ARTICLE pubs.acs.org/EF
Effect of Key Minerals on the Ash Melting Behavior in a Reducing Atmosphere Haigang Wang, Penghua Qiu,* Xu Shi, Jifeng Zhang, Yuqing Chen, and Shaohua Wu School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin, Heilongjiang 150001, People’s Republic of China ABSTRACT: A coal sample was ashed at 600 °C in air for 12 h, then pressed into pellets, and further treated in a reducing mixture of 60% CO and 40% CO2 for 1 h at different temperatures ranging from 700 to 1200 °C in 100 °C intervals. Scanning electron microscopy linked with energy-dispersive X-ray analysis and X-ray diffraction were used to investigate the effects of key minerals on the ash melting behavior. No significant change was found in color and size of ash samples treated at 700900 °C. However, from 1000 to 1100 °C, substantial liquid-phase formation resulted in a greater shrinkage level and closed spherical pore formation in the pellet. A Na illite most likely existed in the parent coal. Most of Na and K coalesced with Si and Al during heating. A small number of NaK-enriched aluminosilicate particles were responsible for the formation of a liquid phase at 900 °C. The FeO phase formed aluminosilicates less readily than lime. The temperature significantly influenced the fragmentation extent of calcite. The formation of substantial Fe minerals and FeCa minerals required a particular temperature condition. In this study, this temperature condition was about 1000 °C. Fe minerals and FeCa minerals played an important role in determining the molten percentage of coal ash at relatively low temperatures. Evidence for this was that some molten FeCa aluminosilicate particles were identified at 1000 and 1100 °C and most Fe minerals and FeCa minerals formed liquid phases.
1. INTRODUCTION Integrated coal gasification combined cycle (IGCC) is attracting considerable interest worldwide. This contributes to the improvement of power generation efficiency compared to conventional pulverized coal-fired plants as well as the reduction of emissions of greenhouse gases and particulates to the atmosphere. The inorganic components in coal are converted into ash during coal gasification. Some ash particles are entrained in the syngas stream and can lead to ash deposition at different locations downstream of the gasifier, especially in the syngas cooling section. The syngas cooling section (mainly including the radiant syngas cooler and the convective syngas cooler) is one of the key sections in the IGCC system. The syngas coolers are developed to generate high-pressure steam by heat transferred from the hot syngas stream and cool the syngas stream before the downstream gas cleanup processes. In comparison to a gasifier without the syngas coolers but with direct water quench, the syngas coolers increase the process efficiency by approximately 5 percentage points.1 Meanwhile, when the syngas stream flows across the syngas coolers, some entrained ash particles can also be removed to clean the syngas stream if the syngas coolers are working well.2 However, poor knowledge on the ash melting behavior under the operating condition of the syngas coolers results in unplanned outages for maintenance to the syngas coolers. As an example, during early operation, severe deposition at the inlet of the convective syngas cooler led to a number of gasifier outages in the Polk Power Station.3 Other reports on the fouling of the syngas coolers can also be found.4,5 Table 1 shows operating temperature ranges of the syngas coolers applied in different gasification technologies in four commercial-scale stations. As seen from Table 1, most of the syngas coolers are operated at relatively low temperatures (less r 2011 American Chemical Society
than 1100 °C). However, in recent years, most researchers have focused on studying coal ash behavior at high temperatures in a reducing atmosphere, such as high-temperature melting characteristics of coal ash and ash formation mechanisms. Huffman et al.7 investigated the high-temperature behavior of coal ash in a reducing and an oxidizing atmosphere. They found that significant partial melting of the ashes occurred at temperatures below the initial deformation temperature (IDT) and iron played an important role in determining the ash melting behavior. Bai et al.8 found that the residence time of coal ash at high temperatures (1300 and 1400 °C) had great influence on the composition of coal ash. The ash formation mechanistic study highlighted the transformation behavior of individual minerals at high temperatures in a reducing atmosphere.912 To ensure that the molten slag flows freely down the gasifier walls, some other researchers made great efforts on the study of the viscositytemperature characteristic of slag1318 and flux requirement.19,20 Additionally, Shannon et al.21 studied the ash composition distribution by particle size and density to investigate its effect on the mineral phase type formed for a range of different temperatures (10271627 °C) in a reducing atmosphere. A limited number of studies has concentrated on examining the coal ash melting behavior at operating temperatures of the syngas coolers in a reducing atmosphere. Despite a large temperature range of 7001500 °C used in ref 7, the use of 200 °C intervals possibly lost some details on the ash melting behavior. In the present study, a small temperature interval of 100 °C (the temperatures ranged from 700 to 1200 °C) was used Received: April 8, 2011 Revised: June 21, 2011 Published: June 27, 2011 3446
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Table 1. Operating Temperature Ranges of the Syngas Coolers Applied in Different Gasification Technologies36 Polk power station
Buggenum power station
Wabash River station
Puertollano station
in U.S.A.
