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The physical character of coal char formed during rapid pyrolysis at high pressure
Fuel 84 (2005) 63–69 www.fuelfirst.com
The physical character of coal char formed during rapid pyrolysis at high pressure Koichi Matsuokaa,1, Hiroyuki Akihoa, Wei-chun Xub, Rajender Guptac, Terry F. Wallc, Akira Tomitaa,* a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan b Institute of Research and Innovation, 1201 Takada, Kashiwa 277-0861, Japan c Cooperative Research Center for Coal in Sustainable Development, Department of Chemical Engineering, School of Engineering, University of Newcastle, Callaghan, NSW 2308, Australia Received 16 January 2004; revised 24 June 2004; accepted 5 July 2004 Available online 13 August 2004
Abstract Rapid pyrolysis was conducted in a drop tube reactor using seven coals under various operating conditions. In addition to dense char, porous chars (network char and cenospheric char) were formed by the rapid pyrolysis under certain conditions. Porous char was mainly composed of film-like carbon and skeleton carbon. The pyrolyzed coal char particles were characterized in detail. Morphology and bulk density of porous char were quite different from the dense char formed under the same conditions, but elemental composition and BET surface area were similar to each other. CO2 gasification reactivity of porous char was lower than dense char in the later gasification stage, and this was ascribed to the low reactivity of skeleton carbon. q 2004 Elsevier Ltd. All rights reserved. Keywords: Coal; Char; Pyrolysis; Cenosphere; Morphology
1. Introduction At the early stage of coal combustion and gasification, devolatilization reaction occurs. During the devolatilization, volatiles are released in the form of gas and tar, and then resultant char is generated. The characteristics of the resultant char control the subsequent combustion and gasification processes. Morphology is one of the important characteristics, because it affects aerodynamic behavior of char in a reactor like entrained-bed combustor or gasifier. The morphology of char depends on the rank and the petrographic composition of parent coal as well as experimental conditions. It is reported that not only * Corresponding author. Tel.: C81-22-217-5625; fax: C81-22-2175626. E-mail address: [email protected] (A. Tomita). 1 Present address: Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba 305-8569, Japan. 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2004.07.006
non-porous char (dense char) but also porous char is formed during rapid pyrolysis and combustion of coal [1–10]. There are several shape types of porous chars, and several classifications on the morphology are proposed on the basis of the porosity and geometric parameters [6]. In these classifications, porous char is generally classified as thinwalled cenosphere, thick-walled cenosphere and network char [6]. There have been several studies on formation mechanisms of porous chars [11–16]. Furthermore, the effects of coal type, gas pressure and heating rate on porous char formation have also been examined [4,5,7,12,13]. From these studies, roughly speaking, formation of porous char is favored by high vitrinite content, high pressure and high heating rate conditions. However, there is essentially no systematic study on the properties of porous char. For example, the reactivity of porous char during gasification and combustion processes has only been speculated from the reactivity of mixture of the dense char and porous char. Such reactivity depends on the characteristics of the char, and these characteristics are determined by the initial stage
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of the devolatilization reaction. Thus, we attempt here to characterize char particles by pyrolyzing seven coals under relatively low temperature. The morphology, the elemental composition and the BET surface area, and the gasification reactivity of the chars were determined in this study.
examined by an energy dispersive X-ray analyzer (EDX; EDAX Phoenix), because the amount of char remained after the gasification was too small to provide for ultimate analysis.
3. Results and discussion 2. Experimental 3.1. The effect of the operating condition on char morphology
Analyses of the coals used in this study are listed in Table 1. The size of particle was from 75 to 90 mm for Shenmu coal, and 75–150 mm for all other coals. These coals were pyrolyzed in a drop tube reactor. Details of the experimental apparatus are described elsewhere [17,18]. Coal particles were fed at a rate of 0.1 g/min with a stream of He or H2 at 3.5 l (STP)/min. Heating rate of the coal particles was calculated to be about 104 8C/s. Experiments were carried out in a temperature range from 600 to 850 8C and a pressure from 1.0 to 5.0 MPa. In addition to dense char particles, porous char particles were observed under some conditions as a minor fraction on weight basis. In such a case, the resultant char particles were separated into C100 mesh (!150 mm) fraction and K100 mesh (O150 mm) fraction for further analyses. The char particles were observed by a scanning electron microscope (SEM; Topcon 60). Whole structure image as well as cross sectional image were obtained by SEM observation. Size distribution of porous char was determined from the whole structure images. Elemental composition and ash fraction of the resultant chars were determined from ultimate analysis and proximate analysis, respectively. Bulk density was determined from the volume occupied by a unit weight of char particles in a glass tube with an inner diameter of 8 mm. BET surface area of the sample was determined by N2 adsorption at 77 K using an automatic volumetric sorption analyzer (Quantachrom, Autosorb-1). The dense char and porous char were independently gasified at 900 8C in CO2 by a thermogravimetric analyzer to examine the effect of the char morphology on gasification reactivity. The porous char particles were observed by SEM before and after the gasification. The elemental composition of the remaining char after the gasification was qualitatively
Fig. 1 shows SEM images of Shenmu char particles (O150 mm) formed by rapid hydropyrolysis under different pressures and temperatures. Porous char was abundant in the char with a size over 150 mm. On the other hand, few porous char was found in the char with a size below 150 mm (not shown here). For this reason, the two fractions (!150 mm fraction and O150 mm fraction) are hereafter termed as dense and porous char, respectively. Appearance of porous char could be seen from these images, but detailed structure was unclear. Therefore, the observation of cross sectional images was made with SEM. Fig. 2 shows an example of cross sectional image of Shenmu char particles (O150 mm) prepared at 800 8C under 3.0 MPa-H2. Char particles can roughly be classified into three types, cenospheric char (C), network char (N) and dense char (D) according to the classification proposed by Cloke and Lester [6]. The image in Fig. 1 cannot differentiate two types of porous char, cenospheric char and network char. In this paper, a mixture of cenospheric char and network char is termed porous char. We checked the effect of operating conditions on the formation of porous char. Table 2 lists experimental conditions and whether or not porous char could form under each condition. ‘Yes’ in this Table means the case where porous char particles were found in the whole char. The weight fraction of porous char was 1.0 wt% at the most. The fraction seems to be small at first glance, but it is not small when expressed by vol.%, because the density of porous char particle was very low as expected from Figs. 1 and 2. Porous char was formed in the case of pyrolysis of Shenmu coal in He atmosphere as well as in H2 atmosphere.
