Pore Characteristics and Slurryability of Coal Blends - American

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Pore Characteristics and Slurryability of Coal Blends Jie-Feng Zhu,† Jie Wang,‡ Jian-Zhong Liu,*,† Jun Cheng,† Zhi-Hua Wang,† Jun-Hu Zhou,† and Ke-Fa Cen† †

State Key Lab of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China Zhejiang Energy Group R&D, Hangzhou 310000, China

‡

ABSTRACT: The pore characteristics and slurryability of two coal blends between Shigang anthracite coal and Huangling bituminous coal (SG/HL), and Guizhou anthracite coal and Xiaotun lean coal (GZ/XT), respectively, were investigated. The fractal dimensions of coal were calculated in the two regions of P/P0 < 0.45 and P/P0 > 0.45 and defined as D1 and D2, respectively. Upon an increase in the blending ratio of parent coal with smaller BET surface area (SBET) and total pore volume (TPV), the SBET and TPV of coal blends monotonously decreased. D1 was mainly related to the Smeso/macro(10−220 nm)/Stotal and mineral phase within coal while D2 was closely affected by the Vmeso(2−10 nm)/Vtotal. D1 of SG/HL coal blends had no apparent linear correlation with the pore structure parameters whereas D1 of GZ/XT coal blends changed linearly with the pore structure parameters. Both D2 of SG/HL coal blends and that of GZ/XT coal blends changed linearly with the pore structure parameters. The slurry quality of coal water slurry (CWS) prepared from coal blends is comprehensively affected by the physicochemical properties and blending ratio of parent coals. Therefore, the maximum solid loading (MSL) and water separation ratio (WSR) of CWS prepared from coal blends do not always change linearly with the blending ratio of parent coal.

1. INTRODUCTION Pulverized coal-fired power plants are usually designed to burn a specific coal, which is very important for the safe, efficient, and economical operation of the unit.1,2 However, in many cases, the boilers utilize a variety of coal types and/or coal blends in view of fuel shortage, fuel cost, and environmental protection. Coal blending is a kind of feasible and effective clean coal utilization technology and has been applied to many power plants. The application of coal blending has several advantages, including enhancing the fuel flexibility, improving combustion behavior, reducing the costs and pollutant emission, and utilizing the low-quality coal.2−5 Although coal blending is achieved by mechanical mixing, the properties of coal blends are difficult to predict as a linear combination of the properties of parent coals and the blending ratio in many cases.6−10 Therefore, much attention has been paid to the physicochemical properties and combustion characteristics of coal blends, including coal properties, grindability, ignition and combustion characteristics, pollutant emissions, slagging and fouling, and ash disposal.4−15 Coal blending is also a kind of simple and economical technology for improving the slurryability of coals.16−21 However, on the basis of the MSL of parent coals and the blending ratio, linear prediction of the MSL of coal blends cannot be applied easily. Coal has very complex and irregular surface morphology and pore structures because of the existence of a variety of pores, including capillaries, through-holes, cracks, and closed and open pores. The surface morphology and pore structures of coal not only play important roles in determining the physical properties (i.e., adsorption, wetting, mechanical, and permeation properties), but also provide diffusion paths and reaction sites for the reactant and product during coal processing. Moreover, the surface morphology and pore structures of coal exert important influences on the slurryability. Therefore, it is meaningful to accurately and quantitatively describe the surface morphology © XXXX American Chemical Society

and pore structures of coal. Because the surface morphology and pore structures of coal have the property of self-similarity for a certain range of scales, fractal geometry can be used to describe their irregularity and disorder.22−29 Fractal geometry is a powerful and reasonable tool that provides a new insight in studying the pore structures of coal. Bale et al.30 did pioneering work in applying fractal geometry to the study of the surface morphology of Beullah lignite. Different methods, including mercury porosimetry, gas adsorption, and small-angle X-ray scattering, were employed by Cai et al.26 to characterize the microscopic pore structures and fractal characteristics of bituminous and sub-bituminous coals from Northeast China. The relationships between pore properties and CH4 adsorption isotherms and coal permeability were studied. Fractal dimensions calculated with different methods were comparable. The more irregular a surface was, the more heterogeneous pore structures were, indicating larger pore area and stronger adsorption capability, especially for the micropores with sizes in the range 2−10 nm and the mesopores. Liu et al.31 analyzed the fractal characteristics of superfine pulverized coal particles based on N2 gas adsorption and small-angle X-ray scattering. They observed that the surface fractal dimension increased with increasing pulverized coal particle size, while the structure fractal dimension increased with decreasing average pore diameter (APD). After the analysis of fractal dimensions of superfine pulverized coal particles, they concluded that the superfine pulverized coal particles were beneficial to the coal combustion process. N2 gas adsorption was employed by Cheng et al.24 to study the pore fractal structures and combustion dynamics of cokes obtained from the pyrolysis of 18 typical Chinese power coals. Results revealed that the cokes obtained Received: June 11, 2016 Revised: August 15, 2016

A

DOI: 10.1021/acs.energyfuels.6b01424 Energy Fuels XXXX, XXX, XXX−XXX

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was employed to study the pore structures and fractal characteristics of two different coal blends (i.e., SG/HL and GZ/XT coal blends). The piecewise linear fitting method was used to calculate their fractal dimensions. The slurryability of SG/HL and GZ/XT coal blends was also investigated. The evolution of pore structures in coal blends was discussed, and the relationship between fractal dimensions and different types of pore structures in coal blends was quantitatively analyzed. The mechanisms affecting the slurryability of coal blends were also obtained. The aim is to provide references for the processing and utilization of coal blends.

