Separation and Upgrading of Fine Lignite in a Pulsed Fluidized Bed. 2

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Separation and Upgrading of Fine Lignite in Pulsed Fluidized Bed. Part 2: Experimental Study on Lignite Separation Characteristics and Improvement of Separation Efficiency Cheng Sheng, Chenlong Duan, Yuemin Zhao, Panpan Zhang, and Liang Dong Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02941 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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Separation and Upgrading of Fine Lignite in Pulsed Fluidized Bed. Part 2: Experimental Study on Lignite Separation Characteristics and Improvement of Separation Efficiency Cheng Sheng,†,‡ Chenlong Duan,∗,† Yuemin Zhao,∗,† Panpan Zhang, and Liang Dong †School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, China ‡Forschungszentrum Juelich, Juelich 52428, Germany E-mail: [email protected]; [email protected] Phone: +86 516 83591102. Fax: +86 516 83591101 Abstract A pulsed fluidized bed system was established, targeting the analysis of the drying and separation characteristics of fine lignite. Experiments, involving factors of air velocity, pulsating frequency and bed height were conducted under room temperature to analyse the influence of each factor on the separation characteristics of -6+3 mm lignite and -3+1 mm lignite in pulsed fluidized bed, respectively. The combustible recovery and standard deviation of ash content were implemented to evaluate the separation efficiency. Results show three factors have significant impact on the separation efficiency of fine lignite. Afterwards, the thermal energy characterized by inlet temperature was imported into pulsed fluidized bed system to study the influence of inlet temperature, air velocity, pulsating frequency and bed height on separation

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ACS Paragon Plus Environment

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efficiency of fine lignite under thermal condition. Results show that ash content of top layer product decreases significantly from 14.44% to 9.82% and from 15.52% to 7.31% when the pulsating frequency increases from 0 Hz to 3.93 Hz under room temperature and thermal condition for -6+3 mm lignite, respectively. For -3+1 mm lignite, the ash content of top layer product decreases from 17.22% to 10.25% and from 16.77% to 10.01% under room temperature and thermal condition, respectively. Optimal operation parameters were determined. With the air velocity of 1.09 m/s, inlet temperature of 80 ◦ C, pulsating frequency of 3.93 Hz and bed height of 120 mm, the optimal separation efficiency was achieved for -6+3 mm lignite. For -3+1 mm lignite, the optimal is achieved when air velocity, inlet temperature, pulsating frequency and bed height are 0.55 m/s, 100 ◦ C, 3.49 Hz and 80 mm. Separation efficiency of lignite with pulsed fluidized bed was dramatically improved.

1 INTRODUCTION Coal occupies a leading position in primary energy consumption. Combined with crude oil and natural gas, coal and lignite make the largest contribution to covering around 80% of German primary energy consumption. 1 Meanwhile, it is estimated that coal accounts for 39% of global fuel consumption for electricity generation in next ten years. 2,3 Energy study (2016) 1 reports coal has the largest potential of all non-renewable energy resources with a share of around 55% of reserves and around 89% of resources and is predicted to play a major role against the practice of the expected rise in global primary energy consumption. 1 Lignite is an abundant raw coal resource, which is estimated to account for approximately 13% of coal reserves in China and is the unique resource of all of the non-renewable energy resources that is available in large economically extractable amounts in Germany. 1,4–6 The lignite is increasingly implemented as a primary energy supply due to the large reserves and low mining cost worldwide. 4–6 Germany is the largest lignite producer and consumer worldwide in 2015. 1 23.1% of the Germany’s gross electricity is generated by the combustion of lignite. 1 However, due to characteristics of high moisture content (20-55 wt%) 6 , high ash content, high volatile content and 2


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low calorific value, etc., the direct combustion of lignite results in low combustion efficiency, high transportation cost and scorification in the combustor. Hence, prior to direct use, the upgrading of lignite by discarding ash and removing moisture can significantly increases calorific capacity of lignite and further improve combustion efficiency. 6,7 Coal beneficiation before utilization is the most effective and economical method for discarding hazardous components and gangue, 6 thereby alleviating environmental pollution and improving coal utilization efficiency. Hence, prior to the combustion, drying and separation of lignite significantly are capable of reducing moisture content and ash content and improving the combustion efficiency. As lignite is easy to slime, wetting separation methods are inapplicable for lignite separation. In contrast, dry coal separation technology has the superiority of avoiding lignite slimming in separation process. Thus, dry coal separation methods are promising in the separation and upgrading of lignite. The liquid bridge between lignite particles due to high water content jeopardizes the separation. Onefold separation without drying of lignite cannot eliminate the adverse effect of liquid bridge on the separation. Otherwise, clean lignite product still contains large amount of water content after the separation, which reduces the calorific capacity of clean lignite. Hence, it is necessary to include the drying operation in lignite separation process. In fact, dry coal drying and simultaneously separation technologies have become increasingly significant for lignite drying and separation throughout the world. Among these technologies, fluidized bed drying and separation have numerous advantages of high processing capacity, low cost, and low energy consumption. 8–10 Fluidized bed has been widely studied in order to verify the capability of drying and separating lignite. The fluidized bed was initially reported as a combustor, which offers a means of burning coal, other low grade fuels and waste materials with high moisture and ash content in an economical and environmentally accepted way due to the outstanding drying and separation characteristics of fluidized bed. 11,12 Calban and Ersahan (2003) 13 and Çalban (2006) 14 explored the influence of a number of factors, like bed height, temperature, air velocity, initial moisture concentration on drying lignite and particle size on the drying of lignite and obtained that efficient drying results can be achieved under certain operation conditions in conventional fluidized bed. 15 A number of sce-

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narios of drying low rank coal with superheated stream in fluidized bed have been reported 16 and showed that superheated steam fluidized bed technology is a mature and advanced process of lignite drying. Compared with conventional fluidized bed dryers, additional field fluidized bed drying technologies have been developed and occupy significant advantages. Tahmasebi et al. (2014) 17 investigated the drying kinetics of lignite in microwave-assisted fluidized bed dryer. Compared with conventional fluidized bed dryer, the microwave-assisted fluidized bed dryer can penetrate the materials and create temperature uniformity, hence enhancing the drying rate. 15 Other additional field assisted fluidized bed dryer, like vibration-assisted fluidized bed dryer, 18,19 agitation-assisted fluidized bed dryer 20 and acoustic-field assisted fluidized bed dryer have been widely studied and reported to have significant effect on improving the drying efficiency. Likewise, separation and upgrading of lignite with fluidized bed has become research hotspot in recent years. With purposes of drying lignite and simultaneously discarding gangue, numerous experiments have been conducted with conventional fluidized bed and additional field fluidized beds. Lubin et al. (2014) 10 analyzed the influence of multi factors, like inlet temperature, air velocity, surface moisture, particle size on the drying and separation of lignite with fluidized bed and reported that the surface moisture can be reduced by 94% with separation probable error value of 0.046 g/cm3 . Zhang and You (2011) 4 investigated an appropriate multi-scale approach to describe lignite particle drying in a fixed bed. The approach was verified by experimental results. Zhao et al. (2014) 6 studied the feasibility of sequential drying and simultaneously separation of fine lignite in a vibrated fluidized bed. It is reported that promising results were achieved. 6 The vibrated gassolid fluidized bed was investigated and utilized for the drying and simultaneously separation of lignite. 21 By introducing the vibrated energy, the diffusion and stratification of lignite particles in pulsed fluidized bed are promoted, as well as the density-based hindered settling process. Hence, the separation performance was improved correspondingly. 21 Pulsed fluidized bed is a typical additional field fluidized bed, 22–24 which implements pulsating air flow to promote the diffusion and stratification of particles based on density difference and improve the drying performance and separation efficiency. Researchers from China University and

