Elucidation of the Pumping Effect during Microwave Drying of Lignite

Article pubs.acs.org/IECR

Elucidation of the Pumping Effect during Microwave Drying of Lignite Zhanlong Song, Liansheng Yao, Chuanming Jing, Xiqiang Zhao,* Wenlong Wang, Jing sun, Yanpeng Mao, and Chunyuan Ma National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong Provincial Key Lab of Energy Carbon Reduction and Resource Utilization, Shandong University, Jinan 250061, China ABSTRACT: Lignite could be a potentially valuable energy resource, but high moisture contents in the raw material hinder its use in industrial applications. While various studies have investigated the use of microwave radiation for pretreating and dehydrating lignite, the drying mechanism has still not been clearly elucidated. In this study, a digital video recorder was used to document progressive moisture migration on the surface of subdecimeter-sized lignite particles and track the reduction in moisture contents. The temperature and pressure history inside the samples during microwave drying was also investigated. The results showed that the drying process could be categorized into the following three stages: preheating, constant-rate drying, and falling-rate drying. The microwave pumping effect was observed on the surface of the samples at the constant-rate drying stage, thus indicating that the moisture inside the samples could be partly driven out in liquid form. An interesting observation was that the moisture content at the exterior zone of the sample was even higher than the initial moisture content at the constant-rate drying stage. Meanwhile, the highest temperature at the center core of the particles reached 120 °C and the corresponding internal vapor pressure equaled 2 atm, according to the acceptable vapor−liquid equilibrium. This high internal vapor pressure may have been responsible for generation of the jet flow of moisture in the solid matrix, namely, the microwave pumping effect. This knowledge should be very instructive for the development of efficient lignite drying technologies, especially from the perspective of energy conservation.

1. INTRODUCTION Lignite, a type of low-quality coal with a high moisture content, accounts for approximately 45% of global coal reserves.1 Despite its abundance, utilization of raw lignite in industrial applications is not beneficial for several reasons. First, transportation and storage of lignite is difficult, and, second, lignite has a low calorific value and is inefficient when used for heating. Thus, lignite is a costly energy resource. However, it may be possible to improve the quality of lignite through dehydration technologies. While conventional mechanical dewatering methods can be used to pretreat lignite with a limited efficiency of water removal, better lignite dehydration technologies are needed. Thermal-drying methods have been widely used to achieve increased dehydration in wet coal. During thermal drying, the samples are heated externally. Energy transfer takes place along a temperature gradient whereby the heat flux flows from the surface to the interior of the coal particles, whereas the moisture is transported from the interior to the surface of the particles with the assistance of the mass concentration gradient. Insufficient heat flux can lead to an uneven distribution of temperature, and at times the moisture content may remain higher in the interior compared to the exterior parts of coal © XXXX American Chemical Society

particles. Such nonuniform drying will not only lower the overall drying rate but also lead to potential risks for the combustion and explosion of lignite because of its high content of volatile matter. Consequently, the traditional thermal-drying process should be improved so that inefficient drying problems can be avoided. Microwave heating has several advantages. One key advantage involves the application of internal heating, which often results in a more uniform temperature distribution. Hence, drying can be conducted at lower bulk temperatures than those in conventional heating. Moreover, microwave heating is especially suitable for drying large-sized lignite pieces. The homodromous transfer of heat and moisture in lignite samples during microwave heating results in highly efficient dehydration reactions. Overall, the low-temperature operation conditions and high drying efficiency make microwave heating an ideal dehydration technology for lignite pretreatments; i.e., anticipated energy savings can be achieved without affecting the Received: December 22, 2015 Revised: February 25, 2016 Accepted: February 28, 2016

A

DOI: 10.1021/acs.iecr.5b04881 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

occur with large-sized samples, so such samples were employed in this study. Samples were collected from the eastern region of Inner Mongolia in China, and selected samples were polished into cubic shapes (Figure 1a). Figure 1b shows a half portion of

