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A differential scanning calorimetric (DSC) study on the characteristics and behavior of water in low-rank coals
Fuel 135 (2014) 243–252
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A differential scanning calorimetric (DSC) study on the characteristics and behavior of water in low-rank coals Arash Tahmasebi a, Jianglong Yu a,b,⇑, Huaixing Su a, Yanna Han a, John Lucas b, Hanglin Zheng a, Terry Wall b a Key Lab for Advanced Coal and Coking Technology of Liaoning Province, School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051, China b Chemical Engineering, University of Newcastle, Callaghan, NSW 2308, Australia
h i g h l i g h t s Forms of water in low-rank coals were investigated by DSC and XRD methods. Two types of water namely free water and freezable bound water were detected by DSC. Presence of non-freezable water was also confirmed during heating in DSC. XRD analysis showed that part of ‘‘freezable water’’ was not detected in DSC. The findings advance the present knowledge in literature on coal–water interactions.
a r t i c l e
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Article history: Received 6 April 2014 Received in revised form 25 June 2014 Accepted 26 June 2014 Available online 9 July 2014 Keywords: Lignite Coal–water interaction Low temperature DSC Freezable water Non-freezable water
a b s t r a c t High moisture content in low-rank coals significantly limits their application and there is a R&D need for advanced drying technologies and upgrading of lignite. Fundamental understanding of coal–water interactions is a critical part of the development of drying technologies. In this study, forms of water in low-rank coals were investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses based on their freezing characteristics. Two types of free and freezable bound water were detected during freezing process in DSC experiments. The amount of free and freezable bound water was experimentally determined from their heat of freezing. These two types of water accounted for 18.22–78.25% of total water in coal, which indicated the presence of the third type of water, namely ‘‘non-freezable water’’. The presence of the third type of water was confirmed in DSC heating process experiments. Moisture reabsorption experiments on heat treated samples showed that the physical and chemical structures of coal have a significant effect on its interaction with water. When coal samples with 14.75% water contents were subjected to low temperature XRD analysis, water in frozen state was detected at 80 °C indicating that part of ‘‘freezable bound water’’ that was not detected by DSC was in frozen state at low temperatures. Amount of heat generated by phase transition of the third type of water for coal samples with water contents less than 18% was very small and therefore was not detected by the DSC. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction It is estimated that nearly 45% of the world coal reserves consist of low-rank coals [1]. Low-rank coals are primary source of energy in developing countries such as China. Low-rank coals contain high moisture content (25–60%) which has a significant effect on downstream utilization processes [2,3]. High water content in lignite ⇑ Corresponding author at: Chemical Engineering, University of Newcastle, Callaghan, NSW 2308, Australia. Tel.: +61 2 40333902; fax: +61 2 40339302. E-mail address: [email protected] (J. Yu). http://dx.doi.org/10.1016/j.fuel.2014.06.068 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
results in a lower efficiency, increased transportation cost, and higher carbon dioxide emissions [4]. On the other hand, low-rank coals have some advantages such as low mining cost, high reactivity, high amount of volatiles, and low pollution-forming impurities such as sulfur, nitrogen, and heavy metals [5,6]. New drying or dewatering technologies need be developed to ensure the competitiveness of lignite in energy market. In order to design and implement advanced drying systems, it is necessary to have a better understanding of the coal–water interactions, particularly the behavior of water in low-rank coals.
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The ease of water removal depends upon the forms of water in low-rank coals. Water is retained in coals in different forms, and it is commonly accepted that water exists in a bound phase and a free phase [7,8]. Researchers have used different definitions for various types of water in coal such as freezable and no-freezable water, free water, pore water, surface water, molecular water, internal water, capillary water, adsorbed water and crystal water [2]. Allardice and co-workers [9–11] suggested that at least two types of water exist in brown coal at any particular temperature: (1) water which can be removed by evacuation, and (2) chemisorbed water which can be released only by raising the temperature to cause thermal decomposition of functional groups. Unsworth et al. [12] used microwave attenuation, pulsed and continuous wave NMR techniques to observe the behavior of water in coals with different ranks as a function of temperature and water content. Their results showed three types of water, i.e., tightly bound water, loosely bound water, and bulk-like water. More detailed classification of water in coal involves [2,13]: (1) Interior adsorption water is contained in micropores and micro-capillaries within each coal particle, deposited during coal formation; (2) Surface adsorption water forms a layer of water molecules adjacent to coal molecules but on the particle surface only; (3) Capillary water is contained in capillaries; (4) Interparticle water is contained in small crevices found between two or more particles; and (5) Adhesion water forms a layer or film around the surface of individual or agglomerated particles. Water types (4) and (5) (i.e. surface moisture) can be removed by mechanical dewatering methods. Water type (3) can be removed partially, depending upon the size of the openings in the coal surface and the drying temperature. Water types (1) and (2) are inherent moisture and can be removed by thermal drying processes [1]. A similar classification was given by Dahlstrom et al. [14] in which water was classified as chemisorbed, physisorbed, micro- and macropore water, inter-particle and intra-particle held water. Norinaga et al. [15] investigated the enthalpy peaks during the cooling process of eight coals, ranging from brown coal to bituminous with the water content ranging from 57.5% to 4.6% by using DSC and NMR methods. They reported two types of freezable water (free water and freezable bound water) and non-freezable water in coals, based on congelation characteristics. In general, the classification methods and terms used in literature for water in coals are not consistent. A coherent classification system is therefore necessary in order to enhance the future study and technology development. The classification system and terms should take into account the nature of the water as well as the interaction of water with coal structure. The changes in different types of water as a function of coal moisture content and the critical moisture contents at which every type of water is removed have not been fully understood in the literature. A better understanding of water–coal interaction is crucial in order to design more advanced and efficient drying technologies. Low-temperature high resolution Differential Scanning Calorimetry (DSC) is a powerful tool for investigating water behavior in porous materials such as lignite [16,17]. In order to better understand the nature of water in coal, characteristics and behavior of water in two lignite coals and interaction of water with coal structure are investigated in this study by using DSC, TGA-MS, and XRD techniques.
Table 1 Proximate and ultimate analyses of raw coal samples. Coal sample
HL
YN
Moisture content (wt.%, ar) Moisture content (wt.%, ad) Volatile matter (wt.%, ad) Ash (wt.%, ad) C, wt.% (daf) H, wt.% (daf) N, wt.% (daf) S, wt.% (daf) O (by diff.), wt.% (daf)
32.10 25.56 33.98 12.12 72.28 5.89 0.93 0.18 20.59
35.00 25.41 37.1 2.30 71.51 4.94 1.03 0.24 22.25
Notes: ar – as received; ad – air dried; daf – dry ash free.
assigned as ‘‘YN’’ and ‘‘HL’’ coals, respectively. The properties of the as-received coals are shown in Table 1. Ba(OH)2, NaOH, HCl and Ca(CH3COO)2 (Sinopharm Chemical Reagent Co. Ltd.) as analytical reagents were used to measure the concentration of functional groups in coal. All coal samples were crushed and sieved to particle size of less than 125 lm. Part of the coal samples were dried under nitrogen at the temperatures of 110, 150, and 190 °C in order to obtain samples with different moisture contents. The water contents of coal samples were determined from the fractional mass loss measured upon drying at 105 °C under nitrogen for 2 h. The as received moisture content of coals was around 32–35%. In order to obtain coals with lower moisture contents, the as received samples were dried in a vacuum drying oven at 30 °C and 0.1 MPa. By controlling the residence time in oven, coal samples with different moisture contents were obtained. In order to obtain moisture contents higher than that of as received amount, the coal samples were soaked in de-ionized water in a sealed beaker at room temperature for 20 min. The water–coal mixture was then filtered for different times to obtain moisture contents in the range of around 32– 50%. The coal samples with different water content were immediately stored in airtight containers for further analysis. The water contents of HL and YN coal samples after treatments ranged between 17.42–50.22% and 17.83–46.10%, respectively. 2.2. DSC analysis DSC experiments were carried out by using a NETZSCH DSC200-F3 with internal cooling system incorporated with Thermal Analysis System Software to control experimental runs. The instrument was also equipped with an external liquid nitrogen cooling accessory. The coal sample (8 mg) was placed in an aluminum sample pan. The pan was then placed in the DSC head together with a reference pan. The sample was cooled down from 25 °C to 150 °C and heated up again to 300 °C at a rate of 3 °C/min under 80 ml/min of nitrogen. It has been reported that the quantity of heat evolved during DSC experiments is independent of the cooling rate in the range of 2–8 °C/min [15]. Experiments were repeated at least twice to ensure the reproducibility of the data and the error in the enthalpies associated with the phase transitions were calculated. 2.3. TGA analysis
2. Experimental 2.1. Sample preparation An Indonesian low-rank coal (supplied by Banpu Public Co. Ltd., Thailand) and a Chinese lignite (supplied by Shenhua Coal Co. from Hulunbeier, Inner Mongolia) were used in this study and were
TGA (NETZSCH STA-449-F3) equipped with NETZSCH QMS-403C was used to study the mass changes and evolution of gases during drying of coal samples. Approximately 10 mg of coal sample (<125 lm size) were used in the TGA experiments. The TGA runs were carried out at the rate of 5 °C/min from 25 °C up to 300 °C under 80 ml/min nitrogen gas flow. The gases generated during the heating process were detected and quantified by MS.