from lignite pyrolysis had higher fractal dimensions, and therefore, these cokes had lower ignition temperatures and higher burnout efficiencies than those from lean coal and anthracite pyrolysis. Because fractal geometry provides a new perspective in investigating the effects of the surface morphology and pore structures of coal on its processing and utilization, it is useful to study the pore fractal structures of coal blends. However, up to now, few literature studies reported the fractal characteristics of pore structures in coal blends or evaluated the effects of the pore fractal structures of coal blends on their processing and utilization. Cheng et al.32 focused on the effects of pore fractal structures on the combustion characteristics of Shenhua coal blends and obtained some conclusions. The pore fractal dimensions of Shenhua coal blends monotonously changed with the blending ratio, which was consistent with the changes in the SBET and TPV. The coal blends with higher fractal dimensions had lower ignition temperatures and smaller unburned carbon mass fractions in the fly ash, thus indicating that these coal blends were easier to ignite and burn out. However, because the pore structures of coal are complicated and irregular, application of piecewise linear fitting to the calculation of fractal dimensions is necessary to reveal the real fractal characteristics of coal.23,29,33−38 Moreover, the mechanisms affecting the slurryability of coal blends are still unclear and need to be further studied. This study is an extension of our previous work on the pore characteristics of coal.38,39 In this paper, N2 gas adsorption

2. EXPERIMENTAL SECTION 2.1. Materials. The parent coals used in this study include Huangling (HL) bituminous coal, Xiaotun (XT) lean coal, and Shigang (SG) and Guizhou (GZ) anthracite coal. The proximate and ultimate analyses of parent coals are listed in Table 1, and their ash compositions are listed in Table 2. The HL, SG, and XT coal samples were ground and sieved to less than 74 μm, while the GZ coal was sieved to less than 154 μm. Figure 1 shows the particle size distributions of parent coals. Coal blends between SG and HL, and GZ and XT, respectively, were prepared with blending ratio of 8:2, 6:4, 5:5, 4:6, and 2:8. 2.2. Methods. 2.2.1. N2 Gas Adsorption. The pore structures of coal blends were measured with an automatic surface area and pore analyzer (TriStar 3020, Micromeritics). N2 gas adsorption/desorption isotherms at 77 K were measured for relative pressure (P/P0) ranging from 0.01 to 0.99. The multipoint Brunauer−Emmett−Teller method was applied to

Table 1. Proximate and Ultimate Analyses of Parent Coalsa proximate analysis (wt %)

ultimate analysis (wt %)

coal sample

Mad

Aad

VMdaf

FCdaf

Cdaf

Hdaf

Ndaf

St,daf

Odaf

O/C ratio (%)

HL SG XT GZ

3.48 2.48 1.70 3.59

29.57 28.33 38.53 22.95

34.38 17.78 18.22 13.25

65.62 82.22 81.78 86.75

81.97 84.74 85.59 87.80

5.27 4.12 4.08 3.74

1.27 1.66 1.87 1.43

0.70 2.41 3.58 3.35

10.78 7.07 4.87 3.68

13.16 8.34 5.68 4.19

Mad and Aad refer to the moisture and ash content on an air-dried basis, respectively; VMdaf and FCdaf refer to the volatile and fixed carbon content on a dry ash-free basis, respectively.

a

Table 2. Ash Compositions of Parent Coals ash composition (wt %) coal sample

SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

SO3

HL SG XT GZ

53.01 49.95 57.45 55.89

20.27 26.68 25.87 17.05

6.96 11.48 7.57 20.43

6.98 4.81 2.98 1.87

4.67 0.97 1.97 2.18

1.54 0.64 1.71 0.67

1.12 0.69 0.91 0.59

5.45 4.78 1.54 1.32

Figure 1. Particle size distributions of parent coals. B


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Figure 2. N2 adsorption/desorption isotherms of SG/HL coal blends. calculate the SBET. Pore size distribution in the range 1.9−220 nm was determined according to the Barrett−Joyner−Halenda method. 2.2.2. Preparation of CWS and Properties Characterization. CWS was prepared by mixing pulverized coal, deionized water, and an additive (0.8 wt % based on the weight of air-dried coal powder). A rotational viscometer (NXC-4C, Chengdu Instrument Factory, China) was employed

to measure the CWS apparent viscosity. Details of the preparation and determination of CWS properties can be found in ref 40. MSL, which was defined as the solid content of CWS with a viscosity of 1 Pa s at a shear rate of 100 s−1, was adopted to evaluate the slurryability of CWS. A higher MSL value implies better slurryability of CWS.41,42 The static stability of CWS was appraised by water separation ratio (WSR), which C


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Figure 3. N2 adsorption/desorption isotherms of GZ/XT coal blends. Here, the following abbreviations apply: V is the volume of the gas adsorbate at equilibrium pressure P, V0 is the monolayer volume of gas, P is the gas equilibrium pressure, P0 is the saturation pressure of the gas, A is a power-law exponent, depending on D and the adsorption mechanism, and C is the constant of gas adsorption. The value of A can be calculated by plotting the gas adsorption isotherm data in terms of ln V versus ln[ln(P0/P)]. The slope of the straight line should be equal to A, and fractal dimension D can be calculated with D = 3 + A in this study.

was defined as the mass ratio of the supernatant to the total water in the test sample after storing still for 7 days.41 A lower value of WSR indicates better static stability of CWS. 2.2.3. Calculation of Fractal Dimension. The fractal dimension can be calculated with the formula based on the fractal Frenkel−Halsey− Hill (FHH) model:43,44

