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High-temperature drying behavior and kinetics of lignite tested by the micro fluidization analytical method
Fuel 253 (2019) 180–188
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Full Length Article
High-temperature drying behavior and kinetics of lignite tested by the micro fluidization analytical method
T
⁎
Xi Zenga,b, Fang Wanga, , Mohammed Haruna Adamua,b, Lijuan Zhanga,c, Zhennan Hand, ⁎ Guangwen Xua,d, a
State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China c School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China d Institute of Industrial Chemistry and Energy Technology, Shenyang University of Chemical Technology, Shenyang 110142, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Lignite Thermal conversion Drying Kinetics Micro fluidized bed
To simulate the drying behavior of lignite in the high-temperature thermal conversion process, a micro fluidized bed reactor was adopted to measure the drying characteristics of moist char from 773 K to 1173 K and calculate the drying kinetics. Both temperature and the initial moisture strongly affected the drying behavior by changing the curve shape and drying time. The higher the initial moisture, the more obvious the influence of drying temperature. Compared to the conventional low-temperature lignite drying, the rate curve tested by MFBRA only had the growing-rate zone and falling-rate zone, but did not include the common constant-rate zone. Under the experimental condition, the Lewis drying model can describe the drying characteristics of wet char in MFBRA very well. The apparent diffusion coefficient and activation energy ranged from 1.765 E−10 to 4.756 E−10 m2/ s, and 12.83–14.97 kJ/mol, respectively. The activation energy was much lower than those in literature.
1. Introduction Now, lignite is an important resource and primary energy throughout the world. With the property of large reserve (430 billion tons), cheap mining cost, good reactivity, and low impurities for pollutant formation, lignite is widely used as feedstock in coal chemical industry and as fuel in power generation [1–3]. But even so, high moisture in lignite, generally in the range of 30−65%, presents a great challenge for its clean and high-efficient utilization, such as increasing transportation cost, lowering energy utilization efficiency, raising CO2 emission, and enhancing the risk of spontaneous combustion during storage [4–6]. Generally, for the direct utilization of lignite, the process of water evaporation accounts for the energy of 20−25% in raw coal [7–9]. Moreover, for most of the high-temperature thermal conversion processes, such as pyrolysis, gasification, and combustion, coal drying or water evaporation is always considered as an initial step, limiting the total conversion rate and energy utilization efficiency [10]. So, examining the drying behavior of lignite and calculating its kinetics at high temperature, become very necessary and essential to understand the thermal-chemical conversion process and guide the design of a novel reactor.
The numerous literature about coal drying can be divided into evaporative drying and non-evaporative drying. For the former, hot air, effluent gas, and superheated steam can be adopted as the heating medium in the dryers of fixed bed, fluidized bed, rotary kiln, entrained bed, and microwave [11–13]. For the latter, it mainly includes hydrothermal dewatering, mechanical/thermal dewatering, and solvent extraction [14]. Pusat et al. summarized the evaporative drying of low rank coal comprehensively, and investigated the main influencing parameters of drying media, coal property and drying method [15]. He et al. found that, in the mechanical thermal treatment, temperature effect (473 K and 513 K) on the drying behavior of soft lignite (Loy Yang from Australia) was more evident than that of hard lignite (Shengli from China) [16]. While the utilization of hydrothermal treatment for lignite drying in the temperature range of 473–623 K was still limited by its harsh experimental conditions [17]. As for the drying kinetics, Stokie et al. [18] compared the effect of superheated steam and air (below 473 K) in the drying characteristics of Victorian brown coal by a fluidized bed dryer, and found that Midilli-Kucuk model can accurately describe the drying kinetics. Kang et al. [19] examined the drying kinetics of Indonesian lignite below 460 K by a lab scale fixed bed reactor and thermobalance to apply the catalytic gasification process.
⁎ Corresponding authors at: State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China (F. Wang). E-mail addresses: [email protected] (F. Wang), [email protected] (G. Xu).
https://doi.org/10.1016/j.fuel.2019.05.025 Received 1 April 2019; Received in revised form 27 April 2019; Accepted 5 May 2019 Available online 09 May 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Tahmasebi et al. [20] found that the Midilli-Kucuk model fitted well the drying kinetics of lignite in a fluidized bed dryer regardless of in the atmosphere of nitrogen and superheated steam, while the page model was more suitable for the microwave drying. After reviewing the existing researches on drying behavior and kinetics, one can see that most of them were conducted at low temperature adopting to the different drying processes, but not at high temperature corresponding the actual thermal-chemical conversion processes. This is much attributed to the easy decomposition property of lignite even at very low operational temperature, such as dehydration below 623 K, and decarboxylation even below 433 K [21,22]. Actually, Zheng et al. [23] tried to investigate the high-temperature (873–1073) drying kinetics of lignite by a horizontal fixed-bed reactor in N2, but only analyze the initial drying stage for the moisture fraction of above 0.7. As we know, the drying behavior and kinetics obtained at low temperature, such as below 473 K, are very useful for the design and improvement of the whole drying system [9,24]. However, are these researches also effective at high temperature, such as above 773 K? Can they still help greatly to understand the behavior of coal particle in pyrolyzer, gasifier, and combustor operated in the temperature range of 773–1273 K? Actually, at present, there are not direct research and obvious evidence to solve these issues. So, probing drying behavior of lignite at high temperature and calculating the corresponding kinetics will be much more beneficial to understand the whole behavior of coal particle in a thermal conversion process. Recently, to test the gas-solid reaction characteristics and kinetics, the micro fluidization analytical method has been proposed by the Institute of Process Engineering (IPE), Chinese Academy of Sciences [25,26], and the corresponding micro fluidized bed reaction analyzer (MFBRA) has been also developed. The adopted micro fluidized bed (MFB) reactor with an inner diameter of about 20 mm can effectively enable high transfer rate of heat and mass, and realize the minimum diffusion inhibition. The online feeding system by pulse jetting method guarantees to initiate the chemical reaction of thermal-unstable materials instantaneously at the preset temperature. All the gaseous products from the gas-solid reaction will be measured continuously by a fast process mass spectrometer (MS). Up to now, this analyzer has been employed in many reactions successfully, such as coal/biomass pyrolysis, char gasification and combustion, iron ore reduction, CO2 adsorption, methanation, and so on [27,28]. In the present research, the micro fluidized bed reaction analyzer was used to measure drying behavior and kinetics of lignite in the temperature range of 773–1173 K. To avoid the decomposition of lignite, char samples with different initial moisture were employed to simulate wet lignite in a gasifier or combustor operated at high temperature. Several empirical models were applied to fit the drying behavior and kinetics. Then, the apparent diffusion coefficient and activation energy of char drying in MFBRA were further analyzed and compared with the literature. As a consequence, this study hopes to provide a method for probing lignite drying behavior at high temperature, and thus enhance the understanding of coal particle in the high-temperature thermal conversion processes.
