Improving the slurrying ability of XiMeng brown coal by medium- to low-temperature thermal treatment

Fuel Processing Technology 119 (2014) 218–227

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Improving the slurrying ability of XiMeng brown coal by medium- to low-temperature thermal treatment Jiefeng Zhu, Jianzhong Liu ⁎, Wangjun Shen, Junhong Wu, Ruikun Wang, Junhu Zhou, Kefa Cen State Key Lab of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China

a r t i c l e

i n f o

Article history: Received 10 July 2013 Received in revised form 28 October 2013 Accepted 17 November 2013 Available online 6 December 2013 Keywords: Brown coal Thermal treatment Coal water slurry Slurrying ability Dewatering

a b s t r a c t The slurrying ability of XiMeng brown coal thermally modified at medium- to low-temperature (100 °C–350 °C) was investigated. We discussed the mechanism for improving the slurrying ability of brown coal by thermal treatment considering the coal properties, surface micro-topography, microscopic pore structure, and the moisture reabsorption characteristics. The results suggest that medium- to low-temperature thermal treatment can remove the internal moisture content of brown coal, which decreases from 19.42% (raw coal) to 1.8% after treatment at 350 °C for 0.5 h. The coal rank is increased as AC/O increases from 2.99 of raw coal to 4.88 of 350 °C. The coal surface becomes smooth and the shape becomes regular. With increasing thermal modification temperature, the surface area and BJH desorption pore volume of the coal samples first increased and then decreased, and the average pore diameter increased. All these phenomena improve the slurrying ability of brown coal. Higher thermal modification temperatures and longer time duration result in lower equilibrium moisture content of coal after moisture reabsorption. However, when the time is more than 0.5 h, its effect on the equilibrium moisture content is limited. The most suitable thermal modification temperature for improving the slurrying ability is between 200 °C and 250 °C. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Presently, the global energy consumption heavily relies on fossil fuel, which accounts for 87% of the energy consumption. Oil dominates the fossil fuels; however, coal is the fastest-growing. In 2011, coal accounted for 30.3% of the global energy consumption and 70.4% of the Chinese energy consumption [1]. With the consumption of coal resources, high-rank coal, such as anthracite and bituminous, is in short supply. Therefore, the utilization of low-rank coal, such as brown coal, has received greater attention. Brown coal, which is formed by peat, is low-rank coal. China has abundant brown coal resources. The proved brown coal reserves are about 130 billion tons, which account for about 13% of the total coal reserves [2]. Because of the high moisture and low fixed carbon content, its calorific value is low. Brown coal weathers easily and suffers from spontaneous combustion owing to its high volatile content. Hence, it is not suitable for long-distance transport and storage [3]. Most of brown coal is used as fuel in nearby power plants and raw material in the chemical industry. The use of brown coal in practical applications has several disadvantages, including low energy efficiency, environmental pollution, and high cost, because of its high moisture content and low calorific value.

⁎ Corresponding author. Tel.: +86 571 87952443x5302; fax: +86 571 87952884. E-mail address: [email protected] (J. Liu). 0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.11.010

Coal water slurry (CWS) was developed in the 1970s and is a kind of new clean coal-based liquid fuel and raw material for gasification. CWS consists of pulverized coal, water, and additives and is a highly concentrated and heterogeneous liquid–solid suspension [4–6]. CWS has been widely used as a substitute for oil and gasification material [7]. As a fuel, brown coal water slurry is deemed ideal and has been used on the Texaco gasifier in Inner Mongolia, China [8]. Brown coal is a kind of low-rank coal of low metamorphic grade, high moisture content, abundant pore structure, and high oxygencontaining functional groups. Therefore, it is very hard to prepare CWS of high solid concentration, low viscosity, and good fluidity from brown coal. Generally, the maximum solid loading of untreated brown coal CWS is only about 40–45% [9]. Therefore, it is necessary to dewater and modify brown coal. Favas G et al. [10], studied on the effects of hydrothermal dewatering on the intra-particle porosity of a kind of Victorian brown coal. Among the various process conditions, including types of autoclave, reaction temperature, residence time, solid concentration of CWS and particle size of pulverized coal, only the reaction temperature significantly affected the intra-particle porosity, which decreased with increasing temperature. They also found that hydrothermal dewatering significantly improved the slurrying ability of the brown coal, i.e., the maximum solid concentration of CWS could reach as high as 63.9% [11]. M. Sakaguchi et al. [12], studied on the effect of treatment conditions on the characteristics of treated brown coal by hydrothermal treatment. They reported that upgrading using the separation method yielded more effective drying especially during

