Correlation between Drying Behaviors of Brown Coal and Its Pore

Jun 16, 2019 - Drying behaviors of brown coal are affected by internal water diffusion, which is controlled by pore structures. To diminish the effect...
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Article Cite This: Energy Fuels 2019, 33, 6027−6037

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Correlation between Drying Behaviors of Brown Coal and Its Pore Structures Guorui Feng,† Xiaohong Niu,† Junjie Liao,‡ Yanna Han,*,† Zongqing Bai,§ and Wen Li§ College of Mining Engineering and ‡State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China § State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China

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†

ABSTRACT: Drying behaviors of brown coal are affected by internal water diffusion, which is controlled by pore structures. To diminish the effect of functional groups on drying, drying behaviors and pore structures of only one kind of coal dried at different heating rates and atmospheres were investigated. The final drying temperature was kept constant. The relationships between pore structures, including specific surface area (SBET), volume of mesopores (Vmeso), volume of macropores (Vmacro), volume of total pores (Vtotal), and two fractal dimensions (DI and DII), and drying characteristics, including maximum drying rate (vmax), activation energy of drying surface water (E1), activation energy of drying pore water (E2), apparent diffusion coefficient surface water (Deff‑1), and apparent diffusion coefficient pore water (Deff‑2), were correlated. The relationship between basic pore parameters and fractal dimensions was examined. For coals dried in N2, the SBET was confirmed as one of the key factors influencing DI. The vmax increased with heating rates, resulting from the higher temperature at the same drying time and larger SBET and Vmacro. The E1 was higher at faster heating rates, because of the higher temperature gradient and the bigger DI. The E2 increased as heating rates increased from 3 to 20 °C/min and then decreased for flash drying (directly dried at 200 °C). The increase could be also due to the higher temperature gradient at higher heating rates. The decrease could be because of the effect of Vmacro on Deff‑2. For coals dried in different atmospheres, the higher E1 and E2 in air were due to oxidation reaction. The relatively large heat conductivity of N2 led to the lower E1. The small molecular diameter of CO2 led to the lower E2. There was no consistent relationship between SBET and DI and between pore structures and drying characteristics for coals dried in different atmospheres.

1. INTRODUCTION Brown coal resources in China are relatively abundant and have the potential of main energy and chemical feed stocks under the trend of tight supplies of high-quality coal.1−3 However, the high content of water (25−65 wt %) in brown coal restricts its large-scale application, and thus the upgrading is critical for the development of clean technologies of brown coal.3,4 In many brown coal drying processes, the samples are directly or indirectly dried by N2, air, or flue gas. During the process, the pores in coals shrink or collapse and the contents of oxygen containing functional groups decrease.5,6 Pore structures are essential to the transportation of gas (water vapor), so that pore variations during drying could influence drying parameters such as drying rate, energy consumption, and water transfer rate.2,7,8 Thus, in order to save drying energy consumption and enhance drying rate, the relationship between pore structures and drying characteristics was investigated in this study. Water in brown coals exists as different forms. Generally, it is classified into three forms: free water, water bound in pores (capillary water), and water hydrogen-bonded to surface functional groups (multilayer or monolayer water).2,9,10 The different forms of water have different desorption heats because of the variety of environments in which water occurs in brown coal, due to the large range of pore sizes and the presence of functional groups of varying affinity with water. It is reported that the contents of free, capillary, and molecular water for coal chars almost without oxygen functional groups © 2019 American Chemical Society

were mainly controlled by the volume of macropores and mesopores and surface area, respectively.4 Therefore, drying behaviors of different forms of water would be changed because of the variations of pore structures and chemical functional groups during drying. Fractal dimension has been used to describe the pore wall and pore space roughness of particles including coal chars and dewatered brown coals.11−14 Based on N2 adsorption data, two fractal dimensions of D1 and D2 at relative pressures of 0−0.5 and 0.5−0.95, respectively, were calculated by Tang et al.15 and Liu et al.8 using the Frenkel−Halsey−Hill (FHH) method. Tang et al.15 reported that the changes in D1 reflected the roughness of mesopore surface. The variation trend of D2 indicated the volumetric roughness of micropores. Liu et al.8 reported that the changes in D1 were similar to those in average pore diameter development, whereas the changes in D2 were consistent with those in specific surface area development. Zhu et al.16 also based on the N2 adsorption data calculated the values of D1 and D2 at relative pressures of 0−0.45 and 0.45−1, respectively. They found that the D1 can be utilized to quantitatively describe the surface roughness of these mesopores and macropores in coal and D2 can be utilized to quantitatively describe the volumetric roughness of 2−10 nm mesopores in coal. It is found that the relationship between Received: March 5, 2019 Revised: May 16, 2019 Published: June 16, 2019 6027

DOI: 10.1021/acs.energyfuels.9b00657 Energy Fuels 2019, 33, 6027−6037

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Energy & Fuels Table 1. Proximate and Ultimate Analyses of YNa proximate analysis (wt %)

ultimate analysis (wt %, daf)

A (ad)

M (ar)

V (ad)

C

H

N

O

St

7.88 ± 0.04

24.80 ± 0.15

37.55 ± 1.23

59.22 ± 2.03

3.42 ± 0.06

1.02 ± 0.03

35.94 ± 1.01

0.40 ± 0.01

a

ar: as received basis; ad: air-dried basis; daf: dry ash-free basis; St: total sulfur content.