coal quality. Because microwave heating does not require a long preheating period and also achieves instantaneous heating, it is more efficient than conventional heating. In addition, much higher drying rates are expected because of the rapid evaporation of liquid water. Hence, the drying time may be significantly shortened compared with that in traditional drying methods. Because microwave heating offers many advantages, considerable research has been undertaken into the effects of microwave radiation on low-rank coals, especially on lignite,2−11 brown coal,12 subbituminous,13 and coal slime.14 Most studies were conducted in a bench- or pilot-scale application. There have been attempts to scale up the microwave-drying process. Drycol and Coaltek processes, representing commercial-scale processes of the microwave drying of coal, were successively proposed. However, these industrial applications were experiencing an ongoing development regardless of some crucial hurdles, e.g., economic case and safety issues.15−18 The water, which is an excellent microwave receptor, present in lignite is warmed up after absorbing microwave energy and vaporizes. Previous studies have been focused mainly on the sample mass and temperature changes as well as variations in the structure of the coal particles during microwave heating. Such studies, however, have not clarified the transportation pathways of moisture from the internal regions to the surface area of the particles during the drying of lignite. It has been assumed that an internal pressure gradient due to rapid vapor generation inside fresh lignite, and hence a distinct “liquid movement period”,19 will appear and that this process drastically improves the moisture diffusion rates, in contrast to conventional convective heating. This special feature of moisture transportation can be designated as the “pumping effect”.13 To date, however, there has been no direct verification of this mechanism for the dehydration of lignite, and very limited information is readily available. Indeed, research on microwave pumping is of great significance because if water from lignite can be removed in a nonevaporable process (i.e., mechanical dehydration without the need for vaporization), the efficiency of dewatering may be sharply increased, possibly exceeding the maximum theoretical value of 1.40 kg of water/ kWh.20,21 This means that dehydration can be achieved without spending the entire bulk of heat energy on the vaporization of water in the samples, thus potentially resulting in great energy savings. The objective of the present work was to confirm the pumping effect during microwave drying of lignite. Extensive details were captured regarding the morphology of lignite particles through photographs obtained during the dehydration process, and these data were used to evaluate the drying process and further corroborate the above assumptions about the pumping effect. Furthermore, the moisture content, temperature, and pressure distribution inside the lignite samples were studied in order to illuminate the overall drying mechanism of microwave radiation pretreatment. These studies are expected to promote the accelerated development of microwave-drying technologies for lignite, which could help lignite become a better energy resource.

Figure 1. Coarse lignite samples and the temperature and moisture content measurement positions: (a) an entire sample; (b) a halfsample where A is the position near the side surface, B is the middle position between A and C, C is the geometric center of the sample, and D is the center zone of the upper surface.

a sample after it was cut across the geometric center and parallel to the lignite fibers. Points A−D reflect different positions, and points A−C were located along the same longitudinal section. Samples measuring 100 mm in length (L), 65 mm in width (W), and 25 mm in height (H) were used in our tests. The properties of the samples are given in Table 1. These data showed that the lignite samples had relatively low contents of volatile matter and fixed carbon; moreover, extremely low ash and sulfur contents were detected. Table 1. Ultimate and Proximate Analyses of the Samples Presented ultimate analysis

wt % dry basis

proximate analysis

wt % as-received

carbon hydrogen nitrogen sulfur oxygen

66.04 3.92 0.75 0.40 18.13

volatile matter fixed carbon mean total water ash

25.13 32.63 35.01 7.23

2.2. Process. A schematic diagram of the microwave-drying system is illustrated in Figure 2. A domestic microwave oven (Galanz G70F20CN1L-DG) was modified to serve as the microwave thermogravimetric analysis system by adding temperature and weight measurement units as well as a data acquisition system. Detailed information about this apparatus can be found in our previous studies.14,22,23 The microwave oven, which operates at a frequency of 2.45 GHz, can be run at different power outputs, with a maximum power of 800 W. A JJ500 digital balance with an accuracy of 0.01 g was used to continuously record the sample mass during progression of the

2. EXPERIMENTAL SECTION 2.1. Materials. Because large-sized samples require lower crushing costs and are preferable for industrial applications, relatively large-sized (subdecimeter) particulate lignite was used in this study. Additionally, microwave pumping is more likely to B


