Energy & Fuels 2008, 22, 4029–4033
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Differences in Gasification Behaviors and Related Properties between Entrained Gasifier Fly Ash and Coal Char Jing Gu, Shiyong Wu, Youqing Wu, Ye Li, and Jinsheng Gao* Department of Chemical Engineering for Energy Resources and Key Laboratory of Coal Gasification of Ministry of Education, East China UniVersity of Science and Technology, Shanghai 200237, China ReceiVed July 2, 2008. ReVised Manuscript ReceiVed September 8, 2008
In the study, two fly ash samples from Texaco gasifiers were compared to coal char and the physical and chemical properties and reactivity of samples were investigated by scanning electron microscopy (SEM), SEM-energy-dispersive spectrometry (EDS), X-ray diffraction (XRD), N2 and CO2 adsorption method, and isothermal thermogravimetric analysis. The main results were obtained. The carbon content of gasified fly ashes exhibited 31-37%, which was less than the carbon content of 58-59% in the feed coal. The fly ashes exhibited higher Brunauer-Emmett-Teller (BET) surface area, richer meso- and micropores, more disordered carbon crystalline structure, and better CO2 gasification reactivity than coal char. Ashes in fly ashes occurred to agglomerate into larger spherical grains, while those in coal char do not agglomerate. The minerals in fly ashes, especial alkali and alkaline-earth metals, had a catalytic effect on gasification reactivity of fly ash carbon. In the low-temperature range, the gasification process of fly ashes is mainly in chemical control, while in the high-temperature range, it is mainly in gas diffusion control, which was similar to coal char. In addition, the carbon in fly ashes was partially gasified and activated by water vapor and exhibited higher BET surface area and better gasification activity. Consequently, the fact that these carbons in fly ashes from entrained flow gasifiers are reclaimed and reused will be considered to be feasible.
1. Introduction It is well-known that coal gasification technology has become one of the leading clean coal technologies. Among many of the gasification methods, the entrained flow coal gasification technology, such as Texaco, Shell, and GSP gasification technology, is considered as the most suitable one to meet largescale gasification demands. Therefore, it has been winning a wide market, especially in China. Carbon-containing fly ash from this type of gasifier is an inevitable solid waste, which is fine powder, entrained out by hot gas and then filtrated up in the gas purification system. Generally, fly ashes from entrained flow gasifiers contain a large amount of carbon (about 30-40%) instead of less than 5-7% in fly ashes from combustion. These carbons are usually used as fuels for combustion or raw materials for other use. If these carbons from gasifiers are reclaimed and reused as feed for gasifiers, it will be quite favorable to improve the efficiency of gasifiers. When fly ashes are used as fuel in gasifiers, the gasification characteristics of coal chars cannot be completely used for reference. Because the carbons in fly ashes from gasifiers has been partially gasified and activated by water vapor, their relative properties and gasification reactivity are obviously different from those of coal chars. Moreover, the most previous works,1-3 thus far concentrated on the combustion fly ash from power station boiler, have been published on physicochemical properties, morphology, size distribution, and combustion reactivity, while there is only very little study on those of fly ashes from gasifiers. Obviously, it * To whom correspondence should be addressed. Telephone: 86-2164252058. Fax: 86-21-64252058. E-mail:
[email protected]. (1) Ghosal, S.; Ebert, J. L.; Self, S. A. Fuel Process. Technol. 1995, 44, 81–94. (2) Goodarzi, F. Fuel 2006, 85, 1418–1427. (3) Kutchko, B. G.; Kim, A. G. Fuel 2006, 85, 2537–2544.
