Article pubs.acs.org/EF
Cite This: Energy Fuels 2017, 31, 11884-11891
Analysis of Water Forms in Lignite and Pore Size Distribution Measurement Utilizing Bound Water as a Molecular Probe Keji Wan,†,‡ Pengchao Ji,†,‡ Zhenyong Miao,*,†,‡ Zishan Chen,†,‡ Yongjiang Wan,†,‡ and Qiongqiong He†,‡ †
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu China National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou 221116, Jiangsu China
‡
ABSTRACT: Based on the congelation characteristics, different types of water in lignite and their drying behavior were investigated by using low-temperature differential scanning calorimetry over a temperature range from 25 to −80 °C. The water in lignite was classified into three types: free water, bound water, and nonfreezable water. It was shown that during drying, the free water in lignite was removed first, and then the bound water began to be evaporated. With the moisture decreasing, the congelation peak of free water moves toward lower temperature, which may be explained by the formed capillary force distribution in the pores driving the free water to flow toward the smaller pores and increasing the confinement effect of the pores. However, for bound water, this mobility was restricted by the stronger coal−water interaction; thus, its congelation peak remained at around −42 °C. The amounts of three types of water were determined by a new method. It shown that all of the bound water and more than 30% of nonfreezable water needs to be removed in real drying cases. Based on the melting point depression of bound water in lignite, using bound water as a molecular probe, the pore size distribution of lignite was also determined. The maximum and minimum measured pore diameters were 86.47 and 7 nm, respectively. water.2,9,12 Nevertheless, different from free water, the increasing coal−water interaction forces greatly restrict the mobility of bound water and sharply increase the energy consumption of water removal. Based on the enthalpy of congelation in DSC results, the amounts of the water that undergo a phase change can be calculated.2,12 These two types of freezable water account for 35−78% of the total moisture content, and the rest is thought to be the nonfreezable water, which is incapable of forming crystalline ice because of the effects of internal surfaces or micropores.2 Nonfreezable water may bind to the acidic sites through hydrogen bonds and then form a water molecule−strong acid complex, which results in the loss of mobility for the water molecular.12,13 Bound water is the only water form that has strong interaction with coal and can be experimentally measured. It is always considered that the bound water condenses in the capillaries with diameters less than several micrometers,2 and Hayashi et al.14 even thought that the bound water and nonfreezable water are present in pores with diameters smaller than 10 nm. Hence, the behavior of bound water is easy to influence by the confinement effect of small pore space which would decrease the equilibrium melting temperature of water and produce melting point depression.15,16 The relationship between this depression and its relative pore diameter can be described by the Gibbs−Thomson equation,17,18 which has been used to examine the alteration of the pore size distributions (PSDs) of cellulose as a function of the degree of drying of the fibers.19,20 Therefore, because of the interaction
1. INTRODUCTION Lignite is an intermediate stage in the transformation over time of accumulated vegetable debris into black coals. The coalification process is accompanied by the elimination of water, but low-rank coals including lignite still have high residual moisture content in the range of 30−65 wt % on a wet basis.1 High moisture content has significantly limited its economic benefits and competitiveness in different industrial utilization such as pyrolysis, gasification, and combustion. Therefore, new drying technologies need be developed to ensure the competitiveness of lignite in the energy market.2 A better understanding of the water forms in lignite and their behavior in drying can help people to design and implement advanced drying systems. Because lignite contains large amounts of oxygen functional groups which result in material with hydrophilicity,3−6 different coal−water interaction forces classify the water retained in lignite into several forms, and it is commonly accepted that water exists in a freezable and nonfreezable state on the basis of congelation characteristics.2,3,7,8 Low-temperature high-resolution differential scanning calorimetry (DSC) is a powerful tool for investigating water forms in coal.9−11 When the temperature is decreased to below 0 °C, phase transitions occur, and the enthalpy of congelation released in this process can be accurately determined. Norinaga et al.12 observed two exothermic peaks in DSC results, namely, the free water peak at 258 K and bound water peak at 226 K, when cooling a series of moist coals. Because of the weak coal−water interaction, free water has properties similar to those of bulk water and shows the characteristic of strong mobility. The freezing point and congelation heat of capillary-condensed water is much less than that of bulk water, and this kind of water is referred as bound © 2017 American Chemical Society
