Separation and Upgrading of Fine Lignite in Pulsed Fluidized Bed. 1

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Separation and Upgrading of Fine Lignite in Pulsed Fluidized Bed. Part 1: Experimental Study on Lignite Drying Characteristics and Kinetics Cheng Sheng, Chenlong Duan, Yuemin Zhao, Panpan Zhang, and Liang Dong Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02885 • 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 1: Experimental Study on Lignite Drying Characteristics and Kinetics 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 The pulsating flow was introduced into the gas-solid fluidized bed to enhance the diffusion and stratification of lignite particle and avoid channeling and short circuits with attempt to achieve a better drying effect for fine lignite. A pulsed fluidized bed system was established, targeting the analysis of the drying and segregation characteristics of fine lignite. With the thermal input characterized by inlet temperature, thermal energy was transported into the fluidized bed and lignite particles, which accelerates the evaporation of surface water and interior water of lignite particles. Under the combination effect of thermal and pulsating energy, the drying efficiency was significantly improved. This paper mainly focuses on the influence of various operating factors on drying characteristics of fine lignite in pulsed fluidized bed and

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exploring the appropriate kinetic model for drying process under different inlet temperatures in pulsed fluidized bed. In the drying process, the drying rate was influenced dramatically in a positive way by the inlet temperature, gas velocity and pulsating frequency, while drying rate decreases with the increase of bed height. The increase of inlet temperature, gas velocity and pulsation frequency reduced the moisture content and increased the drying rate. When the inlet temperature, air velocity, pulsating frequency and bed height are 100 ◦ C, 1.09 m/s, 3.06 Hz and 120 mm, the water content of -6+3 mm lignite decreases dramatically from 31.66% to approximately under 8% after 12 min drying. The water content of -3+1 mm lignite plummets from 29.99% to around 4% after 12 min drying when inlet temperature, air velocity, pulsating frequency and bed height are set as 100 ◦ C, 0.61 m/s, 2.62 Hz and 80 mm, respectively. Infinitesimal effect on the lignite drying can be detected when the gas velocity and the pulse frequency exceeded a certain range. In the study of drying kinetics, fitting results under different temperatures combined with thin layer drying model showed that the Logarithmic model was the optimal model for interpreting drying characteristic of fine lignite. After drying and separation, the calorific capacities of -6+3 mm lignite and -3+1 mm lignite increase by up to 60% and 67%, respectively. Hence, drying of fine lignite with pulsed fluidized bed offers a feasible and economical method to enable further industrial application of lignite.

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

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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 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, conventional 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 intensively. 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 increases the transportation cost and reduces the calorific capacity of clean lignite. Hence, it is necessary to involve drying treatment in lignite separation process. In fact, coal drying and simultaneously separation technologies have become increasingly significant 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 Initially, the fluidized bed was reported as a combustor, which offers a means of burning coal, other low grade fuels and waste materials with high moisture and

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ash content in an economical and environmentally accepted way due to the outstanding drying and segregation characteristics of fluidized bed. 11,12 Afterwards, fluidized bed has been widely studied in order to investigate the drying and separation characteristics and verify the capability of drying and separating lignite. 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 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 scenarios 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 technique for lignite drying. Compared with conventional fluidized bed dryers, additional field fluidized bed drying technologies have been developed and gradually exhibited significant advantages in drying and separating lignite. 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 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

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fine lignite in a vibrated fluidized bed. It is reported that promising results were achieved. 6 The vibrated gas-solid fluidized bed was investigated and utilized for the drying and simultaneously separation of lignite. 21 By introducing the vibrated energy, the fluidization stability, as well as separation performance was improved significantly. 21 The 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 and improve the drying performance and separation efficiency. In terms of drying characteristics and kinetics, pulsed fluidized bed has been widely studied and verified. Reyes et al. (2006, 2007a, 2007b, 2008, 2010, 2012) 25–30 from University of Santiago investigated the drying characteristics and drying kinetics of different materials experimentally and theoretically with pulsed fluidized bed, which was validated as feasible and efficient technology for drying different materials, like turnip seeds, potato slices, suspensions, sawdust, etc. In order to improve the fluidization quality and drying performance, Jia et al. (2015, 2016) investigated the influence of gas pulsation and bed vibration on fluidization quality and drying performance with respect to biomass particles in fluidized bed without bed materials. 31,32 By introducing gas pulsation into a vibrating fluidized bed, Jia et al. (2015) demonstrated that pulsation has proven to be effective in promoting the dispersal of irregular biomass particles, regardless of strong inter-particle forces. 31 It is reported that the heat and mass transfer could be improved with the presence of gas pulsation, which is capable of enhancing gas solid contact. 32 Researchers from China University and 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, the pulsed fluidized bed occupies characteristics of no use of magnetite powder, low energy consumption and high processing capacity and has been proven to be efficient dryer and separator. Therefore, in the proposed work, the pulsed fluidized bed technology was implemented in a way of combining both advantages as a dryer and advantages as a separator with attempts to dry and separate the fine lignite simultaneously. The proposed work focuses on the first part of the

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research-experimental study on lignite drying characteristics and kinetics. A novel pulsed fluidized bed system was designed and established. The tentative experimental researches on the influence of each single factor on the drying characteristics were studied. Afterwards, a number of drying models were explored to characterize drying kinetics of fine lignite with pulsed fluidized bed and ascertain the optimal mathematical model for predicting moisture content variation of -6+3 mm lignite and -3+1 mm lignite under certain operation condition.