pore structures and fractal dimension for different coals showed an obvious discrepancy. Thus, in this study, the relationship between pore structures and fractal dimensions needs to be further investigated. The effect of irregular matrix morphology on gas diffusion during coal char combustion has been analyzed using fractal dimension in our previous study.17 It was found that the more irregular and rough the char surface, the more difficult it was for adsorbate gas to transfer into the char. The pore surface roughness showed apparent influence on diffusion activation energy of char combustion. In this study, detailed investigations on the relationship between pore structures and drying behaviors of brown coal, particularly, the effect of fractal dimension on the drying kinetic of different forms of water were pursued. In order to study the effect of pore structures on the drying behaviors of brown coal, several brown coals dried under different heating rates and drying atmospheres were used. To keep the content of functional groups that might impact drying behaviors of molecular water constant, the samples were derived from the same parent coal and the final drying temperature was set as 200 °C.

the brown coal was heated from room temperature to 200 °C at heating rates of 3, 5, 10, and 20 °C/min. Also, a flash dried experiment was performed, in which the furnace temperature was raised to 200 °C before drying, and then brown coal was directly dried under 200 °C. The CO2 and air atmospheres were used in order to compare with the drying characteristics of brown coal dried in N2. The brown coal was heated from room temperature to 200 °C at a heating rate of 3 °C/min for brown coals dried at different atmospheres. The nomenclature of dried brown coals and their corresponding drying conditions was shown in Table 2.

2. EXPERIMENTAL SECTION

a

Table 2. Nomenclature of Dried Brown Coals and Their Corresponding Drying Conditionsa

2.1. Fix-Bed Drying Experiments. Chinese Yunnan brown coal (YN) was selected due to the abundant reserves of brown coal in Yunnan province. Before use, it was crushed and sieved to a particle size under 100 mesh. Its proximate and ultimate analyses data and total sulfur content are shown in Table 1. A self-made experimental setup was designed for the drying experiment, as shown in Figure 1. The setup was mainly composed of

nomenclature

atmosphere

heating rate (°C/min)

temperature (°C)

N3 N5 N10 N20 N200 A3 C3

N2 N2 N2 N2 N2 air CO2

3 5 10 20 3 3

25−200 25−200 25−200 25−200 25−200 25−200 25−200

-: too fast to be measured.

2.2. Pore Structures Analysis of Samples. Pore structures of raw brown coal and dried coals were measured by an N2 adsorption analyzer (ASAP2020, Micromeritics Instruments, Norcross, United States). The temperature was controlled at −196 °C and the vacuum was 4 × 10−7 Pa. The raw brown coal was dried under vacuum at 30 °C for 24 h before starting its pore analysis. During the pore structure analysis, all samples in the study were degassed at 105 °C for 12−15 h under vacuum. The pores diameter from 0.8 to 300 nm was measured.18 The specific surface area was calculated according to the Brunauer−Emmett−Teller (BET) equation. These experiments were duplicated at least twice. 2.3. Fractal Dimensions of Samples. Based on the N2 adsorption curves, the fractal dimensions of pores in dried brown coals were calculated by the fractal FHH method. The method has been successfully used to study the fractal characters of pores in dried brown coal (Song et al.).12,19 The fractal dimension was calculated by eq 1 É ÄÅ ij V yz ÅÅ i P yÑÑÑ lnjjj zzz = C + A′ÅÅÅlnjjjln 0 zzzÑÑÑ ÅÅÅ k P {ÑÑÑ j V0 z (1) Ö Ç k { where V is the amount of N2 adsorbate at pressure P, m3/g; V0 is the amount of N2 adsorbate on the monolayer, m3/g; P0 is the saturation pressure of N2, Pa; A′ is a power-law exponent based on fractal dimension (D) and the adsorption mechanism; and C is a constant number. The value of A′ can be computed by plotting the N2 isotherm adsorption data in terms of lnV versus ln(ln(P0/P)). A′ is the slope of the straight line. The fractal dimension can be calculated based on eq 2.

Figure 1. Schematic diagram of the self-made brown coal drying apparatus. a drying reactor, an electrically heated furnace, and an electric balance. For each run, 2.0 g of brown coal was used. The reactor was purged with drying gas at 200 mL/min for 15 min before drying, to ensure a constant drying atmosphere so as to obtain precise and steady data from the electronic balance. When the sample temperature arrived at 200 °C, the furnace was removed immediately. Afterward, the reactor was cooled down to room temperature under the gas flow of 200 mL/ min. To study the influence of heating rates on drying characteristics,

D = A′ + 3

(2)

2.4. Kinetic Modeling. The calculation of non-isothermal kinetic analysis consisted with the studies of Jin et al.20 and Ertekin and Yaldiz.21 A number of mechanisms with different f(α) and g(α) values (Table 3) were trialed to find which gave the best fit to the 6028

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Energy & Fuels Table 3. Some Kinetic Models Used in This Study22−24 model

f(α)

g(α)

Coats−Redfern Mampel unimolecular law two-dimensional growth three-dimensional growth Prout−Tompkins branching nuclei Valensi equation Jander equation two-dimensional three-dimensional

