Dry Separation of

Jun 16, 2014 - ABSTRACT: The combined dewatering process and dry separation in a vibration fluidized-bed dryer was developed to upgrade. Chinese ...
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Feasibility Studies of the Sequential Dewatering/Dry Separation of Chinese Lignite in a Vibration Fluidized-Bed Dryer: Effect of Physical Parameters and Operation Conditions Pengfei Zhao,†,‡ Yuemin Zhao,*,‡ Zhenfu Luo,‡ Zengqiang Chen,‡ Chenlong Duan,‡ Shulei Song,‡ and Liang Dong‡ †

School of Electric Power Engineering, and ‡School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou, Jiangsu 221116, People’s Republic of China S Supporting Information *

ABSTRACT: The combined dewatering process and dry separation in a vibration fluidized-bed dryer was developed to upgrade Chinese lignite in terms of moisture and ash for effective utilization in power plants. The influence of operational parameters (fluidizing gas velocity, vibration strength, bed height, and drying temperature) and lignite properties (particle size and moisture content) on the drying or separation characteristics of −6−1 mm lignite was examined in a laboratory-scale vibration fluidizedbed dryer. Both operational parameters and lignite properties had a significant effect on the drying and separation performance. The drying and separation behaviors of a vibration fluidized bed were both enhanced in comparison to the case without vibration under the same operating conditions. Moreover, with the change in gas velocity, the drying and separation processes could be combined in the same vibration fluidized-bed dryer. Both processes achieved promising results compared to those of the fixed gas velocity condition, which was confirmed by the low calorific value analysis. A sequential drying and separation system in a vibrofluidized-bed dryer was also designed on the basis of the separation and drying characteristic results. fluidized bed could facilitate fluidization for irregular and widely distributed materials (such as lignite particles). Furthermore, this process could overcome such obstacles as channeling, defluidization, or slug flow, thus possibly improving the drying kinetics. More importantly, this process could also process moist material with 30−50% less air than that in a conventional fluidized bed, thereby reducing energy consumption and mitigating the elutriation of moist materials.22 However, limited research is available to consider this particular fluidized bed as a lignite dryer, which could be beneficial in providing a promising dewatering method of lignite. Several coal-cleaning technologies have also been examined to reduce the high amount of ash in coals. Wet cleaning technologies based on froth flotation23 or enhanced gravity separation are highly efficient in terms of coal recovery and sticky run-of-mine coals.21 However, these technologies inevitably encountered problems because of the use of large quantities of water and the loss of millions of tons of tailings ponds.21 Given the sliming character of lignite, wet cleaning technologies could be unsuitable for lignite. Dry coal beneficiation technologies, including an air dense medium fluidized-bed separator,24,25 air jigging,26 and a FGX separator,27 provided an alternative solution to cleaning coals and did not suffer from the problems confronting wet cleaning technologies. In particular, the air dense medium fluidized bed could efficiently beneficiate high-ash coarse coals (−6−50 mm)22 and low-ash coals.6,28 Several researchers24,29,30 also

1. INTRODUCTION Lignite is an abundant raw energy material that is considered as a major component of global coal reserves.1 In China, lignite accounts for approximately 13% of proven reserves and serves an increasingly important function in supplying primary energy because of its accessibility and low mining cost.2,3 However, the high moisture content (20−55 wt %) of lignite usually results in a low calorific value, high fuel consumption, low efficiency, risk of spontaneous combustion, and high transportation cost.4 Meanwhile, the high ash content, low ash melting point, and difficulty in coal washing entailed by lignite commonly cause a lignite-fired boiler to exhibit fouling, slagging, and agglomeration, which could be critical limitations in the design and operation of lignite power plants.5 Direct run of lignite coals through treatment by removing moisture and a substantial amount of ash-forming mineral could not only enhance the efficiency of power plants but also reduce additional particulate materials, SOx, and trace element emission.6 Thus, these coals need to be upgraded in terms of moisture and ash prior to use. Various technologies, such as rotary tube drying,7 fluidizedbed drying,8−12 mechanical thermal dewatering,13−15 hydrothermal dewatering,16,17 and solvent dewatering,18 have been proposed for lignite drying. Among these technologies, the fluidized-bed dryer was considered as an attractive choice for its low energy consumption (0.33 MJ/kg of H2O), high processing capacity (greater than 15 tons/h), and low capital cost (far less than 90 dollars/kg of coal).19 However, this method was unsuitable for irregularly shaped particles or particles with a wide size distribution20 and was proven to produce substantial amounts of coal fines.21 Thus, the vibration fluidization bed received further attention. Applying mechanical vibration to the © 2014 American Chemical Society