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. Coal samples of two granular levels, namely -6+3 mm and -3+1 mm, were sampled and analyzed by proximate analysis and ultimate analysis 33 under a N2 atmosphere. The moisture content is determined by the mass loss of coal samples undergoes after lignite has been heated to 110 ◦ C under a N2 atmosphere. After the exterior water is eliminated, coal samples were comminuted into lignite powder of 1 mm, which then is measured with proximate analysis in terms of internal moisture content. 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 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 environmentfriendly way, lignite is required to be dried and separated in order to reduce moisture content and ash content and increase the calorific capacity correspondingly.

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Table 1. Proximate Analysis and Ultimate Analysis Results of Lignite Coal Samples Total Granularity level moisture (mm) Mt (%) -6+3 -3+1

2.2

31.66 29.99

Internal moisture Mad (%) 13.79 10.34

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

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. 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 gas temperature is lower than the given temperature, the controller switches on the electrical heater 7


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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. 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 8


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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 under the effect of gravity. Hence, feeding lignite particles are dried and separated simultaneously. 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. The lignite was sampled by a sampler every 1 min. After the equilibrium moisture was reached, drying process continued at time steps of 1 min. The as-received lignite sample was immediately weighted. The moisture content, which is defined by a dry basis, is the ratio of the weight of water to the weight of dry lignite. Moisture content (Mt ) and drying rate (U) were calculated in virtue of 9


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Equation 2 and Equation 3, 6 respectively. Wwet −Wdry × 100% Wdry

(2)

(Wwet )1 − (Wwet )2 × 100% Wdry (t2 − t1 )

(3)

Mt =

U=

where Mt is the moisture content; Wwet is the initial sampled lignite mass; Wdry denotes the mass of dried lignite; U represents drying rate; t is drying time. Prior to drying and separation, -6+3 mm lignite and -3+1 mm lignite were sampled and analyzed by thermogravimetric analysis method to study the thermogravimetric property of lignite.

3 RESULTS AND DISCUSSIONS 3.1

Scanning Electron Microscope (SEM) Analysis of Lignite Clean Coal and Gangue

In order to get a better understanding of the difference of superficial characteristics and porous structure between clean lignite and gangue, the clean coal and gangue were sampled and analyzed by Scanning Electron Microscope (SEM). The results are shown in Figure 2.

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Figure 2. SEM Analysis Results of Lignite Clean Coal and Gangue (a-Clean Coal; b-Gangue)

From Figure 2, it can be seen that the porous structure of clean coal (low ash content) is relatively well developed. Thus, the water binding capacity of clean coal is stronger than that of gangue. Clean coal binds with large amount of water. The fluidity of clean coal is slightly lower at the drying and separation process. The surface structure of the gangue is relatively compact, the porous structure is barely developed. Hence, gangue has a stronger mobility in the drying and separation process.

3.2

Thermogravimetric Analysis of Lignite

The thermogravimetric analysis of lignite was conducted in order to study the variation trend of lignite mass under different temperatures and determine the reasonable range of drying temperature for lignite drying in pulsed fluidized bed. Lignite samples were heated and pyrolyzed with a lab-scale furnace in a nitrogen atmosphere under non-isothermal conditions at heating rate of 10 ◦ C/min

until the furnace wall temperature reached 900 ◦ C. Mass losses of lignite samples were

measured during the heating process with a thermo-gravimetric analyzer. Results are illustrated in Figure 3. On the basis of the results, the temperature range for lignite drying can be determined. 11


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The TG represents thermogravimetry, which characterizes the mass variation of lignite against

Figure 3. Thermogravimetric Analysis of -6+3 mm Lignite and -3+1 mm Lignite temperature. The DTG denotes the differential thermogravimetric, which is a plot of the change rate of mass with respect to temperature. Figure 3 shows that the drying of -6+3mm and -3+1mm lignite have been completed fundamentally when the temperature rises up to around 150 ◦ C. When the temperature continues increasing, the TG curve is approximately horizontal between 150 ◦ C and 250 ◦ C, interpreting that the mass of lignite maintain an approximate constant at the temperature range. The mass of lignite decreases dramatically after 250 ◦ C, especially in the range of 400 ◦C

and 700 ◦ C. It indicates that lignite starts the pyrogenic decomposition when the temperature

is around 250 ◦ C. The curve tends to decrease gradually after 700 ◦ C. At about 450 ◦ C, a negative peak value is detected on DTG curve, which denotes local maximum mass loss rate of lignite. At this point, lignite has the largest pyrogenic decomposition rate. From the thermogravimetric analysis of lignite, the reasonable temperature range can be determined for experimental purposes of drying lignite. From the perspective of achieving a better experimental result and saving energy, the drying temperature is set within 150 ◦ C in the study of lignite drying characteristics.

3.3

Study on Drying Characteristics of Lignite

The drying process of moisture content change against temperature was analyzed. Initially, lignite sample was weighted and analyzed with proximate analysis method in terms of water content to 12


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acquire the moisture content of lignite. In the drying experiment, the lignite was sampled by a sampler every 1 min. Sampled lignite was immediately weighted and afterwards analyzed with proximate analysis method in terms of moisture content. The moisture contents of lignite over time were achieved. The result is shown in Figure 4. From Figure 4, the drying process of lignite in pulsed fluidized bed can be approximately divided into three stages, which are described as follows:

Figure 4. Drying Curve of Moisture Content Variation With Respect To Time

• Stage 1: Initial drying stage: At this stage, the lignite is dried under a certain temperature. A small amount of water is evaporated. • Stage 2: Constant drying rate stage: From the Fiugre 4, it can be seen the line segment in Stage 2 in general has a constant gradient, which implies a constant drying rate. The energy provided by air is basically implemented for the evaporation of water. The exterior temperature of materials maintains constant. Thus, the enhancement of heat diffusion is capable of accelerating the drying process. 13


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• Stage 3: Falling-rate drying stage: The energy carried by air flow is divided into two parts: one portion of the energy is used for the evaporation of internal water; the other part of the energy is responsible for increasing the temperature of materials, promoting the formation of temperature gradient between the surface and interior of lignite particle and then accelerating the outward migration of internal water. The migration speed of internal water hinges on the particle porous structure. The heat transfer emerges when lignite particles are in contact with the hot air in the pulsating fluidized bed. The surface water and interstitial water are evaporated during the heat transfer process. Different blending modes and movements of lignite particles, which differ the extent of contact of particles with hot air, has great influence on the drying of lignite. In order to improve the effect of lignite separation and upgrading, the drying characteristics of lignite in pulsed fluidized bed were studied to achieve a better separation result and drying result, simultaneously. A number of factors, inlet temperature, air flow velocity, pulsating frequency and bed height were selected. The influence of these factors on drying characteristics of lignite has been studied.