J. Zhu et al. / Fuel Processing Technology 119 (2014) 218–227

219

Table 1 Proximate and ultimate analyses of coal sample. Sample

XM brown coal

Qb,ad/(J/g)

Proximate analysis/% Mad

Aad

Vad

FCad

19.42

13.17

31.97

35.44

18926

treatment at 350 °C and hydrothermal treatment at 300 and 350 °C led to a net increase in the calorific value compared to the raw coal. Ge Lichao et al. [13], investigated the effects of microwave irradiation treatment on physicochemical characteristics of Chinese low-rank coals and found that the upgrading process significantly reduced the coals' inherent moisture, and increased their calorific value and fixed carbon content. Moreover, the upgrading process generated micropore and increased pore volume and surface area of the coals. However, the existing methods are all having the same problems, i.e. high cost of modification, and complex technologies and systems, which makes unsuitable for China. In this paper, XiMeng (XM) brown coal was modified thermally at medium- to low-temperature (100–350 °C) and the slurrying ability, rheological behavior, and stability of CWS prepared by treated brown coal were investigated. Then, based on the analysis results of coal property, surface micro-topography, microscopic pore structure and the characteristics of reabsorption, the mechanism for improving the slurrying ability of brown coal by medium- to low-temperature thermal modification were discussed. The aim was to find a simple, practical, and economical brown coal dewatering and upgrading method. 2. Experimental 2.1. Material The brown coal used in this paper was from the XiMeng region, Inner Mongolia, which is the largest brown coal producer in China. Proximate and ultimate analyses of the raw coal sample can be found in Table 1. As shown in Table 1, the volatile content of the raw coal sample is high, the internal moisture content and oxygen content are relatively high, and the contents of ash and fixed carbon are relatively low, which means that the brown coal is a typical low-rank coal. The additive used in this paper is sodium methylene naphthalene sulfonate– sodium styrene sulfonate–sodium maleate dispersant. 2.2. Experimental section 2.2.1. Medium- to low-temperature thermal modification The naturally dried XM brown coal was ground in a ball mill until the coal powder size was less than 2.5 mm. Then, the coal powder was sieved through a 125 μm sieve. Finally, the sieved coal powder was sealed and stored. The brown coal was thermally modified using a controlled atmosphere furnace (HMX1700-30, HaoYue High-Temperature Equipment Company, China). A coal sample of specified thickness was neatly placed in the oven. After the oven leakage test, N2 was continuously passed

Ultimate analysis/%

[C]/[O]

Cad

Had

Nar

St,ad

Oad

47.34

2.99

0.71

0.53

15.84

2.99

through the oven at velocity of 1 L/min to drive off the moisture and gases generated, and to prevent the spontaneous combustion of brown coal. The temperature was rapidly increased to the final value (100, 150, 200, 250, 300, and 350 °C), under which the brown coal was thermally treated for 0.5 h. Then, the oven was cooled to room temperature and the treated coal sample was put into a dry vessel. 2.2.2. CWS preparation CWS was prepared using the following steps. First, the coal powder, deionized water, and additive were weighted according to the design solid concentration. Then, the additive was dissolved well in the deionized water in a 1000 mL stainless steel beaker. The coal powder was then slowly poured into the beaker. The mixture was thoroughly stirred with a mechanical mixer operating at a constant speed of 1000 rpm for 10 min. Before measuring, the prepared CWS was left undisturbed for 5 min to release the entrapped air. 2.2.3. The viscosity and rheological behavior of CWS The viscosity and rheological behavior of CWS were obtained by using a rotational viscometer (NXC-4C, Chengdu Instrument Factory, China), according to the Chinese National Standard GB/T18856.4-2008 [14]. A moderate amount of CWS was poured into the sample container. The shear rate increased from 10 to 100 s−1 and then held constant at 100 s−1 for 72 s. The viscosity data were recorded every 12 s. The apparent viscosity at 100 s−1 was then calculated as the average of the six recorded viscosity values. Through the procedure, the temperature was held constant within 20 ± 0.1 °C controlled by a water bath. The solid content of CWS was calculated from the weight difference before and after drying in an oven at 105 °C for 3 h according to the Chinese National Standard GB/T 18856.2-2008 [15]. The maximum solid loading (MSL), which is defined as the solid content of CWS with a viscosity of 1000 mPa s at a shear rate of 100 s−1, is used to appraise the slurrying ability of CWS. The higher the MSL of CWS, the better the slurrying ability of CWS [16]. 2.2.4. The static stability of CWS The static stability of CWS is appraised by the water separation ratio (WSR), which is defined as the mass ratio of separated water to the total water in the test slurry after storing still for 7 d [17]. The higher the WSR, the poorer the static stability of CWS. 2.2.5. Field-emission scanning electron microscope (FESEM) analysis The surface micro-topography of the coal samples was measured by FESEM (ULTRA 55, Zeiss, Germany). The surface micro-topography can be used to analyze the slurrying ability mechanism of brown coal by medium- to low-temperature thermal modification.