⎛V ⎞ ⎡ ⎛ P ⎞⎤ ln⎜ ⎟ = C + A⎢ln⎜ln 0 ⎟⎥ ⎣ ⎝ P ⎠⎦ ⎝ V0 ⎠

(1) D


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Energy & Fuels 2.2.4. Particle Size Distribution and Contact Angle of Coal. The particle size distribution of coal was measured with a laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd., U.K.). A cylinder (20 mm in diameter and 2 mm in thickness) of 1 g of coal powder was first formed using a 769YP-15A tablet press at 10 MPa for 30 min. Then, the contact angle of coal was measured with a contact angle analyzer (JC2000C, Shanghai Zhongchen Ltd., China). During measurement, a droplet of deionized water was dropped on the smooth end face of the prepared cylinder, and the droplet was photographed at the exact moment when it contacted the cylinder.45,46 The contact angle was then calculated using the built-in image processing software.

pore structures of coal are more complex and various; therefore, the above-mentioned pores may exist together in coal. In conclusion, the adsorption/desorption isotherms of coal blends, which were related to the isotherms of parent coals and the blending ratio, monotonously changed with the blending ratio of parent coal. Because the gas adsorption mechanisms in the two regions of P/P0 < 0.45 and P/P0 > 0.45 were different, the fractal dimensions of coal should be calculated in these two regions and defined as D1 and D2, respectively. According to refs 23, 35, and 39, D1 revealed the surface roughness of coal pore structures and could be used to describe the surface morphology of coal, while D2 reflected the volumetric roughness of coal pore structures and could be utilized to describe the pore structure morphology of coal. 3.2. Pore Structures of Coal Blends. The coal pores are classified according to the IUPAC recommendation:49 micropore with a pore size range 50 nm. Table 3 shows that the SBET and TPV of HL were the highest and were 6.3567 m2/g and 1.192 × 102 cm3/g, respectively, while

3. RESULTS AND DISCUSSION 3.1. N2 Gas Adsorption/Desorption Isotherms. According to Figure 2, there was an obvious difference between the adsorption/desorption isotherm of HL and that of SG. On the basis of the classification method proposed by the International Union of Pure and Applied Chemistry (IUPAC),47 the adsorption isotherm of HL showed a type IV(a) with H3 type hysteresis loop, while the adsorption isotherm of SG belonged to a type IV(b) with inconspicuous hysteresis loop. At a relative pressure range of 0 < P/P0 < 0.45, the adsorption curves of HL and SG slowly increased with increasing relative pressure, meaning that the mono- and multilayer gas adsorption was developed in the micro- and mesopores. Therefore, the gas adsorption was controlled by van der Waals forces. The adsorption/desorption isotherms of HL and SG were reversible at low relative pressure range, revealing the existence of semiopen micro- and mesopores (except ink-bottle pores). At a relative pressure range of 0.45 < P/P0 < 0.99, the increment rates of the adsorption curves sharply increased, and adsorption saturation did not appear until the pressure was near the saturated vapor pressure. This phenomenon indicated that a certain amount of meso- and macropores existed in HL and SG, and the gas molecules caused capillary condensation in mesopores, resulting in a significant increase in the amount of adsorbed gas. Therefore, the gas adsorption was dominated by capillary condensation. The H3 type hysteresis loop existing in HL indicated the existence of slit-like and/or parallel-plate meso- and macropores in coal,47,48 while the hysteresis loop of SG was inconspicuous, revealing the existence of semiopen meso- and macropores (except ink-bottle pores). Because the increment rate and gas adsorption capacity of HL were higher than those of SG, the pore structures of HL were more abundant, resulting in the higher SBET and TPV of HL. As for SG/HL coal blends, with increasing blending ratio of SG, the adsorption isotherm of the coal blend gradually transformed from IV(a) to IV(b), and its hysteresis loop and gas adsorption capacity became smaller, indicating the decrease in SBET and TPV. Figure 3 reveals that the adsorption isotherms of XT and GZ showed a type IV(a) with H3 type hysteresis loop. On the basis of the above-mentioned analysis, there were semiopen micro- and mesopores (except ink-bottle pores) existing in XT and GZ because of the existence of a reversible adsorption/desorption isotherm at low relative pressure range. At a relative pressure range of 0.45 < P/P0 < 0.99, the existence of H3 type hysteresis loop in XT and GZ revealed that slit-like and/or parallel-plate meso- and macropores existed in these coals. The pore structures of XT were more abundant than those of GZ, and therefore, XT had higher SBET and TPV. As for GZ/XT coal blends, with increasing blending ratio of GZ, the increment rate and gas adsorption capacity of coal blend were reduced, meaning there was a decrease in SBET and TPV. It should be noted that the real

Table 3. Pore Structure Parameters of SG/HL Coal Blends coal sample

SBET (m2/g)

APD (Å)

TPV (×102 cm3/g)

HL SG/HL = 2/8 SG/HL = 4/6 SG/HL = 5/5 SG/HL = 6/4 SG/HL = 8/2 SG

6.3567 ± 0.0341 5.7081 ± 0.0482 5.6177 ± 0.0290 5.1312 ± 0.0322 4.5708 ± 0.0312 3.9499 ± 0.0285 3.0306 ± 0.0136

74.9968 74.8763 72.3152 75.4876 79.0496 80.4767 94.1287

1.192 1.069 1.016 0.968 0.903 0.795 0.713

the APD of SG was the highest and was 94.1287 Å. According to Table 4, the SBET and TPV of XT were the highest and were Table 4. Pore Structure Parameters of GZ/XT Coal Blends coal sample

SBET (m2/g)

APD (Å)

TPV (×102 cm3/g)

