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Effect of temperature and pressure on the mineralogical and fusion characteristics of Jincheng coal ash in simulated combustion and gasification environments
Fuel 104 (2013) 647–655
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Effect of temperature and pressure on the mineralogical and fusion characteristics of Jincheng coal ash in simulated combustion and gasification environments Nijie Jing a,b, Qinhui Wang a,⇑, Leming Cheng a, Zhongyang Luo a, Kefa Cen a, Dongke Zhang b,⇑ a b
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38, Hangzhou, Zhejiang 310027, China Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
h i g h l i g h t s " Temperature effect on ash fusion characteristics strongly depends on atmospheres. " High temperature minerals form in combustion, increasing ash fusion temperatures. " Low-melting eutectics form in gasification, decreasing the ash fusion temperatures. " Pressure influences mineral transformation by affecting reactions between minerals. " Increasing pressure accelerates the formation of high-temperature minerals.
a r t i c l e
i n f o
Article history: Received 27 December 2011 Received in revised form 19 May 2012 Accepted 23 May 2012 Available online 6 June 2012 Keywords: Ash fusibility HPTGA XRD analysis FSEM–EDS analyses Low-temperature eutectics
a b s t r a c t The effects of the pressure and temperature on the fusibility of coal ash during combustion and gasification were investigated. Experimentation was conducted using a high pressure thermogravimetric analyzer (HPTGA) apparatus. In order to observe the conversion of minerals with changing temperature and pressure in different atmospheres, the resulting ash samples were analyzed using an X-ray diffractometry (XRD) analyzer. In addition, the quantitative XRD analyses of ash samples and a field emission scanning electron microscope (FSEM), together with X-ray energy dispersive spectroscopy (EDS) were employed to verify the detailed mechanisms of ash fusion. The results indicated that temperature and type of atmosphere had a dominant effect on the ash fusion characteristics while the effect of pressure was somewhat more complicated, depending on the temperature and atmosphere being experienced by the ash. The high-temperature minerals such as mullite were formed with increasing temperature under both combustion and gasification atmospheres. However, in gasification atmosphere, there were more fluxing minerals and feldspar minerals present, such as muscovite, anhydrite and K-feldspar, decreasing the fusion temperature. The effect of pressure on the ash fusibility showed different behavior at different temperatures. At 900 °C, the decompositions of low-temperature minerals, such as muscovite, anhydrite and oldhamite, were suppressed with increasing pressure, resulting in a decrease in the fusion temperatures. On the other hand, at 1000 °C, the low-temperature minerals transformed into hightemperature minerals such as mullite and sanidine with rising pressure. However, the presence of fluxing minerals and the melting of iron-containing minerals resulted in the lowering of the fusion temperatures. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction Fluidized bed combustion and gasification are widely considered to be most promising technologies for conversion and utilization of solid fuels such as coal with advantages of fuel flexibility, lower power production costs and thus reduced greenhouse gas emissions. However, they have several operational challenges due ⇑ Corresponding authors. Tel.: +86 571 87952802; fax: +86 571 87951616 (Q.H. Wang), tel.: +61 8 6488 7600; fax: +61 8 6488 7235 (D.K. Zhang). E-mail addresses: [email protected] (Q. Wang), [email protected] (D. Zhang).
to the behavior and transformation of minerals and inorganic matter in these fuels, which can cause problems such as bed agglomeration, deposition on gas circuits and heat exchange surfaces, and bridging on hot gas filtration systems as a result of ash build-up. These problems can cause reduced operability, thermal efficiency and in the extreme cases, unplanned outage of the plant [1,2]. The ash characteristics are responsible for a series of problems related to coal use in fluidized bed reactor systems [3,4]. The ash fusibility is an important parameter in determining the behavior of coal ash in fluidized bed combustion and gasification of coal as well as their ash utilization [5,6]. Hence, a scientific understanding of the fusion behaviors of coal ash is the basis for effective and clean utilization of
0016-2361/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.05.040
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coal. In addition, in fluidized bed combustion and gasification, the mineral matter in coal gives detailed information on the fusion characteristics of coal ash [7–10]. Therefore, the study of transformation of mineral matter in coal is crucial to the determination of the ash fusion characteristics. The relationships between ash fusibility and the chemical composition of coal minerals have been studied in the literature using a number of experimental techniques including fusion tests, X-ray diffractometry, light microscopy, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy analyzer (EDX), differential–thermal and thermogravimetric analyzers and chemical analyses [10–15]. To date, however, limited work has been conducted on the fusion characteristics of coal ash with regard to mineral transformation under high pressure combustion and gasification conditions. The present work was aimed to examine the relationship between ash fusibility and mineralogical transformation of a Chinese anthracite, Jincheng coal, under high pressure combustion and gasification atmospheres simulated using a high pressure thermogravimetric analyzer, aided with X-ray diffraction (XRD) analyses and FSEM– EDS analyses to investigate coal ash fusion characteristics.
Table 2 Ash fusion temperatures of Jingcheng coal ash. Ash fusion temperatures (°C)
Reducing atmosphere Oxidizing atmosphere
DT
ST
HT
FT
1349 1401
1409 1449
1421 1457
1438 1466
800–1090 °C and at different pressures of 0.1, 0.5, 1.0, 1.5 and 2.0 MPa, from which different ash samples were retrieved for further characterization. A schematic diagram of the HPTGA system is shown in Fig. 1. The gas compositions of the simulated combustion and gasification atmospheres used in the HPTGA experiments are shown in Table 3. During a typical experiment, around 500 mg of an ash sample was heated from room temperature to a preset final temperature at a rate of 20 °C min 1 in a desired reaction atmosphere at a given pressure in the HPTGA. The sample was maintained at the final temperature for 30 min. The ash sample was then cooled down to room temperature and retrieved and ground to a particle size fraction less than 74 lm in the agate mortar for further analysis. The ash samples prepared under the different conditions described above were subjected to mineralogical characterization using a Rigaku D/Max-2550PC X-ray diffractometer using copper Ka radiation (neoConfucianism Company, Japan). The qualitative powder XRD analysis employed an accelerating voltage was of 40 kV, a current of 250 mA and a scan rate of 10 deg min 1 between 2h = 5° and 85°. The relative intensities of main characteristic XRD peaks for major mineral phases were measured and recorded. This method assumes, qualitatively, that the ratios of the peak heights in the XRD patterns are proportional to the concentrations of the minerals in a sample [11,17,18]. Quantitative XRD analyses of the ash samples prepared under the simulated gasification atmosphere at 900 °C and different pressures were also performed. The morphological characteristics and the corresponding chemical compositions of the ash samples prepared under the simulated gasification atmosphere at 1000 °C and at pressures of 0.1 MPa and 2.0 MPa were also studied using a FEI – SIRION-100 field emission SEM equipped with an EDAX GENESIS4000 X-ray energy dispersive spectroscopy (EDS). The EDS is allowed for qualitative element identification and semiquantitative analysis of ash samples. The EDS analyses were performed on the ash samples by operating the FSEM at an accelerating voltage of 25 kV. The intensity ratios of various constituent elements present on the EDS spectra were also used to identify the chemical composition of some partially fused particles in the ash.