Table 2 Proximate and ultimate analyses of upgraded coal samples. Modification temperature/°C

100 150 200 250 300 350

Proximate analysis (%) Mad

Aad

Vad

FCad

3.54 3.49 2.98 2.80 2.42 1.80

16.09 16.22 16.57 16.78 17.87 18.76

37.09 36.15 34.58 33.35 30.96 26.12

43.28 44.14 45.87 47.07 48.75 53.32

Qnet,ad/ (J/g)

Ultimate analysis (%) Cad

Had

Nad

St,ad

Oad

AC/O

22518 22631 22970 23082 23780 24316

56.46 56.46 56.46 58.47 59.70 61.47

3.72 3.72 3.72 3.44 3.38 3.17

0.98 1.03 1.10 1.05 1.05 1.11

1.16 1.10 1.16 1.09 1.19 1.09

18.05 17.98 17.93 16.37 14.39 12.60

3.13 3.14 3.15 3.57 4.15 4.88

220

J. Zhu et al. / Fuel Processing Technology 119 (2014) 218–227

Apparent Viscosity(mPas)

1400 1200

Raw Coal 200 350

100 250

3.2. Slurrying ability of brown coal

150 300

1000 800 600 400 45

50

55

60

65

Solid Concentration (%) Fig. 1. Apparent viscosity–solid concentration dependence of coal samples.

2.2.6. The microscopic pore structure of the coal samples The microscopic pore structure of the coal samples was determined by an automatic surface area and pore analyzer (TriStar II3020, Micromeritics, USA). The samples were pre-heated at 200 °C for 4 h in vacuum and then nitrogen adsorption isotherm (77 K) was used to measure the surface area based on the Brunauer–Emmett–Teller (BET) method and the pore size distribution based on the Barrett– Joyner–Halenda (BJH) method. 2.2.7. The moisture reabsorption of brown coal from thermal process The effect of the thermal treatment temperature and the duration of the treatment time on the moisture reabsorption performance of brown coal were investigated. 3 g of thermally processed coal sample was placed into a constant temperature and humidity atmosphere (temperature of 20 °C, relative humidity of 75%) and the moisture reabsorption of brown coal was studied. The mass changes of the brown coal samples were recorded for 72 h. 3. Results and discussion 3.1. Coal property analyses of upgraded coal samples The proximate and ultimate analyses of thermal treated coal samples are shown in Table 2. As the thermal treated coal samples were put into dry vessel rather than in air. As a result, the moisture content of treated coal did not get equilibrium with air and were lower than the equilibrium content. The variation of equilibrium moisture, which got equilibrium in a constant temperature and humidity atmosphere, can be found in Section 3.7.2. Compared with Table 1, the internal moisture content of the coal samples significantly decreased after medium- to low-temperature thermal treatment. After treatment at 350 °C for 0.5 h, the internal moisture content decreased from 19.42% for raw coal to 1.8%, i.e., the relative decrease was as high as 90.73%. This implies that the water-holding capacity of brown coal decreased sharply and the medium- to low-temperature thermal treatment can efficiently remove the internal moisture of brown coal. Moreover, the ratio of carbon to oxygen (AC/O) increased with increasing thermal modification temperature, from 2.99 for raw coal to 4.88, suggesting the coal rank also increased. The decrease in the water-holding capacity and the increase in AC/O both benefit the slurrying ability of brown coal.

