Abstract
The high moisture content of lignite restricts its extensive and efficient use. Furthermore, the reabsorption of lignite is also a factor that affects lignite spontaneous combustion. Therefore, it is of great importance to study the process and mechanism of water molecule desorption and adsorption on lignite and coke (25–950°C) to achieve the clean and efficient utilization of lignite and environmental protection. Proton nuclear magnetic resonance (1H-NMR), thermogravimetric analysis, and other techniques were used in this study to explore the water molecule absorption and desorption processes of lignite pyrolysis at different temperatures (25–950°C) and the special contributions of ether bonds to water molecule adsorption. A mechanism of lignite water molecule adsorption was proposed. The results showed that ether bonds played a special role in the water molecule adsorption by pyrolyzed lignite. The ether bond content was greater in the coal samples at 300 and 950°C, which changed the trend of lignite water molecule absorption and the distribution of water (T2) detected in the 1H-NMR experiments and delayed the escape of water molecules during moisture desorption. The total amount of adsorbed water decreased first and then increased in the coal samples as the pyrolysis temperature increased. However, the maximum adsorption interactions of each coal sample increased first and then decreased. This was the result of the interactions between the pores and the oxygen-containing functional groups. Based on the above analysis, water molecule adsorption mechanism models of lignite and coke were constructed. This study offers a new approach for investigating the water molecule adsorption and adsorption mechanisms of lignite and coke.
1. Introduction
The energy crisis is becoming more and more serious around the world. Lignite is an important coal reserve. However, owing to its high moisture content, low heat value, easy weathering, and spontaneous combustion, the utilization efficiency of lignite is relatively low [1]. At present, the common method to improve the utilization efficiency is lignite upgrading, but the current technologies fail to solve the problems of moisture adsorption and spontaneous combustion of lignite [2].
The microstructural and adsorption characteristics of lignite have been the focus of many scholars. Tao et al. [3] compared the material composition, pore structure, and adsorption capacity of lignite and candle coal and found that lignite has a higher inertinite content, larger pore volume, better connectivity, and greater specific surface area (SSA) than candle coal. In another article, a continuous distribution model of pore space was constructed for coal reservoirs based on the measured data of mercury intrusion porosimetry (MIP) and low-temperature nitrogen adsorption (LTNA) experiments. This model can obtain the complete pore size distribution from nanopores to microfractures [4]. Series and parallel seepage simulations of coal matrix pore samples, natural fracture samples, and artificial fracture samples with three permeability ranges were designed and performed [5]. Another analysis showed that after water immersion, the content of hydroxyl and aromatic hydrocarbons in the coal increased significantly, and the temperature at which fat-based and oxygen-containing functional groups participated in the reaction decreased [6]. The monolayer water in soft brown coal comprises only about one twentieth of the total water present, and it is attached to coal by hydrogen bonds in suitable polar functional groups [7]. Gutierrez-Rodriguez et al. [8] estimated the hydrophobicity of coal. In recent years, the research on this subject has focused more on the adsorption effect of the functional groups on water. Liu et al. [9] suggested that the potential to form more hydrogen bonds is the key factor influencing the interaction energy between model compounds and water molecules. The behavior of water in coal depends on many factors, such as intermolecular hydrogen bonding [10]. Water molecules interact with the oxygen-containing functional groups on the surface of coal by hydrogen bonds [11–14]. The research on the moisture adsorption of lignite has mainly focused on the water occurrence state and the influence of -OH, C=O, and other functional groups [7, 15–17]. However, there are few studies on the effect of a single functional group, such as -O-, on water molecule adsorption in the existing literature.
In this study, the pore distribution, the effect of oxygen-containing functional groups on water molecule adsorption, and the relationship between the oxygen-containing functional groups, pores, and water molecule adsorption were examined [18]. Based on previous studies, the influence of ether bonds on the water molecule adsorption in lignite and coke (<950°C) was investigated through proton nuclear magnetic resonance (1H-NMR) and water molecule absorption and desorption experiments to lay a foundation for the inhibition of lignite reabsorption and spontaneous combustion.