appears imperative to investigate the relative properties and gasification reactivity of fly ashes from gasifiers. Here, two fly ashes from Texaco gasifiers at different operation conditions were used for raw materials, and their physical and chemical properties and CO2 gasification behavior were investigated. At the same time, a rapid heating coal char was used for comparison. It aims to obtain the richer knowledge of physical and chemical properties and the gasification characteristics of fly ashes. Moreover, it is more significant to provide valuable information for proper design and operation of gasifiers, in which fly ashes are used as fuel. 2. Experimental Section 2.1. Raw Materials and Preparation of Samples. Two fly ashes and a coal char were used in the study. Two fly ashes were respectively collected from Texaco gasifiers of the Shanghai Coking Chemical Company and the Nanjing Chemical Company under their normal operation conditions and were respectively designated as SCFA and NCFA. In general, over 1.5 tons of fly ash is generated when 100 tons of coal is consumed in Texaco gasifiers. A coal char was prepared at rapid pyrolysis conditions (the pyrolysis rate is up to 103 K/s) in a small drop tube furnace (inner diameter of 50 mm)4 and was designated as RPCC. The detailed descriptions of the two fly ashes and the coal char can be seen in Table 1. In the table, SH No. 1 and SH No. 2 are from the same coal mine (Shenhua Coal Mine in China), and they present slightly different properties, which are presented in Table 2. All fly ashes and coal chars are ground (particle size < 73 µm) and then are used as experimental samples. To investigate the catalytic effect of minerals on the gasification reactivity of the carbon in fly ashes, two demineralized fly ash (4) Wu, S. Y.; Gu, J.; Zhang, X.; Wu, Y. Q.; Gao, J. S. Energy Fuels 2008, 22, 199–206.
10.1021/ef800527x CCC: $40.75 2008 American Chemical Society Published on Web 10/15/2008
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Gu et al.
Table 1. Detailed Descriptions of Two Fly Ashes and a Coal Char sample
raw coal
gasifier
operation pressure (MPa)
operation temperature (K)
SCFA NCFA RPCC
Shenhua coal (SH No. 1) Shenhua coal (SH No. 2) Shenhua coal (SH No. 1)
Texaco, Shanghai Coking Chemicals Company Texaco, Nanjing Chemicals Company a small drop tube furnace4
∼3.9 ∼8.7 atmospheric pressure
∼1573 ∼1593 ∼1673
Table 2. Properties of SH No. 1 and SH No. 2a proximate analysis (wt %, d) sample
A
V
FC
SH No. 1 SH No. 2
6.50 7.67
35.21 33.10
58.29 59.23
chemical components of ash (wt %) sample
SiO2 Al2O3 Fe2O3 CaO MgO TiO2 Na2O K2O SO3
SH No. 1 42.68 14.64 11.76 15.00 1.54 0.72 SH No. 2 43.56 14.32 7.65 18.34 2.22 0.81
1.09 2.31 10.01 1.76 2.69 8.13
ash fusion temperature (K) sample
DT
ST
FT
SH No. 1 SH No. 2
1398 1446
1483 1491
1513 1522
a A, ash; VM, volatile matter; FC, fixed carbon; d, dry basis. DT, deformation temperature; ST, soften temperature; FT, flow temperature.
Table 3. Conventional Analysis of Five Experimental Samplesa proximate analysis (wt %, d)
ultimate analysis (wt %, d)
sample
A
V
FC
C
H
St
N
SCFA NCFA DSCFA DNCFA RPCC
53.53 62.31 18.48 1.68 11.32
9.55 6.01 12.70 6.68 3.07
36.92 31.68 68.82 91.64 85.61
46.73 30.21 66.37 78.94 87.05
0.74 0.46 1.05 1.20 0.51
0.24 0.46 0.34 1.21 0.38
0.29 0.22 0.41 0.58 0.08
ash fusion temperature (K) sample
DT
ST
FT
SCFA NCFA
1173 1248
1398 1488
1433 1508
a
A, ash; VM, volatile matter; FC, fixed carbon; d, dry basis; DT, deformation temperature; ST, soften temperature; FT, flow temperature.
