ARTICLE pubs.acs.org/EF
Effects of Mineral Matter and Coal Blending on Gasification Jin Bai,* Wen Li, and Zongqing Bai State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China ABSTRACT: Two Chinese coals were used for coal blending to study the effects of mineral matter and coal blending on gasification. The transformational behaviors of blending coals were studied with FactSage thermodynamic calculation. Most mineral matter was transformed into Ca aluminosilicates and mullite at high temperatures. High-temperature gasification was performed from 1100 to 1500 °C to examine the gasification behavior of blended chars. Considering the viscosity results of coal ash from blending coals, the melting ash was assumed to hinder the gasification reactions at high temperatures. The Fourier transform infrared (FTIR) spectrum indicated the existence of iron oxides, which was verified by FactSage, and iron oxides led to thermal carbon reactions that promoted the gasification reactions.
1. INTRODUCTION Coal is one of the most commonly used primary energy resources in China.1 In recent years, the mining depth of coal has increased because of the expanding demand for energy; therefore, the quantity of poor-quality coal has significantly increased.1 The ash content and ash-melting temperatures of coal increases with the mining depth; therefore, poor-quality coal is often underutilized.2 The most common method to improve the ash-melting behavior is through the addition of flux; however, flux, such as limestone, increases the ash content and heat loss. Conversely, coal blending is a feasible method for the use of poor-quality coal.3 Considering that the ash content of poor-quality coal is as high as 30-40%, the transformation behavior of mineral matter in the coal has an important influence on coal use, especially on coal gasification at high temperatures.4,5 With the development of the entrained-flow gasifier in the coal conversion field, high-temperature coal gasification is becoming more and more important. The operating temperature is usually above 1450 °C for entrained-flow gasification, which is far above the ash-melting temperature.6 Hence, the transformation of mineral matter at high temperatures is significant for blending coal gasification. Qiu et al. studied the relationship between mineral-matter transformation behavior and the coal-blending ratio. It was found that the transformation of mineral matter was predictable with a CaO-Al2O3-SiO2 ternary phase diagram.2 Fourier transform infrared (FTIR) spectroscopy was recognized to be useful for observing variations of Fe-O present in the coal ash after gasification.7,8 Some researchers have specified that the melting minerals obstruct coal gasification at high temperatures, while others have illustrated that the carbon thermal reaction between char and iron oxides promotes the gasification reaction.5,9 The effect of mineral matter at high temperatures is very complex, and detailed research is required to evaluate and explain the influence of mineral transformation on the gasification behavior of blending coals. In this study, high-temperature gasification of the blending chars prepared from two Chinese coals was investigated in a bench-scale fixed-bed reactor. The mineral transformation was also evaluated from 1100 to 1500 °C with FactSage. The ash viscosity of blending coals was analyzed with a high-temperature rotating viscometer to demonstrate the behavior of melting mineral matter. The activation energy was obtained with kinetic r 2011 American Chemical Society
Table 1. Proximate and Ultimate Analyses of Coal Samples A and B proximate analysis (wt %, ad)
ultimate analysis (wt %, daf)
sample
ash
volatile
fixed carbon
C
H
Stotal
N
A
16.2
30.6
51.0
62.0
6.1
0.4
1.5
B
16.4
31.3
51.8
62.3
4.8
0.9
0.4
models to evaluate the effects of mineral matter and coal blending on coal gasification.