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Mining and Technology studied the characteristics of fine coal beneficiation using a pulsing air dense medium fluidized bed. The ash content of fine coal has been reported to be reduced significantly. 23,24 A separation probable error value of 0.085 g/cm3 was achieved, which indicates an excellent separation result. 24 Compared with conventional fluidized bed, pulsed fluidized bed occupies characteristics of no use of magnetite powder, low energy consumption and high processing capacity. In the proposed work, the pulsed fluidized bed technology was implemented with attempts to dry and separate the fine lignite simultaneously. The proposed work focuses on the second part of the research-experimental study on lignite separation characteristics and improving the separation efficiency. A novel pulsed fluidized bed system was designed and established. The tentative experimental researches on the influence of each single factor on the separation characteristics were studied under both room temperature and thermal condition. Results achieved under thermal condition were compared with results obtained under room temperature to verify the feasibility and efficiency of drying and simultaneously separating lignite with pulsed fludized bed.

2 MATERIALS AND METHODS 2.1

Experimental Materials

Lignite samples were taken from Shengli coal field, which is located in Inner Mongolia, China. Coal samples were comminuted and then analyzed by sieving analysis. Raw lignite samples of two granular levels, namely -6+3 mm (the size of lignite particle is large than 3 mm and less than 6 mm) and -3+1 mm (the size of lignite particle is large than 1 mm and less than 3 mm), were sampled and analyzed by proximate analysis and ultimate analysis. Results are illustrated in Table 1. From the proximate analysis, it can be seen the total moisture contents (dry basis) of -6+3 mm lignite and -3+6 mm lignite are 31.66% and 29.99%, which are high initial moisture contents. High moisture content results in adverse effect in the combustion of lignite as the evaporation of water requires excess input of thermal energy. Calorific capacities for -6+3 mm lignite and -3+1 mm lignite are 5


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2667.50 Cal/kg and 2757.31 Cal/kg, which are much lower than that required for thermal coal. Prior to implementing lignite in an economical and environment-friendly way, lignite is required to be dried and separated in order to reduce moisture content and ash content and increase the calorific capacity correspondingly. Table 1. Proximate Analysis and Ultimate Analysis Results of Lignite Coal Samples Granularity Total level moisture (mm) Mt (%) -6+3 -3+1

2.2

31.66 29.99

Internal moisture Mad (%)

Ash Volatile content content Aad (%) Vad (%)

13.79 10.34

27.51 27.81

47.12 46.44

Fixed carbon FCad (%)

Calorific capacity (Cal/kg)

52.88 53.56

2667.50 2757.31

Experimental Apparatus

The experimental apparatus used for separating and upgrading lignite coal samples in the pulsed fluidized bed is shown in Figure 1.

12 3

6

P

TC1

11

4

1

5

2

7 FI

9

1

8

10

Figure 1. Schematic Diagram of the Experimental Setup 1-Draught fan; 2-Air bellow; 3-Valve; 4-Gas heater; 5-Ball valve; 6-Manometer; 7-Flowmeter; 8-Electromotor; 9-Frequency converter; 10-Butterfly valve; 11-Fluidized bed; 12-Thermocouple The core of the apparatus involves: gas heater, pulsating system and fluidized bed with thermocouple measuring equipment. The gas heater equipment includes electrical heater and controller. 6


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The column of the fluidized bed has a height of 0.4 m and inner diameter of 0.08 m. The electrical heater consists of electrical heating tubes. Controller is responsible for setting the range of heating temperature in order to satisfy experimental requirements. The electrical heater ceases working until the given inlet gas temperature value is achieved. Otherwise, when the inlet temperature is lower than the given temperature, the controller switches on the electrical heater automatically. In the heating process, it is imperative to have the gas involved inside the heater. The pulsating system possesses electromotor, decelerator, frequency converter and butterfly valve. The operation principle of pulsating system is elaborated: When the air flow and inlet temperature are steady, the electromotor is activated. The electromotor is connected with butterfly valve. There is a disc positioned in the center of butterfly valve. A rod passes through the disc to an actuator on the outside of the valve. The actuator is connected with electromotor. The rotation of actuator turns the disc periodically from the position that is perpendicular to the direction of air flow to the position that is parallel to the direction of air flow and then back to the position that is perpendicular to the direction of air flow. When the disc is parallel to the direction of air flow, air flow passing through butterfly valve reaches the maximum value. When the disc is perpendicular to the direction of air flow, air flow is 0 m3 /s. Air flow exhibits periodical characteristics under the effect of periodical rotation of butterfly disc. As the speed of an Alternating Current (AC) induction motor depends on the frequency of the supply voltage, it is possible to alter electromotor speed by transforming the frequency of the voltage of power supply. The frequency converter, which is variable-frequency drive, connects electromotor and power supply and is capable of adjusting the frequency of power supply. Thus, frequency converter is implemented as an efficient device to adjust the speed of electromotor and further control the pulsating frequency of air flow. The air flow entering pulsed fluidized bed can be calculated based on Equation 1.

V = V0 (1 − |cos(α)|)

(1)

where V represents the butterfly valve exit air flow; V0 is the butterfly valve inlet air flow; α denotes rotating angle of valve disc. 7


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The air flow was measured by the flowmeter, which is illustrated in Figure 1. After acquiring the air flow valve, air velocity was calculated by dividing air flow with cross section area of pulsed fluidized bed body. Hence, the variation of air velocity is in positive correlation with periodical variation of air flow. However, in order to study the influence of air velocity on drying characteristics of lignite with pulsed fluidized bed, the air velocity in the proposed work is defined as the ratio between the air flow value measured when butterfly valve is fully opened and cross section area of pulsed fluidized bed body. The maximum air flux passing through butterfly valve was controlled by ball valve, which is illustrated in Figure 1. When rotating angle of valve disc is from 0 to 1π, the air flow experiences one period. When the butterfly valve disc rotates back from the angle of 1π to 0, the air flow experiences the second period. Therefore, the frequency of pulsed air flow can be calculated as twice the value of rotating period of butterfly valve disc, which is determined by the rotating speed of the electromotor. In order to achieve desired pulsating frequency of air flow, the frequency converter (Figure 1) was implemented in the experiment to adjust the rotating speed of the electromotor.