samples were also prepared by acid treatment with 40 wt % HCl and 36 wt % HF at the reaction temperature of 373 K and the reaction time of 24 h. The demineralized fly ash samples from SCFA and NCFA, with a particle size below 73 µm, are respectively designated as DSCFA and DNCFA. Above all, five experimental samples, which are SCFA, NCFA, RPCC, DSCFA, and DNCFA, were obtained. Their conventional analyses are shown in Table 3. 2.2. Conventional Analysis of Samples. The proximate and ultimate analysis and ash fusion temperatues (ash fusion temperatures include deformation temperature, soften temperature, hemispheric temperature, and flow temperature) of raw coals or experimental samples were conducted according to the criterions of GB/T212-2001 (Chinese standard), GB/T476-2001 (Chinese standard), and ISO540-1995 (International standard), respectively. 2.3. SEM/SEM-EDS/XRD Test of Samples. A scanning electronic microscopy (SEM, JEOL JSM-6360LV) analysis was used to investigate the porosity and the ash fusion of samples. The SEM-energy-dispersive spectrometry (EDS) test was also performed, using a scanning electron microscope equipped with backscattered and secondary electron detectors coupled with EDS. The SEM-EDS analysis provides detailed imaging information about the morphology, structureal texture, and elemental composition in the surface of samples. An X-ray diffraction analyzer (JSM6360LV XRD, D/max-rA12Kw, 40 kV, 100 mA, Cu KR radiation) was employed to determine the carbon crystalline structure of samples.
2.4. Measurement of Brunauer-Emmett-Teller (BET) Surface Areas and Pore Structures of Samples. Porosimetric analysis of samples was carried out by means of a Micromeritics and Quantasorb Sorption System, using N2 at 77 K and CO2 at 273 K as adsorption gas, respectively. 2.5. Measurement of CO2 Gasification Reactivity of Samples. Measurements of sample gasification reactivity were performed in a SETARAM TG-DTG/DSC thermogravimetric analyzer using an isothermal method. Experiments were conducted in the temperature range of 1223-1673 K. In each run, a weighted sample was placed in an alumina crucible with 5 mm inner diameter and 3 mm height and heated at a nominal heating rate of 30 K/min from room temperature to the prescribed temperature. During the heating period, a nitrogen flow of 25 mL/min was used to sweep the sample. Upon reaching the level-off temperature, the isothermal gasification reaction of the sample was initiated by switching to the carbon dioxide. The sample was then allowed to react with CO2 until its weight was diminished no longer. At this time, the experiment was stopped by switching to nitrogen again. The weight loss was recorded by a highly sensitive electronic balance located in the apparatus. Thermocouples for the determination of sample temperatures were connected to the crucible at the bottom part, precisely underneath the sample layer. The experimental conditions were as follows: sample mass, 2 mg; carbon dioxide flow, 60 mL/min; particle diameter of samples e 73 µm. In the present study, the carbon conversion (X) and gasification reaction rate (R) of samples were calculated as follows:
W0 - W W0 - Wash
(1)
1 dW dt W0 - Wash
(2)
X) R)
where W0 is the initial mass of the sample, W is the instantaneous sample mass, Wash is the mass of the ash, and R is the reaction rate at the time of t. The gasification reactivity is usually presented by a reactivity index (Rs), which is defined as
Rs )
0.5 t0.5
(3)
where t0.5 is the time when the carbon conversion is up to 50 wt %. This definition is commonly used in the literature5,6 for the comparison of gasification reactivity of different coals.
3. Results and Discussion 3.1. Analysis Results on Relative Properties of Sample. The conventional properties of raw coals (SH No. 1 and SH No. 2) are listed in Table 2. From the table, It is shown that the ash content, carbon content, chemical components, and ash fusion temperatures of SH No. 1 and No. 2 are very similar. Consequently, it is approximately considered that the parent coals, from which SCFA, NCFA, and RPCC are obtained, belong to the same coals. The proximate and ultimate analysis of SCFA, NCFA, RPCC, DSCFA, and DNCFA are shown in Table 3. In comparison to (5) Takarada, T.; Tamai, Y.; Tomita, A. Fuel 1985, 64, 1438–1442. (6) Zhang, L. X.; Huang, J. J.; Fang, Y. T.; Wang, Y. Energy Fuels 2006, 20, 1201–1210.
Entrained Gasifier Fly Ash and Coal Char
Energy & Fuels, Vol. 22, No. 6, 2008 4031
Figure 1. SEM micrographs of SCFA, NCFA, and RPCC.
