Effect of pyrolysis temperature on the grindability of semi-cokes

Subscriber access provided by University of Florida | Smathers Libraries

Article

Effect of pyrolysis temperature on the grindability of semi-cokes produced by two kinds of low-rank coal Yumeng Yang, Jianzhong Liu, Jie Wang, Jun Cheng, Zhihua Wang, and Kefa Cen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03285 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 21, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Effect of pyrolysis temperature on the grindability of semi-cokes produced by two kinds of low-rank coal Yumeng Yang a, Jianzhong Liu a*, Jie Wang b, Jun Chen a, Zhihua Wang a, Kefa Cen a a

State Key Lab of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, China b

Zhejiang Energy Group R & D, Hangzhou, 310000, China

ABSTRCT: Variations in the grindability of semi-cokes produced by two low-rank coals at different pyrolysis temperatures were studied. The grindability of semi-cokes prepared by XiMeng lignite (XL) had minimal changes at varying pyrolysis temperature and was nearly similar to that of a dehydrated sample. The pore structure of XL was fully developed after dehydration and had no significant change during pyrolysis, as indicated by the observations obtained through scanning electron microscopy and results of the mercury intrusion porosimetry experiment. Thus, its grindability showed little change during pyrolysis. Meanwhile, the grindability of ShenHua sub-bituminous coal (SC) decreased considerably when pyrolysis temperature reached 550 °C. As shown by the results of thermal gravimetric (TG) experiments, X-ray diffraction (XRD) and fractal analysis of pore structure, the coal matrix of SC had a plastic stage. Meanwhile, its volatile matter rapidly releases in this stage. After the plastic stage, considerable changes occur in the pore and chemical structure of SC. All these factors eventually altered the grindability of SC. Keywords：：Pyrolysis; Low-rank coal; Semi-coke; Grindability; Fractal 1. Introduction Low-rank coal remain to have limited applications in various industries because of its undesirable characteristics, such as high moisture content, low calorific value, and prone to spontaneous combustion. However, owing to the rapid increase in demand for energy, exploitation and utilization of low-rank coal as an important fossil fuel component are becoming increasingly necessary. However, the direct combustion of low-rank coal in power generation is ineffective and releases greenhouse gases. Through staged conversion methods, low-rank coal can be used to produce synthetic liquid fuels and chemical feedstock, which can be a partial replacement to oil and natural gases [1–3]. Pyrolysis is a complicated thermal conversion process and often the first stage in many coal utilization processes, such as combustion, gasification, and liquefaction. Thus, a coal-staged conversion process based on pyrolysis is a clean and effective method for coal utilization [4–7]. Moisture in low-rank coal can be removed by pyrolysis, and gas, tar, and semi-coke in coal can be separated and utilized [8]. Furthermore, coal gas, tar, and other high-value resources can be generally used effectively. Semi-coke, a kind of low-volatile solid product, is always considered as a kind of by-product of the pyrolysis process. Nowadays vertical furnaces are still widely used for mid–low-temperature pyrolysis in China [9]. The further utilization of semi-coke is the key of the whole process of low-rank-coal step conversion. Mixed Combustion of semi-coke and raw coal in power stations is a kind of techno-economic and feasible method [10]. In coal power stations, typically 75% of the particles must be reduced to sizes finer than 0.075 mm, and 99.5% of the particles required to be less than 0.3 mm, thereby requiring further pulverization of the semi-coke [11]. Meanwhile, a milling system consumes a large amount of

*

Corresponding author. Tel.: +86 571 87952884. E-mail address: [email protected].

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

energy and is directly associated with the operational economy of a power plant [12].Besides that, Pulverized Coal Injection (PIC) which is widely used in iron industry needs to consume a large number of anthracite[13]. Semi-coke is considered as a kind of replacement for it and the grindability of raw materials is an important index for PIC. Semi-coke can also be used as raw material of coal water slurry for gasification. This also need the pulverized process. Therefore, research on the grindability of semi-coke is of great practical significance. Semi-coke as a kind of porous solid material, its grindability or mechanical strength is largely dominated by its own matrix strength and pore structure. Many studies on structures of semi-coke have been conducted. Combined with the electron spin resonance spectroscopy (ESP) experiment, the pyrolysis studies by heating coal sample showed maxima in spin concentrations around 400–500 °C, suggesting that the populations of free radicals first increased with temperature and eventually declined at higher temperatures [14]. It indicated that there are polycondensation reactions happened in the higher temperature. It could change the matrix strength of semi-coke. Lu et al. [15] performed X-ray diffraction (XRD) and used several instruments to investigate the characteristics of chars prepared from various pulverized coals at different temperatures. Their results showed that chars tend to be more ordered and condensed as pyrolysis temperature increases, and the atomic ratios of chars H/C and O/C decrease considerably at increased pyrolysis temperatures. Karen et al. [16] combined rheometry and micro-CT analysis studied the pore structure of several kinds of coals and found that the sample with high fluidity showed rapid expansion and bubble coalescence. Mercedes et al [17] investigated the plasticity development for a low-volatile Australian bituminous coal by using high-temperature in-situ 1

H NMR (nuclear magnetic resonance), and they found that the extractable material and the low

molecular mass species make comparable contributions to the development of fluidity of coke. Pyrolysis is a complicated thermal conversion process at high temperatures in an inert environment in which significant changes occur not only in the carbon crystalline structures but also in pore structures. These changes influence the grindability of the resulting semi-coke. Furthermore, several studies on pyrolysis mostly focused on coking coal and paid less attention to low-rank coal. Therefore, in the present paper, two kinds of low-rank coals were selected and studied. In particular, changes in their grindability at different pyrolysis temperatures were investigated. The mechanisms involved were analyzed by a variety of means, including scanning electron microscope (SEM), mercury intrusion porosimetry (MIP), TG analysis, nitrogen absorption, gas chromatography (GC) method and XRD. We believe that this work can provide the basis for further processing and utilization of low-rank coal. 2 Experimental section 2.1 Materials The sub-bituminous coal and the lignite used in this study were obtained from the Shenfu region in the Northern Shanxi Province and Ximeng region in Inner Mongolia, respectively. Naturally dried XLs and SCs were crushed with a jaw crusher and then screened into a monosize range of 2.0–4.0 mm. 2.2 Pyrolysis and drying experiments Pyrolysis and drying experiments were performed in an atmosphere furnace (GF14Q- II, Nanjing Boyuntong Instrument Co., Ltd, China) with a power input of 7 KW and maximum operating temperature of 1400 °C. During the experiment, 300 g of air-dried coal were placed in each of the six