3.3.1 The Influence of Inlet Temperature on Drying Characteristics of Lignite The inlet temperature denotes the air temperature measured at air chamber of the fluidized bed. Hot air is the main energy source used to evaporate water in the drying process. At certain operation condition, when the inlet temperature rises, more energy was transported into the fluidized bed. The evaporation rate was increased. The outward migration of internal water was intensively promoted. Hence, the drying rate was facilitated. High temperature is beneficial for lignite drying. It is also verified that a higher temperature favored the dry characteristics in conventional fluidized bed dryer. 34 However, when the inlet temperature is larger than 200 ◦ C, the organic structure of lignite particle was damaged. 6 In the experiment, the drying temperature was set in a certain range to avoid high temperature and devastating influence caused by high temperature. In virtue of thermogravimetric analysis results, the inlet temperatures for both -6+3 mm and -3+1 mm lignite 14


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were set between 80 ◦ C and 140 ◦ C. The operation conditions for the study of drying characteristics of -6+3 mm and -3+1 mm lignite are listed in Table 2. The influence of different inlet temperature on drying characteristics of -6+3 mm and -3+1 mm lignite was studied. Results are shown in Figure 5. Table 2. Parameters for Studying the Influence of Inlet Temperature Lignite particle Air velocity (m/s) Pulsating frequency (Hz) Bed height (mm) -6+3mm -3+1mm

1.09 0.61

3.06 2.62

120 80

Figure 5. Effects of Inlet Temperatures on Moisture Content Variation of -6+3 mm Lignite and -3+1 mm Lignite From Figure 5, it can be seen that water content of lignite decreases gradually as the inlet temperature rises at each time point. The drying process takes approximate 16 min, 12 min, 11 min and 9 min to dehydrate the -6+3 mm lignite to a water content of 8% under inlet temperatures of 80 ◦ C, 110 ◦ C, 120 ◦ C and 140 ◦ C, respectively. It shows that the increase of inlet temperature in a certain range is beneficial for the drying of lignite. When the inlet temperature is 140 ◦ C, the drying has fundamentally completed at 17 min, while Figure 5 shows apparently that more time is imperative to complete the drying of -6+3 mm lignite under other three inlet temperatures. Generally, the drying curve of the -3+1 mm lignite has the same trend as that of -6+3 mm lignite. As the inlet temperature rises, the water content of lignite particle decreases dramatically; 15


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water content loss per unit time increases at the meantime. After 5 min of drying, the water contents of -3+1 mm lignite are 21.50%, 17.00%, 14.50% and 11.87% under inlet temperatures of 80 ◦ C, 100 ◦ C, 110 ◦ C and 125 ◦ C, respectively. It can be seen that for the -6+3 mm, -3+1 mm lignite, the water content decreases monotonically with the increase of the inlet temperature. As the inlet temperature increases, the hot air transfers more heat to the bed material. As the drying continues, part of the heat is used for the vaporization of water; the other part is transferred to the lignite particles, increasing particle temperature correspondingly. The increase of particle temperature promotes the diffusion and evaporation of internal water. A number of closed holes in lignite particles are reopened with the increase of inlet temperature, expanding the contact area of lignite particle with hot air. The evaporation is accelerated. In the drying process, the influence of inlet temperature on drying rate is likewise researched. The results are shown in Figure 6.

Figure 6. Drying Rate Variation with Respect to Temperature From Figure 6, the variation of drying rate follows the approximately identical trends for both -6+3 mm and -3+1 mm lignite under different inlet temperatures. At each moisture content point, drying rate rises as the increase of inlet temperature in general. Drying rate under 80 ◦ C increases moderately until water content decreases to around 18%, while drying rates under other three temperatures decrease dramatically from the maximum values to around 0 g · g−1 · min−1 . This 16


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is probably because that when the inlet temperature is equal to or larger than 100 ◦ C, which is the boiling point of water, the evaporation of free water is predicted to be much more intensive than that under 80 ◦ C in the initial drying stage. As it can be seen from Figure 5, there are still large amount of water remaining in lignite after 18 min and 14 min drying with pulsed fluidized bed under 80 ◦ C for -6+3 mm lignite and -3+1 mm lignite, respectively. Hence, it is reasonable that longer time is needed to dry the lignite to identical moisture content under 80 ◦ C than time needed under other three temperatures. As it can be seen from Figure 6, the drying rates under other two temperatures are slightly higher than that under inlet temperatures of 80 ◦ C and 100 ◦ C for both -6+3 mm and -3+1 mm lignite. This is because the energy carried by hot air is responsible for evaporation of lignite surface water. High inlet temperature results in more energy input into the pulsed fluidized bed, which accelerates the evaporation of water. When the water content of lignite particle is lower than 6%, it is predicted to be the falling-rate drying stage, where energy is transported into interior of the particles, increasing the outward diffusion rate of internal water. When the water content of lignite is lower than 5%, the drying rate curves under different inlet temperature overlap dramatically; this indicates that it is difficult to eliminate internal water. The water content of lignite is almost independent of inlet temperature when it decreases to 5%. The drying rate stagnates as well. At this stage, the surface water of lignite is evaporated totally. While a small amount of internal water is trapped in the porous structure. Due to the existing of hydrogen bond, internal water is absorbed by the surface of internal pore and cluster topology is formed through interaction of water with oxygen containing functional groups. In the cluster topology structure, a small amount of water is secured under high inlet temperature, which interprets the emergence of moderate drying rate under low water content scenario.