2. Method
All of the data obtained in this experiment were collected through three parallel experiments, and their average values were calculated and reported as the final values.
2.1. Sample Preparation
Lignite samples were collected from the Shengli Mine of China. The coal samples were crushed and screened, and those with particle sizes of 200–400 mesh were stored for future use. Raw coal was stored directly at room temperature (25°C), and other coal samples were extracted by pyrolysis in a fixed-bed reactor. Coal samples were selected and heated to 50, 75, 105, 200, 300, 500, 700, and 950°C at 5°C/min under a nitrogen atmosphere for 1 h. The obtained sample was labeled with the corresponding temperature, and the temperature label of the raw coal (untreated coal sample) was 25°C. Table 1 displays the results of the proximate and ultimate analyses of all coal samples obtained by an industrial analyzer (5E-MAG6700), an element analyzer (5E-CHN2000), and an infrared sulfur meter (5E-IRS II).
2.2. Moisture Adsorption and Desorption
The moisture adsorption experiments were performed in an SHBY-40B standard curing box (manufactured by Jiangsu Wuxi Southern China Experimental Instrument Inc.). The coal samples were spread on a 35 mm culture dish, which was then placed into a constant-temperature (30°C) and constant-humidity (85%) box. The first 6 h was divided into three phases, with 2 h for each phase and an interval of 30 min. The samples were weighed every 1 or 2 h. Equilibrium moisture adsorption was reached if the difference between the two adjacent weights was smaller than 0.01 g. The adsorption of water and the equilibrium adsorption ratio were calculated as follows:where m0 is the mass of the lignite sample and mt is the mass of the lignite sample after water absorption.
Lignite and coke samples after the adsorption of water molecules were desorbed with a temperature rise rate of 2°C/min and a temperature range of 25–200°C. A thermogravimetric analyzer was used to obtain the thermogravimetry (TG), derivative thermogravimetry (DTG), and differential thermal analysis (DTA) curves of the desorbed lignite after adsorption.
2.3. Proton Nuclear Magnetic Resonance (1H-NMR)
1H-NMR (VTMR20-010V-T; Shanghai Niumag) was performed to test the water contents of the samples. The testing parameters were a resonance frequency of 21.306 MHz, magnetic intensity of 0.5 T, coil diameter of 10 mm, and a magnetic temperature of 35.00°C. The sample signal values were collected using NMR analysis software, and Carr–Purcell–Meiboom–Gill (CPMG) sequences were obtained. On this basis, the T2 spectrum was obtained through inversion with the simultaneous iterative reconstruction technique (SIRT). The coal samples (1–1.25 g) were placed into a detector oven, and the detection limit of water was 10 mg.
The coal samples were immersed in water for 24 h under standard conditions to fill all the pores with water such that they became saturated water samples. The coal samples were placed in an environment with a specified humidity, and water was adsorbed to the surface of the lignite. This kind coal sample is called an adsorbed water sample.
The interaction strength between the lignite and water molecules is expressed as a transverse relaxation time T2 (ms), as follows [19]:where Mxy is the component of the macroscopic magnetization vector on the x-y axis, M0 is the initial magnetization vector, and T2 is the transverse relaxation time. The interaction strength Mxy is related to the relaxation time t by equation (2).
2.4. Oxygen-Containing Functional Groups
Oxygen-containing functional groups were tested using Fourier-transform infrared spectroscopy (FTIR, NEXUS670; American Nicolet Corporation). The KBr method was performed to prepare the samples. The samples and KBr were mixed at a ratio of 1 : 120. Infrared spectra were obtained within the range of 400–4000 cm−1, and the scanning time was set at 25 s. The resolving power and wavenumber accuracy were 0.125 and 0.001 cm−1, respectively. The calculation of the content of oxygen-containing functional groups is shown in Supplementary Materials S1.
