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Tandem fluidized bed elutriator—Pneumatic classification of coal particles in a fluidized conveyer
Particuology 10 (2012) 600–606
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Tandem fluidized bed elutriator—Pneumatic classification of coal particles in a fluidized conveyer Zhouen Liu a , Yimin Xie b , Yin Wang a , Jian Yu a , Shiqiu Gao a , Guangwen Xu a,∗ a b
State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China School of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China
a r t i c l e
i n f o
Article history: Received 24 March 2011 Received in revised form 16 December 2011 Accepted 14 March 2012 Keywords: Tandem fluidized bed elutriator Coking Coal classification Moisture control Pneumatic bed Fluidized bed
a b s t r a c t Coal moisture control (CMC) in coking process, which reduces coal moisture before loading the coal into the coke oven, allows substantial reduction in coking energy consumption and increase in coke productivity. The technology is seeking to integrate the coal classification, thus calling it the coal classifying moisture control (CCMC), to separate the fine and coarse coal fractions in the CMC process so that the downstream coal crushing can only treat the coarse fraction. CCMC adopts a reactor that integrates a fluidized bottom section and a pneumatic conveying top section. The present work investigates the pneumatic classification behavior in a laboratory CCMC reactor with such a configuration by removing the coal fraction below a given size (e.g., 3.0 mm) from a 0 to 20.0 mm coal feed. The results show that the coal classification were dominated by the gas velocity in the top conveying section, and the required gas velocity for ensuring the maximal degree of removing a fine coal fraction could be roughly predicted by the Richardson and Zaki equation. The effect of bottom fluidization on the performance of CCMC is also examined. © 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction To improve coke quality, to save energy, and to reduce emission (e.g., wastewater) in the coal coking process, it is desirable to limit the coal feedstock for coking ovens (i.e., the tamping coking oven) to sizes smaller than 3.0 mm and to moisture contents between 6.0 wt% and 9.0 wt% (Liu et al., 2009; Yang, 2003). However, actual raw coal feedstock used in coking usually has a wide size distribution of, e.g., 0–20.0 mm, and an average moisture content of 11.0–20.0 wt%, thus calling for both coal crushing and coal moisture control (CMC) before loading into the coking oven. If all the coal particles, including those already smaller than 3.0 mm, are crushed, higher energy consumption is inevitable. As to coal moisture content, it was found that the coal fraction (e.g., <3.0 mm) is dominant in forming the moisture content of the raw coal, and the moisture content of the coarse fraction (e.g., >3.0 mm) is usually below 10.0 wt% (Liu et al., 2010). Therefore, in coking process the coal crushing should primarily be used for the coarse fraction, whereas the moisture control should be mainly applied to the fine fraction. This justifies the combination of coal size classification and moisture content control so that coal is classified into fine (e.g.,
∗ Corresponding author. Tel.: +86 10 82629912; fax: +86 10 82629912. E-mail addresses: [email protected] (Z. Liu), [email protected] (G. Xu).