XT GZ/XT = 2/8 GZ/XT = 4/6 GZ/XT = 5/5 GZ/XT = 6/4 GZ/XT = 8/2 GZ

5.4879 ± 0.0123 4.6086 ± 0.0211 4.1669 ± 0.0246 3.7635 ± 0.0254 3.3781 ± 0.0261 2.7926 ± 0.0228 1.0153 ± 0.0211

78.0647 74.9004 77.2172 80.3691 85.3629 88.9797 102.4807

1.071 0.863 0.804 0.756 0.729 0.620 0.260

5.4879 m2/g and 1.071 × 102 cm3/g, respectively. However, the APD of GZ was the highest and was 102.4807 Å. The results were consistent with the above-mentioned analysis shown in the previous section. As for SG/HL and GZ/XT coal blends, the SBET and TPV of coal blends monotonously decreased with increasing blending ratio of parent coal with smaller SBET and TPV. According to Figure 4a,c, the pore size distributions of HL, SG, and SG/HL coal blends were similar in shape, which were mainly distributed in mesopores. However, the pore structures of HL were more abundant than those of SG. Therefore, with the increase in the blending ratio of SG, the number of pore structures in SG/HL coal blends, especially the number of fine mesopores with an average pore size range 2−10 nm, was gradually reduced, resulting in the decrease in the SBET and TPV. Figure 4b,d shows that meso- and macropores with an average pore size range 10−220 nm mainly contributed to the pore volume of HL, SG, and SG/HL coal blends, whereas the pore E


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Figure 4. Pore size distributions: (a) pore volume vs APD, (b) incremental pore volume vs APD, (c) pore area vs APD, and (d) incremental pore area vs APD of SG/HL coal blends.

Figure 5. Pore size distributions: (a) pore volume vs APD, (b) incremental pore volume vs APD, (c) pore area vs APD, and (d) incremental pore area vs APD of GZ/XT coal blends.

area of these coals was mainly controlled by mesopores. The effects of fine mesopores with an average pore size range 2−10 nm on the pore area of these coals were comparable to those of mesopores with an average pore size range 10−50 nm.

Figure 5a,c reveals that mesopores with an average pore size range 2−40 nm were the dominant pores in XT and GZ/XT coal blends, whereas the pore structures of GZ were mainly distributed in mesopores with an average pore size range 6−50 nm. F


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Figure 6. Effects of different pore structures on the (a) pore area and (b) pore volume of SG/HL coal blends.

Figure 7. Effects of different pore structures on the (a) pore area and (b) pore volume of GZ/XT coal blends.

important effects on the pore volume of HL, SG, and their coal blends. With increasing the blending ratio of SG, the Vmeso/Vtotal of coal blends first increased and then decreased, while the Vmacro/Vtotal showed the opposite trend. Figure 7a reveals that the Smeso/Stotal of XT, GZ, and GZ/XT coal blends was over 79%, indicating that mesopores had dominant effects on the pore area of these coals. With an increase in the blending ratio of GZ, the Smeso/Stotal of GZ/XT coal blends monotonously decreased whereas the Smacro/Stotal of coal blends monotonously increased. Moreover, the Smeso/Stotal of XT was much higher than that of GZ, and therefore, the effect of mesopores on the pore area of XT was more remarkable than that on the pore area of GZ. According to Figure 7b, the pore volume of XT, GZ, and GZ/XT coal blends was mainly controlled by mesopores and macropores. However, the Vmacro/Vtotal of GZ was much higher than that of XT, indicating that the effect of macropores on the pore volume of GZ was more significant than that on the pore volume of XT. With an increase in the blending ratio of GZ, the Vmeso/Vtotal of coal blends monotonously decreased while the Vmacro/Vtotal showed the opposite trend.

In addition, the pore structures of XT were more abundant than those of GZ. Therefore, the number of pore structures in GZ/XT coal blends was reduced with increasing blending ratio of GZ, resulting in the decrease in the SBET and TPV. According to Figure 5b,d, the pore volume of XT, GZ, and their coal blends was mainly controlled by meso- and macropores with an average pore size range 10−220 nm. However, the pore area of XT and GZ/XT coal blends was mainly controlled by mesopores, whereas mesopores with an average pore size range 6−50 nm mainly contributed to the pore area of GZ. In comparison with GZ, the effects of fine mesopores with an average pore size range 2−10 nm on the pore area and pore volume of XT were more remarkable. The effects of different pore structures in coal on the pore area and pore volume were quantitatively analyzed. Figure 6a shows that the Smeso/Stotal of HL, SG, and SG/HL coal blends was over 89% and much higher than the Smicro/Stotal and Smacro/Stotal, indicating that mesopores had dominant effects on the pore area of these coals. With the increase in the blending ratio of SG, the Smeso/Stotal of SG/HL coal blends showed a decreasing trend. According to Figure 6b, both mesopores and macropores had G


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Energy & Fuels In conclusion, the pore structures of SG/HL and GZ/XT coal blends were related to the pore structures and blending ratio of parent coals. With an increase in the blending ratio of parent coal with smaller SBET and TPV, the SBET and TPV of coal blends monotonously decreased. Different pore structures in coal blends had differential effects on the pore area and pore volume. The pore area of SG/HL and GZ/XT coal blends was controlled by mesopores whereas mesopores and macropores had the dominant effects on the pore volume of these coal blends. 3.3. Relationship between Fractal Dimensions and Pore Structure Parameters. Some representative FHH plots of coals are shown in Figure 8. According to Tables 5 and 6, all the coal samples exhibited remarkable fractal characteristics in the entire relative pressure range, and the R2 values of D1 and D2 were both higher than 0.96, with the exception of GZ. The R2 value of D1 of GZ was 0.2544, indicating that the fractal characteristic of GZ at a low relative pressure range of 0 < P/P0 < 0.45

Table 5. Fractal Dimensions of SG/HL Coal Blends coal sample

D1

R2

D2

R2


2.5990 2.6446 2.6154 2.6160 2.6247 2.6517 2.5757

0.9852 0.9706 0.9741 0.9744 0.9752 0.9627 0.9709

2.6436 2.6331 2.6437 2.6304 2.6175 2.5961 2.5773

0.9926 0.9916 0.9916 0.9921 0.9928 0.9924 0.9931

was inconspicuous. However, the D1 value of GZ will be discussed to evaluate the effects of the fractal dimensions and blending ratio of parent coals on the fractal dimensions of coal blends. According to Table 5, the D1 values of SG/HL coal blends were higher than those of HL and SG, indicating that the surface morphology of these coal blends became more irregular.