2.5. Microstructure of Lignite Coke
The specific surface areas and pore distributions of the coal samples were determined by a low-temperature nitrogen adsorption method using a 3H-2000PS2 tester from the Baystar Instrument Technology (Beijing) Co., Ltd. The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were used to calculate the specific surface area, pore volume, and pore distribution for each sample. The surface morphologies and particle sizes of the coal samples were observed using scanning electron microscopy (SEM, s-3400N, Hitachi company). The accelerating voltage was 20 kV, and backscattered electron imaging was performed.
3. Results and Discussion
3.1. Abnormal Changes of Moisture Absorption and Desorption in Lignite
3.1.1. Moisture Content of Coal Samples after Equilibrium Adsorption in Difference Method
In this study, we found that under conditions with a constant temperature of 30°C and humidity of 85% and after the coal samples absorb water to achieve equilibrium adsorption, the moisture content of coal samples decreased first and then increased with the increase in treatment temperature (Figure 1). The moisture content of raw coal at 25°C is the highest, reaching 26.44%. The moisture content of heat-treated coal sample at 105°C was higher than that at 75°C, which may be due to the exposure of functional groups after the escape of water molecules with strong binding force. Comparing Figure 1 with Table 1, we can see that the coal sample heat treated at 105°C achieved a maximum adsorption rate of 22.80%. At higher heat-treatment temperatures, the adsorption rate decreased. In the 200–300°C range, the coal sample equilibrium adsorption rate was relatively low, and at 700°C, the heat-treated coal sample equilibrium adsorption rate was the lowest, with a value of 12.66%, and the moisture content was 13.28%. At 950°C, the equilibrium adsorption rates of the coal samples increased up to 22.82%, and the moisture content was 23.29%. The moisture adsorption capacities of the coke at 300 and 950°C were abnormal as well (Figure 1: A300 and B950).
3.1.2. Moisture Content of Coal Samples after Equilibrium Adsorption in 1H-NMR Analysis
Table 2 shows the T2 distribution and spectral intensity of the equilibrium adsorbed water. The total water content (Atotal) decreased with the increase in the temperature below 700°C. The water content of the coal sample increased at 950°C (Figure 2).
When the heat-treatment temperature of the coal sample was lower than 105°C, the total amount of adsorbed water (Atotal) of the coal sample decreased with the increase in the heat-treatment temperature (Table 2 and Figure 2). This is similar to the trend in Figure 1. However, the coal sample heated at 105°C showed its particularity, which may be caused by its strong binding force with water molecules after the exposure of the functional group, which cannot be detected by 1H-NMR. When the heat-treatment temperature was 105–700°C, the amount of adsorbed water decreased, the rate of decrease of the amount of adsorbed water of the coal sample heated at 300°C slowed (Figure 2: A300), and the amount of adsorbed water of the coal sample heated at 950°C increased sharply (Figure 2: B950). With the increase in the heat-treatment temperature, the coal samples for heat treatment at 300 and 950°C showed abnormal moisture adsorption.
3.1.3. Water Desorption
The TG curves of the water desorption from lignite after adsorption were obtained by desorption experiments (Figure 3). The amount of desorbed water of raw coal (25°C) after water absorption was the largest, approaching 20%. As the pyrolysis temperature of lignite increased, the amount of desorbed water of lignite decreased gradually. The amounts of desorbed water of the coal samples treated at 50, 75, and 105°C were almost the same, and those of the coal samples treated at 300 and 950°C were abnormal. The amount of desorbed water of the coal sample treated at 300°C was greater than that of coal sample treated at 200°C, and the amount of desorbed water of the coal sample treated at 950°C was greater than that of coal sample treated at 500°C, which was consistent with the results of the adsorption process described above (Figures 1 and 2).