The apparent viscosity–solid concentration dependence of the coal samples is shown in Fig. 1. From Fig. 1, the apparent viscosity of all types of CWS increased with increase in solid concentration. According to Debadutta Das et al. [18–21], this increase is attributed to the increase in particle–particle interaction. As a result, the viscosity of CWS increased because of the decrease in space and the increase in the friction forces between particles, especially when the solid loading density approached maximum [22]. MSL is a very important index to appraise the slurrying ability of CWS. The higher the MSL, the better the slurrying ability of CWS. The MSL of the coal samples is shown in Table 3 and the data are averages from three independent samples of each coal. The medium- to lowtemperature thermal modification clearly improved the slurrying ability of brown coal. After treating at 350 °C, the MSL increased from 50.38% of raw coal to 60.22%. Moreover, when the modification temperature was lower than 150 °C, the increase rate of MSL was relatively slow, whereas the MSL noticeably improved when the modification temperature was higher than 150 °C. The reason was that the carboxyl, which is the most active among the oxygen-containing groups, decomposes at temperatures greater than 150 °C [23]. As a result, the hydrophilicity of brown coal greatly decreased and thus the slurrying ability of brown coal improved clearly. The higher the thermal modification temperature, the higher the MSL of brown coal. The increase in the treatment temperature helped improve the slurrying ability of brown coal, because the decrease in internal moisture and the increase in AC/O of the treated brown coal (From Table 2). On the other hand, the changes in the physical and chemical properties of the treated brown coal also affected the slurrying ability of brow coal [24,25]. 3.3. The rheological properties of CWS 3.3.1. Effect of thermal modification temperature on the rheological behavior The rheological properties of CWS, which are characterized by the flow and deformation caused by some external force, mainly reflect the dependence between the shear rate and the shear stress or the dependence between shear rate and apparent viscosity. It is a very important index to appraise the quality of CWS and directly affects the storage, transportation, atomization, and combustion characteristics of CWS; therefore, it is very important in the industrial applications of CWS [26]. Plots of the rheological characteristics of CWS of the coal samples are shown in Fig. 2, where all slurry samples of raw coal were pseudoplastic fluids with evident shear-thinning properties, similar to earlier observation [27]. The experimental results also reveal a sharp increase in the apparent viscosity of all CWS samples with the concentration of coal. This increase can be explained with the frictional forces between the particles becoming significant, and the accompanying resistance is reflected in the increase in viscosity [28]. Besides, Ting and Luebbers [29] reported that this increase in viscosity might be due to occlusion of water in the coal particles. According to Table 1, the internal moisture content and oxygen-containing groups of raw coal were high. Thus, the surface of the raw coal powder is hydrophilic and free water is easily absorbed. When sheared, the water absorbed on the surface of the coal powder is released by the shear stress as the shear rate increases. As a result, the amount of free water in the suspension system increases, thereby improving fluidity and reducing viscosity. When thermally

Table 3 The maximum solid loading (Mean ± SE) of coal samples. Sample

Raw coal

100 °C

150 °C

200 °C

250 °C

300 °C

350 °C

MSL

50.38 ± 6.17%

50.99 ± 7.17%

52.33 ± 1.00%

54.80 ± 1.00%

57.22 ± 1.73%

58.86 ± 6.69%

60.22 ± 9.07%

J. Zhu et al. / Fuel Processing Technology 119 (2014) 218–227

b) 100 oC

a) Raw coal

3500

Apparent Viscosity(mPa s)

Apparent Viscosity(mPa s)

3500

50.55% 50.37% 49.95%

3000 2500 2000 1500 1000 500 0

20

40

60

80

51.55% 50.99% 50.72%

3000 2500 2000 1500 1000 500

0

0

100

0

20

40

-1

60

80

100

Shear Rate/s-1

Shear Rate/s

c) 150 oC

d) 200 oC

3000

Apparent Viscosity(mPa s)

Apparent Viscosity(mPa s)

221

52.41% 52.21% 51.91%

2500 2000 1500 1000 500

4000

55.28% 54.77% 54.35%

3500 3000 2500 2000 1500 1000

0 0

20

40

60

80

100

0

20

40

Shear Rate/s-1

e) 250 oC

80

100

f) 300 oC

2400

1800

2200 2000

Apparent Viscosity(mPa s)

58.01% 56.97% 56.39%

1800 1600 1400 1200 1000 800 600

59.47% 58.80% 56.38%

1600 1400 1200 1000 800 600 400 200

0

20

40

60

80

100

0

20

40

-1

g) 350 oC 2000

61.02% 60.09% 59.84%

1800 1600 1400 1200 1000 800 600 0

60

Shear Rate/s-1

Shear Rate/s

Apparent Viscosity(mPa s)

Apparent Viscosity(mPa s)

60

Shear Rate/s-1

20

40

60

80

Shear Rate/s-1 Fig. 2. CWS rheological characteristics.