2. Experimental 2.1. Preparation of ash samples and determination of ash fusion temperatures A Chinese anthracite from Shanxi Province, Jincheng coal, was chosen for this study. The proximate and ultimate analysis data as well as the ash chemistry are presented in Table 1. The ash samples were prepared according to the Chinese Standard GB/T2122001, in which coal sample was heated to 540 °C in air in a muffle furnace for ashing slowly. The residue was heated to 820 °C in air and kept for 2 h. Finally, the resultant ash sample was cooled down to room temperature and then ground to a particle size less than 200 lm in a mortar. The fusion temperatures of the coal ash were determined using a 5E-AFIII auto-analyzer (Changsha, China) under reducing and oxidizing atmospheres according to Chinese Standard GB219-74. Air was used as the oxidizing atmosphere. The reducing atmosphere was gained by incomplete combustion of graphite and active carbon in a ceramic ark within the 5E-AFIII auto-analyzer during the heating of the ash cones. There are four characteristic temperatures, deformation temperature (DT), soft temperature (ST), hemisphere temperature (HT) and flow temperature (FT), for each cone [16]. The results, averaged from five measurements for each sample, are presented in Table 2.
3. Results and discussion
2.2. Experimental setup and characterisations of mineralogy and morphology
3.1. Effect of temperature on the mineralogy in different atmospheres A high pressure thermogravimetric analyzer (HPTGA), Thermax 500 (USA), was employed to simulate the conditions of the combustion and gasification of Jincheng coal in the temperature range
Fig. 2 presents the XRD patterns of the ash samples from the combustion atmosphere at 0.5 MPa and temperatures between
Table 1 Proximate and ultimate analysis and ash composition of Jincheng coal. Proximate analysis (wt%) (air dried basis)
Ultimate analysis (wt%) (air dried basis)
M
V
A
FC
C
H
O
N
S
2.23
8.46
20.1
69.21
65.81
3.25
6.7
0.94
0.95
Composition (wt%) SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
SO3
48.1
31.84
5.9
4.96
0.98
1.14
1.09
3.07
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Fig. 1. A schematic diagram of the HPTGA system.
Table 3 Gas composition of the simulated atmospheres. Atmosphere
Gas composition (%) (vol.)
Combustion Gasification
H2
O2
CO
CH4
CO2
N2
– 40.8
2.1 –
– 36.5
– 1.9
49.7 20.8
48.2 –
800 and 1090 °C, showing the variations in mineral phases with increasing temperature. The main mineral phases found in the ash at 800 and 900 °C were quartz, muscovite (KAl2(AlSi3)O10 (OH)2), anhydrite (CaSO4) and K-feldspar (KAlSi3O8). When the temperature was raised to 1000 °C, the K-feldspar phase disappeared and the intensities of the peaks of anhydrite and muscovite [19] also decreased slightly. In the meantime, new peaks, believed to be hematite and mullite (Al6Si2O13) [20,21] appeared. This indicated that at the high temperature, muscovite began to transform into mullite. At 1090 °C, the sanidine ((K,Na)(Si,Al)4O8) phase also
Q-Quartz M-Mullite Ah-Anhydrite K-K-feldspar Sa-Sanidine He-Hematite Mus-Muscovite
Q
Q M
Ah
He S a S aM H e
0
20
Q
Q
M
An
M QH eQ H e M H e Q
Q Ah
M us K M us M Q Ah K M us M us Q Ah M us M us K
appeared, which is a more stable structure and high temperature (>900 °C) form of the K-feldspar [22]. The feldspar minerals such as sanidine are capable of reacting with other minerals to form low temperature eutectics [21,9], which can reduce the melting temperature of the ash; on the other hand, the high-temperature minerals such as mullite can increase the melting temperature under the combustion atmosphere. Fig. 3 shows that the quartz, anhydrite, muscovite and sanidine ((K,Na)(Si,Al)4O8) phases are presented in the ash samples from the gasification atmosphere at 0.5 MPa and at 800 and 900 °C, respectively. However, at 1000 °C, the peak of muscovite disappeared and that of sanidine decreased. In the meantime, the peaks of hematite and mullite appeared. As the temperature was further increased to 1090 °C, the sanidine and hematite phases completely disappeared while the anorthite and hercynite (FeAl2O4) were observed. This indicates that the transformations from sanidine to anorthite and from the hematite to hercynite have occurred at the high temperatures. Anorthite and hercynite are also high-temperature minerals and they are able of reacting with other minerals to produce
A h H e A hM
AhM us Ah
Q M u s M u s MM u s Q
Q M u sQ M u s Q M u s Q
M
A h M u sA h
40
60
Ah Sa Sa He M
M us
M us
80
Sa
M us
Q M us Q
Ah Sa
M
1090
Sa
100
Fig. 2. XRD patterns of ash samples from the combustion atmosphere at 0.5 MPa and at different temperatures.
0
20
Ah
QM M Q
M usQ
M
1000
Q M us M us
900
Sa
M us Q Q M us
Sa
800
Two-Theta (deg)
QH e
Ah Sa M us Q Sa
Q
900 Q
M H Q An Q M H
M
M us Q
1000
M us
Q M iQ M u s Q M u s Q
M
M
Q
K
M An H M M H M
Q
1090 Q
Q-Quartz,Sa-Sanidine M-Mullite,Ah-Anhydrite He-Hematite,H-Hercynite An-Anorthite Mus-Muscovite
40
Q M us
Q
Q M us
60
M us
800
80
100
Two-Theta (deg) Fig. 3. XRD patterns of ash samples from the gasification atmosphere at 0.5 MPa and at different temperatures.
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low-temperature eutectics under reducing gas environments, leading to decreasing fusion temperatures of the ash. Hence the fusion temperatures in the gasification atmosphere are lower than those in the combustion atmosphere, as shown in Table 2 above, which is consistent with the discussion above. The trends of changes to the main mineral phases based on the XRD intensities of the minerals under the combustion and gasification atmospheres, respectively, as a function of temperature at 0.5 MPa, are shown in Fig. 4. From Fig. 4a and b, significant variations in the XRD intensities of muscovite, K-felspar, sanidine, hercynite, anorthite and mullite with increasing temperature are evident. Note that muscovite and K-felspar are fluxing minerals [23,24] and the mullite, hercynite, anorthite and sanidine are high temperature minerals. The fluxing minerals melted gradually with increasing temperature and transformed to the high temperature minerals.
Q
M us M us
Q-Quartz Ah-Anhydrite K-K-feldspar He-Hematite Mus-Muscovite
Q
Ah K K Ah He Ah Q M us H e Q H e Q Ah M us Ah H e Ah M usQ H e Q H e Q Q Ah M us AhH e Ah M us Q H e Q He Q QAh A h M u sA h M u s Q M us M i Q M usQ QAh K K M u s Q Q M us A h Ah M us M i M u sQ M us
Q
Q
M us
M us M us M us
0
20
40
2.0 MPa
Q
1.5 MPa Q
1.0 MPa Q
0.5 MPa Q
60
0.1 MPa
80
100
Two-Theta (deg) 3.2. Effect of pressure on the mineralogy in different atmospheres Fig. 5 presents the XRD patterns of ash samples from the combustion atmosphere 900 °C with varying pressure from 0.1 MPa to 2.0 MPa. The main mineral phases observed in the ash samples were quartz, muscovite, anhydrite and K-feldspar at 0.1 and 0.5 MPa. At 1.0 MPa, however, the peak of hematite appeared and, as the pressure was further increased, the low-temperature minerals such as anhydrite was observed to increase slightly. In addition, it can be seen that the peak of muscovite becomes slightly sharper as pressure rises. This can be explained by the fact that the pressure affects the geochemical transformation of minerals [25] and restrains the low-temperature minerals from decomposition. It is also apparent from Fig. 5 that the changes in the main minerals phases in the ash with pressure in the combustion atmosphere are not very significant.