3.3.2 The Influence of Air Velocity on Drying Characteristics of Lignite The operation conditions for the study of drying characteristics of -6+3 mm and -3+1 mm lignite are listed in Table 3. 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 -

17


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3+1 mm lignite are capable of diffusing and mixing distinctly. Under these air velocities, sufficient diffusion and stratification promote and intensify the drying process of lignite with pulsed fluidized bed. The influence of different air velocities on drying characteristics of -6+3 mm and -3+1 mm lignite was studied. Results are shown in Figure 7. Table 3. Parameters for Studying the Influence of Air Velocity Lignite particle Inlet Temperature (◦ C) Pulsating frequency (Hz) Bed height (mm) -6+3mm -3+1mm

100 100

3.06 2.62

120 80

Figure 7. Effects of Air Velocities on Moisture Content Variation of -6+3 mm Lignite and -3+1 mm Lignite From the Figure 7, it can be seen that there is correlation between air velocity and moisture content of lignite, which can be basically described as moisture content decreases dramatically with the increase of air velocity at each time point. An efficient drying effect has been achieved. Before the moisture content reaching 8%, where exterior water is totally evaporated, the time required for the drying rises with the decrease of air velocity. In Figure 7, the moisture contents of -6+3 mm lignite and -3+1 mm lignite are monotonic decreasing functions of air velocity. The time required for lignite to be dried to 8% decreases with the increase of air velocity. The moisture content variation curves under air velocities of 0.96 m/s and 1.09 m/s for -6+3 mm lignite and 0.61 m/s and 0.68 m/s for -3+1 mm lignite overlap with each other generally. 18


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0,10

=0.68m/s =0.82m/s =0.96m/s =1.09m/s =1.23m/s

0,08

0,06

0,04

0,02

0,00

0.41m/s 0.48m/s 0.55m/s 0.61m/s 0.68m/s

-6+3mm

U(gg-1min-1)

0,08

U(gg-1min-1)

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

0,06

-3+1mm

0,04

0,02

0

5

10

15

20

25

30

35

0

5

Mt (%)

10

15

20

25

30

Mt (%)

Figure 8. Drying Rate Variation with Respect to Air Velocity Collision friction between particle and particle decreases with the decrease of air velocity. Under small air velocity, materials in the bed cannot be significantly mixed in the pre-heating stage. The local dead zones probably occur in this scenario. The energy obtained by materials per unit time is predicted to be decreased and cannot be transported to the lignite particle homogeneously, which results in inferior heat transfer efficiency and low drying rate in the initial stage. As a result, the elimination of exterior water is retarded; the drying effect is inconspicuous. With the increase of drying time, gradually, materials tend to blend homogeneously due to the reduction of moisture. The drying rate is improved simultaneously. When the air velocity gradually increases, the perturbation, collision and friction rate between particles in the bed are accelerated. Particles in the fluidized bed are capable of mixing homogeneously. In the fluidization process, a portion of violently perturbed particles act as heating transfer medium, intensifying the heat transfer between particle and particle, particle and hot air. Meanwhile, the drying rate of lignite is accelerated. Additionally, under the identical inlet temperature, the time required for the elimination of exterior water from the -6+3 mm lignite is 10.2 min, which is much longer than 7.8 min, which is the time required for -3+1 mm lignite. This is because the surface area of particles and heat exchange surface increase with the decrease of particle size. The drying rate curves of -6+3 mm, -3+1 mm lignite basically experience the identical trend, 19


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as shown in Figure 8. At each moisture content point, with the air velocity increases, the drying rate gradually increased. For the -6+3 mm lignite and -3+ 1 mm lignite, the drying rate gradually decreases when the lignite particles enter the falling-rate drying stage. When the lignite particles are sampled and dried for the first time under the experimental conditions, the drying rate is the largest and the water vaporization rate is the largest. When the moisture content of -6+1 mm lignite is less than 8% and the moisture content of -3+1 mm lignite is less than 5%, the variation of drying rate basically follows the identical trend, indicating that the amount of water evaporated increase in the amount over drying time. Due to the collapse of capillary structure in the lignite, a part of the internal water is trapped inside the coal particle and diffuses slowly outwards, which is in line with the small drying rate showed in the Figure 8. It can be seen that to some extent, the collapse of capillary structure influences the variation of drying rate. In virtue of saving energy and achieving a better drying result, the air velocity is necessary to be set in a reasonable range.

3.3.3 The Influence of Pulsating Frequency on Drying Characteristics of Lignite The pulsating energy is introduced to improve the fluidization effect and heat transfer efficiency in fluidized bed. As the increase of pulsating frequency, fluidization quality is predicted to be improved. The void ratio in fluidized bed decreases correspondingly. The high pulsating frequency enhances the contact between fluid phase and particle phase, thus improving the heat transfer efficiency. The operation conditions for the study of drying characteristics of -6+3 mm and -3+1 mm lignite are listed in Table 4. The influence of different pulsating frequencies on drying characteristics of -6+3 mm and -3+1 mm lignite was studied. Results are shown in Figure 9. Table 4. Parameters for Studying the Influence of Pulsating Frequency Lignite particle Inlet Temperature (◦ C) Air Velocity (m/s) Bed height (mm) -6+3mm -3+1mm

100 100

1.09 0.61

120 80

The moisture content variation curves of -6+3 mm lignite and -3+1 mm lignite have analogical 20