Figure 2. SEM-EDS analysis results of melted ashes in SCFA.
coal char, the two fly ash samples are reasonably rich in ash content and poor in carbon content and it indicates that both have experienced a partial gasification process. However, there is an obvious difference between NCFA and SCFA; the former has a lower carbon content (30.21%) than that of the latter (46.73%). It is shown that the gasifier in Nanjing, compared to that in Shanghai, was operated in a better working status: much higher pressure (about 8.7 MPa) and slightly higher temperature (about 1593 K). It illuminates that the impact of the operating temperature of gasifier is more than that of pressure. Figure 1 is the SEM micrographs of NCFA, SCFA, and RPCC. From the table, it can be seen that the ashes in the three samples are evidently melted to form spherical grains, that those in NCFA and SCFA occur to agglomerate, while those in RPCC do not agglomerate, and that those in NCFA are agglomerated into larger spherical grains than those in SCFA. Seeing the main preparation conditions of the three samples, the difference in the temperature is less, while the pressure is obviously different (NCFA, 8.7 MPa; SCFA, 3.9 MPa; RPCC, atmospheric pressure). Thereby, it can be considered that the differences of the three samples in the ash fusion characteristics should be due to the difference of pressures and that the higher pressure can make the ashes melt and agglomerate more easily. To understand the form of melted ashes in fly ashes, the SEM-EDS analyzer is employed to investigated the elemental composition of melted ashes in SCFA, which is used for an example. The results are shown in Figure 2. From the figure, it can be seen that the melted ashes contain oxygen, iron,
Table 4. BET Surface Area and Total Pore Volume of SCFA, NCFA, and RPCC CO2 adsorption sample
SCFA (m2/g)
BET surface area pore volume (cm3/g)
NCFA
N2 adsorption SCFA
NCFA
RPCC
59.09 57.59 187.99 201.09 4.38 0.04144 0.03974 0.1968 0.2166 0.0079
magnesium, silicon, aluminum, calcium, manganese, etc., which suggests that the melted ash belongs to eutectic compounds, which are composed of the above elements. Accordingly, when the aforementioned results are combined (Figure 1), it is considered that, the higher the pressure, the more easily ashes form eutectic compounds. In Table 3, in comparison to the change in the ash content of fly ashes after demineralization, it can be found that the ash contents of the two fly ashes decrease. It is the reason that some minerals containing alkali and alkaline-earth metals, such as Na and K, are removed. In addition, those are also found that DT, ST, and FT of SCFA is respectively lower to 75, 90, and 75 K than those of NCFA and the ashes in SCFA were removed more difficultly than those in NCFA by acid treatment (the ash contents in SCFA and NCFA decrease from 53.53 to 18.48% and from 62.31 to 1.68%, respectively). These results suggest that there are also some differences in properties, which probably resulted from the operation conditions of gasifiers. The BET surface areas and total pore volumes of SCFA, NCFA, and RPCC in the conditions of N2 and CO2 adsorption are listed in Table 4, and their pore size distributions in the
4032 Energy & Fuels, Vol. 22, No. 6, 2008
Figure 3. Pore size distribution of SCFA, NCFA, and RPCC (9, NCFA; b, SCFA; 2, RPCC).
Gu et al.
Figure 5. Rs of samples (9, SCFA; b, NCFA; 2, DSCFA; 1, DNCFA; +, RPCC).
Figure 6. Gasification reactivity of NCFA. (a) Carbon conversion versus time and (b) reaction rate versus carbon conversion (9, 1223 K; b, 1273 K; 2, 1373 K; 1, 1473 K; +, 1573 K; ×, 1673 K).
Figure 4. XRD patterns of SCFA, NCFA, and RPCC.