2. EXPERIMENTAL SECTION 2.1. Coal Sources and Properties. Two typical Chinese coals were selected for this study. Coal sample A was a poor-quality coal with a high ash content and ash-melting temperature. Coal sample B was from the Yunnan province with a high content of calcium oxides and iron oxides, which was chosen for blending with coal A. The properties of coal samples A and B, including ash compositions and ash-melting temperatures, and the properties of the blending coals are shown in Tables 1-3 Blending coals with different mixing ratios were pyrolyzed at 950 and 1200 °C in a fluidized-bed reactor under a N2 atmosphere. The blending samples . with different ratios of A and B were denoted as A2B8 (20% A þ 80% B, by weight), A4B6 (40% A þ 60% B, by weight), A6B4 (60% A þ 40% B, by weight), and A8B2 (80% A þ 20% B, by weight). Ash samples were prepared at 815 °C in a muffle furnace. The de-ashed coal was prepared with HCl and HF solution according to GB/T7560-2001. The coal sample was treated in the HCl solution for 50 min at 55-60 °C and washed several times with distilled water. Then, the coal sample was treated in HF solution for 50 min at 55-60 °C and washed several times. The de-ashed sample was dried at around 50 °C. Ash-free coal was pyrolyzed at 950 °C in a fluidized-bed reactor, and char samples generated after pyrolysis were regarded as ash-free char. 2.2. Mineral Transformation. FactSage was used to investigate the behavior of coal ash at different temperatures under a reducing atmosphere. This thermodynamic model is able to evaluate the composition of mineral matter at high temperatures. The mineral matter in both solid and liquid phases can be identified. The chemical composition, including CaO, Al2O3, Received: November 10, 2010 Revised: January 21, 2011 Published: February 25, 2011 1127
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Table 2. Ash Compositions of Coal Samples A and B chemical composition of ashes (wt %)
a
sample
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
SO3
A
55.6
33.5
3.8
2.2
0.7
1.4
0.5
0.5
A8B2a
51.6
30.4
5.7
8.4
1.3
1.4
0.7
0.6
A6B4a
46.3
26.3
7.6
15.0
2.0
1.3
0.9
0.6
A4B6a
40.6
22.0
9.6
22.1
2.7
1.2
1.2
0.7
A2B8a
34.4
17.4
11.8
29.7
3.5
1.1
1.5
0.7
B
25.3
11.3
12.9
34.6
3.9
0.9
1.6
0.7
By calculation.
Table 3. Ash-Melting Temperaturesa ash-fusion temperature (°C) sample
DT*
ST*
FT*
A
1250
1510
1600
B2A8 B4A6
1250 1380
1500 1411
1550 1442
Figure 1. Mineral matters in B2A8 at different temperatures.
B6A4
1241
1251
1263
B8A2
1136
1159
1178
B
1125
1210
1240
was controlled from 1100 to 1150 °C at 25 °C/min. The procedure was repeated until the gasification at 1500 °C was finished. The CO2 reduction ratio was used to evaluate the char reactivity by gasification for 3 min at high temperature. The CO2 reduction ratio X was calculated by the following equation: X = (100 - v)/(100 þ v) 100, where v is the content of CO2 in the outlet gas after gasification.
DT*, deformation temperature; ST*, soften temperature; FT*, fluid temperature. a
SiO2, and Fe2O3, was input into the model for multi-phase equilibrium prediction. The reducing atmosphere of 60% CO and 40% CO2 was employed in the FactSage calculation. The temperatures were set from 1100 to 1500 °C at intervals of 50 °C. The pressure for calculation was 0.1 MPa. The FactOxide and Solid Soulution databases in the FactSage database were selected for calculation. A Nicolet NEXUS 470 FTIR was used for mineral analysis from 4000 to 400 cm-1. 