Energy Fuels 2011, 25, 130–135 Published on Web 01/05/2011
: DOI:10.1021/ef1014346
Experiments and Modeling of Ash Mineral Evolution in Burning High-Sulfur Coal with Lime Wenlong Wang,*,† Zhongyang Luo,‡ Zhenglun Shi,‡ and Kefa Cen‡ †
National Engineering Laboratory for Coal-Fired Pollutants Emission Reduction, Shandong University, Jinan 250061, People’s Republic of China, and ‡State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China Received October 21, 2010. Revised Manuscript Received December 17, 2010
The mineral phase evolution of ash was studied when high-sulfur coal was burnt with CaO at different proportions. Experiments were carried out in a drop-tube furnace. Modeling via FACTSAGE was performed by obtaining the absent thermochemical property data of the key mineral 3CaO 3 3Al2O3 3 CaSO4 and expanding the database. It was found that the ash mineral phases follow a certain rule to evolve and have an immense potential for modification. At low CaO proportion, 2CaO 3 Al2O3 3 SiO2 might be the dominant mineral phase in ash. However, with the increase of CaO addition, the amount of 2CaO 3 Al2O3 3 SiO2 may decrease and the amount of 2CaO 3 SiO2 and 3CaO 3 3Al2O3 3 CaSO4 may increase noticeably. Owing to the excellent hydraulic characteristics of the latter two minerals, coal ash is very possible to become cement-like. The findings may open up an efficient way to burn the high-sulfur coal. The burning of high-sulfur coal with lime may result in an attractive sulfur fixation as well as a high-quality byproduct.
has been sparsely discussed despite the fact that the sulfation products of sorbents are widely concerned.7,8 When the addition of calcic reagents varies in a wide range, the mineral compositions of coal ash may evolve greatly because of the formation of some cement minerals, such as 2CaO 3 SiO2 or 3CaO 3 3Al2O3 3 CaSO4. In our previous work, an interesting idea to co-generate cement clinker in pulverized coal combustion (PCC) boilers was developed. It has been proven that, when coal is first ground together with lime and then injected into boilers for burning, the mineral phases of fly ash may be entirely changed and become cement-like.9,10 If some certain rules of evolution for coal ash can be found, the in situ desulfurization may become more attractive by the combination of ash modification. To understand how the mineral phases of coal ash evolve, this paper gives a thorough study via experiments and modeling. The experiments have been carried out mainly by burning a high-sulfur coal with different lime addition. The modeling depends upon the application of the software package FACTSAGE. The results may be consistently helpful whether studying the sulfur fixation in high-temperature combustion or discussing the mineral modification of coal ash.11-13
1. Introduction Because of the increasing demand for energy worldwide, more use of low-rank coal, such as high-sulfur type, is challenging the present desulfurization technologies.1,2 Some calcic reagents, e.g., lime or limestone, are often used to mix with high-sulfur coal during the combustion for in situ desulfurization.3 Careful studies have been carried out for the effects of sorbent type, sorbent ratio, temperature, particle sizes, and other factors and for the sulfur removal mechanisms by many researchers.4-6 However, the previous work has mostly focused on the sulfur removal efficiency and tried to improve the desulfurization technique in the furnace. The evolution of ash mineral phases *To whom correspondence should be addressed: Energy and Environment Institute, Energy and Power Engineering School, Shandong University, Jinan 250061, People’s Republic of China. Telephone: þ86-531-88399372. Fax: þ86-531-88395877. E-mail:
[email protected]. (1) Zhang, L.; Sato, A.; Ninomiya, Y.; Sasaoka, E. In situ desulfurization during combustion of high-sulfur coals added with sulfur capture sorbents. Fuel 2003, 82 (3), 255–266. (2) Li, S.; Xu, T. M.; Sun, P.; Zhou, Q. L.; Tan, H. Z.; Hui, S. E. NOx and SOx emissions of a high sulfur self-retention coal during air-staged combustion. Fuel 2008, 87 (6), 723–731. (3) Cheng, J.; Zhou, J. H.; Liu, J. Z.; Zhou, Z. J.; Huang, Z. Y.; Cao, X. Y.; Zhao, X.; Cen, K. F. Sulfur removal at high temperature during coal combustion in furnaces: A review. Prog. Energy Combust. Sci. 2003, 29 (5), 381–405. (4) Ye, Z. C.; Wang, W. Y.; Zhong, Q.; Bjerle, I. High temperature desulfurization using fine sorbent particles under boiler injection conditions. Fuel 1995, 74 (5), 743–750. (5) Zhou, J. H.; Cheng, J.; Cao, X. Y.; Liu, J. Z.; Zhao, X.; Huang, Z. Y.; Cen, K. F. Experimental research on two-stage desulfurization technology in traveling grate boilers. Energy 2001, 26 (8), 759–774. (6) Wang, W. Y.; Bjerle, I. Modeling of high-temperature desulfurization by Ca-based sorbents. Chem. Eng. Sci. 1998, 53 (11), 1973–1989. (7) Cheng, J. Mechanisms of two-stage desulfurization technology at high temperature during coal combustion in furnaces. Ph.D. Thesis, Zhejiang University, Hangzhou, China, 2002. (8) Yang, T. H.; Li, R. D.; Li, Y. J.; Zhou, J. H.; Cen, K. F. The desulfurization behavior of mineral matter in ash during coal combustion at high temperature. J. Fuel Chem. Technol. 2007, 35 (1), 23–26. r 2011 American Chemical Society
(9) Wang, W. L.; Luo, Z. Y.; Shi, Z. L.; Cen, K. F. A preliminary study on zero solid waste generation from pulverized coal combustion (PCC). Waste Manage. Res. 2003, 21 (3), 243–248. (10) Wang, W. L.; Luo, Z. Y.; Shi, Z. L.; Cen, K. F. Experimental study on cement clinker co-generation in pulverized coal combustion boilers of power plants. Waste Manage. Res. 2006, 24 (3), 207–214. (11) Liu, H.; Qiu, J. R.; Xiong, Q. J.; Kong, F. H.; Zhang, X. P.; Wang, Q. H.; Xiao, X. Q. Transportation and heterogeneous reactions of calcium containing minerals in coal combustion solid residues. Proc. CSEE 2005, 25 (11), 72–78. (12) Zhao, G. J.; Xie, J. L.; Lu, C. M.; Han, K. H. Study on formation process of sulphoaluminate calcium during high temperature desulfurization. J. Fuel Chem. Technol. 2006, 34 (6), 665–669. (13) Song, W. J.; Tang, L. H.; Zhu, X. D.; Wu, Y. Q.; Zhu, Z. B.; Koyama, S. Effect of coal ash composition on ash fusion temperatures. Energy Fuels 2010, 24 (1), 182–189.
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: DOI:10.1021/ef1014346
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Figure 2. Ash XRD diffractogram for Changguang coal alone. Figure 1. Sketch of the drop-tube furnace: 1, air compressor; 2, flowmeter; 3, charging bucket; 4, fluidized-bed material feeder; 5, combustor; 6, water-cooled jacket; 7, ash collector; 8, induced draft fan. Table 1. Industrial and Elemental Analysis Results of Changguang Coal (%) parameter Mad Aad
Qnet,ad Vad FCad Cad Had Nad Sad Oad (kJ/kg)