The corresponding DTA (Figure 4) and DTG (Figure 5) data were further analyzed. Most of the adsorbed water was removed at 80°C. Small amounts of water in the coal samples at 300 and 950°C needed more energy to release from the sample surfaces, and the removal temperature was around 100°C. Strong binding interactions occurred in the coal samples heated at 300°C, similar to that in the coal samples heated at 950°C.
(a)
(b)
(a)
(b)
In the moisture absorption and desorption experiments of lignite coke, with the increase in the heat-treatment temperature, the equilibrium amount of adsorbed water of the coal coke increased first (<105°C), then decreased (105–700°C), and then increased dramatically (700–950°C). At the same time, the coal samples for heat treatment at 300 and 950°C showed abnormal moisture adsorption behaviors. Water desorption experiments showed that some of the water in the coal samples for heat treatment at 300 and 950°C was difficult to remove, and the water had relatively stronger bonds.
3.2. State of Adsorbed Water Occurrence
1H-NMR was used to investigate the differences in the state of adsorbed water under saturated and unsaturated conditions. The primary moisture adsorbed on the surfaces of the lignite and coke was detected by 1H-NMR, and the peak shifts of the adsorption curves were used to show the main binding interaction. Figure 6 shows the water molecule adsorption capacities of the coke samples with different degrees of pyrolysis. When the treatment temperature was below 500°C, the relaxation time (T2) moved to the left as the temperature increased (curve a-b in Figure 6). T2 showed a decreasing trend, but the H2O adsorption capacity was increasing. T2 reflects the degrees of freedom of the protons in the binding interactions between coal and water molecules, which was gradually increasing. The value of T2 at 700 and 950°C shifted to the right (curve b-c in Figure 6), and the binding interactions of coal samples with adsorbed water molecules decreased. In the pyrolysis process, the oxygen-containing functional groups with weak binding interactions escaped first as the temperature increased. The residual binding interactions in coal were strong, and the absorption of water molecules was strong. However, the total amount of adsorption was reduced. Therefore, the maximum adsorption interactions of each coal sample increased first and then decreased, and this process was closely related to the evolution of oxygen-containing functional groups [20].
The distribution T2 in the saturated water lignite samples was investigated (Figure 7). The T2 values of the coal samples at 700 and 950°C significantly shifted to the right, indicating that the water molecule binding interactions in the coal samples were weak, and the relaxation times were long.
(a)
(b)
Meanwhile, the T2 of the coal sample heated at 950°C shifted to the left relative to that of the coal sample heated at 700°C, and there were significant peaks at relaxation times between 0.2 and 0.5 ms (Figure 7(a): B950). This indicated that the adsorption capacity of the coal sample heated at 950°C was different from that of the coal sample heated at 700°C. The T2 distribution in the adsorbed of lignite was investigated (Figure 7(b)). The peak of the coal sample heated at 950°C was significantly different from that of the coal sample heated at 700°C for relaxation times between 0.2 and 0.5 ms (Figure 7(b): B950). Based on these results and the 1H-NMR results described above, there were strong binding interactions in the coal sample heated at 950°C, similar to that in the coal sample heated at 300°C (Figure 7).
The results of the 1H-NMR experiment and lignite coke absorption and desorption experiments had good consistency. With the increase in the heat-treatment temperature, the structure of the lignite coke changed at around 300 and 950°C, which led to an enhanced binding effect of the lignite coke with some of the water molecules, a rise in the desorption temperature, and a relative increase in the amount of moisture adsorbed.
3.3. Physical and Chemical Structure Changes during Lignite Pyrolysis
3.3.1. Oxygen-Containing Functional Groups
Infrared analysis was used to detect the changes of the oxygen-containing functional groups in the lignite coke [21]. The calculation of the content of oxygen-containing functional groups is shown in Supplementary Materials S1.