<3.0 mm) and coarse (e.g., >3.0 mm) fractions to allow the former for moisture removal and the latter for further crushing. A great deal of work has been conducted on developing CMC technology in Japan, China and European countries, resulting in three generations of technologies, which are oil CMC, steam CMC and fluidized bed CMC (Chen, Agarwal, & Agnew, 2001; Li, Tan, & Lan, 2010; Li, 2005; Mito, Komatsu, Hasegawa, & Mae, 2005; Potter & Keogh, 1981; Zheng, 2002). Of these, the former two generations were based on indirect heat exchange of the coal with oil and with steam in the contactors, e.g., the rotary oven. The fluidized bed CMC was first developed in Japan and recently further updated in China, by using the sensible heat of the coking oven flue gas in direct contact with wet coal in a fluidized bed. The first fluidized bed CMC in Japan was placed downstream of the coal crusher, whereas in more recent development in China the CMC was set before the coal crusher. In the latter case, the preheating, moisture control and pneumatic classification for coal are implemented simultaneously in a single process, which represents a fresh development of the CMC technology. The fluidized bed CMC adopts either a vibration fluidized bed (VFB) (Yang, Zheng, Zhou, Zhao, & Chen, 2002) or a fluidized bed with an internal grate chain to move non-fluidized large particles (Palappan & Sai, 2008). In these cases, pneumatic classification is possible only for the fine particles, e.g., smaller than 1.0 mm. While separate treatment of these fine particles may cause serious
1674-2001/$ – see front matter © 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.partic.2012.03.005
Z. Liu et al. / Particuology 10 (2012) 600–606
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Nomenclature AF C1 C2 dp F Ho RQP SF UF Umf UP Ut Ut,a Wi,Pr Wi,Fs ε f p
accumulative fraction (wt%) coefficient, C1 = 33.7 coefficient, C2 = 0.048 particle diameter (m) coal feeding rate (t/(m2 h)) height of overflow outlet (m) ratio of primary airflow to total airflow fraction of a sieving size range (wt%) superficial gas velocity in fluidized bottom (m/s) minimum fluidization velocity (m/s) superficial gas velocity in pneumatic top (m/s) transport velocity for a single particle (m/s) entrainment velocity in a gas–solid flow (m/s) mass (weight) of a fraction i in product (kg) mass (weight) of a fraction i in feedstock (kg) voidage viscosity (Pa s) fluid density (kg/m3 ) particle density (kg/m3 ) classification degree
Fig. 1. Distributions of particle size and moisture content of tested coal. (AF = accumulative fraction; SF = sieving fraction.)
2. Experimental 2.1. Material characterization
dusting, the coke oven allows the direct use of the 0–3.0 mm coal fines from classification. This (having 0–3.0 mm fines) consequently reduces the working load required for the coal crushing (saving more energy) and avoids the dust elutriation as well. Therefore, a coal classifying moisture control (CCMC) process was proposed to implement simultaneously the classification for 0–3.0 mm particles and coal moisture control (CMC). Based on the concept of tandem fluidized bed elutriator (TFBE), the proposed process adopts an integrated bed consisting of a dense fluidized or moving-bed bottom section and a dilute, pneumatic conveyer top section to entrain coal particles below a given size (e.g., <3.0 mm), whereas the large particles, with low moisture content, are withdrawn from the bottom of the bed. By interacting with hot coke oven flue gas supplied from the bottom, moisture removal for the fine and coarse fractions occurs in the top and bottom sections, respectively. The present article investigates the pneumatic classification of 0–20.0 mm raw coal feed in a test rig simulating the bed structure for the CCMC process. Numerous prior studies described pneumatic classification (Baeyens, van Gauwbergen, & Vinckier, 1995; Eswaraiah, Kavitha, Vidyasagar, & Narayanan, 2008; He & Liu, 2009; Innocentini, Barizan, Alves, & J, 2009; Mohanty, Palit, & Dube, 2002; Mohanty, 2003; Morimoto & Shakouchi, 2003; Rogers, 1982; Shapiro & Galperin, 2005; Yang et al., 2002). Innocentini et al. (2009) pneumatically separated hulls and meats from cracked soybeans, and Eswaraiah et al. (2008) studied the pneumatic separation of metal and plastic particles. Morimoto and Shakouchi (2003) tested ultra-fine powder elutriation in an air-blown classifier. However, most of the particles tested in these studies had narrow size distributions and were small (e.g., <1.0 mm). For feedstock with sizes of up to several tens of millimeters (e.g., 20.0 mm in this work), there have been but limited studies. Integrating a fluidized bed or moving bed bottom and a pneumatic conveying top represents a special structure for particle classification that has rarely been studied. This study intends to clarify the suitable conditions and bed configuration that ensure the maximal separation of coal particles smaller than 3.0 mm from coarser particles. The results will provide a basis for designing CCMC employing such integral configuration.
