Effect of CS2 extraction on the physical properties and gasification activity of liquid-phase carbonization cokes

Journal of Analytical and Applied Pyrolysis 93 (2012) 33–40

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Effect of CS2 extraction on the physical properties and gasification activity of liquid-phase carbonization cokes Sheng Huang, Shiyong Wu, Yamin Ping, Youqing Wu, 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, 130 Meilong Road, Shanghai 200237, China

a r t i c l e

i n f o

Article history: Received 13 July 2011 Accepted 19 September 2011 Available online 25 September 2011 Keywords: Liquid-phase carbonization coke CS2 extraction Physical properties Gasification activity

a b s t r a c t Two kinds of liquid-phase carbonization cokes (LPCCs) were prepared using vacuum residue (VR) and coal tar pitch (CTP) at 350–500 ◦ C, at which the carbonaceous mesophase appears typically. And the effect of CS2 extraction on the physical properties (carbon crystalline structures, surface morphologies, surface areas and pore structures) and gasification activity of LPCCs were mainly investigated. The increasing carbonization temperature (CT) was favorable for LPCCs to form more ordered carbon crystallite structures, but their graphitization was far from natural graphite. The surface or inner pores of unextracted LPCCs conglutinated a large quantity of CS2 -soluble fractions, which severely blocked the pore structure of LPCCs. Although the carbon crystalline structures of LPCCs after extraction were significantly unchanged, the BET surface areas and total pore volumes of extracted LPCCs increased significantly, which resulted in a greatly increasing gasification activity of LPCCs. And a higher extraction yield led to a higher increasing extent of gasification activity of LPCCs after extraction, regardless of precursors (VR and CTP). The soluble fractions conglutinated on the surface or inner pores of LPCCs were the most important factor of affecting on the gasification activity of LPCCs. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Petroleum coke is a carbonaceous solid derived from oil refinery units. With the development of crude oil refining technology and the continuous increasing of crude oil supply in worldwide, its output is increasing steadily [1,2]. Thus, how to use petroleum coke (especially high-sulfur petroleum coke) in a reasonable, efficient, and clean way becomes a subject worthy of being studied in depth. Gasification technology can convert a wide variety of carbonaceous materials (such as coal, biomass and petroleum coke) into useful syngas (mainly CO and H2 ) with nearly zero pollution emissions [3,4]. Therefore, it is considered to be an effective way to utilize petroleum coke for the production of syngas. It has been reported that the surface area of petroleum coke is low [5,6] and the carbon crystalline structure of petroleum coke is relatively ordered [5,7], resulting in its low gasification activity, which greatly restricts the applicability of petroleum coke as the feedstock of gasifiers. Petroleum coke is a kind of liquid-phase carbonization cokes [8]. Consequently, it is very imperative to understand the physicochemical properties of liquid-phase carbonization cokes (LPCCs) and their effects on the gasification activity of LPCCs.

∗ Corresponding author. Tel.: +86 21 64252058; fax: +86 21 64252058. E-mail address: [email protected] (J. Gao). 0165-2370/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2011.09.008

It is well-known that the gasification activity of carbonaceous materials is closely related to their chemical compositions and structures [5–7,9]. Solvent extraction is a traditional and useful method for investigating the chemical compositions and structures of carbonaceous materials, and has been extensively used to investigate the physicochemical structures of carbonaceous materials [10–15]. However, few reports are focused on the effects of solvent extraction on the physicochemical properties and gasification activity of LPCCs. Therefore, the present study mainly made some investigations on carbon crystalline structures, surface morphologies, surface areas, pore structures and gasification activities of unextracted and extracted LPCCs. These investigations can not only provide valuable information for the efficient utilization of liquid-phase carbonization materials (such as petroleum coke) as the feedstocks of gasifiers, but also enrich the knowledge of the liquid phase carbonization materials.