(1 − α)n 1−α 2(1− α)[(−ln(1 − α))]1/2 3(1 − α)((−ln(1 − α))2/3 α(1 − α) [−ln(1 − α)]−1 3(1 − α)1/3/2[(1 − α)−1/3 − 1] 2(1 − α)1/2 3(1 − α)2/3

[1 − (1 − α)1−n]/(1 − n) n = 0.2, 0.4, 0.6, 0.8, 1.2, 1.4, 1.6, 1.8, 2.0 −ln(1 − α) [−ln(1 − α)]1/2 [−ln(1 − α)]1/3 ln(α(1 − α)) (1 − α) ln(1 − α) + α [1 − (1 − α)1/3]2 1 − (1 − α)1/2 1 − (1 − α)1/3

experimental data. In these functions, α is the ratio of water weight loss (0 < α < 1) at drying time t, as shown in eq 3. w −w α= 0 w0 (3) where w0 is initial water content, (g/100 g dry basis); w is water content at any drying time, (g/100 g dry basis). The value of apparent activation energy E and the pre-exponential factor A is calculated by eq 4. ÄÅ É ÅÅ AR 1 − 2RT ÑÑÑ ÄÅ É ÅÅ ÑÑ ÅÅ g (α) ÑÑÑ E ÑÑÑ − E × 1 lnÅÅÅÅ 2 ÑÑÑÑ = lnÅÅÅÅ ÑÑ R ÅÅ ÅÅÇ T ÑÑÖ E T β ÅÅÇ ÑÑÑÖ (4)

(

)

where T is drying temperature (K), β is drying rate (K/min), and R is universal gas constant (kJ/mol·K). As 2RT/E ≪ 1, the ln(g(α)/T2) varies approximately linearly with 1/T. Thus, the value of E and A can be calculated according to the slope and intercept of the linear plot of ln(g(α)/T2) versus 1/T. Several kinetics equations represented to different mechanism functions of f(α) & g(α) were used, as shown in Table 3. The apparent diffusion coefficient was calculated by Fick’s second law. The details of the calculation process were presented by our previous study.22,25 ij π 2D yz i8y ln(α) = lnjjj 2 zzz − t jjjj 2eff zzzz kπ { k L { 2 ji L zy Deff = − m′jjj 2 zzz jπ z k {

(5)

(6)

where Deff is the effective diffusion coefficient (m2/s); L, which represents the diameter of coal particle, was taken as 8.6 × 10−4 m. The value was estimated by that the percentage of coal particle size smaller than 0.75 mm was approximately 85 wt %. The percentage of coal particle size smaller than 0.75 mm was analyzed by a 200-mesh sieve. m′ is the slope of the linear regression between ln(α) and drying time t (s). Deff can be calculated from m′ and L.

3. RESULTS AND DISCUSSION 3.1. Drying Characteristics of brown Coal. The effect of heating rates on the drying characteristics of brown coal is shown in Figure 2. As can be seen from Figure 2a, at the same drying time, the weight loss of samples dried at higher heating rates was higher. This was because the drying temperatures of brown coals were higher for the higher heating rates at the same drying time, so that the evaporation rate increased and more water was removed. As can be seen from Figure 2b, the maximum drying rate (vmax) increased as heating rates increased. The values of vmax increased from 5.14 to 34.51%/ min as the drying rates increased from 3 °C/min to flash drying. The results indicated that the vmax was significantly affected by heating rates. As can be seen from Figure 2c, as the water content decreased, the drying rate as a function of water

Figure 2. Weight loss (a) and drying rate (b) vs drying time and drying rate vs water content (c) for brown coal dried at different heating rates.

content initially increased, then was constant, and finally decreased. The critical water content (Xcr) is defined as the water content at the point when the drying rate begin to fall,4,10 which can be obtained from the arrow sign in Figure 2c. The content of critical water increased with heating rates from 3 to 20 °C/min (11.97 wt % for 3 °C/min, 16.72 wt % for 20 6029

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Energy & Fuels °C/min) and then decreased under flash drying conditions (13.41 wt %). In order to investigate the effect of atmospheres on the drying characteristics of brown coal, three different atmospheres of air, CO2, and N2 were employed at 200 mL/min gas flow rate, 3 °C/min drying rate, and 200 °C final drying temperature. Results are shown in Figure 3. As can be seen

and N2 0.364 nm) and that of N2 (0.364 nm),29 so that the CO2 could more easily diffuse to the pores in brown coal. This resulted in more water being removed from pores at the same drying temperature. After drying times longer than 35 min, the weight loss of samples dried at air was higher than that of samples dried in N2. The results could be due to the decomposition of hydroperoxides formed in the initial drying stage. As can be seen from Figure 3b, the vmax of samples dried in air was slightly higher than that of samples dried in CO2; the value for samples dried in N2 was the smallest. The intermediate vmax in CO2 could be due to the combination of the smaller heat conductivity of CO2 (1.66 × 102 W/m·K at 300 K, 2.43 × 102 W/m·K at 400 K and 3.25 × 102 W/m·K at 500 K) than that of air (2.62 × 102 W/m·K at 300 K, 3.38 × 102 W/m·K at 400 K and 4.07 × 102 W/m·K at 500 K) and that of N2 (2.59 × 102 W/m·K at 300 K, 3.27 × 102 W/m·K at 400 K and 3.89 × 102 W/m·K at 500 K)30 at the same temperature, which would tend to lower the temperature of the sample and the drying rate relative to that in N2 and air, balanced against an increase in the rate of removal of water from the pores due to the smaller molecular diameter of CO2, which would tend to increase the drying rate relative to that in N2 and air. The slight higher vmax in air could be due to its relative bigger heat conductivity, which would tend to higher the temperature of the coal. Also, the formation of hydroperoxides at the initial drying time and its decomposition after drying time longer than 35 min could be another reason of the higher vmax in air.27,31 As can be seen from Figure 3c, the content of critical water of coals dried in air was bigger than that of samples dried in N2. The value for coal dried in CO2 was the smallest one. The values of Xcr and vmax of brown coals dried at different conditions were summarized in Table 4. For brown coals dried Table 4. Drying Parameters of Dried Brown Coals sample