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2.4. XRD analysis XRD was used to characterize the changes in crystalline nature of ice [18]. In order to study the crystal structure of frozen water in coals, the D8 ADVANCE X-ray diffraction equipped with a liquid nitrogen cooling accessory was used. XRD measurements were carried out at 80 °C. The X-ray patterns were recorded with a stepscanning method in the range of 2h = 10–90°.
2.5. Measurement of acidic functional groups The total amount of acidic functional groups in samples was measured to study the difference in chemical structure changes between coal samples [19]. Coal sample was mixed with 0.1 M Ba(OH)2 solution in water bath and was magnetically stirred at 100 °C for 2 h. The hydrochloric acid solution was poured onto the filtrate from the Ba(OH)2 mixture. The mixture was titrated with a standard solution of 0.2 M NaOH solution. The primary procedure to measure the quantity of carboxylic acid groups were similar to the steps above, but the reacting solution was changed to 0.2 M Ca(CH3COO)2 and in the last step the mixture was titrated with 0.1 M NaOH. The blank experiments without coal sample were carried out under the same experimental conditions for calculation of the acidic groups. The amount of phenolic hydroxyl groups was obtained from the difference between the total acidic groups and carboxyl groups.
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the melting of ice and evaporation of water at around 0 °C and between 0 and 100 °C, respectively. Coal samples with different water contents were prepared by partial drying or addition of deionized water into HL lignite sample with initial moisture content of 32.1%. Figs. 2 and 3 show the DSC thermograms of HL lignite with different water contents under cooling and heating processes, respectively. Water content of coal samples ranged between 17.42% and 50.22% on wet basis. The exothermic peaks in Fig. 2 were attributed to the phase transition of the water into ice, i.e., freezing under cooling. As seen in Fig. 2, two sets of exothermic peaks appeared at around 8 °C and 42 °C with decreasing temperature. The appearance of two peaks during the cooling process revealed the presence of at least two distinct types of freezable water, i.e., ‘‘free water’’ (8 °C) and ‘‘freezable bound water’’ (42 °C) [15]. Free water corresponds to intra-particle water and water condensed in large capillaries. Freezable bound water is present in pores with diameters smaller than 10 nm and is associated with coal chemical structure via hydrogen bonding [20]. When the water content in samples decreased from 50.22% to 30.92% the intensity of the peak at 8 °C decreased monotonically. This peak was not observed in
3. Results and discussion 3.1. DSC analysis of coal samples with different water contents DSC analysis of HL and YN lignites with different moisture contents was carried out in order to better understand the forms of water in coal. Fig. 1 shows the DSC thermogram of HL lignite containing 32.1% water. In DSC experiments, temperature was decreased from 25 °C down to 150 °C and then increased up to 300 °C at a rate of 3 °C/min. The DSC thermogram showed four peaks including two exothermic and two endothermic. The two exothermic peaks generated as a result of water freezing were also reported by Norinaga et al. [15] who proposed the concept of free and bound water. The two endothermic peaks were attributed to
Fig. 2. The DSC thermograms during cooling process for HL lignite with different moisture contents of: (a) 50.22%; (b) 42.37%; (c) 40.16%; (d) 37.49%; (e) 33.13%; (f) 31.93%; (g) 30.92%; (h) 29.56%; (i) 27.84%; (j) 27.63%; (k) 24.55%; (l) 21.16%; (m) 17.42%.
Fig. 1. DSC thermogram of HL lignite with 33.13% moisture content.
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Fig. 3. The DSC thermograms during heating process for HL lignite with different moisture contents of: (a) 50.22%; (b) 42.37%; (c) 40.16%; (d) 37.49%; (e) 33.13%; (f) 31.93%; (g) 30.92%; (h) 29.56%; (i) 27.84%; (j) 27.63%; (k) 24.55%; (l) 21.16%; (m) 17.42%.
samples with moisture content less than 29.56%. The low intensity of the second exothermic peak at around 42 °C indicated the freezing of water condensed in capillaries with diameters less than several micrometers [21]. The variation in the size of second peak was negligible in samples with water content of more than 32.1%. However, the intensity of the second peak decreased when moisture content was decreased from 27.84% to 17.42% and disappeared thereafter. Fig. 3 shows the DSC thermograms of HL lignite as a function of moisture content during heating process. Two endothermic peaks appeared in the temperature range of 0–100 °C. The smaller endothermic peak at around 0 °C was generated with melting of ice. The larger endothermic peak in the temperature range of 0–100 °C was attributed to the evaporation of water. It should be noted that although ice was formed at two different temperatures (8 °C and 42 °C), only one exothermic peak corresponding to melting of ice appeared at around 0 °C, suggesting the similarity between the ice crystals from free and freezable bound water. The size of endothermic peaks in the temperature range of 0– 100 °C was proportional to water content in coal samples. As mentioned above, no exothermic peaks were observed for coal samples with moisture content of less than 17.42% during cooling process (Fig. 2). However, during heating process of these samples, an endothermic peak appeared due to evaporation of water at the same temperature range as free water. It seems that 17.42% of the water in coal sample was non-freezable. Therefore, the presence of the third type of water, namely ‘‘non-freezable water’’ was also confirmed. Similar results have been reported by Mraw and Naas-O’rourke [22] by using DSC analysis. However, this conclusion needs to be verified with combination of XRD analysis, as discussed in the subsequent sections. In order to confirm the results obtained for HL lignite, further experiments were carried out on YN lignite. The proximate and ultimate analysis of HL and YN lignites showed the similarity of these two coals. Fig. 4 shows the DSC thermograms of YN lignite during cooling process. Similar to HL lignite, two exothermic peaks were observed for YN lignite. The first and second peaks appeared at around 10 °C and 42 °C, respectively. When the water content in YN coal samples decreased to 30.75%, the first exothermic peak disappeared. However, there was a slight difference in freezing temperature of ‘‘free water’’. The first exothermic peak in YN lignite occurred at 10 °C compared to 8 °C in HL coal. The nature of the mechanism affecting the freezing temperature of water in different types of coals is not fully understood [2]. However, this
Fig. 4. The DSC thermograms during cooling process for YN lignite with different moisture contents of: (a) 46.10%; (b) 39.75%; (c) 38.64%; (d) 34.30%; (e) 33.27%; (f) 30.98%; (g) 30.75%; (h) 29.83%; (i) 27.49%; (j) 26.67%; (k) 20.02%; (l) 19.50%; (m) 17.83%.
Fig. 5. The DSC thermograms during heating process for YN lignite with different moisture contents of: (a) 46.10%; (b) 39.75%; (c) 38.64%; (d) 34.30%; (e) 33.27%; (f) 30.98%; (g) 30.75%; (h) 29.83%; (i) 27.49%; (j) 26.67%; (k) 20.02%; (l) 19.50%; (m) 17.83%.
difference can be attributed to the difference in chemical and physical structure of different types of coals. Fig. 5 shows the DSC thermograms of YN coal samples with different water content during the heating process. Similar to HL coal, the first endothermic peak corresponding to the melting of ice appeared at around 0 °C regardless of the different freezing temperature of two types of freezable water. The larger endothermic peak (due to water evaporation in the range of about 0–100 °C) was also similar to that of HL lignite. 3.2. Effects of coal chemical structure on coal–water interactions The fact that freezing of free water in lignite did not occur at 0 °C but at lower temperatures can be attributed to its association with the chemical structure of lignite. It has been reported that ‘‘free water’’ is present in the spaces between the coal particles or in pores with diameter greater than 105 cm [23]. Allardice [9] reported that with decreasing water content from 60 to 15 g water per l00 g dry coal, the heat of desorption increased gradually and became higher than the latent heat of vaporization of pure water. This implies that part of the water is associated with physical
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and chemical structure of lignite. Water may interact with coal chemical structure via hydrogen bonds, electrostatic force, Debye force, and London dispersion force. These forces are short-range forces, and the long-range forces are formed when the distance between water molecules and coal surface increases. The monolayer water is adsorbed on coal surface mainly by hydrogen bonds. At low water contents, monolayer water is formed around the functional groups and the short-range forces come into effect [24]. With further increasing the water content, water clusters are formed and the long-range forces are formed. The formation of hydrogen bonds is mainly through oxygen-containing functional groups which plays an important role in coal–water interaction [2]. Švábová et al. [25] reported a linear relationship between the sum of the hydroxyl and carboxyl groups and the water uptake capacity of coal, with a high correlation coefficient (R2 > 0.998). The difference in freezing temperature between YN and HL coal was attributed to the difference in their chemical structures and was investigated by measuring the total acidic groups including carboxyl and phenolic hydroxyl in these coals. The results are given in Table 2. The total acidic groups of YN lignite were considerably higher than HL lignite, rendering it more hydrophilic. The higher hydrophilicity of YN lignite increased the strength of forces between water and coal, delaying the freezing of ‘‘free water’’. As a result, the freezing of ‘‘free water’’ occurred at lower temperatures in YN lignite (10 °C) compared to HL lignite (8 °C).
and freezable bound water accounted for about 18.22–78.25% of the total water in coal samples with water contents in the range of 21.16–50.22%. These results confirmed the presence of another type of water for which freezing was not observed, i.e. ‘‘non-freezable water’’. The fraction of non-freezable water estimated by difference is also given in Tables 3 and 4. It has been reported that non-freezable water is retained in small water clusters with less than 10 water molecules involved in the cluster. This type of water in associated with coal structure via hydrogen bonding [15,26]. The symbols ‘‘w’’ and ‘‘X’’ represent water content on a wet and dry basis, respectively. The relationship between these two is shown in Eq. (1). When the exothermic peaks of free and freezable bound water separately disappeared at 30% and 18% water content, the critical points 0.4286 and 0.2195 were defined as X1 and X2. The percentage of free and freezable bound water in coal samples with different water content was calculated from Eqs. (2)–(4), respectively.