2. Experimental 2.1. Preparation of LPCCs Two kinds of LPCCs were separately prepared from vacuum residue (VR) and coal tar pitch (CTP), which were respectively from Gaoqiao Petrochemical Co. Ltd. of China and Baosteel Co. Ltd. of China, in the carbonization temperature (CT) of 350–500 ◦ C, at

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S. Huang et al. / Journal of Analytical and Applied Pyrolysis 93 (2012) 33–40

Table 1 Proximate and ultimate analysis of VR and CTP. Samples

VR CTP

extraction yield (Y) was calculated according to the following equation:

Proximate analysis (wt%)

Ultimate analysis (wt%)

Ad

Vd

FCd

Cd

Hd

Nd

St,d

0.11 0.21

63.49 27.41

36.40 72.38

80.23 87.14

11.45 5.27

1.03 0.56

6.24 0.57

A: ash; V: volatile matter; FC: fixed carbon; C: carbon; H: hydrogen; N: nitrogen; St : total sulfur; d: dry basis.

Y (wt%) =

Wc,daf − Wr,daf Wc,daf

(1)

where Wc,daf and Wr,daf are the mass of LPCCs and LPCC residues (i.e., extracted LPCCs) on the dry ash-free basis, respectively. The LPCC residues from X350, X400, X450 and X500 were separately termed as X350E, X400E, X450E and X500E. 2.3. Characterization of LPCCs

Table 2 LPCC yields from VR and CTP at different CTs. Carbonization temperature (◦ C)

LPCCs from VR (wt%)

LPCCs from CTP (wt%)

350 400 450 500

27.24 19.32 16.27 15.03

92.69 86.78 77.16 74.41

which the carbonaceous mesophase typically appears. The proximate and ultimate analysis of VR and CTP are shown in Table 1. From the table, it can be found that the VR and CTP are two kinds of high carbonaceous materials, and can be used as the feedstock of gasifier. LPCCs from X (VR or CTP), whose CT were 350 ◦ C, 400 ◦ C, 450 ◦ C and 500 ◦ C, were respectively termed as X350, X400, X450 and X500. The LPCC yields from VR and CTP at different CTs and the proximate and ultimate analysis of various LPCCs are shown in Tables 2 and 3. The LPCCs were prepared at the conditions similar to the industrial delayed coking in a tubular reactor with an inner diameter 55 mm and a length of 1500 mm. For each run, about 75 g of raw material (VR or CTP) was loaded in a corundum boat located in the center of the tubular reactor. An N2 (99.999%, V/V) flow of about 400 ml/min as a sweeping gas was entrained into the tubular reactor. After sweeping for about 10 min, the reactor was heated at a heating rate of 3 ◦ C/min from room temperature to the prescribed temperature by a programmed-temperature controller and held at this temperature for 12 h under an N2 atmosphere. In the end of each run, the resultant cokes were cooled to room temperature under an N2 atmosphere, and then stored for LPCC samples.

2.2. CS2 extraction of LPCCs In each run, about 10 g of LPCCs was extracted by 150 ml of CS2 under an N2 atmosphere (99.999%, V/V) until the CS2 solvent appeared colorless, using a Soxhlet extractor with an oil bath where the temperature was controlled at 60 ◦ C. After extraction, the extracted LPCCs, namely, the LPCC residues were dried for 12 h at 80 ◦ C under ambient pressure to remove the CS2 solvent. The

A scanning electron microscopy (JEOL JSM-6360LV) was employed to observe the surface morphologies of unextracted and extracted LPCCs at the conditions of a 15 kV voltage and amplified 5000 times. The carbon crystallite structures of LPCCs were analyzed using an X-ray diffraction analyzer (JSM-6360LV; D/maxrA12Kw, 12 kW, 40 kV, 100 mA, CuK␣ radiation). The analysis of pore structure properties of unextracted and extracted LPCCs was conducted by an N2 adsorption (77 K) technique using the pore structure analyzer (ASAP 2400). 2.4. CO2 gasification activity of LPCCs Measurement of CO2 gasification activity for extracted and unextracted LPCCs was carried out by an isothermal method using a thermo-gravimetric analyzer (SETARAM TG-DTG/DSC). The detailed description of the analyzer and the procedures can be seen in our previous studies [16]. Before the measurement of gasification activity of LPCCs, a set of preliminary experiments were performed to establish the experimental conditions under which the gasification rate of LPCCs was kinetically controlled. Chemical control conditions were attained as the following: sample mass, 8 mg; carbon dioxide flow, 60 ml/min; particle diameter of samples, ≤73 ␮m [16]. So under our experimental conditions (about 5 mg of samples, a particle diameter of below 73 ␮m and a carbon dioxide flow of 60 ml/min), all the gasification experiments were performed under a regime controlled by chemical reaction at the gasification temperature of 1000 ◦ C [16–19]. In the present study, the carbon conversion (X) was calculated using the following equation: X=