Page 2 of 19

Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

porcelain boats, which were evenly placed in the furnace. Pyrolysis atmosphere contained nitrogen (99.999% purity), and the flow rate was 1 L/ min to eliminate pyrolysis gas and maintain the pressure of the furnace at 1 atm. Coal samples were heated from room temperature to five target pyrolysis temperatures (450-650 ℃, every 50 ℃) successively at heating rate of 10 °C/min and then held for 30 min. The furnace was then naturally cooled to room temperature under a nitrogen atmosphere. For the study of the dehydration process at the early stage of pyrolysis, the coal samples were dried for 2 h at 105 °C under the same atmosphere. The grindability of each of the obtained dehydrated samples was compared with those of raw coal and semi-cokes to investigate the influence of the dehydration process. The samples of the corresponding working conditions were designated in the same manner as the following example: SC dried at 105 °C was named as SC-105. Thus, in a sample designated as XL-500, the pyrolysis temperature is 500 °C.

2.3 Grindability tests Previous researchers accumulated a large number of grindability evaluation standards for the study of coal crushing characteristics; these standards include the traditional Hardgrove grindability index (HGI), improved grindability test, t10 index method, and first-order grinding kinetics [11–12]. A standard Hardgrove apparatus (FTHM-60. Zhenjiang Fengtai Laboratory Equipment Co., Ltd. China) was used to grind all the samples. Each sample (60 mL) was ground in the mill for 20 revolutions. Owing to the expansion and shrinkage of coal particles during pyrolysis [18], conducting tests on a constant coal volume is necessary to maintain the same bed level in the ball-race grinding system for various density components. HGI tests based on the same particle volume are heavily reported in literature [19]. The t10 index of material can be used to evaluate grindability. For example, for a 2.0–4.0 mm (geometrical mean size 3.0 mm) feed coal, t10 = 20 indicates that 20% of the coal particles are smaller than 0.3 mm (1/10th of 3.0 mm) in the product. A larger t10 value indicates a finer product. 2.4 Analytical method 2.4.1 SEM , nitrogen absorption and MIP experiments Surface microtopography was measured using a SEM (SIRION-100, FEI, Netherlands). Its maximum acceleration voltage is 30KV, and the highest resolution is 1.5nm. In this paper, 100x magnification was used to investigate samples. The internal pore structure was determined with an automatic mercury porosimeter (AutoPore IV 9510, Micromeritics, USA). Its pressure range is from 3.45 to 413685kPa and the smallest measurable pore diameter is 30 Å. In the MIP experiments, when the pore diameter was smaller than 24,000 nm (corresponding to a pressure higher than 51.57 kpa), mercury entered the internal pores of the coal. When the pressure was less than 51.57 kpa, the corresponding large pore was considered as interparticle void or special big fractures on the surface of the sample. These pores had a strong randomness property and could not represent the regular change rules of the internal structure of the coal. Thus, 24,000 nm (51.57 kpa) was regarded as the upper limit. When the pressure exceeded at 90793.36 kpa, the corresponding pore diameter was smaller than 13 nm. In this diameter, the mercury was immersed in the gap between molecules. Therefore, pores of this size had no effect on the crushing process at the millimeter scale. Therefore, 13 nm (90793.36 kpa) was considered the lower limit. The microscopic pore structure of selected samples was determined with an automatic surface area and pore analyzer (TriStar II 3020, Micromeritics, USA). Samples were degassed at 150 ℃ for 12 h under vacuum to ensure that any residual volatiles were removed. N2 gas


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 19

adsorption/desorption isotherms at 77 K were measured for the relative pressure (P/P0) range from 0.01 to 0.99. Adsorption data were used to measure the specific surface area and average pore diameter according to the Brunauer–Emmett–Teller (BET) method and the total pore volume. The test range in this paper is from 19 to 4000Å. 2.4.2 TG and XRD experiments TG experiments were carried out with thermogravimetric analyzer (SDT Q600, TA Instruments, USA) under N2 atmosphere with flow rate of 100 mL/min. Approximately 10 mg of coal samples heated from room temperature to 1000 °C at a heating rate of 10 °C/min under ambient pressure. The carbon crystalline structure of SC semi-cokes was analyzed by XRD (X-pert Powder, PANalytical B.V., Netherlands) experiment. The following operating conditions of the X-ray tube were used: U = 40 kV and I = 30 mA. The powdered sample was grinded through a 300-mesh sieve. 2.4.3 GC analysis experiments For chromatographic experiments, 30 g of each coal sample (less than 200 mesh) was pyrolyzed in a tube furnace (OTF-1200X, MTI Corporation, USA). Before pyrolysis began the furnace was flushed with 500 ml/min N2 for 2 h. Next, the furnace was heated from room temperature to 1000 °C at the rate of 10 °C/min, and the Nitrogen flow was maintained at 500 ml/min. From 150 °C, every 50 °C, pyrolysis gases were collected in sample bags and analyzed by gas chromatography (Micro GC 490, Agilent, USA). The volatile release volume was measured using a water displacement method. 2.4.4 Calculation of fractal dimension Fractal geometry is an extremely powerful tool for studying irregular microscopic pore structures [20]. A pressure P(r) must be exerted on mercury before it fills a pore size of r to overcome the interfacial tension between mercury and a solid. In a cylindrical pore, P and r are related by the well-known Washburn equation. According to this equation and on the Menger Sponge fractal model, the fractal dimension can be calculated through the following equation [21]: ௗ௏

݈݊ ቀௗ௉ ቁ ∝ ሺ‫ ܦ‬− 4ሻ݈݊ܲ

(1)

where P is the pressure, V is the cumulative intrusion volume that corresponds to P, and D is the fractal dimension. The fractal dimension of the surface of the porous medium can be obtained simply from the slope of the ln–ln plot of dV/dP versus P. 3. Results and discussion 3.1 Repetitive experiments Repetitive experiments on the SC and XL raw coal samples were carried out to ensure the accuracy of the grindability tests. The results are listed in Table 2. Each sample was carefully split with a sample divider to reduce the errors in preparation. The experiment was repeated four times for the SC and XL samples. The detailed data are listed in Table 1. The standard error of SC raw coal was 0.238, and its 95% confidence limit was 0.466. These results indicate that, for SC raw coal, 95% of all experiments were located in the range of 24.95 (mean number) ± 0.466 or 95% ± 1.8% in percentage terms. For the XL raw coal, the standard error was 0.368, and the 95% confidence limit was 0.721. Therefore, its repeatability was 95% ± 5.2%. The high repeatability shown in the experiment results indicated that the grindability test data were highly reliable.


