J. Cent. South Univ. Technol. (2008) 15: 840-844

DOI: 10.1007/s11771-008-0155-z

Slagging characteristics of molten coal ash on

silicon-aluminum combustion liners of boiler

HE Jin-qiao(何金桥)^{1, 2}, SHI Zhang-ming(时章明)¹, CHEN Dong-lin(陈冬林)^{1, 2},

JIANG Xian-liang(蒋显亮)³, YAN Xiao-zhong(鄢晓忠)²

 (1. School of Energy Science and Engineering, Central South University, Changsha 410083, China;

2. Institute of Energy Source and Power Engineering, Changsha University of Science and Technology,

Changsha 410076, China;

3. School of Materials Science and Engineering, Central South University, Changsha 410083, China)

Abstract: In order to study the slagging characteristics of boiler combustion liners during pulverized coal stream combustion, the slag samples on the surface of combustion liner were investigated by X-ray diffractometry, scan electron microscopy and energy dispersive X-ray analysis, and the transformation characteristics of the compositions and crystal phases were studied. The results show that the size of slag granules decreases as the slagging temperature increases; the crystallinity of coal ashⅠreduces to about 48.6% when the temperature is increased up to 1 350 ℃, and that of the coal ash Ⅱ reduces to about 65% when the temperature is increased up to 1 500 ℃; the encroachment of molten coal ash to the combustion liner is strengthened. At the same time, the diffusion and the segregation of the compositions in combustion liners have selectivity, which is in favor of enhancing the content of crystal phases, weakening the conglutination among molten slag compositions and combustion liner, and avoiding yielding big clinkers. But the diffusion of the compositions in combustion liners increases the porosity and decreases the mechanical intensity of combustion liner, and makes the slag encroachment to the liner become more serious.

Key words: pulverous coal ash; slagging characteristics; crystallinity; combustion liner

1 Introduction

When pulverized coal stream combusts in boiler, the temperature will continuously increase and may reach above 1 600 ℃ in flame center. Thus, some minerals of coal ash will transform into vapor state and molten liquid state^[1-4], and the compositions of coal ash will change as well. The non-uniformity of pulverized coal particles and that of the mixing between air for combusting and pulverized coal make the change of ash composition become more complex^[5]. When the pulverized coal stream burns away itself, the flue gas will decrease, and the vapor and molten ash will agglomerate again. Once the fluid dynamic conditions, especially the mixing between air for combusting and pulverized coal, are illogical, it is easy for ash to slag on boiler’s combustion liner and heating surface^[1,6]. In China, the quality of coal for power is worse, and there are much ash and sulfide in it, so the maldistribution for temperature and velocity of flue gas become more severe for large-scale capacity boiler^[1,7], and the slagging becomes more severe. So it is important to avoid the slagging between molten coal ash and combustion liner in order to assure the safe running for combustion and adequate efficiency for the thermodynamic system of power plant^[1,8].

The flue gas temperature in firebox is very high, and coal ash in flue gas and combustion liner both contain oxide aluminum^[9]. So the slagging between molten coal ash and combustion liner is a complicated multiphase agglomeration process and inclines to make the free energy for combining decrease to the minimum^[5,10-12], which will make molten coal ash transformed into not only crystals with coagulating core but also vitrescent substance. Crystals cannot be transformed into any other matter even at very high temperature^[13], and the combination between different crystals is poor. But the vitrescent substance is contrary. It is easy to form vitreous clinker in molten coal ash and boiler’s combustion liner at high temperature^[14]. So in this work the characteristics of micro-phase transformation of molten coal ash were studied to prevent coal ash slagging and improve resistance to slag for liner.

2 Experimental procedure

There were two ash samples in this investigation.

Their granularities were both 0.074 mm, and their compositions of minerals were measured by chemical method. The compositions of ash samples are given in Table 1. The slagging experiments were performed in a high temperature silicon-molybdenum rods muffle furnace in the mild oxidation condition. The ash samples were placed on the surface of combustion liner made of fire-resistant materials, the diameter of the sample was 30 mm, and the thickness was 2-3 mm. Then the ash and the liner were put into the furnace to be heated.

Table 1 Ash compositions of typical coal for power plant (mass fraction, %)

The samples were heated at a certain temperature for certain time, then cooled, and the micro-phases of slagging ash samples were analyzed by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA), which were completed in State Key Laboratory of Powder Metallurgy, Central South University, China. The combustion liners were classified into types B and C. Their main components are listed in Table 2, which were made according to the same compositions of boiler’s combustion liners in power plant.