Received: July 25, 2017 Revised: October 24, 2017 Published: October 24, 2017 11884
DOI: 10.1021/acs.energyfuels.7b02180 Energy Fuels 2017, 31, 11884−11891
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Energy & Fuels
temperature was maintained for 5 min to ensure that all of the water was frozen. After that, the sample was heated to 15 °C at a heating rate of 1, 0.5, or 0.2 °C/min. The temperature at the onset of the melting peak (the ice starts to melt) was calibrated to 0.0 °C. PSD measurement was performed by measuring the amount of water that had its melting point depression at each isothermal step procedure. Measurement of freezing process was excluded because the supercooling phenomenon of water would result in poor repeatability of measured freezing point. At first, the samples were cooled to −80 °C to ensure all of the water in lignite was frozen. After that, the temperature was then increased to −40 °C and maintained for 5 min. To prevent the thermal delay of the melting transition, the low heating rates of 0.2, 0.5, and 1 °C/min were selected in subsequent heating steps. Finally, subsequent heating steps to higher temperatures (−30, −15, −10, −6, −4, −2, −1.5, −1.1, −0.8, and −0.5 °C) were then performed in succession. When the temperature was increased to the target temperature, it was maintained isothermally until the heat flow returned to the baseline. An example of this melting program and the resulting heat flow is shown in Figure 1. Assuming that the pore shape of lignite is cylindrical, the relationship between a pore diameter (D) and the depressed melting temperature (Tm) can be described by Gibbs−Thomson equation,17,18 as shown in eq 1
between coal and bound water, a new method to determine PSD of moist coal may be feasible on the basis of the Gibbs− Thomson equation. The ease of water removal depends on the forms of water in lignite, and the varying strength of the water bond will produce different drying behavior. Although several studies have reported on the water forms in low-rank coals, the drying behavior of different types of water in lignite is still not wellunderstood, and the critical moisture contents of different type of water in drying also need to be clarified. In addition, based on the coal−water interaction, the method using bound water as a molecular probe to measure the PSD of lignite should produce more representative results than the mercury intrusion method (MIP) because it is performed at atmospheric pressure which makes it less likely to crush pore structure during the characterization. Therefore, the objectives of this study are (1) to analyze the variation of different types of water in lignite as a function of total moisture to investigate their drying behavior and (2) to investigate a new method to determine PSD in lignite utilizing bound water as a molecular probe.
2. MATERIALS AND METHODS ΔT = T0 − Tm =
2.1. Samples. Two lignite samples from different regions of China were used in this study, which were assigned as XLT and SL. All lignite samples were crushed and sieved to particle size below 150 μm. The proximate and ultimate analysis of samples are shown in Table 1. The
moisture (wt %, as received basis) 29.98 Proximate Analysis (wt %, dry basis) volatile matter 44.41 fixed carbon 36.01 ash 19.58 Ultimate Analysis (wt %, dry ash free basis) C 68.18 H 3.47 N 1.89 S 3.08 O (by diff.) 23.38
(1)
where ΔT is the depression of the melting temperature of water in a pore (K), T0 the melting temperature of pure water (273.15 K), θ the contact angle of ice and water (180°),20,21 γs,l the surface energy between ice and water (12.1 mJ/m2),3,4 ρ the density of ice (916.7 kg/ m3), and ΔHm the melting enthalpy of water in the unconfined (bulk) state (334 J/g).22 Finally, the total volume of the pores with the relevant diameter for an isothermal step can be obtained from the peak area of melting water. Thus, on the basis of the Gibbs−Thomson equation, the water held in a smaller pore would have a larger melting temperature depression (Table 2). However, it should be acknowledged that real pore structure in lignite will be complicated and irregularly shaped, and the DSC method just provides a way of estimating effective pore diameters. The measured pore size distribution was compared with that measured by MIP.