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sample holder located in the microwave oven; after that, the drying tests were initiated. When the drying time reached 20 s, the microwave unit was shut off. The sample mass at this moment was recorded through the digital balance connected with the microwave oven, and with these results, moisture contents were then calculated according to the differences in sample mass between the raw and dried samples. After the sample mass was recorded, the sample was immediately removed and bisected vertically parallel to the texture of the lignite by using a knife. A sampling spoon was used to rapidly extract relevant samples located at points A−D (as illustrated in Figure 1). The moisture content of the sample was then measured according to the microwave methods catalogued in China’s national regulatory document GB/T 211-2007. After the moisture contents at the four points were acquired, similar tests were conducted after additional 20-s heating times, i.e., testing occurred after 40, 60, ..., 240 s of microwave exposure. It should be noted that the removal, bisection, and extraction of the samples occurred very quickly (usually less than 30 s) to ensure that there was a minimal drop in the moisture content that could skew the measurement results. The samples that were employed in one experiment were not reused in later experiments because the structure and moisture distribution of a sample was altered once it had been dried with microwave radiation. Accordingly, a sufficient number of similar samples in terms of the shape and particle size were prepared in advance. These samples were obtained easily from the local coal yard, which contained countless decimeter-sized pieces of fresh lignite. Admittedly, however, the selection of similar samples was somewhat difficult because of the anisotropy of lignite, in terms of both the surface shape and the internal structure. In order to obtain samples that were very similar in shape and size, the selected samples were hand shaped by removing the angular and irregular textures; thus, most of the samples used in this study resembled a cuboid. Further, these samples ideally needed to have identical initial moisture contents. Fortunately, our preliminary tests indicated that our samples had nearly identical moisture contents, with a maximum error rate of less than 2%; thus, these carefully selected samples qualified as parallel samples. Each test was repeated three times, and the average result was used to minimize data discrepancies.

Figure 2. Simplified schematic of the microwave-drying system: (1) microwave generator; (2) microwave control system; (3) fiber-optic sensors; (4) temperature acquisition system; (5) samples and plastic tray; (6) microwave cavity; (7) digital video recorder; (8) digital balance.

drying process. Three 3-mm-diameter GX-1 fiber-optic sensors capable of measuring temperatures of 0−200 °C were employed to detect the in situ temperatures of the sample at various points between the surface and central region (i.e., at points A−C shown in Figure 1). In order to detect liquid migration on the sample surface, progressive changes of the sample surface appearance were monitored by using a highdefinition digital video recorder (Sony HDR-SR11) at certain test stages. The recorder was placed outside the oven, and filming took place through a hole (about 50 mm in diameter) on the side of the oven (see Figure 2). 2.3. Temperature Measurement Method. In most cases, traditional temperature measurement techniques cannot be applied directly in microwave heating. For instance, an IR thermometer can merely be used to measure the sample surface temperature, but it may be interfered with by the variable sample emissivity. Furthermore, a conventional thermocouple device cannot normally be used in a microwave field because of the possible generation of electrical discharges, which would prevent it from collecting reliable values. In this study, three fiber-optic thermometers, which are unaffected by microwave interference, were used in our experiment to acquire the temperature distribution and capture thermal transitions within large-sized samples. During the experiments, the heating process had to be terminated when the measured temperature reached 180 °C because of the limitations imposed by the measuring range of the thermometers used in the analyses; however, this did not affect the research on the pumping effect at all. In order to measure internal temperatures, preliminary tests were performed. These tests involved drilling 3 mm holes vertically from the top surfaces to the geometric centers of the samples, and then fiber probes were inserted through the holes to measure internal temperatures. The three measurement points A−C, which are shown in Figure 1, represent the near side surface, the 1/4 radius region, and the center core area, respectively. The drying experiments started when the power of the microwave unit was turned on, and the microwave power was set at 800 W. The temperature was recorded automatically by computers at 2 s intervals. 2.4. Measurements of the Moisture Content. It is essential to obtain variations of the internal moisture contents of coarse lignite samples to elaborate on the mechanism of water migration in the samples. The measurement procedure was as follows. The sample was first weighed and placed in the

3. RESULTS AND DISCUSSION 3.1. Observational Study of Water Migration on the Particle Surface. Observational studies of water migration on the particle surface during microwave radiation were carried out. Gradual changes of the coarse subdecimeter sample surface are shown in Figure 3. It should be noted that there was no liquid water on the sample surface at the initial state (A). As drying progressed, there were no obvious changes until 50 s (B), and at this time, clear water began to appear on the sample surface. At 55 s (C), bubbles appeared on the sample surface and the sound of vapor erupting from the sample could be heard, thus indicating that vapor evaporated under the influence of microwave heat was being transferred from the interior to the surface of the sample. These processes characterized by water migration and the emergence of bubbles do not appear as readily with conventional drying methods. As the drying process proceeded, the water yield increased and the maximum value was obtained around the 75-s mark of the heating time (D). At this moment, water covered the entire observable surface, and some water droplets noticeably began to roll and flow at the surface. Moreover, at this point in time, C


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was only used during the initial drying period (0−160 s) of the present work to avoid microwave interference. In order to explain the liquid movement on the sample surface, it is essential to discuss the structure of lignite. Scanning electron microscopy (SEM) photographs for the dried lignite samples are shown in Figure 4. As illustrated in

Figure 4. Typical SEM image of microwaved lignite.