Xcr (wt %)

vmax (%/min)

N3 N5 N10 N20 N200 A3 C3

11.97 13.22 14.48 16.72 13.41 12.78 9.45

5.11 8.40 13.94 27.74 34.38 5.66 5.39

in N2, the values of vmax apparently increased as heating rates increased. The results suggested that the maximum drying rate was highly affected by heating rates, and the drying efficiency was enhanced as heating rates increased. For samples dried at the same heating rate but different atmospheres, the vmax of coals dried in air was slightly higher than that of coals dried in CO2 and that of coals dried in N2 was the smallest. The reason has been discussed in above paragraphs. As for the variation of Xcr, the values obviously increased with heating rates increasing from 3 to 20 °C/min in N2 and then decreased at flash drying condition. Generally, the water removed when the drying rate was increasing and when it was constant is only loosely bound to the coal, and its properties are similar to those of bulk liquid water, whereas water removed in drying rate decreasing stage is the more strongly bound water (capillary water and molecular water).4,10,22 So that, the changes of Xcr in N2 as a function of heating rates

Figure 3. Weight loss (a) and drying rate (b) vs drying time and drying rate vs water content (c) for brown coal dried at different drying atmospheres.

from Figure 3a, at the initial 35 min, the weight loss of samples dried in air was smaller than that of samples dried in N2 and the weight loss of samples dried in CO2 was the highest one. The smaller weight loss of samples dried in air could be due to the oxygen in the air being absorbed, particularly on methylene groups adjacent to aromatic rings, and then hydroperoxides being formed, which are converted into the usual oxygen functional groups and are thus incorporated into the coal.26,27 The highest weight loss of samples dried in CO2 could be because the molecular diameter of CO2 (0.33 nm)28 was smaller than the average diameter of air molecules (the molecular diameter of major components of air: O2 0.346 nm 6030

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Figure 4. Nitrogen adsorption/desorption isotherms of raw coal and dried coals obtained at different heating rates.

increased as heating rates increased, implying that the collapse of pore structures decreased with heating rates increasing. As can be seen from Figure 5, the total amount of nitrogen adsorption for coals dried in air and N2 was significantly smaller than that of the raw coal, whereas that of coals dried in CO2 was only slightly lower than that of raw coal. The results indicated that the larger pores of the coal tended to collapse after drying in air and N2, whereas more of the larger pores remained after drying in CO2. The results could be due to the swelling effect of CO2 molecules on pores.34,35 All hysteresis loops (Figures 4 and 5) are H3-type of the IUPAC classification, indicating slit-shaped pores.36 As can be seen from Figure 4, the hysteresis loops of dried coals increased as heating rate increased from 3 to 10 °C/min and then decreased at heating rate at 20 °C/min and flash drying. As can be seen from Figure 5, the significant smaller hysteresis loops of coals dried in N2 and air than that of raw coal indicated lower porosity. The hysteresis loops for C3 were wide in the P/P0 range of 0.5−1.0, indicating that the number of meso- or macropores that inhibited the desorption of N2 molecules increased. The results suggested that a small number of closed and semiclosed micro- or mesopores of raw brown coal could be opened during drying in CO2. Table 5 summarizes the volume of mesopores (Vmeso), macropores (Vmacro), and total pores (Vtotal) and specific

indicated that the amount of water with properties similar to those of bulk water removed decreased as heating rates increased from 3 to 20 °C/min and then increased under flash drying. The results suggested that the drying energy consumption initially increased as heating rates increased and then decreased. Tahmasebi et al.27 found that the critical moisture content depends mainly on drying temperature. In our study, the higher heating rate means the higher drying temperature at the same drying time. As for the decrease of Xcr at flash drying, the reason could be because of the pore structure changes of N200. The Xcr variation of samples dried at different atmospheres indicated that the amount of water removed in the form of free water in CO2 was higher than that in N2, and the amount for air atmospheres was the smallest. The results could be due to the more amount of oxygen functional groups for samples dried in air, which leads to more amount of water hydrogen-bonded to the brown coal surface. The smaller molecule diameter of CO2 could result in the free water in smaller pores to be quickly removed. 3.2. Pore Structures of Raw Coal and Dried Coals. The N2 adsorption/desorption isotherms of raw coal and brown coals dried under N2 at different heating rates are shown in Figure 4 and those of raw coal and brown coals dried under N2, air, and CO2 at a heating rate of 3 °C/min are shown in Figure 5. The adsorption isotherms of all samples were similar