2.3 Experimental Procedure The separation and upgrading procedure of fine lignite coal is demonstrated as follows: The air is transported into the air bellow under the effect of draught fan. And then constant air flow is delivered through gas heater, where the air is heated up to a certain temperature. Then, through the pipe, the heated air enters the butterfly section, where pulsating air flow is generated with the periodical rotation of butterfly valve. After that, the pulsating air flow is ejected into the fluidized bed across air distributing chamber. Due to the density difference between coal particle and gangue particle, Coal particles are drifted by the force of air flow and have the analogous trend of movements as air flow, while gangue particles sink towards the lower part of the bed in the density-based hindered settling process. Hence, feeding lignite particles are dried and separated simultaneously. In order to conduct researches under room temperature, the gas heater was switched off during the separation of lignite. 8


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Otherwise, in the experimental process, the inlet and outlet temperatures and temperature inside the bed are measured simultaneously by the thermocouple. With the measurement, the variation of temperatures at different locations can be recorded and analyzed. After each experiment, the bed was divided equally into four layers in the axial direction. The product of each layer has the same productivity, which is 25%. The product of each layer was sampled and analyzed to obtain the ash content of each layer product. Then, the standard deviation of ash content of each experiment was calculated, as well as combustible recovery. Results are projected to be compared with further experimental results achieved under thermal condition to determine the optimized operation conditions. The segregation standard deviation of ash content σash and combustible recovery ε were combined and implemented to investigate the effect of lignite separation efficiency in pulsed fluidized bed. s σash =

1 n ¯ 2 ∑ (A(i) − A) n − 1 i=1

(2)

n

A¯ = ∑ A(i)γ(i)

(3)

i=1

ε = γ(i)(

100 − A(i) ) × 100% 100 − A¯

(4)

where A(i) (%) is the ash content of i-layer product; γ(i) (%) denotes productive rate of i-layer product; A¯ (%) is weighted mean of ash contents of all layer products. A¯ is equal with the ash content of raw lignite coal.

2.4

Sample Preparation and Characterization

In order to get a better understanding of coal wash-ability characteristics, float-and-sink experiments for -6+3 mm lignite and -3+1 mm lignite were conducted. The feed coal was successively separated in dense solutions with densities of 1.3 g/cm3 , 1.4 g/cm3 , 1.5 g/cm3 , 1.6 g/cm3 , 1.8 g/cm3 , 2.0 g/cm3 , respectively. Corresponding float and sink products were collected, weighted and analyzed to get the productivity and ash content, correspondingly. The heavier product after 9


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the separation with a density of 2.0 g/cm3 was collected and dried. The float-and-sink results for -6+3 mm lignite and -3+6 mm lignite are shown in Table 2 and Table 3. The wash-ability characteristics curves of feed coal were achieved and shown in Figure 2. Table 2. Float-sink Analysis for -6+3 mm Lignite Density (g/cm3 )

Yield (%)

Ash content (%)

Float accumulation

Sink accumulation

Separation density±0.1

Yield (%)

Ash content (%)

Yield (%)

Ash content (%)

Density (g/cm3 )

Yield (%)

-1.30 1.30-1.40 1.40-1.50 1.50-1.60 1.60-1.80 1.80-2.00 +2.00 Total

49.41 14.53 4.82 2.68 1.05 9.20 18.31 100.00

9.95 10.98 27.37 35.75 43.16 59.94 69.67

49.41 63.94 68.76 71.44 72.49 81.69 100.00

9.95 10.18 11.39 12.30 12.75 18.06 27.51

100.00 50.59 36.06 31.24 28.56 27.51 18.31

27.51 44.67 58.24 63.00 65.56 66.42 69.67

1.30 1.40 1.50 1.60 1.80 1.90 2.10

63.94 19.35 7.50 3.21 5.13 9.20 17.18

Table 3. Float-sink Analysis for -3+1 mm Lignite Density (g/cm3 )

Yield (%)

Ash content (%)

Float accumulation

Sink accumulation

Separation density±0.1

Yield (%)

Ash content (%)

Yield (%)

Ash content (%)

Density (g/cm3 )

Yield (%)

-1.30 1.30-1.40 1.40-1.50 1.50-1.60 1.60-1.80 1.80-2.00 +2.00 Total

45.21 19.98 7.52 4.15 2.89 3.20 17.05 100.00

11.23 16.74 30.15 40.78 50.78 66.13 69.45

45.21 65.19 72.71 76.86 79.75 82.95 100.00

11.23 12.92 14.70 16.11 17.37 19.25 27.81

100.00 54.79 34.81 27.29 23.14 20.25 17.05

27.81 41.48 55.69 62.72 66.66 68.93 69.45

1.30 1.40 1.50 1.60 1.80 1.90 2.10

65.19 27.50 11.67 5.60 3.05 3.20 17.18

In the wash-ability characteristics curve, λ curve is the characteristic ash curve; β curve is the cumulative float curve; θ curve denotes the cumulative sink curve; δ curve represents the density curve and τ curve is the δ ±0.1 curve. Generally, the wash ability curve demonstrates difficulty levels of coal separation. It can be seen from the Figure 2 that when the theoretical separation density is located in 1.6∼2.00 g/cm3 , the density±0.1 values for -6+3 mm lignite and -3+1 mm 10


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Density(g/cm3)

1,9

1,8

1,7

1,6

1,5

1,4

1,3

-6+3mm

10 20

1,2 100 90 80



30

70

40

60



50

50 

60 70

40





2,2 0

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90

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1,4

0 100

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1,2 100 90 80 70 60

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1,6

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10 30

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90 20

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-3+1mm

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100

2,1

10

Float productivity (%)

2,0

Sink productivity(%)

2,1

30



Sink productivity (%)

Density(g/cm3) 2,2 0

Float productivity(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

20 10

0

10

20

30

40

50

60

70

80

90

0 100

Ash content (%)

Ash content(%)

Figure 2. Washability Curves of -6+3 mm Lignite and -3+1 mm Lignite lignite are around 6%∼15%. Thus -6+3 mm lignite and -3+1 mm lignite can be clarified as easyto-wash coal.

3

RESULTS AND DISCUSSIONS

3.1 Study on the Separation Characteristics under Room Temperature Initially, the separation characteristics of the pulsed fluidized bed were explored without the input of thermal source. The bed height, air velocity and pulsating frequency were selected as factors in the experiment in order to explore the influence of each factor on separation effect of lignite in pulsed fluidized bed under room temperature.

3.1.1 The Influence of Bed Height on Separation Characteristics of Lignite The pulsating energy carried by pulsating air flow depletes along the direction from the bottom to the top of the bed. Other influence factors are set as constants. To achieve the decompaction and stratification of lignite particles, the bed height of a reasonable range is capable of obtaining a better separation result. -6+3 mm lignite and -3+1 mm lignite were used in the experiment. The Operation parameter is shown in Table 4.

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Table 4. Parameters for Studying the Influence of Bed Height under Room Temperature Lignite particle Air velocity (m/s) Pulsating frequency (Hz) -6+3 mm -3+1 mm

1.09 0.55

2.18 1.75

Different bed heights (e.g. 60 mm, 100 mm, 120 mm, 140 mm, 180 mm) were set in the experiment for -6+3 mm lignite. For separation of -3+1 mm lignite, bed heights were set as 60 mm, 80 mm, 100 mm, 120 mm, 140 mm. The separation time was set as 3 min. Experimental results are shown in the Figure 3 and Figure 4. H ∗ denotes the height of each product layer. Horizontal coordinate represents the ash content. The dash lines in the middle of each sub-figure denotes the ash content of raw lignite. After the experiment, the bed was divided equally into successive four layers from the top to the bottom. The products were sampled from different layers. In the Figure 3 and Figure 4, the ash content of each layer lignite product increases with decrease of bed height, indicating a density based separation was achieved in the experiment. Under the effect of pulsating air flow, coal particles are drifted by the force of pulsating air flow and move upwards generally, while gangue particles sink towards the bottom of the bed due to the relatively larger density. The ash content of the first layer coal sample decreases firstly and then increases with the increase of bed height from 60 cm to 180 cm. For -6+3 mm lignite, when the bed height is 60 mm, the ash content of first layer sample is 12.02%. The ash content of the first layer sample reaches the lowest value at bed height of 120 mm. Afterwards, it rises up to 15.50% at bed height of 180 mm. The ash contents of other layer samples experience the analogous variation trend.