Figure 4, transverse and parallel pore tunnels dominate the surface structure of the samples, and it can be speculated further that the internal area is also filled with this tubular structure, which is responsible for the layered formation of lignite. The pores are mostly less than 10 μm in diameter and hence can be categorized as macropores (having a pore diameter larger than 0.1 μm24). Apparently, this feature is very conducive to the movement of moisture inside the samples, provided the driving force is strong enough. The inner moisture locked within the capillary pores can be released by highintensity microwave heating, and microwave pumping can be regarded as the result of the high internal vapor pressure induced by the generation of jet flow of moisture in the solid matrix.7,13,19,25 The corresponding mechanism is elaborated in section 3.3. Accordingly, the moisture can be forced to migrate to the surface along the straight and smooth pore channels under the influence of the vapor pressure. As the drying time was prolonged, small cracks and fissures in the samples that were created by the previous crushing and grinding process began to expand. Thereafter, the cracks also became more numerous. On the basis of the findings of the pumping effect, it is believed that the rapid expansion of moisture along with relatively high internal pressure within the particles was the reason for crack expansion and new crack formation. The shrinkage, due to the loss of water and other volatiles during coal drying, decreased the volume of the sample, partly contributing to the formation of these cracks. The numerous openings that make up the layered structure in the coal may be another reason for the large horizontal cracks.11,13 Consequently, when the samples were dried completely, relatively large and parallel random cracks were observed, as can be seen in Figure 5; this is in accordance with the knowledge of the way the layered structure of lignite behaves, as illustrated in Figure 4. 3.2. Variations of the Moisture Content and Temperature in Samples. As mentioned above, water migrates from

Figure 3. Photographs of water generated progressively on sample surfaces during microwave drying.

the sound of steam being ejected seemed to be at its loudest. Subsequently, the water at the surface gradually vaporized because of the relatively high temperatures caused by microwave heating, and it eventually disappeared at the 95-s mark (E). Simultaneously, the sound of vapor escaping gradually diminished until it was gone, thus indicating that the water migration process had finished, and the samples continued to dry through conventional evaporation, which was characterized by the transition from liquid water to vapor. In general, water existed at the surface of the sample for about 40 s, starting at the 55-s mark, and the presence of water lasted until the 95-s mark, according to the video evidence. Furthermore, some relatively large water drops, which likely did not originate from vapor condensation, could be found at the bottom of the tray. This confirms that the moisture inside the samples was partially driven out in the liquid form, with the driving force being the vapor pressure caused by the expansion of moisture during the transition initiated by the microwave drying. Because it was possible that a small quantity of microwave radiation could have leaked through the observation hole when the video was being recorded and interfered with the measurements, especially in the later stages of the drying period when there was less water in the samples, the recorder D


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Figure 5. Photograph of the microwave-dried sample.

the interior to the surface, and the liquid collects for a time at the surface. Because evaporation of water at the sample surface is obviously a more efficient process than evaporation from the interior of the samples, the drying rates may be accelerated once the pumping effect takes place. In order to explain the findings for liquid water migration, the transient distribution of the moisture content against the drying time is detailed in Figure 6. The positions of sampling points of A−D can be

Figure 7. TG and DTG curves of microwave drying of the samples.

Figure 8. Temperatures at different locations in the samples, as described in Figure 1.