Table 5. Pore Parameters of YN and Dried Coala pore volume×10−3 (cm3/g) sample

SBET (m2/g)

Vmeso

Vmacro

Vtotal

YN N3 N5 N10 N20 N200 A3 C3

1.57 1.24 1.28 1.33 1.36 1.37 1.21 1.56

3.51 3.34 4.07 3.42 3.45 3.13 2.92 3.14

11.35 11.84 12.02 15.63 15.69 22.64 9.85 16.15

15.13 15.37 16.11 19.12 20.05 26.54 12.87 19.47

Note: error of SBET is ±0.0006 m2/g; error of pore volume is ±0.0002 cm3/g. a

Figure 5. Nitrogen adsorption/desorption isotherms of raw coal and dried coals obtained in different drying atmospheres.

surface area (SBET) of dried coal. For coals dried in N2, the values of Vmacro were somehow bigger than that of raw coal. The reason could be that the mesopores of raw coal were gradually enlarged into macropores during drying, resulting in the higher value of Vtotal. The values of Vmacro, SBET, and Vtotal roughly increased as heating rates increased. The results suggested that the faster heating rates were of benefit to

and exhibited a reverse S shape, which belonged to type II according to IUPAC (International Union of Pure and Applied Chemistry) classification.32,33 The total amount of nitrogen adsorption for raw coal was higher than that of the dried coals, indicating that the pores collapsed during drying. As can be seen from Figure 4, the total amount of nitrogen adsorption 6031

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Figure 6. lnV vs ln(ln(P0/P)) by nitrogen adsorption of raw coal and dried coals.

porosity of dried coals. The obvious big values of SBET, Vmacro, and Vtotal could be the reason of the low value of Xcr of N200, which results in a larger overall drying rate, so that the drying energy consumption decreased at N200. As for coals dried in N2, air, and CO2, the values of SBET, Vmeso, Vmacro, and Vtotal for coals dried in air were significantly smaller than those of samples dried in CO2. The values of SBET and Vmacro for samples dried in N2 were intermediate. The relative abundant pore structures of coals dried in CO2 could be because of the swelling effect of CO2 molecules on pores31,32 and the smaller diameter of CO2 molecules. The amount of pores or micropores penetrated by CO2 could be increased because of the smaller diameter of CO2. Also, the swelling effect of CO2 could lead to the volume of these pores that were swelled. The

low temperature oxidation because of the reaction between air and surface oxygen-containing functional groups of coal could lead to the relative severer pore collapse.37,38 The fractal dimension of raw coal and dried coals was calculated by eqs 1 and 2. The ln V versus ln(ln(P0/P)) based on N2 adsorption of all samples is shown in Figure 6. All fitting results were divided into two distinct linear segments, including regions at 0 < P/P0 < 0.5 (region I) and 0.5 < P/ P0 < 1 (region II). The reason of the separate fits is that P/P0 = 0.5 corresponds to the transition from monolayer to multilayer adsorption. At a low relative pressure (region I), monolayer adsorption was observed because nitrogen molecules were orderly adsorbed and arranged on the surface of char samples. At a higher relative pressure (region II), monolayer adsorption 6032

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surface area could be one of key factors on surface roughness. Similar findings have been reported by Song et al.12 and Liu et al.8 and also by our previous studies.17 In this study, the relationship between SBET and DI was further confirmed. But for coals dried at different atmospheres and the same heating rate (N3, A3, and C3), the changes of SBET and DI did not show a clear relationship. This could be due to that the pore changes of brown coals dried under different atmospheres are significantly different. As for coals dried under air, the low temperature oxidation because of the reaction between air and surface oxygen-containing functional groups could lead to the relative severer pore collapse. But for coals dried under CO2, the relative abundant pore structures could be as results of the swelling effect of CO2 molecules on pores. The changes of coals dried under N2 were intermediate.27,31,37,38 3.3. Drying Kinetics of brown Coals Dried at Different Conditions. The drying activation energies and the apparent diffusion coefficient of water removed during drying rate increasing and short constant stages and decreasing stage were calculated separately. Based on the activation energy values and R2, models with g(α) = −(ln(1 − α))1/2 were optimum for the increasing-rate and constant-rate stages and models with g(α) = −ln(1 − α) were optimum for the decreasing-rate stage. We referenced the activation energy values of brown coal drying in other studies.22,25 If the R2 was very high, but the activation energy was too big or small compared with that in references, then the model would not be used. The acceptable range of activation energies could be 15−45 kJ/mol. The fitting results of ln((−ln(1 − α))1/2/T2) versus 1/T for drying rate increasing and short constant stage are shown in Figure 8. The fitting results of ln(−ln(1 − α)/T2) versus 1/T for drying rate decreasing stage are shown in Figure 9.

changed into multilayer adsorption, where gas molecules gradually filled into the pores of char samples. The good linear relationship between ln V and ln(ln(P0/P)) was found at the two segments. The results demonstrated that the FHH method could effectively study the fractal characters of pores in dried brown coal. According to our previous reports, the DI values which were calculated from region I could reflect the surface roughness of coals, resulting from the N2 molecules that were adsorbed on the surface at low relative pressure. The DII values, which were computed from region II, could represent the pore space roughness of coals, as N2 molecules mainly filled into pores at high relative pressure.15 The values of DI and DII are shown in Table 6. Table 6. Fractal Dimensions of Raw Coal and Dried Coals sample YN N3 N5 N10 N20 N200 A3 C3