W0 − W W0 − Wash

(2)

where W0 and W are the initial and instantaneous mass of the sample, respectively, and Wash is the mass of the ash in the sample. For the quantitative comparison, the gasification reactivity index Rs (Rs = 0.5/t0.5 , t0.5 denotes the time needed for the carbon conversion of 50%) was used to characterize the gasification activity of LPCCs. 3. Results and discussion 3.1. Elemental compositions of LPCCs

Table 3 Proximate and ultimate analysis of LPCCs from VR and CTP. Samples

VR350 VR400 VR450 VR500 CTP350 CTP400 CTP450 CTP500

Proximate analysis (%)

Ultimate analysis (%)

Ad

Vd

FCd

Cd

Hd

Nd

St,d

0.31 0.33 0.30 0.31 0.26 0.25 0.28 0.25

14.08 12.17 11.64 11.36 11.76 10.53 9.15 8.86

85.61 87.50 88.06 88.33 87.98 89.22 90.57 90.89

86.21 86.87 86.82 87.71 90.82 92.37 93.05 93.89

5.49 5.07 4.74 4.32 4.21 3.81 3.58 3.44

0.72 0.76 0.71 0.74 0.44 0.41 0.45 0.40

5.49 5.63 5.51 5.45 0.60 0.51 0.45 0.39

From Table 3, it was observed that the LPCCs from VR and CTP have the characteristics of high carbon content and low ash and volatile matter, which are similar to the petroleum coke and pitch coke reported in the literatures [1,2,5–7]. The LPCCs (from VR and CTP) produced in the experimental range are two kinds of high carbonaceous materials similar to the coal chars from high temperature pyrolysis [9,16]. Besides, it can be seen that the carbon content increased and the hydrogen content decreased gradually with the increasing CT, which suggests that the higher CT is favorable to produce the higher carbon content LPCCs. While the N and S contents of LPCCs from VR and CTP were nearly kept unchanged or slightly

S. Huang et al. / Journal of Analytical and Applied Pyrolysis 93 (2012) 33–40

15

80

CTP500

LPCCs from CTP

002

LPCCs from VR

12

3

Intensity ( 10 CPS)

60 Y (wt%)

35

40

9

6

3

20

0

0 350

400

450

500

Carbonization temperature (ºC)

γ

10

100

20

30

2θ(° )

40

50

60

Fig. 2. XRD patterns of CTP500.

Fig. 1. Extraction yields of various LPCCs at different CTs.