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Energy & Fuels

Figure 9. Effects of Pulsating Frequency on Moisture Content Variation of -6+3 mm Lignite and -3+1 mm Lignite

Figure 10. Drying Rate Variation with Respect to Pulsating Frequency

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trend. Generally, the moisture content decreases with the increase of pulsating frequency at each time point. When pulsating frequency exceeds 1.31 Hz, moisture content curves of -6+3 mm lignite coincide with each other. The same trend can be detected when the pulsating frequency exceeds 1.05 Hz for -3+1 mm lignite, which is shown in the Figure 9. The emergence of the overlap of moisture content variation curves obtained under different pulsating frequencies is probably due to the fact that the increase of pulsating frequency decreases time interval of two contiguous maximum air velocities. Hence, large pulsating frequency enabled frequently consecutive air flow pulse, which is capable of maintaining the intensified diffusion and stratification of lignite particles in pulsed fluidized bed and promotes the drying process. However, when pulsating frequency exceeds a certain value and continued increasing, air flux entering the pulsed fluidized bed merely increased slightly. Hence, the drying characteristics of lignite in pulsed fluidized bed were slightly influenced by the increase of pulsating frequency, which probably can interpret the overlap between moisture content variation curves under large pulsating frequencies. Compared with ordinary fluidized bed ( f = 0.00 Hz), the introduction of pulsating energy apparently improves the drying effect. The periodical air flow is introduced into the fluidized bed, which alleviates the clustering of wet particles. Under the effect of periodical flow, particles in the fluidized bed behaviour periodically correspondingly. This is benefit for drying. From Figure 10, it can be seen that the drying rates increase with the increase of pulsating frequency for both -6+3 mm lignite and -3+1 mm lignite at each moisture content point, respectively. However, when the pulsating frequency exceeds 1.31 Hz for -6+3 mm lignite and 1.05 Hz for -3+1 mm lignite, the drying rate remains invariable. When the moisture content is lower than internal water content, the drying rate curves under different pulsating frequencies coincide with each other. From the Figure 10, the drying rate curves have analogical trend. When the moisture content is lower than 10%, drying rate curves for -6+3 mm lignite overlap with each other. For -3+1 mm lignite, the drying rate curves basically overlap with each other under the effect of pulsating frequency. When the moisture content is less than 10%, the drying rates have infinitesimal variation with respect to pulsating frequency globally. Ultimately, the import of pulsating energy

22


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Energy & Fuels

is significantly benefit for lignite drying.

3.3.4 The Influence of Bed Height on Drying Characteristics of Lignite In the pulsating fluidized bed, bed height is a factor that influences the drying of lignite. In order to explore the influence of bed height on drying characteristics of lignite, drying experiments have been conducted for -6+3 mm lignite and -3+1 mm lignite. The operation conditions for the study of drying characteristics of -6+3 mm and -3+1 mm lignite are listed in Table 5. The influence of different bed heights on drying characteristics of -6+3 mm and -3+1 mm lignite was studied. Results are shown in Figure 11. Table 5. Parameters for Studying the Influence of Bed Height Lignite particle Inlet Temperature (◦ C) Air Velocity (m/s) Pulsating frequency (Hz) -6+3mm -3+1mm

100 100

1.09 0.61

3.06 2.62

Figure 11. Effects of Bed Height on Moisture Content Variation of -6+3 mm Lignite and -3+1 mm Lignite At a certain time, the moisture contents of -6+3 mm lignite and -3+1 mm lignite increase as the bed height rises at each time point. In the initial stage of drying, the reduction magnitudes of water rise with the increase of bed height. For -6+3 mm lignite, as drying proceeds, the reduction 23


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Figure 12. Drying Rate Variation with Respect to Bed Height magnitudes gradually gravitate towards a constant. The times required to eliminate the exterior water are 6 min, 8 min, 9 min, 10 min, 11 min under different bed heights, respectively. For -3+1 mm lignite , it can be seen that under bed heights of 60 mm, 80 mm, 100 mm, 120 mm, 140 mm, the times required to eliminate exterior water are 7 min, 8 min, 10 min, 11 min, 12 min, respectively. Figure 12 shows the variation of the drying rate of fine lignite under different bed heights. At each moisture content point, the drying rate decreases with the increase of bed height. In the pre-heating stage, under the condition of identical air velocity, particles in the fluidized bed with lower bed height are capable of contacting with air flow thoroughly. Thus, the energy carried by hot air can be transferred to particles sufficiently. The moisture content reduces correspondingly in larger scale. A better drying result is predicted to be achieved in low bed height. However, with the evolving of time, particles in pulsed fluidized beds with different bed heights have already mixed with air flow sufficiently. The heat transfer rate at this time remains a constant. Then the moisture content is independent of bed height in general.

24


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Energy & Fuels

3.4

The Analysis on Drying Kinetics of Fine Lignite with Pulsed Fluidized Bed

The drying of lignite is an extremely complicated process. There are no integrated theories developed. In contrast, a number of drying kinetics models have been explored in order to describing drying process. In the proposed work, five drying models have been selected to fit the experimental results in order to achieve a proper model and obtain optimized model parameters for mathematical description of drying process of lignite. The correlation coefficient R2 , F-value, Root Mean Square Error (RMSE) were implemented as evaluation methods for fitting effect. r RSME =

∑ni=1 (Mexp,i − Mexp, j )2 n

(4)

where Mexp,i is the experimental moisture content of lignite; Mexp, j denotes the fitting value and n is the number of sample selected for the analysis. A number of drying kinetic models, which are listed in Table 6, were explored to fit the drying results of -6+3 mm lignite and -3+1 mm lignite under different inlet temperatures with attempt of ascertaining the optimized model for describing lignite drying in pulsed fluidized bed. Table 6. Drying models Model name