percentage 1.63 34.94 26.31 37.12 52.60 3.92 1.0 3.69 2.22 21, 431
Table 2. Ash Composition of Changguang Coal at 1600 K (%) componds
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
total
contents
53.18
32.46
4.58
3.16
0.72
0.25
95.1
2. Experimental Section Figure 3. Ash XRD diffractogram for Changguang coal with 20% CaO.
2.1. Materials and Methods. Because PCC is the typical style of coal combustion, the experiments were carried out in a 2 m high drop-tube furnace (see Figure 1), which was heated by electricity. The performance temperature was set to 1600 K, and the residence time of material particles in the furnace was about 7 s. The fine coal particles were fed into the combustor by the fluidized-bed material feeder (4), and the final ash was collected by the collector (7) after rapid cooling in water-cooled jacket (6). A high-sulfur coal, Changguang coal, obtained from Zhejiang, China, was adopted. Table 1 gives the results of industrial and elemental analyses for the coal. Table 2 is the chemical analysis results of the coal ash, which was obtained at 1600 K. The adopted lime additive was chemically pure. Five batches of experiments were performed. In the first batch, the pulverized Changguang coal was burnt alone. In the next three batches, the coal was first mixed with lime and the mass ratios of lime/coal equaled 20, 30, and 40%, respectively. Then, the mixed materials were ground in a ball mill for 3 h before burning. In the fifth batch, the Changguang coal and lime were ground separately and then blended thoroughly in the proportion of 100:40, which was the same as in the fourth batch. The sizes of the particles fed into the furnace were all controlled less than 90 μm by screening. Thereupon, five ash samples were generated in the furnace. Then, their chemical compositions were measured by titration following the Chinese national standard method. X-ray diffraction (XRD) analyses were applied to determine the final mineral phases. A Rigaku B/Max diffractometer was operated at 40 kV and 40 mA, and the XRD patterns were recorded from 10° to 70° of 2θ and at a rate of 0.06° per step. The scanning electron microscopy (SEM) analysis was also applied to observe the effect of the co-grinding step.
Figure 4. Ash XRD diffractogram for Changguang coal with 30% CaO.
2.2. Results and Discussion. Figures 2-6 show the diffractograms of the five ash samples. With a rough contrast, it can be found that the XRD peaks and mineral patterns differ entirely in the five figures. When the Changguang coal was burnt alone, as shown in Figure 2, the dominant mineral was mullite (3Al2O3 3 2SiO2) in the ash; the peaks of quartz (SiO2) and gehlenite (2CaO 3 Al2O3 3 SiO2) were also visible. Because there was some inherent CaO in the ash, some 2CaO 3 Al2O3 3 SiO2 was generated and became the only calcic product. Because SiO2 and Al2O3 were completely 131
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Thermodynamic analysis has shown that, in the CaO-Al2O3-SiO2 ternary system, reaction 2 to form 2CaO 3 Al2O3 3 SiO2 has the minimum Gibbs free energy and, accordingly, is the easiest reaction to proceed.14 Therefore, it can be understood why the peaks of 2CaO 3 Al2O3 3 SiO2 become the strongest in Figure 3. From the changes of Figures 2 and 3, it is reasonable to say that 2CaO 3 Al2O3 3 SiO2 is the most easily formed mineral in coal ash at low CaO proportions. Then, the percentage of CaO addition was increased to 30% in Figure 4. There was a decline in the peak intensity of 2CaO 3 Al2O3 3 SiO2. SiO2 could not be detected any longer, but the peaks of 2CaO 3 SiO2 and 3CaO 3 3Al2O3 3 CaSO4 emerged clearly. It meant that the CaO addition had already consumed all SiO2 and its increase contributed to further reactions, such as reactions 3 and 4. In the fourth batch, even more CaO, 40%, was added to the coal. Figure 5 showed that the peaks of 2CaO 3 SiO2 and 3CaO 3 3Al2O3 3 CaSO4 became the strongest in the diffractogram and there was a further decline for those of 2CaO 3 Al2O3 3 SiO2. On one hand, it meant that reactions 3 and 4 might have been enhanced in this case; on the other hand, 2CaO 3 Al2O3 3 SiO2 was very likely to