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H = 100 mm

60

12.02

100

10.90

45

14.16

75

13.67

50

28.95

30

28.95

15

45.83 0

H*

H*

H = 60 mm

25 10

20

30

40

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46.90 0

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20

Aad (%)

140

10.91

90

12.35

105

13.77

30

52.24 0

H*

H*

9.97

25.54

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50

H = 140 mm

120

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Aad (%)

H = 120 mm

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48.67 0

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Aad (%)

H = 180 mm

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

180

15.50

135

16.01

90

27.90

45

42.33 0

10

20

30

40

50

Aad (%)

Figure 3. Ash Content Distribution under Different Bed Heights (-6+3 mm Lignite)

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Energy & Fuels

H = 60 mm

H = 80 mm 80

12.65

45

14.02

60

11.18

30

27.08

40

24.81

H*

12.92

H*

60

15

51.24

20

45.66 0

10

20

Aad (%)

30

40

50

0

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H = 100 mm

20

Aad (%)

30

40

50

40

50

H = 120 mm 120

13.32

75

13.54

90

14.90

50

25.91

60

26.98

25

48.18

30

47.50

H*

14.27

H*

100

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20

Aad (%)

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40

50

40

50

0

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20

Aad (%)

30

H = 140 mm 140

14.87

105

17.11

70

28.41

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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43.79

35 0

10

20

Aad (%)

30

Figure 4. Ash Content Distribution under Different Bed Heights (-3+1 mm Lignite)

In contrast, the ash content of first layer sample possesses the lowest value (11.18%) at bed height of 80mm for -3+1 mm lignite, while the fourth layer sample has the largest ash content, which is 51.25%. The standard deviation of ash contents ( σash ) of four layer products and corresponding combustible recoveries (ε) were investigated. Results are shown in Figure 5.

14


20

54 20



curve

ashcurve

-3+1mm

ash curve

-6+3mm

52

19

 curve

53

18

 (%)

52 16 51

ash (%)

18

ash (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

17

51

50

16

 (%)

Page 15 of 42

49 15

14

14

50

48

13 12

47

49 60

80

100

120

140

160

60

180

80

100

120

140

H (mm)

H (mm)

Figure 5. Effects of Bed Height on σash and ε for -6+3 mm Lignite and -3+1 mm Lignite From the Figure 5, it can be seen that the segregation standard deviations of ash content (σash ) and combustible recoveries (ε) of -6+3 mm lignite and -3+1 mm lignite possess the analogous variation trend: initially, σash and ε rise generally and then decrease with the increase of bed height. σash and ε for -6+3 mm lignite occupy the largest values at bed height of 120 mm, while the largest value occurs at bed height of 80 mm for -3+1 mm lignite. The best separation effect is achieved when the bed height is 120 mm for -6+3 mm lignite and 80 mm for -3+1 mm lignite, respectively. When the bed height is 60 mm, the pulsating air can penetrate the bed easily. There is no enough space for sufficient decompaction and stratification of coal particles in the fluidized bed. Thus, effective separation of lignite particles cannot be achieved. The standard deviation of ash content is small. When the bed height increases up to 120 mm for -6+3 mm lignite and 80 mm for -3+1 mm lignite, the resistance force generated when air flow passes through the bed is relatively smaller than these under even higher bed heights. Materials tend to mix homogeneously, which is beneficial for stratification and separation of lignite particles. The appearance of swarming motion of particles due to the effect of pulsating air flow accelerates the hindered settling process, thus promoting the separation of lignite particles. However, extra high bed height deteriorates the separation effect. Suffice to mention the ap15


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

pearance of channelling, dead zone, which adverse to the separation of lignite particles. When the bed height is extra high, the energy carried by pulsating air flow dissipates in the penetrating process through the bed from bottom to the top. Meanwhile, high bed height provides opportunity for the generation of large irregular bubbles, which have negative effect for the separation of lignite particles. In conclusion, a mediate bed height is capable of achieving better separation effect.

3.1.2

The Influence of Air Velocity on Separation Characteristics of Lignite

The air velocity is a key factor that influences the extent of particle mixing, which directly affects the separation of lignite particles. The influence of air velocity on separation characteristics of lignite is studied in this section. Likewise, the -6+3 mm lignite and -3+1 mm lignite were used in the experiment. Operation parameters are shown in Table 5. The separation time was set as 3min. Different air velocities have been selected and implemented in the experiment for -6+3 mm lignite and -3+1 mm lignite on the basis of lignite particles of -6+3 mm lignite and -3+1 mm lignite are capable of diffusing and mixing distinctly. Under these air velocities, sufficient diffusion and stratification of lignite particles promote and intensify the separation process of lignite in pulsed fluidized bed. Experimental results are shown in the Figure 6 and Figure 7. In the Figure 6, the ash content decreases from top layer to the bottom layer under each air velocity, indicating the density based layering is achieved in the experiment. Different air velocities correspond with different ash content gradients. The largest ash content gradient occurs when the air velocity is 1.09 m/s, when the top layer sample has the smallest ash content compared to the ash contents of top layers under other air velocities. The ash content of the top layer lignite sample decreases first and then rises with the increase of air velocity in general. In Figure 7 (-3+1 mm), the analogous variation trend to that of -6+3 mm lignite can be detected. The ash content decreases from top layer to the bottom layer under each air velocity. The ash content of the top layer lignite sample decreases first and then rises with the increase of air velocity. The largest ash content gradient occurs when the air velocity is 0.55 m/s, where the top layer

16


Page 16 of 42

Page 17 of 42

sample has the smallest ash content compared to the ash contents of top layers under other air velocities. Table 5. Parameters for Studying the Influence of Air Velocity under Room Temperature Lignite particle Bed height (mm) Pulsating frequency (Hz) -6+3 mm -3+1 mm

120 80

2.18 1.75

 = 0.96 m/s

120

24.81

120

14.69

90

27.40

90

22.98

60

27.70

30

29.62 0

H*

H*

 = 0.82 m/s

28.28

60

30 10

20

Aad (%)

30

40

50

38.90 0

10

 = 1.09 m/s 9.97

90

12.35

60

25.54

30

120

13.67

90

15.04

60

24.45

10

20

Aad (%)

30

Aad (%)

30

40

50

40

50

48.82

30

52.24 0

20

 = 1.23 m/s

H*

H*

120

40

50

40

50

0

10

20

Aad (%)

30

 = 1.37 m/s

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

120

18.57

90

18.58

60

23.98

30

46.23 0

10

20

Aad (%)

30

Figure 6. Ash Content Distribution under Different Air Velocities (-6+3 mm Lignite)

17


Energy & Fuels

80

 =0.48 m/s

19.91

80

15.25

60

22.20

60

11.23

40

29.07

40

25.50

20

H*

H*

 = 0.41 m/s

20

38.87 0

10

20

30

40

50

48.11 0

10

20

Aad (%)