condition of CO2 adsorption are shown in Figure 3. The N2 surface area and pore volume of samples provided a measure of the amount of mesopores and small macropore (2-450 nm), and the CO2 surface area and pore volume of samples provided a measure of the amount of micropores and small mesopores (0.5-5 nm). From Table 4, it is very clear to see that BET surface areas (N2) and pore volumes of two fly ash samples, through partial gasification and activation by water vapor, respectively reach a high level, closing to 200 m2/g and 0.2 cm3/g, which are much higher than those of RPCC, with only 4.38 m2/g and 0.0079 cm3/g, respectively. The fly ash samples also have a rather larger BET surface area (CO2) with near 60 m2/g. From Figure 2, it can be found that two peaks are present in the pore size distributions of two fly ashes and that the larger peak is at pore diameter of about 4 nm, while no peaks are present in the pore size distributions of coal char, which indicates that pores of fly ashes are rich in meso- and micropores. To examine the carbon crystalline structures of fly ashes and coal char, XRD determination of SCFA, NCFA, and RPCC was conducted and the results are shown in Figure 4. In the XRD patterns of fly ashes, the 002 and 100 bands can hardly be recognized because of its very short residence time and partial gasification in gasifiers, while the two bands are much more pronounced for RPCC, indicating that the carbon in RPCC presents a more ordered crystalline structure than that in two fly ashes. As mentioned above, the partially gasified fly ash samples exhibit obviously different properties in comparison to the ungasified coal char. Now, their CO2 gasification behavior is investigated in the following sections. 3.2. CO2 Gasification Behavior of Samples. 3.2.1. Comparison of CO2 Gasification ReactiVity of Samples. Figure 5 presents Rs of various samples for the CO2 gasification reaction in the gasification temperature range of 1223-1673 K. According to Figure 5, it can be obtained that the CO2 gasification
reactivity of fly ash carbon is better than that of RPCC, especially at higher temperatures. This is probably owing to a larger specific surface area (Table 4) and more disordered carbon crystalline structure (Figure 4) of fly ashes. The significant influence of specific surface area and carbon crystalline structure on the gasification reactivity of carbon has been demonstrated previously, and the results obtained here are in accordance with previous studies.7,8 In addition, the gasification reactivity of SCFA is higher than that of NCFA, which was probably due to more micropores of SCFA. From Figure 5, it is also found that the CO2 gasification reactivity of fly ash carbon with acid washing is lower than that without acid washing in the whole temperature range of 1223-1673 K and even lower than that of coal char. However, the surface area of fly ash samples is about 43-46 times larger than that of coal char (see Table 4), and the carbon crystalline structure of fly ash samples is more disordered than that of coal char (see Figure 4). The main reason may be the catalytic action of minerals contained in fly ash samples, such as alkali and alkaline-earth metals.9 Accordingly, it is deduced that, at the treatment conditions of acid washing, only these minerals with catalytic activity are removed, while eutectic minerals remain. 3.2.2. Effects of the Gasification Temperature on the ReactiVity of Fly Ash Carbon. The relationship between the carbon conversion and the gasification time and that between the reaction rate and carbon conversion for SCFA is similar to those for NCFA; therefore, NCFA, taken for an example, is investigated here, as shown in Figure 6. Figure 6a shows that the carbon conversion of NCFA increases with the increase of the gasification temperature, while the change of carbon conversion in a higher temperature range (1473-1673 K) is intensitive to the gasification temperature. In Figure 6b, the gasification reaction rate of NCFA increases initially with the increase of (7) Feng, B.; Bhatia, S. K.; Barry, J. C. Carbon 2002, 40, 481–496. (8) Lee, C. W.; Jenkins, R. G.; Schobert, H. H. Energy Fuels 1992, 6, 40–47. (9) Samaras, P. E. Fuel 1996, 75, 1108–1114.