undergo reaction 5 because of the sufficient CaO addition.15,16 The emergence of peaks for CaO indicated that its addition had already exceeded the demand of relevant reactions. However, the diffraction peaks of 2CaO 3 Al2O3 3 SiO2 and CaSO4 have not disappeared entirely, which might be explained by the incompleteness of solid-phase reactions in the short residence time. Therefore, the above phase evolution shows that the different CaO additions have great effects on the mineral compositions of coal ash. Although the mineral phases of ash may vary to some extent when different types of coal or different proportions of CaO addition are applied, the general tendency will not change. Without CaO, mullite and quartz would be the main crystals in the common fly ash. At low CaO proportions, 2CaO 3 Al2O3 3 SiO2 would be the most easily formed mineral phase, while the amount of 2CaO 3 SiO2 and 3CaO 3 3Al2O3 3 CaSO4 will increase noticeably with high CaO addition. 2CaO 3 Al2O3 3 SiO2 has no hydraulicity and exerts little mechanical strength during hydration, but 3CaO 3 3Al2O3 3 CaSO4 and 2CaO 3 SiO2 are both important cement minerals. 3CaO 3 3Al2O3 3 CaSO4 is a rapid-hardening mineral and shows excellent early mechanical strength; 2CaO 3 SiO2 hydrates slowly, but its mechanical strength can exert continuously on a long-term basis. Thus, the mineral evolution results give us strong indications to improve the mineral compositions and characteristics of coal ash. Although the same amount of CaO was added in the fifth batch as in the fourth one, a quite different diffractogram (Figure 6) was obtained. Only because co-grinding was not adopted, the solid-phase reactions became very incomplete and much CaO was left without reacting with SiO2 or Al2O3. The peaks of CaO were also visible in Figure 5 but very weak. Therefore, the measure of co-grinding has an important effect on the eventual ash mineral phases. Figure 7 showed the SEM micrographs of the coal after co-grinding with 40% lime. The left one was magnified 5000 times, and the right one was magnified 10 000 times. The energy spectrum analyses indicated that the brighter points were lime and the weaker points were coal. Clearly, the particles of coal and lime adhered together in most cases. This feature might have given many chances for the lime to react with the mineral substances in the coal. Therefore, to realize the above mineral evolution, appropriate measures should be taken to ensure that added CaO reacts with the ash contents in coal.
Figure 5. Ash XRD diffractogram for Changguang coal with 40% CaO.
Figure 6. Ash XRD diffractogram for Changguang coal with 40% CaO by separate grinding.
excessive compared to CaO, reaction 1 to form Al2O3 3 2SiO2 was given the chance to proceed. Then, the surplus SiO2 took the form of quartz. It can be deduced that Al2O3 3 2SiO2 and quartz would be the main crystals if there were little inherent CaO in the ash. Actually, the mineral structure in Figure 2 is in accordance with the theory of formation of the common fly ash.14 3Al2 O3 þ 2SiO2 f 3Al2 O3 3 2SiO2
ð1Þ
Al2 O3 þ SiO2 þ 2CaO f 2CaO 3 Al2 O3 3 SiO2
ð2Þ
SiO2 þ 2CaO f 2CaO 3 SiO2
ð3Þ
3CaO þ 3Al2 O3 þ CaSO4 f 3CaO 3 3Al2 O3 3 CaSO4
ð4Þ
3CaO þ 3ð2CaO 3 Al2 O3 3 SiO2 Þ þ CaSO4 f 3CaO 3 3Al2 O3 3 CaSO4 þ 3ð2CaO 3 SiO2 Þ
ð5Þ
When 20% CaO was added to the coal, the dominant mineral phase of ash became 2CaO 3 Al2O3 3 SiO2 in Figure 3. The peaks of Al2O3 3 2SiO2 disappeared entirely. However, SiO2 still existed possibly because no more CaO could participate in the further reactions. The distinguishable peaks of CaSO4 indicate that the sulfur fixation reaction could not be neglected in this case.
(15) Sahu, S.; Majling, J. Phase compatibility in the system CaO-SiO2-Al2O3-Fe2O3-SO3 referred to sulphoaluminate belite cement clinker. Cem. Concr. Res. 1993, 23 (6), 1331–1339. (16) Wang, Y. M.; Su, M. Z.; Zhang, L. Sulfoaluminate Cement; Beijing University of Technology Press: Beijing, China, 1999.
(14) Wang, W. L.; Luo, Z. Y.; Shi, Z. L.; Cen, K. F. Thermodynamic study on formation mechanism of coal ash minerals at high calcium content in PCC boilers. Proc. CSEE 2005, 25 (18), 116–120.