Table 1. Proximate and Ultimate Analysis of Lignite XLT lignite
− 4γ s,lT0 cos θ DρΔHm
SL lignite 28.18 35.72 42.95 21.33 65.71 4.92 1.26 2.42 25.69
3. RESULTS AND DISCUSSION 3.1. Water Forms in Lignite and Their Drying Behavior. Lignite samples, XLT, with different moisture contents were obtained by partial drying or addition of ultrapure water into as-received lignite sample with initial moisture content of 29.98% (w.t. in wet basis). The DSC thermograms of XLT lignite with different moisture contents (74.8−29.9% in dry basis) in the cooling process are shown in Figure 2. With temperature decreasing, there were two exothermic peaks appearing at around −12.8 to −28.7 °C and −41.3 °C. The appearance of these two peaks illustrates the presence of at least two different forms of freezable water in lignite, namely, the “free water” (−12.8 to −28.7 °C) and “bound water” (−41.3 °C), which indicate different intensities of the interaction between coal and water. When the moisture content in lignite decreased from 74.8% to 50.6%, the intensity of the free water peak declined correspondingly and the peak disappeared at the moisture content less than 46.9%. When the moisture content was higher than 46.9%, the variation in the intensity of the exothermic peak of bound water can be negligible with moisture. However, as the moisture content continued to decline to 42.9%, the intensity of the bound water peak began to decrease and then disappeared at the moisture content of around 29.9%. Therefore, it indicates that, during lignite drying process, the free water is first removed, and then
moisture contents of raw coals were approximately 28−30%, and a series of samples with expected values of moisture content could be obtained by using a halogen moisture detector to remove different quantities of water from raw lignite. In order to obtain the lignite with moisture contents higher than 30%, the samples were soaked in ultrapure water in a sealed beaker and then filtered. The samples with different moisture were immediately stored in airtight containers for further analysis. 2.2. Measurement of Water Forms in Lignite. DSC experiments were performed by using a Netzsch DSC-200-F3 instrument with an external liquid nitrogen cooling accessory. Several pure chemicals including C10H16, indium, bismuth, tin, and zinc were used to do the temperature and sensitivity calibration. The lignite sample (about 5−8 mg) was placed in a flat bottom aluminum sample pan. After that, the sample was cooled from 25 to −80 °C at a cooling rate of 3 °C/min under 60 mL/min of nitrogen and then heated to room temperature. The quantity of heat evolved during DSC experiments was independent of the cooling rate in the range of 2−8 °C/min,2,12 and the lower cooling rate, 3 °C/min, was expected to obtain more information on water behavior in coal. Experiments were repeated at least three times to ensure the reproducibility of the data. 2.3. Measurement of Pore Size Distribution in Lignite. About 5 mg of ultrapure water was also used for the temperature calibration of DSC in this section. The water was cooled to −40 °C, and the 11885
DOI: 10.1021/acs.energyfuels.7b02180 Energy Fuels 2017, 31, 11884−11891
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Figure 1. Melting of water in XLT lignite (moisture ratio of 0.58 g/g dry coal) during an isothermal step melting program. The heating rate was 0.5 °C/min.
Table 2. Relationship between the Melting Temperature Depression and the Pore Diameter Based on the Gibbs− Thomson Effect, Equation 1 ΔT (°C)
D (nm)
−30 −15 −10 −6 −4 −2 −1.5 −1.1 −0.8 −0.5
1.44 2.88 4.32 7.21 10.81 21.62 28.82 39.31 54.05 86.47
Figure 3. Moisture transport mechanism in capillary pore of lignite.
Figure 4. DSC thermal profile during cooling process for SL lignite with different moisture contents (dry basis).
disappeared, and it was almost consistent with literature resuls,2 but overall, the position of free water peak was changed with moisture decrease and slowly moved toward the bound water peak (there was a small fluctuation at the moisture content of 61.3%). The shift of free water indicates that the free water is moving to the smaller pores and the interaction between free water and coal structure is increasing as the drying progresses.