AI′ −0.47 −0.64 −0.64 −0.63 −0.63 −0.62 −0.40 −0.62

± ± ± ± ± ± ± ±

AII′ 0.01 0.01 0.02 0.02 0.01 0.01 0.03 0.01

−0.57 −0.59 −0.58 −0.62 −0.59 −0.67 −0.58 −0.63

± ± ± ± ± ± ± ±

DI = AI′ + 3 0.01 0.01 0.02 0.02 0.01 0.02 0.03 0.01

2.53 2.36 2.36 2.37 2.37 2.38 2.60 2.38

± ± ± ± ± ± ± ±

0.01 0.01 0.02 0.02 0.01 0.01 0.03 0.01

DII = AII′ + 3 2.43 2.41 2.42 2.38 2.41 2.33 2.42 2.37

± ± ± ± ± ± ± ±

0.01 0.01 0.02 0.02 0.01 0.02 0.03 0.01

As can be seen from Table 6, the DI values of coals dried in N2 and CO2 were smaller than that of raw coals. But for coals dried in air, the value was bigger than that of raw coal. The results suggested that the surface roughness increased after drying in air. For coals dried in N2, the value did not show obvious changes. This result indicated that the effect of drying atmospheres on the surface roughness of dried coals was bigger than that of the heating rates. As for the DII, the values of all dried coals were smaller than that of the raw coal, indicating that pore structures of coals after drying became regularly. The DII values of coals dried in CO2 was the smallest in comparison with those of coals dried in air and N2, indicating that the roughness of pores decreased. This could be due to that more amount of pores or micropores was penetrated by CO2 molecules because of the smaller diameter of CO2, resulting that the number of pores or micropores swelled by CO2 increased. In order to investigate the relationship between pore structures and fractal dimensions, the results obtained from Tables 5 and 6 were compared. For coals dried under N2 and raw coal, the changes trend of S and values of DI were very similar, as shown in Figure 7. The results indicated that the

Figure 8. Fitting results of ln((−ln(1 − α))1/2/T2) vs 1/T of increasing and short constant drying-rate stages for samples dried at different heating rates (a) and different drying atmospheres (b).

Figure 7. Relationship between S and DI for coals dried in N2. 6033

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faster as heating rates increased. The results could be due to that the faster heating rates resulted in higher temperatures at the same drying time, so that the concentration of the water vapor on the surface of brown coal increased, which enhanced the diffusion coefficient of surface water.22,25 Also, the increase of SBET could result in the faster evaporation rate of water,40 which could enhance the diffusion coefficient of surface water. As can be seen from Table 8, the value of E2 for coals dried in N2 initially increased with heating rates increasing from 3 to 20 °C/min and then decreased for flash drying. The decreasing drying stage mainly corresponds to the transportation of water in the pores to the surface of the particle and then its evaporation.4,10 Thus, the variation of E2 values indicated that the energy consumption for removing water in pores increased as heating rates increased from 3 to 20 °C/min and then decreased for flash drying. The increase of E2 at the range of 3−20 °C/min could also be due to the higher temperature gradient at higher heating rates. Whereas, the decrease at N200 would be due to the bigger Vmacro and Vtotal of N200 (Table 5), which could lead to the easier removal of pore water, balanced against the effect of the higher temperature gradient on the drying temperature and the drying rate of the coal. Deff‑2 increased with increasing heating rate. On the one hand, the relative higher drying temperature increased the mass evaporation rate of pore water. On the other hand, the increase of surface area and macropore volume (Table 5) could result in the easier diffusion of pore water to coal surface. Comparing different atmospheres, the E1 values of coals dried in air were bigger than those of coals dried in CO2, and the values for coals dried in N2 were the smallest. The results indicated that the energy consumption for drying surface water in N2 was the smallest. The higher E1 of coals dried in air could be due to the oxygen reaction between air and functional groups on the coal surface.27,31,38 The reaction resulted in a small weight loss at increasing and short constant drying-rate stages. The lower E1 of coals dried in N2 compared to CO2 could be the result of the higher heat conductivity of N2. The E2 values of coals dried in air were bigger than those of coals dried in N2, and the values for coals dried in CO2 were the smallest. The small E2 values of coals dried in CO2 can be attributed to its smaller molecular diameter, which increased the diffusion rate of pore water to surface.37,38 The higher E2 values of coals dried in air could be because of the relative abundant oxygen functional groups resulting from the reaction between oxygen and the coal surface, so that more water was strongly bound to the coal surface, for example, by hydrogen bonding and so required more energy to remove.2,27,31 3.4. Correlation between Drying Characteristics and Pore Structures of Dried brown Coal. Brown coal drying

Figure 9. Fitting results of ln(−ln(1 − α)/T2) vs 1/T of drying rate decreasing stage for samples dried at different heating rates (a) and different drying atmospheres (b).