Figure 8. Some representative FHH plots of ln V versus ln[ln(P0/P)] based on gas adsorption isotherms. H


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with an average pore size range 10−220 nm on the pore area, whereas D2 was mainly related to the effect of fine mesopores with an average pore size range 2−10 nm on the pore volume. However, in this paper, D1 of SG/HL coal blends exhibited no apparent linear correlation with the Smeso/macro(10−220 nm)/Stotal, indicating that not only the Smeso/macro(10−220 nm)/Stotal but also other factors had effects on D1. According to Table 1, SG was a high-rank coal with high ash and low volatile matter content. On one hand, the polycondensation and aromatization of organic macromolecules occurred in high-rank coals because of the overburden heat and pressure during the coalification process, making coal skeleton compact and reducing the pore diameter. As a result, the mesopores of high-rank coals were evolved into the predominant ultramicropores and micropores that could not be effectively detected by N2 gas adsorption, thus making coal porosity reduced.50,51 Hence, not only the number of fine mesopores with an average pore size range 2−10 nm within SG but also the effects of these fine mesopores on the pore area and pore volume were significantly reduced (see Figures 4 and 10d). On the other hand, as for high-rank coals with high ash and low volatile matter content, uneven bulk compaction might occur in coal structures and the strength of coal could not withstand the internal stress, thereby inducing the cracks in coal structures and increasing the coal porosity.52 Therefore, both the number of meso- and macropores with an average pore size range 10−220 nm within SG and the influence of these pores on the pore area and pore volume were increased (see Figures 4a and 9d). By contrast, HL was a bituminous coal with high volatile matter. The pore structures of HL were loosely reticular and mainly distributed in fine mesopores with an average pore size range 2−10 nm. Therefore, the Smeso/macro(10−220 nm)/Stotal of SG was higher than

Table 6. Fractal Dimensions of GZ/XT Coal Blends coal sample

D1

R2

D2

R2


2.5048 2.6175 2.6421 2.6464 2.6552 2.6585 2.9075

0.9985 0.9914 0.9846 0.9844 0.9788 0.9640 0.2544

2.6540 2.6474 2.6189 2.6106 2.5873 2.5656 2.3982

0.9985 0.9908 0.9933 0.9926 0.9932 0.9937 0.9785

Moreover, D2 of HL was higher than that of SG. With the increase in the blending ratio of SG, D1 of SG/HL coal blends showed a nonlinear change while D2 showed a decreasing trend. Table 6 shows that D1 of GZ was much higher than that of XT whereas XT had much higher D2. With increasing blending ratio of GZ, D1 of GZ/XT coal blends monotonously increased, while D2 monotonously decreased. The relationships between fractal dimensions of coal blends and the pore structure parameters were quantitatively analyzed. Figure 9 shows that D1 of SG/HL coal blends had no apparent linear correlation with the pore structure parameters. However, according to Figure 11, D1 of GZ/XT coal blends not only had a negative linear correlation with the SBET and TPV, but also exhibited a positive linear correlation with the APD and Smeso/macro(10−220 nm)/Stotal. Figures 10 and 12 reveal that D2 of SG/HL and GZ/XT coal blends had a positive linear correlation with the SBET, TPV, and Vmeso(2−10 nm)/Vtotal and exhibited a negative linear correlation with the APD. According to our previous research,38 the relationship between D1 and the Smeso/macro(10−220 nm)/Stotal, and D2 and the Vmeso(2−10 nm)/Vtotal, respectively, was qualitatively discussed. D1 was mainly affected by the influence of meso- and macropores

Figure 9. Plots of D1 versus (a) SBET, (b) APD, (c) TPV, and (d) Smeso/macro(10−220 nm)/Stotal of SG/HL coal blends. I


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Figure 10. Plots of D2 versus (a) SBET, (b) APD, (c) TPV, and (d) Vmeso(2−10 nm)/Vtotal of SG/HL coal blends.

Figure 11. Plots of D1 versus (a) SBET, (b) APD, (c) TPV, and (d) Smeso/macro(10−220 nm)/Stotal of GZ/XT coal blends.

higher and the majority of mineral matters within SG might exist under a crystalline phase, thereby making the surface morphology

that of HL. However, Table 2 reveals that the number of highvalent oxides (including SiO2, Al2O3, and Fe2O3) within SG was J


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Figure 12. Plots of D2 versus (a) SBET, (b) APD, (c) TPV, and (d) Vmeso(2−10 nm)/Vtotal of GZ/XT coal blends.