Dramatically different mineralogical behavior of the ash samples from the gasification atmosphere was observed. Fig. 6 shows the XRD patterns of the ash samples at 900 °C with varying pressure from 0.1 MPa to 2.0 MPa. It is worth noting that oldhamite (CaS) was identified to present at low pressures, in addition to quartz, muscovite, anhydrite. As pressure was raised, the oldhamite and anhydrite phases disappeared and the peak of sanidine appeared at 0.5 MPa and the peak of mullite appeared at 1.5 MPa. Interestingly, at 2.0 MPa, the anhydrite phase appeared again. This indicates that the oldhamite might have been converted to anhydrite. Notwithstanding the fact that the presence of mullite would increase the ash fusion temperatures, the intensity of mullite peak
4000 3000 2000 1000 0
Hematite
Hercynite
4000 3000 2000 1000 0
Mullite
3000 1500 0
4000 3000 2000 1000 0
Mullite
Sanidine
4500 3000 1500 0
Sanidine
6000 4000 2000 0
6000 4000 2000 0
K-feldspar
4000 3000 2000 1000 0
Intensity / CPS
Intensity / CPS
4000 3000 2000 1000 0
Fig. 5. XRD patterns of ash samples from the combustion atmosphere at 900 °C and at different pressures.
2400 1600 800 0
Anhydrite
4000 3000 2000 1000 0
12000 9000 6000 3000
Quartz
12500 10000 7500 5000
6000 4000 2000
Muscovite
6000 4000 2000 0
800
850
900
950
1000
1050
Temperature/
(a) Combustion atmosphere
1100
Hematite
Anorthite Anhydrite
Quartz
Muscovite 800
850
900
950
1000
1050
Temperature/
(b) Gasification atmosphere
Fig. 4. Changes to the main mineral phases of the ash samples as a function of temperature at 0.5 MPa.
1100
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Table 4 Contents of main mineral phases in ash samples from gasification atmosphere at 900 °C.
Q-Quartz Ah-Anhydrite M-Mullite Ol-Oldhamite Mus-Muscovite Sa-Sanidine
Q
Q M us Ah Q S a S aM u s M M Q Q M M us M us M Q Q Q M us Sa S a M M us M M u s S a MM u s Q M u s M Q M Q Q M Q Sa O lM u s Q M u s M us Ol Q Ol Q Q Ol Q Ol Q M us Ah S a Q M us M us M us Q Q Sa S a Q M u sQ M u s Q Q Ol M us M us Ah M us Q O l Q M us Q M us Q O l Q O l M us M
0
20
40
60
2.0 MPa 1.5 MPa
0.1 MPa
1.0 MPa
2.0 MPa
Quartz Muscovite Mullite Anhydrite Oldhamite Sanidine
29.5 56.1 6.3 1.2 0.9 5.9
31.0 57.4 1.5 0.8 0.9 8.4
31.8 37.8 21.0 1.4 0.5 7.5
1.0 MPa 0.5 MPa 0.1 MPa
80
100
Two-Theta (deg) Fig. 6. XRD patterns of ash samples from the gasification atmosphere at 900 °C and at different pressures.
was weakened in the reducing atmosphere and therefore, in the presence of the fluxing minerals such as muscovite and anhydrite the ash fusion temperatures would be significantly reduced, as observed from the results in Table 2. It is also very interesting to note that, under gasification atmosphere the sanidine phase increased with pressure from 0.1 MPa to 1.0 MPa, but weakened as the pressure was further increased to 2.0 MPa. The muscovite phase showed a similar trend. To understand this phenomenon, quantitative XRD analysis was performed with the crystalline minerals and their relative contents presented in Table 4. It can be seen that the contents of muscovite and
2000 1500 1000 500 0
Mineral phase (%)
sanidine phase were increased while that of mullite decreased from 0.1 MPa to 1.0 MPa. Both the increase in the low melting point minerals and the decrease in the high temperature minerals will lead to a reduction in the ash fusion temperature in the gasification atmosphere. On the other hand, the muscovite and sanidine phase decreased whilst the mullite phase increased as the pressure was increased to 2.0 MPa. This finding suggests that the pressure affects the mineral transformation more dramatically in the gasification environment than in the combustion environment, although the influence extent decreases slightly with an increase in pressure above 1.0 MPa. This is because that, as Monson et al. [26] and Liua et al. [27] reported, increasing the pressure can lead to an increase in the rate of mineralogical reactions but the rate of the rate increase decreases with further increases in pressure [26,27]. Therefore, the fusion temperatures of ash in reducing environment would be most influenced by pressure below 1.0 MPa. The trends of changes to the main mineral phases under both combustion and gasification atmospheres at 900 °C are shown in Fig. 7. It can be seen that the mineral phases under the gasification
2400 1600 800 0
Hematite
Mullite
2400 1800 1200 600 0
2400
K-feldspar 2100
Sanidine
3300
Intensity / CPS
Intensity / CPS
3000 1800
Anhydrite
3000 2700 6000
2000
Oldhamite
1000 0 3000
Anhydrite
2000 1000 0
5500 5000
6000 5000 4000 3000
Muscovite
4500 4000
Muscovite
13500 13000 12000
Quartz
12000
Quartz
11000
10500
10000 0.0
0.5
1.0
1.5
2.0
2.5
0.0
0.5
1.0
1.5
Pressure / MPa
Pressure / MPa
(a) Combustion atmosphere
(b) Gasification atmosphere
Fig. 7. Changes to the main mineral phases of the ash samples as a function of pressure at 900 °C.
2.0
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Q
Q
Q-Quartz Ah-Anhydrite M-Mullite He-Hematite Mus-Muscovite
Q Ah
He Ah M us M M M us H e Q H e M H e Q M us Q Ah M us A hH e M u s A h H e Q H e M H e Q M us M M us Q M us Ah Q A hH e M u s A h Q M He He M He M us M us Q M us Ah He Ah M us Ah Q Q M us M M He H e Q M us He Q Ah M us A h H eM u s Ah H eQ H e M QH eQ M us M us M
0
20
Q Q Q
Q Q
60
40
M
Q Ah M Sa Sa
M
Q Ah Sa Sa M M
M
Q Ah Sa Sa M Q
2.0 MPa Sa
1.5 MPa Sa
1.0 MPa
Sa
0.5 MPa
80
Ah
M
Sa Q
M
Sa
Ah
Sa
0.1 MPa
100
0
Q-Quartz M-Mullite Sa-Sanidine Ah-Anhydrite He-Hematite M
M
A h Q M A hQ M M Q
M M
S aM H e
M
Sa M M
MQ
20
M
Q Ah Q M Ah M M Q
M
Sa
Q
M
QM M Q
Sa
M
QM M Q
Sa
2.0 MPa 1.5 MPa 1.0 MPa 0.5 MPa
Ah
40
Two-Theta (deg)
M
Q MAh Q M M Q
Ah
Q
60
0.1 MPa
80
100
Two-Theta (deg)
Fig. 8. XRD patterns of ash samples from the combustion atmosphere at 1000 °C and at different pressures.
Fig. 9. XRD patterns of ash samples from the gasification atmosphere at 1000 °C and at different pressures.
atmosphere (Fig. 7b) as compared to those under the combustion atmosphere (Fig. 7a) changed more significantly in terms of both the types and intensities of the minerals with majority being fluxing minerals, and, as the fluxing minerals normally became more resistant to decomposition with increasing pressure, the low temperature minerals would be more readily restrained at high pressures. The combined effects of the above phenomena would reduce the ash fusion temperatures. Quartz, muscovite, anhydrite, mullite and hematite are the major mineral phases in ash samples from the combustion atmosphere at 1000 °C at 0.1 and 0.5 MPa as shown in Fig. 8. As pressure increased, the peak of muscovite weakened and the peak of mullite strengthened. This indicates that muscovite was transformed to
mullite with rise in pressure. The high-temperature minerals like mullite were formed with further increase in pressure, and thereby raising the ash fusion temperatures. It may be noted that muscovite was still detected at highest pressure, suggesting that the decomposition of muscovite was incompletely at 1000 °C within the time of the experiment. Fig. 9 shows the XRD patterns of the main mineral phases in ash samples from the gasification atmosphere at 1000 °C and at pressures from 0.1 MPa to 2.0 MPa, including quartz, mullite, anhydrite and sanidine ((K,Na)(Si,Al)4O8). The peak of hematite only appeared at 0.5 MPa and its absence at higher pressures suggests that hematite might have gradually melted into the glass phase forming a low temperature fluxing [24] with increasing pressure at 1000 °C.