Figure 2. DSC thermal profile during cooling process for XLT lignite with different moisture contents (dry basis).
the bound water begins to be evaporated. As seen in Figure 2, for bound water, the peak was stabilized at −41.3 °C before it 11886
DOI: 10.1021/acs.energyfuels.7b02180 Energy Fuels 2017, 31, 11884−11891
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Energy & Fuels Table 3. Summary of DSC Results for XLT Lignite Samplesa water content
a
water type (w.t., dry basis)
wet basis (w.t.)
dry basis (w.t.)
ΔHf (J/g coal, dry basis)
0.428 0.380 0.368 0.361 0.347 0.319 0.300 0.290 0.280 0.260 0.250 0.230
0.748 0.613 0.581 0.566 0.531 0.469 0.429 0.408 0.389 0.351 0.333 0.299
75.093 33.180 23.548 19.366 5.744 − − − − − − −
ΔHb (J/g coal, dry basis) ΔHtotal (J/g coal, dry basis) 3.666 3.495 5.006 3.236 3.577 4.252 2.547 1.965 1.771 0.503 0.277 −
78.759 36.675 28.553 22.602 9.321 4.252 2.547 1.965 1.771 0.503 0.277 0
free water
bound water
nonfreezable water
0.239 0.104 0.072 0.057 0.022 − − − − − − −
0.179 0.179 0.179 0.179 0.179 0.139 0.099 0.078 0.059 0.021 0.030 −
0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.299
ΔHf, freezing heat of free water; ΔHb, freezing heat of bound water; ΔHtotal, sum of ΔHf and ΔHb.
Table 4. Summary of DSC Results for SL Lignite Samplesa water content
a
water type (w.t., dry basis)
wet basis (w.t.)
dry basis (w.t.)
ΔHf (J/g coal, dry basis)
0.37 0.34 0.33 0.29 0.28 0.25 0.23 0.17
0.587 0.515 0.493 0.408 0.389 0.332 0.301 0.205
33.546 16.198 7.422 − − − − −
ΔHb (J/g coal, dry basis) ΔHtotal (J/g coal, dry basis) 4.434 4.392 4.435 3.908 2.977 1.892 1.056 −
37.980 20.590 11.857 3.908 2.977 1.892 1.056 0.000
free water
bound water
nonfreezable water
0.127 0.055 0.033 − − − − −
0.193 0.193 0.193 0.141 0.122 0.065 0.034 −
0.267 0.267 0.267 0.267 0.267 0.267 0.267 0.205
ΔHf, freezing heat of free water; ΔHb, freezing heat of bound water; ΔHtotal, sum of ΔHf and ΔHb.
water flow within the pore of lignite may be also governed by Darcy’s law in drying. The transport of free water drives plenty of water from large pores to small pores, and the confinement effect of pores for water molecules is increased,25 which results in the shift of the free water peak. Meanwhile, for bound water in lignite, its freezing temperature was constant, because the stronger interaction between coal and water greatly restricted its mobility and the liquid water flow was becoming impossible. Thus, diffusion may be the major method of mass transfer for bound water in drying. According to Table 1, the basic properties of two samples, SL and XLT lignites, were different. To confirm the above results and the conclusion obtained by using XLT lignite, the measurements of water forms in SL lignite were also performed. As shown in Figure 4, similar to XLT lignite, the exothermic peaks of free water and bound water were observed for SL lignite during the cooling process. When the moisture content of SL lignite declined to 38.6% and 20.5%, the free water and bound water peaks disappeared, respectively. The bound water peaks maintained at −42 °C, which approached to those of XLT, −41.3 °C, but the free water peaks in SL lignite were in the range of −8 to −20 °C compared to −12.8 to −28.7 °C in XLT lignite. This difference may be attributed to the difference in chemical and physical structure of different types of lignites. With moisture content decreasing from 58.7% to 45.6%, the free water peak of SL lignite moved slightly toward bound water peak and showed the similar phenomenon with XLT lignite.