sample forms, and structural changes over the course of the various stages. 3.2.1. Preheating Period. During the preheating period (about 0−50 s), the moisture content of the coal was relatively high, which meant that more microwave energy could be absorbed by the wet coal, as theoretically indicated by the difference in the loss factors of water and dry lignite (10 and 0.1, respectively).2 The generated thermal energy elevated the whole temperature of the sample to approximately 100−120 °C in less than 50 s, which was significantly higher than the boiling point of pure water at ambient pressure (see Figure 8). Meanwhile, no evident sample mass losses were found according to Figures 6 and 7. Moreover, the different sampling points exhibited similar behaviors regardless of some different aspects; i.e., temperatures for curve A seemed to have no obvious turning point, unlike curves B and C, where the turning points in the temperature occurred at 60 and 40 s, respectively. At the same time, minor changes in the sample mass (Figure 7) and hence the moisture content (Figure 6) could be found at this stage because of the hysteresis effect of water removal. 3.2.2. Constant-Rate Drying Stage. After the preheating stage, drying continued and the samples quickly lost mass. Owing to the selective heating, the water, rather than the lignite itself, absorbed most of the microwave energy and the internal vapor pressure increased drastically, which drove the internal moisture to migrate to the surface in an unusual way. This led to enhanced mass and heat transfer, and, consequently, the moisture content of the samples was quickly reduced by over

Figure 6. Moisture contents at different locations in the samples, as described in Figure 1.

found in Figure 1. On the basis of the data of real-time sample masses, the general moisture content of samples was easily measured, and the results are expressed with the letter E in Figure 6 for comparative purposes. In light of variations in the sample masses, the thermogravimetric (TG) and differential thermogravimetric (DTG) analysis curves are presented in Figure 7. Additionally, the temperature was charted at different positions in the samples, as shown in Figure 8. It needs to be pointed out that the aim of this paper was to investigate the microwave pumping effect during lignite dehydration, and because we found that the pumping effect usually occurred during the earlier stage of the drying process, the samples did not necessarily need to be completely dried out within the 250-s test period. Regardless, the moisture contents of microwaved samples at the end of the test period reached relatively low values of 5%; lignite with this level of moisture can be directly used in most industrial applications. As can be seen from the moisture content profiles in Figure 6, curves A−D exhibited varying behaviors, but on the whole, similar trends were apparent. Specifically, the drying process consisted of the three main stages of preheating, constant-rate drying, and falling-rate drying, which are denoted by the symbols of α, β, and γ, respectively, in Figure 7. In actuality, the boundaries of these sections were not always clear because of the complex drying characteristics of water, variations in the E


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time to microwave radiation, the internal moisture content of the samples remained at relatively high values, e.g., 20−30%, as shown in Figure 6. This implies that vapor may have been generated in the internal regions of the large-sized lignite particles, and the vapor as well as the liquid would have coexisted in the samples at the same time; hence, vapor−liquid equilibrium could have possibly been achieved during this period. Under this assumption, the corresponding saturation pressures were easily estimated according to the phase equilibrium equation, and the results are presented in Figure 9.

10 percentage points within less than 1 min according to curve E (Figure 6), which represents the overall moisture content change of the samples. We also found that the moisture content varied linearly against the drying time between 55 and 105 s (see Figure 7). This indicates that the removal rate of water was approximately a constant, and this time period is referred to hereafter as the constant-rate drying period. Coincidentally, this duration was consistent with that of the presence of liquid water, as can be seen in Figure 3. Therefore, it can be speculated that when the liquid water discharges from the internal area to the surface via induction by the internal vapor pressure as a result of microwave radiation, then the water existing on the sample surface can be readily evaporated in the form of bulk water;1,2,26 this will be described in more detail in later sections. As a result, it can be expected that the evaporation of bulk water will proceed at a constant rate. This process merely requires a minimum amount of energy, i.e., the latent heat of water without additional energy. Thus, the removal rate of water was the largest at this stage, and it ranged from 0.002 to 0.0025 s−1 (Figure 7). Consequently, the constant-rate period was most efficient for dewatering the samples, and this was when most of the free water in the samples was removed. It should be noted that the overall temperature of lignite decreased noticeably at first because water evaporation is endothermic at the early stage. The slight change in the temperature can be explained as follows: when the free water vaporizes at the surface of the sample, the surface temperature of the sample is always kept at a constant level. This occurs because the energy required to evaporate water is approximately equal to the heat generated during this period. This is well in accordance with the aforementioned analyses. In addition, regarding internal heating during microwave radiation exposure, the internal temperature is usually higher than that at the surface; e.g., the corresponding temperatures were around 100, 105, and 110 °C for points A−C, respectively, as shown in Figure 8. This result can be attributed to the combined impact of the focusing effect caused by internal heating from microwave radiation2 and the fact that the exterior area loses more heat than the interior of the sample. Furthermore, a seemingly paradoxical phenomenon whereby the moisture contents during the constant-rate stage of drying were even higher than the initial values could be observed for curves A and B. The moisture content of curve A declined slightly at first and subsequently started to rise again to a maximum value of 37.6%, which was significantly higher than that of the raw samples (around 35%); then afterward it dropped rapidly. Similarly, the moisture content of curve B slightly exceeded the initial value of the samples by 1.3 percentage points. This was likely due to the fact that generation of the internal vapor pressure caused by the highintensity heating of the microwave compelled the inner moisture (partly in a liquid state) of the sample to migrate toward the surface; thus, the liquid water increased at the sample surface, which coincided exactly with the aforementioned analyses, as shown in Figure 3. Because pressure-driven flow is one of the most important features for moisture migration in lignite, measurements of the pressure distribution inside the samples are crucial. Unfortunately, the pressure inside the samples could not be measured directly because of a lack of suitable measurement equipment. Nevertheless, the internal pressure was still obtained according to the following procedure. After a certain amount of exposure