X¼
w 1w
ð1Þ
3.3. Quantification of freezing heat
Table 2 The amount of acidic groups of raw coals (mmol/g). Coal
Carboxyl groups
Phenolic hydroxyl groups
Total acidic groups
YN HL
1.21 1.14
10.44 3.15
11.65 4.28
ð2Þ
Water content > 30% : Freezable bound water ð%Þ ¼
Further DSC analysis was carried out on samples with different moisture contents in order to measure the quantity of heat release during freezing process as a function of moisture content. Tables 3 and 4 summarize the quantity of freezing heat and the percentage of free and freezable bound water for HL and YN lignites, respectively. The freezing enthalpy contains two sections, i.e., freezing heats of free water (DHf) and freezable bound water (DHb). DHf and DHb were the average value obtained by integration of the peak areas using DSC thermo-analysis software. The sum of free
X X1 100 X
Free water ð%Þ ¼
X1 X2 100 X
ð3Þ
Water content < 30% : Freezable bound water ð%Þ ¼
X X2 100 X
ð4Þ
The variation of different types of water was similar in HL and YN lignite samples. As discussed above, in case of YN coal with decreasing water content from 46.10% to 30.75%, the percentage of free water gradually decreased and disappeared when the water content in coals reached to around 29.83%. With further decreasing the water content, the freezable bound water also decreased monotonically. Since the exothermic peaks in samples with different water content appeared at similar temperatures, the variation in the size of these peaks reflected the amount of heat and water content. The quantity of heat DH for free and freezable bound water showed a linear relationship with the water content, as can be seen in Fig. 6. For both coals, DH decreased linearly with decreasing
Table 3 Summary of DSC results for HL coal. Water content
DHf (kJ/(kg coal))
w (g/(g coal))
X (g/(g coal))
50.22% 42.37% 40.16% 37.49% 33.13% 31.93% 30.92% 30.00% 29.56% 27.84% 27.63% 24.55% 22.97% 21.16% 18.00% 17.42%
1.009 0.7352 0.6711 0.5997 0.4954 0.4691 0.4476 0.4286 0.4196 0.3858 0.3818 0.3254 0.2982 0.2684 0.2195 0.2109
DHb (kJ/(kg coal))
DHm (kJ/(kg coal))
158.4 2.873 161.2 75.64 2.480 75.53 54.23 2.422 55.77 45.45 2.289 45.94 16.82 2.185 20.46 9.249 2.167 11.59 3.625 2.501 6.541 The free water exothermic peak disappears 0 2.665 0.2510 0 2.138 0.1940 0 1.926 0.2006 0 1.345 0.0740 0 1.175 0.05443 0 0.9836 0.03931 The bound water exothermic peak disappears 0 0 0
Water type (%) Free water
Freezable bound water
Non-freezable water
57.52 41.70 36.14 28.54 13.49 8.63 4.24
20.73 28.44 31.16 34.86 42.21 44.58 46.72
21.76 29.86 32.71 36.60 44.30 46.79 49.04
0 0 0 0 0 0
47.69 43.11 42.51 32.54 26.39 18.22
52.31 56.89 57.49 67.46 73.61 81.78
0
0
100.00%
DHf – freezing enthalpy of free water; DHb – freezing enthalpy of bound water; DHm – melting enthalpy of ice.
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Table 4 Summary of DSC results for YN coal. Water content
DHf (kJ/(kg coal))
w (g/(g coal))
X (g/(g coal))
46.10% 39.75% 38.64% 34.30% 33.27% 30.98% 30.75% 30.00% 29.83% 27.49% 26.67% 23.24% 20.02% 19.50% 18.00% 17.83%
0.8553 0.6598 0.6297 0.5221 0.4986 0.4489 0.4440 0.4286 0.4251 0.3791 0.3637 0.3028 0.2503 0.2422 0.2195 0.2170
DHb (kJ/(kg coal))
DHm (kJ/(kg coal))
106.90 3.896 109.3 73.69 3.547 76.55 49.69 3.453 54.34 17.95 3.426 22.37 13.39 3.433 18.58 11.99 3.480 16.36 3.41 3.760 8.44 The free water exothermic peak disappears 0 5.368 0.2993 0 4.670 0.2571 0 4.346 0.2526 0 2.924 0.06042 0 1.754 0.02966 0 1.677 0.01984 The bound water exothermic peak disappears 0 0 0
Water type (%) Free water
Freezable bound water
Non-freezable water
49.89 35.04 31.94 17.90 14.04 4.51 3.48
24.45 31.69 33.20 40.05 41.94 46.59 47.09
25.66 33.27 34.86 42.04 44.03 48.90 49.43
0 0 0 0 0 0
48.37 42.10 39.65 27.50 12.31 9.39
51.63 57.90 60.35 72.50 87.69 90.61
0
0
100%
DHf – freezing enthalpy of free water; DHb – freezing enthalpy of bound water; DHm – melting enthalpy of ice.
water content in the range between 1.1 and 0.45 g/(g coal) on dry basis. From the slope of straight lines in Fig. 6, enthalpy of free water for HL and YN lignites were calculated as 272.60 kJ/kg and 265.54 kJ/kg, respectively. The enthalpy of free water was similar in HL and YN lignite probably due to similarity in their physical and chemical structures. The enthalpy of freezing of 333.88 kJ/kg
for bulk water at 0 °C has been reported in literature [17]. When the nucleation occurs at temperatures below 0 °C, a correction (Eq. (5)) should be made to the enthalpy of freezing at 0 °C to allow for the heat capacity [17,27].
Fig. 6. Quantity of heat generated by freezing as a function of water content in HL and YN lignites: (a) free water; (b) freezable bound water.
Fig. 7. The comparison of enthalpy of freezing and melting in: (a) HL lignite; (b) YN lignite.
DH ¼ 334 2:05 ðT 273:15Þ
ð5Þ
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where DH is the enthalpy of fusion (kJ/kg) and T (K) is freezing onset temperature. The corrected phase transition enthalpy of free water at 8 °C and 10 °C for HL and YN lignites was calculated as 317.6 kJ/kg and 313.5 kJ/kg, respectively. The difference between the freezing enthalpy of bulk water and ‘‘free water’’ in lignite can be attributed to the interaction of water with coal structure. The interaction between coals and water must be overcome during freezing, and
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therefore part of the heat will be consumed, resulting in a decrease in freezing enthalpy. For water contents ranging between 0.45 and 0.24 g/(g coal) on dry basis and where the peak at 42 °C (freezable bound water) decreases with decreasing water content below 30%, the enthalpies of 10.85 kJ/kg and 21.13 kJ/kg were obtained in HL and YN lignite, respectively (Fig. 6). The DH values calculated for freezable bound water were much smaller compared to free water. Similar results were reported by Fei et al. [17], who reported the specific enthalpy of 18 ± 6 kJ/kg for bound water frozen at 40 °C. When the bound water condensed in coal pores is about to freeze, the coal–water interaction forces (strong hydrogen bonds) that need to be overcome are larger, consuming a significant amount of heat and decreasing the freezing enthalpy. Based on these results, it can be concluded that physical and chemical structures of coal have a significant effect on its association with water. As mentioned above, the ice formed at both around 8 °C and 42 °C, melted at around 0 °C during heating process. To confirm these results, DH of the endothermic peak at 0 °C was calculated. Fig. 7a and b show the enthalpy of freezing and melting of water in HL and YN lignites, respectively. It can be seen that the enthalpy of melting of ice was equivalent to the sum of freezing enthalpies of free and freezable bound waters. Since the heat release during freezing of bound water was significantly smaller than free water, the DH of ice melting was mainly attributed to free water.
3.4. Effect of heat treatment and chemical structure change on coal– water interactions
Fig. 8. The DSC thermograms of heat treated HL lignite under cooling process with original moisture contents of: (1) more than 30%; (2) 20–30%.
The pore structure and oxygen-containing functional groups have a significant impact on association of water with low-rank coals [2]. Modification of these structures may alter the water–coal interactions. The effects of changes in coal structure on its association with water was investigated by drying of coal samples with water contents of above 30% and between 30–20% at 110, 150, and 190 °C under nitrogen. Dried coals were subsequently subjected to water reabsorption. Fig. 8 shows the DSC thermograms of coal samples with different water content which were subjected to drying at different drying temperature and subsequent moisture reabsorption. For comparison, the DSC curves of the coal sample without heat treatment are also given. Fig. 8-1 shows the heat treated coal samples with water contents more than 30%. As expected, two exothermic peaks were observed under cooling process after drying and moisture reabsorption. The first exothermic peak in all the heat treated coal samples appeared at around 8 °C, similar
Fig. 9. The TG, DTG and MS curves of raw HL lignite under heating: (a) DTG; (b) Steam evolution; (c) TG; (d) CO2 evolution.