Received: March 9, 2014 Revised: June 13, 2014 Published: June 16, 2014 4383

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employed force fields to improve the efficiency of fine coals (−6 mm). Although these enhanced air dense medium fluidized-bed technologies exhibited excellent separation results, they all encountered obstacles in dense medium recovery, product purification, and low processing capacity. In a recent study, Yang et al.31 developed a simple dry separation method based on the “segregation” of fine high-rank coals in a vibration fluidized bed. Without any dense medium, this method could thus eliminate the obstacles confronted by air dense medium fluidized-bed technologies. However, the dry separation performance of lignite using this method remains unclear. Although the reliability of the structure and the development of low-energy-consumption vibration equipment were the main concerns,32 this vibro-fluidized bed could enhance the drying or segregation behavior of particles when compared to the case without vibration.33 The common upgrading system could be separated into two different processes, which were dewatering and dry separation processes, using two vibro-fluidized beds in series, but it made this system complicated. In the integration upgrading system, which was developed in this work, the encouraging performances in both drying and dry separation processes were achieved only with the change in gas velocity. The use of this system could replace two vibro-fluidized beds in series and avoided this process complicacy. Meanwhile, this proposed pre-combustion removal process system produced a higher calorific value of clean lignite, which was beneficial in improving the efficiency of the power plants and reducing particulate materials, SOx, and emissions of trace elements.6 In addition, this integration upgrading technology with a vibrofluidized bed differed from the “DryFining” process34 that used a common fluidized bed. This study is part of a series of works that investigate the performance of the combined drying and dry separation of Chinese lignite, which has a typical raw moisture and ash content of approximately 30 and 30 wt %, respectively, in a gas vibro-fluidized bed prior to combustion. Many studies31,32 demonstrated that drying or separation characteristics depend upon the effect of physical parameters of materials and operation conditions. Lignite was a special multicomponent irregular and moist particle mixture. Although several kinds of ceramics,35 mining,36 and feeding products37,38 were dewatered or separated in vibro-fluidized beds, limited information was available and could probably not be applied to lignite drying or separation because of the characteristic differences of the abovementioned materials. Meanwhile, minimal research was conducted to examine the effect of operating conditions in a vibro-fluidized bed on the drying or separation characteristics of lignite. Therefore, this work focused on studying the effects of operational parameters (vibration strength, drying temperature, fluidizing gas velocity, and bed height) and lignite properties (particle size and moisture content) on the separation or drying performance of −6−1 mm lignite in a laboratory-scale vibrofluidized-bed dryer. This information was essential in establishing the optimization design of a lignite upgrading system. Under the optimal conditions, the feasibility of combining drying/dry separation Chinese lignite was evaluated.