2.3. Analysis of Viscosity. The viscosity of ash for blending coals was analyzed with a Theta high-temperature rotating viscometer under a reducing atmosphere of 60% CO and 40% CO2. The viscosity was recorded continuously, and a viscosity-temperature curve was generated.10 The molybdenum rotors and cylinder crucibles were used for tests at high temperatures, and after tests, the sample was analyzed for Mo content. The parameters for the rotor crucible combination had previously been found from the measurement of the standard reference material 717A glass. The sample temperature was recorded using a type-B platinum thermocouple in an alumina pedestal, supporting the graphite crucible holder and corrected using a previously determined temperature calibration experiment with a thermocouple inside the crucible filled with magnesium oxide. 2.4. Char Gasification at High Temperatures. The gasification method is designed according to the Chinese national standard GB/T 2202001, which is widely used for evaluated the reactivity of coal or char under CO2. The CO2 gasification of chars, including raw chars and ash-free chars, was performed in a bench-scale fixed-bed reactor at 50 °C intervals from 1100 to 1500 °C. The length of the constant temperature area was about 4 cm. The inner diameter of the inner tube was 4 cm. The excessive char, 8 g, was used for gasification. When the sample was placed into the reactor, the air in the reactor was removed by argon blowing. The gasification agent was a mixture of 60% CO2 and 40% Ar. The flow rate was 200 cm3 min-1 to reduce the influence of internal and external diffusions. When the temperature reached 1100 °C with 10 °C/min, the mixture gas was purged into the reactor at 200 cm3 min-1 and stayed for 3 min. The outlet gas gathering by gas chromatography (GC) was performed at the last 10 s of 3 min. After the gas was gathered, the purge of mixture gas was suspended and the temperature
3. RESULTS AND DISCUSSION 3.1. Mineral Transformation of Blending Coals. At high temperatures, clays decompose to form calcium oxides and alumina. In Figure 1, the major components in high-temperature ash of B2A8 were mullite (3Al2O3 3 2SiO2), SiO2, and anorthite (CaAl2Si2O8), which was close to the ash composition of coal A. However, an increase of calcium led to the formation of anorthite by adding coal B. The content of mullite was reduced because of the formation of anorthite. The melting temperature of B2A8 was lower because of the decrease of mullite, and the melting temperature of mullite is over 1800 °C. The decrease of mullite above 1300 °C is due to slag formation. As the heating temperature increased from 1100 to 1500 °C, the content of SiO2 decreased. SiO2 was consumed in the reactions between mullite and anorthite. When the temperature was above 1400 °C, most mineral matter turned into the liquid phase, except mullite. The following reactions were postulated to occur:11
NaAl3 Si3 O11 f Na2 O þ Al2 O3 3 2SiO2 Al2 O3 3 2SiO2 f 2SiO2 þ Al2 O3 CaO þ SiO2 þ Al2 O3 f CaAl2 Si2 O8 SiO2 þ Al2 O3 f 3Al2 O3 3 2SiO2 The content of CaAl2Si2O8 increased at first and then disappeared when the temperature is over 1350 °C. At around 1300 °C, the major components were anorthite and mullite, which greatly increased in concentration at elevated temperatures. The content of iron oxides decreased gradually with increasing temperature because of the formation of iron aluminosilicates and iron silicates. When the 1128
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Figure 2. Mineral matters in B8A2 at different temperatures.
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Figure 4. Mineral matters in B4A6 at different temperatures.
Figure 5. Content of iron oxide in different blending coal ashes.
Figure 3. Mineral matters in B6A4 at different temperatures.