Table 1 Analyses of coals used Coal
Berau Adaro Pasir Taiheiyo Shenmu Blair Athol Datong a
CSNa (K)
Proximate analysis (wt%, dry)
Ultimate analysis (wt%, daf)
Ash
VM
FC
C
H
N
SCO (diff.)
Vitrinite
Exinite
Inertinite
2.9 1.6 4.4 10.6 6.4 6.6 10.0
46.6 45.4 44.4 46.9 31.8 30.3 29.7
50.5 53.0 51.2 42.5 61.8 63.1 60.3
68.4 70.6 71.7 76.1 77.4 78.4 82.7
4.7 4.8 5.1 6.4 4.5 4.6 4.7
1.6 1.2 1.3 1.4 1.1 2.0 1.1
25.3 23.4 21.9 16.1 17.0 15.0 11.6
92.8 94.8 86.6 89.6 62.3 37.1 47.8
4.6 1.8 5.4 8.3 2.2 5.8 2.7
2.6 3.4 8.0 2.1 35.5 57.1 49.5
Crucible swelling number (JIS M8801).
Maceral (vol.%, dmmf)
0 0 0 0 0 0 0
K. Matsuoka et al. / Fuel 84 (2005) 63–69
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Fig. 1. SEM images of Shenmu char formed in hydropyrolysis under different conditions: (a) 700 8C, 3.0 MPa; (b) 800 8C, 3.0 MPa; (c) 850 8C, 3.0 MPa; (d) 800 8C, 1.0 MPa; (e) 800 8C, 5.0 MPa.
Though not shown in this paper, the morphology of porous char formed in He atmosphere was similar to that in H2 atmosphere. Moreover, the elemental composition and the ash content of Shenmu porous char formed under 3 MPa-He and at 800 8C were similar to those of char formed in H2 atmosphere (Table 3). In contrast to the pyrolysis in He atmosphere, no porous char was formed from Berau, Adaro, Pasir and Blair Athol in H2 atmosphere. Formation of porous char is likely to be related to caking property. The crucible swelling number (CSN; JIS, M8801) of all the coals used in this study is zero (Table 1), indicating that all the coals do not swell under an atmospheric pressure. This does not necessarily mean that these coals do not swell under the present experimental conditions, because the condition for CSN measurement is far from the present pyrolysis condition. In fact it is well known that the caking property is affected by gas atmosphere. Kaiho and Toda observed, for
example, the maximum fluidity was significantly affected by surrounding gas as well as gas pressure. At higher pressure, even non-swelling coal greatly swells especially under hydrogen atmosphere [20]. Therefore, it is quite probable that a small portion of coal particles softened under rapid heating at high-pressure, and the softened particle formed porous char particle. The effect of pressure on the formation of porous char is not straightforward. High pressure is generally preferable for the formation of porous char [21,22]. Softening of coal particle and increasing of internal pressure are essential to form porous char [20,23]. The external pressure is also an important factor. At a high pressure, even a coal having CSN value of 0 can form porous char. We examined here the pressure effect in a range from 1.0 to 5.0 MPa using Shenmu coal, which shows very little softening property at ambient pressure. Fig. 3 shows the particle size distribution of Shenmu porous char
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Fig. 2. Cross sectional images of Shenmu char formed in hydropyrolysis at 800 8C and 3.0 MPa. C, cenospheric char; N, network char; D, dense char.
formed under different H2 pressures. The size distribution was obtained from the several SEM images of whole char (Fig. 1(b, d and e)). Particle size range of the raw coal (75–90 mm) was also shown in this Figure. About 50% of
porous char formed at 1.0 MPa was more than 275 mm, while the size of most porous chars prepared at 3.0 and 5.0 MPa were less than 275 mm. The gas evolution from the inside of coal particle would be suppressed at a high external pressure [21,22], and therefore swelling of coal particle would be suppressed under a pressure of 5 MPa. Size of a porous char particle depends on the balance between the pressure from the inside of particle and the external pressure. The preparation temperatures for the porous chars shown in Fig. 1(a–c) were 700, 800 and 850 8C, respectively. Morphology of porous char seems to be independent of pyrolysis temperature. The effect of temperature on the morphology is rather insignificant in the range temperature used in this study. Heating rate is also one of the important factors to control the char morphology [12]. About 120 mg of Shenmu coal was set in a tiny quartz bottle and it was heated at a low heating rate (heating rate: 25 8C/min, maximum temperature: 800 8C, gas: He, pressure: 0.1 or 1.0 MPa). After holding at 800 8C for 10 min, it was allowed to cool down and then appearance of the char was observed. No porous char particles were observed in the resultant char irrespective of pressure, while porous char was found when heated the same coal at 104 8C/s in various gas atmosphere (Table 2). Rapid heating leads to sudden formation of volatile matter that is in favor of balloon formation, whereas slow heating may help volatile matter to slowly escape from a particle without forming balloon. Many researchers insist that cenospheric char is formed from vitrinite [12–14,19]. In the present study, porous char was formed not only from vitrinite-rich coals but also inertinite-rich Shenmu and Blair Athol coals (Table 2). However, this observation cannot exclude the possibility of porous char formation from vitrinite, because the weight fraction of porous char particles in Shenmu char was at most 1 wt%, while the weight percentages of vitrinite part in Shenmu and Blair Athol coal were 62 and 37 wt%, respectively. These amounts were sufficient to form the small amount of porous char as reported here. It is of interest
Table 2 Formation of porous char under different experimental conditions Coal
Temperature (8C)
Gas
Pressure (MPa)
Porous chara
Coal
Temperature (8C)
Gas
Pressure (MPa)
Porous chara
Berau Adaro Pasir Taiheiyo
800 800 800 600 700 800 850 800 800 800
He He He He He He He He He He
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 3.0 1.0
Yes Yes Yes No No No No Yes Yes Yes
Berau Adaro Taiheiyo
800 800 800 850 700 800 800 800 850 800 800
H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2
1.0 1.0 1.0 1.0 3.0 1.0 3.0 5.0 3.0 1.0 1.0
No No No No Yes Yes Yes Yes Yes No No
Shenmu Blair Athol a
Shenmu
Blair Athol Datong
Yes: the weight fraction of porous char was 1.0 wt% at the maximum. No: porous char was absent.