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Figure 7. SEM micrographs of the coal after co-grinding with lime. Table 3. Chemical Compositions of Coal Ash with Different CaO Proportions (%) proportion of CaO addition (%)
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
CaSO4 converted
CaO excluded that in CaSO4
0 20 30 40
53.18 29.91 22.51 16.55
32.46 18.26 13.74 10.10
4.58 2.58 1.94 1.42
3.16 41.95 53.18 60.24
0.72 0.40 0.30 0.22
0.25 1.91 3.32 6.46
0.43 3.25 5.65 10.98
2.99 40.61 50.85 55.72
absent data could be obtained theoretically according to the previous studies.20-24 Our working group has performed the job, and the details have been introduced in other papers.25,26 The standard formation enthalpy ΔHf298 K = -8393.19 kJ/ mol; the Gibbs free energy of formation ΔGf298 K = -7929.54 kJ/mol; the standard entropy S298 K = 450.2 J K-1 mol-1; the molar heat capacity Cp = a þ b 10-3T þ c 105T -2, where a = 554.05, b = 143.34, and c = -113.4. With these data, the database of FACTSAGE can be expanded. Then, it becomes possible to conduct the calculation for the reactions in which calcium sulfoaluminate is involved. Table 3 gives the final chemical compositions of the ash samples when Changguang coal was burnt with different CaO addition. Because CaO, SiO2, Al2O3, and SO3 are the main compositions, the calculation can be simplified into a CaOSiO2-Al2O3-SO3 quarternary system by neglecting the other trace compositions. Then, whereas CaSO4 is the common form in the initial sulfur fixation, the calculation and discussion can finally be carried out in the CaO-SiO2-Al2O3CaSO4 system. In Table 3, the percentages of CaSO4 were obtained from the contents of SO3, and the new CaO contents were also calculated by excluding those combined by SO3. Clearly, it can be noticed that more sulfur has been fixed in the ash with the increase of CaO addition. In the calculation, CaO, SiO2, Al2O3 and CaSO4 are set as the initial reactants. Their mass proportions are set according
Conclusively, when high-sulfur coal is burnt with enough lime addition and if the coal ash minerals have sufficient reaction chances with CaO, the mineral phases of fly ash will have a great modification potential. The final ash may be mainly made up of hydraulic cement minerals, such as 2CaO 3 SiO2 and 3CaO 3 3Al2O3 3 CaSO4. In this way, the fly ash of high-sulfur coal is quite possible to become cement-like and might be directly applied in construction engineering as cement.17 In addition, because 3CaO 3 3Al2O3 3 CaSO4 is more stable than CaSO4 at high temperatures, better sulfur fixation may be achieved by sufficiently using the effects of lime.
3. Modeling via FACTSAGE To verify the experimental results, FACTSAGE, a typical thermodynamic software package, was employed to calculate the possible mineral phases when different proportions of lime are added to the Changguang coal. The core module of FACTSAGE, Equilib, is set up by minimizing the Gibbs free energy, which can work out the type and proportion of each substance under chemical equilibrium conditions.18,19 The calculation in this paper is based on FACTSAGE 5.2. Although FACTSAGE is a very strong calculation tool, its barrier to study the ash system of high sulfur coal is the absence of thermodynamic data for some key minerals, such as 3CaO 3 3Al2O3 3 CaSO4. No experimentally measured data have been reported thus far for this mineral. However, these (17) Sharp, J. H.; Lawrence, C. D.; Yang, R. Calcium sulfoaluminate cement;Low-energy cement, special cement or what? Adv. Cem. Res. 1999, 11 (1), 3–14. (18) Cao, Z. M.; Song, X. Y.; Qiao, Z. Y. Thermodynamic modeling software FactSage and its application. Chin. J. Rare Met. 2008, 32 (2), 216–219. (19) Kondratiev, A.; Jak, E. Predicting coal ash slag flow characteristics (viscosity model for the Al2O3-CaO-“FeO”-SiO2 system). Fuel 2001, 80 (14), 1989–2000. (20) Wen, Y. K.; Shao, J.; Wang, S. S.; Chen, D. W. A simplified formula to calculate heat of formation of oxyacid salt and mineral. Acta Metall. Sin. 1979, 15, 98–108. (21) Wen, Y. K.; Shao, J.; Chen, D. W. Calculation of standard free energies of formation of oxyacid salt minerals. Sci. Geol. Sin. 1978, 4, 348–357.