50

11.18

80

12.14

60

12.65

60

17.80

40

24.08

40

26.73

20

51.24

20

46.74 40

50

H*

H*

40

 = 0.61m/s

80

0

30

Aad (%)

 = 0.55 m/s

10

20

30

40

50

0

10

20

Aad (%)

30

Aad (%)

H*

 = 0.68 m/s 80

16.12

60

20.62

40

28..44

20

41.62 0

10

20

30

40

50

Aad (%)

Figure 7. Ash Content Distribution under Different Air Velocities (-3+1 mm Lignite)

 curve

54 20

53

 curve

10

 (%)

51 50 5

50 49 10 48

1,0

1,1

1,2

1,3

47

5

48 0,9

52

15

49

0,8

53

51

52

15

0

ashcurve

-3+1 mm

 (%)

ashcurve

-6+3mm

ash (%)

20

ash (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 42

0,40

1,4

0,45

0,50

0,55

0,60

0,65

0,70

 (m/s)

 (m/s)

Figure 8. Effects of Air Velocity on σash and ε for -6+3 mm Lignite and -3+1 mm Lignite From the Figure 8, the highest segregation standard deviation of ash content appears when the 18


Page 19 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

air velocity is 1.09 m/s for -6+3 mm lignite, as well as the combustible recovery, while it is true for -3+1 mm lignite when the air velocity is 0.55 m/s. When the air velocity is infinitesimal, it is impossible for sufficient diffusion and stratification of lignite particles. Dead zones occur in parts of the fluidized bed. It is difficult for lignite particles to mix and segregate comprehensively. The separation effect is unsatisfactory under the condition of small air velocity. Overlarge air velocity results in the formation of irregular large bubbles and intensive swarming motion in the fluidized bed, which deteriorates the separation effect. Thus, the air velocity is a key factor that influences the extent of particle mixing, which directly affects the separation of lignite particles. A reasonable range of air velocity is beneficial for the separation of lignite particles.

3.1.3

The Influence of Pulsating Frequency on Separation Characteristics of Lignite

Pulsating frequency is considered as a significant factor that influences the separation efficiency of lignite in fluidized bed. The introduction of pulsating energy is beneficial to improve the diffusion and stratification ability and promote the mixing of lignite particles. To verify the effect of pulsating frequency on separation efficiency of lignite, control experiment was conducted by setting pulsating frequency to 0 Hz for one experiment and nonzero for others. Operation parameters are shown in Table 6. The separation time was set as 3min. Experimental results are shown in the Figure 9 and Figure 10. Table 6. Parameters for Studying the Influence of Pulsating Frequency under Room Temperature Lignite particle Bed height (mm) Air velocity (m/s) -6+3 mm -3+1 mm

120 80

19


1.09 0.55

Energy & Fuels

 = 1.31Hz

120

14.44

120

12.61

90

26.19

90

16.75

60

31.66

60

27.20

30

34.29

30

46.26

0

H*

H*

 = 0.00 Hz

10

20

30

40

50

0

10

20

Aad (%)

120

90

12.35

90

12.95

60

25.54

60

25.21

30

52.24

30

50.80

H*

H*

9.97

10

20

30

40

50

0

10

20

 = 3.93 Hz

40

50

40

50

 =4.80 Hz 120

11.67

90

11.15

90

16.21

60

23.99

30

52.90

H*

9.82

20

30

Aad (%)

120

10

50

9.97

Aad (%)

0

40

 = 3.06 Hz

120

0

30

Aad (%)

 = 2.18 Hz

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 42

30

40

50

60

25.71

30

49.16 0

10

Aad (%)

20

30

Aad (%)

Figure 9. Ash Content Distribution under Different Pulsating Frequencies (-6+3 mm Lignite)

20


Page 21 of 42

 = 1.05 Hz

80

17.22

80

11.39

60

23.89

60

14.58

40

26.04

40

25.22

20

38.60

20

44.22

0

H*

H*

 = 0.00 Hz

10

20

30

40

50

0

10

20

Aad (%)

80

11.05

60

12.65

60

12.03

40

24.81

40

24.40

20

51.24

20

52.20

H*

H*

11.18

10

20

30

40

50

0

10

20

Aad (%)

60

11.13

60

15.75

40

22.55

40

23.08

20

53.46

20

48.28

H*

10.25

14.50

10

20

30

40

50

40

50

 = 4.36 Hz 80

0

50

Aad (%)

 = 3.49 Hz 80

40

 = 2.62 Hz

80

0

30

Aad (%)

 = 1.75 Hz

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

30

40

50

0

10

Aad (%)

20

30

Aad (%)

Figure 10. Ash Content Distribution under Different Pulsating Frequencies (-3+1 mm Lignite)

The ash contents of -6+3 mm lignite and -3+1 mm lignite have the analogous variation trend under each pulsating frequency. Compared with conventional fluidized bed, the introduction of pulsating energy improves the separation effect. When the pulsating frequency is 0 Hz, the ash contents of top layer samples of -6+3 mm lignite and -3+1 mm lignite are 14.44% and 17.22%, respectively, while the ash contents of top layer samples decrease significantly when the pulsating frequency is imported. The results indicate that pulsating frequency significantly promote the separation effect for both -6+3 mm lignite and -3+1 mm lignite. Because there is no magnetite powders serving as heavy medium, void ratio of fine lignite particles is relatively larger in conventional fluidized bed, the air flow tends to escape from porous structure of the lignite bed with small resistance. Dead zones emerge in the conventional fluidized

21


Energy & Fuels

bed due to the inadequate contact of air flow with fine lignite particles. Hence, fluidization state can barely be achieved, which renders the insufficient diffusion and stratification of lignite particles. The introduction of pulsating energy reduces the void ratio by enhancing particles mixing with particles, which is beneficial to the separation of lignite particles. With the increase of pulsating frequency, ash content of the top layer sample experiences a slight decrease and then rise. The smallest values are 9.82% and 10.25% for -6+3 mm lignite and 3+1 mm lignite, respectively. Corresponding frequencies for -6+3 mm lignite and -3+1 mm lignite are 3.93 Hz and 3.49 Hz, respectively. As pulsating frequency continues increasing, the adverse effect deteriorates the separation results. Thus, the pulsating frequency is better to be optimized to maximize the separation effects for both -6+3 mm lignite and -3+1 mm lignite.

24

55

ash curve

24

ash curve

-6+3mm

 curve

20

-3+1mm

 curve

54 20

54 53

51

12

52 16 51

 (%)

52

ash (%)

16

 (%)

53

ash (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 42

50

12

50

49

8

8

0

1

2

 (Hz)

3

4

5

49 0

1

2

 (Hz)

3

4

5

48

Figure 11. Effects of Pulsating Frequency on σash and ε for -6+3 mm Lignite and -3+1 mm Lignite

From the Figure 11, it can be seen that with the increase of pulsating frequency, the standard deviation of ash content increases dramatically and then decreases. The largest standard deviations emerge when frequencies are 3.93 Hz for -6+3 mm lignite and 3.49 Hz for -3+1 mm lignite. When the pulsating frequency locates in the range of 2.18 Hz to 3.93 Hz, the standard deviation of ash content for -6+3 mm lignite is between 19% and 20% and merely experiences slight variation. The analogous scenario can be detected for -3+1 mm lignite when pulsating frequency is between 1.75 Hz and 3.49 Hz.