K. Matsuoka et al. / Fuel 84 (2005) 63–69
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Table 3 Analyses of porous char and dense char formed on pyrolysis of Shenmu coal Temperature (8C)
Gas
Pressure (MPa)
Char type
C (wt%, daf)
H (wt%, daf)
N (wt%, daf)
SCO (diff; (wt%, daf))
H/C (K)
Ash (wt%, dry)
700
H2
3
800
H2
1
800
H2
3
800
H2
5
850
H2
3
800
He
3
Porous char Dense char Porous char Dense char Porous char Dense char Porous char Dense char Porous char Dense char Porous char Dense char
91.7 85.2 88.4 88.9 89.1 91.1 89.6 90.6 90.4 92.7 87.4 85.8
3.6 3.3 3.0 2.9 3.1 3.1 3.3 3.2 2.7 2.8 2.9 2.7
1.6 1.0 1.2 0.9 1.3 0.9 1.2 0.8 1.1 0.8 1.2 0.9
3.2 10.4 7.4 7.2 6.5 4.9 6.0 5.4 5.8 3.7 8.5 10.5
0.47 0.46 0.41 0.39 0.42 0.41 0.44 0.42 0.36 0.36 0.40 0.38
10.7 7.9 6.5 8.1 5.4 8.1 5.4 9.8 6.5 8.7 5.0 10.0
to note that no porous char was found in vitrinite-rich Taiheiyo coal char even if temperature and pressure were widely changed. One possible reason is the peculiar maceral composition of Taiheiyo coal; degradinite, which exhibits low plasticity upon heating, is abundant in vitrinite [24]. Anyway, the relationship between porous char formation and maceral composition in the parent coal is not straightforward. 3.2. Characterization of porous char The elemental composition of porous char determined by ultimate analysis was similar to that of dense char formed under the same conditions (Table 3). H/C atomic ratio of the dense char decreased with increasing pyrolysis temperature, while it was almost independent of pressure. This trend was also seen in porous char. SEM/EDX mapping analysis was carried out to clarify the elemental distribution of porous char particle embedded in the carnauba wax. Porous char consists of skeleton part and film part. This difference can be more clearly seen later in Fig. 5. The main component of the skeleton was carbon, but some ash particles were also observed in this part. The ash content in porous char determined from the proximate analysis was relatively lower than that in dense char in most cases (Table 3). Table 4 shows bulk density of raw coal, dense char and porous char. The bulk density of porous char ranged from 0.03 to 0.07 g/cm3, which was one order smaller than that of dense char. This was expected from the balloon type shape (Figs. 1 and 2). The bulk density of porous char was quite low irrespective of operating conditions and coal type. This difference between dense char and porous char would have significant influence on overall reactivity and aerodynamic behavior of char particles in combustor or gasifier. We carried out N2 adsorption experiment at 77 K for Shenmu chars listed in Table 4. The BET surface area of porous char and dense char ranged 13–51 and 11–61 m2/g, respectively. Unfortunately, the amount of porous char used for N2 adsorption experiment was too small to obtain reliable BET surface area. The data do not show any
meaningful trend as a function of temperature and pressure. We can only note that the BET surface area of porous char was not very different from that of dense char. 3.3. Gasification reactivity As described above, properties of porous char were similar to those of dense char in many aspects. The examination of reactivity of porous char is also of interest. Maloney and Jenkins [3] determined combustion reactivity in air at 395 8C of dense char as well as a mixture of dense char and porous char (the mixing ratio was not reported). They found that the time required to attain a fractional burn-off of 0.5 was independent of the presence of porous char. Benfell et al. [22] investigated the combustion reactivity of mixture of dense char and porous char in 50% O2/N2K1.5 MPa at 375 8C. Apparent combustion rate was lower for the mixture containing a larger amount of porous char. Oka et al. [25] examined the combustion reactivity of seven coals at 1200–1300 8C. The reactivity of coal chars was low when many porous char particles were formed. They suggested that the low combustion reactivity
Fig. 3. Size distribution of Shenmu porous char formed at 800 8C in different H2 pressures.