in Netherlands
in U.S.A.
in Spain
IGCC stations gasification technology
Texaco
Shell
E-GAS
Prenflo
temperature range (°C)
2501350
350900
3701040
250900
Table 2. Coal Ash Composition Analysis T (°C, reducing atmosphere)
composition (%, by mass) SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
SO3
P2O5
TiO2
DT
ST
HT
FT
42.95
14.27
10.49
16.28
0.96
1.94
1.4
6.65
0.03
0.84
1130
1157
1162
1197
Figure 3. Shrinkage curve of the ash sample plotted against the temperature. Figure 1. Schematic diagram of the experimental system.
Figure 2. Appearance of ash samples treated for 1 h at different temperatures from 700 to 1200 °C in 100 °C intervals.
to investigate in detail the effect of some key minerals on the ash melting behavior.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation and Analyses. A representative bituminous coal was used in this study. The coal was crushed to less than 97 μm and then ashed at 600 °C in air for 12 h. Coal ash composition analysis used in this study is given in Table 2. Loose ash powder was pressed into cylindrical pellets before further heat treatment. The dimensions of the cylindrical pellet were about 13 mm in diameter and 2 mm in height. The pressing step made the surface characteristics of ash samples clearly observable after heat treatment. 2.2. Experimental Procedures. The schematic diagram of the experimental system is shown in Figure 1. The ash pellet was placed in a ceramic crucible, quickly placed in a preheated tube furnace for 1 h, and
Figure 4. SEM images of ash samples after heat treatment at different temperatures. then quenched into liquid nitrogen. The temperatures ranged from 700 to 1200 °C in 100 °C intervals, and a reducing mixture of 60% CO and 3447
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Figure 5. Pores in the cross-section of the ash pellet after heat treatment at 1100 °C. 40% CO2 was used in this study. All of the quenched ash samples shown in Figure 2 were also examined using scanning electron microscopy linked with energy-dispersive X-ray analysis (SEMEDX) and X-ray diffraction (XRD). To approximately estimate the shrinkage level of each ash sample after heat treatment, the following formula was used to calculate the shrinkage, x: x¼
V ð600Þ V ðTÞ 100% V ð600Þ
the pore walls; thus, gas in the pores contains sulfur. To judge whether further sintering can proceed or not, the traveling time of the bubble or pore through the top surface of the ash pellet from the inside of this pellet is calculated. The traveling time is directly determined by the terminal velocity of the bubble movement. The terminal velocity can be described by the following equation:23
ð1Þ
where V(600) is the bulk volume of the ash sample before further heat treatment and V(T) is the bulk volume of the corresponding ash sample after heat treatment at temperature T in °C. The mean results of at least five duplicate experiments are presented in Figure 3.