The coal tested was a raw feedstock taken before crushing from the Henan Shuncheng Coking Chemical Corporation. The size distribution of the coal was determined by using a vibratory sieve consisting of sieve sizes (di ) of 1.0, 1.5, 2.0, 3.0, 4.0, 6.0, 8.0, 10.0 and 12.0 mm, and the Sauter mean diameter (dmn ) for the fraction between the sieve sizes dm and dn was estimated by
n Wi dmn = n i=m+1 i=m+1
(Wi /di )
, n ≥ m + 1,
m = 0, 1, 2, . . . , N − 1
(1)
where Wi is the mass fraction of particles in sizes between di−1 and di ; m and n represent the series number of the selected sieves; N is the total number of sieves used. The moisture content of the coal was measured according to the GB/T211-1996 method (drying at 105–110 ◦ C for approximately 2 h). The size distribution and the corresponding moisture content of the tested coal samples are shown in Fig. 1, where SF is the sieving fraction, and AF is the accumulative fraction. The overall size range was 0–20.0 mm, and approximately 60.0 wt% was smaller than 3.0 mm (see the AF) and did not need crushing for use in coking. The Sauter mean diameters for the coal fractions smaller than and larger than 3.0 mm were approximately 1.0 and 6.0 mm, respectively (calculated by Eq. (1)). Fig. 1 shows that the coal moisture decreased with increasing coal particle size. The moisture content for the fraction smaller than 3.0 mm was as high as 10.5–13.0 wt%, but for the fraction larger than 6.0 mm, it was less than 8.0 wt%, illustrating again the necessity of CMC based on size classification so that moisture control is implemented primarily for the fine coal fraction (i.e., <3.0 mm). Using helium pycnometry (AccuPyc 1330), the coal particle density was found to be 1398 kg/m3 . The minimum fluidization velocity Umf and the entrainment velocity Ut for a given size fraction were calculated using the following Eqs. (2) and (3), respectively (Kwauk & Li, 2008):
Umf =
C
1/2
+ C2
dp3 f (p − f )g 2
1/2 − C1
, dp f
(2)
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Fig. 2. Minimum fluidization and entrainment velocities, Umf and Ut , for differently sized particles determined by both calculation and experimental measurements.
Ut =
⎧ 2 dp (p − f )g ⎪ ⎪ , Ret < 0.4 ⎪ ⎪ 18 ⎪ ⎪ ⎪ ⎨ 0.5 2dp (p − )gRe
f t , 0.4 < Ret < 500 ⎪ 15f ⎪ ⎪ ⎪ ⎪ ⎪ d ( − f )g ⎪ ⎩ 4/3 p p , Ret > 500
(3)
0.43f
In the equations, p and f denote the particle and fluid densities; g is the acceleration of gravity; is the fluid viscosity; dp is the average particle diameter for a sieve size range; and C1 = 33.7 and C2 = 0.048 are two constant coefficients. Meanwhile, both Umf and Ut were experimentally measured in a 60.0-mm column in I.D. and 3.0 m high to verify the calculated values of the two velocities, considering that the employed large particle sizes of the coal might not be accurately predicted by Eqs. (2) and (3). The Ut was measured according to the approach reported by Innocentini et al. (2009), and the Umf was determined using the standard velocity reduction method (Felipe & Rocha, 2007). From the comparison in Fig. 2, one can see good agreement between measurement and empirical calculation. Therefore, to classify particles smaller than 3.0 mm via elutriation, the required superficial gas velocity in the conveyer may be approximately 10.0 m/s, and the fluidization velocity for the dense bottom of the proposed CCMC technology should be over 1.5 m/s (the minimum fluidization velocity based on the Sauter diameter of the coarse coal larger than 3.0 mm) to fully fluidize the coal fraction larger than 3.0 mm. 2.2. Apparatus and procedure The experimental apparatus had two different configurations, which are schematically shown in Fig. 3(a) and (b). The configuration shown in Fig. 3(a) refers to the normal design featured in a fluidized dense bottom and a conveying column above it. The top conveying column had an I.D. of 60.0 mm and a height of 3.0 m, and the bottom fluidized bed section had an 80.0 mm I.D. and a height of 1.5 m. Between these two sections there was a conical transition connection. Three particle overflow points were set in the fluidized bed section, with heights from the bottom of the fluidized bed at 28.0, 56.0 and 84.0 cm. Coal was continuously fed into the bed via a screw feeder from a position just at the bottom of the conveying section. Fig. 3(b) shows a modified configuration with a narrow bottom section to enable turbulent fluidization at the bottom for large