The oxygen-containing functional group content in the lignite increased first and then decreased (Figure 8), and the change of the water molecule adsorption quantity (Figures 1 and 2) was in good agreement. In particular, the ether bonds at 300 and 950°C were abnormal. In the pyrolysis process, the content of ether bonds generally showed a downward trend, but at 300 and 950°C, the downward trend slowed and even changed to an upward trend (Figure 8(c): A300, B950), which was consistent with previous experimental results [22]. The moisture adsorption capacity of the 300 and 950°C lignite coke (Figure 1: A300, B950; Figure 2: A300, B950) was consistent with the changes of the ether bond content. This indicated that the increase in the ether bond contents in the coal samples at 300 and 950°C was an important reason for the increase of the adsorbed moisture in the coal coke.
(a)
(b)
(c)
Meanwhile, the 1H-NMR data showed that a small amount of moisture with strong adsorption was present in the lignite coke at 300 and 950°C (Figure 7), and there was also a small amount of moisture present during the process of water desorption, whose removal temperature was higher than that of the other water (Figures 3, 5, and 6). Based on the above analysis, the generation of ether bonds in the lignite coke at 300 and 950°C enhanced the binding of the lignite coke surface to some of the moisture.
3.3.2. Pore Size Distribution
The pore distributions of coal samples were determined by 1H-NMR, and the pore size distribution is shown in Figure 9 and Table 3.
The microstructural evolution of the heat-treated lignite was further investigated using SEM (Figure 10). The lignite coke below the heat-treatment temperature of 200°C exhibited no chemical differences from the raw coal. With the removal of water in the pores, more pores were exposed, resulting in a significant increase in the pore content. When the heat-treatment temperature was higher than 200°C, the heat-treatment process caused significant weight loss and gas escape [18]. In particular, condensation polymerization occurred at about 300°C, resulting in ether bonds and pore contraction (Figure 10). Under 500°C heat treatment, the number of lignite coke pores increased (Table 3 and Figures 9 and 10). At this point, the bridge bond fracture reached its maximum value. The porosity of lignite coke treated at 700°C continued to increase (Table 3 and Figures 9 and 10), and it increased significantly, which was caused by condensation polymerization [23]. When the heat-treatment temperature was 950°C, the proportion of pores below 10 nm was significantly reduced, and the pore size distribution moved toward the direction of large pores, which also indicated that condensation polymerization and pore collapse occurred at this stage.
(a)
(b)
(c)
(d)
(e)
Based on the above changes of the amount of water molecule adsorption and the water binding state in the lignite coke, as well as the physical and chemical changes that occurred in the lignite coke heat-treatment process, it was inferred that the abnormal changes of the water molecule adsorption on the lignite coke heated at 300 and 950°C were caused by the formation of ether bonds and pore changes.
3.4. H2O Adsorption Mechanism of Lignite and Coke
According to above analysis and previous research [18], the water molecule adsorption mechanisms of lignite and coke were determined, and they are shown in Figure 11.
(a)
(b)
When the heat-treatment temperature was below 105°C, with the increase in temperature, moisture escaped and oxygen-containing functional groups were exposed in the lignite. The number of active sites or oxygen-containing functional groups was the highest [18]. The moisture was absorbed on the active sites, which led to increased water molecule absorption in lignite in this temperature range. However, most of the absorbed water molecules were single layered. This was associated with a strong absorption force. The relaxation time (T2) decreased, and the pre−105°C heat treatment of the coal sample moisture relaxation time decreased (Figure 6). When the heat-treatment temperature was 105–500°C, the content of oxygen functional groups -OH and C=O continued to decrease and the adsorbed moisture decreased with the increase in temperature. However, the bonding interactions of water molecules were relatively strong, with relaxation time decreasing (Figure 6). The increase in the ether bonds in the coal samples at 300°C slowed the decreasing trend of water molecule absorption (Figures 1 and 2). The oxygen-containing functional groups (OH and -O-) of the coal samples heated at 500 and 700°C continued to decrease, and C=O disappeared (Figure 8). As a result, an insufficient number of water molecules were adsorbed to form the first layer water. Significant multilayer water appears in 500°C coal samples (Figure 6). The amount of water absorption in the 500°C and 700°C coal samples was relatively small for all the coal samples, as the content of oxygen functional groups in those two samples was small (Figures 1 and 2). However, the T2 of the lignite coke heated at 500°C was the shortest, and the adsorption force was the strongest (Figure 6). The coal sample heated at 950°C contained a certain number of ether bonds, and the water molecule absorption increased significantly (Figures 1 and 2). However, the maximum adsorption force was the weakest (Figure 6), because the functional groups were almost gone (Figure 8).
