6, the lines of the same color represent all compositions having a given liquidus temperature. The red point indicates that the SiO2-Al2O3-CaO-Fe2O3 system composition varies with changes of Fe2O3 content. From inspection of Figure 4, it can be seen that the synthetic slag sample with an Fe2O3 content of 5% is located in the high melting temperature composition region. The samples gradually move into the lower melting temperature composition region as the Fe2O3 content is increased up to 35%. Finally, the samples move back into the high melting temperature composition. This trend is similar to the changes in the AFTs of the coal ash samples as the Fe2O3 content is increased. Figure 7 shows the relative mineral content of coal ash samples with Fe2O3 contents of 4.02%, 15%, 25%, and 40% between the solid and liquidus temperatures. It can be seen that the subliquidus crystallized mineral changed from mullite to cristobalite. Analysis of Figure 7 indicates the effect of Fe2O3 content on the sensitivity of phase equilibria of coal ash samples to changes in temperature by calculating the proportions of the various phases present as a function of
Figure 4. Micrographs of slowly cooled ash-fusion cones with different CaO contents.
forms ferrous iron and even metallic iron in reducing atmosphere. In our work, the Fe2O3 content was varied from 4.02% to 40%, which also covers the range of Fe2O3 contents of typical Chinese coal ash samples. Figure 5 is a plot of the AFTs against percent Fe2O3. The temperatures drops as Fe2O3 content is about 35%, and at higher concentrations of Fe2O3, the AFTs remain constant or increase slightly. Also shown in Figure 5 is a curve representing the change in liquidus temperature with the increasing Fe2O3 content. This trend is similar to that displayed by the AFTs of coal ash samples as the Fe2O3 content is increased, which also give rise to a parabolic curve. The liquidus temperatures of coal ash samples are higher than AFTs of coal ash samples. The pseudoternary phase diagram (Figure 6) displays the range of composition and liquidus temperatures of synthetic 185
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temperature. For example, the proportion of the liquid phase of this system with the Fe2O3 contents of 4.02%, 15%, and 25% increases rapidly with decreasing temperature. For example, the proportion of the liquid phase of this system with the Fe2O3 contents of 4.02% decreases from 100% to 0% with a temperature decrease of 720 °C. However, the Fe2O3 content increased up to 30%, and the proportion of the liquid phase of those systems with Fe2O3 contents of 25% increase rapidly with decreasing temperature, which may provide an explanation for the change in the AFTs of the coal ash samples giving rise to a parabolic curve as the Fe2O3 content is increased. Figure 8 presents the microstructures and crystalline phases of quenched synthetic melt slag samples with Fe2O3
contents of 4.02%, 15%, 25%, and 40%. It can be seen that the crystalline phases consist mainly of white particles when the Fe2O3 content is 4.02% (Figure 8a). The number of these white particles clearly decreases, while the number of red crystalline particles increases concomitantly, when the Fe2O3 content is increased to 25% (Figure 8b,c). When the Fe2O3 content is further increased to 40%, there are a number of red and white crystalline particles, some of which appear on the surface of the samples (Figure 8d). According to the calculation results by FactSage (Figure 7), we surmise that the white crystalline particle in Figure 8a most probably consisted of mullite, that the red and white crystalline particles in Figure 8b,c were likely to consist of a mixture of mullite and iron oxide, and that the red and white crystalline particles in Figure 8d were probably composed of a mixture
Figure 6. Calculated liqudius temperatures in the SiO2-Al2O3CaO-Fe2O3 system on the pseudoternary section with a SiO2/ Al2O3 weight ratio of 3.16.
Figure 8. Micrographs of slowly cooled ash-fusion cones with different Fe2O3 contents.
Figure 7. Phase assemblage-temperature curves for SiO2-Al2O3-CaO-Fe2O3-MgO-TiO2-Na2O-K2O with different Fe2O3 contents.
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system composition varies with changes of MgO content. From inspection of Figure 10, it can be seen that the coal ash samples with an MgO content of 1.52% are located in the high melting temperature composition region. The samples gradually move into the lower melting temperature composition region as the MgO content is increased up to 5%. Finally, the samples move back into the high melting temperature composition. This trend is similar to the changes in the AFTs of coal ash samples as the MgO content is increased. Figure 11 presents the phase assemblage curves for the coal ash samples with different MgO contents. It can be seen that the subliquidus crystallized mineral does not change and is still mullite as the MgO content is increased from 1% to 7%. When the MgO content is further increased to 11%, the subliquidus crystallized mineral changes to spinel. Figure 12 presents surface micrographs of quenched melt coal ash samples with MgO contents of 1.52%, 7%, 11%, and 15%. It can clearly be seen that the morphology changes as the MgO content is increased. The crystalline phase consists mainly of white agglomerated particles, when the MgO content is 1.52% (Figure 12a). When the MgO content is further increased to 11% and 15%, respectively, the crystalline phase consists of yellow particles (Figure 12c,d). According to the calculation results by FactSage, we surmise that the white circular crystalline particles in Figure 12a most probably consisted of mullite, that the small white crystalline particles in Figure 12b were likely to consist of a mixture of leucite and mullite, and that the yellow crystalline particles in Figure 12c,d probably consisted of spinel containing Mg. Effect of S/A Content. S/A is an important parameter that affects the flow properties of coal ash slag. In our experiments, the S/A ratio was varied from 1.6 to 4.0, which covers the range of S/A ratios found in typical Chinese coal ash samples. Figure 13 presents the liquidus temperatures and the AFTs of coal ash samples as a function of the S/A ratio. It can be seen that the AFTs of coal ash samples increase with increasing S/A ratio until the S/A ratio reaches 4.0. This trend is similar to that displayed by the liquidus temperatures as S/A ratio increases. Figure 14 shows the projection of the liquidus surface and the composition of coal ash samples with different S/A ratios on to the pseudoternary section (SiO2-Al2O3-(CaO þ Fe2O3)) at a CaO/Fe2O3 weight ratio of 1.15. From inspection of Figure 14, it can be seen that the coal ash samples with an S/A ratio of 1.6 are located in the low melting temperature composition region. The samples gradually move into the higher melting temperature composition region as the S/A ratio is increased up to 4.0 This trend is similar to the changes in the AFTs of coal ash samples as the S/A ratio is increased. To illustrate in detail the crystallized minerals and their relative content, the phase assemblage of synthetic slag samples for three different S/A ratio levels of 1.6, 2.2, 3.2, and 4.0 as a function of temperature was calculated by FactSage (Figure 15). Observations indicate that the subliquidus phase changes from low-melting anorthite into high-melting mullite as the S/A ratio is increased from 1.6 to 2.2, and when the S/A ratio is further increased to 4.0, the subliquidus phase reverts to high-melting corundum, which may account for the fact that the AFTs of coal ash samples with S/A ratios of 1.6, 2.2, 3.2, and 4.0 always increase as the S/A ratio is increased.