Model

Reference

Henderson and Pabis Logarithmic Midilli-Kucuk Wang and Singh Page

Mt = a exp(−kt n ) Mt = a exp(−kt n ) + c Mt = a exp(−kt n ) + bt Mt = 1 + at + bt 2 Mt = exp(−kt n )

Pabis and Henderson, 1961 To˘grul and Pehlivan, 2002 Midilli et al., 2002 Wang and Singh, 1978 Page,1949

The model fitting results for -6+3 mm lignite and -3+1 mm lignite are shown in Table 7 and Table 8, respectively. From Table 7, the Logarithmic model has largest R2 values, which are larger than 0.97. Compared with other model predictions, Logarithmic model possesses the largest F-value and smallest 25


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Table 7. Model Fitting Analysis of -6+3 mm Lignite Drying Experiment Model

Temperature (◦ C)

Coefficient

R2

RMSE

F-value

Henderson and Pabis

80 100 120 140

a=34.9656, k=0.0837 a=32.0095, k=0.1153 a=31.7209, k=0.1315 a=31.7460, k=0.1683

0.9381 0.9586 0.9480 0.9548

2.0452 1.8070 2.1036 2.0201

749.6821 709.2120 461.3063 405.6412

0.9929

0.6114

3279.7516

0.9864

0.9407

1091.9611

0.9824

1.1068

691.8058

0.9785

1.2249

531.4444

0.9923

0.6396

3041.8410

0.9865

0.9685

1095.4816

0.9824

1.1476

689.9346

0.9777

1.2282

415.8181

80 Logarithmic

100 120 140

80 Midilli-Kucuk

100 120 140

a=25.6075, k=0.0053 n=2.2010, c=4.6189 a=80.1214, k=0.8656 n=0.8656, c=-49.3838 a=79.5141, k=0.0450 n=0.8306, c=-49.6390 a=36.9314, k=0.0955 n=1.0424, c=-7.1045 a=30.2091, k=0.0074 n=2.0263, b=0.1880 a=31.3734, k=0.1092 n=0.5459, b=-1.0141 a=31.3479, k=0.1433 n=0.4732, b=-1.0764 a=29.5860, k=0.1003 n=1.1100, b=-0.2371

Wang

80 100 120 140

a=5.1469, b=-0.3023 a=3.9426, b=-0.2425 a=3.6268, b=-0.2290 a=3.1927, b=-0.2185

-0.8944 -0.5363 -0.3844 -0.3455

9.7086 8.3161 8.0953 8.1387

16.2916 10.7460 9.1556 5.8906

Page

80 100 120 140

k=-3.6049, n=-0.1229 k=-3.4540, n=-0.1658 k=-3.4024, n=-0.1840 k=-3.3526, n=-0.2244

-0.0345 4.9300 0.0201 4.6619 5.0507 4.35837 4.9685 3.57996

36.8984 21.8316 17.1061 12.1820

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Table 8. Model Fitting Analysis of -3+1 mm Lignite Drying Experiment Model

Temperature (◦ C)

Coefficient

R2

RMSE

F-value

Henderson and Pabis

80 100 110 125

a=32.5390, k=0.1020 a=29.6237, k=0.1387 a=28.5769, k=0.1676 a=27.9644, k=0.2123

0.9486 0.9658 0.9582 0.9719

1.8153 1.5397 1.6730 1.2693

703.7793 707.5665 458.0699 566.9360

0.9939

0.6063 3010.8907

0.9851

0.9277

817.6997

0.9756

1.2069

395.6642

0.9856

0.9084

559.2042

0.9940

0.6014 3044.4202

0.9851

0.9459

813.8300

0.9753

1.2223

390.2961

0.9848

0.9351

527.2384

80 Logarithmic

100 110 125

80 Midilli-Kucuk

100 110 125

a=51.4366, k=0.0237 n=1.2387, c=-1.8643 a=80.0400, k =0.0548 n=0.7551, c=-50.7840 a=80.1244, k=0.0841 n=0.6206, c=-50.6068 a=44.3556, k=0.2264 n=0.5921, c=-14.5151 a=29.5407, k=0.0248 n=1.3438, b=-0.4830 a=29.6436, k=0.1555 n=0.5822, b=-0.8374 a=29.7020, k=0.2482 n=0.5282, b=-0.6947 a=29.8147, k=0.3392 n=0.6316, b=-0.313

Wang

80 100 110 125

a=5.2182, b=-0.3494 a=3.8204, b=-0.2651 a=3.1463, b=-0.2243 a=2.3824, b=-0.1734

-0.7620 -0.5550 -0.4373 -0.3964

8.6166 7.4669 6.9296 6.3425

13.2665 8.2114 6.0408 4.0661

Page

80 100 110 125

k=-3.5158, n=-0.1434 k=-3.3182, n=-0.1854 k=-3.2168, n=-0.2208 k=-3.1404, n=-0.2796

-0.0561 -0.0572 -0.0397 -0.0072

4.4463 4.1721 4.0649 3.2697

27.1479 15.6086 11.2180 8.5351

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RSEM value under the identical temperature, which elaborates that the fitting result of Logarithmic model is furthest in line with actual experimental result.Hence, the Logarithmic model is the most suitable model for predicting the drying characteristics of -6+3 mm lignite. The analogous result can be derived from Table 8 for -3+1 mm lignite. Under the same temperature, the fitting result of Logarithmic model has the largest R2 value and F-value. Meanwhile, the RSEM value is smaller than that of other models. Therefore, the Logarithmic model is selected as the most suitable model for predicting the drying characteristics of -3+1 mm lignite. In order to explore the fitting results of Logarithmic model under other operation parameters and verify the model, model fittings for drying experimental under different operation parameter were conducted. Bed height, air velocity and pulsating frequency were selected as operation parameters.