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to coal samples without heat treatment. This implied that drying and subsequent pore collapse did not affect the reabsorption of free water. However, heat treatment significantly affected the intensity of freezable bound water at 42 °C. The intensity of second peak decreased with increasing heat treatment temperature. The exothermic peak corresponding to freezable bound water was observed in the coal sample dried at 110 °C (Fig. 8-1). However, with further increasing the temperature to 150 and 190 °C, this peak nearly disappeared. When the water content of samples was below 30%, only the second exothermic peak corresponding to freezable bound water was detected (Fig. 8-2). As can be seen, all the DSC curves disappeared after heat treatment of samples at temperatures above 150 °C, implying that moisture reabsorption on these dried samples was rather limited and certainly not to the level detectable for DSC. The decrease in reabsorption of water after heat treatment can be attributed to the changes in coal physical (pore collapse) and chemical (functional groups) structures. The destruction of coal physical structure after drying is well documented in the literature [28–30]. The decrease in the porosity of coal is induced by the irreversible shrinkage in macropores [31,32]. Pore shrinkage and pore emptying occur due to the counteraction of particle contraction and moisture removal [2]. The intensity of pore collapse depends on the drying temperature. Mraw and Naas-O’rourke [8] reported that drying process alters the original pore structure of the lignite, leaving few small or medium-sized water clusters in coal pores. This was attributed to the irreversibility of gel-like pore structure of lignite [28]. The lower tendency of heat treated coal to reabsorb moisture was therefore in part attributed to destruction of its porous structure. The changes in coal chemical structure at high temperatures were investigated by drying of coal and study of gas evolution during drying by using TGA-MS. TG and DTG curves and the changes in the evolution of water vapor and CO2 generated during heating of HL raw coal in the temperature range of 25–300 °C under nitrogen is shown in Fig. 9. The highest weight loss appeared in the temperature range of 30–100 °C and the maximum weight loss rate (DTGmax) appeared at around 50 °C. The weight loss up to 150 °C was mainly due to the removal of moisture. The difference between the peak position of DTG (50 °C) and moisture release (60 °C) was due to the slight delay in MS measurement compared to TG. CO2 evolution increased significantly with increasing the temperature above 150 °C, mainly due to the decomposition of oxygen-containing functional groups such as carboxyl, carbonyl, carboxylate and phenolics. Decomposition of oxygen functional groups and subsequent release of light gases such as CO and CO2 during drying has been reported in literature [33–36]. The decrease in concentration of oxygen functional groups resulted in lower intensity of hydrogen bonds between water molecules and active sites on coal structure, which in turn resulted in higher hydrophobicity and lower water reabsorption tendency in coals dried at high temperatures (Fig. 8).
47.78%, 25.92% and 14.75% were prepared from YN lignite. The XRD patterns of coal samples with different water contents are shown in Fig. 10. The XRD patterns in Fig. 10 show three phases of H2O, SiO2, and Al2(Si2O3)(OH)4. The diffraction peaks of ice were studied. Interestingly, although different types of water in coal were frozen at different temperatures, only hexagonal crystals were formed. This observation was consistent with the simultaneous melting of all types of water at the same temperature around 0 °C during heating process in DSC experiments.
3.5. XRD analysis As discussed above, low-temperature DSC experiments showed two types of freezable water in coals, i.e. free water and freezable bound water. The non-freezable water was not detected by DSC method. However its existence was confirmed by the difference between the water detected during freezing and total water content in coals and also by the DSC thermograms obtained during the heating process. In order to confirm the presence of the nonfreezable water, low-temperature XRD analysis was used to investigate the crystalline structures of ice at 80 °C. Low-temperature X-ray diffraction study of ice structures has been reported in literature [37,38]. Three coal samples with different water contents of
Fig. 10. The XRD patterns of YN lignite with different water contents: (a) 47.78%; (b) 25.92%; (c) 14.75%.
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It should be noted that DSC results showed two exothermic peaks (free and freezable bound water) for coal sample with 47.78%, one exothermic peak (freezable bound water) for coal sample with 25.92%, and no exothermic peaks for coal sample with 14.75%. In XRD analysis for coal samples with 47.78% and 25.92% shown in Fig. 10a and b, respectively, ice crystals were detected due to presence of free and/or freezable bound water. However, surprisingly ice crystals were also detected in samples with 14.75% water content at 80 °C (Fig. 10c). This implies that part of ‘‘freezable bound water’’ in coal sample with moisture content below 18% that was not detected by DSC was in fact in frozen state, contradicting the results obtained in DSC experiments. This finding advances the present knowledge in literature on coal–water interaction. The fact that the third type of water was not detected in DSC thermograms can be attributed to very small amount of heat generated during phase transition of this type of water. It seems that the heat generated during freezing of the third type of water was just enough to offset the very strong coal–water interaction forces. However, the nature of these forces is not fully understood. Based on these results, the classification of the coal water in literature needs to be updated and the difference between freezable and non-freezable water needs to be further investigated.
4. Conclusions (1) Two types of water namely free water and freezable bound water were identified in DSC analysis. The variation of DSC thermograms was similar for the two coals used in this study. The intensity of two exothermic peaks corresponding to free and bound water decreased with decreasing water content. The critical moisture contents at which the free and freezable bound water disappeared were around 30% and 18%, respectively. These results were consistent in both types of coals used and can further improve the understanding of coal–water interaction. The sum of free and bound water accounted for 18.22–78.25% of total amount of water in coal. Therefore, the presence of the third type of water, namely ‘‘non-freezable water’’ was also confirmed. (2) The quantity of heat DH for free and freezable bound water were determined. The enthalpy of free water for HL and YN lignites was 317.6 kJ/kg and 313.5 kJ/kg, respectively. These values were in good agreement with the enthalpy of bulk water at 0 °C (333.88 kJ/kg). The DH values calculated for freezable bound water were much smaller compared to free water. For freezable bound water, enthalpies of 10.85 kJ/kg and 21.13 kJ/kg were obtained in HL and YN lignite, respectively. The low enthalpy of bound water indicated the strong interaction of freezable bound water with coal chemical structure through hydrogen bonds. (3) Heat treatment did not show any significant effect in reabsorption of free water. However, the change in reabsorption of freezable bound water was dramatic. Heat treatment altered the physical (pore structure) and chemical (oxygen functional groups) structure of coal, resulting in significantly reduced reabsorption of bound water. (4) In addition to DSC results, XRD analysis clearly identified the ice crystals in coal samples with the water content less than 18%, indicating that part of the ‘‘freezable bound water’’ that has not been picked-up in DSC analysis was in fact in frozen state at low temperatures. This finding further advanced the present knowledge on the nature of water in coals and their interaction.