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Table 4 Bulk density of raw coal, dense char and porous char Coal Berau Adaro Pasir Taiheiyo Blair Athol Shenmu
Temperature (8C) 800 800 800 800 800 700 800 800 800 850
Gas He He He He He H2 H2 H2 H2 H2
Pressure (MPa) 1 1 1 1 1 3 1 3 5 3
Bulk density (g/cm3) Raw coal
Dense char
Porous char
0.66 0.66 0.64 0.69 0.61 0.59 0.59 0.59 0.59 0.59
0.33 0.28 0.17 0.40 0.23 0.30 0.37 0.31 0.35 0.38
0.04 0.06 0.04 N.D. 0.07 0.05 0.03 0.05 0.03 0.06
was due to the presence of porous char, but they did not mention the reason for the low reactivity of porous char. Thus, there is essentially no report so far on gasification reactivity of porous char by itself. We carried out here CO2 gasification at 900 8C using separated porous char particles and dense char particles prepared from Shenmu coal at 800 8C in 1.0 MPa-He. Fig. 4 represents CO2-gasification profiles. Initial gasification rate was almost the same for both the chars. On the contrary, in the later stage (gasification conversion of over 50%), the gasification rate of porous char was lower than that of dense char. The specific gasification rate of porous char was derived from the first-order kinetic plot, and it was three times lower in the later region than in the initial region. The gasified porous char was analyzed by SEM/EDX to clarify the reason for this difference. SEM images of porous char before and after the gasification are shown in Fig. 5. At a 50% conversion (Fig. 5(b)), film portion (this is composed of carbon according to the SEM/EDX analysis) was partly disappeared but skeleton portion seemed to remain as they were. At a high conversion of 90% (Fig. 5(c)), film-like portion disappeared almost completely and the width of remaining skeleton became thin. Since film-like portion is very thin, it may be easy to break down and the fragments
Fig. 4. CO2-gasification profiles of Shenmu dense char and porous char formed at 800 8C in 1.0 MPa-He.
Fig. 5. SEM images of Shenmu porous char before and after CO2 gasification. Char conversion: (a) 0%, (b) 50%, (c) 90%.
K. Matsuoka et al. / Fuel 84 (2005) 63–69
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char particles, cenosphere and network char, were formed in addition to dense char under some pyrolysis conditions. The physical characteristics of porous char were similar to those of dense char in many aspects, except for morphology and bulk density. In addition to morphology and bulk density, gasification reactivity of the porous char was different from that of the dense char, especially in the later stage. During gasification of porous char, film-like carbon was preferentially gasified and remaining skeleton carbon was gasified in the later stage. The porous char was more graphitic than the dense char, and the calcium species which may act as catalyst was less abundant in the porous char. Therefore, the reactivity of the porous char was lower than that of dense char.
Acknowledgements Fig. 6. XRD profile for Shenmu char formed in hydropyrolysis at 800 8C and 3.0 MPa. C, calcite (CaCO3); Q, quartz (SiO2).
were either gasified or swept away from the skeleton. Therefore, the main reason for the decrease in reactivity of porous char in the later stage is that the skeleton carbon is less reactive than film carbon. We attempted to clarify the reason for low reactivity of skeleton carbon. Fig. 6 shows the XRD patterns for the porous char and dense char after hydropyrolysis and before CO2-gasification. A broad 002 band of carbon was observed at around 258 for both the chars. The peak of porous char was slightly sharper than that of dense char, indicating that the porous char was slightly more graphitic. The main ash components are quartz and calcite for both chars and the peak intensity is lower for porous char. This is in accordance with the lower ash content in porous char as shown in Table 3. Thus, a higher ordering of carbon layers and a lower content of catalytic mineral component like calcium species may be the reason for the lower reactivity of porous carbon. Among porous carbon, film carbon and skeleton carbon have different structure and the latter remained in the later gasification stage. In entrained-bed combustion and gasification processes, the presence of porous char may strongly affect aerodynamic behavior of char. In some cases, this may leads to too much carry-over, plugging at narrow equipment, and others. Therefore, the understanding of formation mechanism is of importance for more smooth operation. Furthermore, the presence of less-reactive portion in char may control overall reactivity of char. It is thus of interest to clearly identify the reactivity of various components in char. The presence of rather inactive skeleton part in porous char may be the least reactive part in the whole char.
4. Conclusion Coal chars formed by rapid pyrolysis under various operating conditions were characterized in detail. Porous
This study was partially supported by New Energy and Industrial Technology Development Organization (NEDO) under BRAIN-C program. The authors thank Dr H. Orikasa in Institute of Multidisciplinary Research for Advanced Materials, Tohoku University for carrying out XRD analysis.
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Newall HE, Sinnatt FF. Fuel 1924;3:424. Lightman P, Street PJ. Fuel 1968;47:7. Maloney DJ, Jenkins RG. Fuel 1985;64:1415. Tsai CT, Scaroni AW. Fuel 1987;66:200. Bailey JG, Tate A, Diessel CFK, Wall TF. Fuel 1990;69:225. Cloke M, Lester E. Fuel 1994;73:315. Wu H, Bryant G, Benfell K, Wall TF. Energy Fuels 2000;14:282. Wu H, Wall TF, Liu G, Bryant G. Energy Fuels 1999;13:1197. Liu G, Wu H, Gupta RP, Lucas JA, Tate AG, Wall TF. Fuel 2000; 79:627. Yu J, Lucas JA, Strezov V, Wall TF. Energy Fuels 2003;17:1160. Shibaoka M. Fuel 1969;48:285. Hamilton LH. Fuel 1981;60:909. Jones RB, McCourt CB, Morley C, King K. Fuel 1985;64:1460. Bend SL, Edwards AS, Marsh H. Fuel 1992;71:493. Thomas CG, Shibaoka M, Gawronski E, Gosnell ME, Brunckhort LF, Phong-anart D. Fuel 1993;72:907. Thomas CG, Shibaoka M, Gawronski E, Gosnell ME, Phong-anart D. Fuel 1993;72:913. Matsuoka K, Ma ZX, Akiho H, Zhang ZG, Tomita A, Fletcher TH, Wojtowicz MA, Niksa S. Energy Fuels 2003;17:984. Xu WC, Matsuoka K, Akiho H, Kumagai M, Tomita A. Fuel 2003; 82:677. Alvarez D, Borrego AG, Mene´ndez R. Fuel 1997;76:1241. Kaiho M, Toda Y. Fuel 1979;58:397. Wall TF, Liu GS, Wu HW, Roberts DG, Benfell KE, Gupta S, Lucas JA, Harris DJ. Prog Energy Combust Sci 2002;28:405. Benfell KE, Liu GS, Roberts DG, Harris DJ, Lucas JA, Bailey JG, Wall TF. Proc Combust Inst 2000;28:2233. Matsuoka K, Kumagai T, Chiba T. ISIJ Int 1996;36:40. Kimura H, Fujii S. Sekitankagaku-to-Kogyo.. Tokyo: Sankyo Pub. Co.; 1979. p. 28. Oka N, Murayama T, Matsuoka H, Yamada S, Yamada T, Shinozaki S, Shibaoka M, Thomas CG. Fuel Process Technol 1987; 15:213.