Table 1. Proximate Analysis of Chinese Lignite sample

d (mm)

Mad (%)

Aad (%)

Vad (%)

FCad (%)

Dayan

1−6

10.27

31.46

23.13

35.14

performed with an equal mass of lignite on an as-received basis. To determine the initial moisture content, a 2 g sample was dried in an oven at 105 °C for 2 h using a digital balance (Sartorius BT 224 S, with a precision of 0.1 mg). 2.2. Experimental Apparatus. A schematic of the experimental setup, which consisted of a gas supply system, flow meter, gas distribution plate, fluidized bed, pressure scanner, and vibration system, is shown in Figure 1. Compressed air from a Roots-type blower was regulated by a valve installed on the parallel pipe, which had rotameters for flow rate measurements. The fluidizing gas flow was then passed through an electric heater using a temperature controller. The resulting hot air that entered the distributor plate was used to distribute the inlet gas uniformly. This distributor plate was also supported on a high-temperature filter cloth to prevent the particles from falling. The fluidized bed was a stainless-steel column with a cross-section of 0.2 × 0.2 m and height of 0.8 m. The column was covered with the glass wool to prevent heat loss. Meanwhile, a manometer was placed at different locations on the bed to measure pressure drops, and a bag filter was placed on top of the bed to separate fines and gas. The vibration motions were induced by a vibration system with amplitude ranging from 0 to 10 mm and frequency ranging from 1 to 400 Hz, made by China STI Co., Ltd. 2.3. Experimental Procedure. During the separation experiments, the lignite particles were first fluidized for a certain period, after which the fluidizing air was suddenly shut down. The static bed was divided into five layers in the axial direction, and each layer was tested for its ash content. Similar to our previous studies,31 the separation performance was evaluated on the basis of the statistical indicator (S), as defined in eq 1 n

∑i = 1 (Ai /A 0 − 1)2

S=

(1)

n−1

where Ai is the ash content of coal of the ith sampling point, A0 is the initial ash content of the feed coal, and n is the total sampling number. A larger value of S suggests that better segregation is favorable for fine coal separation. Drying was continued at time steps of 20 s until equilibrium moisture was reached under experimental conditions. Every 20 s, dried material was collected by a sampler. The as-received material was immediately weighed. The moisture content (MR) and the drying rate (R) values were calculated on a dry basis using several equations

MR =

R=

Wwet − Wdry Wdry

⎛ g of water ⎞ ⎟ ⎜ ⎝ g of dry coal ⎠

(Wwet)1 − (Wwet)2 Wdry(t 2 − t1)

⎡ (g of evaporation) ⎤ ⎢ ⎥ ⎣ (g of dry coal)(sed) ⎦

(2)

(3)

where MR is the moisture content, Wwet is the initial sample mass, Wdry is the mass of the dry coal, R is the drying rate, and t is the drying time.

3. RESULTS AND DISCUSSION 3.1. Separation Performance. Particle segregation performance largely depended upon the operating conditions and feed particle properties. Thus, the influence of operational parameters, including fluidizing gas velocity, vibration strength, bed height, and lignite properties (particle size and moisture content), on separation performance was examined in detail. 3.1.1. Effects of the Superficial Air Velocity. To study the effect of superficial air velocity on segregation performance, a dimensionless superficial air velocity (U* = U/Umf) was

2. EXPERIMENTAL SECTION 2.1. Lignite Properties. A typical Chinese lignite sample (Dayan coal in east of Inner Mongolia) was used in this work, and its proximate analysis is shown in Table 1. The sample was crushed and sieved to two size fractions (1−3 and 3−6 mm), and their density distribution is provided in Tables 2 and 3. All drying experiments were 4384

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Table 2. Results of the Sink−Float Experiment of the −6−3 mm Size Fraction of Lignite cumulative float

cumulative sink

separation density

density fraction (kg L−1)

weight fraction (%)

ash content (%)

product (%)

ash (%)

product (%)

ash (%)

density (kg L−1)

product (%)

−1.4 1.4−1.5 1.5−1.6 1.6−1.7 1.7−1.8 +1.8 total

28.81 16.77 8.91 7.71 7.98 29.82 100

11.41 21.72 36.42 43.84 65.88 80.44 42.90

28.81 45.58 54.49 62.20 70.18 100.00

11.41 15.20 18.67 21.79 26.81 42.80

100.00 71.19 54.42 45.51 37.80 29.82

42.80 55.50 65.91 71.69 77.37 80.44

1.4 1.5 1.6 1.7

45.58 25.68 16.62 15.69

Table 3. Results of the Sink−Float Experiment of the −3−1 mm Size Fraction of Lignite cumulative float

cumulative sink

separation density

density fraction (kg L−1)

weight fraction (%)

ash content (%)

product (%)

ash (%)

product (%)

ash (%)

density (kg L−1)

product (%)