Entrained Gasifier Fly Ash and Coal Char
Energy & Fuels, Vol. 22, No. 6, 2008 4033
Figure 7. Arrhenius plots of samples in the range of 1223-1673 K (9, SCFA; b, NCFA; 2, DSCFA; 1, DNCFA; f, RPCC). Table 5. Apparent Activation Energy and Pre-exponential Factors of Samples at the Gasification Temperature of 1223-1673 K gasification temperature range 1223-1373 K
1373-1673 K
samples
k0 (min-1)
Ea (kJ/mol)
k0 (min-1)
Ea (kJ/mol)
SCFA NCFA DSCFA DNCFA RPCC
11708 155833 8039 91620 10111
103.05 135.82 107.25 139.05 105.53
230 172 47 100 48
58.59 58.61 49.49 62.25 45.22
carbon conversion, passes through a maximum at a carbon conversion between 40 and 60%, and then gradually declines, which is similar to the our previous reports on coal chars.4 The variation of the reaction rate with carbon conversion attributes to the evolution of particle structure (e.g., particle surface area) and activates sites of samples during gasification.10,11 3.2.3. CO2 Gasification Reaction Kinetics of Samples. Generally, the analysis of CO2 gasification reaction kinetics is carried out by fitting experimental data (the data of X versus t or R versus X) with various kinetic models based on either weight (or volume) or surface area. Here, the modified volumetric model (MVM) is used to describe the gasification process. The equation of the model, which was described in our previous study,4 is given as follow: (4) X ) 1 - exp(-atb) The Arrhenius plots of ln(Ks) (Ks ) average reaction rate constant) versus 1/T for various samples are presented in Figure 7, and the calculated apparent activation energies of CO2 gasification for various samples are listed in Table 5. From Figure 7, it can be seen that from the low-temperature range (1223-1373 K) to the high-temperature range (1373-1673 K), the curves of Arrhenius plots of all samples turn. From Table 5, it is obtained that the apparent activation energies of CO2 gasification for various samples range from 45.22 to 62.25 kJ/ mol at higher temperatures of 1373-1673 K and from 103.05 to 139.05 kJ/mol at lower temperatures of 1223-1373 K. These (10) Adschiri, T.; Furusawa, T. Fuel 1986, 65, 927–931. (11) Liu, G. S.; Benyon, P.; Benfell, K. E.; Bryant, G. W.; Tate, A. G.; Boyd, R. K.; Harris, D. J.; Wall, T. F. Fuel 2006, 79, 617–626.
results indicate that the gasification proceeds differently for T < 1373 K and > 1373 K and that, in the low-temperature range, the gasification process is mainly in chemical control, while in the high-temperature range, it is mainly in gas diffusion control, which are in accordance with that of previous reports.4,12,13 This appears to result from ash fusion. Kasaoka et al.14 studied the gasification reactivity of various coal chars with carbon dioxide and concluded that the gasification process above the ash fusion temperature was different compared to that below the ash fusion temperature. Luo et al.15 conducted the measurements of CO2 gasification kinetics of SS005 char at temperatures of 1273-1873 K by using a bench-scale fluidized bed reactor and also found that the reaction rate became almost unchanged above the temperature of 1673 K, which was around the ash fusion temperature. 4. Conclusions (1) The carbon crystalline structure of fly ashes was more disordered than that of coal char. The BET surface area of fly ashes was about 40 times higher than that of coal char, and pores of fly ashes were much richer than those of coal char in meso- and micropores. Ashes in fly ashes occurred to agglomerate into larger spherical grains, while those in coal char do not agglomerate. The CO2 gasification activity of fly ashes was obviously higher than that of coal char, especially in the higher temperature range (>1373 K). (2) The minerals in fly ashes, especial alkali and alkaline-earth metals, had a catalytic effect on the gasification reactivity of fly ash carbon. The carbon content of gasified fly ashes exhibited 31-37%, which was less than the carbon content of 58-59% in the feed coal. However, the carbon in fly ashes was partially gasified and activated by water vapor and exhibited higher BET surface area and better gasification activity. Consequently, if the carbons in fly ashes from entrained flow gasifiers are reclaimed and reused, it will be considered to be feasible. (3) Through CO2 gasification kinetics analysis of all samples, it was obtained that the apparent activation energies of CO2 gasification for various samples range from 45.22 to 62.25 kJ/mol at higher temperatures of 1373-1673 K and from 103.05 to 139.05 kJ/mol at lower temperatures of 1223-1373 K and it was concluded that, in the low-temperature range, the gasification process of fly ashes is mainly in chemical control, while in the high-temperature range, it is mainly in gas diffusion control, which was similar to coal char. Acknowledgment. The authors thank the National Basic Research Program of China for financial support for this work under Project 2004CB217704. EF800527X (12) Cui, H.; Xu, X. F.; Gu, Y. D. Coal ConVers. 1996, 19, 75–79. (13) Liu, H.; Luo, C. H.; Kato, S.; Uemiya, S.; Kaneko, M.; Kojima, T. Fuel Process. Technol. 2006, 87, 775–781. (14) Kasaoka, S.; Sakata, Y.; Tong, C. Int. Chem. Eng. 1985, 25, 160– 175. (15) Luo, C.; Watanabe, T.; Nakamura, M.; Uemiya, S.; Kojima, T. Energy Resour. Technol. 2001, 123, 21–26.