The physical character of coal char formed during rapid pyrolysis at high pressure Koichi Matsuokaa,1, Hiroyuki Akihoa, Wei-chun Xub, Rajender Guptac, Terry F. Wallc, Akira Tomitaa,* a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan b Institute of Research and Innovation, 1201 Takada, Kashiwa 277-0861, Japan c Cooperative Research Center for Coal in Sustainable Development, Department of Chemical Engineering, School of Engineering, University of Newcastle, Callaghan, NSW 2308, Australia Received 16 January 2004; revised 24 June 2004; accepted 5 July 2004 Available online 13 August 2004
Abstract Rapid pyrolysis was conducted in a drop tube reactor using seven coals under various operating conditions. In addition to dense char, porous chars (network char and cenospheric char) were formed by the rapid pyrolysis under certain conditions. Porous char was mainly composed of film-like carbon and skeleton carbon. The pyrolyzed coal char particles were characterized in detail. Morphology and bulk density of porous char were quite different from the dense char formed under the same conditions, but elemental composition and BET surface area were similar to each other. CO2 gasification reactivity of porous char was lower than dense char in the later gasification stage, and this was ascribed to the low reactivity of skeleton carbon. q 2004 Elsevier Ltd. All rights reserved. Keywords: Coal; Char; Pyrolysis; Cenosphere; Morphology
1. Introduction At the early stage of coal combustion and gasification, devolatilization reaction occurs. During the devolatilization, volatiles are released in the form of gas and tar, and then resultant char is generated. The characteristics of the resultant char control the subsequent combustion and gasification processes. Morphology is one of the important characteristics, because it affects aerodynamic behavior of char in a reactor like entrained-bed combustor or gasifier. The morphology of char depends on the rank and the petrographic composition of parent coal as well as experimental conditions. It is reported that not only * Corresponding author. Tel.: C81-22-217-5625; fax: C81-22-2175626. E-mail address: [email protected] (A. Tomita). 1 Present address: Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba 305-8569, Japan. 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2004.07.006
non-porous char (dense char) but also porous char is formed during rapid pyrolysis and combustion of coal [1–10]. There are several shape types of porous chars, and several classifications on the morphology are proposed on the basis of the porosity and geometric parameters [6]. In these classifications, porous char is generally classified as thinwalled cenosphere, thick-walled cenosphere and network char [6]. There have been several studies on formation mechanisms of porous chars [11–16]. Furthermore, the effects of coal type, gas pressure and heating rate on porous char formation have also been examined [4,5,7,12,13]. From these studies, roughly speaking, formation of porous char is favored by high vitrinite content, high pressure and high heating rate conditions. However, there is essentially no systematic study on the properties of porous char. For example, the reactivity of porous char during gasification and combustion processes has only been speculated from the reactivity of mixture of the dense char and porous char. Such reactivity depends on the characteristics of the char, and these characteristics are determined by the initial stage
64
K. Matsuoka et al. / Fuel 84 (2005) 63–69
of the devolatilization reaction. Thus, we attempt here to characterize char particles by pyrolyzing seven coals under relatively low temperature. The morphology, the elemental composition and the BET surface area, and the gasification reactivity of the chars were determined in this study.
examined by an energy dispersive X-ray analyzer (EDX; EDAX Phoenix), because the amount of char remained after the gasification was too small to provide for ultimate analysis.