Page 5 of 19

Table 2 Standard errors associated with the t10 values in the breakage tests. Sample Name

SC RAW t10 (%)

XM RAW t10 (%)

Repeat1

25.55

13.74

Repeat2

26.35

14.48

Repeat3

26.17

12.91

Repeat4

25.37

14.43

Mean

25.86

13.89

Standard error

0.238

0.368

95% Confidence limit

0.466

0.721

95% Confidence limit relative to the

1.8

5.2

mean (%) 3.2 Grindability analysis Fig. 1 and Table 3 shows that SC and XL showed completely different change rules in grindability. For XL, the grindability of its raw coal was significantly improved and the t10 index number increased from 13.89 to 31.25 after dehydration. At increasing pyrolysis temperature, the grindability of the semi-coke of XL did not change significantly. Their t10 numbers fluctuated around the number of dehydrated raw coal. The most remarkable among the samples was XL-600 with t10 number of 32.83, and the worst was XL-550 with t10 number of 28.41. The gap between the two was only 4.42. The grindability of SC raw-coal was relatively good and improved to a certain extent after dehydration. The t10 index number increased from 24.95 to 30.37 only. At increasing of pyrolysis temperature, the grindability of the SC semi-coke did not change significantly until the temperature reached 500 °C. For the SC-450 and SC-500, the change was nearly similar to that in the dehydrated raw coal. When the temperature reached 550 °C, a significant deterioration in grindability was observed, as indicated by the decrease in the t10 index number from 29.95 (SC-500) to 12.07 (SC-550). In the subsequent pyrolysis (550–650 °C), the grindability of the SC semi-coke had no considerable change. SC-650 was the most remarkable sample. It had a t10 index value of 15.20. Meanwhile, SC-550, with t10 index number of 12.07, was the worst for having only a difference of 3.13. 40

40

Dehydrated Coal

Dehydrated Coal 30

25.86 20

30

30.37

Raw Coal

t10(%)

t10(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

31.25

20

SC 10

Raw Coal

XL

13.89

10 450

500

550

600

650

450

Temperature (°C)

500

550

600

650

Temperature (°C)

Fig.1 The variation trend of t10 values with the change of experimental conditions in the breakage tests. Table 3 The t10 values in the breakage tests of all the samples. Sample Name

SC t10 (%)


XM t10 (%)

Energy & Fuels

Raw Coal

25.86

13.89

Dehydrated Coal

30.37

33.72

450°C semi-coke

31.34

29.13

500°C semi-coke

29.95

29.77

550°C semi-coke

12.07

28.41

600°C semi-coke

15.20

32.83

650°C semi-coke

13.71

31.58

3.3 TG analysis TG is widely used in studying the pyrolysis of low-rank coals [5, 22]. The TG and differential thermogravimetric (DTG) curves of SC and XL are illustrated in Fig. 2. Two mass loss rate peaks in the DTG curves were observed in both SC and XL. The first sharp peak located in the position lower than 100 °C was mainly attributed to moisture loss. The second peak was attributed to the volatile matter release peak formed during fast pyrolysis, in which many intense pyrolysis reactions occurred and large amounts of volatile matter were generated. More concretely, XL was dehydrated rapidly at the early stage of heating. The maximum dehydration rate of XL was −8.84%/min at 61.84 °C, which was considerably greater than the maximum devolatilization rate of 2.83%/min at 448.45 °C. XL contains a large amount of water (Mad%=20.41). Thus, when a large amount of moisture within XL escapes rapidly, a steam jet flow is generated [12], which subsequently destroys internal pore structure. The entire dehydration process caused numerous fractures in the coal matrix and eventually improved the grindability of the sample. The t10 index number increased from 13.89 to 31.25 after dehydration. Owing to the intense dehydration process, the pore structure developed well, and the volatile matter was smoothly released without causing more fractures. Further, lignite does not undergo thermoplastic deformation during pyrolysis because of the small amount of metaplast [23]. The minimal change in the grindability of XL semi-coke at increasing pyrolysis temperature can be attributed to the internal pore structure model of XL. The large fractures, which can influence particle strength, exhibited minimal change after dehydration. Thus, the XL semi-coke and dehydrated raw coal had similar grindability. This finding was supported by subsequent SEM and MIP experiments. 100

SC

Mass (%)

Mass (%)

100

80 55.22°C

452.62°C

60 0.0

XL 448.45 °C

80

60

61.84 °C

0

DTG (%/ min)

DTG (%/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 19

-0.5 -1.0 -1.5

-5

-10

0

200

400

600

800

1000

0

200

400

600

800

1000

T (°C)

T (°C)

Fig. 2 TG and DTG curves of different coal samples. The DTG curve of the SC indicated the rapid removal of the moisture, but the maximum dehydration rate was −1.69%/min at 55.22 °C, which was considerably smaller than that of XL. The maximum volatile matter release rate of SC was −1.11%/min at 452.62 °C. Mild dehydration did not destroy the internal pore structure. Therefore, the t10 index number of SC only increased from 24.95 to