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Acknowledgements This study was supported by the Natural Science Foundation of China (21176109, U1361120 and 21210102058). The funding support by Liaoning Outstanding Professorship Funding Program (2012) of Liaoning Province is also gratefully acknowledged. References [1] Karthikeyan M, Zhonghua W, Mujumdar AS. Low-rank coal drying technologies—current status and new developments. Dry Technol 2009;27: 403–15. [2] 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. [3] Allardice DJ, Clemow LM, Favas G, Jackson WR, Marshall M, Sakurovs R. The characterisation of different forms of water in low rank coals and some hydrothermally dried products. Fuel 2003;82:661–7. [4] Karthikeyan M, Kuma JVM, Hoe CS, Ngo DLY. Factors affecting quality of dried low-rank coals. Dry Technol 2007;25:1601–11. [5] Willson WG, Walsh D, Irwin BW. Overview of low-rank coal (LRC) drying. Int J Coal Prep Util 1997;18:1–15. [6] Tahmasebi A, Yu J, Han Y, Zhao H, Bhattacharya S. A kinetic study of microwave and fluidized-bed drying of a Chinese lignite. Chem Eng Res Des 2014;92:54–65. [7] Lynch IJ, Webster DS. Effect of thermal treatment on the interaction of brown coal and water: a nuclear magnetic resonance study. Fuel 1982;61:271–5. [8] Mraw SC, O’Rourke DF. Water in coal pores: the enthalpy of fusion reflects pore size distribution. J Colloid Interf Sci 1982;89:268–71. [9] Allardice DJ. The Water in Brown Coal. Ph.D. Thesis. University of Melbourne. Melbourne, Australia; 1968. [10] Allardice DJ, Evans DG. The brown-coal/water system. part 1. the effect of temperature on the evolution of water from brown coal. Fuel 1971;50:201–10. [11] Allardice DJ, Evans DG. The brown-coal/water system. Part 2. Water sorption isotherms on bed-moist Yallourn brown coal. Fuel 1971;50:236–53. [12] Unsworth JF, Fowler CS, Heard NA, Weldon VL, McBrierty VJ. Moisture in coal 1. Differentiation between forms of moisture by N.M.R. and microwave attenuation techniques. Fuel 1988;67:1111–9. [13] Le Roux M, Campbell QP. An investigation into an improved method of fine coal dewatering. Miner Eng 2003;16:999–1003. [14] Dahlstrom DA, Klepper RP. In: Liu YA, editor. Physical cleaning of coal: present and developing methods. New York: Marcel Dekker; 1982. [15] Norinaga K, Kumagai H, Hayashi J-i, Chiba T. Classification of water sorbed in coal on the basis of congelation characteristics. Energy Fuels 1998;12:574–9. [16] Homshaw LG. High resolution heat flow DSC: application to study of phase transitions, and pore size distribution in saturated porous materials. J Therm Anal 1980;19:215–34. [17] Fei Y, Chaffee AL, Marshall M, Roy Jackson W. Lignite–water interactions studied by phase transition–differential scanning calorimetry. Fuel 2005;84:1557–62. [18] Saikia BK, Boruah RK, Gogoi PK, Baruah BP. A thermal investigation on coals from Assam (India). Fuel Process Technol 2009;90:196–203. [19] Boehm HP. Some aspects of the surface chemistry of carbon blacks and other carbons. Carbon 1994;32:759–69. [20] Hayashi JI, 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;15:903–9. [21] Thomas DG, Staveley LAK. A study of the supercooling of drops of some molecular liquids. J Chem Soc (Resumed) 1952;4569–77. [22] Mraw SC, Naas-O’Rourke DF. Water in coal pores: low-temperature heat capacity behavior of the moisture in Wyodak coal. Science 1979;205:901–2. [23] Yu JS. Coal chemistry. Beijing: Metallurgical Industry Press; 2006. [24] Charriere D, Behra P. Water sorption on coals. J Colloid Interface Sci 2010;344:460–7. [25] Švábová M, Weishauptová Z, Prˇibyl O. Water vapour adsorption on coal. Fuel 2011;90:1892–9. [26] Sheng P, Cohen RW, Schrieffer JR. Melting transition of small molecular clusters. J Phys C: Solid 1981;14:L565. [27] Rasmussen DH, MacKenzie AP. Clustering in supercooled water. J Chem Phys 1973;59:5003–13. [28] Evans DG. Effects of colloidal structure on physical measurements on coals. Fuel 1973;52:155–6. [29] Gan H, Nandi SP, Walker Jr PL. Nature of porosity in American coals. Fuel 1972;51:272–7. [30] Li X, Song H, Wang Q, Meesri C, Wall T, Yu J. Experimental study on drying and moisture re-adsorption kinetics of an Indonesian low rank coal. J Environ Sci Supplement 2009:127–30. [31] Mahidin, Ogaki Y, Usui H, Okuma O. The advantages of vacuum-treatment in the thermal upgrading of low-rank coals on the improvement of dewatering and devolatilization. Fuel Process Technol 2003;84:147–60. [32] Deevi SC, Suuberg EM. Physical changes accompanying drying of western US lignites. Fuel 1987;66:454–60.
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A differential scanning calorimetric (DSC) study on the characteristics and behavior of water in low-rank coals Arash Tahmasebi a, Jianglong Yu a,b,⇑, Huaixing Su a, Yanna Han a, John Lucas b, Hanglin Zheng a, Terry Wall b a Key Lab for Advanced Coal and Coking Technology of Liaoning Province, School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051, China b Chemical Engineering, University of Newcastle, Callaghan, NSW 2308, Australia
h i g h l i g h t s Forms of water in low-rank coals were investigated by DSC and XRD methods. Two types of water namely free water and freezable bound water were detected by DSC. Presence of non-freezable water was also confirmed during heating in DSC. XRD analysis showed that part of ‘‘freezable water’’ was not detected in DSC. The findings advance the present knowledge in literature on coal–water interactions.
a r t i c l e
i n f o
Article history: Received 6 April 2014 Received in revised form 25 June 2014 Accepted 26 June 2014 Available online 9 July 2014 Keywords: Lignite Coal–water interaction Low temperature DSC Freezable water Non-freezable water
a b s t r a c t High moisture content in low-rank coals significantly limits their application and there is a R&D need for advanced drying technologies and upgrading of lignite. Fundamental understanding of coal–water interactions is a critical part of the development of drying technologies. In this study, forms of water in low-rank coals were investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses based on their freezing characteristics. Two types of free and freezable bound water were detected during freezing process in DSC experiments. The amount of free and freezable bound water was experimentally determined from their heat of freezing. These two types of water accounted for 18.22–78.25% of total water in coal, which indicated the presence of the third type of water, namely ‘‘non-freezable water’’. The presence of the third type of water was confirmed in DSC heating process experiments. Moisture reabsorption experiments on heat treated samples showed that the physical and chemical structures of coal have a significant effect on its interaction with water. When coal samples with 14.75% water contents were subjected to low temperature XRD analysis, water in frozen state was detected at 80 °C indicating that part of ‘‘freezable bound water’’ that was not detected by DSC was in frozen state at low temperatures. Amount of heat generated by phase transition of the third type of water for coal samples with water contents less than 18% was very small and therefore was not detected by the DSC. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction It is estimated that nearly 45% of the world coal reserves consist of low-rank coals [1]. Low-rank coals are primary source of energy in developing countries such as China. Low-rank coals contain high moisture content (25–60%) which has a significant effect on downstream utilization processes [2,3]. High water content in lignite ⇑ Corresponding author at: Chemical Engineering, University of Newcastle, Callaghan, NSW 2308, Australia. Tel.: +61 2 40333902; fax: +61 2 40339302. E-mail address: [email protected] (J. Yu). http://dx.doi.org/10.1016/j.fuel.2014.06.068 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
results in a lower efficiency, increased transportation cost, and higher carbon dioxide emissions [4]. On the other hand, low-rank coals have some advantages such as low mining cost, high reactivity, high amount of volatiles, and low pollution-forming impurities such as sulfur, nitrogen, and heavy metals [5,6]. New drying or dewatering technologies need be developed to ensure the competitiveness of lignite in energy market. In order to design and implement advanced drying systems, it is necessary to have a better understanding of the coal–water interactions, particularly the behavior of water in low-rank coals.
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The ease of water removal depends upon the forms of water in low-rank coals. Water is retained in coals in different forms, and it is commonly accepted that water exists in a bound phase and a free phase [7,8]. Researchers have used different definitions for various types of water in coal such as freezable and no-freezable water, free water, pore water, surface water, molecular water, internal water, capillary water, adsorbed water and crystal water [2]. Allardice and co-workers [9–11] suggested that at least two types of water exist in brown coal at any particular temperature: (1) water which can be removed by evacuation, and (2) chemisorbed water which can be released only by raising the temperature to cause thermal decomposition of functional groups. Unsworth et al. [12] used microwave attenuation, pulsed and continuous wave NMR techniques to observe the behavior of water in coals with different ranks as a function of temperature and water content. Their results showed three types of water, i.e., tightly bound water, loosely bound water, and bulk-like water. More detailed classification of water in coal involves [2,13]: (1) Interior adsorption water is contained in micropores and micro-capillaries within each coal particle, deposited during coal formation; (2) Surface adsorption water forms a layer of water molecules adjacent to coal molecules but on the particle surface only; (3) Capillary water is contained in capillaries; (4) Interparticle water is contained in small crevices found between two or more particles; and (5) Adhesion water forms a layer or film around the surface of individual or agglomerated particles. Water types (4) and (5) (i.e. surface moisture) can be removed by mechanical dewatering methods. Water type (3) can be removed partially, depending upon the size of the openings in the coal surface and the drying temperature. Water types (1) and (2) are inherent moisture and can be removed by thermal drying processes [1]. A similar classification was given by Dahlstrom et al. [14] in which water was classified as chemisorbed, physisorbed, micro- and macropore water, inter-particle and intra-particle held water. Norinaga et al. [15] investigated the enthalpy peaks during the cooling process of eight coals, ranging from brown coal to bituminous with the water content ranging from 57.5% to 4.6% by using DSC and NMR methods. They reported two types of freezable water (free water and freezable bound water) and non-freezable water in coals, based on congelation characteristics. In general, the classification methods and terms used in literature for water in coals are not consistent. A coherent classification system is therefore necessary in order to enhance the future study and technology development. The classification system and terms should take into account the nature of the water as well as the interaction of water with coal structure. The changes in different types of water as a function of coal moisture content and the critical moisture contents at which every type of water is removed have not been fully understood in the literature. A better understanding of water–coal interaction is crucial in order to design more advanced and efficient drying technologies. Low-temperature high resolution Differential Scanning Calorimetry (DSC) is a powerful tool for investigating water behavior in porous materials such as lignite [16,17]. In order to better understand the nature of water in coal, characteristics and behavior of water in two lignite coals and interaction of water with coal structure are investigated in this study by using DSC, TGA-MS, and XRD techniques.