−1.4 1.4−1.5 1.5−1.6 1.6−1.7 1.7−1.8 +1.8 total

28.76 16.79 9.12 8.70 8.99 27.64 100

12.14 22.32 35.42 44.21 65.73 79.56 42.22

28.76 45.55 54.67 63.37 72.36 100.00

12.14 15.89 19.15 22.59 27.95 42.22

100.00 71.24 54.45 45.33 36.63 27.64

42.22 54.36 64.24 70.03 76.17 79.56

1.4 1.5 1.6 1.7

45.55 25.81 17.82 17.69

Figure 1. Schematic diagram of the experimental apparatus: (1) air filter, (2) Roots blower, (3) electric heater, (4) pressure gauge, (5) valve, (6) rotameter, (7) vibrated bed, (8) air chamber, (9) air distributor, and (10) vessel.

Figure 2. Segregation degree at different superficial gas velocities.

intensified the solid flow around the larger bubbles, which improved the circulation of lignite particles and resulted in poor segregation performance.40,41 3.1.2. Effects of the Vibration Intensity. To quantify the effects of vibration intensity on the segregation behavior of particles, the dimensionless vibration number (K) was defined as follows: K = A(2πf)2/g, where A and f are the amplitude and the frequency of vibration, respectively, and g is the acceleration of gravity. The −6−3 mm and −3−1 mm size fractions of coal were processed at U* = 1.3 under different vibration intensities, with their segregation performance shown in Figure 3. The segregation degree for both types peaked when K = 1.5, which indicated that the lignite particles reached the maximum segregation condition. Under these appropriate vibrated fluidizing conditions, an improved degree of segregation was achieved because the channeling or slug flow within the bed was eliminated in the presence of vibration.42 However, when the introduced mechanical vibration exceeded the critical value, the motions of lignite particles became too intense to preserve the original segregation behaviors.43 Consequently, it was not

adopted, where U and Umf are the superficial and minimum fluidizing air velocities, respectively. The separation results of the two size fractions of air-dried lignite (−6−3 mm and −3−1 mm) at different U* are shown in Figure 2. As the superficial gas velocity increased after the minimum fluidization velocity, the values of S on both fractions reached their peaks when U* = 1.3, which indicated that this velocity resulted in the maximum segregation. Thereafter, the level of particle segregation on both size fractions gradually decreased, which suggested that the lignite particles began to mix. These observations suggested that an optimum air velocity appeared at U* = 1.3 to maximize segregation in this study. The dependence of segregation on air velocity could be ascribed to the difference in the size of air bubbles,39 which affected the segregation or mixing characteristics. When the superficial air velocity was lower than Umf (U* < 1.3), the excess bubbles with smaller size induced less effective solid flow, which positively affected segregation. However, further increasing the gas flow velocity (U* > 1.3) 4385

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Figure 3. Segregation degree at different vibration intensities.

Figure 5. Segregation degree at different surface moisture contents.