Vmeso(2−10 nm)/Vtotal, revealing that the volumetric morphology of pore structures in coal became rough and irregular. 3.4. Slurryability of Coal Blends. According to Table 7, as for HL, SG, and SG/HL coal blends, the MSL and θ of SG were

of SG smoother. By contrast, the number of low-valent oxides (including CaO, MgO, K2O, and Na2O) within HL was higher and the majority of mineral matters within HL might exist under amorphous state,53 thus making the surface morphology of HL rougher. Taking account of the comprehensive effects of two above-mentioned factors, D1 of SG was smaller than that of HL. Likewise, on one hand, the Smeso/macro(10−220 nm)/Stotal of GZ was much higher than that of XT. On the other hand, there was little difference between the number of high-valent oxides within GZ and that within HL. Therefore, D1 of GZ was much higher than that of XT. As for SG/HL and GZ/XT coal blends, the SBET, TPV, and APD of coal blends had a good linear correlation with the blending ratio. Because the Smeso/macro(10−220 nm)/Stotal of SG/HL coal blends exhibited a nonlinear change with increasing blending ratio of SG, D1 of SG/HL coal blends had no apparent linear correlation with the pore structure parameters. However, with an increasing blending ratio of GZ, the Smeso/macro(10−220 nm)/Stotal of GZ/XT coal blends monotonously increased, and therefore, D1 of GZ/XT coal blends had a linear correlation with the pore structure parameters. D2 was mainly related to the Vmeso(2−10 nm)/ Vtotal, and both the Vmeso(2−10 nm)/Vtotal of SG/HL coal blends and that of GZ/XT coal blends monotonously changed with the blending ratio; therefore, D2 of coal blends had a good linear correlation with the pore structure parameters. In conclusion, D1 was mainly related to the influence of mesoand macropores with an average pore size range 10−220 nm on the pore area and the mineral phase within coal. With the decrease in the Smeso/macro(10−220 nm)/Stotal and the increase in the number of crystalline mineral matters, D1 of coal was decreased, indicating that the surface morphology of pore structures in coal became smooth and regular, while D2 was mainly affected by the effect of fine mesopores with an average pore size range 2−10 nm on the pore volume. D2 of coal increased with increasing

Table 7. MSL, Contact Angle, and WSR of SG/HL Coal Blends coal sample

MSL (%)

θ (deg)

WSR (%)


61.91 ± 0.33 62.97 ± 0.27 64.46 ± 0.31 65.47 ± 0.23 66.31 ± 0.36 68.28 ± 0.29 69.42 ± 0.40

59.52 ± 0.42 62.75 ± 0.36 65.46 ± 0.44 67.33 ± 0.31 68.63 ± 0.40 70.84 ± 0.47 72.43 ± 0.34

5.29 ± 0.11 9.51 ± 0.20 9.88 ± 0.22 8.01 ± 0.17 5.86 ± 0.12 6.26 ± 0.13 8.47 ± 0.15

the highest and were 69.42% and 72.43°, respectively, while the MSL and θ of HL were the smallest and were 61.91% and 59.52°, respectively. However, the WSR of HL was 5.29% and lower than that of SG. Therefore, the slurryability of SG was better than that of HL, whereas the hydrophilicity and static stability of HL were better. With an increase in the blending ratio of SG, the MSL and θ of SG/HL coal blends monotonously increased while the WSR appeared to exhibit a nonlinear trend. Table 8 shows that, as for XT, GZ, and GZ/XT coal blends, the MSL and θ of XT were the highest and were 71.08% and 67.44°, respectively, while the MSL and θ of GZ were the smallest and were 66.39% and 61.75°, respectively. However, the WSR of GZ was lower than that of XT and was 4.74%. Therefore, the slurryability of XT was better than that of GZ while the hydrophilicity and static stability of GZ were better. With increasing blending ratio of GZ, the θ of GZ/XT coal blends monotonously decreased, while the MSL and WSR changed nonlinearly. K


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Energy & Fuels

Fe3+ within SG was lower, indicating that the mineral compositions of SG were beneficial to preparing CWS. Moreover, the SBET, TPV, and D2 of SG were smaller. As a result, the water absorption and storage capacity of SG coal particles were reduced, which was also good for improving the MSL of SG. Overall, the physicochemical properties of SG were beneficial to preparing CWS, and hence the MSL of SG was higher than that of HL. As for SG/HL coal blends, with an increased blending ratio of SG, the changes in coal properties, surface characteristics, mineral compositions, and pore characteristics of coal blends were good for preparing CWS. Therefore, the MSL of SG/HL coal blends monotonously increased. Likewise, in comparison with GZ, XT had lower moisture content and higher θ, revealing the weaker hydrophilicity of XT coal particles. In addition, the ash and indissoluble solid content of XT were much higher, while the amount of dissoluble mineral matters containing Ca2+, Mg2+, Fe2+, and Fe3+ within XT was much lower. Therefore, the ash content and mineral compositions of XT were also beneficial to improving its MSL. However, according to Figure 1, the particle size distribution of GZ exhibited a bimodal distribution, and fine particles might fill the space between the coarse particles, thereby increasing the maximum packing fraction. Moreover, the SBET, TPV, and D2 of GZ were much smaller, thus reducing the water absorption and storage capacity of GZ coal particles. With consideration of the comprehensive effects of above-mentioned factors, the MSL of XT was higher than that of GZ. As for GZ/XT coal blends, with increasing blending ratio of XT, the hydrophilicity of coal blends became weaker. The ash and indissoluble solid content of coal blends were increased, while the amount of dissoluble mineral matters containing Ca2+, Mg2+, Fe2+, and Fe3+ was decreased. Moreover, the fine XT coal particles could fill the space between the coarse GZ coal particles, thereby increasing the packing efficiency and reducing the viscosity of CWS. However, when the mass fraction of XT was high in the coal blends, the space between the coarse GZ coal particles was filled with some fine XT coal particles, while other remaining fine XT coal particles would flow in the free water, thus reducing the lubricating effects of free water and increasing the viscosity of CWS. In addition, the SBET, TPV, and D2 of coal blends increased with increasing blending ratio of XT. As a result, the water absorption and storage capacity of coal blends were also enhanced, which had a harmful impact on preparing CWS. With consideration of the comprehensive effects of the above-mentioned factors, the MSL of GZ/XT coal blends, which exhibited a nonlinear change with increasing blending ratio of XT, first increased and then decreased. The coal particles dispersed in CWS will settle and form agglomerates during the storage, which goes against the industrial utilization of CWS. Therefore, the stability is an important index to evaluate the quality of CWS, and the static stability can be used to appraise the storage property of CWS. The instability of CWS is mainly related to the agglomeration and sedimentation of coal particles in the suspension system.70 Many factors are responsible for the static stability of CWS, including coal properties, particle size distribution, mineral and maceral compositions, mass fraction and type of additives, the solid concentration of slurry, pH value, and temperature.71,72 The basic mechanism of stabilization of CWS is to decrease the coal−coal interaction or to promote coal−water interaction, which includes the following: (i) increasing the surface charge (electrostatic repulsion), such as adding additives; (ii) introducing groups that provide a mechanical barrier (steric repulsion), such as optimizing the coal particle size distribution; and (iii) increasing the