These results may be due to water migration behavior in the capillary, as shown in Figure 3. During drying, the nearly watersaturated lignite (Figure 3a) became unsaturated and began to form capillary force distribution in a complicated pore structure, as shown in Figure 3 b. For a single concentric capillary pore structure, pore I in Figure 3 b, according to the Young−Laplace equation,23 the pressure difference, ΔPmn, between two meniscuses with different radii of curvature can be described by eq 2 ⎛ 1 1 ⎞ ΔPmn = Pm − Pn = 2γ ⎜ − ⎟>0 Rm ⎠ ⎝ Rn
(2)
where Pm and Pn are the liquid water pressure in position m and n, respectively; Rm and Rn are the radii of curvature of meniscuses in m and n, respectively; Rm > Rn, and γ is the surface tension. Because Pm > Pn, the larger curvature radius of meniscus can result in higher liquid water pressure, and the water would flow from m to n. Based on this principle, for a series of parallel connected capillary pore structure as shown in Figure 3b, the pressure relationship of liquid water in different radii of pores can be expressed by eq 3 Pa = Pb > Pc > Pd
(3)
and R a = R b > Rc > Rd
(4)
Hence, the water would flow from a and b to c to d. As described by Whitaker24 in porous media drying, the liquid free 11887
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3.2. Quantitative Analysis of Different Types of Water in Lignite. Based on the freezing peak areas, the quantities of freezing heat of different types of water in XLT and SL lignite are shown in Tables 3 and 4, respectively. The evolution of freezing heat with moisture content is shown in Figure 5. It can be seen that there is a linear relationship between the freezing heat and moisture content in SL and XLT lignites. Figure 5 shows the results of linear fitting, and the free water region and bound water region were defined based on different slopes of straight lines, and the intersection between two lines was the boundary of two types of water. Based on this method, the moisture contents of different types of water are also summarized in Tablea 3 and 4, and the nonfreezable water content was then calculated by the intersection of bound water line and abscissa axis. From Figure 5, it can be seen that the maximum nonfreezable water contents of SL and XLT are 0.267 and 0.330 g/g dry coal, respectively, and the maximum bound water contents are 0.193 and 0.179 g/g dry coal for SL and XLT lignites, respectively. However, in Table 1, the total moisture contents of raw lignites of SL and XLT (0.392 and 0.429 g/g dry coal) are smaller than the sum of nonfreezable and bound water contents (0.460 and 0.510g/g dry coal), and these results illustrate that the free water is absent in raw lignite. To meet the industrial demand of gasification, combustion, carbonization, etc., the target moisture content of lignite should be below 0.176 g/g dry coal (15% in wet basis26). Therefore, in a real drying case, all of the bound water and more than 30% of nonfreezable water needs to be removed for XLT and SL lignite. From Figure 5, the freezing enthalpy of free water, i.e., the slope of line in the free water region, was 294.75 J/g and 316.14 J/g for SL and XLT, respectively, which was smaller than the enthalpy of freezing of 334 J/g for bulk water at 0 °C.22 The differences of free water enthalpies may be due to the different pore structure of lignites. For SL lignite, more free water existed in the macropores, and for XLT lignite, some of free water existed in the surface of particles. Meanwhile, the freezing enthalpy of bound water in SL and XLT lignite reached 26.88 and 29.76 J/g, respectively. The sharp decreases of freezing enthalpy of bound water may be due to the strong interaction between coal pore structure/oxygen functional groups and water, which consumes large amounts of energy during freezing. 3.3. Pore Size Distribution Measurement of Lignite Utilizing Bound Water as a Molecular Probe. The water in XLT lignite was completely frozen first and then heated to room temperature by using DSC. As expected, because of the total moisture content below the critical moisture point of free water, 0.510 g/g dry coal, only one bound water peak was observed during cooling as shown in Figure 6, and the total freezing heat (the peak area) was 4.566 J/g dry coal. In the heating process, one small melting peak of onset of 0 °C was found, and the corresponding melting heat was 0.077 J/g dry coal, which almost can be neglected compared to the freezing heat. Hence, most of the bound water melted below 0 °C in heating and did not show any apparent peaks. In addition, compared to the cooling process, the larger slope of baseline in heating process also indicates that some extra heat is produced below 0 °C. There is melting point depression for most of the bound water in lignite relative to pure water, and a similar phenomenon was also observed in SL and XLT lignite with different moisture contents, which is not shown here. Actually, this melting point depression of water in porous media reflects the pore structure.27,28 Moore et al.25 investigated the
Figure 5. Quantity of heat generated by freezing as a function of water content in SL and XLT lignites: (a) XLT and (b) SL.