22


Page 23 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

In the experiment, the pulsating air flow is generated from the rotation of butterfly valve. The rotation frequency determines the time gap between two consecutive air streams that pass though the value. Frequent contact of the air stream with the lignite particles promotes the decompaction, mixing of lignite particles. Ultimately, a hierarchical structure can be achieved based on density separation. Therefore, a certain range of pulsating frequency is capable of acquiring a better separation effect.

3.2

Study on the Separation Characteristics of Lignite under Thermal Condition

After exploring the separation characteristics of lignite under room temperature, this section focuses on lignite separation and drying under thermal condition. The optimized operation parameters for lignite separation and drying are projected to be investigated and analyzed.

3.2.1

The Effect of Bed Height on Separation Characteristics of Lignite

We consider bed height as an independent variable and other variables as constants. Operation parameters are shown in Table 10. Table 7. Parameters for Studying the Influence of Bed Height Lignite particle Air velocity (m/s) Pulsating frequency (Hz) Inlet temperature (◦ C) -6+3 mm -3+1 mm

1.09 0.55

2.18 1.75

80 80

The duration for each separated experiment was set as 3 min. Ash content distributions for -6+3 mm lignite and -3+1 mm lignite are shown in Figure 12 and Figure 13.

23


Energy & Fuels

H = 100 mm

60

11.17

100

9.23

45

12.73

75

12.16

30

26.40

50

26.08

15

45.23 0

H*

H*

H = 60 mm

25 10

20

30

40

50

51.15 0

10

20

Aad (%)

140

10.55

90

12.00

105

13.16

60

25.28

70

26.95

H*

H*

9.24

10

20

50

51.60

35

53.50 0

40

H = 140 mm

120

30

30

Aad (%)

H = 120 mm

30

40

50

0

10

Aad (%)

20

30

40

50

Aad (%)

H =180 mm

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 42

180

11.86

135

14.01

90

27.24

45

49.00 0

10

20

30

40

50

Aad (%)

Figure 12. Ash Content Distribution under Thermal Condition with Respect to Bed Height (-6+3 mm Lignite)

24


Page 25 of 42

H = 80 mm

60

12.92

80

10.79

45

14.02

60

11.64

30

27.08

40

22.50

15

45.66 0

H*

H*

H = 60 mm

20 10

20

30

40

50

54.18 0

10

20

Aad (%)

120

11.37

75

11.28

90

14.69

50

25.14

H*

H*

11.19

51.47 0

60

20

50

40

50

26.38

30 10

40

H = 120 mm

100

25

30

Aad (%)

H = 100 mm

30

40

50

48.95 0

10

Aad (%)

20

30

Aad (%)

H = 140 mm

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

140

12.24

105

14.70

70

26.45

35

48.42 0

10

20

30

40

50

Aad (%)

Figure 13. Ash Content Distribution under Thermal Condition with Respect to Bed Height (-3+1 mm Lignite)

Compared with results achieved under room temperature, the ash content of the top layer is relatively smaller under thermal condition. For each independent experiment, from the top layer to the bottom layer, analogous variation trend emerges: the ash contents of each layer increases with the decrease of bed height, which demonstrates density based separation has been achieved. Otherwise, the standard deviation of ash content has been analyzed for each independent experiment. Results are shown in Figure 14. In the Figure 14, the cool in the legend demonstrates the data was achieved under room temperature; thermal refers to experimental results achieved under the input of thermal energy, which is characterized by inlet temperature.

25


Energy & Fuels

56

22

Cool Thermal

-6+3mm

18

(%)

54 53



ash (%)

Cool Thermal

-6+3mm

55

20

52 51

16

50 14

49 60

80

100

120

140

160

180

60

80

100

120

140

160

180

H (mm)

H (mm) 22 Thermal Cool

-3+1mm

Thermal Cool

-3+1mm

52

51

18

(%)

(%)

20



ash

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 42

50

16

14

49 60

80

100

120

60

140

H (mm)

80

100

120

140

H (mm)

Figure 14. Effects of Bed Height on σash and ε for -6+3 mm Lignite and -3+1 mm Lignite

In the thermal condition, the standard deviation of ash content and combustible recovery occupy the analogous variation trend as these under cool condition. Both values rise first and then decrease with the increase of bed height. The optimal bed heights for -6+3 mm lignite and -3+1 mm lignite are 120 mm and 80 mm, where the best separation effect was achieved. Generally, the combustible recovery and standard deviation of ash content under thermal condition are higher than these under cool condition. Under root temperature, due to the existence of large amount of water, liquid bridge between lignite particles deteriorates the diffusion and stratification of lignite particles. The bonding between clean coal particle and gangue particle due to liquid bridge has adverse effect on lignite separation as bonded particle has the identical moving trend and is difficult to be separated during the separation process. Due to drying effect, the thermal

26


Page 27 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

energy carried by hot air actuates the evaporation of water within lignite and therefore, reduces the viscous force between particles. Hence, the agglomeration due to viscous force can be alleviated. Particles in the pulsed fluidized bed are more inclined to mix with each other homogeneously and segregate thoroughly correspondingly.

3.2.2

The Effect of Air Velocity on Separation Characteristics of Lignite

Operation parameters implemented in the research are listed in the Table 8. Table 8. Parameters for Studying the Influence of Air Velocity under Thermal Condition Lignite particle Inlet temperature (◦ C) Pulsating frequency (Hz) Bed height (cm) -6+3 mm -3+1 mm

80 80

2.18 1.75

120 80

The air velocity is set as independent invariable. The lignite in each experiment is sampled and analyzed after 3 min separation. Results are shown in Figure 15 and Figure 16. The ash content of each top layer is significant lower compared with separation results under room temperature, which demonstrates that thermal condition is better for the separation of lignite. Under room temperature, there are liquid bridges between lignite particles. In the separation process, particles bonded by viscous force of liquid are difficult to be separated under the effect of upstream air flow. With the input of thermal energy, the exterior water of lignite particle is projected to be evaporated, which weakens the adverse effect of liquid bridge on the separation effect. Thus, bonded lignite particles can be disconnected sufficiently under the effect of air flow, which promotes the diffusion and stratification of lignite particles and improve the separation effect. In both conditions, the optimal air velocities for -6+3 mm lignite and -3+1 mm lignite are 1.09 m/s and 0.55 m/s, respectively. The standard deviation of ash content of each layer has been investigated to compare with results under room temperature, as well as combustible recovery. Results are shown in Figure 17.