100

80

100

222

J. Zhu et al. / Fuel Processing Technology 119 (2014) 218–227

treated at temperatures lower than 200 °C, the modification effect was not evident because of the low temperature and thus the rheological properties of the CWS of treated coal are similar with that of CWS of raw coal. When the treated temperature is more than 200 °C, the shear thinning properties of the CWS of treated coal weakened with the increase in the treatment temperature. This is because the modification effect was more evident with the increase in treatment temperature. As a result, the hydrophilicity of the coal powder sharply decreased and the hydrophobicity increased. Although the coal particles were dispersed by stirring, granule conglomeration in CWS still existed at some areas in the suspension system owing to the water-repulsive and Van der Waals forces between the coal particles. As a result, many coal particle clusters are surrounded by water film. When subjected to shearing, the water film broke and the coal particles in the clusters were dispersed into the water, and the unwetted surface of the coal particles increased. As a result, some free water was needed to wet this dry surface, decreasing the ratio of free water in the suspension system and increasing the viscosity of CWS. In highly concentrated CWS, most of the coal particles connect to each other to form a cover, wherein the free water is trapped. The state of the water surrounded by the coal particles is broken at high shear rates and the free water is released into the suspension system, resulting in a viscosity decrease of CWS [30]. 3.3.2. CWS rheological model The rheological characteristics of CWS can be described by the Bingham plastic model [18–21,31], whichever one gave a statistically better fit, from which the yield stresses were calculated, τ ¼ τ0 þ η •γ

ð1Þ

where τ is the shear stress (Pa), τ0 is the yield stress (Pa), η is the coefficient of rigidity (Pa·s), γ is the shear rate (s−1). A high-viscosity CWS typically has high values of η. The rheological parameters of the coal samples, which are averages from three independent CWS samples of each coal, are listed in Table 4 and the rheological fitting curves of CWS of the coal samples are shown in Fig. 3. All individual data on the average represent a linear relationship with regression coefficient (R2) around 1 and the values are within the experimental error. Therefore, the Bingham plastic model is deemed appropriate to describe the rheological properties of CWS. The value of η increased as the solid concentration of CWS increased. This phenomenon is consistent with that of viscosity in Fig. 2. The yield stress of

each coal samples increases with the increase in solid concentration of CWS, which is good for the static stability of CWS, but may have negative effect on the initiate flow of CWS [6]. What's more, we can see that the tendency of yield stress is decreasing with the increase in treated temperature, which is good for pipeline transport of the slurry. 3.4. The static stability of CWS The static stability is an important property for CWS storage. The dispersed coal particles in CWS will set and form flocculation when their interaction among themselves is stronger than with the surrounding medium [32]. The primary factors responsible for the optimum stability of CWS depend on the physicochemical properties of coal, including its (i) surface hydrophobicity [33,34], (ii) oxygen content [35], (iii) particle size distribution [35–37], (iv) zeta potential (surface charge) [19,38], (v) pH sensitiveness [18–20,39], and (vi) temperature sensitiveness of the viscosity of the coal–water slurry [21,40] etc. Actually, the basic mechanism of stabilization of CWS is to increase the net interparticle repulsion, in other words, to decrease the coal–coal interaction and õpromote coal–water interaction [19], which is usually included in the following ways: [41] (i) increasing the surface charge (electrostatic repulsion), such as adding the additives (ii) introducing groups that provide a mechanical barrier (steric repulsion), such as optimizing the coal particle size distribution or (iii) increasing the steric wettability of the solid surface to eliminate the hydrophobic interaction. The WSR reflects the static stability of CWS. The higher the WSR, the harder the sediment, the worse the static stability of CWS. The WSRs of CWS stored for 7 d, which are averages from three independent samples of each coal, are shown in Fig. 4. The WSRs of each CWS decreased with the solid concentration of CWS, meaning that the higher solid concentration resulted in better static stability of CWS. Because the coal particles, water molecules and additives in the suspension system connected with each other and formed large amounts of constitutional units. These constitutional units formed a three-dimensional network structure, which can prevent the coal particles from uniting and settling. When the solid concentration of CWS increased, the coal particles in the suspension system accumulated closely and the three-dimensional network structure strengthened, and the hindrance preventing the coal particles from settling increased [19]. 3.5. FESEM analysis

Table 4 CWS rheological parameters of coal samples. Samples

Solid concentration (%)

Apparent viscosity (mPa·s)

τ0(Pa)