surprising to observe that the smaller values of S performed at higher vibration intensity. 3.1.3. Effects of the Bed Height. The segregation results of the two sizes of lignite with different initial bed heights (H) are shown in Figure 4. When the bed height was less than 70 mm,

segregation of lignite particles, especially at a higher surface moisture content (Mf > 4.5 wt % for −6−3 mm, and Mf > 2.0 wt % for −3−1 mm). However, with a low surface moisture content, separation characteristics presented no significant effect when compared to the case without surface moisture (Mf = 0). These results were consistent with those of related studies on the common fluidized bed33 and further indicated that the suppression of separation characteristics in a vibro-fluidized bed was limited to certain ranges of Mf. The suppression effect could be ascribed to the liquid bridge between lignite particles. When Mf increased, the liquid bridge between lignite particles was enhanced and the flow properties of these particles deteriorated, which generated poor and irregular fluidization.45 Therefore, the redundant moisture should be dewatered prior to the separation process. 3.1.5. Optimized Separation Performance. Optimized separation performance was achieved when U* = 1.3, K = 1.4, H = 50 mm, Mf = 4% (−6−3 mm), and Mf = 2% (−3−1 mm). The segregation patterns under these optimal conditions are shown in Figure 6. As expected, the ash contents gradually

Figure 4. Segregation degree at different bed heights.

the segregation performance of both sizes of lignite did not significantly change. However, when the bed height was greater than 70 mm, the segregation performance dramatically declined. This unfavorable result could be explained by the gradually diminishing effect of mechanical vibration when the bed height exceeded a critical value. Meanwhile, the large size of air bubbles in the upper layers with a large bed height was so intense that segregation failed to occur. This effect also contributed to the decline in segregation performance. 3.1.4. Effects of the Moisture Content. During the separation process, lignite particles contained a high moisture content, although the air may remove a small amount of moisture. The stickiness of the lignite particles tended to be heightened by the amount of surface moisture (Mf), which also increased particle agglomeration and prevented the onset of fluidization.44 This unbeneficial behavior could affect separation performance. To study the effect of surface moisture on separation performance, different surface moisture contents of lignite were realized through different drying times in the oven. The segregation results of the two types of lignite with different surface moisture contents are show in Figure 5. The moisture content served an important function in the

Figure 6. Optimal segregation patterns of the two sizes of feed coal.

increased as the bed height increased. This result indicated that the bulk density also exhibited an increasing trend, thus suggesting the occurrence of density segregation. These data are used to plot the partition curves of two sizes of feed coal, as shown in Figure 7. The probable error E values of −6−3 mm and −3−1 mm size fractions of feed coal were 0.202 and 0.225, respectively. Considering the small differences in separation 4386

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Figure 7. Partition curves of the two sizes of feed coal.

performance between these two particle sizes, the sample of −3−1 mm was analyzed in the subsequent study. 3.2. Drying Performance. The results in Figure 5 show that, prior to the separation process, the evaporation of the redundant surface moisture could mitigate the suppression effect of the liquid bridge between lignite particles. Therefore, a vibro-fluidized-bed dryer was introduced. In a vibro-fluidizedbed dryer, many studies demonstrated that operation conditions significantly affected drying characteristics.46 Consequently, the effects of the inlet air temperature, superficial air velocity, vibration strength, and bed height on the drying performance of lignite particles were systematically investigated. 3.2.1. Effects of the Inlet Air Temperature. The changes in the moisture content and drying rate with three different temperatures are shown in Figure 8. Figure 8a shows that, during the drying process of lignite particles, a rapid constant rate stage initially occurred, followed by a slow falling rate period. Meanwhile, the moisture content presented a decreasing trend until it reached the critical level. In addition, the drying rate reached the maximum value after the constant rate stage, after which the drying rate gradually declined and finally approached zero (Figure 8b). Similar results could be obtained under other conditions. Therefore, only the moisture content curves are shown in the subsequent study for concision purposes. Figure 8 shows that an increase in inlet temperature resulted in increased moisture content reduction and reduced drying time. Higher drying rates were observed in both periods. These results indicated that a higher temperature favored the drying characteristics, which was also observed in the ordinary fluidized-bed dryer.12 A higher drying temperature resulted in an increased evaporation rate at a constant rate period and in enhanced intraparticle moisture transport at the falling rate period, which resulted in improved drying performance. However, a very high inlet temperature (>200 °C) could decompose the organic structure of lignite.47 Given that the temperature of coal-fired flue gas was approximately 110−160 °C, the appropriate inlet temperature in this study should be 160 °C to use the residual heat of flue gas. 3.2.2. Effects of the Superficial Air Velocity. The effects of the superficial air velocity on the moisture content are shown in Figure 9. An increase in the superficial air velocity resulted in a greater reduction in the moisture content and shorter drying time. This observation could be attributed to the higher mixing behavior of lignite particles and the enhancement of moisture