Table 2 Compositions of combustion liners (mass fraction, %)

3 Results and discussion

3.1 Compositions and crystal transformation of molten coal ash on combustion liners

The reactions of slagging at high temperature differ from single composition action for ash, especially in molten state. Intricate chemical change will take place among compositions of coal ash and combustion liner, and new substances will be produced. The free energy among compositions of ash and fire-resistant materials during the slagging process will tend spontaneously to the minimum in order to reach a new stable state. Alkali metal compounds of Na and K in ash are very easy to gasify and pyrolyze under the thermodynamic effect, so they have few erosion to the combustion liners while combusting the pulverized coal stream in boiler.

Fig.1 shows the XRD patterns for the coal ashⅠ heated to different temperatures to slag on liner B. In Fig.1(a) the coal ash is heated up to 900 ℃ for 5 h in muffle furnace, the conditions of Fig.1(b) are 1 250 ℃ in muffle furnace and 20 h, and those of Fig.1(c) are 1 350 ℃ and the same heated time as that of Fig.1(b).

Fig.1 XRD patterns of coal ash Ⅰ at different slagging temperatures: (a) 900 ℃ for 5 h; (b) 1 250 ℃ for 20 h; (c) 1 350 ℃ for 20 h; 1—QuartzⅠ; 2—HematiteⅠ; 3—MulliteⅠ; 4— Rutile; 5—Anatase; 6—MulliteⅡ; 7—HematiteⅡ; 8— QuartzⅡ; 9—Mullite Ⅲ; 10—Hematite (eskolaite) Ⅲ;  11— Quartz Ⅲ

The results in Fig.1 imply that the dimensions of mullite crystal are 7.588 0 μm×7.688 0 μm×2.889 5 μm, and the density is 3.00 g/cm³ at 900 ℃. When the temperature increases, at the same location the diffraction peak becomes sharper and intenser. The structure of mullite crystal is changed at 1 250 ℃, and its density is decreased to 2.87 g/cm³ without bulk change. The transformation of quartz crystal diversifies its structure. In Fig.1(a) the diffraction peaks of quartz are very sharp and their diffraction intensity is strong, which suggests that the crystal property of quartz is obvious, and the form of the quartz is steady tridymite with dimensions of 4.913 4 μm×5.405 2 μm. But in Fig.1(b) the intensity of relevant diffraction peaks at the same place becomes weaker on the same scale. When the temperature increases to 1 350 ℃, the steady tridymite is transformed into metacristobalite.

The tridymite crystal is a tabular shape and consists of crystallites with size of 1-2 μm by reuniting. Tridymite will strengthen reuniter with increasing temperature, which makes random tropism intensify and the X-ray to the tridymite dispel, so the diffraction peaks for quartz in Fig.1(b) become complanate. And their location departs compared with that in Fig.1(a). Coal slag is a mixture with multi-phases, its phase change can be reflected by the diffraction peaks of XRD, and the mass fractions (w_j) of phases in coal slag have direct relation with the cumulation intensity of certain diffraction line in the XRD pattern^[15]. Fig.2 was obtained from the diffraction in Fig.1, which reflects the relative contrast of the main phases in coal ashⅠat different slagging temperatures.

Fig.2 shows that the contents of SiO₂ and mullite decrease. However, hematite content increases when the temperature rises. This suggests that the eutectic and the diffusion between molten ash and combustion liner become stronger with increasing temperature.

Fig.2 Phase mass fraction of coal ash Ⅰ at different slagging temperature

Cr₂O₃ in combustion liner B is rich, and its diffusion is evident. However, it can yield eutectic with hematite. Because Cr₂O₃, hematite and their eutectic have the same diffraction peaks in XRD pattern, the content of hematite in Fig.2 is the total of the three phases above. But when the temperature rises the granule of mullite and quartz trails off continuously, the vitreous phase of their eutectics cannot keep the crystal characteristics, so their crystal phases decrease.

Figs.3 and 4 show the XRD patterns and the contents of main phases in the slag samples of coal ash Ⅱ which were heated up to different temperatures to slag upon liner C. Fig.3(a) corresponds to the coal ash heated to 900 ℃ for 5 h in muffle furnace. The conditions of Fig.3(b) are 1 350 ℃ for 40 h in muffle furnace, and those of Fig.3(c) are 1 500 ℃ for 40 h.