2700 2400 2100 1800 1500
Mullite
Hematite
3300
Anhydrite
3000 2700 2400
Intensity / CPS
Intensity / CPS
2400 2100 1800 1500 1200
3600 3000 2400 1800 1200
Mullite
2000 1500 1000 500 0
Hematite
1600 1500 1400 1300 1200 1100
Anhydrite
4000
Muscovite
3000
3000
Sanidine
2400 2000
1800
1000 14000
1200 14000
Quartz
12000 10000
Quartz
12000 10000
8000
8000 0.0
0.5
1.0
1.5
Pressure / MPa
(a) Combustion atmosphere
2.0
0.0
0.5
1.0
1.5
Pressure / MPa
(b) Gasification atmosphere
Fig. 10. Changes to the main mineral phases of the ash samples as a function of pressure at 1000 °C.
2.0
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(a) 0.1 MPa
(b) 0.1 MPa
(e) 2.0 MPa
(f) 2.0 MPa
(c) 0.1 MPa
(g) 2.0 MPa
(d) 0.1 MPa
(h) 2.0 MPa
Fig. 11. FSEM images of the ash samples from the gasification atmosphere at 1000 °C at different pressure.
As the pressure increased, the quartz phase decreased in intensity while the sanidine and mullite increased slightly. It may be postulated that quartz reacted with other oxides to form sanidine and mullite at the high temperature under reducing environment. It follows that the low-temperature fluxing minerals may have begun to transform into high-temperature minerals. However, the presence of the fluxing minerals and the melting of the iron-containing minerals still contributed to the lowering of the ash fusion temperatures under the gasification atmosphere. The trend of changes to the main mineral phases under both combustion and gasification atmospheres at 1000 °C are shown in Fig. 10. At this higher temperature, pressure led to an increase in the silicate and aluminosilicate minerals present, thereby affecting the melting temperature causing an increase in the ash fusion temperature in combustion due to more mullite and a decrease in the ash fusion temperature in gasification atmosphere. The FSEM–EDS imaging was applied to the ash samples from gasification atmosphere at 1000 °C at 0.1 MPa and 2.0 MPa, respectively, to further elucidate the ash fusion mechanism. Typical FSEM images are presented in Fig. 11 and it could be seen that more fine nonspherical particles [15] were observed along with the pressure. This indicates that these nonspherical particles are not fully molten. The variations in mineralogy and textural of the ash samples from the gasification atmosphere at 1000 °C and at 0.1 MPa and 2.0 MPa were examined using FSEM imaging and EDS analysis,
(a) 0.1 MPa
(b) 0.1 MPa
and the results are presented in Figs. 12 and 13. The nonspherical aluminosilicate minerals rich in Fe, K, and Na are highlighted in Fig. 12a–c, and the crystallized phase of quartz is featured in Fig. 12d. It is these Fe, K and Na rich aluminosilicate minerals that can react with other oxides and/or minerals to form low temperature eutectics [21], reducing the ash fusion temperatures in the reducing gasification environments. At the higher pressure of 2.0 MPa (Fig. 13), some aluminosilicate minerals with Ca, K and Na were detected in the ash samples. Some nonspherical Fe-rich and Ca-rich minerals (Fig. 13b and c) were also observed. These iron and calcium-rich minerals could result in the formation of low melting silicate and/or aluminosilicates [8] in a gasification atmosphere, whereas there were no Ferich minerals detected in XRD patterns. This observation suggests the Fe-rich minerals melts into the other silicate and/or aluminosilicate as the pressure increased. It can be therefore concluded that more Fe and Ca containing minerals can melt and agglomerate together with the increasing pressure in reducing gas environment and the effect of pressure on the fusibility is related not only to the atmospheres, but also to the temperature. Under the combustion atmosphere at 900 °C, the decompositions of fluxing minerals such as muscovite, anhydrite and K-feldspar are suppressed with increasing pressure, reducing the ash fusion temperatures. At 1000 °C, however, no new minerals can be present at high pressures, however, the peak
(c) 0.1 MPa
Fig. 12. FSEM images of ash samples of 1000 °C from the gasification atmosphere at 0.1 MPa.
(d) 0.1 MPa
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(a) 2.0 MPa
(b) 2.0 MPa
(c) 2.0 MPa
(d) 2.0 MPa
Fig. 13. FSEM images of ash samples from the gasification atmosphere at 1000 °C and 2.0 MPa.
intensities of the high temperature minerals increase and thus the fusion temperature also increase. On the other hand, in the gasification atmosphere at 900 °C, the fluxing minerals such as muscovite, anhydrite and oldhamite can still be present. Furthermore, the feldspar minerals such as sanidine appear with rising pressure. The fluxing minerals and feldspar minerals present in Jincheng coal ash will reduce the fusion temperatures as pressure is increased. On the other hand, at 1000 °C, the intensities of the mullite and sanidine phases are observed to increase slightly with rising pressure. This suggests that the lowtemperature fluxing minerals will transform into high-temperature minerals at high temperature, even in reducing environment. However, the presence of the fluxing minerals and the melting of iron-containing minerals may result in the lowering of the fusion temperatures. 4. Conclusions The effect of temperature on the ash fusion characteristics is strongly dependent on the atmospheres. In combustion atmospheres, more high temperature minerals such as mullite are formed and dominate, leading to increases in the fusion temperatures. In gasification atmospheres, iron-bearing minerals and feldspar minerals, such as hercynite and anorthite, become abundant and react with other minerals to form low-temperature eutectics, thus decreasing the fusion temperatures. The pressure influences the mineral transformation by affecting the reactions between minerals, which also depend on the gas environment. Increasing pressure accelerates the formation of high-temperature minerals such as mullite and sanidine and the melting of iron-bearing minerals such as hematite. At the same time, the decomposition of the low-temperature minerals like muscovite and anhydrite is suppressed. In general, the presence of mullite increases the fusion temperatures, while the iron-based minerals and feldspar minerals react with other minerals to form the low temperature eutectics, decreasing the fusion temperatures. The degree of the influence of the high temperature minerals under combustion conditions is stronger than that under gasification conditions. Therefore, the effect of mullite dominates in the combustion atmosphere, while the effect of the iron-bearing minerals and feldspar minerals dominate in the gasification atmosphere. These observations have been confirmed by both FSEM–EDS and XRD analyses. The effects of temperature and pressure on the coal ash fusion characteristics are very complicated issues and to understand their combined effect has to consider the nature of the atmospheres the ash is subjected to. Acknowledgements The authors acknowledge the financial and other supports provided by the U.S. – China Clean Energy Research Center (Project No. 2010DFA24580-202), the New Century Excellent Talents in
Universities of China Program (Project No. NCET-09-0696) and the Introducing Discipline Talents of to Universities of China Program (Project No. B08026). Dongke Zhang also thanks Chinese Academy of Sciences for providing funding under the Outstanding Overseas Chinese Talent Funds Scheme, Chinese Government for the China’s One Thousand Chinese Talents Award, and the Australian Research Council (ARC) for partial funding under the ARC Linkage Project Scheme (Project No. ARC LP100200135) and the ARC Discovery Project Scheme (Project No. ARC DP110103699).