27


Energy & Fuels

 = 0.82 m/s

 = 0.96 m/s 120

11.67

90

22.11

90

15.50

60

32.60

60

27.27

30

H*

16.55

H*

120

30

35.50 0

10

20

30

40

50

50.03 0

10

20

Aad (%)

 = 1.09m/s

90

12.00

90

13.67

60

25.28

60

26.81

30

53.50

30

49.61

H*

11.93

H*

120

10

20

40

50

40

50

 = 1.23 m/s

9.24

0

30

Aad (%)

120

30

40

50

0

10

Aad (%)

20

30

Aad (%)

 = 1.37 m/s 120

14.54

90

20.59

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 42

60

27.36

30

43.65 0

10

20

30

40

50

Aad (%)

Figure 15. Ash Content Distribution under Thermal Condition with Respect to Air Velocity (-6+3 mm Lignite)

28


Page 29 of 42

 = 0.48 m/s

80

13.17

80

11.82

60

25.10

60

13.65

40

31.50

40

22.81

20

H*

H*

 = 0.41m/s

52.45

20

35.27 0

10

20

30

40

50

0

10

20

Aad (%)

80

11.80

60

11.64

60

14.83

40

22.50

40

23.39

20

54.18

20

50.89

H*

H*

10.79

10

20

40

50

40

50

 = 0.61 m/s

80

0

30

Aad (%)

 = 0.55 m/s

30

40

50

0

10

Aad (%)

20

30

Aad (%)

 = 0.68 m/s

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

80

16.89

60

18.15

40

27.83

20

43.26 0

10

20

30

40

50

Aad (%)

Figure 16. Ash Content Distribution under Thermal Condition with Respect to Air Velocity (-3+1 mm Lignite)

29


Energy & Fuels

22 20

55 54 53 52

12

(%)

14 10



ash (%)

16

51

8

50

6

49

4

48

2 0

Thermal Cool

-6+3mm

Thermal Cool

-6+3mm

18

0,8

0,9

1,0

1,1

1,2

1,3

47

1,4

0,8

0,9

1,0

1,1

1,2

1,3

1,4

 (m/s)

 (m/s) 22

53

Thermal Cool

-3+1mm

20

Thermal Cool

-3+1mm

52 18 (%)

14

50



(%)

51 16

ash

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 42

49 12

48 10

47 8

0,45

0,50

0,55

0,60

0,65

0,70

0,75

0,80

0,40

0,45

0,50

0,55

0,60

0,65

0,70

 (m/s)

 (m/s)

Figure 17. Effects of Air Velocity on σash and ε for -6+3 mm Lignite and -3+1 mm Lignite

From the Figure 17, it can be seen that the standard deviation of ash content and combustible recovery experience a significant increase and then decrease moderately with the increase of air velocity. For -6+3 mm lignite, the optimal standard deviation of ash content is 20.45% and the corresponding combustible recovery is 53.98% when the air velocity is 1.09 m/s. In contrast, the optimal standard deviation of ash content and combustible recovery emerge at air velocity of 0.55 m/s for -3+1 mm lignite.

3.2.3

The Influence of Pulsating Frequency on Separation Characteristics of Lignite

The influence of pulsating frequency on separation characteristics of lignite was studied to investigate the cooperative effect of pulsating frequency and thermal input on improving the separation

30


Page 31 of 42

efficiency in pulsed fluidized bed. Operation parameters implemented in the research are listed in Table 9. Table 9. Parameters for Studying the Influence of Pulsating Frequency under Thermal Condition Lignite particle Inlet temperature (◦ C) Air velocity (m/s) Bed height (cm) -6+3mm -3+1mm

80 80

1.09 0.55

120 80

The air velocity is set as independent invariable. The lignite in each experiment is sampled and analyzed after 3 min separation. Results are shown in Figure 18 and Figure 19.  = 1.31 Hz

120

15.52

120

11.40

90

25.16

90

16.94

60

29.66

60

26.73

30

H*

H*

 = 0.00 Hz

30

36.30 0

10

20

30 Aad (%)

40

50

60

48.01 0

10

20

120

7.42

90

12.00

90

9.84

60

25.28

60

24.80

30

53.50

30

56.53

H*

H*

9.24

10

20

30

40

50

60

0

10

20

Aad (%)

 = 3.93 Hz 7.88

90

9.50

90

10.24

60

24.23

60

27.94

30

58.23

30

54.77

H*

120

20

30

60

30

40

50

60

 = 4.80 Hz

7.31

10

50

Aad (%)

120

0

40

 =3.06 Hz

120

0

30

Aad (%)

 = 2.18 Hz

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

40

50

60

0

10

Aad (%)

20

30

40

50

Aad (%)

Figure 18. Ash Content Distribution under Thermal Condition with Respect to Pulsating Frequency (-6+3 mm Lignite)

31


Energy & Fuels

 = 1.05 Hz

80

16.77

80

11.27

60

21.76

60

12.98

40

26.02

20

39.48 0

H*

H*

 = 0.00 Hz

40

24.58

20 10

20

30

40

50

49.43 0

10

20

Aad (%)

80

10.64

60

11.64

60

11.64

40

22.96

40

21.96

20

54.56

H*

H*

10.80

20

30

40

50

0

10

20

Aad (%)

60

11.08

60

14.50

40

21.84

40

19.39

20

55.75

20

54.28

H*

10.01

12.35

10

20

40

50

40

50

 = 4.36 Hz 80

0

30

Aad (%)

 = 3.49Hz 80

50

55.04

20 10

40

 =2.62 Hz

80

0

30

Aad (%)

 = 1.75 Hz

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 42

30

40

50

0

10

Aad (%)

20

30

Aad (%)

Figure 19. Ash Content Distribution under Thermal Condition with Respect to Pulsating Frequency (-3+1 mm Lignite)

It can be apparently seen that the ash content gradient from the top layer to the bottom layer of each single independent experiment differs with the variation of pulsating frequency. Compared with conventional fluidized bed ( f = 0Hz), the pulsed fluidized bed results in larger ash content gradient in ash content distribution of the product. Ash content of the top layer decreases dramatically with the increase of pulsating frequency. Ultimately, it remains essentially constant when the pulsating frequency is between 3.06 Hz and 4.80 Hz for -6+3 mm lignite. For -3+1 mm lignite, when the pulsating frequency is 3.49 Hz, the optimal separation result was achieved. The experimental results show that the pulsating energy is beneficial for improving the separation effect. Compared with results achieved under room temperature, ash contents of top layer products are slight higher when pulsating frequency is less than 2.18 Hz for -6+3 mm lignite. When pulsating 32


Page 33 of 42

frequency is equal or larger than 2.18 Hz, ash contents of top layer products are lower than these obtained under room temperature for -6+3 mm lignite. For -3+1 mm lignite, the ash content of top layer product is lower than theses achieved under room temperature under each pulsating frequency.

26

57

Thermal Cool

-6+3mm

24

56

22

54 (%)

18 16

53



ash (%)

Thermal Cool

-6+3mm

55

20

14

52

12

51

10

50

8 0

24

1

2

 (Hz)

3

4

49

5

55

Thermal Cool

-3+1mm

22

2

 (Hz)

3

4

5 Thermal Cool

-3+1mm

53 52 (%)

16

1

51



(%)

18

0

54

20

ash

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

50

14

49

12

48 10

47 8 0

1

2

3

4

0

5

1

 (Hz)

2

 (Hz)

3

4

5

Figure 20. Effects of Pulsating Frequency on σash and ε for -6+3 mm Lignite and -3+1 mm Lignite

The standard deviation and combustible recovery are shown in the Figure 20. Two evaluation indices rise significantly and then decrease with the increase of pulsating frequency. Maximum values of standard deviation and combustible recovery can be reached when pulsating frequencies are 3.93 Hz for -6+3 mm lignite and 3.49 Hz for -3+1 mm lignite. When the frequency is 0 Hz, σash and ε under cool condition is comparable with these under thermal condition. In contrast, both σash and ε are improved under thermal condition with the

33


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 42

increase of pulsating frequency, which indicates that the pulsating energy is beneficial for the separation of lignite. In thermal condition, the standard deviation of ash content for -6+3 mm lignite basically remains unchanged when the pulsating frequency is between 2.18 Hz and 3.93 Hz. For -3+1 mm lignite, the identical trend can be detected when the pulsating frequency is between 1.75 Hz and 3.49 Hz.