temperature was over 1300 °C, the iron oxides left turned into liquid to form A-slag-liquid based on the FactSage model.12 Fe2 O3 þ Al2 O3 þ SiO2 f Fe2 Al4 Si5 O18 Fe2 O3 þ CaCO3 þ SiO2 f CaFeSi2 O6 The mineral transformations of A2B8, A4B6, and A6B4 are illustrated in Figures 2-4. The major components for blending coal were similar; therefore, the reactions and transformation are not discussed. As mentioned above, most mineral matter was transformed into silicates and aluminosilicates, which did not affect the gasification reaction, except for the iron oxides. Iron oxides have been proven to promote the gasification reaction with thermal carbon at high temperatures.9 The content of iron oxides in both the solid and liquid phases is shown in Figure 5. The proportion of iron oxides in blending coal ashes was 1:2.6:4:4.7, and it is obvious that the proportion increased with adding coal B. The ratios between iron oxides in the solid and liquid
phases were calculated, and they were 1.1, 0.9, 1.7, and 0.9, respectively. Although the iron oxide in B8A2 was highest, the highest ratio of iron oxides between the solid and liquid phases was B6A2. Because no other iron silicate in solid was found, all iron in solid was in iron oxides and was regarded as reactive with char. The distribution of iron in the solid and liquid phases is related to the structure of aluminosilicates at high temperatures. When the structure of aluminosilicate needs more modifier former, the iron goes into the structure to fill up the empty. Otherwise, iron oxides existed stably. 3.2. Char Gasification Characteristics. As shown in Figure 6, the initial gasification reactivity was improved by adding coal B. The conversion of carbon dioxide increased with the ratio of coal B because of the high reactivity of coal B. The variation trend for gasification reactivity of different samples was generally similar. The reactivity of samples increased at first and decreased with reaction temperatures. Below 1300 °C, the rate of coal gasification was controlled by the reaction between coal and gasification agents. The increasing temperature accelerates the reaction; therefore, the conversion of coal increased at first. The decrease was not able to be explained by the reactivity of organic matter in the chars because the gasification reaction at high temperatures is influenced by diffusion. The decrease should be caused by the melting ash covering the surface of the coal 1129
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Figure 6. Char reactivity with different blending ratios.
Figure 7. Viscosity-temperature curve for raw coal and blending coals.
char. However, the influences by melting ash were different because the maxima values of gasification reactivity were very different. The temperature order of the maxima value was B, B8A2, B4A6, B2A8, B6A4, and A. The sequence was coordinated with the ratio of coal B, except B6A4. The contact of liquid and solid may be evaluated by surface tension, and the surface tension is in direct proportion with the viscosity of liquid. The status of the melting mineral matter at high temperatures could be described by the viscosity of the ash. To explain the order of the maxima value of gasification reactivity, the ash viscosity tests of four samples were performed. The viscosity-temperature curve is shown in Figure 7. It was found that 20 Pa s represented a critical level for melting the mineral matter. The temperatures corresponding to 20 Pa s for the blending coals are listed in Figure 7. When the viscosity of the slag was about 20 Pa s, it was possible for the melting ash to extend on the surface of the coal chars.13,14 When the viscosity was above 20 Pa s, the melting ash displayed low fluidity and tended to agglomerate into spheres,
Figure 8. FTIR for residual ashes from gasification of 950 °C char.
which did not influence the char surface. In summary, the melting mineral matter influenced the surface of the chars at temperatures 1130
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Energy & Fuels according to approximately 20 Pa s. In comparison to the temperature maxima value of gasification reactivity (temperatures marked in Figure 6), it was found that those temperatures almost corresponded to 20 Pa s of the blending coal ash, except for B6A4. The melting ash exhibited the least influence on B6A4; therefore, the ash fluidity at high temperatures was not the reason for the unexpected performance of B6A4. The gasification reactivity of ash-free B6A4 was evaluated under the same conditions. The difference between B6A4 and ash-free B6A4 was not obvious, which indicated that melted ash did not affect the gasification reactivity or the effect was controlled. Considering the transformation of mineral matters, the exception was attributed to the content of iron oxides. Iron oxides are able to hinder the covering of mineral matter on the surface of the chars through carbon thermal reactions. The content of iron oxide was extremely high in B6A4, in both the solid and liquid phases. Although the content of iron oxides was close to that of B6A4, the effects of the iron oxides seemed different for B8A2. The possible reason is that the content of calcium oxides in B8A2 is much higher. The ash-melting temperature for B8A2 was much lower than that of B6A4, which indicated that the ratio of the liquid phase of B8A2 was much higher. Therefore, there was a balance between the content of iron oxides and the liquid phase, which is the reason for the temperature order of the maxima value of gasification reactivity, as discussed above. To verify that the content of iron oxides was high in B6A4, the residue after gasification was separated and the ash was analyzed with FTIR, as shown in Figure 8. For B2A8-950, the peak at 912 cm-1 was assigned as Si-O-Al vibrations and peaks at 1090, 790, 550, and 460 cm-1 were assigned as mullite,7 which were quite strong. The content of mullite increased with elevated temperatures, which was confirmed by the stronger peaks at 1100 and 460 cm-1. When the temperature was over 1300 °C, the intensity of the peak at 912 cm-1 became weak. The variation of this peak was coincident with the change of the anorthite content. Peaks at around 1100 and 550 cm-1 were assigned to the Fe-O bond. The Fe-O variation exhibited by FTIR among all of the samples showed that the variation increased with the ratio of coal B, meaning that the iron oxides in the char sample increased with the ratio of coal B.