Figure 6. DSC thermal profile of XLT lignite with moisture content of 0.459 g/g dry coal in cooling and heating process at a cooling−heating rate of 6 °C/min.
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DOI: 10.1021/acs.energyfuels.7b02180 Energy Fuels 2017, 31, 11884−11891
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Figure 7. Pore size distributions of lignites by MIP and DSC on different heat rates and moisture contents: (a) XLT (0.572 g/g dry coal), (b) XLT (0.616 g/g dry coal), (c) SL (0.569 g/g dry coal), and (d) SL (0.649 g/g dry coal).
mercury in MIP measurements. In addition, the anisotropism of pores in the same sample might also result in these differences. For both SL and XLT lignites, the moisture content had little effect on the measurements of PSD in the small water content ranges (SL, 0.569−0.649 g/g dry coal; XLT, 0.572−0.616 g/g dry coal). However, when the moisture content of lignite is too high, the swelling properties are more likely to appear, which may change the pore volume. There was no apparent relationship between the heating rate and PSD. In Figure 7a,b, PSDs measured by using the heating rate of 0.5 °C/min were in good agreement with those measured in 1.0 °C/min, but PSDs measured in 0.2 °C/min showed a small difference with the above results in the pore size range of 7−11 nm. In Figure 7c,d, the results from the heating rate of 1.0 °C/min produced relatively large deviation with the PSDs measured by using 0.5 and 0.2 °C/min. Therefore, the heating rate of 0.5 °C/min would be a better choice to measure the PSD of lignite. In the pore size range from 7 to 86.47 nm, the total cumulative pore volumes were around 6.0 × 10−5 and 5.0 × 10−5 mL/mg dry coal for XLT and SL lignites, respectively, and the corresponding amounts of bound water would be 0.06 and 0.05 g/g dry coal. Compared to the total bound water content of 0.18 and 0.193 g/g dry coal (according to the results of section 3.2), this part of bound water accounted for only about 33% and 26% of total bound water contents of XLT and SL,
coexistence between confined ice and liquid water as a function of temperature for a series of nanopores with hydrophilic and hydrophobic surface and suggested that the melting temperature of the ice is strongly dependent on the radius of the pore but rather insensitive to the hydrophilicity of the pore surface. Using bound water as a molecular probe, the PSD of lignites with different moisture contents was measured in DSC heating process, and the results are shown in Figure 7. According to section 3.2, the total moisture content of the sample should be higher than the critical bound water content to ensure that bound water can completely occupy the volume of pores and improve the reliability of measurement. To avoid the thermal delay of the melting transition due to rapid heating, the slow heating rates 1, 0.5, and 0.2 °C/min were selected to measure the PSD of coal. From Table 2, the maximum and minimum pore diameters were 86.47 and 1.44 nm, respectively. However, assuming the shape of the pore to be a cylinder would be one of the drawbacks of the DSC method because of the complicated pore structure in coal. Thus, the results of MIP were used for comparison to ensure the reliability of data in the pore size range of 7−86.47 nm. From Figure 7a,b, it can be observed that the PSD values measured by DSC were slightly larger than those measured by MIP, and this may be due to the deformation of lignite particles under the high pressure of 11889
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and 111 project (Grant No. 12030) for which the authors express their appreciation. The authors also gratefully acknowledge financial support from China Scholarship Council.
respectively. Thus, there are still large amounts of bound water existing in larger pores, and improving the measurement accuracy of the DSC instrument can extend the PSD measurement range.