Figure 9. Internal vapor pressure profiles versus drying time.

From Figure 9, it can be observed that the internal pressures (curves B and C) were in excess of the standard atmospheric pressure (1 atm) during the early stage corresponding to 35− 60 s. Obviously, such high pressures can only result from the vaporization of moisture in the inner pores of the samples. The pressures at the center position (curve C) were remarkably higher than those at the outer region of the particles, which coincides with the temperature profiles in Figure 8. As shown in Figure 9, the internal temperature of curve C reached 100 °C at about 30 s, which is when the corresponding saturation pressure of 1 atm occurred. The pressure at position C rose quickly and reached a maximum value of 2 atm at 40−45 s; subsequently, the pressure decreased because of the pressure release that occurred along with the water and vapor shifts from the internal regions to the surfaces of the samples. The reduced residual moisture, and hence the reduced vapor amount in the sample, may also have caused the pressure to drop. In the case of curve B, the maximum pressure approached 1.3 atm at about 55 s, which was substantially lower than that of curve C. Additionally, the occurrence time for the maximum value was about 10 s later than that for curve C. Meanwhile, the surface temperature (curve A) fluctuated at about 100 °C, which also suggests that the evaporation of water from the sample surfaces occurred at ambient pressure during the “liquid movement period”. On the basis of these analyses, the pressure distribution of position A was excluded in Figure 9. Because of the lack of channels for water migration along the vertical direction, which was perpendicular to the direction of the capillary tubes (see Figure 4), obvious water discharge was not observed on the upper surface (point D). Thus, there were no observations of a rise in the moisture content of point D within a certain time period, as was expected, and the moisture content consistently dropped during the drying process. Hence, it is believed that moisture mainly migrated along the large pore F


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Figure 10. Schematic diagram of moisture migration and pressure variation during microwave drying.

of the coarse particles, and the easily removed free liquid was the first to dewater. As the drying proceeded, the moisture content of the samples continued to decrease. In addition, the heat of water desorption increased gradually because the inherent water (bound water) that existed in the samples became more and more difficult to evaporate as a result of the requirement for additional energy, except for the latent heat of vaporization,27 which was responsible for weak migration and evaporation of the moisture in the lignite during the later stages. Thus, a falling-rate drying period occurred, as illustrated in Figure 7. During this period, there was no liquid water on the external surfaces of the samples. The interior moisture content was higher than that of the exterior region at certain times during the last period. Because the internal moisture diffusion

channels, and this confirms the aeolotropy of the lignite used in our experiments. Eventually, the profiles of curve E were consistent with the variations of point C. Both curves first exhibited a constantweight period, and subsequently the moisture contents declined rapidly as a result of water evaporation driven by microwave radiation. It needs to be emphasized that point D exhibited the fastest drying rate among these five points during the early stages of drying. After 170 s, however, these points seemed to be almost identical with each other; i.e., they shared similar overall moisture contents (point E). 3.2.3. Falling-Rate Drying Period. During the constant-rate drying period, as shown in Figure 6, the moisture content of the exterior zone was usually higher than that of the internal region G