Figure 8. Drying characteristics for various inlet air temperatures (particle size, 1−3 mm; bed height, 40 mm; air velocity, 0.47 m/s; and vibration strength, 1.4): (a) moisture content versus time and (b) drying rate curves.

Figure 9. Drying characteristics for various air velocities (particle size, 1−3 mm; bed height, 40 mm; inlet air temperature, 160 °C; and vibration strength, 1.4).

transport. When the superficial air velocity was further increased from U* = 2.0 to 2.2, the drying cures showed no considerable improvement, which was also observed in the ordinary fluidized bed.12 To save energy, the suitable fluidization velocity for maximizing the drying behavior should be determined for drying lignite particles. 3.2.3. Effects of the Vibration Strength. To examine the effects of the intensity of mechanical vibration on the drying of lignite particles, a vibration strength (K) is adopted in this study to characterize the intensity of mechanical vibration. Figure 10 4387

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researchers49 also argued that the tendency of agglomerations, which could counteract the improvement in mechanical vibration, was more serious than that with an increasing bed height. Therefore, the relatively low bed height was considered in the vibro-fluidized-bed dryer. 3.3. Performance of Combined Drying/Dry Separation Chinese Lignite. Figure 2 shows that increasing the gas velocity resulted in two states of the vibro-fluidized bed: segregation and mixing. The segregation benefited the dry separation process, whereas the mixing was favorable for the drying process. On one hand, the separation process reached the maximum segregation condition at lower gas velocity (U* = 1.3) (Figure 2). Moreover, considering the appropriate vibrated fluidizing conditions, the magnitude of segregation in a vibrofluidized bed was more than that of the case without vibration (Figure 3), which suggested that the separation performance was improved by vibration. On the other hand, the drying rate exhibited the maximum value at the larger gas velocity (U* = 2.0) (Figure 9), which implied that the maximum mixing condition was also achieved. More importantly, considering the suitable vibrated fluidizing conditions, the drying performance of the vibro-fluidized bed was also enhanced under vibration when compared to that of without vibration (Figure 10). These results confirmed that the vibration effectively reduced the sizes of the bubbles and enhanced the mixing and segregation behavior of lignite particles in the fluidized bed.33 As a result, with the difference in gas velocity, the drying and dry separation behaviors improved under vibration. These promising results led to the combination of the drying and dry separation processes in a vibro-fluidized-bed dryer. On the basis of these results, three typical situations were selected: lower gas (U* = 1.2), larger gas (U* = 2.0), and changed gas [U* = 2.0 (one stage = 0−150 s), followed by U* = 1.2 (the other stage > 150 s)] velocity conditions. Meanwhile, the constant values of the other parameters were set as 160 °C air temperature, 1−3 mm particle size, K = 1.4 vibration strength, and 40 mm bed height. The drying and separation results of these three typical conditions are shown in Figure 12 and Table 4, respectively. At the lower gas velocity condition, the drying characteristic showed a longer drying time and lower drying rate (Figure 12), with separation presenting a favorable behavior (Table 4). This observation, which verified the

Figure 10. Drying characteristics for various vibration strengths (particle size, 1−3 mm; bed height, 40 mm; inlet air temperature, 160 °C; and air velocity, 0.47 m/s).