Figure 9. Effect of MgO on ash fusion temperature and liquidus temperature.
Figure 10. Liqudius for the system Si-Al-Ca-Fe-Mg-O with SiO2/Al2O3 weight ratio of 2.98 and CaO/ Fe2O3 weight ratio of 3.6.
of cristobalite and iron oxide. The surfaces of the quenched melt slag samples show uniform porosity because on heating the samples the open pore network is transformed to one of micropores, resulting in a minimum porosity level. Effect of MgO Content. MgO is classified as a basic oxide and can decrease the ash fusion temperatures and improve the flow behavior of coal ash slag samples.32 We measured the AFTs of five guojiawan coal ash-MgO mixtures, in which the MgO content was varied between 1% and 15%, which covers the range of MgO contents of most Chinese coal ash samples. Figure 9 presents the liquidus temperatures and the AFTs of coal ash samples as a function of the MgO content. From inspection of Figure 9, it can be seen that the AFTs of the coal ash samples decrease as the MgO content is increased up to 3%. At higher MgO contents, the AFTs increase once more. The trends of liquidus and AFTs show similarities, but not a close parallelism. Figure 10 shows the coal ash composition with different MgO contents and the liquidus temperatures calculated using FactSage for an S/A ratio of 2.98 and a CaO/Fe2O3 ratio of 3.6. In Figure 10, the lines of the same color represent all compositions having a given liquidus temperature. The red point indicates that the SiO2-Al2O3-CaO-Fe2O3-MgO (32) Benson, S. A. Inorganic transformations and ash deposition during combustion; American Society of Mechanical Engineers: New York, 1992.
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Figure 11. Phase assemblage-temperature curves for SiO2-Al2O3-CaO-Fe2O3-MgO-TiO2-Na2O-K2O with different MgO contents.
Figure 12. Micrographs of slowly cooled ash-fusion cones with different MgO contents.
Figure 14. Liqudius for the system SiO2-Al2O3-CaO-Fe2O3 with a CaO/Fe2O3 weight ratio of 1.15.
Figure 16 presents surface micrographs of quenched melt slag samples with S/A ratios of 1.6, 2.2, 3.2, and 4.0. It can be seen that there are clear differences in morphology as the S/A ratio is increased. The crystallized phase consists mainly of red and white particles as the S/A ratio is 1.6 (Figure 16a). The extent of the liquid phase region clearly decreases and the number of crystalline particles increases when the S/A ratio is 2.2 (Figure 16b). When the S/A ratios are further increased to 4.0, the crystallized phase consists of white particles (Figure 16d). According to the calculation results by FactSage, we can deduce that the white crystalline particles seen in Figure 16a were most probably composed of a mixture of leucite and anorthite, while the white crystalline particles seen in Figure 16b were most likely composed of a mixture
Figure 13. Effect of S/A ratio on ash fusion temperature and liquidus temperature.
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Figure 15. Phase assemblage-temperature curves for SiO2-Al2O3-CaO-Fe2O3-MgO-TiO2-Na2O-K2O with different S/A ratios.
Conclusion In our work, we have measured the AFTs of mixtures of selected coal ashes and SiO2, Al2O3, CaO, Fe2O3, and MgO and then have studied the effect of these oxides on the AFTs. The computer software package FactSage has been used to calculate the liquidus temperatures of coal ash samples and the proportions of the various phases present as a function of temperature. The AFTs of coal ash samples decrease with increasing CaO, Fe2O3, and MgO contents, then reach a minimum value, before increasing once more, thereby resulting in parabolatype curves. For the effect of S/A, the AFTs are always increased as the S/A ratios are increased. Liquidus temperatures calculated by FactSage and liquidus surfaces in phase equilibrium diagrams for the pseudoternary systems have been found to correlate well with the trends of AFTs for coal ash additive (SiO2, Al2O3, CaO, Fe2O3, and MgO) mixtures.
Figure 16. Micrographs of slowly cooled ash-fusion cones with different MgO contents.
of mullite and leucite. The white crystalline particles in Figure 16c,d are likely to consist of corundum containing leucite.
Acknowledgment. The authors acknowledge the financial support provided by National Basic Research Program of China (20576040).
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