3.4.1 Model Fitting for Drying Characteristics under Different Air Velocities The operation condition is set according to the Table 3. Model fitting for drying characteristics of -6+3 mm lignite and -3+1 mm lignite under different air velocities was conducted. The fitting results are shown in Figure 13.

Figure 13. Fitting Curves of Drying Characteristics under Different Air Velocities From Figure 13, it can be seen fitting values of Logarithmic model are generally in line with the 28


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experimental values under each air velocity for -6+3 mm lignite and -3+1 mm lignite. Although a small amount of data point deviate from the fitting lines, deviation attitudes locate in the acceptable range. The variation trend of experimental data points under each air velocity corresponds with that of the fitting line, which suggests that the model fitting result is satisfactory. Meanwhile, the optimized model parameters were achieved under different air velocities for -6+3 mm lignite and -3+1 mm lignite. The fitting results of Logarithmic model under different air velocities for -6+3 mm lignite and -3+1 mm lignite are shown in Table 9. From Table 9, it can be seen that R2 values of fitting results are larger than 0.97 under different air velocities for -6+3 mm lignite and -3+1 mm lignite. Corresponding RMSE values are less than 1.2 for -6+3 mm lignite and -3+1 mm lignite under different air velocities. Both R2 values and RMSE values demonstrate that Logarithmic model is capable of describing the drying characteristics of lignite drying with pulsed fluidized bed with significant high accuracy.

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Table 9. Fitting Results of Logarithmic Model under Different Air Velocities for -6+3 mm Lignite and -3+1 mm Lignite Air velocity (m/s)

R2

Coefficient

RMSE

F-value

-6+3 mm lignite 0.68 0.82 0.96 1.09 1.23

a=78.5131, k=0.0036 n=1.5876, c=-51.1157 a=80.1201, k=0.0082 n=1.3302, c=-50.6107 a=33.6016, k=0.0331 n=1.3391, c=-4.2528 a=40.2623, k=0.0549 n=1.0732, c=-10.3168 a=71.8212, k=0.0770 n=-0.6759, c=-40.9087

0.9845 0.8357 1547.0909 0.9825 1.0144 1176.8529 0.9875 0.8135 1271.2158 0.9863 0.9457 1011.0048 0.9858 0.9767

864.3242

-3+1 mm lignite 0.41 0.48 0.55 0.61 0.68

a=26.0138, k=0.0001 n=3.6666, c=0.7451 a=49.5001, k=0.0276 n=1.2266, c=-20.7922 a=34.7151, k=0.0265 n=1.4459, c=-7.3855 a=80.0000, k=0.0573 n=0.7430, c=-50.9085 a=48.6637, k=0.0677 n=-0.9191, c=-19.9561

0.9850 0.7177 1125.2489 0.9915 0.6857 1718.4958 0.9760 1.0813

557.2236

0.9845 0.9461

745.7247

0.9787 1.1093

567.7790

3.4.2 Model Fitting for Drying Characteristics under Different Pulsating Frequency The operation condition is set according to the Table 4.To study the feasibility of model fitting with Logarithmic model under different pulsating frequency, experimental data points were compared with model fitting curves. Results are shown in Figure 14. For -6+3 mm lignite, the fitted values corresponds significantly with experimental data with respect to different pulsating frequencies, while a slight deviation of experimental data from fitted values is detected for -3+1 mm lignite when there is no pulsating frequency ( f = 0Hz). It can see from Figure 14 that a small amount of experimental data points deviate from fitted values when the

30


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Figure 14. Fitting Curves of Drying Characteristics under Different Pulsating Frequency pulsating frequency is no greater than 1.05 Hz for -3+1 mm lignite. When the pulsating frequency is larger than 1.05 Hz, experimental data is in line with fitted values for -3+1 mm lignite. The fitting results of Logarithmic model under different pulsating frequencies for -6+3 mm lignite and -3+1 mm lignite are shown in Table 10. The fitting results show that the correlation coefficient R2 are approaching 1 and RMSE values are less than 0.9. It can be interpreted from fitting results that Logarithmic model demonstrates significantly accurate fitting results under different pulsating frequencies for -6+3 mm lignite and -3+1 mm lignite, respectively.

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Table 10. Fitting Results of Logarithmic Model under Different Pulsating Frequencies for -6+3 mm Lignite and -3+1 mm Lignite Pulsating frequency (Hz)

Coefficient

R2

RMSE

F-value

-6+3 mm lignite 0.00 1.31 2.18 3.06 3.93

a=31.77591, k=0.07399 c=0.06456, n=1.11588 a=31.59876, k=0.08414 c=0.11743, n=1.27572 a=31.22272, k=0.09275 c=-0.00331, n=1.32269 a=31.36557, k=0.11823 c=-0.01269, n=1.29125 a=31.49114, k=0.14229 c=-0.02898, n=1.24473

0.9970 0.4208 125574.8430 0.9900 0.8081

6447.4191

0.9887 0.8017

1668.6362

0.9886 0.8712

1421.2712

0.9906 0.7663

1279.6323

0.9846 0.7016

2223.2560

0.9867 0.7960

1123.3894

0.9885 0.8464

1046.7953

0.9905 0.7478

1341.9091

0.9881 0.8506

990.7740

-3+1 mm lignite 0 1.05 1.75 2.62 3.49

a=200.1242, k=0.0599 c=-170.7589, n=1.0875 a=35.1176, k=0.0661 c=-6.8440, n=1.4626 a=130.8708, k=0.0270 c=-101.3667, n=0.8260 a=116.5361, k=0.1003 c=-86.9673, n=0.7820 a=54.2079, k=0.0741 c=-24.8191, n=0.8520