decreased with the increasing CT, implying that the nitrogen- and sulfur-containing species in the VR and CTP were relatively thermally stable at the CT of 350–500 ◦ C. 3.2. Extraction yields of LPCCs The CS2 extraction yields of various LPCCs from VR and CTP at different CTs are shown in Fig. 1. From the figure, it was found that the CS2 extraction yields of LPCCs from both VR and CTP decreased monotonously with the increasing CT, suggesting that the extraction yields of LPCCs strongly depended on the CT. It was agreed with the influence of the CT on the extraction yields of gas-phase carbonization chars (e.g., carbon black) [20] and coal chars [10]. This was probably ascribed to that the increasing CT was favorable for the further decomposition and polymerization of soluble fractions contained in LPCCs, resulting in the decrease of the CS2 -extractable fractions [10,21]. Also, it was clearly presented that the extraction yields of LPCCs from CTP were always higher than those of VR at the same CT, although the difference between the two became quite little at the high CT of 500 ◦ C. Therefore, it could be concluded that the liquid-phase carbonization of VR was easier than that of CTP. Besides, it was obvious that the extraction yields (14.12–66.69%) of LPCCs were much higher than those of coal chars, which were reported to be not more than 10% in the literature [10]. Due to the high contents of soluble fractions in LPCCs, it was affirmed that the physicochemical properties and gasification activity of LPCCs would be different from those of coal chars [5,6,9,16], and the high soluble fractions might be one of the reasons leading to the low gasification of LPCCs. 3.3. Carbon crystalline structure of LPCCs Fig. 2 shows a typical diffraction pattern of LPCCs (CTP500). The characteristic peak in the low angle region (2 = 20–35◦ ) corresponding to the (0 0 2) peak of graphite, was generally attributed to the stacking of the graphitic basal plan of carbon crystallites [22,23]. However, the characteristic peak appeared asymmetric (solid line) due to the existence of a ␥ band (on its left-hand side) associated to the isotropic matters, which included premesogenic structures [22,23] and/or non-aromatic structures (polynaphthenic or aliphatic). Fig. 3 shows the XRD patterns of unextracted and extracted LPCCs from CTP at different CTs. The carbon crystalline structures of LPCCs from VR were also detected by XRD and the results were almost the same as those of LPCCs from CTP. Therefore, the diffraction patterns of LPCCs from VR were not presented in the investigation. From Fig. 3, it was concluded that various extracted

and unextracted LPCCs contained two types of carbons: a crystalline carbon with graphite-like structure and a amorphous carbon (highly disordered) contributing to the ␥ band. Lu et al. [22] also obtained the similar conclusion. There were no significant differences in the position of the characteristic peaks (0 0 2 and 1 0 0 peak) or in the values of FWHM (full width at half maximum of the characteristic peaks of XRD patterns) between unextracted and extracted LPCCs. However, the intensity of the ␥ band of LPCCs was decreased after extraction. This was probably owing to the decrease of isotropic matters (CS2 -soluble fractions) contained in LPCCs after extraction. Furthermore, the decreasing extent of the ␥ band intensity of LPCCs after extraction became smaller and smaller with the increasing CT. This was probably ascribed to that less CS2 -soluble fractions could be extracted with the increasing CT, as shown in Fig. 1. The intensity of the ␥ band of LPCCs presented a decreasing trend with the increasing CT. This was due to the progressive elimination or transformation of isotropic matters (CS2 -soluble fractions) contained in LPCCs [15,24]. From the occurrence of wide (1 0 0) peaks of LPCCs and almost no significant changes of the peak intensity with the increasing CT, it was inferred that the coalescence growth of longitudinally adjacent basic structure units dominated the growth process of carbon crystalline structure of LPCCs in the experimental temperature range, and the condensation growth of basic structure units within the aromatic nuclear of carbon crystalline could be negligible [25,26]. That was to say, with the increasing CT, the orientation degree of carbon crystalline lamellar was higher and higher, while there were nearly no changes in the diameter of carbon crystalline lamellar [26]. In order to further understand the crystallite structures of LPCCs, the microcrystalline structure parameters of extracted and unextracted LPCCs from VR and CTP (d002 : the interplanar spacing; Lc : the stacking height) were calculated according to method described in our previous investigation [16]. Due to the complexity of carbonaceous part of LPCCs (or coal chars), many investigators [16,27,28] considered that LPCCs (or coal chars) contained several different carbon structures and that the normalized XRD curves were separated into several deconvoluted Gauss curves. In the studies of Oya et al. [27] and Wang et al. [28], the carbon structures of coal chars were separated into a broad amorphous component (A component) and a relatively broad turbostratic component (T component) and that the normalized XRD curves were separated into two deconvoluted Gauss curves. In the study, due to the poor crystalline structure of LPCCs, the carbon crystalline structure of LPCCs could be divided into two components of a relatively poor crystalline structure (P component) and a relatively good crystalline structure (G component). The separated Gauss curves for the 0 0 2 peaks of CTP450 and CTP450E are presented in Fig. 4. The detailed calculation method and procedure of microcrystalline structure