3. RESULTS AND DISCUSSION
v¼
ð2Þ
where R is the bubble radius, F is the melt density, g is the gravitational constant, and μ is the melt viscosity. In this study, the main melt components (SiO2, Al2O3, CaO, FeO, Na2O, K2O, MgO, and TiO2) are considered to determine the melt density and viscosity. The melt density can be expressed as24
3.1. Appearance Characteristics of Ash Samples after Heat Treatment. Few changes are identified in the color and size of
ash samples after heat treatment at temperatures from 700 to 900 °C (Figures 2 and 3). The color of the ash sample treated at 1000 °C is gray and distinctly different from that treated at 900 °C. The greater shrinkage level at 1100 °C indicates that a substantial liquid phase has been formed, and the color of the ash sample becomes much darker. The ash sample treated at 1200 °C does not maintain its original shape because of the flow of the liquid phase. Figure 4 shows SEM observations on surfaces of ash samples treated at 8001100 °C. As seen in Figure 4a, most particles are irregular at 800 °C but a few particles are more rounded. Some obvious molten particles are easily observed at 900 °C in Figure 4b. In comparison to the 900 °C results in Figure 4b, the presence of some big pores in the surface of the ash sample treated at 1000 °C is mainly attributed to the permeation of the liquid phase through the surface of the ash sample (Figure 4c). This indicates that a local liquid phase of low viscosity has been formed. Figure 4d shows further densification resulting from a larger amount of liquid phase completely filling the pore spaces at 1100 °C. These microscopic observations are reasonably consistent with the bulk results described in Figures 2 and 3. Although no pores are found on the surface of the ash sample treated at 1100 °C (Figure 4d), large numbers of pores occur in the cross-section of this ash pellet (Figure 5). Existence of large numbers of closed pores will possibly result in low thermal conductivity.22 From Figure 5, it can also be seen that these pores are closed, spherical, and isolated, which implies that the ash sample has already arrived at the final stage of sintering. EDX analysis in Figure 5 shows that a large amount of sulfur exists on
2R 2 Fg 9μ
F¼
∑n i M i ∑n i V i
ð3Þ
where Vi is molar volume of the oxide i and ni is its molar fraction. Mi is the molar weight of the oxide i. On the basis of the data in Table 1, the melt density evaluated by eq 3 is 2788.2 kg/m3. A viscosity model developed by Browning et al.25 is used to determine the viscosity of coal ash in this study. The model predicts the viscosity of coal ash accurately in the viscosity range of less than 103 Pa s. The viscosity determined using the viscosity model with coal ash composition analysis given in Table 1 is 103.1 Pa s at 1100 °C. The diameter of typical bubbles is about 30 μm observed by SEM in Figure 5. According to the above data and eq 2, the time required to travel through the pellet with the 1.5 mm height and reach the surface can be derived. The calculated result shows that the time is far more than 1 h, which implies that, at 1100 °C, further densification is difficult. 3.2. Analyses of NaK Minerals. XRD patterns of the samples after the heat treatment at 600900 °C are shown in Figure 6. There is a relatively high content of illite in coal ash at 600 °C in Figure 6. The most obvious diffraction intensity peak of illite is at 2θ = 19.8°, which gradually decreases with an increasing heat treatment temperature and disappears at 900 °C. This indicates that illite has entirely melted at 900 °C. A molten particle as shown in Figure 4b at 900 °C is analyzed in detail in Figure 7a. In comparison to the bulk coal ash composition, the molten particle is rich in Na and K and with small amounts of Fe and Ca. It is observed that most K occurs in illite in bituminous coals.26 Therefore, these molten NaK-enriched aluminosilicate particles should be derived from illite. As seen in Figure 7b, the surface of a NaK enriched aluminosilicate particle at 1000 °C is 3448
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Table 3. Minerals Marked by Numerals 018 numerals
minerals
numerals
minerals
numerals
minerals
0
quartz
7
anorthite
14
fayalite
1
calcite
8
gehlenite
15
almandite
2
illite
9
oldhamite
16
sekaninaite
3
kaolinite
10
lime
17
hedenbergite
4
pyrite
11
hematite
18
grossular ferrian
5
siderite
12
magnetite
6
anhydrite
13
wuestite
concave because of low viscosity. At 1100 °C, the lower viscosity causes the formation of a smooth surface. The EDX analysis shows that the smooth surface is enriched in Na and K and with higher Ca and Fe contents (area 1 in Figure 7c) than the particles described in panels a and b of Figure 7. The higher Ca and Fe contents are a result of the molten NaK-enriched particle contacting its surrounding CaFe minerals. Generally, the K content in illite is higher than the Na content.27 However, the EDX analyses in Figure 7 show that the Na content exceeds the K content. In fact, illite containing a significant amount of sodium and potassium (5.22% Na2O and 2.58% K2O) has been described in the literature.27 To obtain more information on the particles containing Na and K, area map scanning analysis of an arbitrary zone on the surface of the ash sample treated at 800 °C was carried out. A RGB image of each element obtained was further transformed into a corresponding gray-scale image, and the global threshold value was determined by the method described in ref 28 for each element. The intensity level of the element x at a pixel is noted as g(x). In this study, the number of total pixels N is 200 256 in a gray-scale image. Using the Matlab software platform, a computer program has been compiled to calculate the number of pixels satisfying specific conditions (n), such as g(x) > 0. The percentage of the pixels satisfying specific conditions is expressed as n ð4Þ P ¼ 100% N The average intensity of the element x in a gray-scale image is written as gðxÞave ¼
Figure 6. XRD patterns of the samples treated at temperatures of 600900 °C. Minerals marked by numerals are given in Table 3.