3. Results and discussion 2. Experimental 3.1. The effect of the operating condition on char morphology
Analyses of the coals used in this study are listed in Table 1. The size of particle was from 75 to 90 mm for Shenmu coal, and 75–150 mm for all other coals. These coals were pyrolyzed in a drop tube reactor. Details of the experimental apparatus are described elsewhere [17,18]. Coal particles were fed at a rate of 0.1 g/min with a stream of He or H2 at 3.5 l (STP)/min. Heating rate of the coal particles was calculated to be about 104 8C/s. Experiments were carried out in a temperature range from 600 to 850 8C and a pressure from 1.0 to 5.0 MPa. In addition to dense char particles, porous char particles were observed under some conditions as a minor fraction on weight basis. In such a case, the resultant char particles were separated into C100 mesh (!150 mm) fraction and K100 mesh (O150 mm) fraction for further analyses. The char particles were observed by a scanning electron microscope (SEM; Topcon 60). Whole structure image as well as cross sectional image were obtained by SEM observation. Size distribution of porous char was determined from the whole structure images. Elemental composition and ash fraction of the resultant chars were determined from ultimate analysis and proximate analysis, respectively. Bulk density was determined from the volume occupied by a unit weight of char particles in a glass tube with an inner diameter of 8 mm. BET surface area of the sample was determined by N2 adsorption at 77 K using an automatic volumetric sorption analyzer (Quantachrom, Autosorb-1). The dense char and porous char were independently gasified at 900 8C in CO2 by a thermogravimetric analyzer to examine the effect of the char morphology on gasification reactivity. The porous char particles were observed by SEM before and after the gasification. The elemental composition of the remaining char after the gasification was qualitatively
Fig. 1 shows SEM images of Shenmu char particles (O150 mm) formed by rapid hydropyrolysis under different pressures and temperatures. Porous char was abundant in the char with a size over 150 mm. On the other hand, few porous char was found in the char with a size below 150 mm (not shown here). For this reason, the two fractions (!150 mm fraction and O150 mm fraction) are hereafter termed as dense and porous char, respectively. Appearance of porous char could be seen from these images, but detailed structure was unclear. Therefore, the observation of cross sectional images was made with SEM. Fig. 2 shows an example of cross sectional image of Shenmu char particles (O150 mm) prepared at 800 8C under 3.0 MPa-H2. Char particles can roughly be classified into three types, cenospheric char (C), network char (N) and dense char (D) according to the classification proposed by Cloke and Lester [6]. The image in Fig. 1 cannot differentiate two types of porous char, cenospheric char and network char. In this paper, a mixture of cenospheric char and network char is termed porous char. We checked the effect of operating conditions on the formation of porous char. Table 2 lists experimental conditions and whether or not porous char could form under each condition. ‘Yes’ in this Table means the case where porous char particles were found in the whole char. The weight fraction of porous char was 1.0 wt% at the most. The fraction seems to be small at first glance, but it is not small when expressed by vol.%, because the density of porous char particle was very low as expected from Figs. 1 and 2. Porous char was formed in the case of pyrolysis of Shenmu coal in He atmosphere as well as in H2 atmosphere.
Table 1 Analyses of coals used Coal
Berau Adaro Pasir Taiheiyo Shenmu Blair Athol Datong a
CSNa (K)
Proximate analysis (wt%, dry)
Ultimate analysis (wt%, daf)
Ash
VM
FC
C
H
N
SCO (diff.)
Vitrinite
Exinite
Inertinite
2.9 1.6 4.4 10.6 6.4 6.6 10.0
46.6 45.4 44.4 46.9 31.8 30.3 29.7
50.5 53.0 51.2 42.5 61.8 63.1 60.3
68.4 70.6 71.7 76.1 77.4 78.4 82.7
4.7 4.8 5.1 6.4 4.5 4.6 4.7
1.6 1.2 1.3 1.4 1.1 2.0 1.1
25.3 23.4 21.9 16.1 17.0 15.0 11.6
92.8 94.8 86.6 89.6 62.3 37.1 47.8
4.6 1.8 5.4 8.3 2.2 5.8 2.7
2.6 3.4 8.0 2.1 35.5 57.1 49.5
Crucible swelling number (JIS M8801).
Maceral (vol.%, dmmf)
0 0 0 0 0 0 0
K. Matsuoka et al. / Fuel 84 (2005) 63–69
65
Fig. 1. SEM images of Shenmu char formed in hydropyrolysis under different conditions: (a) 700 8C, 3.0 MPa; (b) 800 8C, 3.0 MPa; (c) 850 8C, 3.0 MPa; (d) 800 8C, 1.0 MPa; (e) 800 8C, 5.0 MPa.
Though not shown in this paper, the morphology of porous char formed in He atmosphere was similar to that in H2 atmosphere. Moreover, the elemental composition and the ash content of Shenmu porous char formed under 3 MPa-He and at 800 8C were similar to those of char formed in H2 atmosphere (Table 3). In contrast to the pyrolysis in He atmosphere, no porous char was formed from Berau, Adaro, Pasir and Blair Athol in H2 atmosphere. Formation of porous char is likely to be related to caking property. The crucible swelling number (CSN; JIS, M8801) of all the coals used in this study is zero (Table 1), indicating that all the coals do not swell under an atmospheric pressure. This does not necessarily mean that these coals do not swell under the present experimental conditions, because the condition for CSN measurement is far from the present pyrolysis condition. In fact it is well known that the caking property is affected by gas atmosphere. Kaiho and Toda observed, for
example, the maximum fluidity was significantly affected by surrounding gas as well as gas pressure. At higher pressure, even non-swelling coal greatly swells especially under hydrogen atmosphere [20]. Therefore, it is quite probable that a small portion of coal particles softened under rapid heating at high-pressure, and the softened particle formed porous char particle. The effect of pressure on the formation of porous char is not straightforward. High pressure is generally preferable for the formation of porous char [21,22]. Softening of coal particle and increasing of internal pressure are essential to form porous char [20,23]. The external pressure is also an important factor. At a high pressure, even a coal having CSN value of 0 can form porous char. We examined here the pressure effect in a range from 1.0 to 5.0 MPa using Shenmu coal, which shows very little softening property at ambient pressure. Fig. 3 shows the particle size distribution of Shenmu porous char
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K. Matsuoka et al. / Fuel 84 (2005) 63–69
Fig. 2. Cross sectional images of Shenmu char formed in hydropyrolysis at 800 8C and 3.0 MPa. C, cenospheric char; N, network char; D, dense char.