(22) Li, X. B.; Li, Y. F.; Liu, X. M.; Liu, G. H.; Peng, Z. H.; Zhai, Y. C. A simple method of estimation of Gibbs free energy and enthalpy of complicate silicates. J. Chin. Ceram. Soc. 2001, 29, 232–237. (23) Kubaschewski, O.; Unal, H. An empirical estimation of the heat capacities of inorganic compounds. High Temp.-High Pressures 1977, 9, 361–365. (24) Spencer, P. J. Estimation of thermodynamic data for metallurgical applications. Thermochim. Acta 1998, 314, 1–21. (25) Wang, W. L.; Wang, P.; Ma, C. Y.; Luo, Z.Y. Calculation for mineral phases in the calcination of desulfurization residue to produce sulfoaluminate cement. Ind. Eng. Chem. Res. 2010, 49, 9504–9510. (26) Wang, W. L.; Chen, X. D.; Chen, Y.; Ma, C. Y. Calculation and verification for the thermodynamic data of 3CaO 3 3Al2O3 3 CaSO4. Chin. J. Chem. Eng., manuscript submitted for publication.
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Table 4. Mineral Phases of Changguang Coal Ash Calculated by FACTSAGE mineral compositions at different CaO additions (%) item
initial reactants
probable resultants
mineral
0% CaO
20% CaO
30% CaO
40% CaO
CaO SiO2 Al2O3 CaSO4 CaO 3 SiO2 β-2CaO 3 SiO2 3CaO 3 2SiO2 3CaO 3 SiO2 CaO 3 Al2O3 3CaO 3 Al2O3 12CaO 3 7Al2O3 CaO 3 2Al2O3 2CaO 3 Al2O3 3 SiO2 CaO 3 Al2O3 3 2SiO2 3Al2O3 3 2SiO2 3CaO 3 3Al2O3 3 CaSO4 CaSO4 f-CaO SiO2
2.99 53.18 32.46 0.43
40.61 29.91 18.26 3.25 26.04
50.85 22.51 13.74 5.65
55.72 16.55 10.10 10.98
64.53 3.08
47.44
25.10 0.05
20.15 6.48 19.27
13.64
14.83
49.11
37.64 0.43
3.25
36.16
structure via FACTSAGE. However, the existence of about 19% free CaO reflects that the lime addition has already exceeded its practical demand. Because of the excessive CaO addition, the sulfur fixation is also enhanced and more CaSO4 is generated in the ash. The surplus CaO and CaSO4 in resultants indicate that the reaction to form 3CaO 3 3Al2O3 3 CaSO4 could not proceed any more. 2CaO 3 Al2O3 3 SiO2 that ought to have but did not disappear in Figure 5 can also be explained in the same way as in the previous case. Therefore, the modeling via FACTSAGE is in accordance with the XRD analyses in the experiments basically. The emergence of dominant minerals in the diffractograms can be very well-explained with the modeling results. The deviation for accidental minerals can be mostly attributed to the incomplete solid-phase reactions. The modeling does not indicate the decomposition of CaSO4, which is treated as a pure substance in the calculation and does not decompose at 1600 K. Actually, if CaSO4 is not pure or reactions happen in reductive atmospheres, its decomposition may be noticeable. However, both the experiments and the modeling tell us that CaSO4 is more likely to form 3CaO 3 3Al2O3 3 CaSO4 than to decompose even at the high temperature of 1600 K when sufficient CaO is added. Because 3CaO 3 3Al2O3 3 CaSO4 is a stable mineral below 1673 K,16 the above results are quite meaningful to study either the sulfur fixation in high-temperature combustion or the mineral modification of coal ash.