η

R

Raw coal

49.95 50.37 50.55 50.72 50.99 51.55 51.91 52.21 52.41 54.35 54.77 55.28 56.39 56.97 58.01 56.38 58.80 59.47 59.84 60.09 61.02

796.2 989.8 1112.0 747.8 999.8 1154.5 700.7 878.0 1086.7 872.2 981.0 1260.5 761.8 924.5 1243.5 473.5 988.2 1179.2 862.3 974.3 1155.5

14.39233 22.76236 27.50329 12.39496 23.0743 29.4366 8.05271 10.01447 23.12071 11.36619 14.64175 40.936 8.26192 13.32466 21.1126 2.58745 12.72373 12.70055 7.1606 8.11879 11.70805

0.65589 0.77115 0.8349 0.62561 0.80143 0.90507 0.6465 0.77798 0.8797 0.7572 0.83648 1.1404 0.71203 0.82378 1.19347 0.51705 0.92902 1.07515 0.86767 0.98867 1.15372

0.97922 0.97972 0.98705 0.98317 0.98626 0.97654 0.98121 0.98488 0.97781 0.96999 0.96792 0.93494 0.97212 0.9605 0.96099 0.98759 0.96211 0.97186 0.9774 0.98302 0.97911

100 °C

150 °C

200 °C

250 °C

300 °C

350 °C

The microstructure of the coal samples is shown in Fig. 5. Many researchers [42–50] have reported that the physical and chemical properties of low-rank coal changed during modification. The internal moisture of coal particles was released and the pore structure of coal particles shrunk under pressure [42–45]. The pore wall experienced the following successive stages: softening, hardening etc., and the pore structure of the coal particles thickened and solidified. From Fig. 5, the surface of the raw coal is loose and the external contour is inconspicuous. When treated under temperatures lower than 200°C, the surface of the coal samples changed a little and remained loose. However, the surface partially fractured and hardened, and several holes were generated because of evaporation and the moisture escaped. As treatment temperature further increased, the surface of the treated coal samples became smooth, hard and dense, and the external contours of the coal particles became gradually clear. The CWS prepared by dense coal particles can reach higher solid concentration, because the dense coal particles occupy less space in the suspension system, and thus more coal particles can be dispersed in CWS. Moreover, we can see that the amount of open pore of coal particles decreases after thermal modification, especially when the treatment temperature exceeds 200 °C, which is good for improving the solid concentration of

J. Zhu et al. / Fuel Processing Technology 119 (2014) 218–227

b) 100 oC

a) Raw coal

80

51.55% 50.99% 50.72%

100

Shear Stress/Pa

Shear Stress/Pa

120

50.55% 50.37% 49.95%

100

223

60 40 20

80 60 40 20

0 0

20

40

60

80

100

0

20

c) 150 oC

80

100

80

100

80

100

120

55.28% 54.77% 54.35%

120

52.41% 52.21% 51.91%

100

Shear Stress/Pa

140

Shear Stress/Pa

60

d) 200 oC

160

100 80 60 40

80 60 40 20

20

0

0 0

20

40

60

80

100

0

20

40

60

Shear Rate/s-1

Shear Rate/s-1

(e) 250 oC

f) 300 oC

120

140

59.47% 58.80% 56.38%

80

58.01% 56.97% 56.39%

120

Shear Stress/Pa

100

Shear Stress/Pa

40

Shear Rate/s-1

Shear Rate/s-1

60 40 20

100 80 60 40 20

0 0

20

40

60

80

100

0

0

20

40

60

Shear Rate/s-1

Shear Rate/s-1

g) 350 oC 120

61.02% 60.09% 59.84%

Shear Stress/Pa

100 80 60 40 20 0 0

20

40

60

80

100

Shear Rate/s-1 Fig. 3. Rheological fitting curves for the CWS of the coal samples.

CWS. The clear and regular external contours of the coal particles suggest that the brittleness of the coal increases and thus the coal rank increases, because high-rank coal is typically brittle. This is

consistent with the analysis results in Table 2. All in all, the microstructure of the coal samples well explains the slurrying ability of the treated brown coal.

J. Zhu et al. / Fuel Processing Technology 119 (2014) 218–227

WSR/%

224

7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 50

100 200 300

51

52

53

54

55

56

150 250 350

57

58

59

60

61

Solid Concentration/% Fig. 4. The WSRs of CWS of treated coal samples stored for 7 d.