formed under different H2 pressures. The size distribution was obtained from the several SEM images of whole char (Fig. 1(b, d and e)). Particle size range of the raw coal (75–90 mm) was also shown in this Figure. About 50% of
porous char formed at 1.0 MPa was more than 275 mm, while the size of most porous chars prepared at 3.0 and 5.0 MPa were less than 275 mm. The gas evolution from the inside of coal particle would be suppressed at a high external pressure [21,22], and therefore swelling of coal particle would be suppressed under a pressure of 5 MPa. Size of a porous char particle depends on the balance between the pressure from the inside of particle and the external pressure. The preparation temperatures for the porous chars shown in Fig. 1(a–c) were 700, 800 and 850 8C, respectively. Morphology of porous char seems to be independent of pyrolysis temperature. The effect of temperature on the morphology is rather insignificant in the range temperature used in this study. Heating rate is also one of the important factors to control the char morphology [12]. About 120 mg of Shenmu coal was set in a tiny quartz bottle and it was heated at a low heating rate (heating rate: 25 8C/min, maximum temperature: 800 8C, gas: He, pressure: 0.1 or 1.0 MPa). After holding at 800 8C for 10 min, it was allowed to cool down and then appearance of the char was observed. No porous char particles were observed in the resultant char irrespective of pressure, while porous char was found when heated the same coal at 104 8C/s in various gas atmosphere (Table 2). Rapid heating leads to sudden formation of volatile matter that is in favor of balloon formation, whereas slow heating may help volatile matter to slowly escape from a particle without forming balloon. Many researchers insist that cenospheric char is formed from vitrinite [12–14,19]. In the present study, porous char was formed not only from vitrinite-rich coals but also inertinite-rich Shenmu and Blair Athol coals (Table 2). However, this observation cannot exclude the possibility of porous char formation from vitrinite, because the weight fraction of porous char particles in Shenmu char was at most 1 wt%, while the weight percentages of vitrinite part in Shenmu and Blair Athol coal were 62 and 37 wt%, respectively. These amounts were sufficient to form the small amount of porous char as reported here. It is of interest
Table 2 Formation of porous char under different experimental conditions Coal
Temperature (8C)
Gas
Pressure (MPa)
Porous chara
Coal
Temperature (8C)
Gas
Pressure (MPa)
Porous chara
Berau Adaro Pasir Taiheiyo
800 800 800 600 700 800 850 800 800 800
He He He He He He He He He He
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 3.0 1.0
Yes Yes Yes No No No No Yes Yes Yes
Berau Adaro Taiheiyo
800 800 800 850 700 800 800 800 850 800 800
H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2
1.0 1.0 1.0 1.0 3.0 1.0 3.0 5.0 3.0 1.0 1.0
No No No No Yes Yes Yes Yes Yes No No
Shenmu Blair Athol a
Shenmu
Blair Athol Datong
Yes: the weight fraction of porous char was 1.0 wt% at the maximum. No: porous char was absent.
K. Matsuoka et al. / Fuel 84 (2005) 63–69
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Table 3 Analyses of porous char and dense char formed on pyrolysis of Shenmu coal Temperature (8C)
Gas
Pressure (MPa)
Char type
C (wt%, daf)
H (wt%, daf)
N (wt%, daf)
SCO (diff; (wt%, daf))
H/C (K)
Ash (wt%, dry)
700
H2
3
800
H2
1
800
H2
3
800
H2
5
850
H2
3
800
He
3
Porous char Dense char Porous char Dense char Porous char Dense char Porous char Dense char Porous char Dense char Porous char Dense char
91.7 85.2 88.4 88.9 89.1 91.1 89.6 90.6 90.4 92.7 87.4 85.8
3.6 3.3 3.0 2.9 3.1 3.1 3.3 3.2 2.7 2.8 2.9 2.7
1.6 1.0 1.2 0.9 1.3 0.9 1.2 0.8 1.1 0.8 1.2 0.9
3.2 10.4 7.4 7.2 6.5 4.9 6.0 5.4 5.8 3.7 8.5 10.5
0.47 0.46 0.41 0.39 0.42 0.41 0.44 0.42 0.36 0.36 0.40 0.38
10.7 7.9 6.5 8.1 5.4 8.1 5.4 9.8 6.5 8.7 5.0 10.0
to note that no porous char was found in vitrinite-rich Taiheiyo coal char even if temperature and pressure were widely changed. One possible reason is the peculiar maceral composition of Taiheiyo coal; degradinite, which exhibits low plasticity upon heating, is abundant in vitrinite [24]. Anyway, the relationship between porous char formation and maceral composition in the parent coal is not straightforward. 3.2. Characterization of porous char The elemental composition of porous char determined by ultimate analysis was similar to that of dense char formed under the same conditions (Table 3). H/C atomic ratio of the dense char decreased with increasing pyrolysis temperature, while it was almost independent of pressure. This trend was also seen in porous char. SEM/EDX mapping analysis was carried out to clarify the elemental distribution of porous char particle embedded in the carnauba wax. Porous char consists of skeleton part and film part. This difference can be more clearly seen later in Fig. 5. The main component of the skeleton was carbon, but some ash particles were also observed in this part. The ash content in porous char determined from the proximate analysis was relatively lower than that in dense char in most cases (Table 3). Table 4 shows bulk density of raw coal, dense char and porous char. The bulk density of porous char ranged from 0.03 to 0.07 g/cm3, which was one order smaller than that of dense char. This was expected from the balloon type shape (Figs. 1 and 2). The bulk density of porous char was quite low irrespective of operating conditions and coal type. This difference between dense char and porous char would have significant influence on overall reactivity and aerodynamic behavior of char particles in combustor or gasifier. We carried out N2 adsorption experiment at 77 K for Shenmu chars listed in Table 4. The BET surface area of porous char and dense char ranged 13–51 and 11–61 m2/g, respectively. Unfortunately, the amount of porous char used for N2 adsorption experiment was too small to obtain reliable BET surface area. The data do not show any
meaningful trend as a function of temperature and pressure. We can only note that the BET surface area of porous char was not very different from that of dense char. 3.3. Gasification reactivity As described above, properties of porous char were similar to those of dense char in many aspects. The examination of reactivity of porous char is also of interest. Maloney and Jenkins [3] determined combustion reactivity in air at 395 8C of dense char as well as a mixture of dense char and porous char (the mixing ratio was not reported). They found that the time required to attain a fractional burn-off of 0.5 was independent of the presence of porous char. Benfell et al. [22] investigated the combustion reactivity of mixture of dense char and porous char in 50% O2/N2K1.5 MPa at 375 8C. Apparent combustion rate was lower for the mixture containing a larger amount of porous char. Oka et al. [25] examined the combustion reactivity of seven coals at 1200–1300 8C. The reactivity of coal chars was low when many porous char particles were formed. They suggested that the low combustion reactivity
Fig. 3. Size distribution of Shenmu porous char formed at 800 8C in different H2 pressures.