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Fuel journal homepage: www.elsevier.com/locate/fuel
Effect of temperature and pressure on the mineralogical and fusion characteristics of Jincheng coal ash in simulated combustion and gasification environments Nijie Jing a,b, Qinhui Wang a,⇑, Leming Cheng a, Zhongyang Luo a, Kefa Cen a, Dongke Zhang b,⇑ a b
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38, Hangzhou, Zhejiang 310027, China Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
h i g h l i g h t s " Temperature effect on ash fusion characteristics strongly depends on atmospheres. " High temperature minerals form in combustion, increasing ash fusion temperatures. " Low-melting eutectics form in gasification, decreasing the ash fusion temperatures. " Pressure influences mineral transformation by affecting reactions between minerals. " Increasing pressure accelerates the formation of high-temperature minerals.
a r t i c l e
i n f o
Article history: Received 27 December 2011 Received in revised form 19 May 2012 Accepted 23 May 2012 Available online 6 June 2012 Keywords: Ash fusibility HPTGA XRD analysis FSEM–EDS analyses Low-temperature eutectics
a b s t r a c t The effects of the pressure and temperature on the fusibility of coal ash during combustion and gasification were investigated. Experimentation was conducted using a high pressure thermogravimetric analyzer (HPTGA) apparatus. In order to observe the conversion of minerals with changing temperature and pressure in different atmospheres, the resulting ash samples were analyzed using an X-ray diffractometry (XRD) analyzer. In addition, the quantitative XRD analyses of ash samples and a field emission scanning electron microscope (FSEM), together with X-ray energy dispersive spectroscopy (EDS) were employed to verify the detailed mechanisms of ash fusion. The results indicated that temperature and type of atmosphere had a dominant effect on the ash fusion characteristics while the effect of pressure was somewhat more complicated, depending on the temperature and atmosphere being experienced by the ash. The high-temperature minerals such as mullite were formed with increasing temperature under both combustion and gasification atmospheres. However, in gasification atmosphere, there were more fluxing minerals and feldspar minerals present, such as muscovite, anhydrite and K-feldspar, decreasing the fusion temperature. The effect of pressure on the ash fusibility showed different behavior at different temperatures. At 900 °C, the decompositions of low-temperature minerals, such as muscovite, anhydrite and oldhamite, were suppressed with increasing pressure, resulting in a decrease in the fusion temperatures. On the other hand, at 1000 °C, the low-temperature minerals transformed into hightemperature minerals such as mullite and sanidine with rising pressure. However, the presence of fluxing minerals and the melting of iron-containing minerals resulted in the lowering of the fusion temperatures. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction Fluidized bed combustion and gasification are widely considered to be most promising technologies for conversion and utilization of solid fuels such as coal with advantages of fuel flexibility, lower power production costs and thus reduced greenhouse gas emissions. However, they have several operational challenges due ⇑ Corresponding authors. Tel.: +86 571 87952802; fax: +86 571 87951616 (Q.H. Wang), tel.: +61 8 6488 7600; fax: +61 8 6488 7235 (D.K. Zhang). E-mail addresses: [email protected] (Q. Wang), [email protected] (D. Zhang).
to the behavior and transformation of minerals and inorganic matter in these fuels, which can cause problems such as bed agglomeration, deposition on gas circuits and heat exchange surfaces, and bridging on hot gas filtration systems as a result of ash build-up. These problems can cause reduced operability, thermal efficiency and in the extreme cases, unplanned outage of the plant [1,2]. The ash characteristics are responsible for a series of problems related to coal use in fluidized bed reactor systems [3,4]. The ash fusibility is an important parameter in determining the behavior of coal ash in fluidized bed combustion and gasification of coal as well as their ash utilization [5,6]. Hence, a scientific understanding of the fusion behaviors of coal ash is the basis for effective and clean utilization of
0016-2361/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.05.040
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coal. In addition, in fluidized bed combustion and gasification, the mineral matter in coal gives detailed information on the fusion characteristics of coal ash [7–10]. Therefore, the study of transformation of mineral matter in coal is crucial to the determination of the ash fusion characteristics. The relationships between ash fusibility and the chemical composition of coal minerals have been studied in the literature using a number of experimental techniques including fusion tests, X-ray diffractometry, light microscopy, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy analyzer (EDX), differential–thermal and thermogravimetric analyzers and chemical analyses [10–15]. To date, however, limited work has been conducted on the fusion characteristics of coal ash with regard to mineral transformation under high pressure combustion and gasification conditions. The present work was aimed to examine the relationship between ash fusibility and mineralogical transformation of a Chinese anthracite, Jincheng coal, under high pressure combustion and gasification atmospheres simulated using a high pressure thermogravimetric analyzer, aided with X-ray diffraction (XRD) analyses and FSEM– EDS analyses to investigate coal ash fusion characteristics.
Table 2 Ash fusion temperatures of Jingcheng coal ash. Ash fusion temperatures (°C)
Reducing atmosphere Oxidizing atmosphere
DT
ST
HT
FT
1349 1401
1409 1449
1421 1457
1438 1466
800–1090 °C and at different pressures of 0.1, 0.5, 1.0, 1.5 and 2.0 MPa, from which different ash samples were retrieved for further characterization. A schematic diagram of the HPTGA system is shown in Fig. 1. The gas compositions of the simulated combustion and gasification atmospheres used in the HPTGA experiments are shown in Table 3. During a typical experiment, around 500 mg of an ash sample was heated from room temperature to a preset final temperature at a rate of 20 °C min 1 in a desired reaction atmosphere at a given pressure in the HPTGA. The sample was maintained at the final temperature for 30 min. The ash sample was then cooled down to room temperature and retrieved and ground to a particle size fraction less than 74 lm in the agate mortar for further analysis. The ash samples prepared under the different conditions described above were subjected to mineralogical characterization using a Rigaku D/Max-2550PC X-ray diffractometer using copper Ka radiation (neoConfucianism Company, Japan). The qualitative powder XRD analysis employed an accelerating voltage was of 40 kV, a current of 250 mA and a scan rate of 10 deg min 1 between 2h = 5° and 85°. The relative intensities of main characteristic XRD peaks for major mineral phases were measured and recorded. This method assumes, qualitatively, that the ratios of the peak heights in the XRD patterns are proportional to the concentrations of the minerals in a sample [11,17,18]. Quantitative XRD analyses of the ash samples prepared under the simulated gasification atmosphere at 900 °C and different pressures were also performed. The morphological characteristics and the corresponding chemical compositions of the ash samples prepared under the simulated gasification atmosphere at 1000 °C and at pressures of 0.1 MPa and 2.0 MPa were also studied using a FEI – SIRION-100 field emission SEM equipped with an EDAX GENESIS4000 X-ray energy dispersive spectroscopy (EDS). The EDS is allowed for qualitative element identification and semiquantitative analysis of ash samples. The EDS analyses were performed on the ash samples by operating the FSEM at an accelerating voltage of 25 kV. The intensity ratios of various constituent elements present on the EDS spectra were also used to identify the chemical composition of some partially fused particles in the ash.