shows that the vibration strength improved the drying of lignite particles at higher vibration strengths (K > 1). Similar observations were also reported by previous drying studies using other types of moist materials.22,48 These studies demonstrated that applying mechanical vibration to the fluidized bed was an effective method of reducing the sizes of bubbles and enhancing fluidization quality, which finally resulted in improving the heat and mass transfer. However, at the low level of vibration strength (K < 1), the drying curves poorly performed, with a reduction in the moisture content and a longer drying time compared to the case without vibration (K = 1). These results, which agree with those of related studies,44,49 indicated that the vibro-fluidized-bed dryer enhanced the drying performance only at certain ranges of K. 3.3.4. Effects of the Bed Height. The effect of the bed height was investigated in the range of 20−60 mm at 160 °C air temperature, 1−3 mm particle size, Umf = 2.0 superficial air velocity, and K = 1.4 vibration strength. These drying results are shown in Figure 11. The increase in the settled bed height decreased the reduction in the moisture content but increased the drying time. The results of the conventional fluidized bed support the contention of this study,9 which suggested that both external and internal diffusion controls were reinforced when the bed height was increased. Furthermore, several

Figure 11. Drying characteristics for various bed heights (particle size, 1−3 mm; inlet air temperature, 160 °C; air velocity, 0.47 m/s; and vibration strength, 1.4).

Figure 12. Drying characteristics for various conditions (particle size, 1−3 mm; inlet air temperature, 160 °C; bed height, 40 mm; and vibration strength, 1.4). 4388

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of dried and de-ashing lignite increased to 5200 kcal/kg, which implied that this upgraded lignite could be used as steam coal. A sequential drying and separation process in a vibrofluidized-bed dryer was designed on the basis of the analysis of the separation and drying characteristic results, as shown in Figure 14. The bed could be divided into three stages: I, II, and III. Crushed lignite was fed to the first stage of the fluidized-bed dryer with a high gas velocity (U* = 2.0). This bed with hot air (160 °C) was completely fluidized at a mixing state to evaporate the surface moisture. In this stage, the lignite was dried with a rapid constant rate and most of the moisture was eliminated by drying gas. This stage obviously required high gas velocity. The residence time of this stage could be approximately 150 s until the moisture content was 2−4 wt % more than that of the inherent moisture content. In stage II, this bed was fluidized with a low gas velocity (U* = 1.2) at a segregation state where non-fluidizable materials, such as higher density fractions (gangues), were segregated to the bottom of the dryer. These gangues were discharged as a stream of higher mineral matter and sulfur and mercury contents. According to our previous study,31 this stage could last at least 120 s. Meanwhile, approximately 2−4 wt % of the moisture could also be removed. In stage III, the inherent moisture of lignite was dried with a slow falling rate. A small drying gas flow (U < Umf) was introduced to further dry the lignite to the required content. According to the research by Liu et al.,50,51 when 0 < U < Umf, both the vibration and the gas flow affected the particles, which resulted in an interesting mixing state. Applying this special state instead of an ordinary mixing state (U > Umf) could save the energy of drying moist materials (such as lignite particles) in the slow falling rate period. In addition, to prevent the autoignition of lignite, an inert gas (typically nitrogen from an air separation unit or a low-pressure CO2 stream) was needed. In comparison to the conventional drying or separation process, this proposed system may have several advantages: (1) In comparison to treatment with only the dewatering or separation process, this proposed system produced a higher calorific value of clean lignite. This method can not only improve the efficiency of the power plants but also reduce particulate materials, SOx, and emissions of trace elements. (2) With the change in gas velocity, the separation and drying processes could be combined in the same vibro-fluidized-bed dryer. Both processes also achieved promising results. Thus, this proposed system could improve the utilization efficiency of equipment, reduce equipment size, and diminish the cost of upgrading. (3) In the conventional separation process, the particles are fluidized by the ambient air. The preliminary study (shown in Figure S1 of the Supporting Information) suggested that the separation behavior could be improved with the hot gas. Hence, this proposed system (stage II) could enhance the separation performance. (4) In the slow falling rate period, the conventional drying process adopted an ordinary mixing state (U > Umf). The preliminary tests (shown in Figures S2 and S3 of the Supporting Information) confirmed that this proposed system (stage III) could not only realize appreciable drying behavior but also save energy.