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S. Huang et al. / Journal of Analytical and Applied Pyrolysis 93 (2012) 33–40

a

b

15 CTP350

15 CTP400

CTP350E

CTP400E 12

3 Intensity ( 10 CPS)

3 Intensity ( 10 CPS)

12

9 002 6 γ 3

9 002 6 γ 3 100

100 0 10

0 20

30

40

50

60

10

20

30

2θ(° )

c

d

15 CTP450

50

60

15 CTP500 002

CTP450E

CTP500E

12

3 Intensity ( 10 CPS)

12

3 Intensity ( 10 CPS)

40

2θ(° )

002

9

6 γ

9

6

γ

3

3

100

100 0

0 10

20

30

40

50

60

10

20

2θ(° )

30

40

50

60

2θ(° )

Fig. 3. XRD patterns of various extracted and unextracted LPCCs from CTP.

parameters of LPCCs base on the two separated components can be found in our previous investigation [16]. Table 4 shows the microcrystalline structure parameters of extracted and unextracted LPCCs from VR and CTP at different CTs.

CTP450

002

It could be seen that with the increase of CT, d002 of unextracted and extracted LPCCs decreased and Lc of unextracted and extracted LPCCs increased, suggesting that the increasing CT was favorable for unextracted and extracted LPCCs to form more ordered carbon

CTP450E

P+G

002

G

P+G

G P

10

15

20

25 2 θ(° )

30

P

35

10

15

20

25 2 θ(° )

Fig. 4. Normalized XRD curves for the 0 0 2 peaks of CTP450 and CTP450.

30

35

S. Huang et al. / Journal of Analytical and Applied Pyrolysis 93 (2012) 33–40

37

Fig. 5. SEM images of several unextracted and extracted LPCCs from VR and CTP (5000×).

crystallite structures. It was in accordance with the influence of CT on the carbon crystallite structures of coal chars [5,16,26] and gasphase carbonization chars [20]. d002 of various LPCCs (coal pitch and petroleum residue as the precursors) ranged from 0.343 nm to 0.358 nm, which was in accordance with those reported by these literatures [23,25], depending on the precursor and the experimental conditions used for the carbonization. Furthermore, it can be found that the d002 and Lc of extracted LPCCs are almost the same as those of unextracted LPCCs, indicated that the ordering degree of carbon crystalline structures of LPCCs were significantly unchanged after the removal of CS2 -soluble fractions. Besides, in comparison with natural graphite, it was found that d002 and Lc of unextracted and extracted LPCCs produced in the experimental temperature range were far from those of natural graphite (d002 : 0.3363 nm; Lc : 17.3129 nm), suggesting that the “graphitization” extent of unextracted and extracted LPCCs was quite low. 3.4. Surface morphologies of LPCCs The surface morphologies of several extracted and unextracted LPCCs from VR and CTP were observed by SEM and shown in Fig. 5. It could be observed that a large quantity of fine particles (CS2 -soluble fractions) conglutinated on the surface of unextracted Table 4 Carbon crystallite structure parameters of extracted and unextracted LPCCs from VR and CTP. Samples

d002 (nm)

Lc (nm)

Samples

d002 (nm)

Lc (nm)