Page 7 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

33.37 after the dehydration process. At increasing pyrolysis temperature, the change rule of SC was different from XL. Furthermore, the SC semi-coke exhibited a considerable deterioration in grindability at 550 °C. This may be associated with the intermediate plastic state forms during coal pyrolysis. Kidena [24] investigated the plastic properties of coal by thermogravimetric experiment (TG) analysis and Gieseler plastometry, and they found that a good linear relationship exists between the temperatures at the maximum rate of weight loss and maximum fluidity temperature. Barriocanal [25] believed that the temperature of maximum volatile matter release is slightly lower or equal to the maximum fluidity temperature of coals at the plastic stage during pyrolysis. The softening and resolidification temperatures are symmetrically located at 30–50 °C of the maximum fluidity temperature. The plastic stage is usually defined as the period between the softening and resolidification temperatures. The maximum volatile matter release temperature of SC was 452.62 °C. For SC-450 and SC-500, the coal matrix was still at the plastic stage. By contrast, for SC-550, SC-600, and SC-650, the coal matrix was already resolidified. As indicated by the results of the grindability analysis on the SC semi-coke, the softening and resolidification processes caused the deterioration of the grindability of SC semi-coke at 550 °C. Thus, the former two and the latter three samples had extremely different internal structures. The differences among the microscopic structures possibly caused the changes in grindability. Later chapters will prove this idea. 3.4 Chemical structure analysis The plastic stage is closely relative with hydrogen element and the molecular species released by pyrolysis at relatively low temperatures. The development of plasticity can be considered as a pseudo-liquefaction process and the stabilization of the free radical species formed by hydrogen transfer determine the formation of plastic stage [26-27].Laser desorption time of flight imaging mass spectrometry (LDI-TOF-IMS) [28] experiment showed that the plastic layer has a prevalence for midrange molecules below 1000 Da in size and a rise in the abundance of larger structures (4000 Da) in coal coincides with the end of plastic stage. This indicated that at the end of plastic stage, the available (donatable) hydrogen is exhausted and the polycondensation reaction become dominant, forming larger molecules and light gas (such as H2). Pyrolysis and electron spin resonance (ESR) studies [27, 29] showed the maximum spin concentration is around 400–500 °C, and good correlations were found between hydrogen donor ability and change in spin concentration. So the hydrogen element has positive influence on the development of plastic stage. However, for low-rank-coal, the presence of oxygen cross-links prevents fusion [30]. Marzec and co-workers used field ionization mass spectrometry on a wide range of coals and found that the hydrogenated aromatic compounds and oxygen plus nitrogen species that gave positive and negative correlations, respectively, with both fluidity development [31]. The small molecules were mainly investigated in this study. The proximate and ultimate analysis of all the samples are given in Tables 4 and 5. And the gas chromatography result of lightweight volatile gases for SC and XL are showed in Fig. 3. Table 4 Coal property analyses of SC coal and its semi-cokes Samples

Proximate analysis (% ad) M

A

V

FC

Qb, ad (MJ/kg)

Ultimate analysis (% ad) C

H

N

S

O

Raw coal

8.26

4.74

29.91

57.09

28.13

71.42

4.84

0.71

0.35

9.68

SC-450

4.14

4.89

24.03

66.94

29.77

76.28

4.07

0.87

0.38

9.37

SC-500

3.98

5.06

19.48

71.48

29.99

77.26

3.68

0.91

0.41

8.70


Energy & Fuels

SC-550

3.94

5.62

15.10

75.34

SC-600

2.91

5.91

11.00

SC-650

2.29

6.51

8.86

30.29

77.11

3.28

0.94

0.72

7.39

80.18

31.22

83.07

2.93

0.93

0.38

3.87

82.34

31.56

84.64

2.69

0.92

0.55

2.40

Note: M refers to moisture content. A, V, and FC refer to ash, volatile, and fixed carbon contents, respectively. Qb refers to the bomb calorific value. “ad” refers to air dry basis. Table 5 Coal property analyses of XL coal and its semi-cokes Samples

Proximate analysis (% ad)

Qb, ad

Ultimate analysis (% ad)

M

A

V

FC

(MJ/kg)

C

H

N

S

O

Raw coal

20.41

13.65

30.57

35.37

18.94

47.24

3.35

1.18

0.61

13.56

XL-450

9.65

12.03

26.72

51.60

22.37

63.16

2.84

1.01

0.62

10.69

XL-500

9.20

14.88

22.65

53.27

22.61

63.64

2.33

1.07

0.64

8.24

XL-550

8.57

14.12

18.77

58.54

23.69

65.42

2.18

1.10

0.64

7.97

XL-600

7.42

15.14

14.42

63.02

25.16

68.86

2.09

1.09

0.60

4.80

XL-650

7.13

16.37

11.19

65.31

25.55

69.01

1.73

1.01

0.37

4.38

As shown in tables, the content of oxygen in XL is higher than in SC, but the content of hydrogen is on the contrary. Both the content of the two kinds of element in SC and XL decreases with the increase of pyrolysis temperature. Meanwhile the moisture and volatile matter content decrease and fixed carbon content increases. The volatile fraction is produced by the drop and decompositions of side chains in coal macromolecular structure at low temperature. The drop of oxygen content is the result of the oxygen-containing functional groups decompose into volatile matter [32]. 4 H2

4

SC-GC

CH4

3

ml/(min*g)

CO CO2 C2H6

2

XL-GC

H2

CH4

3

ml/(min*g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 19

1

CO CO2 C2H6

2 1

0

0 0

200

400

600

800

1000

0

200

Temperature (°C)

400

600

800

1000

Temperature (°C)

Fig .3. Evolution profiles of the lightweight volatile gases during the pyrolysis process As shown in figures, the release amount and the maintaining time of CO and CO2for XL are all greater than SC. It means that XL contains more oxygen-containing functional groups. And they are negative to the development of plastic stage. Methane formation is related to the breakage of methylene bridges around 550 °C and it could also be produced from secondary devolatilization at 700 °C [33]. The two methane peaks could be found in both SC and XL. C2H6 was mainly consistent with radical transfer after the primary coal decomposition step. [34]. In the pyrolysis process of SC, it mainly appeared in about 500 °C. For both CH4 and C2H6, their yield in SC is higher than XL. It indicates that SC contains more aliphatic hydrocarbon and hydrogenated aromatic compounds. They have positive influence on the formation and the maintaining of plastic stage. Hydrogen begins to be produced in large quantities at 550°C for both SC and XL. This temperature is also the end point of plastic stage because the significant H2 emission is due to the condensation of aromatic structures [33].The hydrogen production of XL is obviously less than SC. First, it is because the content of hydrogen in XL