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Table 4 Bulk density of raw coal, dense char and porous char Coal Berau Adaro Pasir Taiheiyo Blair Athol Shenmu
Temperature (8C) 800 800 800 800 800 700 800 800 800 850
Gas He He He He He H2 H2 H2 H2 H2
Pressure (MPa) 1 1 1 1 1 3 1 3 5 3
Bulk density (g/cm3) Raw coal
Dense char
Porous char
0.66 0.66 0.64 0.69 0.61 0.59 0.59 0.59 0.59 0.59
0.33 0.28 0.17 0.40 0.23 0.30 0.37 0.31 0.35 0.38
0.04 0.06 0.04 N.D. 0.07 0.05 0.03 0.05 0.03 0.06
was due to the presence of porous char, but they did not mention the reason for the low reactivity of porous char. Thus, there is essentially no report so far on gasification reactivity of porous char by itself. We carried out here CO2 gasification at 900 8C using separated porous char particles and dense char particles prepared from Shenmu coal at 800 8C in 1.0 MPa-He. Fig. 4 represents CO2-gasification profiles. Initial gasification rate was almost the same for both the chars. On the contrary, in the later stage (gasification conversion of over 50%), the gasification rate of porous char was lower than that of dense char. The specific gasification rate of porous char was derived from the first-order kinetic plot, and it was three times lower in the later region than in the initial region. The gasified porous char was analyzed by SEM/EDX to clarify the reason for this difference. SEM images of porous char before and after the gasification are shown in Fig. 5. At a 50% conversion (Fig. 5(b)), film portion (this is composed of carbon according to the SEM/EDX analysis) was partly disappeared but skeleton portion seemed to remain as they were. At a high conversion of 90% (Fig. 5(c)), film-like portion disappeared almost completely and the width of remaining skeleton became thin. Since film-like portion is very thin, it may be easy to break down and the fragments
Fig. 4. CO2-gasification profiles of Shenmu dense char and porous char formed at 800 8C in 1.0 MPa-He.
Fig. 5. SEM images of Shenmu porous char before and after CO2 gasification. Char conversion: (a) 0%, (b) 50%, (c) 90%.
K. Matsuoka et al. / Fuel 84 (2005) 63–69
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char particles, cenosphere and network char, were formed in addition to dense char under some pyrolysis conditions. The physical characteristics of porous char were similar to those of dense char in many aspects, except for morphology and bulk density. In addition to morphology and bulk density, gasification reactivity of the porous char was different from that of the dense char, especially in the later stage. During gasification of porous char, film-like carbon was preferentially gasified and remaining skeleton carbon was gasified in the later stage. The porous char was more graphitic than the dense char, and the calcium species which may act as catalyst was less abundant in the porous char. Therefore, the reactivity of the porous char was lower than that of dense char.
Acknowledgements Fig. 6. XRD profile for Shenmu char formed in hydropyrolysis at 800 8C and 3.0 MPa. C, calcite (CaCO3); Q, quartz (SiO2).
were either gasified or swept away from the skeleton. Therefore, the main reason for the decrease in reactivity of porous char in the later stage is that the skeleton carbon is less reactive than film carbon. We attempted to clarify the reason for low reactivity of skeleton carbon. Fig. 6 shows the XRD patterns for the porous char and dense char after hydropyrolysis and before CO2-gasification. A broad 002 band of carbon was observed at around 258 for both the chars. The peak of porous char was slightly sharper than that of dense char, indicating that the porous char was slightly more graphitic. The main ash components are quartz and calcite for both chars and the peak intensity is lower for porous char. This is in accordance with the lower ash content in porous char as shown in Table 3. Thus, a higher ordering of carbon layers and a lower content of catalytic mineral component like calcium species may be the reason for the lower reactivity of porous carbon. Among porous carbon, film carbon and skeleton carbon have different structure and the latter remained in the later gasification stage. In entrained-bed combustion and gasification processes, the presence of porous char may strongly affect aerodynamic behavior of char. In some cases, this may leads to too much carry-over, plugging at narrow equipment, and others. Therefore, the understanding of formation mechanism is of importance for more smooth operation. Furthermore, the presence of less-reactive portion in char may control overall reactivity of char. It is thus of interest to clearly identify the reactivity of various components in char. The presence of rather inactive skeleton part in porous char may be the least reactive part in the whole char.
4. Conclusion Coal chars formed by rapid pyrolysis under various operating conditions were characterized in detail. Porous
This study was partially supported by New Energy and Industrial Technology Development Organization (NEDO) under BRAIN-C program. The authors thank Dr H. Orikasa in Institute of Multidisciplinary Research for Advanced Materials, Tohoku University for carrying out XRD analysis.
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