Table 8. MSL, Contact Angle, and WSR of GZ/XT Coal Blends coal sample

MSL (%)

θ (deg)

WSR (%)


71.08 ± 0.43 67.98 ± 0.35 68.30 ± 0.28 68.20 ± 0.26 68.40 ± 0.24 67.67 ± 0.31 66.39 ± 0.37

67.44 ± 0.37 66.23 ± 0.31 64.98 ± 0.25 63.77 ± 0.32 63.07 ± 0.28 62.29 ± 0.19 61.75 ± 0.22

5.28 ± 0.11 13.05 ± 0.24 10.22 ± 0.21 9.75 ± 0.15 3.89 ± 0.13 4.68 ± 0.10 4.74 ± 0.12

Actually, the slurryability of coal is comprehensively affected by many factors, including coal properties (e.g., inherent moisture, oxygen-containing functional groups, and ash), mineral and maceral compositions, surface characteristics (e.g., contact angle, zeta potential, and wetting properties), pore structures (e.g., SBET, TPV, and APD), particle size distribution, and mass fraction and type of additives.54−62 In general, with increasing coal rank or with decreasing moisture and oxygen-containing functional group content, the θ and isoelectric point of coal are increased, which is good for preparing CWS. Moreover, it is easier for coal particles with higher packing efficiency and smaller SBET, TPV, and D2 to prepare high-quality CWS. The ash content and mineral compositions of coal have complex effects on the slurryability, and different conclusions can be obtained from different research studies. Some research studies reveal that the CWS prepared from high ash coal has a poorer slurryability and rheological property due to the strong interaction of the hydrophilic ash with water forming gels.61,63 However, other research studies show that the increase in ash content has a positive effect on the slurryability and rheological behavior of coal. The reason is that the clay minerals contained in coal can easily form the cross-linked network structures in the slurry because of the strong interparticle electrical repulsive forces.54,64 According to the existing conclusions, the effects of ash content and mineral compositions on the slurryability of coal are as follows: On one hand, the specific gravity of indissoluble mineral matters (such as quartz, kaoline) contained in coal is usually higher than that of coal organic matter. Therefore, the volume of high ash coal is smaller than that of low ash coal at the same coal mass, resulting in a larger fraction of free water available in the slurry to increase the separation distance between the coal particles and the decrease in the viscosity of CWS.65−67 On the other hand, the metal cations extracted from the mineral matters are mainly Ca2+, Mg2+, Fe2+, and Fe3+. These dissolved cations will react with the additives in the slurry and make the additives deposited on the surface of coal particles, leading to the increase in the viscosity of CWS.65,68,69 In a word, the abovementioned factors influence each other and have comprehensive effects on the slurryability of coal. Therefore, it is difficult to determine the slurryability of coal with the change of a single factor. According to Tables 1 and 2, in comparison with HL, SG was a high-rank anthracite coal with low moisture and oxygencontaining functional group content. Therefore, the θ of SG was higher while its hydrophilicity was weaker. Therefore, the amount of free water absorbed by SG coal particles was decreased, and hence, a larger fraction of free water was available to increase the separation distance between the coal particles, thus reducing the viscosity of CWS prepared from SG. On the other hand, the indissoluble solid content of SG was higher while the amount of dissoluble solid containing Ca2+, Mg2+, Fe2+, and L


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GZ:XT blending ratio of 6:4. This might be because the interspaces between the coarse GZ coal particles were fully filled with the majority of fine XT coal particles, thereby minimizing the voidage among the coal particles and optimizing a stable medium in the CWS. Hence, both the slurryability and static stability of CWS prepared from GZ/XT coal blend were best at the GZ:XT blending ratio of 6:4. In conclusion, coal blending is a kind of simple and economical technology to improve the slurry quality of CWS. However, coal blends are “new” coal, and therefore, the slurry quality of CWS prepared from coal blends is comprehensively affected by the physicochemical properties of coal blends, including coal properties, surface characteristics, mineral compositions, particle size distribution, pore characteristics, and so on. Therefore, the MSL and WSR of CWS prepared from coal blends, which are mainly related to the physicochemical properties and blending ratio of parent coals, do not always change linearly with the blending ratio of parent coal.