Page 9 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

is less than SC. In addition, a large amount of complex side chains in its aromatic structure are not conducive to the condensation of aromatic nuclei. This is reflected in the phenomenon that after the temperature point of highest release rate, with the pyrolysis process continuing, the decrease of H2 release rate of XL is not as vigorous as it in SC. 3.5 Morphology and internal structure analyses SEM images of all of the XL samples (magnified 100 times) are shown in Fig. 4. The structure of XL raw coal was free of visible fractures. Large fractures and cracks were induced within the XL-105 because of the extreme dehydration process. Many studies on the effect of dehydration on the grinding characteristics of lignite have been reported [12, 35]. The destruction of the pore structure caused by the steam jet flow generated through the rapid removal of moisture helped improve the grindability of raw coal. When the pyrolysis temperature reached 450 and 550 °C, the cracks enlarged and developed fully. The SEM images of XL-450 and XL-500 showed the results of the volatile rapid release process. These results were combined with the prior TG analysis results. When the pyrolysis temperature was increased further, the morphological surface of the semi-coke became rough and pulverized. However, numerous large fractures were observed on the surface.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig.4 The SEM images of all of the XL samples with enlargement of 100 times. The SEM images of all the SC samples are shown in Fig.5. Notably, all the samples, except the dehydrated raw coal (SC-105), were cut out at a certain depth to observe the internal structure. The internal morphology of the SC raw coal was complete, and its structure was free of visible micro fractures. After dehydration, the micro fractures and cracks were induced within the SC-105 but were considerably smaller than those formed in XL-105. This phenomenon conforms to the differences between the dehydration rates of the two kinds of coal. When the pyrolysis temperature reached 450 °C, many small bubble structures were observed in the image, indicating that the SC coal matrix started to soften. After the temperature increased further, the size of the bubble structures increased and became evident. The growth, coalescence, and rupture of the bubble structures in the bituminous were caused by the fluidic metaplast and volatile gas releases [36].


Page 10 of 19

Page 11 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Fig.5 The SEM images of all of the SC samples with enlargement of 100 times. Fractal theory can be used to describe complex systems. When a system has fractal characteristics, it exhibits self-similarity, and the only difference is the scale. For instance, when fractals are zoomed, no new detail appears, no change occurs, and the same pattern repeats. In some fractals, nearly the same pattern reappears repetitively. Mandelbrot set is a famous artificial fractal. Regardless of the number of times of magnification, Mandelbrot set fine detail resembles the detail at low magnification. During the development of pore structure, when a large fracture originates from a smaller fracture, which in turn originates from an even smaller fracture, the pore structure exhibits self-similarity or it has fractal characteristic. Similarly, when a pore structure has fractal characteristics, its inner structure is self-similar, and no sudden change occurs. Therefore, basing on the above discussions, we performed MIP experiments to investigate the pore structures of XL and SC. Menger Sponge fractal model was used to investigate whether their pore structures have the fractal characteristic. The MIP experiment required completely dehydrated samples for the absorption of mercury into the pores. Thus, the raw coal samples of XL and SC were not included in this experiment. The operation of the mercury intrusion porosimeter was divided into two steps of high and low pressures, generating an obvious discontinuity in data point between the two steps. The parting point (3600 nm, 319.31 kpa) of the high and low pressures was denoted as the segmentation point of the research range. The piecewise fractal fitting curves of the mercury porosimetry data of all the samples are shown in Figs. 6 and 7.


Energy & Fuels

XL-105°C

-5

XL-450°C

-5 2

2

y1=-1.1898x-3.8371; R =0.9883

ln(dv/dp)

ln(dv/dp)

y1=-1.1552x-3.8633; R =0.9738

-10

-10

2

2

y2=-0.8977x-4.7614; R =0.9776

y2=-1.0310x-3.8239; R =0.9819

-15

0

5

-15

10

0

5

ln(p)

XL-500°C

XL-550°C

-5 2

2

y1=-1.1415x-3.7373; R =0.9844

ln(dv/dp)

ln(dv/dp)

y1=-1.2037x-3.6087; R =0.9901

-10

-10

2

y2=-0.9515x-4.2650; R =0.9782

-15

2

y2=-1.0724x-3.3143; R =0.9840

-15

0

10

ln(p)

-5

5

10

0

5

10

ln(p)

ln(p)

XL-600°C

-5

XL-650°C

-5

2

2

y1=-1.2251x-3.6263; R =0.9802

ln(dv/dp)

ln(dv/dp)

y1=-1.1886x-3.7575; R =0.9846

-10

-10

2

2

y2=-0.9931x-3.9146; R =0.9707

y2=-1.0086x-3.8113; R =0.9642

-15

0

5

-15

10

0

5

10

ln(p)

ln(p)

Fig. 6. Linear fitting of fractal dimension for XL samples

-5

SC-105°C

SC-450°C

-5

2

2

y1=-1.0214x-5.6771; R =0.9744

y1=-1.1394x-4.5352; R =0.9549

ln(dv/dp)

ln(dv/dp)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 19

-10

-10

2

y2=-0.8260x-5.6717; R =0.9110 2

y2=-1.0810x-43.3774; R =0.9467

-15

0

5

10

-15

0

ln(p)

5

ln(p)


10

Page 13 of 19

SC-500°C

-5

SC-550°C

-5 2

2

y1=-1.4715x-2.9995; R =0.9443

ln(dv/dp)

ln(dv/dp)

y1=-1.2813x-3.6250; R =0.6932

-10

-10

2

y2=-1.1326x-3.0529; R =0.9521

-15

0

2

y2=-1.1284x-2.9640; R =0.9433

5

-15

10

0

5

SC-600°C

-5

SC-650°C

-5

2

y1=-1.4534x-3.4317; R =0.9189

2

ln(dv/dp)

y1=-1.2694x-3.8786; R =0.7825

-10

-10

2

2

y2=-1.0721x-3.3460; R =0.9261

-15

y2=-1.1063x-3.1900; R =0.9408

-15 0

10

ln(p)

ln(p)

ln(dv/dp)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

5

10

0

5

10

ln(p)

ln(p)

Fig. 7. Linear fitting of fractal dimension for SC samples First, the high pressure section of each sample was analyzed. Although the fitting condition was relatively good, high pressure data showed a wave-like change. The XL samples were less obvious compared with the SC samples, which were more obvious in the images. The fractal dimensions (D) of XL-450, XL-500, XL-650, and SC-105 were all >3 (3.10, 3.05, 3.01, and 3.17, respectively). These values are nonphysical from a geometric point of view and can be attributed to the compressibility of the coal [21]. The particle size of the coal samples used in this paper was 2.0–4.0 mm, which was relatively large. Thus, the coal structure was destroyed even through the pressure of the MIP experiment was not extremely high. Therefore, the present paper mainly investigated the fractal structure of the low pressure section. The fractal dimensions and goodness of fit (R2) of all the samples at low pressure are shown in Table 6. Table 6 Fractal dimension and fitting precision data at low pressure of low pressure section XL Coal