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4. CONCLUSIONS (1) Based on the congelation characteristics, two different types of freezable water (free water and bound water) were determined. During drying, the free water in lignite was first removed and then the bound water began to be evaporated. As water was removed progressively, free water was transferred to the smaller pores, which may be explained by the formed capillary force distribution in the pore driving the free water to flow toward the smaller pore and increasing the confinement effect of pore. However, for bound water, the stronger interaction between coal and water decreased its mobility; thus, its congelation peak remained at around −42 °C. (2) The maximum nonfreezable water contents of SL and XLT were 0.267 and 0.330 g/g dry coal, respectively, and the maximum bound water contents were 0.193 and 0.179 g/g dry coal for SL and XLT lignites, respectively. To meet the industrial demand, in a real drying case, more than 30% of nonfreezable water and all of the bound water needs to be removed. In addition, the freezing enthalpy of freezable water was smaller than that of pure water, and the strong interaction between coal and water may consume part of the energy during freezing. (3) There was melting point depression for bound water in lignite due to the effect of the small radius of the pores; thus, using bound water as a molecular probe, the PSDs of moist XLT lignite were determined in a step-heating process. The measured maximum and minimum pore diameters were 86.47 and 7 nm, respectively. The moisture content had little effect on the measurements of PSDs in the small water content ranges (SL, 0.569−0.649 g/g dry coal; XLT, 0.572−0.616 g/g dry coal). There was no apparent relationship between the heating rate and PSD, and the heating rate of 0.5 °C/min would be a better choice to measure the PSD of lignite. In addition, it was shown that the amounts of bound water existing in the pore size range from 7 to 86.47 nm accounted for about 30% of total bound water content.
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REFERENCES
(1) Durie, R. The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization; Butterworth-Heinemann: Oxford, 2013. (2) Tahmasebi, A.; Yu, J.; Su, H.; Han, Y.; Lucas, J.; Zheng, H.; Wall, T. A differential scanning calorimetric (DSC) study on the characteristics and behavior of water in low-rank coals. Fuel 2014, 135, 243−252. (3) Yu, J.; Tahmasebi, A.; Han, Y.; Yin, F.; Li, X. A review on water in low rank coals: the existence, interaction with coal structure and effects on coal utilization. Fuel Process. Technol. 2013, 106, 9−20. (4) Charrière, D.; Behra, P. Water sorption on coals. J. Colloid Interface Sci. 2010, 344 (2), 460−467. (5) Allardice, D.; Clemow, L.; Favas, G.; Jackson, W.; Marshall, M.; Sakurovs, R. The characterisation of different forms of water in low rank coals and some hydrothermally dried products☆. Fuel 2003, 82 (6), 661−667. (6) Wan, K.; He, Q.; Miao, Z.; Liu, X.; Huang, S. Water desorption isotherms and net isosteric heat of desorption on lignite. Fuel 2016, 171, 101−107. (7) Mraw, S.; Naas-O’Rourke, D. Water in coal pores: Lowtemperature heat capacity behavior of the moisture in Wyodak coal. Science 1979, 205 (4409), 901−902. (8) Mraw, S. C.; O’Rourke, D. F. Water in coal pores: the enthalpy of fusion reflects pore size distribution. J. Colloid Interface Sci. 1982, 89 (1), 268−271. (9) Fei, Y.; Chaffee, A. L.; Marshall, M.; Jackson, W. R. Lignite− water interactions studied by phase transitiondifferential scanning calorimetry. Fuel 2005, 84 (12), 1557−1562. (10) Han, Y.; Liao, J.; Bai, Z.; Chaffee, A. L.; Chang, L.; Li, W. Study on the Relationship Between Pore Structure and Water Forms in Pore Using Partially Gasified Lignite Char. Energy Fuels 2016, 30 (11), 8875−8885. (11) Nwaka, D.; Tahmasebi, A.; Tian, L.; Yu, J. The effects of pore structure on the behavior of water in lignite coal and activated carbon. J. Colloid Interface Sci. 2016, 477, 138−147. (12) Norinaga, K.; Kumagai, H.; Hayashi, J.