LPCCs from VR 0.352 VR350 0.349 VR400 0.345 VR450 0.344 VR500

2.75 3.27 3.61 3.80

VR350E VR400E VR450E VR500E

0.351 0.349 0.347 0.343

2.57 3.18 3.68 3.92

LPCCs from CTP 0.358 CTP350 CTP400 0.352 0.348 CTP450 0.344 CTP500

2.89 3.46 3.82 4.07

CTP350E CTP400E CTP450E CTP500E

0.357 0.350 0.348 0.345

3.04 3.62 3.88 4.02

LPCCs from VR and CTP, and the amount of conglutinated fine particles decreased with the increasing CT, which was in agreement with the results of CS2 extraction yields of various LPCCs at different CTs (Fig. 1). It is well-known that the gasification of carbonaceous materials is a typical gas-solid reaction, and the diffusion resistance of gasifying agents to the active sites is an important factor related to their gasification activities [4–7]. These fine particles which conglutinated on the surfaces or inner pores of LPCCs would hinder the convenient diffusion of reaction gases into pores, resulting in the decreasing gasification activity of LPCCs (as shown in Fig. 9). For extracted LPCCs from VR, the surface was smooth and hardly presented visually identifiable pores. This was also obtained by Tran and Bhatia [6], who found that the pore network of LPCCs from VR were initially largely inaccessible, and the blocked pores were opened only after the gasification of LPCCs at the carbon conversion of about 5%. However, the surface of extracted LPCCs from CTP exhibited a certain amount of visually identifiable pores, which was different from those of extracted LPCCs from VR. This was probably attributed to the differences in the carbonization processes of different raw materials [29,30].

3.5. Pore structures of LPCCs In order to investigate the pore properties of LPCCs, some extracted and unextracted LPCCs from VR and CTP were analyzed by a N2 adsorption method. The results are shown in Figs. 6–8 and Table 5. The pore size distributions in Fig. 6 indicate that the extracted and unextracted LPCCs have a relatively complete pore structure (including micro-, meso- and macro-pores), and the pore size distributions of extracted LPCCs are similar to those of unextracted LPCCs (such as a visible peak of the pore distribution curves at the pore diameter of 2.7 nm and the volumes of pores below 10 nm accounting for a large proportion of total pore volumes) [31–33], which illustrate that the pore size distribution of LPCCs are significantly indeclinable after the removal of CS2 -soluble fractions conglutinated on the surface or inner pores of LPCCs [31,32].

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S. Huang et al. / Journal of Analytical and Applied Pyrolysis 93 (2012) 33–40 Table 5 Pore characteristic parameters of extracted and unextracted LPCCs.

dV/dlgD ( 10-3 cm3 .g-1 .nm-1 )

60 VR350E

Samples

BET surface area (m2 g−1 )

Total pore volume (cm3 g−1 )

Average pore diameter (nm)

VR350 VR350E VR500E CTP500 CTP500E

3.92 42.39 36.86 4.91 29.81

0.0076 0.0477 0.0407 0.0097 0.0352

7.76 4.50 4.41 7.90 4.72

45 VR500E 30

CTP500E CTP500 VR350

15

suggesting that the surface areas of pores below 10 nm make a main contribution to surface areas of LPCCs (especially extracted LPCCs), which is in accordance with the results presented in Fig. 8(a). However, Fig. 7(b) shows that the micro–meso-pores (below 10 nm) and the meso–macro-pores (above 10 nm) separately make different contributions to the pore volumes of extracted and unextracted LPCCs. For extracted LPCCs, the pore volumes below 10 nm are obviously larger than those above 10 nm; for unextracted LPCCs, the pore volumes below 10 nm are smaller than those above 10 nm. This is in accordance with the results presented in Fig. 8(b). Table 5 shows the pore characteristic parameters of extracted and unextracted LPCCs. From the table, two main results can be obtained:

0 1

10

100

D (nm) Fig. 6. Pore size distribution of extracted and unextracted LPCCs (D: average pore diameter; V: pore volume).

However, Figs. 7 and 8 show a obvious increase of total pore volumes of LPCCs after extraction, mainly attributed to the greatly increasing amount of pores after extraction (especially that of micro–meso-pores (below 10 nm)). According to the above results, it can be inferred that the solvent extraction can only undermine various weak forces (such as hydrogen bond, Van der Waals’ force, charge transfer force, etc.) among the basic structure units of LPCCs and cannot undermine the network structure mainly formed by chemical cross-linking bond among the basic structure units of LPCCs, suggesting that the solvent extraction of LPCCs is only a physical process [33,34]. As shown in Fig. 7(a), the surface areas of micro–meso-pores (below 10 nm) are obviously larger than those of meso–macropores (above 10 nm) for extracted and unextracted LPCCs,