Fractal dimension

SC

Samples

(D)

R2

Dehydrated coal

2.8448

450°C semi-coke 500°C semi-coke 550°C semi-coke

Fractal dimension (D)

R2

0.9738

2.9786

0.9744

2.8102

0.9883

2.8606

0.9549

2.7963

0.9901

2.8674

0.6932

2.8585

0.9844

2.5285

0.9443

600°C semi-coke

2.8114

0.9846

2.5466

0.9189

650°C semi-coke

2.7749

0.9802

2.8937

0.7825

All the XL samples had obvious fractal characteristics, and their goodness of fit were all greater


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

than 0.97. The differences among the fractal dimensions of all the XL samples were minimal. The gap between the maximum (SC-550, D=2.8448) and the minimum values (SC-650. 2.7749) was only 0.0836. The strong self-similarity feature and the small difference between their fractal dimensions indicated that the internal pore structure of XL exhibited minimal change during pyrolysis. Thus, grindability basically remained unchanged. The goodness of fit of SC-105 was 0.9744, indicating that the internal pore structure of the dehydrated SC raw coal had fractal property. This finding was similar to that of XL-105. The difference was observed only in the degree of fracture development. Owing to the self-similarity of pore structure, the internal pores of the SC-105 structures were similar to the fractures, which were observed on the surface via the SEM image. In the subsequent pyrolysis process, all the XL semi-coke maintained the fractal feature. Meanwhile, the situation for SC was quite different. First, although SC-450 (D = 2.8606 R2 = 0.9549) exhibited a strong fractal feature, the SEM image of its internal structure showed that its pore structure was composed of small pores rather than fractures. Combining with the TG experiment results of SC and the plasticity temperature range of coal matrix, the temperature of the maximum volatile matter release was near 450 °C. The coal matrix was at the plastic stage in this period, and volatile matter was released rapidly. Given that this was the early period of the entire plastic stage, the merger and rupture of bubbles were relatively less, and the main source of pore formation was the volatile gases trapped in the bubbles. Larger pores were only able to form through the expansion of small pores. Thus, the pore structures were able to exhibit fractal characteristics. SC-500 had a chaotic internal pore structure (R 2 =0.6392) with no fractal characteristic. This finding may be due to the fact that internal pores originated not only from the expansion of small pores but also from the formation of a merger and rupture of the bubbles. At continuous increase of pyrolysis temperature, the resolidification of coal matrix and devolatilization rate decreased, and the pore structure began to harden again. Thus, SC-550 (D = 2.5385, R2 = 0.9443) and SC-600 (D = 2.5466, R2 = 0.9189) exhibited certain fractal characteristics, which were not extremely strong. The external morphologies of SC-550 and SC-650 (Fig. 8) is very different. Because 550 °C is not far from the resolidification temperature, SC-550 could still show a certain degree of swelling state,. However, 650 °C is much higher than resolidification temperature and the bubble structure formed during plastic phase was destroyed due to the effects of gas volatilization and thermal cracking.

Fig. 8. The exterior morphology of SC-550 and SC-650 The detailed pore structure parameters of all samples are listed in Table 7. It could be seen that for SC coal, due to the existing of plastic stage, its porosity firstly increased and then decreased. SC-500 have the largest porosity and average pore diameter. The development of pore structure in plastic stage is mainly attributed to the growth of bubbles and this could be proved by SEM photographs. The


Page 14 of 19

Page 15 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

SC-550 and SC-600 have went through the plastic stage and their pore structure were well developed. Therefore, the internal spaces of SC-550 and SC-600 were occupied by large pores formed in plastic stage. The SC-450 didn’t go through this process, so it contained many undeveloped small pores. The SEM picture of SC samples also can visually show this phenomenon. However, the porosity of XL samples shows a continuing upward trend with the increasing of pyrolysis temperature. This indicates that the XL coal has no plastic phase in the pyrolysis process, which is consistent with the SEM pictures and the fractal analysis of pore structures. Table 7 Average pore diameter and Porosity of all samples XL

SC

Coal

Average pore

Average pore

Porosity

Samples

diameter (nm)

Porosity (%)

diameter (nm)

(%)

Dehydrated coal

124.9

13.9657

77.3

8.2954

450°C semi-coke

101.2

14.1239

126.1

14.5503

500°C semi-coke

112.0

15.8119

176.3

18.6317

550°C semi-coke

124.6

16.1974

158.2

17.3646

600°C semi-coke

125.0

16.1388

135.8

16.4709

650°C semi-coke

120.1

16.2458

142.9

15.2251

Because the particle size (2.0-4.0mm) of samples in this paper is relatively large. The micropores cannot directly affect the grindability of semi-coke. However, micropores are strongly affected by chemical changes during pyrolysis. The chemical structural change of coals which could reflected by the change in micropore is also important for the grindability of the semi-coke. So N2 adsorption experiments were carried out. In order to maximally keep the pore structure from destruction, the original granular samples were adopted in experiments. Finally, three samples for each kind of coal were chosen, including raw coal, 500 °C sample and 600 °C sample. These samples cover raw coal stage, the plastic stage and the polycondensation stage, the pyrolysis process is well represented. Table 8 Pore structure parameters of SC coal extracted from Nitrogen adsorption experiment Condition

BET surface area

Pore volume (cm3/g)

(m2/g)

Average pore diameter(nm)

Raw coal

16.41

0.03288

8.016

SC-500

18.61

0.02551

5.483

SC-600

38.05

0.03771

3.964

Table 9 Pore structure parameters of XL coal extracted from Nitrogen adsorption experiment Condition

BET surface area

Pore volume (cm3/g)

(m2/g)

Average pore diameter (nm)

Raw coal

0.91

0.01199

52.4625

XL-500

3.45

0.01339

15.5132

XL-600

12.64

0.01301

4.1195

As shown in tables, SC samples and XL samples have the similar evolution tendency. The pore diameter of every SC sample is smaller than the corresponding XL sample, however, the BET surface area is larger. It means that SC has a more developed microporous structure. The BET surface area of both SC-500 and XL-500 showed a certain degree of growth comparing to raw coal. This is because of