3.6. The microscopic pore structure of coal samples The BJH desorption pore size distribution of raw coal is shown in Fig. 6 and the variations in surface area, BJH desorption pore volume, and average pore diameter of coal samples are shown in Figs. 7 and 8. From Fig. 6, the BJH desorption pore size distribution of raw coal was mainly mesopore and macropore, whose diameter was above 3 mm. The most concentrated pore was mesopore with diameter between 3 mm and 5 mm. This is consistent with the results reported in literature [51]. Surface area and BJH desorption pore volume are important parameters to appraise the pore structure of coal samples. From Fig. 7, with the increase in treatment temperature, both the surface area and the BJH desorption pore volume first increased and then decreased, and the transition temperature was 100 °C. In the increasing section (b100 °C), the surface area and BJH desorption pore volume slightly increased, whereas in the decreasing section (N 100 °C) they decreased abruptly. The surface area and BJH desorption pore volume of the thermally treated coal samples at 350 °C respectively decreased to 38.2% and 59.1% of that at 100 °C. During the thermal treatment, coal successively experienced the removal of internal water, the decomposition of the oxygen-containing functional groups, and the release of tar and volatiles [52]. The removal of internal water, decomposition of oxygen-containing functional groups and particle thermal contraction can destroy and cause the pore structure to collapse [45]. The release of tar blocks the coal pores, decreasing the coal pores [53]. However, the release of volatiles generates new pores and enriches the pore structure of coal. The change in the pore structure of coal samples from medium- to low-temperature thermal treatment depends on the combined effects of the abovementioned factors. From Figs. 7 and 8, we see that because the internal moisture of the coal particles is evaporated and removed from room temperature up to 100 °C [54],which enlarges the pore structure. As a result, the surface area and BJH desorption pore volume of the coal samples increased and the average pore diameter slightly increased. From 100 °C to 150 °C, the surface area and BJH desorption pore volume began to decrease, and the average pore diameter also decreased slightly. This may be attributed to the removal of internal moisture in the mesopore and big capillaries at this stage, the subsequent collapse of the pore structure, and the filling of the micropore with debris. According to the above two stages, when the treatment temperature is lower than 200 °C, the changes in surface area, BJH desorption pore volume, and average pore diameter of coal samples are not obvious. In other words, the modification effect is unexceptional, which is consistent with the results of the slurrying ability of CWS and FESEM. When the treatment temperature is more than 150 °C, the internal moisture in the micropore is removed and the macromolecular structure such as

oxygen-containing functional groups and methylene starts to thermally decompose, and gases are released. As the temperature further increases, tar is released. Therefore, on one hand, the release of gases enriches the pore structure of the coal samples and enlarges the opening pores, increasing the macropore and resulting in the increase in average pore diameter. On the other hand, micropore collapses, due to the removal of internal moisture, evident particle thermal contraction and the release of tar blocks the coal pores. When these two factors are simultaneously considered [54], the surface area and BJH desorption pore volume decreased and the average pore diameter increased with increasing treatment temperature. The surface area and pore volume significantly affect the slurrying ability of CWS. A larger surface area and pore volume lead to a stronger water-holding capability of coal particles. Thus, the free water in the suspension system decreases and the viscosity of CWS increases. Therefore, the slurrying ability of brown coal worsens. After medium- to lowtemperature thermal treatment, the surface area and BJH desorption pore volume gradually decreased and the average pore diameter increased, meaning that the pore size distribution of the treated coal samples changed from mesopore to macropore and the coal hardened and densified. As a result, the slurrying ability of treated brown coal improved gradually. This is consistent with the variations in MSL of brown coal CWS and of the FESEM results. 3.7. The moisture reabsorption of brown coal 3.7.1. Effect of treatment time duration The moisture reabsorption experiments of coal samples treated at 300 °C for different time duration were investigated, shown in Fig. 9. With increasing treatment time, the equilibrium moisture content (EMC) of the coal samples decreases. When the time duration was above 0.5 h, the EMC decreased very little, i.e., the EMC of coal treated for 0.5 h was very close to that of the 1 h treatment. According to the thermogravimetric curve of brown coal [55], the thermal modification of brown coal is completed within 0.5 h. Therefore, 0.5 h was selected as the primary treatment time duration in this paper. 3.7.2. Effect of treatment temperature The moisture reabsorption of coal samples from thermal treatment at different temperatures is shown in Fig. 10. With increasing treatment temperature, the EMC of the coal samples decreased. When the treatment temperature was above 250 °C, the EMC of the coal samples decreased sharply. Combined with the results of the coal properties, FESEM, and microscopic pore structure, the thermal treatment effect was evident at high temperatures, because the water-repulsive properties of the coal particles improved, and coal became thick and solid. In addition, particle thermal contraction and the released tar blocked the mesopore and micropore, and thus the pores of the treated coal samples transformed from mesopore to macropore, and the surface area and BJH desorption pore volume consequently decreased. Therefore, the waterholding capacity of the coal particles decreased and the EMC of the coal samples became lower. The results can be used to explain why the slurrying ability of the brown coal slowly increased at low temperature and sharply increased with increasing temperature. Combined with the results of the slurrying ability of CWS, FESEM, microscopic pore structure, and moisture reabsorption of treated coal samples, from the perspective of improving the slurrying ability of brown coal and economic efficiency, the most suitable thermal modification temperature for improving the slurrying ability was between 200 °C and 250 °C. 4. Conclusions (1) Thermal modification can effectively remove the internal moisture and improve the rank of the brown coal. After thermal treatment at 350 °C for 0.5 h, the internal moisture content decreased from