2. Experimental 2.1. Preparation of ash samples and determination of ash fusion temperatures A Chinese anthracite from Shanxi Province, Jincheng coal, was chosen for this study. The proximate and ultimate analysis data as well as the ash chemistry are presented in Table 1. The ash samples were prepared according to the Chinese Standard GB/T2122001, in which coal sample was heated to 540 °C in air in a muffle furnace for ashing slowly. The residue was heated to 820 °C in air and kept for 2 h. Finally, the resultant ash sample was cooled down to room temperature and then ground to a particle size less than 200 lm in a mortar. The fusion temperatures of the coal ash were determined using a 5E-AFIII auto-analyzer (Changsha, China) under reducing and oxidizing atmospheres according to Chinese Standard GB219-74. Air was used as the oxidizing atmosphere. The reducing atmosphere was gained by incomplete combustion of graphite and active carbon in a ceramic ark within the 5E-AFIII auto-analyzer during the heating of the ash cones. There are four characteristic temperatures, deformation temperature (DT), soft temperature (ST), hemisphere temperature (HT) and flow temperature (FT), for each cone [16]. The results, averaged from five measurements for each sample, are presented in Table 2.
3. Results and discussion
2.2. Experimental setup and characterisations of mineralogy and morphology
3.1. Effect of temperature on the mineralogy in different atmospheres A high pressure thermogravimetric analyzer (HPTGA), Thermax 500 (USA), was employed to simulate the conditions of the combustion and gasification of Jincheng coal in the temperature range
Fig. 2 presents the XRD patterns of the ash samples from the combustion atmosphere at 0.5 MPa and temperatures between
Table 1 Proximate and ultimate analysis and ash composition of Jincheng coal. Proximate analysis (wt%) (air dried basis)
Ultimate analysis (wt%) (air dried basis)
M
V
A
FC
C
H
O
N
S
2.23
8.46
20.1
69.21
65.81
3.25
6.7
0.94
0.95
Composition (wt%) SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
SO3
48.1
31.84
5.9
4.96
0.98
1.14
1.09
3.07
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Fig. 1. A schematic diagram of the HPTGA system.
Table 3 Gas composition of the simulated atmospheres. Atmosphere
Gas composition (%) (vol.)
Combustion Gasification
H2
O2
CO
CH4
CO2
N2
– 40.8
2.1 –
– 36.5
– 1.9
49.7 20.8
48.2 –
800 and 1090 °C, showing the variations in mineral phases with increasing temperature. The main mineral phases found in the ash at 800 and 900 °C were quartz, muscovite (KAl2(AlSi3)O10 (OH)2), anhydrite (CaSO4) and K-feldspar (KAlSi3O8). When the temperature was raised to 1000 °C, the K-feldspar phase disappeared and the intensities of the peaks of anhydrite and muscovite [19] also decreased slightly. In the meantime, new peaks, believed to be hematite and mullite (Al6Si2O13) [20,21] appeared. This indicated that at the high temperature, muscovite began to transform into mullite. At 1090 °C, the sanidine ((K,Na)(Si,Al)4O8) phase also
Q-Quartz M-Mullite Ah-Anhydrite K-K-feldspar Sa-Sanidine He-Hematite Mus-Muscovite
Q
Q M
Ah
He S a S aM H e
0
20
Q
Q
M
An
M QH eQ H e M H e Q
Q Ah
M us K M us M Q Ah K M us M us Q Ah M us M us K
appeared, which is a more stable structure and high temperature (>900 °C) form of the K-feldspar [22]. The feldspar minerals such as sanidine are capable of reacting with other minerals to form low temperature eutectics [21,9], which can reduce the melting temperature of the ash; on the other hand, the high-temperature minerals such as mullite can increase the melting temperature under the combustion atmosphere. Fig. 3 shows that the quartz, anhydrite, muscovite and sanidine ((K,Na)(Si,Al)4O8) phases are presented in the ash samples from the gasification atmosphere at 0.5 MPa and at 800 and 900 °C, respectively. However, at 1000 °C, the peak of muscovite disappeared and that of sanidine decreased. In the meantime, the peaks of hematite and mullite appeared. As the temperature was further increased to 1090 °C, the sanidine and hematite phases completely disappeared while the anorthite and hercynite (FeAl2O4) were observed. This indicates that the transformations from sanidine to anorthite and from the hematite to hercynite have occurred at the high temperatures. Anorthite and hercynite are also high-temperature minerals and they are able of reacting with other minerals to produce
A h H e A hM
AhM us Ah
Q M u s M u s MM u s Q
Q M u sQ M u s Q M u s Q
M
A h M u sA h
40
60
Ah Sa Sa He M
M us
M us
80
Sa
M us
Q M us Q
Ah Sa
M
1090
Sa
100
Fig. 2. XRD patterns of ash samples from the combustion atmosphere at 0.5 MPa and at different temperatures.
0
20
Ah
QM M Q
M usQ
M
1000
Q M us M us
900
Sa
M us Q Q M us
Sa
800
Two-Theta (deg)
QH e
Ah Sa M us Q Sa
Q
900 Q
M H Q An Q M H
M
M us Q
1000
M us
Q M iQ M u s Q M u s Q
M
M
Q
K
M An H M M H M
Q
1090 Q
Q-Quartz,Sa-Sanidine M-Mullite,Ah-Anhydrite He-Hematite,H-Hercynite An-Anorthite Mus-Muscovite
40
Q M us
Q
Q M us
60
M us
800
80
100
Two-Theta (deg) Fig. 3. XRD patterns of ash samples from the gasification atmosphere at 0.5 MPa and at different temperatures.
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low-temperature eutectics under reducing gas environments, leading to decreasing fusion temperatures of the ash. Hence the fusion temperatures in the gasification atmosphere are lower than those in the combustion atmosphere, as shown in Table 2 above, which is consistent with the discussion above. The trends of changes to the main mineral phases based on the XRD intensities of the minerals under the combustion and gasification atmospheres, respectively, as a function of temperature at 0.5 MPa, are shown in Fig. 4. From Fig. 4a and b, significant variations in the XRD intensities of muscovite, K-felspar, sanidine, hercynite, anorthite and mullite with increasing temperature are evident. Note that muscovite and K-felspar are fluxing minerals [23,24] and the mullite, hercynite, anorthite and sanidine are high temperature minerals. The fluxing minerals melted gradually with increasing temperature and transformed to the high temperature minerals.
Q
M us M us
Q-Quartz Ah-Anhydrite K-K-feldspar He-Hematite Mus-Muscovite
Q
Ah K K Ah He Ah Q M us H e Q H e Q Ah M us Ah H e Ah M usQ H e Q H e Q Q Ah M us AhH e Ah M us Q H e Q He Q QAh A h M u sA h M u s Q M us M i Q M usQ QAh K K M u s Q Q M us A h Ah M us M i M u sQ M us
Q
Q
M us
M us M us M us
0
20
40
2.0 MPa
Q
1.5 MPa Q
1.0 MPa Q
0.5 MPa Q
60
0.1 MPa
80
100
Two-Theta (deg) 3.2. Effect of pressure on the mineralogy in different atmospheres Fig. 5 presents the XRD patterns of ash samples from the combustion atmosphere 900 °C with varying pressure from 0.1 MPa to 2.0 MPa. The main mineral phases observed in the ash samples were quartz, muscovite, anhydrite and K-feldspar at 0.1 and 0.5 MPa. At 1.0 MPa, however, the peak of hematite appeared and, as the pressure was further increased, the low-temperature minerals such as anhydrite was observed to increase slightly. In addition, it can be seen that the peak of muscovite becomes slightly sharper as pressure rises. This can be explained by the fact that the pressure affects the geochemical transformation of minerals [25] and restrains the low-temperature minerals from decomposition. It is also apparent from Fig. 5 that the changes in the main minerals phases in the ash with pressure in the combustion atmosphere are not very significant.