Energy & Fuels

the release of volatile matter. The XL-500 has an increased pore volume compared to XL raw coal. However, the pore volume of SC-500 is smaller than that of raw coal. It may be attributed to the filling of fluidity material. As pyrolysis temperature increased from 500 °C to 600 °C, the BET surface area of both SC and XL both increased sharply. The mechanism is that at this temperature a large amount of H2 is released which increased the quantity of the pores that can adsorb adsorbent in particles [37].The release of a large amount of H2 is resulted from the condensation of aromatic structures. Due to the presence of plastic layer, the polycondensation process caused the SC coal matrix became more compact, eventually resulting in poor grindability. But for XL, its macro pore structure and coal matrix strength changes little after dehydration process, so its grindability only little changed. 3.6 Carbon crystalline structure analysis XRD has been used to investigate carbonaceous materials. Many structural parameters, such as crystallite size, aromaticity, interlayer spacing, and other properties of carbonaceous materials, can be derived from the XRD spectrum. The position of the peak at the low-angle region (2θ = 5°–35°) corresponds to the 002 peak, which is usually considered as the stacking of char crystallite. The 002 peak should be symmetric; however, because of the existence of the γ band, which represents amorphous carbon, such as aliphatic side chains, the appearance of 002 peak is not symmetrical. Origin 7.5 software was used to fit the spectrum in the 2θ range of 15°−30° to resolve 002. The detailed information of 002 peak included position, intensity, area, and FWHM. The crystallite structural stacking heights (Lc) were calculated using the conventional Scherrer equation [15]:

‫ܮ‬௖ = ఉ

଴.଼ଽఒ

(2)

బబమ ௖௢௦ఝబబమ

And d002 which represents the interlayer spacing of aromatic layers [38]. It is calculated by the Bragg’s equation show as below: ఒ

݀଴଴ଶ = ௦௜௡ఝ

బబమ

(3)

Where λ is the wavelength of the radiation used, β002 is the FWHM of 002 peaks, and φ002 is the peak positions. The average number of N (Nc), which was estimated from d002and Lc by means of the following equation [38]: ௅೎

ܰ௖ = ௗ

(4)

బబమ

The example 002 peak fitting of SH-500 was shown in Fig. 9. The fitting results of the 002 peak of all samples are shown in Table 10 and Table 11. Raw data Fit Peak 1 Fit Peak 2 Cumulative Peak Fit

002 peak

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 19

18

0

10

20

30

40

20

22

50

24

26

60

28

70

2θ θ(deg.)

Fig. 9. The Curve-fitting 002 spectrum of SC-500.


Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 10 Structural parameters of SC samples extracted from XRD profile Conditions

Center (deg)

Lc (Å)

d002(Å)

Nc

Raw coal

23.76

9.47

3.74

2.53

SC-450

24.12

9.89

3.69

2.69

SC-500

24.28

10.41

3.66

2.84

SC-550

24.09

10.29

3.69

2.79

SC-600

23.98

9.90

3.71

2.67

SC-650

23.80

9.19

3.73

2.46

Table 11 Structural parameters of XL samples extracted from XRD profile Conditions

Center (deg)

Lc (Å)

d002(Å)

Nc

Raw coal

24.27

7.75

3.66

2.12

XL-450

23.13

7.11

3.84

1.85

XL-500

23.55

6.97

3.77

1.85

XL-550

23.40

6.86

3.80

1.81

XL-600

22.84

6.63

3.89

1.70

XL-650

22.34

6.45

3.97

1.62

It could be seen that with the increase of pyrolysis temperature, Lc of SC samples first increased then decreased and the maximum point is 500°C which is around the maximum fluidity temperature. Meanwhile from 450 °C to 550 °C, its d002 didn’t change a lot. So the evolution of Lc could mainly attributed to the increase of Nc. The mechanism for this phenomenon is that the development of the stacking structure of aromatic layer was accelerated by the increased mobility of the structure at fluidity stage [38]. After 500°C, Lc of SC decreased with the continuing of pyrolysis process. It could be considered that the stacking of aromatic layers developed at fluidity stage was destroyed by the release of lower-molecular weight components incorporated in stacking structure [39].This is coincided with the phenomenon observed in GC experiment. H2 release became remarkable at 550°C which is the ending point of the plastic stage. The remarkable production of hydrogen is a signal of the condensation of aromatic structures. Coal matrix become more compact after polycondensation process, thus semi-cokes would have a greater strength and rigidity. In the point view of macroscopic statistics, this appears as the grindability of semi-coke becomes worse. As the coal matrix becomes more compact after the polycondensation, the strength and rigidity of the semi-coke increases, eventually leading to deterioration grindability of the semi coke. That’s why the grindability of SC-650 did not improve even though its pore structure was destroyed. Because there is no plastic stage in XL pyrolysis process, the evolution of its grindability is relatively simple. As shown in Table 4, its Lc decreased and d002 increased with the increase of pyrolysis temperature. The sustained release of volatile matter caused the destruction to stacking structure of aromatic layers. A large amount of H2 is also released from XL after 550 °C. It indicates that XL coal also have polycondensation process. But due to the lack of plastic stage, the polycondensation process in XL can be considered as occurring in many places separated by cracks. They couldn’t link up with each other to let the whole particle become a more compact material. In the point view of macroscopic, each particle is still full with cracks. This could be proved by the results of SEM and MIP experiments.

4. Conclusion


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The dominant mechanism that improved the grindability of XL was the rapid removal of moisture. This process destroyed the internal pore structure of XL. According to the fractal analysis results, the dehydration process of XL was very vigorous such that the subsequent pyrolysis process has no further development on fractures. As a result, the grindability of XL exhibited insignificant change after dehydration. After the relatively mild dehydration, the fractures of SC were slightly developed, and the grindability was improved. However, the extent of this improvement was insignificant compared with XL. When the temperature reached at 550 °C, a significant deterioration in the grindability of SC semi-coke was observed. The results of the TG experiment, fractal analysis on the pore structure and other methods indicated that a plastic stage is present during SC pyrolysis when the volatile matter rapidly releasing. After this stage, polycondensation happened in its matrix. Due to the existence of plastic stage, after resolidification, the strength of SC coal matrix increased, and the grindability of SC was reduced. The polycondensation becomes the main factor that enhance the mechanical strength of SC semi-coke and finally reduces its grindability. The process can be briefly summed up as after volatile matter rapidly releasing process, the grindability of SC semi-coke deteriorated rapidly.