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found in the small crevices between particles; (5) adhesion water, which refers to a layer of film around the surface of individual or agglomerated particles. Interparticle water and adhesion water, which represent surface water, can be removed easily by using mechanical methods; however, interior adsorption water and surface adsorption water, which are types of inherent moisture, need to be removed by thermal drying methods. The capillary water can be partially desorbed, depending on the drying conditions. These different forms of water associated with coal are illustrated in Figure 10. On the basis of the appearance of water on the sample surface, as well as evidence from the SEM photographs, it was presumed that water can exist in the tubular pore channels (macropores) through weak hydrogen bonds and that there were very few if any interactions at all between the water molecules and the oxygen-containing functional groups at the surface of the lignite; this means that the water can be removed at low temperatures of around 80 °C.28 The majority of water present in the macropores was likely in the form of unbound water, and only a very small fraction of bound water was present.27 During hydration, the unbound water, which usually exists in the spaces between the coal particles or in large pores of the coal, was first transferred to the particle’s surface, where it then evaporates. When this fraction of moisture was mostly desorbed, the bound water, which was relatively resistant to vaporization, evaporated subsequently. In order to explain the mechanism for moisture migration during the drying process, a schematic diagram of moisture migration on the molecular scale is shown in Figure 10. At the initial state (Figure 10a), the tubular pore water, which includes the maximum contents of the unbound and bound water (the green and blue balls are used to reflect both types of water, respectively), can be found in the large capillary-sized pores. As aforementioned, there are five different forms of water present in the lignite matrix. In order to clarify the pumping effect, the internal pressure changes in the tubular pores at different drying periods are illustrated on the left side of Figure 10, where different colors denote the different pressures; e.g., red corresponds to the maximum pressure, while gray denotes the minimum pressure. Meanwhile, progressive changes of the different types of water that exist in the lignite matrix are illustrated on the right side of Figure 10. As the drying proceeds, the preheating, constant-rate period, and falling-rate period (parts b−d in Figure 10, respectively) take place in sequence. The different types of moisture show little change in the first stage for the water both within the lignite matrix and in the large pores. The vapor pressure is also rather stable because of weak evaporation of water during the preheating stage. During the constant-rate period, the internal vapor pressure caused by evaporation of the liquid water in the large pores is generated quickly, and this pressure is possibly the main driving force behind moisture transfer into vapor and/or liquid forms. It should be noted that moisture evaporation, and hence the vapor pressure, at these stages exhibits an uncommon behavior; i.e., the pressure rises rapidly to a maximum value (corresponding to the red area with maximum pressure); at this moment, jet flows of water may occur because of the high internal pressure and tubular structure of the lignite with relatively low flow resistance; i.e., the special structure is beneficial to the water’s movement. During this period (Figure 10c), most of the water in the large capillary pores is evaporated, whereas much of the water present in the lignite

rate was lower than the external surface evaporation rate, the interface of vaporization shifted toward the interior zones of the samples. Thus, the drying rate (Figure 7) became lower, in general, until the drying was actually finished. Moreover, because of the reduced moisture contents in the samples, the driving force of mass transfer, i.e., the vapor pressure, decreased accordingly. Thus, the microwave-absorbing performance of lignite, which can also be referred to as the microwave energy efficiency, decreased. At this moment, it was generally assumed that the lignite, rather than the water, was the main microwave receptor. Consequently, the temperatures at different regions of the lignite started to rise again with relatively small heating rates of 6−15 °C/min. Near the end of the heating period, 235 s into the process, the temperatures approached 140 °C in the samples’ center regions and 120 °C at the surfaces. This finding was in accordance with the temperature profiles (Figure 8). For instance, the temperature of point A was higher than that of B at the later period, possibly for the following reasons. At the last stage, the moisture contents of the samples were relatively lower, and thus the lignite rather than the moisture became the main absorbing material of microwaves; hence, less heat energy is required for evaporation of moisture with less moisture content at point A. The temperature of point A is higher than that of point B accordingly. At the same time, if the sample was dried at a higher temperature, then its water evaporation might be accelerated and its moisture content would be lowered. Additionally, the surface region had a smaller moisture diffusion resistance than the internal region. As a result of both variables, the moisture content of point A became lower than that of point B, and the corresponding transition time was about 125 s, which was slightly prolonged in the case of the data shown in Figure 7. The moisture content of different zones seemed to match up at the later stage, thus indicating that the coarse samples dried in a relatively uniform process. This was likely a distinct feature of microwave radiation acting on large-sized samples, e.g., 30− 100 mm in diameter, and the large sizes were found to be beneficial for dehydration of the lignite. Recall that coarse samples are preferred in industrial applications given that less energy of the coal sample crush is used. Therefore, microwave drying may serve as a promising method to achieve efficient dehydration of the lignite, especially for large-sized particles, prior to its industrial use. 3.3. Possible Mechanism of Moisture Transfer Related to Microwave Pumping. Because of lignite’s complex physical and chemical structure, tracking the movement of water in the material during the heating process was complicated. Several issues with regard to the drying mechanism should be taken into account during this process. For example, how does moisture transport from the interior to the surface and how does the internal pressure affect the jet flow of moisture. Clarification of the nature of water as well as the interactions between water and the coal structure is essential for understanding the characteristics of dewatering in lignite, especially for studies of microwave pumping. In previous studies, water in lignite was classified into the five following types of water:1,27 (1) interior adsorption water, which is contained in the micropores and microcapillaries within each coal particle; (2) surface adsorption water, which is positioned adjacent to the coal molecules but only on the particle surface; (3) capillary water, i.e., the water that is contained in capillaries; (4) interparticle water, which can be H