Dramatically different mineralogical behavior of the ash samples from the gasification atmosphere was observed. Fig. 6 shows the XRD patterns of the ash samples at 900 °C with varying pressure from 0.1 MPa to 2.0 MPa. It is worth noting that oldhamite (CaS) was identified to present at low pressures, in addition to quartz, muscovite, anhydrite. As pressure was raised, the oldhamite and anhydrite phases disappeared and the peak of sanidine appeared at 0.5 MPa and the peak of mullite appeared at 1.5 MPa. Interestingly, at 2.0 MPa, the anhydrite phase appeared again. This indicates that the oldhamite might have been converted to anhydrite. Notwithstanding the fact that the presence of mullite would increase the ash fusion temperatures, the intensity of mullite peak
4000 3000 2000 1000 0
Hematite
Hercynite
4000 3000 2000 1000 0
Mullite
3000 1500 0
4000 3000 2000 1000 0
Mullite
Sanidine
4500 3000 1500 0
Sanidine
6000 4000 2000 0
6000 4000 2000 0
K-feldspar
4000 3000 2000 1000 0
Intensity / CPS
Intensity / CPS
4000 3000 2000 1000 0
Fig. 5. XRD patterns of ash samples from the combustion atmosphere at 900 °C and at different pressures.
2400 1600 800 0
Anhydrite
4000 3000 2000 1000 0
12000 9000 6000 3000
Quartz
12500 10000 7500 5000
6000 4000 2000
Muscovite
6000 4000 2000 0
800
850
900
950
1000
1050
Temperature/
(a) Combustion atmosphere
1100
Hematite
Anorthite Anhydrite
Quartz
Muscovite 800
850
900
950
1000
1050
Temperature/
(b) Gasification atmosphere
Fig. 4. Changes to the main mineral phases of the ash samples as a function of temperature at 0.5 MPa.
1100
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Table 4 Contents of main mineral phases in ash samples from gasification atmosphere at 900 °C.
Q-Quartz Ah-Anhydrite M-Mullite Ol-Oldhamite Mus-Muscovite Sa-Sanidine
Q
Q M us Ah Q S a S aM u s M M Q Q M M us M us M Q Q Q M us Sa S a M M us M M u s S a MM u s Q M u s M Q M Q Q M Q Sa O lM u s Q M u s M us Ol Q Ol Q Q Ol Q Ol Q M us Ah S a Q M us M us M us Q Q Sa S a Q M u sQ M u s Q Q Ol M us M us Ah M us Q O l Q M us Q M us Q O l Q O l M us M
0
20
40
60
2.0 MPa 1.5 MPa
0.1 MPa
1.0 MPa
2.0 MPa
Quartz Muscovite Mullite Anhydrite Oldhamite Sanidine
29.5 56.1 6.3 1.2 0.9 5.9
31.0 57.4 1.5 0.8 0.9 8.4
31.8 37.8 21.0 1.4 0.5 7.5
1.0 MPa 0.5 MPa 0.1 MPa
80
100
Two-Theta (deg) Fig. 6. XRD patterns of ash samples from the gasification atmosphere at 900 °C and at different pressures.
was weakened in the reducing atmosphere and therefore, in the presence of the fluxing minerals such as muscovite and anhydrite the ash fusion temperatures would be significantly reduced, as observed from the results in Table 2. It is also very interesting to note that, under gasification atmosphere the sanidine phase increased with pressure from 0.1 MPa to 1.0 MPa, but weakened as the pressure was further increased to 2.0 MPa. The muscovite phase showed a similar trend. To understand this phenomenon, quantitative XRD analysis was performed with the crystalline minerals and their relative contents presented in Table 4. It can be seen that the contents of muscovite and
2000 1500 1000 500 0
Mineral phase (%)
sanidine phase were increased while that of mullite decreased from 0.1 MPa to 1.0 MPa. Both the increase in the low melting point minerals and the decrease in the high temperature minerals will lead to a reduction in the ash fusion temperature in the gasification atmosphere. On the other hand, the muscovite and sanidine phase decreased whilst the mullite phase increased as the pressure was increased to 2.0 MPa. This finding suggests that the pressure affects the mineral transformation more dramatically in the gasification environment than in the combustion environment, although the influence extent decreases slightly with an increase in pressure above 1.0 MPa. This is because that, as Monson et al. [26] and Liua et al. [27] reported, increasing the pressure can lead to an increase in the rate of mineralogical reactions but the rate of the rate increase decreases with further increases in pressure [26,27]. Therefore, the fusion temperatures of ash in reducing environment would be most influenced by pressure below 1.0 MPa. The trends of changes to the main mineral phases under both combustion and gasification atmospheres at 900 °C are shown in Fig. 7. It can be seen that the mineral phases under the gasification
2400 1600 800 0
Hematite
Mullite
2400 1800 1200 600 0
2400
K-feldspar 2100
Sanidine
3300
Intensity / CPS
Intensity / CPS
3000 1800
Anhydrite
3000 2700 6000
2000
Oldhamite
1000 0 3000
Anhydrite
2000 1000 0
5500 5000
6000 5000 4000 3000
Muscovite
4500 4000
Muscovite
13500 13000 12000
Quartz
12000
Quartz
11000
10500
10000 0.0
0.5
1.0
1.5
2.0
2.5
0.0
0.5
1.0
1.5
Pressure / MPa
Pressure / MPa
(a) Combustion atmosphere
(b) Gasification atmosphere
Fig. 7. Changes to the main mineral phases of the ash samples as a function of pressure at 900 °C.
2.0
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Q
Q
Q-Quartz Ah-Anhydrite M-Mullite He-Hematite Mus-Muscovite
Q Ah
He Ah M us M M M us H e Q H e M H e Q M us Q Ah M us A hH e M u s A h H e Q H e M H e Q M us M M us Q M us Ah Q A hH e M u s A h Q M He He M He M us M us Q M us Ah He Ah M us Ah Q Q M us M M He H e Q M us He Q Ah M us A h H eM u s Ah H eQ H e M QH eQ M us M us M
0
20
Q Q Q
Q Q
60
40
M
Q Ah M Sa Sa
M
Q Ah Sa Sa M M
M
Q Ah Sa Sa M Q
2.0 MPa Sa
1.5 MPa Sa
1.0 MPa
Sa
0.5 MPa
80
Ah
M
Sa Q
M
Sa
Ah
Sa
0.1 MPa
100
0
Q-Quartz M-Mullite Sa-Sanidine Ah-Anhydrite He-Hematite M
M
A h Q M A hQ M M Q
M M
S aM H e
M
Sa M M
MQ
20
M
Q Ah Q M Ah M M Q
M
Sa
Q
M
QM M Q
Sa
M
QM M Q
Sa
2.0 MPa 1.5 MPa 1.0 MPa 0.5 MPa
Ah
40
Two-Theta (deg)
M
Q MAh Q M M Q
Ah
Q
60
0.1 MPa
80
100
Two-Theta (deg)
Fig. 8. XRD patterns of ash samples from the combustion atmosphere at 1000 °C and at different pressures.
Fig. 9. XRD patterns of ash samples from the gasification atmosphere at 1000 °C and at different pressures.
atmosphere (Fig. 7b) as compared to those under the combustion atmosphere (Fig. 7a) changed more significantly in terms of both the types and intensities of the minerals with majority being fluxing minerals, and, as the fluxing minerals normally became more resistant to decomposition with increasing pressure, the low temperature minerals would be more readily restrained at high pressures. The combined effects of the above phenomena would reduce the ash fusion temperatures. Quartz, muscovite, anhydrite, mullite and hematite are the major mineral phases in ash samples from the combustion atmosphere at 1000 °C at 0.1 and 0.5 MPa as shown in Fig. 8. As pressure increased, the peak of muscovite weakened and the peak of mullite strengthened. This indicates that muscovite was transformed to
mullite with rise in pressure. The high-temperature minerals like mullite were formed with further increase in pressure, and thereby raising the ash fusion temperatures. It may be noted that muscovite was still detected at highest pressure, suggesting that the decomposition of muscovite was incompletely at 1000 °C within the time of the experiment. Fig. 9 shows the XRD patterns of the main mineral phases in ash samples from the gasification atmosphere at 1000 °C and at pressures from 0.1 MPa to 2.0 MPa, including quartz, mullite, anhydrite and sanidine ((K,Na)(Si,Al)4O8). The peak of hematite only appeared at 0.5 MPa and its absence at higher pressures suggests that hematite might have gradually melted into the glass phase forming a low temperature fluxing [24] with increasing pressure at 1000 °C.