Table 4. Separation Results at Different Conditions condition larger gas velocity condition

changed gas velocity condition

lower gas velocity condition

product

ash content (%)

clean coal middlings gangue clean coal middlings gangue clean coal middlings gangue

29.14 38.15 38.04 16.53 34.12 65.47 14.54 36.07 67.27

previous results (Figures 2 and 9), demonstrated that a lower gas velocity was disadvantageous for the drying process but was advantageous for the separation process. Conversely, under the condition of a larger gas velocity, this drying characteristic achieved a shorter drying time and higher drying rate. Nevertheless, separation presented poor performance (Table 4). The larger gas velocity appeared to be beneficial for the drying process but was unfavorable for the separation process. Finally, the changed gas velocity condition was examined. In the first stage (0−150 s), the larger gas velocity (U* = 2.0) was adopted. This drying characteristic maintained better behavior, similar to the case of the larger gas velocity condition. Given the evaporation of the redundant surface moisture in the first stage, the lower gas velocity (U* = 1.2) was then introduced. This separation also presented excellent behavior that was almost commensurate with that under the lower gas velocity condition. These encouraging performances in both drying and dry separation processes confirmed the feasibility of the combined drying/dry separation Chinese lignite in a vibrofluidized bed. To confirm the superiority of this combined drying/dry separation process, further low calorific value analysis of the original or upgraded lignite was conducted. Figure 13 shows

Figure 13. Low calorific value analysis of the original or upgrading lignite.

that the low heat value of the original lignite was lower than 3000 kcal/kg, which meant that this lignite was a particularly low-grade coal with higher contents of moisture and ash. When treated with the dewatering process, the low calorific value of dried lignite increased to 4100 kcal/kg. However, this value was lower than 5000 kcal/kg, which indicated that this dried lignite cannot be considered as steam coal. When treated with the combined drying/dry separation process, the low calorific value

4. CONCLUSION An analysis of the results of the separation and drying characteristics showed that separation performance was strongly affected by the superficial air velocity, vibration strength, bed height, and surface moisture content. Meanwhile, 4389

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Figure 14. Schematic representation of a sequential drying and separation process in a vibro-fluidized-bed dryer.

Notes

the drying performance was largely dependent upon the inlet air temperature, superficial air velocity, vibration strength, and bed height. When the gas velocity increased, the vibro-fluidized bed presented two phases of segregation and mixing, which apparently favored dry separation and drying processes, respectively. Furthermore, in comparison to an ordinary fluidized bed, the separation performance and drying characteristics of the vibro-fluidized-bed dryer improved under the same operating conditions. To combine the drying and dry separation processes in a vibro-fluidized-bed dryer, the fixed gas and changed gas velocity conditions were examined. With the fixed gas velocity condition, only one process obtained the maximized behaviors, whereas the other process poorly performed. Fortunately, with the changed gas velocity condition, the separation and drying processes could be combined in the same vibro-fluidized-bed dryer and both processes achieved promising results. The low calorific value analysis was conducted, and the results verified the advantage of this combined process. These encouraging results were applied to design a sequential drying and separation system, which was divided into three stages. This sequential drying/dry separation process may be a new option for the beneficiation and drying of −6 mm lignite. Further economical assessments for this proposed system and verification of the advantages are being conducted.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was sponsored by the Key Project of Chinese National Programs for Fundamental Research and Development (973 Program) (2012CB214904), the National Natural Science Foundation of China for Innovative Research Group (51221462), and the Fundamental Research Funds for the Central Universities (2012QNB06).

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ASSOCIATED CONTENT

S Supporting Information *

Segregation degree for different frequencies at different temperatures (Figure S1), flow patterns for different gas velocities of the vibro-fluidized-bed column (Figure S2), and drying performance of the lignite containing inherent moisture for different gas velocities of the vibro-fluidized bed (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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