Table 1 Property of coal sample used in the experiment. Proximate analysis [wt%, d]
Ultimate analysis [wt%, daf] a
LHV [MJ/ kg]
A
V
FC
C
H
S
O
N
16.7
36.2
47.1
75.1
4.3
1.1
18.7
0.8
20.42
➀ d: dry basis; ➁ daf: dry and ash-free basis;➂ aBy difference.
constant temperature humidifier, and accurately control the water-absorbing time. 2.2. Apparatus and test procedure As schematically illustrated in Fig. 1, the adopted MFBRA mainly included a micro fluidized bed reactor, a pulse injecting system for solid sample, a gas supplying system, an electric heating furnace, and an online process mass spectrometer (AMETEK). The quartz reactor of micro fluidized bed (MFB) with an inner diameter of 20 mm was composed by a bottom stage for gas preheating, a middle stage as the reaction zone, and a top stage for preventing the escape of solid particles from the reaction zone. High-purity quartz sand with a particle size in the range of 0.07–0.09 mm and high-purity Ar (99.9995%) were chosen as the heating carrier and carrying gas, respectively. The reactor was heated by an electrical heating furnace with the temperature control accuracy of ± 2 K. All the experimental temperatures in MFBRA were tested by a series of K-type thermocouples. Prior to each experiment, the MFB reactor was heated to the preset temperature, such as 1073 K, and then a certain flow rate of highquality Ar was introduced into the reactor, forming a good fluidization state of the solid particles. Under the stable state of MFBRA, a certain amount of wet char sample (about 30 mg) was instantaneously jetted into the MFB reactor, initiating the char drying process quickly. The released steam was detected by an online process mass spectrometer with a sampling frequency of 20 times per second. 2.3. Analytical method For each isothermal drying experiment, the beginning point and ending point of drying process can be determined by the steam release curve measured by the mass spectrometer. Fig. 2 takes the drying experiment of wet char with the initial moisture of 10% as an example. During the drying process, the moisture of char sample can be determined according to the equations from (1) to (3).
St − t ⎞ ⎛ mi = m 0 × ⎜1 − 0 i ⎟ St0− t f ⎝ ⎠
2. Experimental
ti
St0→ ti =
∫t
St 0 → t f =
∫t
0
tf
0
ti t0 (Imass − Imass ) dt
ti t0 (Imass − Imass ) dt
(1) (2) (3)
where mo and mi are the moisture in char sample at the initial reaction time (t0) and arbitrary time (ti) respectively; St0→ ti and St0→ t f refer to the integration area between the curve of steam and the baseline of MS (without drying process) from the drying time t0 to ti, and t0 to the tf t0 ti , Imass and Imass denote the signal inending time (tf), respectively; Imass tensity of steam tested by MS at t0, ti, and tf, respectively. After the hightemperature drying, the moisture content (mf) was very close to 0. The moisture fraction (X), as a dimensionless term, is defined as the ratio of residual moisture left in char sample to the total initial moisture in char, while the drying rate (R) represents the change of moisture fraction in a unit time (t), as shown in the equations (4) and (5), respectively. According to the definition of moisture fraction, the conversion of moisture (C) during the drying process can be calculated by
2.1. Material A kind of lignite from Shenli coal mine located in Inner Mongolia Autonomous Region of China was adopted in this study, whose proximate, ultimate and heating value analysis were listed in Table 1. Prior to utilization, to obtain coal sample with a particle size in the range of 0.10–0.15 mm, the massive raw coal was crushed, sieved and dried in an oven at 380 K for 12 h. Then, char was produced in a muffle furnace at 1273 K with a residence time of 1.0 h to release volatile matter completely. Finally, the wet char samples with the initial moisture of 10% or 20% were prepared by putting the dried char sample in a 181
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Fig. 1. Schematic diagram (a) and picture (b) of the adopted MFBRA.
conditions, each release curve can be divided into two stages, namely the growing-rate zone and the falling-rate zone. For the former, the signal intensity of steam increased quickly, reaching its maximum value in a short drying time. The higher the drying temperature, the shorter the drying time for the fast-rising zone. For example, as shown in Fig. 3(a), it only needed 16 s and 10 s for the drying temperature of 973 K and 1173 K, respectively. With the increase of moisture content in the char sample from 10% and 20%, the drying time in the growingrate zone did not have an obvious change, which was much related to the high heating rate and uniform heat transfer in the micro fluidized bed reactor. While for the falling-rate zone, much longer drying time, more than 200 s, was always needed to reach the baseline level of steam. Moreover, in the initial stage of the slow-falling zone, the signal intensity of steam decreased quickly, due to the evaporation occurred mainly from the surface of char sample. Then, the curve gradually declined because of the relatively low transfer rate of moisture inside the char particle [29,30]. Fig. 3(c) and (d) further display the effect of the initial moisture on the release behavior of steam from the wet char sample. For brevity, it just took the drying temperatures of 973 K and 1173 K as examples. At each drying temperature, with the increase of the initial moisture content, the curve shape became much wider, indicating longer drying time needed. For example, for the initial moisture of 10% and 20%, at 973 K, it required about 500 s and 1100 s, respectively, while at 1173 K, they were about 300 s and 400 s, respectively. The increase of moisture content meant more energy required, and inevitably lowered the rate of heat transfer in the char sample during the drying process.
Fig. 2. Isothermal analysis method of wet char drying in MFBRA.
the equation (6).
Xi =
R=
mi − m f m 0 − mf
≈
mi m0
dX dt
C i = 1 - Xi
(4)
(5)
3.2. Drying characteristics of wet char in MFBRA
(6) Fig. 4 shows the variation of moisture conversion in the temperature range of 773 K to 1173 K. During char drying, below the conversion of 0.6, moisture in char sample was removed quickly. For the initial moisture of 10% and 20%, approximate 60–80% and 60–90% of the total moisture were decreased within 75 s and 150 s, respectively. After that, the moisture conversion gradually increased until to the complete evaporation of moisture into the carrier gas. For the adopted char sample, with the increase of drying temperature, the time for complete dewatering became short, indicating good promotion effect. High drying temperature increased the surface and interior temperature of char sample, accelerating the evaporation rate of moisture at the char
3. Results and discussion 3.1. Steam release of wet char in MFBRA Fig. 3(a) and (b) show the effect of drying temperature on the release behavior of steam from the wet char sample. Regardless of the initial moisture of 10% and 20%, with the increase of drying temperature, the steam release curve became much narrower, while the maximum moisture intensity was much higher. Under the experimental 182
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Fig. 3. Effect of temperature and initial moisture on the steam release tested by MFBRA during the wet char drying.