Table 1 Proximate and ultimate analyses of raw coal samples. Coal sample
HL
YN
Moisture content (wt.%, ar) Moisture content (wt.%, ad) Volatile matter (wt.%, ad) Ash (wt.%, ad) C, wt.% (daf) H, wt.% (daf) N, wt.% (daf) S, wt.% (daf) O (by diff.), wt.% (daf)
32.10 25.56 33.98 12.12 72.28 5.89 0.93 0.18 20.59
35.00 25.41 37.1 2.30 71.51 4.94 1.03 0.24 22.25
Notes: ar – as received; ad – air dried; daf – dry ash free.
assigned as ‘‘YN’’ and ‘‘HL’’ coals, respectively. The properties of the as-received coals are shown in Table 1. Ba(OH)2, NaOH, HCl and Ca(CH3COO)2 (Sinopharm Chemical Reagent Co. Ltd.) as analytical reagents were used to measure the concentration of functional groups in coal. All coal samples were crushed and sieved to particle size of less than 125 lm. Part of the coal samples were dried under nitrogen at the temperatures of 110, 150, and 190 °C in order to obtain samples with different moisture contents. The water contents of coal samples were determined from the fractional mass loss measured upon drying at 105 °C under nitrogen for 2 h. The as received moisture content of coals was around 32–35%. In order to obtain coals with lower moisture contents, the as received samples were dried in a vacuum drying oven at 30 °C and 0.1 MPa. By controlling the residence time in oven, coal samples with different moisture contents were obtained. In order to obtain moisture contents higher than that of as received amount, the coal samples were soaked in de-ionized water in a sealed beaker at room temperature for 20 min. The water–coal mixture was then filtered for different times to obtain moisture contents in the range of around 32– 50%. The coal samples with different water content were immediately stored in airtight containers for further analysis. The water contents of HL and YN coal samples after treatments ranged between 17.42–50.22% and 17.83–46.10%, respectively. 2.2. DSC analysis DSC experiments were carried out by using a NETZSCH DSC200-F3 with internal cooling system incorporated with Thermal Analysis System Software to control experimental runs. The instrument was also equipped with an external liquid nitrogen cooling accessory. The coal sample (8 mg) was placed in an aluminum sample pan. The pan was then placed in the DSC head together with a reference pan. The sample was cooled down from 25 °C to 150 °C and heated up again to 300 °C at a rate of 3 °C/min under 80 ml/min of nitrogen. It has been reported that the quantity of heat evolved during DSC experiments is independent of the cooling rate in the range of 2–8 °C/min [15]. Experiments were repeated at least twice to ensure the reproducibility of the data and the error in the enthalpies associated with the phase transitions were calculated. 2.3. TGA analysis
2. Experimental 2.1. Sample preparation An Indonesian low-rank coal (supplied by Banpu Public Co. Ltd., Thailand) and a Chinese lignite (supplied by Shenhua Coal Co. from Hulunbeier, Inner Mongolia) were used in this study and were
TGA (NETZSCH STA-449-F3) equipped with NETZSCH QMS-403C was used to study the mass changes and evolution of gases during drying of coal samples. Approximately 10 mg of coal sample (<125 lm size) were used in the TGA experiments. The TGA runs were carried out at the rate of 5 °C/min from 25 °C up to 300 °C under 80 ml/min nitrogen gas flow. The gases generated during the heating process were detected and quantified by MS.
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2.4. XRD analysis XRD was used to characterize the changes in crystalline nature of ice [18]. In order to study the crystal structure of frozen water in coals, the D8 ADVANCE X-ray diffraction equipped with a liquid nitrogen cooling accessory was used. XRD measurements were carried out at 80 °C. The X-ray patterns were recorded with a stepscanning method in the range of 2h = 10–90°.
2.5. Measurement of acidic functional groups The total amount of acidic functional groups in samples was measured to study the difference in chemical structure changes between coal samples [19]. Coal sample was mixed with 0.1 M Ba(OH)2 solution in water bath and was magnetically stirred at 100 °C for 2 h. The hydrochloric acid solution was poured onto the filtrate from the Ba(OH)2 mixture. The mixture was titrated with a standard solution of 0.2 M NaOH solution. The primary procedure to measure the quantity of carboxylic acid groups were similar to the steps above, but the reacting solution was changed to 0.2 M Ca(CH3COO)2 and in the last step the mixture was titrated with 0.1 M NaOH. The blank experiments without coal sample were carried out under the same experimental conditions for calculation of the acidic groups. The amount of phenolic hydroxyl groups was obtained from the difference between the total acidic groups and carboxyl groups.
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the melting of ice and evaporation of water at around 0 °C and between 0 and 100 °C, respectively. Coal samples with different water contents were prepared by partial drying or addition of deionized water into HL lignite sample with initial moisture content of 32.1%. Figs. 2 and 3 show the DSC thermograms of HL lignite with different water contents under cooling and heating processes, respectively. Water content of coal samples ranged between 17.42% and 50.22% on wet basis. The exothermic peaks in Fig. 2 were attributed to the phase transition of the water into ice, i.e., freezing under cooling. As seen in Fig. 2, two sets of exothermic peaks appeared at around 8 °C and 42 °C with decreasing temperature. The appearance of two peaks during the cooling process revealed the presence of at least two distinct types of freezable water, i.e., ‘‘free water’’ (8 °C) and ‘‘freezable bound water’’ (42 °C) [15]. Free water corresponds to intra-particle water and water condensed in large capillaries. Freezable bound water is present in pores with diameters smaller than 10 nm and is associated with coal chemical structure via hydrogen bonding [20]. When the water content in samples decreased from 50.22% to 30.92% the intensity of the peak at 8 °C decreased monotonically. This peak was not observed in
3. Results and discussion 3.1. DSC analysis of coal samples with different water contents DSC analysis of HL and YN lignites with different moisture contents was carried out in order to better understand the forms of water in coal. Fig. 1 shows the DSC thermogram of HL lignite containing 32.1% water. In DSC experiments, temperature was decreased from 25 °C down to 150 °C and then increased up to 300 °C at a rate of 3 °C/min. The DSC thermogram showed four peaks including two exothermic and two endothermic. The two exothermic peaks generated as a result of water freezing were also reported by Norinaga et al. [15] who proposed the concept of free and bound water. The two endothermic peaks were attributed to
Fig. 2. The DSC thermograms during cooling process for HL lignite with different moisture contents of: (a) 50.22%; (b) 42.37%; (c) 40.16%; (d) 37.49%; (e) 33.13%; (f) 31.93%; (g) 30.92%; (h) 29.56%; (i) 27.84%; (j) 27.63%; (k) 24.55%; (l) 21.16%; (m) 17.42%.
Fig. 1. DSC thermogram of HL lignite with 33.13% moisture content.
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Fig. 3. The DSC thermograms during heating process for HL lignite with different moisture contents of: (a) 50.22%; (b) 42.37%; (c) 40.16%; (d) 37.49%; (e) 33.13%; (f) 31.93%; (g) 30.92%; (h) 29.56%; (i) 27.84%; (j) 27.63%; (k) 24.55%; (l) 21.16%; (m) 17.42%.
samples with moisture content less than 29.56%. The low intensity of the second exothermic peak at around 42 °C indicated the freezing of water condensed in capillaries with diameters less than several micrometers [21]. The variation in the size of second peak was negligible in samples with water content of more than 32.1%. However, the intensity of the second peak decreased when moisture content was decreased from 27.84% to 17.42% and disappeared thereafter. Fig. 3 shows the DSC thermograms of HL lignite as a function of moisture content during heating process. Two endothermic peaks appeared in the temperature range of 0–100 °C. The smaller endothermic peak at around 0 °C was generated with melting of ice. The larger endothermic peak in the temperature range of 0–100 °C was attributed to the evaporation of water. It should be noted that although ice was formed at two different temperatures (8 °C and 42 °C), only one exothermic peak corresponding to melting of ice appeared at around 0 °C, suggesting the similarity between the ice crystals from free and freezable bound water. The size of endothermic peaks in the temperature range of 0– 100 °C was proportional to water content in coal samples. As mentioned above, no exothermic peaks were observed for coal samples with moisture content of less than 17.42% during cooling process (Fig. 2). However, during heating process of these samples, an endothermic peak appeared due to evaporation of water at the same temperature range as free water. It seems that 17.42% of the water in coal sample was non-freezable. Therefore, the presence of the third type of water, namely ‘‘non-freezable water’’ was also confirmed. Similar results have been reported by Mraw and Naas-O’rourke [22] by using DSC analysis. However, this conclusion needs to be verified with combination of XRD analysis, as discussed in the subsequent sections. In order to confirm the results obtained for HL lignite, further experiments were carried out on YN lignite. The proximate and ultimate analysis of HL and YN lignites showed the similarity of these two coals. Fig. 4 shows the DSC thermograms of YN lignite during cooling process. Similar to HL lignite, two exothermic peaks were observed for YN lignite. The first and second peaks appeared at around 10 °C and 42 °C, respectively. When the water content in YN coal samples decreased to 30.75%, the first exothermic peak disappeared. However, there was a slight difference in freezing temperature of ‘‘free water’’. The first exothermic peak in YN lignite occurred at 10 °C compared to 8 °C in HL coal. The nature of the mechanism affecting the freezing temperature of water in different types of coals is not fully understood [2]. However, this
Fig. 4. The DSC thermograms during cooling process for YN lignite with different moisture contents of: (a) 46.10%; (b) 39.75%; (c) 38.64%; (d) 34.30%; (e) 33.27%; (f) 30.98%; (g) 30.75%; (h) 29.83%; (i) 27.49%; (j) 26.67%; (k) 20.02%; (l) 19.50%; (m) 17.83%.