to the data in Table 3. CaSO4, CaO, SiO2, 3CaO 3 3Al2O3 3 CaSO4, and all other possibly existing minerals in the CaO-SiO2-Al2O3 ternary diagram are treated as the likely resultants of the solid-phase reactions. The calcination temperature is set to 1600 K. Table 4 lists all of the possible resultants that are taken into account and gives their calculation results of mass percentages in the final products. It can be found that, when Changguang coal is burnt alone, the main mineral phases in the ash are 3Al2O3 3 2SiO2 and SiO2, 37.64 and 36.16%, respectively. In addition, about 14.83% 2CaO 3 Al2O3 3 SiO2 is also generated. This mineral structure is entirely in accordance with the previous experimental results. The fact that the peaks of quartz were weak in Figure 2 might be explained because some SiO2 had taken the form of glass. When 20% CaO is added to the coal, gehlenite (2CaO 3 Al2O3 3 SiO2) accounts for about 50% in the mineral phases. The rest phases are mainly CaO 3 SiO2 and 3CaO 3 2SiO2 via calculation, but they were not detected in the XRD diffractogram (Figure 3). In fact, CaO 3 SiO2 and 3CaO 3 2SiO2 are not thermodynamically easy to be generated, except that solidphase reactions could proceed completely. Therefore, the fact that the peaks of SiO2 could be seen in Figure 3 was decided by the short residence time and incomplete solid-phase reactions in the experiment. However, the calculation via FACTSAGE reflects the results in the complete equilibrium state. Because of the insufficient CaO, CaSO4 remains unchanged without forming the new mineral 3CaO 3 3Al2O3 3 CaSO4, in accordance with the XRD analysis. At the CaO addition of 30%, the amount of 2CaO 3 SiO2 and 3CaO 3 3Al2O3 3 CaSO4 takes up about 90% of the final phases. However, FACTSAGE shows no 2CaO 3 Al2O3 3 SiO2 in this case, which is the biggest difference between the modeling and the XRD analysis. 2CaO 3 Al2O3 3 SiO2 that ought to have but did not disappear in Figure 4 can also be attributed to the incomplete solid-phase reactions. In the experiment, reaction 5 ought to have proceeded completely but the short residence time had not allowed. In the calculation, most CaSO4 in reactants has already entered 3CaO 3 3Al2O3 3 CaSO4 but the existence of trace 3CaO 3 2SiO2 and CaSO4 indicates that CaO is still insufficient in this case. At the CaO addition of 40%, 2CaO 3 SiO2 and 3CaO 3 3Al2O3 3 CaSO4 are also the most important phases in the final mineral
4. Conclusions Both the experimental results and the modeling via FACTSAGE indicate that the mineral phase evolution follows a certain rule when high-sulfur coal is burnt with CaO. If sufficient solid-phase reactions can be ensured by taking measures, such as co-grinding the coal and CaO, the ash mineral formation may be described within the SiO2-Al2O3CaO-CaSO4 system. At low CaO proportions, 2CaO 3 Al2O3 3 SiO2 will be the most easily formed mineral phase. However, with the increase of CaO addition, 2CaO 3 Al2O3 3 SiO2 may decrease and the amount of 2CaO 3 SiO2 and 3CaO 3 3Al2O3 3 CaSO4 may increase noticeably. 2CaO 3 Al2O3 3 SiO2 has no hydraulicity and exerts little mechanical strength during hydration, but 3CaO 3 3Al2O3 3 CaSO4 and 2CaO 3 SiO2 are important cement minerals. When 134
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hydraulic 3CaO 3 3Al2O3 3 CaSO4 and 2CaO 3 SiO2 become the dominant minerals, the coal ash will be made cement-like. Therefore, the mineral evolution rule may open up an efficient way to burn the high-sulfur coal. The burning of high-sulfur coal with lime may result in an attractive sulfur fixation as well as a high-quality byproduct.
Acknowledgment. The authors thank the support of the National Natural Science Foundation of China (Grant 50906046), National High-Tech Research and Development Program of China (863 Program) (Grant 2009AA05Z303), Program for New Century Excellent Talents in University (NCET-10-0529), and Visiting Scholar Foundation of Key Lab in University.
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