2700 2400 2100 1800 1500
Mullite
Hematite
3300
Anhydrite
3000 2700 2400
Intensity / CPS
Intensity / CPS
2400 2100 1800 1500 1200
3600 3000 2400 1800 1200
Mullite
2000 1500 1000 500 0
Hematite
1600 1500 1400 1300 1200 1100
Anhydrite
4000
Muscovite
3000
3000
Sanidine
2400 2000
1800
1000 14000
1200 14000
Quartz
12000 10000
Quartz
12000 10000
8000
8000 0.0
0.5
1.0
1.5
Pressure / MPa
(a) Combustion atmosphere
2.0
0.0
0.5
1.0
1.5
Pressure / MPa
(b) Gasification atmosphere
Fig. 10. Changes to the main mineral phases of the ash samples as a function of pressure at 1000 °C.
2.0
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(a) 0.1 MPa
(b) 0.1 MPa
(e) 2.0 MPa
(f) 2.0 MPa
(c) 0.1 MPa
(g) 2.0 MPa
(d) 0.1 MPa
(h) 2.0 MPa
Fig. 11. FSEM images of the ash samples from the gasification atmosphere at 1000 °C at different pressure.
As the pressure increased, the quartz phase decreased in intensity while the sanidine and mullite increased slightly. It may be postulated that quartz reacted with other oxides to form sanidine and mullite at the high temperature under reducing environment. It follows that the low-temperature fluxing minerals may have begun to transform into high-temperature minerals. However, the presence of the fluxing minerals and the melting of the iron-containing minerals still contributed to the lowering of the ash fusion temperatures under the gasification atmosphere. The trend of changes to the main mineral phases under both combustion and gasification atmospheres at 1000 °C are shown in Fig. 10. At this higher temperature, pressure led to an increase in the silicate and aluminosilicate minerals present, thereby affecting the melting temperature causing an increase in the ash fusion temperature in combustion due to more mullite and a decrease in the ash fusion temperature in gasification atmosphere. The FSEM–EDS imaging was applied to the ash samples from gasification atmosphere at 1000 °C at 0.1 MPa and 2.0 MPa, respectively, to further elucidate the ash fusion mechanism. Typical FSEM images are presented in Fig. 11 and it could be seen that more fine nonspherical particles [15] were observed along with the pressure. This indicates that these nonspherical particles are not fully molten. The variations in mineralogy and textural of the ash samples from the gasification atmosphere at 1000 °C and at 0.1 MPa and 2.0 MPa were examined using FSEM imaging and EDS analysis,
(a) 0.1 MPa
(b) 0.1 MPa
and the results are presented in Figs. 12 and 13. The nonspherical aluminosilicate minerals rich in Fe, K, and Na are highlighted in Fig. 12a–c, and the crystallized phase of quartz is featured in Fig. 12d. It is these Fe, K and Na rich aluminosilicate minerals that can react with other oxides and/or minerals to form low temperature eutectics [21], reducing the ash fusion temperatures in the reducing gasification environments. At the higher pressure of 2.0 MPa (Fig. 13), some aluminosilicate minerals with Ca, K and Na were detected in the ash samples. Some nonspherical Fe-rich and Ca-rich minerals (Fig. 13b and c) were also observed. These iron and calcium-rich minerals could result in the formation of low melting silicate and/or aluminosilicates [8] in a gasification atmosphere, whereas there were no Ferich minerals detected in XRD patterns. This observation suggests the Fe-rich minerals melts into the other silicate and/or aluminosilicate as the pressure increased. It can be therefore concluded that more Fe and Ca containing minerals can melt and agglomerate together with the increasing pressure in reducing gas environment and the effect of pressure on the fusibility is related not only to the atmospheres, but also to the temperature. Under the combustion atmosphere at 900 °C, the decompositions of fluxing minerals such as muscovite, anhydrite and K-feldspar are suppressed with increasing pressure, reducing the ash fusion temperatures. At 1000 °C, however, no new minerals can be present at high pressures, however, the peak
(c) 0.1 MPa
Fig. 12. FSEM images of ash samples of 1000 °C from the gasification atmosphere at 0.1 MPa.
(d) 0.1 MPa
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(a) 2.0 MPa
(b) 2.0 MPa
(c) 2.0 MPa
(d) 2.0 MPa
Fig. 13. FSEM images of ash samples from the gasification atmosphere at 1000 °C and 2.0 MPa.
intensities of the high temperature minerals increase and thus the fusion temperature also increase. On the other hand, in the gasification atmosphere at 900 °C, the fluxing minerals such as muscovite, anhydrite and oldhamite can still be present. Furthermore, the feldspar minerals such as sanidine appear with rising pressure. The fluxing minerals and feldspar minerals present in Jincheng coal ash will reduce the fusion temperatures as pressure is increased. On the other hand, at 1000 °C, the intensities of the mullite and sanidine phases are observed to increase slightly with rising pressure. This suggests that the lowtemperature fluxing minerals will transform into high-temperature minerals at high temperature, even in reducing environment. However, the presence of the fluxing minerals and the melting of iron-containing minerals may result in the lowering of the fusion temperatures. 4. Conclusions The effect of temperature on the ash fusion characteristics is strongly dependent on the atmospheres. In combustion atmospheres, more high temperature minerals such as mullite are formed and dominate, leading to increases in the fusion temperatures. In gasification atmospheres, iron-bearing minerals and feldspar minerals, such as hercynite and anorthite, become abundant and react with other minerals to form low-temperature eutectics, thus decreasing the fusion temperatures. The pressure influences the mineral transformation by affecting the reactions between minerals, which also depend on the gas environment. Increasing pressure accelerates the formation of high-temperature minerals such as mullite and sanidine and the melting of iron-bearing minerals such as hematite. At the same time, the decomposition of the low-temperature minerals like muscovite and anhydrite is suppressed. In general, the presence of mullite increases the fusion temperatures, while the iron-based minerals and feldspar minerals react with other minerals to form the low temperature eutectics, decreasing the fusion temperatures. The degree of the influence of the high temperature minerals under combustion conditions is stronger than that under gasification conditions. Therefore, the effect of mullite dominates in the combustion atmosphere, while the effect of the iron-bearing minerals and feldspar minerals dominate in the gasification atmosphere. These observations have been confirmed by both FSEM–EDS and XRD analyses. The effects of temperature and pressure on the coal ash fusion characteristics are very complicated issues and to understand their combined effect has to consider the nature of the atmospheres the ash is subjected to. Acknowledgements The authors acknowledge the financial and other supports provided by the U.S. – China Clean Energy Research Center (Project No. 2010DFA24580-202), the New Century Excellent Talents in
Universities of China Program (Project No. NCET-09-0696) and the Introducing Discipline Talents of to Universities of China Program (Project No. B08026). Dongke Zhang also thanks Chinese Academy of Sciences for providing funding under the Outstanding Overseas Chinese Talent Funds Scheme, Chinese Government for the China’s One Thousand Chinese Talents Award, and the Australian Research Council (ARC) for partial funding under the ARC Linkage Project Scheme (Project No. ARC LP100200135) and the ARC Discovery Project Scheme (Project No. ARC DP110103699).
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