with the initial moisture of 10% and 20%, each curve reached its maximum value quickly in the initial stage, and then gradually decreased with the progress of char drying. In the initial fast heating stage, the temperature of coal particle rose rapidly, and the evaporation of surface water was promoted by the intense heat transfer. After that, due to the lower moisture in coal particle, the vaporization rate of steam at the surface of coal particle was much higher than the moisture transfer rate from the interior to its surface, making the internal moisture diffusion as the limiting step and thus forming the falling-rate
surface and the transfer rate of moisture inside char particle. Moreover, one can also see that for the two kinds of char sample, the favorable effect of temperature was much different. With respect to the initial moisture of 10%, compared to the temperature from 773 K to 1073 K, the drying behavior at 1173 K was much promoted. While for the char sample with the initial moisture of 20%, it did not have the similar phenomenon. Fig. 5 displays the relationship between the drying rate and conversion under different temperatures. For the adopted char samples
Fig. 4. Effect of temperature on the drying behavior of char with different initial moistures. 183
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Fig. 5. Drying rate versus conversion for the char samples with different initial moistures in MFBRA.
et al. [35] examined the drying characteristics and kinetics of Shengli lignite using four fluidized bed (FB) reactors, namely bubbling FB (BFB), vibrated FB (VFB), medium FB (MFB) and vibrated medium FB (VMFB). Due to the large inner diameter of the reactor (110 mm) and low drying temperature of 353–433 K, there was also the constant-rate zone in the drying curve. Kim et al. [36] investigated the drying behavior of Loy Yang brown coal in a fluidized bed with an inner diameter of 22 mm, and found that in the curve of reaction rate versus moisture conversion, it still had the constant- rate stage in the range of relative humidity of 0.3–0.7. Considering the similar inner diameter with the micro fluidized bed reactor (20 mm) adopted in this study, it was reasonable to attribute these differences mentioned above to the low drying temperature and heating rate. These analyses mentioned above fully confirmed that the drying characteristics are much related to not only the reactor structure but also the drying temperature adopted. So the low-temperature drying property cannot be used to explain the particle behavior of lignite in the high-temperature thermal conversion process effectively. Moreover, in the previous research of char gasification in the atmosphere of CO2 and steam-N2 tested by MFBRA, the complete conversion of char needed about 8 min and 16 min at 1173 K [27], respectively. While for the initial moisture of 10% and 20%, the complete dewatering required about 4 min and 6 min at 1173 K. By comparing with the drying time and gasification time, it is obvious that, in the practical gasifier, lignite drying will last a relatively long time, and for a single coal particle, the processes of drying, devolatilization, gasification, and combustion always synchronize and interlace. Fig. 7 further compares the difference in drying characteristics for
stage [31]. For both of the two stages, by raising drying temperature, the increase of drying rate was much related to the reinforcement of moisture migration resulting from the enhancement of driving force on heat transfer. Moreover, compared to the low drying temperature, the drying rate at high temperatures, such as above 1073 K, was obviously promoted. And, with the increase of drying temperature, the moisture conversion corresponding to the maximum drying rate increased gradually. To elucidate the differences between conventional low-temperature drying in literature and high-temperature drying in MFBRA, Fig. 6 further displays the typical drying curves of coal particle in literature, mainly consisting of the drying zones of growing-rate, constant-rate and falling-rate [32,33]. Generally, in the constant-rate stage, the energy absorbing from drying medium to coal sample and the energy-requiring for moisture vaporization at the particle surface can reach a balance. Compared to the conventional drying at low temperature, in MFBRA, the fine coal particle was heated to the preset temperature instantaneously due to high heat transfer rate above 1000 K/s, leading to the lack of constant – rate stage and the present of growing-rate stage quickly in the initial stage. Moreover, in a conventional drying process, the constant-rate stage was dominant, accounting for two-thirds of the total drying process and consuming a large amount of energy, while for the high-temperature drying in MFBRA, the rate-falling stage played a leading role [34]. On the other hand, by comparing with coal drying adopting the fluidized bed reactor in literature, one can also see the obvious differences in drying characteristics tested by MFBRA. For example, Zhao
Fig. 6. Conventional drying curves of materials under constant drying conditions [14]. I. Growing-rate zone; II. Constant-rate zone; III. Falling-rate zone. 184
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Fig. 7. Effect of the initial moisture on the drying behavior of wet char in MFBRA.
the initial moisture of 10% and 20% in the char sample at temperatures of 1173 K and 873 K. For each experimental temperature, high moisture led to a long drying time and low drying rate. Compared to the drying temperature of 1173 K, the difference in drying characteristics between the initial moisture of 10% and 20% at 873 K became much more obvious. For example, at 1173 K, the ratio of the maximum drying rate for the initial moisture of 10% and 20% was 1.07, while for 873 K, the ratio increased to 1.55.
Table 2 Typical mathematical models of thin-layer drying in literature. Model
3.3. Mathematical model of wet char drying in MFBRA To analyze the drying behavior of moist char in MFBRA at high temperature, the experimental results were fitted by a drying model. For ease of selection, Table 2 summarizes the well – known mathematical models for drying the thin layer material with the uniform temperature distribution. Most of the models adopted can be used to describe well the mathematical decaying tendency of moisture fraction with the evolution of drying time. According to the exponential relationship between moisture conversion and drying time, as shown in Fig. 5, non-linear fitting analyses on the basis of mathematical models in Table 2 were conducted. Among them, for brevity, Fig. 8 just shows some typical curves between predicting models and experimental results in MFBRA. For the model with the equation of X = 1−a(1−ebt), it was very suitable to describe the experimental result with a correlation coefficient (R2) above 0.99. Table 3 displays the calculated results for the parameters of a, b, and the corresponding R2 for each experiment. From it, one can see that the obtained values were very similar. So, it was reasonable to average them, obtaining the values of 1.002 and 0.013 for the parameters a and
Equation
References [37]
Lewis
X = e−kt
Page
X = e−kt
Modified page
X = e−(kt )
[39]
Henderson and pabis
X = a × e−kt
[40]
Logarithmic
X = a × e−kt + c
[41]
Diffusion approach
X = a × e−kt + (1 − a) × e−kbt
[42]
Verma
X = a × e−kt + (1 − a) × e−gt
[43]
Two-term exponential
X = a × e−kt + (1 − a) × e−kat
[44]
Simplified fick diffusion Midilli-Kucuk
n
[38] n
X=a×
− ct e L2
X=a×
n e−kt
[45]
+b×t
[46]
b, respectively. For simplicity, the final value of parameter a was set to 1. By referring and comparing the existing mathematical models listed in Table 2, Lewis drying model was finally adopted to depict the drying of moist char in MFBRA. So, the exponential relationship between moisture fraction and drying time can be expressed by the following equation:
X = e−0.013t
(7)
3.4. Drying kinetics of wet char in MFBRA To quantitatively analyze the drying kinetics, the apparent diffusion coefficient can be derived by the Fick’s second law with the assumption 185
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Fig. 8. Comparison between experiment results and predication results from models.
Fig. 9. Fitting curve of LnDeff VS. 1/T for the drying of wet char in MFBRA.