Fig. 5. The DSC thermograms during heating process for YN lignite with different moisture contents of: (a) 46.10%; (b) 39.75%; (c) 38.64%; (d) 34.30%; (e) 33.27%; (f) 30.98%; (g) 30.75%; (h) 29.83%; (i) 27.49%; (j) 26.67%; (k) 20.02%; (l) 19.50%; (m) 17.83%.
difference can be attributed to the difference in chemical and physical structure of different types of coals. Fig. 5 shows the DSC thermograms of YN coal samples with different water content during the heating process. Similar to HL coal, the first endothermic peak corresponding to the melting of ice appeared at around 0 °C regardless of the different freezing temperature of two types of freezable water. The larger endothermic peak (due to water evaporation in the range of about 0–100 °C) was also similar to that of HL lignite. 3.2. Effects of coal chemical structure on coal–water interactions The fact that freezing of free water in lignite did not occur at 0 °C but at lower temperatures can be attributed to its association with the chemical structure of lignite. It has been reported that ‘‘free water’’ is present in the spaces between the coal particles or in pores with diameter greater than 105 cm [23]. Allardice [9] reported that with decreasing water content from 60 to 15 g water per l00 g dry coal, the heat of desorption increased gradually and became higher than the latent heat of vaporization of pure water. This implies that part of the water is associated with physical
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and chemical structure of lignite. Water may interact with coal chemical structure via hydrogen bonds, electrostatic force, Debye force, and London dispersion force. These forces are short-range forces, and the long-range forces are formed when the distance between water molecules and coal surface increases. The monolayer water is adsorbed on coal surface mainly by hydrogen bonds. At low water contents, monolayer water is formed around the functional groups and the short-range forces come into effect [24]. With further increasing the water content, water clusters are formed and the long-range forces are formed. The formation of hydrogen bonds is mainly through oxygen-containing functional groups which plays an important role in coal–water interaction [2]. Švábová et al. [25] reported a linear relationship between the sum of the hydroxyl and carboxyl groups and the water uptake capacity of coal, with a high correlation coefficient (R2 > 0.998). The difference in freezing temperature between YN and HL coal was attributed to the difference in their chemical structures and was investigated by measuring the total acidic groups including carboxyl and phenolic hydroxyl in these coals. The results are given in Table 2. The total acidic groups of YN lignite were considerably higher than HL lignite, rendering it more hydrophilic. The higher hydrophilicity of YN lignite increased the strength of forces between water and coal, delaying the freezing of ‘‘free water’’. As a result, the freezing of ‘‘free water’’ occurred at lower temperatures in YN lignite (10 °C) compared to HL lignite (8 °C).
and freezable bound water accounted for about 18.22–78.25% of the total water in coal samples with water contents in the range of 21.16–50.22%. These results confirmed the presence of another type of water for which freezing was not observed, i.e. ‘‘non-freezable water’’. The fraction of non-freezable water estimated by difference is also given in Tables 3 and 4. It has been reported that non-freezable water is retained in small water clusters with less than 10 water molecules involved in the cluster. This type of water in associated with coal structure via hydrogen bonding [15,26]. The symbols ‘‘w’’ and ‘‘X’’ represent water content on a wet and dry basis, respectively. The relationship between these two is shown in Eq. (1). When the exothermic peaks of free and freezable bound water separately disappeared at 30% and 18% water content, the critical points 0.4286 and 0.2195 were defined as X1 and X2. The percentage of free and freezable bound water in coal samples with different water content was calculated from Eqs. (2)–(4), respectively.
X¼
w 1w
ð1Þ
3.3. Quantification of freezing heat
Table 2 The amount of acidic groups of raw coals (mmol/g). Coal
Carboxyl groups
Phenolic hydroxyl groups
Total acidic groups
YN HL
1.21 1.14
10.44 3.15
11.65 4.28
ð2Þ
Water content > 30% : Freezable bound water ð%Þ ¼
Further DSC analysis was carried out on samples with different moisture contents in order to measure the quantity of heat release during freezing process as a function of moisture content. Tables 3 and 4 summarize the quantity of freezing heat and the percentage of free and freezable bound water for HL and YN lignites, respectively. The freezing enthalpy contains two sections, i.e., freezing heats of free water (DHf) and freezable bound water (DHb). DHf and DHb were the average value obtained by integration of the peak areas using DSC thermo-analysis software. The sum of free
X X1 100 X
Free water ð%Þ ¼
X1 X2 100 X
ð3Þ
Water content < 30% : Freezable bound water ð%Þ ¼
X X2 100 X
ð4Þ
The variation of different types of water was similar in HL and YN lignite samples. As discussed above, in case of YN coal with decreasing water content from 46.10% to 30.75%, the percentage of free water gradually decreased and disappeared when the water content in coals reached to around 29.83%. With further decreasing the water content, the freezable bound water also decreased monotonically. Since the exothermic peaks in samples with different water content appeared at similar temperatures, the variation in the size of these peaks reflected the amount of heat and water content. The quantity of heat DH for free and freezable bound water showed a linear relationship with the water content, as can be seen in Fig. 6. For both coals, DH decreased linearly with decreasing
Table 3 Summary of DSC results for HL coal. Water content
DHf (kJ/(kg coal))
w (g/(g coal))
X (g/(g coal))
50.22% 42.37% 40.16% 37.49% 33.13% 31.93% 30.92% 30.00% 29.56% 27.84% 27.63% 24.55% 22.97% 21.16% 18.00% 17.42%
1.009 0.7352 0.6711 0.5997 0.4954 0.4691 0.4476 0.4286 0.4196 0.3858 0.3818 0.3254 0.2982 0.2684 0.2195 0.2109
DHb (kJ/(kg coal))
DHm (kJ/(kg coal))
158.4 2.873 161.2 75.64 2.480 75.53 54.23 2.422 55.77 45.45 2.289 45.94 16.82 2.185 20.46 9.249 2.167 11.59 3.625 2.501 6.541 The free water exothermic peak disappears 0 2.665 0.2510 0 2.138 0.1940 0 1.926 0.2006 0 1.345 0.0740 0 1.175 0.05443 0 0.9836 0.03931 The bound water exothermic peak disappears 0 0 0
Water type (%) Free water
Freezable bound water
Non-freezable water
57.52 41.70 36.14 28.54 13.49 8.63 4.24
20.73 28.44 31.16 34.86 42.21 44.58 46.72
21.76 29.86 32.71 36.60 44.30 46.79 49.04
0 0 0 0 0 0
47.69 43.11 42.51 32.54 26.39 18.22
52.31 56.89 57.49 67.46 73.61 81.78
0
0
100.00%
DHf – freezing enthalpy of free water; DHb – freezing enthalpy of bound water; DHm – melting enthalpy of ice.
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Table 4 Summary of DSC results for YN coal. Water content
DHf (kJ/(kg coal))
w (g/(g coal))
X (g/(g coal))
46.10% 39.75% 38.64% 34.30% 33.27% 30.98% 30.75% 30.00% 29.83% 27.49% 26.67% 23.24% 20.02% 19.50% 18.00% 17.83%
0.8553 0.6598 0.6297 0.5221 0.4986 0.4489 0.4440 0.4286 0.4251 0.3791 0.3637 0.3028 0.2503 0.2422 0.2195 0.2170
DHb (kJ/(kg coal))
DHm (kJ/(kg coal))
106.90 3.896 109.3 73.69 3.547 76.55 49.69 3.453 54.34 17.95 3.426 22.37 13.39 3.433 18.58 11.99 3.480 16.36 3.41 3.760 8.44 The free water exothermic peak disappears 0 5.368 0.2993 0 4.670 0.2571 0 4.346 0.2526 0 2.924 0.06042 0 1.754 0.02966 0 1.677 0.01984 The bound water exothermic peak disappears 0 0 0
Water type (%) Free water
Freezable bound water
Non-freezable water
49.89 35.04 31.94 17.90 14.04 4.51 3.48
24.45 31.69 33.20 40.05 41.94 46.59 47.09
25.66 33.27 34.86 42.04 44.03 48.90 49.43
0 0 0 0 0 0
48.37 42.10 39.65 27.50 12.31 9.39
51.63 57.90 60.35 72.50 87.69 90.61
0
0
100%
DHf – freezing enthalpy of free water; DHb – freezing enthalpy of bound water; DHm – melting enthalpy of ice.
water content in the range between 1.1 and 0.45 g/(g coal) on dry basis. From the slope of straight lines in Fig. 6, enthalpy of free water for HL and YN lignites were calculated as 272.60 kJ/kg and 265.54 kJ/kg, respectively. The enthalpy of free water was similar in HL and YN lignite probably due to similarity in their physical and chemical structures. The enthalpy of freezing of 333.88 kJ/kg
for bulk water at 0 °C has been reported in literature [17]. When the nucleation occurs at temperatures below 0 °C, a correction (Eq. (5)) should be made to the enthalpy of freezing at 0 °C to allow for the heat capacity [17,27].
Fig. 6. Quantity of heat generated by freezing as a function of water content in HL and YN lignites: (a) free water; (b) freezable bound water.
Fig. 7. The comparison of enthalpy of freezing and melting in: (a) HL lignite; (b) YN lignite.