∑ gðxÞ n
ð5Þ
The results are described in Figure 8, in which the white pixels represent points satisfying specific composition criteria. Panels ad of Figure 8 show that most Na and K coalesce with Si and Al and the distribution of pixels on the surface takes on a relatively homogeneous structure. The homogeneous structure means that the small zones in panels a or b of Figure 8, which are arbitrarily selected, have the same or a similar number of the pixels to each other. However, in Figure 8e, there is a non-homogeneous distribution for NaK-enriched pixels under the condition of [g(Si) > 0, g(Al) > 0, g(Na) > g(Na)ave, and g(K) > g(K)ave] and the number of pixels satisfying this condition sharply decreases. Of the white pixels described in Figure 8e, about 20% correspond to g(Na) > g(K). These pixels are most likely from Na illite. 3.3. Analyses of Ca and/or Fe Minerals. Figure 6 shows that new Ca minerals (anhydrite, gehlenite, oldhamite, and anorthite) are formed at temperatures from 600 to 900 °C, the main aluminosilicates are anorthite and gehlenite, but only a small amount of lime is found. This indicates that the majority of the lime from 3449
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Figure 7. NaK-enriched aluminosilicate particles.
decomposition of calcite directly forms these new Ca minerals. Although a high content of the FeO phase is found at 600900 °C in Figure 6, the FeO phase does not form Fe aluminosilicates. Until the temperature rises to 1000 °C, many new Fe minerals and FeCa minerals (hedenbergite, fayalite, almandite, and sekaninaite) occur (Figure 9). A few FeO particles can still be identified by SEMEDX at 1100 °C (the content of FeO could be below the detection limit of XRD), e.g., the one described in Figure 10. On the basis of the above analyses, the FeO phase forms new minerals, especially aluminosilicates, less easily than lime. To deeply understand the above conclusion, thermodynamic analysis was carried out at a constant pressure and temperature. According to the XRD patterns collected in Figures 6 and 9, the following aluminosilicates are considered in this study: anorthite [Ca(Al2Si2O8)], gehlenite [Ca2(Al2SiO7)], hedenbergite [CaFe(Si2O6)], fayalite (Fe2SiO4), almandite [Fe3Al2(Si3O12)], and sekaninaite (Fe2Al4Si5O18). The involved reactions proceed as CaO þ Al2 O3 þ 2SiO2 ¼ CaðAl2 Si2 O8 Þ 2CaO þ Al2 O3 þ SiO2 ¼ Ca2 ðAl2 SiO7 Þ
CaO þ FeO þ 2SiO2 ¼ CaFeðSi2 O6 Þ
ð8Þ
2FeO þ SiO2 ¼ Fe2 SiO4
ð9Þ
3FeO þ Al2 O3 þ 3SiO2 ¼ Fe3 Al2 Si3 O12
ð10Þ
2FeO þ 2Al2 O3 þ 4SiO2 ¼ FeAl4 Si4 O18
ð11Þ
The constant-pressure specific heat is considered to be dependent upon the temperature T in K. One relationship used is CP ¼ a þ b þ cT 2
ð12Þ
According to Kirchhoff’s equation and the second law of thermodynamics, the Gibbs free-energy function of a reaction can be derived 1 1 ΔG°T ¼ ΔH0 ΔaT ln T ΔbT 2 ΔcT 1 2 2
ð6Þ ð7Þ
þ yT 3450
ð13Þ
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1 ° ΔH0 ¼ ΔH298 Δa 3 298 Δb 3 2982 2 þ Δc 3 2981 y ¼ ðΔG°298 ΔH0 Þ2981 þ Δa ln 298 1 1 þ Δb 3 298 þ Δcð298Þ2 2 2
ð14Þ
ð15Þ
Thermodynamic data needed in this study are from ref 29. The change of Gibbs free energy of the reactions 611 with tempera-
Figure 8. Some information of the particles containing Na and/or K (white points represent some pixels satisfying specific conditions).