4π 2Deff t 8 LnX = Ln ⎛ 2 ⎞ − d2 ⎝π ⎠
Table 3 Fitting Analysis of drying characteristics by different models. Moisture
10%
20%
bt
Temperature [K]
Equation:1−X = a(1−e )
2
R
a
b
1173 1073 973 873 773
1.024 1.021 1.014 1.006 0.997
−0.018 −0.015 −0.013 −0.013 −0.013
0.999 0.999 0.998 0.998 0.998
1173 1073 973 873 773
1.001 0.976 0.986 0.994 1.003
−0.015 −0.013 −0.011 −0.009 −0.007
0.997 0.987 0.993 0.994 0.998
1.002
−0.013
–
Average
To calculate kinetical parameters of char drying in MFBRA, Arrhenius equation was used for reference, as shown in the Eq. (10) and the corresponding logarithmic form of the Eq. (11), where Ea is the activation energy of char drying, D0 is the diffusivity value for the infinite moisture content, and R presents the universal gas constant. Ea
Deff = D0 × e− RT
LnDeff = LnD0 −
2
4π Deff t − 8 d2 ×e 2 π
(10)
Ea RT
(11)
By plotting LnDeff versus 1/T, a good linear relationship, with a fitting degree above 0.98, can be observed, as shown in Fig. 9. According to the slopes of the fitting curves, Ea can be calculated with the values of 12.83 kJ/mol and 14.97 kJ/mol for the initial moisture of 10% and 20%, respectively. Generally, Ea denotes the energy barrier to overcome for a certain conversion process. High moisture content means the strong interaction force between moisture and char particle. Table 5 lists the typical drying experiments and kinetic parameter calculated in literature. Due to the difference in reactor structure and heating temperature, the values of Ea in literature were much different with a span in the range of 18.53−106.37 kJ/mol. Compared to the literature research, the value of Ea tested by MFBRA was much lower, just lying in the range of 12.83–14.97 kJ/mol. On the one hand, this was much related to the high heating rate and uniform heat transfer in the adopted micro fluidized bed reactor. On the other hand, the high drying temperature further promotes the heat transfer not only on the surface but also the interior of char sample.
of transfer via diffusion, whose equation and the corresponding logarithmic form can be seen in the Eqs. (8) and (9). In Eq. (9), it displays a linear relationship between LnX and drying time (t). So, by plotting the LnX against the drying time (t), the apparent diffusion coefficients (Deff) for each drying temperature can be calculated, as shown in table 4. For the examined temperature, the value of Deff for the initial moisture of 10% and 20% ranged from 2.388E−10 m2/s to 4.756E−10 m2/s, and 1.765E−10 m2/s to 3.901E−10 m2/s, respectively. With the increase of drying temperature, the value of Deff rose quickly, being consistent with the research in the literature [46]. Similar to the effect of temperature, the type of FB played an important role in Deff, displaying the lowest diffusion resistance.
X=
(9)
(8)
Table 4 Apparent diffusion coefficient at different temperatures for char drying. Moisture
Items
Temperature [K]
2
773
873
973
1073
1173
10%
Deff [m /s] Ln R2
2.388E−10 −22.16 0.992
2.980E−10 −21.93 0.985
3.550E−10 −21.76 0.976
4.100E−10 −21.61 0.986
4.756E−10 −21.47 0.988
20%
Deff [m2/s] Ln R2
1.765E−10 −22.46 0.993
2.180E−10 −22.25 0.987
2.630E−10 −22.06 0.986
3.290E−10 −21.83 0.992
3.901E−10 −21.66 0.992
186
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Table 5 Drying kinetics of coal particle in literature. Number
Reactor
Temperature [K]
Initial moisture [%]
Drying medium
Ea [kJ/mol]
Literature
1
BFB VFB MFB VMFB Horizontal fixed bed dryer TGA Microwave FB
353–639
40
Air
[35]
< 473 < 443 < 433 < 523
45 5–10 35 39
N2 N2 N2 N2 Superheated steam
30.22 29.78 29.23 27.92 49.42–106.37 18.53 28.59 25.05 25.76 25.81
2 3 4 5
Microwave
4. Conclusions
[47] [48] [49] [50]
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In this study, to probe the drying characteristics of wet lignite at a high temperature, a newly designed micro fluidized bed reaction analyzer was adopted by jetting the wet char and analyzing the release curve of steam. The results showed that the drying property of wet char was much related to the drying temperature and the initial moisture. With the increase of drying temperature, the curve shape of drying rate versus moisture conversion became narrow, indicating short drying time and high drying rate. Compared to the initial moisture of 10%, the effect of drying temperature became much more obvious. By non-linear fitting the experimental results and comparing with the existing mathematical models, Lewis drying model can describe the drying property of wet char sample in MFBRA. Then, the apparent diffusion coefficient and drying kinetics were quantitatively analyzed and calculated. For the initial moisture of 10% and 20%, the value of Deff ranged from 2.388E−10 m2/s to 4.756E−10 m2/s, and 2 2 1.765E−10 m /s to 3.901E−10 m /s, respectively, while the value of Ea lay in the range of 12.83–14.97 kJ/mol. Compared to the conventional low-temperature drying of lignite, one can find that drying property was much related to the reactor structure and operational temperature. In MFBRA, the measured curve of drying rate against moisture conversion in coal char at a high temperature just had the rate-growing zone and rate-falling zone without the rate-constant zone. And the calculated activation energy was much lower. All of these not only identified the feasibility of measuring method for drying property by MFBRA, but also indicated the necessity of drying behavior and kinetics of lignite to really reflect the thermal conversion behavior at high temperature. Acknowledgement The authors gratefully acknowledge the financial support provided by the National Key R & D Program of China (No. 2018YFF01011400). References [1] Xia WC, Xie GY, Peng YL. Recent advances in beneficiation for low rank coals. Powder Technol 2015;277:206–21. [2] Zhao HY, Song Q, Liu SC, Li YH, Wang XH, Shu XQ. Study on catalytic co-pyrolysis of physical mixture/staged pyrolysis characteristics of lignite and straw over an catalytic beds of char and its mechanism. Energy Convers Manage 2018;161:13–26. [3] Zhao HY, Wang BZ, Li YH, Song Q, Zhao YQ, Zhang RY, et al. Effect of chemical fractionation treatment on structure and characteristics of pyrolysis products of Xinjiang long flame coal. Fuel 2018;234:1193–204. [4] Wu JH, Wang J, Liu JZ, Yang YM, Cheng J, Wang ZH, et al. Moisture removal mechanism of low-rank coal by hydrothermal dewatering: physicochemical property analysis and dft calculation. Fuel 2017;187:242–9. [5] Yu JL, Tahmasebi A, Han YN, Yin FK, Li XC. A review on water in low rank coals: the existence, interaction with coal structure and effects on coal utilization. Fuel Process Technol 2013;106:9–20. [6] Zhao HY, Li YH, Song Q, Wang XH, Shu XQ. Drying, re-adsorption characteristics, and combustion kinetics of Xilingol lignite in different atmospheres. Fuel 2017;210:592–604. [7] Liu HM, Chen MQ, Han ZL, Fu BA. Isothermal kinetics based on two-periods scheme for co-drying of biomass and lignite. Thermochim Acta 2013;10:25–31.