ARTICLE
’ ACKNOWLEDGMENT Financial support from the National Basic Research Program of China (2010CB227005-02), the National Natural Science Funds (21006121), the Youth Foundation of Shanxi Province (2010021008-2), and the State Key Laboratory of Coal Conversion and State Key Laboratory of Coal Combustion (FSKLCC0909) was gratefully appreciated. ’ REFERENCES (1) Yu, G.; Niu, M.; Wang, Y.; Yu, Z. Mod. Chem. Ind. 2004, 24, 69–73. (2) Benson, S. A.; Jones, M. L.; Harb, J. N. Ash formation and deposition. In Fundamentals of Coal Combustion for Clean and Efficient Use; Smoot, D., Ed.; Elsevier: Amsterdam, The Netherlands, 1993; pp 135-147. (3) Qiu, J.; Li, F.; Zheng, Y.; Zheng, C. Fuel 1999, 78, 963–969. (4) Lin, S. Y.; Hirato, M.; Horio, M. Energy Fuels 1994, 8, 598–606. (5) Wang, J.; Risa, I.; Takayuki, T. Energy Fuels 2000, 14, 1108–1114. (6) Gupta, R. P.; Zheng, C.; Wall, T. F.; Tsumita, Y.; Kajigawi, I.; Suzuki, K. Engineering Foundation Conference on The Impact of Ash Deposition on Coal Fired Plant; Taylor and Francis: Oxford, U.K., 1994; pp 281-304. (7) Bai, J.; Li, W.; Li, B. Fuel 2008, 87, 583–591. (8) Adanez, J.; Diego, L. Fuel Process. Technol. 1990, 24, 298–304. (9) Wu, S.; Zhang, X.; Gu, J.; Wu, Y.; Gao, J. Energy Fuels 2007, 21, 1827–1831. (10) Hurst, H. J.; Patterson, J. H.; Quintanar, A. Fuel 2000, 79, 1797–1799. (11) Tomeczek, J.; Palugniok, H. Fuel 2002, 81, 1251–1258. (12) Van Dyk, J. C.; Waanders, F. B.; Benson, S. A.; Laumb, M. L.; Hack, K. Fuel 2009, 88, 67–74. (13) Hurst, H. J.; Novak, F.; Patterson, J. H. Fuel 1999, 78, 1831–1840. (14) Jiao, F. C.; Li, H.; Deng, S. P.; Dong, Z. Coal Convers. 2006, 29, 11–14.
4. CONCLUSIONS The principle results are summarized as follows: (1) The thermodynamic model FactSage was used to predict the transformation of mineral matter at temperatures from 1100 to 1500 °C. Most mineral matter was transformed into Ca aluminosilicates and mullite, and the only component with potential effects on gasification after blending coal chars was iron oxide. (2) The gasification of blended coal chars was performed from 1100 to 1500 °C at intervals of 50 °C. The results indicated that the melting ash at high temperatures hinders the gasification reaction, and the negative effect on B6A4 was less than the effects on other chars. Considering of the viscosity-temperature curve, the temperature where the effects are shown is delayed. The FTIR of gasification residues proved the existence of iron oxides, which led to a delay of the decrease of gasification reactivity for B6A4 during high-temperature gasification. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: 0086-351-4044335. Fax: 0086-351-4050320. E-mail:
[email protected]. 1131
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