3.4.3 Model Fitting for Drying Characteristics under Different Bed Heights The model fitting for drying characteristics under different bed heights was considered and conducted. the operation condition is set based on parameters illustrated in Table 5. The comparison of fitting results of Logarithmic model and experimental data is illustrated in Figure 15. The comparison result shows that the Logarithmic model is capable of predicting the drying characteristics of -6+3 mm lignite and -3+1 mm lignite under each bed height with infinitesimal deviation. From the Figure 15, it can be seen that experimental data for both -6+3 mm lignite and -3+1 mm lignite is in line with the fitted values derived from Logarithmic model under each bed

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Figure 15. Fitting Curves of Drying Characteristics under Different Bed Heights height, which verifies the feasibility of characterizing the drying of -6+3 mm lignite and -3+1 mm lignite with Logarithmic model. Correlation coefficients R2 and RMSE values of the fitting results were achieved and shown in Table 11. Fitting results show that significant fitting accuracies were obtained under different bed heights for -6+3 mm lignite and -3+1 mm lignite as R2 values are larger than 0.99 and RMSE values are less than 0.9. Hence, the validity and accuracy of implementing Logarithmic model to describe drying characteristics of lignite drying with pulsed fluidized bed have been verified.

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Table 11. Fitting Results of Logarithmic Model under Different Bed Heights for -6+3 mm Lignite and -3+1 mm Lignite Bed height (mm)

R2

Coefficient

RMSE

F-value

-6+3 mm lignite 60 100 120 140 160

a=31.0979, k=0.1813 c=0.2088, n=1.1030 a=29.5565, k=0.1344 c=1.4572, n=1.1643 a=30.4620, k=0.1199 c=0.5333, n=1.0875 a=41.3396, k=0.0924 c=-10.1919, n=0.9197 a=96.0630, k=0.0433 c=-64.7110, n=0.7364

0.9956 0.5631 1820.7757 0.9904 0.7945 1092.8994 0.9916 0.7099 1542.5772 0.9947 0.5658 2657.2152 0.9938 0.5742 2976.5914

-3+1 mm lignite 60 80 100 120 140

a=29.95105, k=0.12917 c=-0.01189, n=1.37801 a=29.89422, k=0.0055 c=0.03809, n=1.4043 a=29.96766, k=0.07502 c=0.01665, n=1.35903 a=29.9433, k=0.07279 c=0.0973, n=1.29984 a=30.06385, k=0.04336 c=0.09442, n=1.41305

0.9923 0.7245 1255.3589 0.9914 0.5104 1446.7873 0.9952 0.5556 3258.9343 0.9941 0.4855 2977.3081 0.9956 0.8763 4884.9145

By comparing model fitting results under different inlet temperatures, air velocities, pulsating frequencies and bed heights, it can be seen that Logarithmic model is satisfactory for the fitting of experimental data with high R2 values and small RSEM values. However, seemingly, Logarithmic model is less reliable to fit experimental data in the beginning of the drying process, which can be seen from Figure 13, Figure 14 and Figure 15. Hence, further research is necessary to adjust and extend the reliable range of Logarithmic model to equally represent experimental data in beginning of the drying process.

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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 separation for fine lignite particles (-6+3 mm lignite and -3+1 mm lignite). The drying characteristics of -6+3 mm lignite and -3+1 mm lignite have been studied in the pulsed fluidized bed to ascertain in what extent moisture content varies with respect to different influence factors. The drying kinetics have been explored to verify the feasibility of various mathematical models in characterizing fine lignite drying in pulsed fluidized bed. On basis of the studies, the following conclusions can be demonstrated: From the thermogravimetric analysis, the drying temperature is better to be set within 150 ◦ C to achieve a better experimental result in an economical way. Three drying stages, namely initial drying stage, constant drying rate stage and falling-rate drying stage, were characterized and demonstrated schematically. The drying rate was influenced dramatically in a positive way by the inlet temperature, gas velocity and pulsating frequency, while drying rate decreases with increase of bed height. The increase of inlet temperature, gas velocity and pulsation frequency reduced the moisture content and increased the drying rate. Results demonstrate that water content of lignite was reduced significantly and the calorific capacity of lignite was increased dramatically. When the inlet temperature, air velocity, pulsating frequency and bed height are 100 ◦ C, 1.09 m/s, 3.06 Hz and 120 mm, the water content of -6+3 mm lignite decreases dramatically from 31.66% to approximately under 8% after 12 min drying. The water content of -3+1 mm lignite plummets from 29.99% to around 4% after 12 min drying when inlet temperature, air velocity, pulsating frequency and bed height are set as 100 ◦ C, 0.61 m/s, 2.62 Hz and 80 mm, respectively. After drying and separation, the calorific capacities of -6+3 mm lignite and -3+1 mm lignite increase by up to 60% and 67%,respectively, which verifies the feasibility and economic efficiency of drying fine lignite with pulsed fluidized bed. The drying kinetics was analysed. The Logarithmic model was selected for the best fitting performance. In the fitting experiment, Logarithmic model has largest R2 values, which are larger than 0.97. Compared with other model predictions, Logarithmic model possesses the largest F-value and smallest RSEM value under the identical temperature, which elaborates 35


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that the fitting result of Logarithmic model is furthest in line with actual experimental results.

Nomenclature Mexp,i

Experimental moisture content of lignite (%)

Mexp, j

Fitting value of moisture content (%)

n

Number of sample selected for the analysis (-)

RSME

Root Mean Square Error (-)

t

Drying time (min)

U

Drying rate (g · g−1 · min−1 )

Wdry

Mass of dried lignite (g)

Wwet

Initial sampled lignite mass (g)

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 Letter α

Rotating angle of valve disc (rad) 36


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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 Pulsed Fluidized Bed. 1

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