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Full Length Article
High-temperature drying behavior and kinetics of lignite tested by the micro fluidization analytical method
T
⁎
Xi Zenga,b, Fang Wanga, , Mohammed Haruna Adamua,b, Lijuan Zhanga,c, Zhennan Hand, ⁎ Guangwen Xua,d, a
State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China c School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China d Institute of Industrial Chemistry and Energy Technology, Shenyang University of Chemical Technology, Shenyang 110142, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Lignite Thermal conversion Drying Kinetics Micro fluidized bed
To simulate the drying behavior of lignite in the high-temperature thermal conversion process, a micro fluidized bed reactor was adopted to measure the drying characteristics of moist char from 773 K to 1173 K and calculate the drying kinetics. Both temperature and the initial moisture strongly affected the drying behavior by changing the curve shape and drying time. The higher the initial moisture, the more obvious the influence of drying temperature. Compared to the conventional low-temperature lignite drying, the rate curve tested by MFBRA only had the growing-rate zone and falling-rate zone, but did not include the common constant-rate zone. Under the experimental condition, the Lewis drying model can describe the drying characteristics of wet char in MFBRA very well. The apparent diffusion coefficient and activation energy ranged from 1.765 E−10 to 4.756 E−10 m2/ s, and 12.83–14.97 kJ/mol, respectively. The activation energy was much lower than those in literature.
1. Introduction Now, lignite is an important resource and primary energy throughout the world. With the property of large reserve (430 billion tons), cheap mining cost, good reactivity, and low impurities for pollutant formation, lignite is widely used as feedstock in coal chemical industry and as fuel in power generation [1–3]. But even so, high moisture in lignite, generally in the range of 30−65%, presents a great challenge for its clean and high-efficient utilization, such as increasing transportation cost, lowering energy utilization efficiency, raising CO2 emission, and enhancing the risk of spontaneous combustion during storage [4–6]. Generally, for the direct utilization of lignite, the process of water evaporation accounts for the energy of 20−25% in raw coal [7–9]. Moreover, for most of the high-temperature thermal conversion processes, such as pyrolysis, gasification, and combustion, coal drying or water evaporation is always considered as an initial step, limiting the total conversion rate and energy utilization efficiency [10]. So, examining the drying behavior of lignite and calculating its kinetics at high temperature, become very necessary and essential to understand the thermal-chemical conversion process and guide the design of a novel reactor.
The numerous literature about coal drying can be divided into evaporative drying and non-evaporative drying. For the former, hot air, effluent gas, and superheated steam can be adopted as the heating medium in the dryers of fixed bed, fluidized bed, rotary kiln, entrained bed, and microwave [11–13]. For the latter, it mainly includes hydrothermal dewatering, mechanical/thermal dewatering, and solvent extraction [14]. Pusat et al. summarized the evaporative drying of low rank coal comprehensively, and investigated the main influencing parameters of drying media, coal property and drying method [15]. He et al. found that, in the mechanical thermal treatment, temperature effect (473 K and 513 K) on the drying behavior of soft lignite (Loy Yang from Australia) was more evident than that of hard lignite (Shengli from China) [16]. While the utilization of hydrothermal treatment for lignite drying in the temperature range of 473–623 K was still limited by its harsh experimental conditions [17]. As for the drying kinetics, Stokie et al. [18] compared the effect of superheated steam and air (below 473 K) in the drying characteristics of Victorian brown coal by a fluidized bed dryer, and found that Midilli-Kucuk model can accurately describe the drying kinetics. Kang et al. [19] examined the drying kinetics of Indonesian lignite below 460 K by a lab scale fixed bed reactor and thermobalance to apply the catalytic gasification process.
⁎ Corresponding authors at: State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China (F. Wang). E-mail addresses: [email protected] (F. Wang), [email protected] (G. Xu).
https://doi.org/10.1016/j.fuel.2019.05.025 Received 1 April 2019; Received in revised form 27 April 2019; Accepted 5 May 2019 Available online 09 May 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Tahmasebi et al. [20] found that the Midilli-Kucuk model fitted well the drying kinetics of lignite in a fluidized bed dryer regardless of in the atmosphere of nitrogen and superheated steam, while the page model was more suitable for the microwave drying. After reviewing the existing researches on drying behavior and kinetics, one can see that most of them were conducted at low temperature adopting to the different drying processes, but not at high temperature corresponding the actual thermal-chemical conversion processes. This is much attributed to the easy decomposition property of lignite even at very low operational temperature, such as dehydration below 623 K, and decarboxylation even below 433 K [21,22]. Actually, Zheng et al. [23] tried to investigate the high-temperature (873–1073) drying kinetics of lignite by a horizontal fixed-bed reactor in N2, but only analyze the initial drying stage for the moisture fraction of above 0.7. As we know, the drying behavior and kinetics obtained at low temperature, such as below 473 K, are very useful for the design and improvement of the whole drying system [9,24]. However, are these researches also effective at high temperature, such as above 773 K? Can they still help greatly to understand the behavior of coal particle in pyrolyzer, gasifier, and combustor operated in the temperature range of 773–1273 K? Actually, at present, there are not direct research and obvious evidence to solve these issues. So, probing drying behavior of lignite at high temperature and calculating the corresponding kinetics will be much more beneficial to understand the whole behavior of coal particle in a thermal conversion process. Recently, to test the gas-solid reaction characteristics and kinetics, the micro fluidization analytical method has been proposed by the Institute of Process Engineering (IPE), Chinese Academy of Sciences [25,26], and the corresponding micro fluidized bed reaction analyzer (MFBRA) has been also developed. The adopted micro fluidized bed (MFB) reactor with an inner diameter of about 20 mm can effectively enable high transfer rate of heat and mass, and realize the minimum diffusion inhibition. The online feeding system by pulse jetting method guarantees to initiate the chemical reaction of thermal-unstable materials instantaneously at the preset temperature. All the gaseous products from the gas-solid reaction will be measured continuously by a fast process mass spectrometer (MS). Up to now, this analyzer has been employed in many reactions successfully, such as coal/biomass pyrolysis, char gasification and combustion, iron ore reduction, CO2 adsorption, methanation, and so on [27,28]. In the present research, the micro fluidized bed reaction analyzer was used to measure drying behavior and kinetics of lignite in the temperature range of 773–1173 K. To avoid the decomposition of lignite, char samples with different initial moisture were employed to simulate wet lignite in a gasifier or combustor operated at high temperature. Several empirical models were applied to fit the drying behavior and kinetics. Then, the apparent diffusion coefficient and activation energy of char drying in MFBRA were further analyzed and compared with the literature. As a consequence, this study hopes to provide a method for probing lignite drying behavior at high temperature, and thus enhance the understanding of coal particle in the high-temperature thermal conversion processes.