Through comprehensive consideration of the pore changes [18], it was found that the cracks were larger (Figures 7(a) and 9) and the multilayer water was not easy to retain. The moisture state of the coal sample heated at 700°C changed, and the amount of multilayer adsorption increased (Figure 7(b)). There should be a critical point between 500 and 700°C at which the moisture adsorption changes from a single layer to multiple layers (Figure 6). Although there was little -OH in 950°C coal samples, the -O- content increased (Figure 8). Meanwhile, large-aperture crannies disappeared (Figures 7(a) and 9), and the apertures (20–1000 nm) were uniformly distributed (Figures 7(a) and 9). As the numbers of -OH and -O- groups were relatively small (1000 and 63, respectively), there was less single-layer water, and most of the adsorbed water was multilayer. At the same time, owing to the well-distributed apertures compared to those of the coal sample treated at 700°C, the multilayer water could be retained. As a result, the number of adsorbed water molecules of the coal sample heated at 950°C surpassed those of the samples heated at 700 and 500°C (Figures 1 and 2). The uniform distribution of pore diameters allowed the multilayer water to be retained, which was the body of the absorbed water (Figure 7(a)).
4. Conclusion
In summary, in this study, a new 1H-NMR method for studying the water molecule adsorption of lignite and coke was proposed. This method was used to explore the water absorption and desorption processes of lignite pyrolysis at different temperatures (25–950°C) and the special contributions of ether bonds to the water adsorption. The adsorption was demonstrated to be dependent on the presence of “ether bonds” in the lignite pyrolysis process, which could significantly improve the adsorption effect of water. The increase in the ether bond content (at 300 and 950°C) resulted in a significant increase in the water adsorption capacity and a significant decrease in the desorption capacity. The reason for this phenomenon was the strong adsorption capacities of the ether bonds. The ether bonds could cause water molecules to form stable adsorption layers in a monolayer water state. In the pyrolyzed coal samples at 950°C, the adsorption capacities of the ether bonds remained high after the monolayer of water was saturated, and the water molecules underwent multilayer adsorption. This study offers a new approach for investigating the water molecule adsorption and the adsorption mechanism of lignite and coke.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no direct financial relationship with the commercial entities mentioned in this study that might lead to conflicts of interest.
Acknowledgments
This work was supported by grants from the National Natural Science Foundation of China (21766023), Science and Technology Program of Inner Mongolia (grant number 2019GG268), Research Project of Inner Mongolia University of Technology (grant number ZZ201906), Science and Technology Plan Project with Applied Technology Research and Development of Ordos (grant number 2019501), Science and Technology Development Fund of Central Guide Place in 2020 (grant number 2020ZY00322), and Scientific Research Project for Higher Education Institution of Inner Mongolia (grant number NJZY21150).
Supplementary Materials
As shown in Supplementary Materials S1, the FTIR spectra of the lignite samples treated under different temperatures exhibited the same shapes but showed different intensities of the absorption peaks. The peak at 1700 cm−1 is the stretching vibration peak of aromatic C=O, and the peak at 1600 cm−1 is the stretching vibration peak of aromatic C=C. The peak at 1110 cm−1 is the stretching vibration peak of -O-. The integral area was acquired by integrating the fitted FTIR spectra in the ranges of 3000–3700 cm−1 (-OH), 1550–1800 cm−1 (C=O), and 1100–1200 cm−1 (-O-). (Supplementary Materials)