steric wettability of the solid surface to form a three-dimensional cross-linked net structure.40,73,74 Mechanical resistance is more effective than Brownian motion, van der Waals’ forces, and electrostatic forces in inhibiting the free settlement of coal particles.75 As mentioned above, in comparison with SG, HL had better hydrophilicity and a higher amount of dissoluble cations (i.e., Ca2+, Mg2+, Fe2+, and Fe3+). Therefore, not only could the hydrophilic oxygen functional groups within HL easily associate with water molecules by hydrogen bonds but also the high-valent cations could built bridges among coal particles, resulting in the formation of cross-linked net structure in the slurry.54,73 The cross-linked net structure could act as an effective barrier around HL coal particles, thus preventing coal particles from uniting and settling and enhancing the static stability of CWS. In addition, the water absorption and storage capacity of HL coal particles were enhanced because of the higher SBET, TPV, and D2, which was beneficial to the formation of a cross-linked net structure. As a result, the static stability of CWS prepared from HL was better than that prepared from SG. As for SG/HL coal blends, when the mass fraction of HL was small in the coal blends, the stable cross-linked net structure was formed around HL coal particles in the slurry and could prevent some SG coal particles from uniting and settling. Therefore, the static stability of CWS prepared from SG/HL coal blends was better than that prepared from SG. However, when the mass fraction of SG was small in coal blends, the SG coal particles could easily aggregate into clusters by hydrophobic interaction and then settled in the slurry because of the gravity effect, thereby destroying the crosslinked net structure formed around HL coal particles. As a result, the static stability of CWS prepared from SG/HL coal blends was worse than that prepared from HL. It is interesting to find that the WSR of CWS prepared from SG/HL coal blend was lowest at the SG:HL blending ratio of 6:4. This might be because the coal particles formed a more stable cross-linked net structure in the CWS, thus inhibiting the free settlement of coal particles. Similarly, in comparison with XT, GZ had better hydrophilicity and a higher amount of dissoluble cations (i.e., Ca2+, Mg2+, Fe2+, and Fe3+), resulting in the formation of a cross-linked net structure around GZ coal particles. More importantly, the particle size distribution of GZ showed a bimodal distribution, and the fine particles could enter the gaps between the coarse particles. As a result, the friction forces between coal particles increased because of the decrease in space, and hence, a stable medium was formed in the CWS, especially when the solid loading density approached maximum. Therefore, the static stability of CWS prepared from GZ was better than that prepared from XT. As for GZ/XT coal blends, when the mass fraction of XT was small in the coal blends, the fine XT coal particles could fill the gaps between the coarse GZ coal particles, thus reducing interstices between the particles and forming a more stable medium in the CWS. As a result, the static stability of CWS prepared from GZ/XT coal blends was better than that prepared from GZ. However, when the mass fraction of GZ was small in GZ/XT coal blends, although some fine XT coal particles could fill the space between coarse GZ coal particles, other remaining XT coal particles could easily aggregate into clusters because of the hydrophobic interaction and higher SBET, thereby destroying the stable cross-linked net structure formed in the CWS. As a result, the static stability of CWS prepared from GZ/XT coal blends was worse than that prepared from XT. It is of interest to note that not only was the WSR of CWS prepared from GZ/XT coal blends lowest but also the MSL of CWS was highest at the

4. CONCLUSIONS With increasing blending ratio of parent coal with smaller SBET and TPV, the number of pore structures in coal blends decreased, and therefore, the SBET and TPV of SG/HL and GZ/XT coal blends monotonously decreased. D1 reflected the surface roughness of coal pore structures and was mainly related to the Smeso/macro(10−220 nm)/Stotal and mineral phase within coal. With decreasing Smeso/macro(10−220 nm)/Stotal and increasing content of crystalline mineral matters, D1 of coal decreased, while D2 reflected the volumetric roughness of coal pore structures and was closely affected by the Vmeso(2−10 nm)/Vtotal. D2 of coal increased with increasing Vmeso(2−10 nm)/Vtotal. D1 of SG/HL coal blends had no apparent linear correlation with the pore structure parameters, while D1 of GZ/XT coal blends had a negative linear correlation with the SBET and TPV and exhibited a positive linear correlation with the APD. Both D2 of SG/HL coal blends and that of GZ/XT coal blends had a positive linear correlation with the SBET and TPV and showed a negative linear correlation with the APD. The slurry quality of CWS prepared from coal blends is comprehensively affected by the physicochemical properties and blending ratio of parent coals. Therefore, the MSL and WSR of CWS prepared from coal blends do not always change linearly with the blending ratio of parent coal.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 571 87952884. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge the financial support provided by the National Basic Research Program of China (Grant 2010CB227001) and the National Key Research and Development Plan (Grant 2016YFB0600505). The authors also wish to thank Mr. Pengfei Liu for his invaluable help in polishing the manuscript.

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ABBREVIATION LIST CWS = coal water slurry SBET = BET surface area APD = average pore diameter TPV = total pore volume DOI: 10.1021/acs.energyfuels.6b01424 Energy Fuels XXXX, XXX, XXX−XXX

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MSL = maximum solid loading WSR = water separation ratio FHH = Frenkel−Halsey−Hill θ = contact angle Smicro/Stotal = ratio of micropore area to total pore area Smeso/Stotal = ratio of mesopore area to total pore area Smacro/Stotal = ratio of macropore area to total pore area Vmicro/Vtotal = ratio of micropore volume to total pore volume Vmeso/Vtotal = ratio of mesopore volume to total pore volume Vmacro/Vtotal = ratio of macropore volume to total pore volume Smeso/macro(10−220 nm)/Stotal = ratio of meso- and macropore (10−220 nm) area to total pore area Vmeso(2−10 nm)/Vtotal = ratio of fine mesopore (2−10 nm) volume to total pore volume IUPAC = International Union of Pure and Applied Chemistry

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Pore Characteristics and Slurryability of Coal Blends - American

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