Table 1 Property of coal sample used in the experiment. Proximate analysis [wt%, d]
Ultimate analysis [wt%, daf] a
LHV [MJ/ kg]
A
V
FC
C
H
S
O
N
16.7
36.2
47.1
75.1
4.3
1.1
18.7
0.8
20.42
➀ d: dry basis; ➁ daf: dry and ash-free basis;➂ aBy difference.
constant temperature humidifier, and accurately control the water-absorbing time. 2.2. Apparatus and test procedure As schematically illustrated in Fig. 1, the adopted MFBRA mainly included a micro fluidized bed reactor, a pulse injecting system for solid sample, a gas supplying system, an electric heating furnace, and an online process mass spectrometer (AMETEK). The quartz reactor of micro fluidized bed (MFB) with an inner diameter of 20 mm was composed by a bottom stage for gas preheating, a middle stage as the reaction zone, and a top stage for preventing the escape of solid particles from the reaction zone. High-purity quartz sand with a particle size in the range of 0.07–0.09 mm and high-purity Ar (99.9995%) were chosen as the heating carrier and carrying gas, respectively. The reactor was heated by an electrical heating furnace with the temperature control accuracy of ± 2 K. All the experimental temperatures in MFBRA were tested by a series of K-type thermocouples. Prior to each experiment, the MFB reactor was heated to the preset temperature, such as 1073 K, and then a certain flow rate of highquality Ar was introduced into the reactor, forming a good fluidization state of the solid particles. Under the stable state of MFBRA, a certain amount of wet char sample (about 30 mg) was instantaneously jetted into the MFB reactor, initiating the char drying process quickly. The released steam was detected by an online process mass spectrometer with a sampling frequency of 20 times per second. 2.3. Analytical method For each isothermal drying experiment, the beginning point and ending point of drying process can be determined by the steam release curve measured by the mass spectrometer. Fig. 2 takes the drying experiment of wet char with the initial moisture of 10% as an example. During the drying process, the moisture of char sample can be determined according to the equations from (1) to (3).
St − t ⎞ ⎛ mi = m 0 × ⎜1 − 0 i ⎟ St0− t f ⎝ ⎠
2. Experimental
ti
St0→ ti =
∫t
St 0 → t f =
∫t
0
tf
0
ti t0 (Imass − Imass ) dt
ti t0 (Imass − Imass ) dt
(1) (2) (3)
where mo and mi are the moisture in char sample at the initial reaction time (t0) and arbitrary time (ti) respectively; St0→ ti and St0→ t f refer to the integration area between the curve of steam and the baseline of MS (without drying process) from the drying time t0 to ti, and t0 to the tf t0 ti , Imass and Imass denote the signal inending time (tf), respectively; Imass tensity of steam tested by MS at t0, ti, and tf, respectively. After the hightemperature drying, the moisture content (mf) was very close to 0. The moisture fraction (X), as a dimensionless term, is defined as the ratio of residual moisture left in char sample to the total initial moisture in char, while the drying rate (R) represents the change of moisture fraction in a unit time (t), as shown in the equations (4) and (5), respectively. According to the definition of moisture fraction, the conversion of moisture (C) during the drying process can be calculated by
2.1. Material A kind of lignite from Shenli coal mine located in Inner Mongolia Autonomous Region of China was adopted in this study, whose proximate, ultimate and heating value analysis were listed in Table 1. Prior to utilization, to obtain coal sample with a particle size in the range of 0.10–0.15 mm, the massive raw coal was crushed, sieved and dried in an oven at 380 K for 12 h. Then, char was produced in a muffle furnace at 1273 K with a residence time of 1.0 h to release volatile matter completely. Finally, the wet char samples with the initial moisture of 10% or 20% were prepared by putting the dried char sample in a 181
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Fig. 1. Schematic diagram (a) and picture (b) of the adopted MFBRA.
conditions, each release curve can be divided into two stages, namely the growing-rate zone and the falling-rate zone. For the former, the signal intensity of steam increased quickly, reaching its maximum value in a short drying time. The higher the drying temperature, the shorter the drying time for the fast-rising zone. For example, as shown in Fig. 3(a), it only needed 16 s and 10 s for the drying temperature of 973 K and 1173 K, respectively. With the increase of moisture content in the char sample from 10% and 20%, the drying time in the growingrate zone did not have an obvious change, which was much related to the high heating rate and uniform heat transfer in the micro fluidized bed reactor. While for the falling-rate zone, much longer drying time, more than 200 s, was always needed to reach the baseline level of steam. Moreover, in the initial stage of the slow-falling zone, the signal intensity of steam decreased quickly, due to the evaporation occurred mainly from the surface of char sample. Then, the curve gradually declined because of the relatively low transfer rate of moisture inside the char particle [29,30]. Fig. 3(c) and (d) further display the effect of the initial moisture on the release behavior of steam from the wet char sample. For brevity, it just took the drying temperatures of 973 K and 1173 K as examples. At each drying temperature, with the increase of the initial moisture content, the curve shape became much wider, indicating longer drying time needed. For example, for the initial moisture of 10% and 20%, at 973 K, it required about 500 s and 1100 s, respectively, while at 1173 K, they were about 300 s and 400 s, respectively. The increase of moisture content meant more energy required, and inevitably lowered the rate of heat transfer in the char sample during the drying process.
Fig. 2. Isothermal analysis method of wet char drying in MFBRA.
the equation (6).
Xi =
R=
mi − m f m 0 − mf
≈
mi m0
dX dt
C i = 1 - Xi
(4)
(5)
3.2. Drying characteristics of wet char in MFBRA
(6) Fig. 4 shows the variation of moisture conversion in the temperature range of 773 K to 1173 K. During char drying, below the conversion of 0.6, moisture in char sample was removed quickly. For the initial moisture of 10% and 20%, approximate 60–80% and 60–90% of the total moisture were decreased within 75 s and 150 s, respectively. After that, the moisture conversion gradually increased until to the complete evaporation of moisture into the carrier gas. For the adopted char sample, with the increase of drying temperature, the time for complete dewatering became short, indicating good promotion effect. High drying temperature increased the surface and interior temperature of char sample, accelerating the evaporation rate of moisture at the char
3. Results and discussion 3.1. Steam release of wet char in MFBRA Fig. 3(a) and (b) show the effect of drying temperature on the release behavior of steam from the wet char sample. Regardless of the initial moisture of 10% and 20%, with the increase of drying temperature, the steam release curve became much narrower, while the maximum moisture intensity was much higher. Under the experimental 182
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Fig. 3. Effect of temperature and initial moisture on the steam release tested by MFBRA during the wet char drying.