-i.; Chiba, T. Classification of water sorbed in coal on the basis of congelation characteristics. Energy Fuels 1998, 12 (3), 574−579. (13) Clemow, L. M.; Jackson, W. R.; Chaffee, A. L.; Sakurovs, R.; Allardice, D. J. Understanding brown coal-water interactions to reduce carbon dioxide emissions. In Environmental Challenges and Greenhouse Gas Control for Fossil Fuel Utilization in the 21st Century; Springer: New York, 2002; pp 203−216. (14) Hayashi, J.-i.; Norinaga, K.; Kudo, N.; Chiba, T. Estimation of size and shape of pores in moist coal utilizing sorbed water as a molecular probe. Energy Fuels 2001, 15 (4), 903−909. (15) Findenegg, G. H.; Jähnert, S.; Akcakayiran, D.; Schreiber, A. Freezing and melting of water confined in silica nanopores. ChemPhysChem 2008, 9 (18), 2651−2659. (16) Erko, M.; Findenegg, G.; Cade, N.; Michette, A.; Paris, O. Confinement-induced structural changes of water studied by Raman scattering. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84 (10), 104205. (17) Furó, I.; Daicic, J. NMR cryoporometry: a novel method for the investigation of the pore structure of paper and paper coatings. Nord. Pulp Pap. Res. J. 1999, 14, 221−225. (18) Anovitz, L. M.; Cole, D. R. Characterization and analysis of porosity and pore structures. Rev. Mineral. Geochem. 2015, 80 (1), 61− 164. (19) Fahlén, J.; Salmén, L. Ultrastructural changes in a holocellulose pulp revealed by enzymes, thermoporosimetry and atomic force microscopy. Holzforschung 2005, 59 (6), 589−597.
AUTHOR INFORMATION
Corresponding Author
*School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China. E-mail:
[email protected]. Tel: +86 0516 83884289. Fax: +86 0516 83884289. ORCID
Keji Wan: 0000-0002-6397-2224 Zhenyong Miao: 0000-0002-4595-8867 Qiongqiong He: 0000-0002-5497-7111 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Nature Science Foundation of China (Grant 51574239), the National Key Basic Research Program of China (Grant 2012CB214902), the Fundamental Research Funds for the Central Universities (Grant 2014XT05), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions 11890
DOI: 10.1021/acs.energyfuels.7b02180 Energy Fuels 2017, 31, 11884−11891
Article
Energy & Fuels (20) Park, S.; Venditti, R. A.; Jameel, H.; Pawlak, J. J. Changes in pore size distribution during the drying of cellulose fibers as measured by differential scanning calorimetry. Carbohydr. Polym. 2006, 66 (1), 97− 103. (21) Zauer, M.; Kretzschmar, J.; Großmann, L.; Pfriem, A.; Wagenführ, A. Analysis of the pore-size distribution and fiber saturation point of native and thermally modified wood using differential scanning calorimetry. Wood Sci. Technol. 2014, 48 (1), 177−193. (22) Jähnert, S.; Chávez, F. V.; Schaumann, G.; Schreiber, A.; Schönhoff, M.; Findenegg, G. Melting and freezing of water in cylindrical silica nanopores. Phys. Chem. Chem. Phys. 2008, 10 (39), 6039−6051. (23) Finn, R. Capillary surface interfaces. Notices AMS 1999, 46 (7), 770−781. (24) Whitaker, S. Flow in porous media I: A theoretical derivation of Darcy’s law. Transp. Porous Media 1986, 1 (1), 3−25. (25) Moore, E. B.; Allen, J. T.; Molinero, V. Liquid-ice coexistence below the melting temperature for water confined in hydrophilic and hydrophobic nanopores. J. Phys. Chem. C 2012, 116 (13), 7507−7514. (26) Karthikeyan, M.; Zhonghua, W.; Mujumdar, A. S. Low-rank coal drying technologiescurrent status and new developments. Drying Technol. 2009, 27 (3), 403−415. (27) Iza, M.; Woerly, S.; Danumah, C.; Kaliaguine, S.; Bousmina, M. Determination of pore size distribution for mesoporous materials and polymeric gels by means of DSC measurements: thermoporometry. Polymer 2000, 41 (15), 5885−5893. (28) Majda, D.; Makowski, W.; Mańko, M. Pore size distribution of micelle-templated silicas studied by thermoporosimetry using water and n-heptane. J. Therm. Anal. Calorim. 2012, 109 (2), 663−669.
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DOI: 10.1021/acs.energyfuels.7b02180 Energy Fuels 2017, 31, 11884−11891