3.2.4

The Effect of Inlet Temperature on Separation Characteristics of Lignite

The inlet temperature is set as independent invariable. Operation parameters are shown in Table 10. The lignite in each experiment is sampled and analyzed after 3 min separation. Results are shown in Figure 21 and Figure 22. Table 10. Parameters for Studying the Influence of Inlet Temperature under Thermal Condition Lignite particle Pulsating frequency (Hz) Air velocity (m/s) Bed height (cm) -6+3 mm -3+1 mm

3.06 2.62

1.09 0.55

34


120 80

Page 35 of 42

T = 80oC

T = 100oC

7.42

120

7.52

90

9.84

90

9.99

H*

H*

120

60

24.80

60

25.12

30

56.53

30

54.47

0

120

10

20

30

40

50

0

10

20

30

Aad (%)

Aad (%)

T =120oC

T = 140oC

7.86

120

9.35

90

14.50

11.00

60

26.02

60

30

51.11

30

40

50

40

50

H*

90

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0

10

20

30

40

50

26.10

47.62 0

10

Aad (%)

20

30

Aad (%)

Figure 21. Ash Content Distribution under Thermal Condition with Respect to Inlet Temperature (-6+3 mm Lignite)

35


Energy & Fuels

T = 80oC

T =100oC 80

11.89

60

13.46

60

12.77

40

21.89

40

20.87

20

52.68

20

56.66

H*

12.60

H*

80

0

10

20

30

40

50

0

10

20

30

Aad (%)

Aad (%)

T = 110oC

T = 125oC

12.61

80

14.00

60

13.03

60

13.99

40

24.56

40

25.52

20

50.55

20

48.00

40

50

40

50

H*

80

H*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 42

0

10

20

30

40

50

0

10

Aad (%)

20

30

Aad (%)

Figure 22. Ash Content Distribution under Thermal Condition with Respect to Inlet Temperature (-3+1 mm Lignite)

Compared with the results demonstrated in section 3.1, the thermal condition is beneficial for achieving a better separation result as the ash contents of the top two layers in Figure 21 and Figure 22 are much less than these under cool condition. With the input of thermal energy, when the air passed the surface of lignite particle, thermal energy carried by air flow accelerates the evaporation of exterior water of lignite, which on the one hand weakens the adverse effect of liquid bridge on lignite separation and promotes the particle blending sufficiently on the other hand. The ash contents of top two layers remain basically constant under thermal conditions of 80 ◦ C and 100 ◦C

for -6+3 mm lignite. For -3+1 mm lignite, optimal results occurs when the inlet temperature is

100 ◦ C , where the ash contents of top two layers have the smallest values. The consecutive increase of inlet temperature deteriorates the separation effect according to the illustration in Figure 23. 36


Page 37 of 42

When the inlet temperature is higher than 80 ◦ C for -6+3 mm lignite and 100 ◦ C for -3+1 mm lignite, it takes less time to eliminate the adverse effect of liquid bridge on lignite separation as the drying of exterior water of lignite particle increases under high inlet temperatures. The lignite particles are dried fast under the high inlet temperatures ( higher than 80 ◦ C for -6+3 mm lignite and 100 ◦ C for -3+1 mm lignite), which weakens the cohesive effect between lignite particles fast. Therefore, in the separation process, due to instantaneous disappearance of cohesive effect between lignite particles, the back-mixing of lignite particle is intensified, which deteriorates the separation effect. Hence, the inlet temperature is better to be maintained as 80 ◦ C for -6+3mm lignite and 100 ◦ C for -3+1mm lignite in order to weaken the adverse effect of liquid bridge and meanwhile maintain a moderate drying rate to achieve an efficient separation result and save energy as well.



24

curve

22

24

52

20

20

40

60

80

100

120

53 52

20 51 50

50 16

54

18

51

18

 curve

22

54 53

ash curve

-3+1mm

55

 (%)

ash curve

-6+3mm

 (%) ash (%)

26

ash (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

16

49 80

140

90

100

110

120

T (oC)

T (oC)

Figure 23. Effects of Inlet Temperature on σash and ε for -6+3 mm Lignite and -3+1 mm Lignite

From the Figure 23, it can be seen that the standard deviation of ash content and combustible recovery experience a significant increase and then decrease dramatically with the increase of inlet temperature. Optimal values for both σash and ε emerge under the inlet temperatures of 80 ◦ C and around 100 ◦ C for -6+3 mm lignite and -3+1 mm lignite, respectively.

37


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4 CONCLUSION A pulsating air flow and thermal energy were introduced into the gas-solid fluidized bed system to generate a pulsed fluidized bed system targeting at drying and simultaneously separation for fine lignite particles (-6+3 mm lignite and -3+1 mm lignite). The separation characteristics of -6+3 mm lignite and -3+1 mm lignite under room temperature and thermal condition have been studied in the pulsed fluidized bed to ascertain in what extent ash content of each layer product, standard deviation of ash content and combustible recovery vary with respect to different influence factors. On basis of the studies, following conclusions can be demonstrated: Air velocity, inlet temperature, pulsating frequency and bed height have significant influence on separation efficiency of lignite in pulsed fluidized bed. The standard deviation of ash content and combustible recovery increase significantly under thermal condition compared with results achieved under room temperature. Optimal bed heights are 120 mm and 80 mm for -6+3 mm lignite and -3+1 mm lignite, respectively. Ash content of top layer product decreases significantly from 14.44% to 9.82% and from 15.52% to 7.31% when the pulsating frequency increases from 0 Hz to 3.93 Hz under room temperature and thermal condition for -6+3 mm lignite, respectively. For -3+1 mm lignite, the ash content of top layer product decreases from 17.22% to 10.25% and from 16.77% to 10.01% under room temperature and thermal condition, respectively. It is apparent that the introduce of pulsating energy and thermal energy dramatically increases the separation of lignite in pulsed fluidized bed. Optimal operation parameters were determined. With the air velocity of 1.09 m/s, inlet temperature of 80 ◦ C, pulsating frequency of 3.93 Hz and bed height of 120 mm, the optimal separation efficiency was achieved for -6+3 mm lignite. For -3+1 mm lignite, the optimal efficiency is achieved when air velocity, inlet temperature, pulsating frequency and bed height are 0.55 m/s, 100 ◦ C, 3.49 Hz and 80 mm.

Nomenclature A¯

Weighted mean of ash contents of all layer products (%) 38


Page 38 of 42

Page 39 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

A(i)

Ash content of i-layer product (%)

Aad

Ash content of lignite (%)

FCad

Fixed carbon (%)

V

Butterfly valve exit air flow (m3 /s)

V0

Butterfly valve inlet air flow (m3 /s)

Vad

Volatile content of lignite (%)

Mad

Internal moisture content (%)

Mt

Total moisture content (%)

Greek Letters α

Rotating angle of valve disc (rad)

σash

The segregation standard deviation of ash content (-)

γ(i)

Productive rate of i-layer product (%)

ε

Combustible recovery (%)

Acknowledgement The authors acknowledge the financial support by the Fundamental Research Funds for the Central Universities (No. 2017BSCXA08), Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. KYCX17_1512) and a fund from the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Separation and Upgrading of Fine Lignite in a Pulsed Fluidized Bed. 2

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