with the initial moisture of 10% and 20%, each curve reached its maximum value quickly in the initial stage, and then gradually decreased with the progress of char drying. In the initial fast heating stage, the temperature of coal particle rose rapidly, and the evaporation of surface water was promoted by the intense heat transfer. After that, due to the lower moisture in coal particle, the vaporization rate of steam at the surface of coal particle was much higher than the moisture transfer rate from the interior to its surface, making the internal moisture diffusion as the limiting step and thus forming the falling-rate
surface and the transfer rate of moisture inside char particle. Moreover, one can also see that for the two kinds of char sample, the favorable effect of temperature was much different. With respect to the initial moisture of 10%, compared to the temperature from 773 K to 1073 K, the drying behavior at 1173 K was much promoted. While for the char sample with the initial moisture of 20%, it did not have the similar phenomenon. Fig. 5 displays the relationship between the drying rate and conversion under different temperatures. For the adopted char samples
Fig. 4. Effect of temperature on the drying behavior of char with different initial moistures. 183
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Fig. 5. Drying rate versus conversion for the char samples with different initial moistures in MFBRA.
et al. [35] examined the drying characteristics and kinetics of Shengli lignite using four fluidized bed (FB) reactors, namely bubbling FB (BFB), vibrated FB (VFB), medium FB (MFB) and vibrated medium FB (VMFB). Due to the large inner diameter of the reactor (110 mm) and low drying temperature of 353–433 K, there was also the constant-rate zone in the drying curve. Kim et al. [36] investigated the drying behavior of Loy Yang brown coal in a fluidized bed with an inner diameter of 22 mm, and found that in the curve of reaction rate versus moisture conversion, it still had the constant- rate stage in the range of relative humidity of 0.3–0.7. Considering the similar inner diameter with the micro fluidized bed reactor (20 mm) adopted in this study, it was reasonable to attribute these differences mentioned above to the low drying temperature and heating rate. These analyses mentioned above fully confirmed that the drying characteristics are much related to not only the reactor structure but also the drying temperature adopted. So the low-temperature drying property cannot be used to explain the particle behavior of lignite in the high-temperature thermal conversion process effectively. Moreover, in the previous research of char gasification in the atmosphere of CO2 and steam-N2 tested by MFBRA, the complete conversion of char needed about 8 min and 16 min at 1173 K [27], respectively. While for the initial moisture of 10% and 20%, the complete dewatering required about 4 min and 6 min at 1173 K. By comparing with the drying time and gasification time, it is obvious that, in the practical gasifier, lignite drying will last a relatively long time, and for a single coal particle, the processes of drying, devolatilization, gasification, and combustion always synchronize and interlace. Fig. 7 further compares the difference in drying characteristics for
stage [31]. For both of the two stages, by raising drying temperature, the increase of drying rate was much related to the reinforcement of moisture migration resulting from the enhancement of driving force on heat transfer. Moreover, compared to the low drying temperature, the drying rate at high temperatures, such as above 1073 K, was obviously promoted. And, with the increase of drying temperature, the moisture conversion corresponding to the maximum drying rate increased gradually. To elucidate the differences between conventional low-temperature drying in literature and high-temperature drying in MFBRA, Fig. 6 further displays the typical drying curves of coal particle in literature, mainly consisting of the drying zones of growing-rate, constant-rate and falling-rate [32,33]. Generally, in the constant-rate stage, the energy absorbing from drying medium to coal sample and the energy-requiring for moisture vaporization at the particle surface can reach a balance. Compared to the conventional drying at low temperature, in MFBRA, the fine coal particle was heated to the preset temperature instantaneously due to high heat transfer rate above 1000 K/s, leading to the lack of constant – rate stage and the present of growing-rate stage quickly in the initial stage. Moreover, in a conventional drying process, the constant-rate stage was dominant, accounting for two-thirds of the total drying process and consuming a large amount of energy, while for the high-temperature drying in MFBRA, the rate-falling stage played a leading role [34]. On the other hand, by comparing with coal drying adopting the fluidized bed reactor in literature, one can also see the obvious differences in drying characteristics tested by MFBRA. For example, Zhao
Fig. 6. Conventional drying curves of materials under constant drying conditions [14]. I. Growing-rate zone; II. Constant-rate zone; III. Falling-rate zone. 184
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Fig. 7. Effect of the initial moisture on the drying behavior of wet char in MFBRA.
the initial moisture of 10% and 20% in the char sample at temperatures of 1173 K and 873 K. For each experimental temperature, high moisture led to a long drying time and low drying rate. Compared to the drying temperature of 1173 K, the difference in drying characteristics between the initial moisture of 10% and 20% at 873 K became much more obvious. For example, at 1173 K, the ratio of the maximum drying rate for the initial moisture of 10% and 20% was 1.07, while for 873 K, the ratio increased to 1.55.
Table 2 Typical mathematical models of thin-layer drying in literature. Model
3.3. Mathematical model of wet char drying in MFBRA To analyze the drying behavior of moist char in MFBRA at high temperature, the experimental results were fitted by a drying model. For ease of selection, Table 2 summarizes the well – known mathematical models for drying the thin layer material with the uniform temperature distribution. Most of the models adopted can be used to describe well the mathematical decaying tendency of moisture fraction with the evolution of drying time. According to the exponential relationship between moisture conversion and drying time, as shown in Fig. 5, non-linear fitting analyses on the basis of mathematical models in Table 2 were conducted. Among them, for brevity, Fig. 8 just shows some typical curves between predicting models and experimental results in MFBRA. For the model with the equation of X = 1−a(1−ebt), it was very suitable to describe the experimental result with a correlation coefficient (R2) above 0.99. Table 3 displays the calculated results for the parameters of a, b, and the corresponding R2 for each experiment. From it, one can see that the obtained values were very similar. So, it was reasonable to average them, obtaining the values of 1.002 and 0.013 for the parameters a and
Equation
References [37]
Lewis
X = e−kt
Page
X = e−kt
Modified page
X = e−(kt )
[39]
Henderson and pabis
X = a × e−kt
[40]
Logarithmic
X = a × e−kt + c
[41]
Diffusion approach
X = a × e−kt + (1 − a) × e−kbt
[42]
Verma
X = a × e−kt + (1 − a) × e−gt
[43]
Two-term exponential
X = a × e−kt + (1 − a) × e−kat
[44]
Simplified fick diffusion Midilli-Kucuk
n
[38] n
X=a×
− ct e L2
X=a×
n e−kt
[45]
+b×t
[46]
b, respectively. For simplicity, the final value of parameter a was set to 1. By referring and comparing the existing mathematical models listed in Table 2, Lewis drying model was finally adopted to depict the drying of moist char in MFBRA. So, the exponential relationship between moisture fraction and drying time can be expressed by the following equation:
X = e−0.013t
(7)
3.4. Drying kinetics of wet char in MFBRA To quantitatively analyze the drying kinetics, the apparent diffusion coefficient can be derived by the Fick’s second law with the assumption 185
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Fig. 8. Comparison between experiment results and predication results from models.
Fig. 9. Fitting curve of LnDeff VS. 1/T for the drying of wet char in MFBRA.