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Our tests confirmed the existence of the pumping effect during the microwave drying of lignite. We expect that our findings will help future investigations of the use of microwave drying for the pretreatment of lignite, and these investigations could encompass studies aimed at both obtaining new knowledge about the drying mechanism and improving the technology. For example, if the dewatering process, without the need for vaporization and hence the change of phase, can be achieved as a result of the pumping effect during microwave drying, then it may be possible to promote the utilization of microwave drying for lignite and other materials such as subbituminous coal and coal slime on a large scale. The implications are that microwave drying could provide a viable, energy-efficient option for coal-drying operations.

matrix remains because of the tight connections with the matrix as well as the large resistance for moisture diffusion between the interior and the surface. When the dewatering process enters the falling-rate drying stage (Figure 10d), the unbound water has been mostly removed and the bound water begins to evaporate at higher temperatures; thus, the internal vapor pressure gradually becomes smaller (corresponding to the yellow region with relatively lower pressure) because of the lower moisture content in the residual sample. At this stage, the surface water (i.e., the interparticle water and adhesion water) has already been removed, and the inherent water (i.e., interior adsorption water and surface adsorption water) as well as the partial capillary water is vaporized. The timing of these events is due to the differences in the difficulty of moisture removal for the different kinds of water. After the drying process is finished, the internal vapor pressure approaches zero and the internal pores become theoretically empty (Figure 10e). In fact, the pores collapse, and marked deformation is likely to occur during the drying process, as exhibited in Figure 5. As a result, shrinkage of the lignite particles and the appearance of crevice marks can occur; however, this topic is beyond the scope of our study, which investigated the microwave pumping effect. This study’s main focus was to collect preliminary evidence of the pumping effect during the microwave-drying process within certain experimental conditions. Microwave pumping is a promising basis for lignite dehydration technologies, and this study can be regarded as a starting point for further research. Additionally, this study aimed to clarify the drying mechanism for lignite and other materials with high moisture content during microwave heating. In our next series of experiments, we plan on using different sample types, particle sizes, and operational conditions; moreover, we hope to employ pressure sensors inside the samples, which may shed more light on the microwave pumping mechanism. More information is also needed regarding efficient removal of the liquid water discharged from the interior zones of lignite in a nonevaporative dehydration manner because this is still a challenging issue.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.-Q.Z.). Tel.: +86 531 88399372. Fax: +86 531 88395877. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China and the Joint Fund for the Development and Utilization of Coal (Grants U1361113 and 51306106).

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REFERENCES

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4. CONCLUSIONS In this study, experiments were conducted in which microwave drying was used to treat lignite, which could be a potentially useful energy resource, and we attempted to clarify the mechanistic aspects of the drying process such as the microwave pumping effect. It was found that the samples experienced three drying stages, namely, a preheating stage, a constant-rate drying stage, and a falling-rate drying stage. Exciting findings regarding the generation of a moisture jet flow as well as the existence of water on the sample surface were obtained with data from a video recorder. What our video confirmed was the occurrence of microwave pumping, which is characterized by a jet flow of liquid water, and it was found to be caused by the internal vapor pressure during the constantrate period. We discovered that the centers of coarse lignite particles experienced maximum temperatures, which reached up to 120 °C, and that the corresponding vapor pressure could reach up to 2 atm based on the vapor−liquid equilibrium. This high pressure was the driving force for much of the moisture transfer, and it induced rapid water migration along the tubular pore texture, thus providing evidence for our presumed water movement mechanism. I


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Elucidation of the Pumping Effect during Microwave Drying of Lignite

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