Fig.3 XRD patterns of coal ash Ⅱ at different slagging temperatures: (a) 900 ℃ for 5 h; (b) 1 350 ℃ for 40 h; (c) 1 500 ℃ for 40 h; 1—QuartzⅠ; 2—Anatase; 3—MulliteⅠ; 4—Alumina; 5—QuartzⅡ; 6—MulliteⅡ; 7—Mullite Ⅲ; 8—Corundum; 9—Titanite

The diffraction peak located at 2θ=26.5? in Fig.3(a) is the sharpest among these patterns in Fig.3, and its intensity outclasses that of the others, which suggests that coal ash Ⅱ remains few compositions. The main phase in coal ash Ⅱ is mullite by contrasting the spectrum with the powder diffraction card flock (PDF), and its mass fraction is about 81%, and the alumina in coal ash Ⅱ is entirely transformed into mullite (see Fig.4). After coal ash Ⅱ slags at 1 350 ℃ for 40 h, the sharpest peak disappears, and the content of Al₂O₃ increases to 43.3%. And in fact the content of Al₂O₃ in coal ash is 28.3% (Table 1) even if it is not transformed into mullite.

Fig.4 Phase mass fraction of coal ash Ⅱ at different slagging temperature

It can be explained that Al₂O₃ in the combustion liner penetrates into molten ash sample by diffusion. When the temperature increases to 1 500 ℃, corundum in liner C penetrates into molten ash, and quartz crystal disappears entirely because of granule lessening and eutectic process. This shows that the slag eutectic characteristics of coal ash Ⅱ will also intensify with increasing temperature, which makes ash and combustion liner melt down together and engender serious slagging.

3.2 Crystal characteristic

Figs.5(a) and (b) show the SEM images of coal ashⅠ at slagging temperatures of 1 250 and 1 350 ℃ respectively. Figs.5(c) and (d) show the SEM images of coal ash Ⅱ at slagging temperatures of 1 350 and 1 500 ℃ respectively.

Fig.5 SEM images of different slag samples: (a) Coal ash Ⅰ, 1 250 ℃; (b) Coal ash Ⅰ, 1 350 ℃; (c) Coal ash Ⅱ, 1 350 ℃; (d) Coal ash Ⅱ, 1 500 ℃

Fig.5 shows that coal ash Ⅰ is prone to slag, and there are a lot of the solidified vitreous materials among the enwrapped solid granules with seed crystal. When the slagging temperature increases to 1 350 ℃, the vitreous matters become well-proportioned, the crystal phases disappear, and the molten vitreous matters with high temperature obviously deposit to combustion liner according to Fig.5(b). The crystal compositions obviously take on secondary assemble of Cr₂O₃ and Fe₂O₃ found by EDXA at the top right corner of Fig.5(a), which illuminates that Cr₂O₃ in combustion liner pervades in slag. Therefore, crystal phases for coal ashⅠ increase and the slagging intensity reduces. On the contrary, coal ash Ⅱ is prone to crystallize. This ash is abundant in SiO₂, and easy to be transformed into vitreous matter, but the main composition of liner C is corundum, which can endure high temperature. When the slagging temperature increases to 1 500 ℃, the crystal granules can be found. In Fig.5(d) the crystal granules become smaller than those in Fig.5(c), but their distribution becomes more well-proportioned, and viscid vitreous matter is not found. The slag in Fig.5(d) can be scratched easily too. This shows that the content of crystal compositions in slags directly decides the slagging intensity between molten ash and combustion liner.

When coal ash slags at certain temperature, the compositions cannot be entirely transformed into crystal phases. Thus there must exist non-crystal phase, especially vitreous phase. The size of non-crystal granula is smaller than that of crystal one, and the non-crystal phase easily bring lattice defects such as dislocation and so on^[15]. In XRD patterns these defects make the X-ray disperse and lack of certain tropism, so the diffraction peaks disappear, or only remain wave peak with weak diffraction intensity and wide full wave at half maximum (FWHM), which will form non-crystal peak. The crystallinity (C) for ash can be computed by

                              (1)

where  C is the crystallinity; I_c is the intensity of all crystal phases diffraction peaks; I_cs is the intensity of all crystals and non-crystals diffraction peaks.

The slag crystallinity of coal ashⅠdecreases with increasing temperature based on Fig.6. When the slagging temperature increases to 1 350 ℃, the crystalli- nity reduces to about 48.6%. This means that the more the vitreous phase, the high the slagging possibility between the molten ash and the combustion liner. But the crystallinity of coal ash Ⅱ has the maximum when slagging at about 1 350 ℃. The reason is that Al₂O₃ from liner C enhances the relative content of crystal phases. However, the crystallinity will decrease when the temperature increases because the crystal granule becomes smaller. And the crystallinity reduces to about 65% when the temperature increases up to 1 500 ℃. On the one hand this strengthens the molten coal ash to erode the combustion liner, on the other hand the compositions separated from the combustion liner add the porosity and change the distribution of porous structure in combustion liner, which makes the erosion to the combustion liner become more serious.