Acknowledgment The authors wish to acknowledge the financial support provided by the National Key Research and Development Program of China (Grant No. 2016YFB0600505).

References [1]

H. Katalambula, R. Gupta, Energy & Fuels 2009, 23, 3392-3405.

[2]

J. Wu, J. Liu, S. Yuan, X. Zhang, Y. Liu, Z. Wang, J. Zhou, Energy & Fuels 2015, 29, 6586-6592.

[3]

F. Meng, J. Yu, A. Tahmasebi, Y. Han, H. Zhao, J. Lucas, T. Wall, Energy & Fuels 2014, In Press, 275-284.

[4]

K. Jayaraman, I. Gökalp, Energy Conversion and Management 2015, 89, 83-91.

[5]

H. Song, G. Liu, J. Wu, Energy Conversion and Management 2016, 126, 1037-1046.

[6]

X. Jia, Q. Wang, L. Cheng, L. Han, C. Ye, M. Fang, Z. Luo, K. Cen, Energy & Fuels 2016, 30, 9589-9595.

[7]

H. Yang, S. Li, T. H. Fletcher, M. Dong, W. Zhou, Energy & Fuels 2014, 28, 3511-3518.

[8]

Z. Guo, Q. Wang, M. Fang, Z. Luo, K. Cen, Applied Energy 2014, 113, 1301-1314.

[9]

F. Xue, D. Li, Y. Guo, X. Liu, X. Zhang, Q. Zhou, B. Ma, Energy Technology 2017.

[10]

Q. Huang, S. Li, G. Li, Y. Zhao, Q. Yao, Fuel 2016, 181, 1162-1169.

[11]

A. N. Sengupta, Fuel Processing Technology 2002, 76, 1-10.

[12]

J.-F. Zhu, J.-Z. Liu, S. Yuan, J. Cheng, Y. Liu, Z.-H. Wang, J.-H. Zhou, K.-F. Cen, Fuel Processing Technology 2016, 147, 2-11.

[13]

S. P. Yang, W. M. Cai, H. A. Zheng, J. Q. Liang, S. J. Zhang, Q. C. Xue, Guocheng Gongcheng Xuebao/the Chinese Journal of Process Engineering 2014, 14, 896-900.

[14]

L. Butuzova, R. Makovskyi, T. Budinova, S. P. Marinov, Fuel Processing Technology 2014, 125, 246-250.

[15]

Liming Lu, A. Veena Sahajwalla, David Harris, Energy & Fuels 2000, 14, 869-876.

[16]

K. M. Steel, R. E. Dawson, D. R. Jenkins, R. Pearce, M. R. Mahoney, Fuel Processing


Page 18 of 19

Page 19 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Technology 2017, 155, 106-113. [17]

M. M. Maroto-Valer, J. M. Andrésen, C. E. Snape, Energy & fuels 1997, 11, 236-244.

[18]

D. G. Evans, Fuel 1973, 52, 186-190.

[19]

F. Shi, W. Zuo, Fuel 2014, 117, 1148-1155.

[20]

J.-Z. Liu, J.-F. Zhu, J. Cheng, J.-H. Zhou, K.-F. Cen, Fuel 2015, 146, 41-50.

[21]

W. I. Friesen, R. J. Mikula, Journal of Colloid & Interface Science 1987, 120, 263-271.

[22]

H. Song, G. Liu, J. Zhang, J. Wu, Fuel Processing Technology 2017, 156, 454-460.

[23]

Q. Zhou, Q. Liu, L. Shi, Y. Yan, Z. Liu, Fuel Processing Technology 2017, 161, 304-310.

[24]

K. Kidena, A. Satoru Murata, M. Nomura, Energy & Fuels 1998, 12.

[25]

C. Barriocanal, M. A. Dı́Ez, R. Alvarez, M. D. Casal, C. S. Canga, Journal of Analytical & Applied Pyrolysis 2003, 67, 23-40.

[26]

H. Marsh, R. C. Neavel, Fuel 1980, 59, 511-513.

[27]

T. Yokono, T. Obara, Y. Sanada, S. Shimomura, T. Imamura, Carbon 1986, 24, 29-32.

[28]

R. Stanger, Q. A. Tran, W. Xie, N. Smith, J. Lucas, J. Yu, E. Kennedy, M. Stockenhuber, T. Wall, Fuel 2016, 165, 33-40.

[29]

L. Butuzova, R. Makovskyi, T. Budinova, S. P. Marinov, Fuel Processing Technology 2014,

[30]

C. Barriocanal, M. Dıez, R. Alvarez, M. Casal, C. Canga, Journal of analytical and applied

125, 246-250. pyrolysis 2003, 67, 23-40. [31]

A. Marzec, S. Czajkowska, J. Moszynski, H. R. Schulten, Energy & fuels 1992, 6, 97-103.

[32]

K. Zhang, Y. Li, Z. Wang, Q. Li, R. Whiddon, Y. He, K. Cen, Fuel 2016, 185, 701-708.

[33]

A. Arenillas, F. Rubiera, J. Pis, M. Cuesta, M. Iglesias, A. Jiménez, I. Suárez-Ruiz, Journal of

[34]

K. Van Heek, W. Hodek, Fuel 1994, 73, 886-896.

[35]

E. Lester, S. Kingman, C. Dodds, Fuel 2005, 84, 423-427.

Analytical and Applied Pyrolysis 2003, 68, 371-385.

[36]

H. Yang, S. Li, T. H. Fletcher, M. Dong, Energy & Fuels 2014, 28, 7216-7226.

[37]

S. Li, H. Yang, T. H. Fletcher, M. Dong, Energy & Fuels 2015, 29, 5322-5333.

[38]

H. Takagi, K. Maruyama, N. Yoshizawa, Y. Yamada, Y. Sato, Fuel 2004, 83, 2427-2433.

[39]

C. L. Spiro, Fuel 1981, 60, 1121-1126.


Effect of pyrolysis temperature on the grindability of semi-cokes

Recommend Documents