J. Zhu et al. / Fuel Processing Technology 119 (2014) 218–227

a) Raw coal

b) 100 °C

c) 150 °C

d) 200 °C

e) 250 °C

f) 300 °C

g) 350 °C

Fig. 5. SEM images of coal samples.

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J. Zhu et al. / Fuel Processing Technology 119 (2014) 218–227

0.00022

16

0.00020

14

0.00018

12

0.00016 10

0.00014

M(%)

dV/dD Pore Volume (cm3/g)

226

0.00012

8

0.00010

6

0.00008

4

0.00006

T=300°C 0.25h 0.5h 1h

2

0.00004 0

0.00002

0

10

20

30

40

50

60

70

t(h)

0.00000 10

50

100

500

1000 Fig. 9. The trend of coal samples at different thermal modification times.

Pore Diameter (Å) Fig. 6. BJH desorption pore size distribution of raw coal.

0.012 3.0 (3)

0.011

Surface Area (m2/g)

Surface Area Pore Volume

2.4

0.010

2.1

0.009

1.8 0.008

1.5 1.2

Pore Volume (cm3/g)

2.7

0.007

(4)

(5)

0.9 0

50

100

150

200

250

300

350

0.006 400

T(°C) Fig. 7. Variations in the surface area and BJH desorption pore volume of coal samples.

(6) 19.42% of raw coal to 1.8% and the AC/O increased from 2.99 of raw coal to 4.88. The decrease in internal moisture and increase in AC/O of brown coal help improve the slurrying ability of brown coal. (2) The slurrying ability of brown coal obviously improved by medium- to low-temperature thermal modification. The MSL increased from 50.38% of raw coal to 60.22% of treated coal at

(7)

350 °C for 0.5 h. With increasing treatment temperature, the MSL slowly increased before 150 °C and greatly increased after 150 °C. After thermal treatment, the pseudoplasticity of the brown coal water slurry decreased and the slurrying stability improved. The surface of raw coal was loose and the external contours inconspicuous. After thermal treatment, the brown coal surface fractured and hardened, and holes were generated from the moisture evaporation. With increasing treatment temperature, the brown coal surface became smooth and hard, and the shape became regular. The external contours of the coal particles gradually cleared and the coal particles became dense. The results improved the slurrying ability of brown coal. Thermal modification transformed the internal pores in brown coal from mesopore to macropore, and the surface area and BJH desorption pore volume gradually decreased. The slurrying ability of brown coal was thus improved. This was consistent with the variation in MSL of brown coal CWS and the FESEM results. A higher treatment temperature and time duration led to lower EMC after moisture reabsorption. A more evident EMC reduction occurred at temperatures higher than 250 °C and time duration of 0.5 h. Taking into account the slurrying ability, microscopic morphology and pore structure, and moisture reabsorption, the most suitable thermal modification temperature was between 200 °C and 250 °C.

18

Average Pore Diameter

16

16

14

15

12

14

M(%)

Average Pore Diameter(nm)

17

13 12 11 10 0

50

100

150

200

250

300

T(°C) Fig. 8. Average pore diameter variations of the coal samples.

350

10 8 6

100 °C

4

200 °C

250 °C

2

300 °C

350 °C

0

0

10

20

30

40

150 °C

50

60

70

t(h) Fig. 10. The trend of coal samples at different thermal modification temperatures.

J. Zhu et al. / Fuel Processing Technology 119 (2014) 218–227

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