The kinetic parameters of water removed during drying rate increasing and short constant stages are summarized in Table 7 and those of water removed during drying rate decreasing stage are summarized in Table 8. The activation energy and apparent diffusion coefficient corresponding to the increasing and short constant stages were denoted as E1 and Deff‑1, respectively, and those corresponding to drying rate decreasing stage were denoted as E2 and Deff‑2 respectively. As can be seen from Table 7, the values of E1 and Deff‑1 for coals dried in N2 apparently increased with heating rates increasing. The changes of E1 indicated that the energy consumption in drying surface water was higher at faster heating rates. The increasing energy consumption for removal of surface water can be attributed to the higher temperature gradient at higher heating rates. The higher temperature gradient would tend to lower the drying temperature and drying rate of samples at the same drying temperature. The other reason could be the increase of surface roughness (Table 6), which could add more difficulty for the transportation of water vapor.39 The variations of Deff‑1 suggested that the transportation rate of surface water became

Table 7. Fitting Parameters of Kinetic Analysis at Drying Rate Increasing and Short Constant Stagea activation energy sample N3 N5 N10 N20 N200 A3 C3

slope −2362.94 −2482.65 −2847.21 −3472.14 −3791.51 −3664.89 −3262.38

± ± ± ± ± ± ±

intercept 61.34 117.87 57.73 129.90 114.26 60.14 60.14

−5.81 −6.21 −5.81 −3.77 −2.26 −1.95 −3.62

apparent diffusion coefficient 2

E1 (kJ/mol)

ln(A)

R

± ± ± ± ± ± ±

3.06 3.22 4.45 5.48 7.08 7.36 5.57

0.99 0.99 0.99 0.99 0.97 0.99 0.99

19.65 20.64 23.67 28.87 31.52 30.47 27.12

0.51 0.98 0.48 1.08 0.95 0.50 0.50

m × 10−6

Deff‑1 × 10−11 (m2/s)

−6.97 −6.96 −11.70 −25.20 −61.30 −5.89 −9.11

5.30 5.29 8.90 19.17 46.63 4.48 6.93

Note: maximum error of Deff‑1 is ±0.01×10−11 m2/s.

a

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DOI: 10.1021/acs.energyfuels.9b00657 Energy Fuels 2019, 33, 6027−6037

Article

Energy & Fuels Table 8. Fitting Parameters of Kinetic Analysis at Drying Rate Decreasing Stagea activation energy sample N3 N5 N10 N20 N200 A3 C3

slope −4139.56 −4457.10 −4934.98 −5521.43 −3767.33 −5306.78 −3689.95

± ± ± ± ± ± ±

intercept −1.27 −1.46 −1.33 −0.14 −1.94 1.58 −2.23

85.39 128.69 67.35 147.94 66.15 125.08 61.34

apparent diffusion coefficient

E2 (kJ/mol)

ln(A)

R2

m × 10−5

Deff‑2 × 10−10 (m2/s)

± ± ± ± ± ± ±

8.16 8.04 8.27 9.58 7.40 11.26 7.08

0.99 0.99 0.99 0.99 0.99 0.99 0.97

−2.78 −5.18 −5.96 −9.63 −14.01 −3.60 −3.09

2.11 3.94 4.53 7.33 10.71 2.74 2.35

34.42 37.06 41.03 45.91 31.32 44.12 30.68

0.71 1.07 0.56 1.23 0.55 1.04 0.51

Note: maximum errors of Deff‑2 is ±0.01 × 10−10 m2/s.

a

Table 9. Linear Correlation between Pore Parameters with Maximum Drying Rate and Kinetic Parameters R2 of coals dried at N2

R2 of all dried coals

pore parameters

vmax

E1

E2

Deff‑1

Deff‑2

vmax

E1

E2

Deff‑1

Deff‑2

SBET DI DII Vmeso Vmacro

0.83 0.78 0.24 0.06 0.75

0.85 0.85 0.23 0.32 0.76

0.24 0.21 0.19 0.71 0.21

0.42 0.77 0.68 0.18 0.91

0.07 0.80 0.46 0.26 0.84

0.16 0.06 0.06 0.09 0.021

0.17 0.15 0.05 0.12 0.04

0.04 0.10 0.17 0.72 0.18

0.15 0.15 0.52 0.14 0.76

0.18 0.13 0.27 0.29 0.62

process mainly involves chemical and physical changes of the sample, or heat and mass transfer between the atmosphere and the sample or from the interior to the surface of sample.22,25 In order to investigate the effect of pore structures on the drying rate and drying kinetic characteristics, the relationships between pore parameters including SBET, Vmeso, Vmacro, DI, and DII and drying characteristics including vmax, E1, E2, Deff‑1, and Deff‑2 were investigated by linear fitting. The R2 values for the fits for different pairs of variables are given in Table 9. The bold font formats in the table means the relative good correlation results. For the N2 atmosphere, the relationship between values of SBET and drying characteristics of vmax and E1 showed relative good linear relationships (R2 = 0.83 for SBET and vmax; R2 = 0.85 for SBET and E1); the relationship between the values of DI and drying characteristics of E1 showed a relative good linear relationship (R2 = 0.85 for DI and E1); the relationship between the values of Vmacro and drying characteristics of Deff‑1 and Deff‑2 showed relative good linear relationships (R2 = 0.91 for Vmacro and Deff‑1; R2 = 0.84 for Vmacro and Deff‑2). Based on the correlation, the changes of E1 and vmax as a function of SBET for coals dried in N2 are shown in Figure 10a. The values of vmax increased with SBET increasing. The results imply that the bigger surface area led to the quicker drying rate. The values of E1 also increased with values of SBET increasing. The behaviors of water adsorbed on carbonaceous materials are affected by the interplay of several factors: the concentration and the type of functional groups, the functional groups location, and its distribution on the surface.41 Therefore, the reason that E1 values increased with SBET values could be because of the location and distribution of oxygen functional groups on larger surface. The distribution of oxygen functional groups on larger surface could be more even, resulting in the cooperative effects of oxygen functional groups on water adsorption coming into play. So that more amount of water was hydrogen-bonded on the surface, which increased the drying activation energy.42,43 Otherwise, the good correlation between E1 and SBET could be a coincidence. The changes of E1 as a function of DI are shown in Figure 10b. The drying activation energy of surface water increased with surface