tures is compared in Figure 11. Figure 11 shows that the change of Gibbs free energy of the reactions 6 and 7 is less than that of the reactions 811. A chemical reaction happens more easily when the change of Gibbs free energy is less. Therefore, anorthite and gehlenite are formed more easily than the formation of Fe and CaFe aluminosilicates at low temperatures. The thermodynamic analysis is consistent with the experimental results. However, once these new Fe minerals and FeCa minerals have been formed, they would play an important role in controlling the molten percentage of coal ash. This can be proven by the molten FeCa aluminosilicate particles shown in Figure 12. The big pore in Figure 12a is one of the typical big pores observed in Figure 4c. According to the EDX analysis, aluminosilicate particles around the big pore are rich in Fe and Ca with a high SiO2/Al2O3 ratio. Within the big pore, a higher content of FeCa in the aluminosilicate particles is expected. In Figure 12b, small FeCa aluminosilicate particles are strongly
Figure 9. XRD patterns of the samples treated at temperatures of 1000 and 1100 °C. Minerals marked by numerals are given in Table 3.
Figure 10. Typical FeO particle at 1100 °C. 3451
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Figure 11. Gibbs free-energy change ΔG plotted against the temperature.
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1100 °C. This indicates that most Fe minerals and FeCa minerals have formed a liquid phase at 1100 °C. This explains why the greater shrinking level occurs from 1000 to 1100 °C in Figure 3. However, it is clear that the formation of these new Fe minerals and FeCa minerals needs a particular temperature condition. In this study, the temperature condition is about 1000 °C.
Figure 13. XRD patterns of parent coal. Minerals marked by numerals are given in Table 3.
Figure 12. Typical FeCa aluminosilicate particles at 1000 °C. 3452
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At 600 °C, the characteristic diffraction peak of anhydrite appears at 2θ = 25.4° in Figure 6, and with an increasing temperature, the diffraction peak of anhydrite disappears gradually. High calcite content is identified in the parent coal in Figure 13, but no gypsum exists. Therefore, anhydrite is most likely from the sulfation reaction of calcite during ash sample preparation. Sulfur should be mainly from pyrite because the major sulfur minerals are pyrite in the parent coal in Figure 13. At 800 °C, the obvious diffraction peak of oldhamite is found, e.g., at 2θ = 44.9°; thus, the following reaction could happen in a reducing atmosphere:30,31 CaSO4 þ 4CO ¼ CaS þ CO2
CaSO particle at 800 °C. The particle fragmentation is verified at 900 °C in Figure 14c. The fragments are smaller in
ð16Þ
Typical CaSO particles treated at 800 and 900 °C are shown in Figure 14. An image of the CaSO particle at 800 °C in Figure 14a is shown in Figure 14b at higher magnification. Numerous pores with size less than 0.1 μm have been formed on the surface of the CaSO particle as a result of the evolution of CO2. However, at 900 °C, the appearance of the CaSO particle is significantly different than that of the
Figure 15. Calcite decomposition equilibrium diagram.
Figure 14. Typical CaSO particle at 800 and 900 °C. 3453
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