DH ¼ 334 2:05 ðT 273:15Þ
ð5Þ
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where DH is the enthalpy of fusion (kJ/kg) and T (K) is freezing onset temperature. The corrected phase transition enthalpy of free water at 8 °C and 10 °C for HL and YN lignites was calculated as 317.6 kJ/kg and 313.5 kJ/kg, respectively. The difference between the freezing enthalpy of bulk water and ‘‘free water’’ in lignite can be attributed to the interaction of water with coal structure. The interaction between coals and water must be overcome during freezing, and
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therefore part of the heat will be consumed, resulting in a decrease in freezing enthalpy. For water contents ranging between 0.45 and 0.24 g/(g coal) on dry basis and where the peak at 42 °C (freezable bound water) decreases with decreasing water content below 30%, the enthalpies of 10.85 kJ/kg and 21.13 kJ/kg were obtained in HL and YN lignite, respectively (Fig. 6). The DH values calculated for freezable bound water were much smaller compared to free water. Similar results were reported by Fei et al. [17], who reported the specific enthalpy of 18 ± 6 kJ/kg for bound water frozen at 40 °C. When the bound water condensed in coal pores is about to freeze, the coal–water interaction forces (strong hydrogen bonds) that need to be overcome are larger, consuming a significant amount of heat and decreasing the freezing enthalpy. Based on these results, it can be concluded that physical and chemical structures of coal have a significant effect on its association with water. As mentioned above, the ice formed at both around 8 °C and 42 °C, melted at around 0 °C during heating process. To confirm these results, DH of the endothermic peak at 0 °C was calculated. Fig. 7a and b show the enthalpy of freezing and melting of water in HL and YN lignites, respectively. It can be seen that the enthalpy of melting of ice was equivalent to the sum of freezing enthalpies of free and freezable bound waters. Since the heat release during freezing of bound water was significantly smaller than free water, the DH of ice melting was mainly attributed to free water.
3.4. Effect of heat treatment and chemical structure change on coal– water interactions
Fig. 8. The DSC thermograms of heat treated HL lignite under cooling process with original moisture contents of: (1) more than 30%; (2) 20–30%.
The pore structure and oxygen-containing functional groups have a significant impact on association of water with low-rank coals [2]. Modification of these structures may alter the water–coal interactions. The effects of changes in coal structure on its association with water was investigated by drying of coal samples with water contents of above 30% and between 30–20% at 110, 150, and 190 °C under nitrogen. Dried coals were subsequently subjected to water reabsorption. Fig. 8 shows the DSC thermograms of coal samples with different water content which were subjected to drying at different drying temperature and subsequent moisture reabsorption. For comparison, the DSC curves of the coal sample without heat treatment are also given. Fig. 8-1 shows the heat treated coal samples with water contents more than 30%. As expected, two exothermic peaks were observed under cooling process after drying and moisture reabsorption. The first exothermic peak in all the heat treated coal samples appeared at around 8 °C, similar
Fig. 9. The TG, DTG and MS curves of raw HL lignite under heating: (a) DTG; (b) Steam evolution; (c) TG; (d) CO2 evolution.
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to coal samples without heat treatment. This implied that drying and subsequent pore collapse did not affect the reabsorption of free water. However, heat treatment significantly affected the intensity of freezable bound water at 42 °C. The intensity of second peak decreased with increasing heat treatment temperature. The exothermic peak corresponding to freezable bound water was observed in the coal sample dried at 110 °C (Fig. 8-1). However, with further increasing the temperature to 150 and 190 °C, this peak nearly disappeared. When the water content of samples was below 30%, only the second exothermic peak corresponding to freezable bound water was detected (Fig. 8-2). As can be seen, all the DSC curves disappeared after heat treatment of samples at temperatures above 150 °C, implying that moisture reabsorption on these dried samples was rather limited and certainly not to the level detectable for DSC. The decrease in reabsorption of water after heat treatment can be attributed to the changes in coal physical (pore collapse) and chemical (functional groups) structures. The destruction of coal physical structure after drying is well documented in the literature [28–30]. The decrease in the porosity of coal is induced by the irreversible shrinkage in macropores [31,32]. Pore shrinkage and pore emptying occur due to the counteraction of particle contraction and moisture removal [2]. The intensity of pore collapse depends on the drying temperature. Mraw and Naas-O’rourke [8] reported that drying process alters the original pore structure of the lignite, leaving few small or medium-sized water clusters in coal pores. This was attributed to the irreversibility of gel-like pore structure of lignite [28]. The lower tendency of heat treated coal to reabsorb moisture was therefore in part attributed to destruction of its porous structure. The changes in coal chemical structure at high temperatures were investigated by drying of coal and study of gas evolution during drying by using TGA-MS. TG and DTG curves and the changes in the evolution of water vapor and CO2 generated during heating of HL raw coal in the temperature range of 25–300 °C under nitrogen is shown in Fig. 9. The highest weight loss appeared in the temperature range of 30–100 °C and the maximum weight loss rate (DTGmax) appeared at around 50 °C. The weight loss up to 150 °C was mainly due to the removal of moisture. The difference between the peak position of DTG (50 °C) and moisture release (60 °C) was due to the slight delay in MS measurement compared to TG. CO2 evolution increased significantly with increasing the temperature above 150 °C, mainly due to the decomposition of oxygen-containing functional groups such as carboxyl, carbonyl, carboxylate and phenolics. Decomposition of oxygen functional groups and subsequent release of light gases such as CO and CO2 during drying has been reported in literature [33–36]. The decrease in concentration of oxygen functional groups resulted in lower intensity of hydrogen bonds between water molecules and active sites on coal structure, which in turn resulted in higher hydrophobicity and lower water reabsorption tendency in coals dried at high temperatures (Fig. 8).
47.78%, 25.92% and 14.75% were prepared from YN lignite. The XRD patterns of coal samples with different water contents are shown in Fig. 10. The XRD patterns in Fig. 10 show three phases of H2O, SiO2, and Al2(Si2O3)(OH)4. The diffraction peaks of ice were studied. Interestingly, although different types of water in coal were frozen at different temperatures, only hexagonal crystals were formed. This observation was consistent with the simultaneous melting of all types of water at the same temperature around 0 °C during heating process in DSC experiments.
3.5. XRD analysis As discussed above, low-temperature DSC experiments showed two types of freezable water in coals, i.e. free water and freezable bound water. The non-freezable water was not detected by DSC method. However its existence was confirmed by the difference between the water detected during freezing and total water content in coals and also by the DSC thermograms obtained during the heating process. In order to confirm the presence of the nonfreezable water, low-temperature XRD analysis was used to investigate the crystalline structures of ice at 80 °C. Low-temperature X-ray diffraction study of ice structures has been reported in literature [37,38]. Three coal samples with different water contents of
Fig. 10. The XRD patterns of YN lignite with different water contents: (a) 47.78%; (b) 25.92%; (c) 14.75%.
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It should be noted that DSC results showed two exothermic peaks (free and freezable bound water) for coal sample with 47.78%, one exothermic peak (freezable bound water) for coal sample with 25.92%, and no exothermic peaks for coal sample with 14.75%. In XRD analysis for coal samples with 47.78% and 25.92% shown in Fig. 10a and b, respectively, ice crystals were detected due to presence of free and/or freezable bound water. However, surprisingly ice crystals were also detected in samples with 14.75% water content at 80 °C (Fig. 10c). This implies that part of ‘‘freezable bound water’’ in coal sample with moisture content below 18% that was not detected by DSC was in fact in frozen state, contradicting the results obtained in DSC experiments. This finding advances the present knowledge in literature on coal–water interaction. The fact that the third type of water was not detected in DSC thermograms can be attributed to very small amount of heat generated during phase transition of this type of water. It seems that the heat generated during freezing of the third type of water was just enough to offset the very strong coal–water interaction forces. However, the nature of these forces is not fully understood. Based on these results, the classification of the coal water in literature needs to be updated and the difference between freezable and non-freezable water needs to be further investigated.
4. Conclusions (1) Two types of water namely free water and freezable bound water were identified in DSC analysis. The variation of DSC thermograms was similar for the two coals used in this study. The intensity of two exothermic peaks corresponding to free and bound water decreased with decreasing water content. The critical moisture contents at which the free and freezable bound water disappeared were around 30% and 18%, respectively. These results were consistent in both types of coals used and can further improve the understanding of coal–water interaction. The sum of free and bound water accounted for 18.22–78.25% of total amount of water in coal. Therefore, the presence of the third type of water, namely ‘‘non-freezable water’’ was also confirmed. (2) The quantity of heat DH for free and freezable bound water were determined. The enthalpy of free water for HL and YN lignites was 317.6 kJ/kg and 313.5 kJ/kg, respectively. These values were in good agreement with the enthalpy of bulk water at 0 °C (333.88 kJ/kg). The DH values calculated for freezable bound water were much smaller compared to free water. For freezable bound water, enthalpies of 10.85 kJ/kg and 21.13 kJ/kg were obtained in HL and YN lignite, respectively. The low enthalpy of bound water indicated the strong interaction of freezable bound water with coal chemical structure through hydrogen bonds. (3) Heat treatment did not show any significant effect in reabsorption of free water. However, the change in reabsorption of freezable bound water was dramatic. Heat treatment altered the physical (pore structure) and chemical (oxygen functional groups) structure of coal, resulting in significantly reduced reabsorption of bound water. (4) In addition to DSC results, XRD analysis clearly identified the ice crystals in coal samples with the water content less than 18%, indicating that part of the ‘‘freezable bound water’’ that has not been picked-up in DSC analysis was in fact in frozen state at low temperatures. This finding further advanced the present knowledge on the nature of water in coals and their interaction.
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