4π 2Deff t 8 LnX = Ln ⎛ 2 ⎞ − d2 ⎝π ⎠
Table 3 Fitting Analysis of drying characteristics by different models. Moisture
10%
20%
bt
Temperature [K]
Equation:1−X = a(1−e )
2
R
a
b
1173 1073 973 873 773
1.024 1.021 1.014 1.006 0.997
−0.018 −0.015 −0.013 −0.013 −0.013
0.999 0.999 0.998 0.998 0.998
1173 1073 973 873 773
1.001 0.976 0.986 0.994 1.003
−0.015 −0.013 −0.011 −0.009 −0.007
0.997 0.987 0.993 0.994 0.998
1.002
−0.013
–
Average
To calculate kinetical parameters of char drying in MFBRA, Arrhenius equation was used for reference, as shown in the Eq. (10) and the corresponding logarithmic form of the Eq. (11), where Ea is the activation energy of char drying, D0 is the diffusivity value for the infinite moisture content, and R presents the universal gas constant. Ea
Deff = D0 × e− RT
LnDeff = LnD0 −
2
4π Deff t − 8 d2 ×e 2 π
(10)
Ea RT
(11)
By plotting LnDeff versus 1/T, a good linear relationship, with a fitting degree above 0.98, can be observed, as shown in Fig. 9. According to the slopes of the fitting curves, Ea can be calculated with the values of 12.83 kJ/mol and 14.97 kJ/mol for the initial moisture of 10% and 20%, respectively. Generally, Ea denotes the energy barrier to overcome for a certain conversion process. High moisture content means the strong interaction force between moisture and char particle. Table 5 lists the typical drying experiments and kinetic parameter calculated in literature. Due to the difference in reactor structure and heating temperature, the values of Ea in literature were much different with a span in the range of 18.53−106.37 kJ/mol. Compared to the literature research, the value of Ea tested by MFBRA was much lower, just lying in the range of 12.83–14.97 kJ/mol. On the one hand, this was much related to the high heating rate and uniform heat transfer in the adopted micro fluidized bed reactor. On the other hand, the high drying temperature further promotes the heat transfer not only on the surface but also the interior of char sample.
of transfer via diffusion, whose equation and the corresponding logarithmic form can be seen in the Eqs. (8) and (9). In Eq. (9), it displays a linear relationship between LnX and drying time (t). So, by plotting the LnX against the drying time (t), the apparent diffusion coefficients (Deff) for each drying temperature can be calculated, as shown in table 4. For the examined temperature, the value of Deff for the initial moisture of 10% and 20% ranged from 2.388E−10 m2/s to 4.756E−10 m2/s, and 1.765E−10 m2/s to 3.901E−10 m2/s, respectively. With the increase of drying temperature, the value of Deff rose quickly, being consistent with the research in the literature [46]. Similar to the effect of temperature, the type of FB played an important role in Deff, displaying the lowest diffusion resistance.
X=
(9)
(8)
Table 4 Apparent diffusion coefficient at different temperatures for char drying. Moisture
Items
Temperature [K]
2
773
873
973
1073
1173
10%
Deff [m /s] Ln R2
2.388E−10 −22.16 0.992
2.980E−10 −21.93 0.985
3.550E−10 −21.76 0.976
4.100E−10 −21.61 0.986
4.756E−10 −21.47 0.988
20%
Deff [m2/s] Ln R2
1.765E−10 −22.46 0.993
2.180E−10 −22.25 0.987
2.630E−10 −22.06 0.986
3.290E−10 −21.83 0.992
3.901E−10 −21.66 0.992
186
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Table 5 Drying kinetics of coal particle in literature. Number
Reactor
Temperature [K]
Initial moisture [%]
Drying medium
Ea [kJ/mol]
Literature
1
BFB VFB MFB VMFB Horizontal fixed bed dryer TGA Microwave FB
353–639
40
Air
[35]
< 473 < 443 < 433 < 523
45 5–10 35 39
N2 N2 N2 N2 Superheated steam
30.22 29.78 29.23 27.92 49.42–106.37 18.53 28.59 25.05 25.76 25.81
2 3 4 5
Microwave
4. Conclusions
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In this study, to probe the drying characteristics of wet lignite at a high temperature, a newly designed micro fluidized bed reaction analyzer was adopted by jetting the wet char and analyzing the release curve of steam. The results showed that the drying property of wet char was much related to the drying temperature and the initial moisture. With the increase of drying temperature, the curve shape of drying rate versus moisture conversion became narrow, indicating short drying time and high drying rate. Compared to the initial moisture of 10%, the effect of drying temperature became much more obvious. By non-linear fitting the experimental results and comparing with the existing mathematical models, Lewis drying model can describe the drying property of wet char sample in MFBRA. Then, the apparent diffusion coefficient and drying kinetics were quantitatively analyzed and calculated. For the initial moisture of 10% and 20%, the value of Deff ranged from 2.388E−10 m2/s to 4.756E−10 m2/s, and 2 2 1.765E−10 m /s to 3.901E−10 m /s, respectively, while the value of Ea lay in the range of 12.83–14.97 kJ/mol. Compared to the conventional low-temperature drying of lignite, one can find that drying property was much related to the reactor structure and operational temperature. In MFBRA, the measured curve of drying rate against moisture conversion in coal char at a high temperature just had the rate-growing zone and rate-falling zone without the rate-constant zone. And the calculated activation energy was much lower. All of these not only identified the feasibility of measuring method for drying property by MFBRA, but also indicated the necessity of drying behavior and kinetics of lignite to really reflect the thermal conversion behavior at high temperature. Acknowledgement The authors gratefully acknowledge the financial support provided by the National Key R & D Program of China (No. 2018YFF01011400). References [1] Xia WC, Xie GY, Peng YL. Recent advances in beneficiation for low rank coals. Powder Technol 2015;277:206–21. [2] Zhao HY, Song Q, Liu SC, Li YH, Wang XH, Shu XQ. Study on catalytic co-pyrolysis of physical mixture/staged pyrolysis characteristics of lignite and straw over an catalytic beds of char and its mechanism. Energy Convers Manage 2018;161:13–26. [3] Zhao HY, Wang BZ, Li YH, Song Q, Zhao YQ, Zhang RY, et al. Effect of chemical fractionation treatment on structure and characteristics of pyrolysis products of Xinjiang long flame coal. Fuel 2018;234:1193–204. [4] Wu JH, Wang J, Liu JZ, Yang YM, Cheng J, Wang ZH, et al. Moisture removal mechanism of low-rank coal by hydrothermal dewatering: physicochemical property analysis and dft calculation. Fuel 2017;187:242–9. [5] Yu JL, Tahmasebi A, Han YN, Yin FK, Li XC. A review on water in low rank coals: the existence, interaction with coal structure and effects on coal utilization. Fuel Process Technol 2013;106:9–20. [6] Zhao HY, Li YH, Song Q, Wang XH, Shu XQ. Drying, re-adsorption characteristics, and combustion kinetics of Xilingol lignite in different atmospheres. Fuel 2017;210:592–604. [7] Liu HM, Chen MQ, Han ZL, Fu BA. Isothermal kinetics based on two-periods scheme for co-drying of biomass and lignite. Thermochim Acta 2013;10:25–31.
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