Figure 10. Values of SBET vs E1 and vmax (a), D1 vs E1 (b), and Vmacro vs Deff‑1 and Deff‑2 (c) for coals dried in N2.

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Energy & Fuels

the Innovative Talents of Higher Learning Institutions of Shanxi Province.

roughness increasing. The results indicated that it was more difficult for the water vapor to transport to the outside as the brown coal surface became more irregular and rougher. The increase of surface roughness affected the drying energy consumption of surface water. The changes of Deff‑1 and Deff‑2 as a function of Vmacro are shown in Figure 10c. The apparent diffusion coefficient of surface water and pore water increased with macropore volume increasing. The results indicated that the macropores was one of the main factors on transportation of water vapor. For brown coals dried in different atmospheres (right side of Table 9), the pore structures did not show good linear correlation with drying characteristics. The results could be because the mechanisms of drying in different atmospheres were different, which may result in both the pore structures and the chemical properties of brown coals changes simultaneously.27,31

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NOMENCLATURE YN = Chinese Yunnan brown coal N3 = coal was heated from room temperature to 200 °C at the heating rate of 3 °C/min under N2 N5 = coal was heated from room temperature to 200 °C at the heating rate of 5 °C/min under N2 N10 = coal was heated from room temperature to 200 °C at the heating rate of 10 °C/min under N2 N20 = coal was heated from room temperature to 200 °C at the heating rate of 20 °C/min under N2 N200 = coal was directly dried under 200 °C under N2 A3 = coal was heated from room temperature to 200 °C at the heating rate of 3 °C/min under air C3 = coal was heated from room temperature to 200 °C at the heating rate of 3 °C/min under CO2 FHH = fractal Frenkel−Halsey−Hill method V = amount of N2 adsorbate at pressure P V0 = amount of N2 adsorbate on the monolayer P0 = saturation pressure of N2 D = fractal dimension DI = value of Fractal dimension calculated from 0 < P/P0 < 0.5 DII = value of fractal dimension calculated from 1 > P/P0 > 0.5 A′ = a power-law exponent based on fractal dimension (D) and the adsorption mechanism C = constant number in FHH α = ratio of water weight loss w0 = initial water content w = water content at any drying time t t = drying time E = apparent activation energy E1 = activation energy of drying surface water E2 = activation energy of drying pore water A = pre-exponential factor T = drying temperature β = drying rate R = universal gas constant Deff = diffusion coefficient Deff‑1 = diffusion coefficient surface water Deff‑2 = diffusion coefficient pore water L = diameter of the coal particle m′ = slope of the linear regression between ln(α) and drying time t Xcr = critical water content SBET = specific surface area Vmeso = volume of mesopore Vmacro = volume of macropores Vtotal = volume of total pores vmax = maximum drying rate

4. CONCLUSIONS The pore structures including SBET, Vmeso, Vmacro, Vtotal, DI, and DII and drying characteristics including vmax, E1, E2, Deff‑1, and Deff‑2 for coals dried at different heating rates in a same atmosphere (N2) and coals dried under three different atmospheres (N2, CO2, and air) at a same heating rate were investigated and correlated. For coals dried in N2, the faster heating rates were benefit to porosity of dried coals. The vmax increased with heating rates, resulting from the higher temperature at same drying time and larger SBET and Vmacro. The E1 was higher at faster heating rates, because of the higher temperature gradient and the bigger DI. The E2 increased as heating rates increased from 3 to 20 °C/ min and then decreased for flash drying. The increase could be also due to the higher temperature gradient at faster heating rates. The decrease could be because of the effect of Vmacro on Deff‑2. For coals dried in different atmospheres, the degree of pore collapse was air > N2 > CO2. Comparing with the effect of heating rates on vmax, drying atmospheres showed the negligible influence on vmax. The oxidation reaction resulted in the higher E1 and E2. The relative bigger heat conductivity of N2 than that of CO2 led to the lower E1. The smaller molecular diameter of CO2 led to the lower E2. There was no consistent relationship between SBET and DI. The pore structures also have no good linear correlation with drying characteristics. These results could be because the drying mechanisms in different atmospheres are different.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 351 6111086. ORCID

Junjie Liao: 0000-0003-0983-4469 Yanna Han: 0000-0002-1198-3749

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Notes

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS This work was financially supported by Natural Science Foundation of China (21805202), Science Foundation for Youths of Shanxi Province (201801D221359), Joint Funds of Natural Science Foundation of China (U1710258), Natural Science Foundation of China (51574172), and Program for 6036

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