a

8

(1) Both BET surface areas and total pore volumes of LPCCs increase greatly after extraction, which is ascribed to that the solvent extraction removes a large amount of CS2 -soluble fractions conglutinated on the surface or inner pores of LPCCs (Fig. 1), especially those on the surfaces of micro–meso-pores (as shown in Figs. 7 and 8). At the CT of 350 ◦ C, the increasing values for the BET surface area and total pore volume of LPCCs (from VR) after extraction are separately up to 38.47 m2 g−1 and

b

4 VR350E

VR350E

ΔV ( 10-3cm3 .g-1 )

ΔS ( m 2 .g -1 )

6

VR500E

4 CTP500E CTP500

2

VR350

0

3

VR500E CTP500E

2 CTP500

1 VR350

0 1

10

100

1

10

D (nm)

100

D (nm)

Fig. 7. Surface area and pore volume distribution of extracted and unextracted LPCCs (S: incremental pore area; V: incremental pore volume).

b

50

Total pore voluem ( 10-3cm3 .g-1)

Specific surface area ( m2.g-1)

a

40 30 20 10 0

VR350 VR350EVR500ECTP500CTP500E meso-macro-pores

60

45

30

15

0

VR350 VR350EVR500ECTP500CTP500E micro-meso-pores

Fig. 8. Specific surface areas and pore volumes of micro–meso-pores and meso–macro-pores of extacted and unextracted LPCCs.

S. Huang et al. / Journal of Analytical and Applied Pyrolysis 93 (2012) 33–40

0.15

LPCCs from VR LPCCs from CTP

Δ Rs

-1

(min )

0.12

0.09

0.06

0.03

0.00 10

Gasification temperature:1000 ºC 20

30

40

50

60

70

Extraction yield (wt%) Fig. 9. Correlation between extraction yields and Rs of LPCCs (Rs = Rs,extracted − Rs,unextracted ).

0.0401 cm3 g−1 . As the CT increases to 500 ◦ C, those of LPCCs (from CTP) after extraction are separately up to 24.9 m2 g−1 and 0.0255 cm3 g−1 . These suggests that the increasing CT results in a lower increasing extent for those of LPCCs after extraction, due to the decrease of CS2 -soluble fractions of LPCCs (as shown in Fig. 1). With the increasing CT (from 350 ◦ C to 500 ◦ C), the BET surface area and total pore volume of extracted LPCCs from VR decreased separately from 42.39 m2 g−1 to 36.86 m2 g−1 and from 0.0477 cm3 g−1 to 0.0407 cm3 g−1 . This illustrates that the increasing CT results in the decrease of those of LPCCs, probably due to the further carbonization of CS2 -soluble fractions conglutinated on the pore surfaces and the further shrinkage of LPCCs with the increasing CT, which are similar to the reasons for those of coal chars [34–36]. (2) The pore size affects the mode of diffusion in a porous material significantly with Kundsen diffusion dominating at smaller pore size and molecular diffusion dominating at larger pore size. Some investigations [34,35] worked out that molecular and Kundsen diffusivities became comparable for a pore size of around 250 nm. In the present investigation, the average pore diameter of extracted and unextracted LPCCs is found to be in the range of 4.41–7.90 nm, which is much smaller than 250 nm. Hence, it may be concluded that the diffusion of gasifying agents during the gasification (or combustion) of LPCCs is mainly dominated by Knudsen diffusion. Furthermore, the average pore diameter of LPCCs decreases greatly after extraction. This is probably due to that the micro–meso-pores account for a larger proportion of total pores after extraction (Figs. 6–8). 3.6. Gasification reactivity of LPCCs In order to clarify the effect of CS2 -soluble fractions on the gasification reactivity of LPCCs, the CO2 gasification reactivity of various unextracted and extracted LPCCs from VR and CTP were analyzed. Fig. 9 shows the correlation between extraction yields and Rs of LPCCs from VR and CTP (Rs denotes the CO2 gasification reactivity index of extracted LPCCs subtract that of unextracted LPCCs at the same CT). From the figure, it could be seen that the Rs of LPCCs from VR and CTP are larger than zero, which means that the CO2 gasification activities of LPCCs from VR and CTP were enhanced greatly after extraction (the CO2 gasification reactivity index of unextracted and extracted LPCCs from VR at different CTs were separately in the range of 0.022–0.039 cm−1 and 0.036–0.059 cm−1 , while that of unextracted and extracted LPCCs from CTP were separately in the range of 0.011–0.023 cm−1 and 0.035–0.151 cm−1 ). This was obviously due to the removal of CS2 -soluble fractions conglutinated on the surface or inner pores of unextracted LPCCs,