Fig.6 Crystallinity of slagging at different temperatures

4 Conclusions

1) Molten coal ash on the surface of fire-resistant materials will yield crystal slag under thermodynamic effect. This is favorable to weakening the conglutination among molten slag compositions and combustion liner. But it decreases the mechanical intensity of fire-resistant board.

2) In molten state the segregation between Fe₂O₃ and Cr₂O₃ or between SiO₂ and Al₂O₃ has selectivity, which can enhance the content of crystal phases. This is favorable to weakening the molten ash slagging to combustion liner.

3) The crystallinity of coal ash slag on the surface of combustion liner will weaken with increasing temperature in furnace. This will strengthen the resistance of molten ash to combustion liner.

References

[1] CEN Ke-fa, FAN Jian-ren, CHI Zuo-he, SHEN Luo-chan. Avoiding principle and computation of accumulating ash, slagging, abrasion and corroding for boiler and heat exchanger [M]. Beijing: Science Press, 1994. (in Chinese)

[2] LARRY L, BAXTER, RICHARD W, DESOLLAR. A mechanistic description of ash deposition during pulverized coal combustion: Predictions compared with observations [J]. Fuel, 1993, 72(10): 1411-1418.

[3] RUSHDI A, SHARMA A, GUPTA R. An experimental study of the effect of coal blending on ash deposition [J]. Fuel, 2004, 83(4/5): 495-506.

[4] HUFFMAN G P, HUGGIN F E, DUNMYRE G R. Investigation of the high-temperature behavior of coal ash in reducing and oxidizing atmospheres [J]. Fuel, 1991, 60(7): 585-597.

[5] XU Zu-yao. Principles of phase transformations [M]. Beijing: Science Press, 2000. (in Chinese)

[6] ERICKSON T A, ALLAN S E, MCCOLLOR D P, HURLEY J P, SKINIVASACHAR S, KANG S G, BAKER J E, MORGAN M E, JOHNSONN S A, BORIO R. Modeling of fouling and slagging in coal fired utility boiler [J]. Fuel Processing Technology, 1995, 44(1/3): 155-171.

[7] CHEN Yin-ying, SHI Hui-fang, YAN Wei-ping. Furnace slagging research on the boiler No.3 of Dalate Power Plant [J]. Power Engineering, 2003, 23(5): 2635-2637. (in Chinese)

[8] BSTEENARI B M, LINDQVIST O, LANGER V. Ash sintering and deposit formation in PFBC [J]. Fuel, 1998, 77(5): 407-417.

[9] JON W, GERRY R, JIM W. Interactions between coal-ash and burner quarls (1): Characteristics of burner refractories and deposits taken from utility boilers [J]. Fuel, 2003, 82(10): 1859-1865.

[10] MISCHA T, BENGT J S, MARIA Z. Fouling tendency of ash resulting from burning mixtures of bio-fuels (2): Deposit chemistry [J]. Fuel, 2006, 85(12): 1992-2001.

[11] BENGT J S, RAINER B, MIKKO H. Characterization of the sintering tendency of ten biomass ashes in FBC conditions by a laboratory test and by phase equilibrium calculations [J]. Fuel Processing Technology, 1998, 56(1): 55-67.

[12] DAI Ling-jiang, CAI Shao-hong. Principle of generalize phase transition and its critical theory [J]. Journal of Guizhou University: Natural Science, 1999, 16(3): 182-192.

[13] XIAO Jin, DENG Hua, WAN Ye, LI Jie, LIU Ye-xiang. Preparation of ultrafine α-Al₂O₃ powders by catalytic sintering of ammonium aluminum carbonate hydroxide at low temperature [J]. Journal of Central South University of Technology, 2006, 13(4): 367-372.

[14] LIANG Shu-quan, TAN Xiao-ping, LI Shao-qiang, TANG Yan, Structure and mechanical properties of ZrO₂-mullite nano-ceramics in SiO₂-Al₂O₃-ZrO₂ system [J]. Journal of Central South University of Technology, 2007, 14(1): 1-6.

[15] LIU Yu-hui. Principle and application of X-ray diffraction analysis [M]. Beijing: Chemical Industry Press, 2003. (in Chinese)

Foundation item: Project(50576005) supported by the National Natural Science Foundation of China

Received date: 2008-03-26; Accepted date: 2008-06-11

Corresponding author: HE Jin-qiao, Doctoral candidate; Tel: +86-13117318879; E-mail: 8710054@163.com

(Edited by CHEN Wei-ping)