39

which lead the BET surface areas and total pore volumes of LPCCs to increasing greatly after extraction (as shown in Figs. 7 and 8 and Table 5). This conclusion was also obtained by Xie [14], who found that the CO2 gasification activity of Pyridine-extracted vitrinite of Pingshuo coal is evidently higher than that of the unextracted sample. Furthermore, the Rs of LPCCs (from VR or CTP), as a whole, is larger and larger with the increasing extraction yield, which is probably ascribed to that the more CS2 -soluble fractions conglutinated on the surface or inner pores of unextracted LPCCs were removed, the higher increasing extent of BET surface area and total pore volume of LPCCs after extraction (Figs. 7 and 8). Besides, a trend line is drawn to shows the correlations between extraction yields and Rs of LPCCs (from VR and CTP), and the equation of the trend line is given in the diagram together with the squared correlation coefficient (R2 ). From the figure, it can be obtained that an approximate linear correlation between extraction yields and Rs of LPCCs from VR and CTP exists, which means that a higher extraction yield led to a higher increasing extent of CO2 gasification reactivity of LPCCs after extraction (Rs ), regardless of precursors (VR and CTP). Accordingly, it can be concluded that the gasification activity of LPCCs is mainly depended on the soluble fractions contained in LPCCs. In summary, although the ordering degree of carbon crystalline structures of LPCCs was significantly unchanged after the removal of soluble fractions which severely blocked the pore structures of LPCCs, their BET surface areas and total pore volumes increased significantly. Besides, after the LPCCs were extracted by the CS2 solvent, their gasification activity could be greatly enhanced. Consequently, it can be concluded that the soluble fractions conglutinated on the pores of LPCCs were the most important factor, which had quite significant effects on the gasification activity of LPCCs.

4. Conclusions (1) In the experimental temperature range, the coalescence growth of longitudinally adjacent basic structure units dominates the growth process of carbon crystalline of LPCCs. The increasing CT was favorable for LPCCs to form more ordered carbon crystallite structures, and their “graphitization” was far from natural graphite. After extraction, the ordering of carbon crystalline structures of LPCCs was significantly unchanged, and only the intensity of the ␥ band decreased. (2) A large quantity of fine particles (CS2 -soluble fractions), which would hinder the convenient diffusion of reagent gases into pores, were conglutinated on the surface and inner pores of unextracted LPCCs. With the increasing CT, the amount of conglutinated fine particles decreased. After CS2 extraction, the surface of LPCCs from CTP exhibited a certain amount of visually identifiable pores, while those from VR were still closed initially. (3) The extracted and unextracted LPCCs had a relatively complete porous structure, including micro-, meso- and macro-pores. The surface areas of pores below 10 nm make a main contribution to surface areas of LPCCs, especially for extracted LPCCs. The micro–meso-pores (below 10 nm) and the meso–macropores (above 10 nm) separately made a main contribution to the pore volumes of extracted and unextracted LPCCs. After extraction, the BET surface areas and total pore volumes of LPCCs increased greatly, which can be mainly attributed to the increase of those of micro–meso-pores. (4) The average pore diameter of LPCCs was found to be very small. Hence, it was inferred that the gas diffusion is dominated by Knudsen diffusion during their gasification (or combustion) reactions with gasifying agents. The soluble fractions

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