particles. In this case, the bottom was a column with 50.0 mm I.D., and an expanded section with a diameter of 80.0 mm and a length of 400 mm was mounted between this fluidized bottom and the top conveyer. Coal was fed into the bed at the same position as shown in Fig. 3(a). For both configurations, the entrained fine coal was collected in a cyclone. Compressed air was used for pneumatic classification. The test was conducted by first setting the gas velocity in the test column and then continuously feeding a given amount of coal into the column via the screw feeder. Both the gas velocity in the column and the coal-feeding rate were varied with the tests. The coarse fraction included both the overflow coal particles and those remaining in the bed bottom. The mass and size distribution of the collected coal at the bed bottom and top were measured to characterize the size classification performance. Of the tests shown here, almost all, unless otherwise specified, were conducted with the primary gas flow and the coal overflow in the bottom section. To evaluate the classification of particles with sizes smaller or larger than di (e.g., 3.0 mm) from the raw coal feedstock, we considered not only the percentage of the entrained particles with respect to the total fed particles but also the fraction of the desired particle size in the product obtained at the top. This leads to the definition of the classification degree : =
2 Wi,Pr
Wi,Fs WPr
,
(4)
where Wi,Fs and Wi,Pr are the mass of the coal particles smaller or larger than di (3.0 mm) in the feedstock and the mass of the product obtained from the column top (di mm), respectively; WPr is the mass of the product at the column top or bottom. The definition indicates that the classification degree can be defined with respect to the collected particles at either the bed top (controlling the sizes of the entrained particles to be di ). The present paper considers di = 3.0 mm, and the value of varies from 0 to 1.0. A large value of means more precise classification. 3. Results and discussion 3.1. Dominance of gas velocity As shown in Fig. 3, pneumatic classification of coal is anticipated due to the high gas velocity in the top pneumatic conveying section. Therefore, the test first clarified the classification performance by varying the gas velocity, and the results are shown in Fig. 4, where the gas velocities UP and UF refer to the superficial gas velocities in the top conveying and bottom fluidization sections, respectively. The coal feeding rate F was 5.3 t/(m2 h), and the classification performance is characterized in Fig. 4(a), with both the sieving fraction (SF) and the accumulative fraction (AF) that refer to the particles entrained into the column top. Raising the gas velocity entrained more particles to the column top, and the entrained particle amounts (mass) were 45.0–85.0% corresponding to a UP of 5.0–9.0 m/s (UF of 2.8–5.1 m/s), respectively. With the higher UP , there were more coarse particles present in the top entrainment. The SF of the coal at sizes between 2.0 and 3.0 mm and the AF for the particles smaller than 3.0 mm in the entrained particles were approximately 0.1 and 99.8 wt% at UP = 5.0 m/s, respectively. These two indices became 5.2 and 86.5 wt% when UP was increased to 8.0 m/s. Compared to the raw feedstock, it is seen that only 1.2 wt% of 2.0–3.0 mm particles was in the entrained fraction at 5.0 m/s, however, this was 83.6 wt% at UP = 8.0 m/s. Concerning all the particles smaller than 3.0 mm as an individual fraction, the entrained percentages of this size fraction
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Fig. 3. Schematic diagram of the experimental columns.
Fig. 4. Classification performance as a function of superficial gas velocity in the conveying top section. (SF = sieving fraction and AF = accumulative fraction.)
reached 77.0 and 96.2 wt% at a UP of 5.0 and 8.0 m/s, respectively. These demonstrate that a UP of 8.0 m/s is sufficient to convey nearly all the particles smaller than 3.0 mm into the top of the tested conveying bed. Under this condition, the fraction of the particles larger than 3.0 mm in the entrained product was 13.5 wt%. Corresponding to the data in Fig. 4(a), the estimated with respect to the entrainment product at the bed top for particles smaller than 1.0–3.0 mm are presented in Fig. 4(b). Within the tested range of the gas velocity Up (5.0–9.0 m/s) in the top conveying section, the value of for the particles smaller than 1.0 mm () decreased with increasing UP , whereas for the other three sizes (, , and ), a peak value of , denoted as max , appeared at a critical velocity UPC . The velocities of UPC corresponding to all max in Fig. 4(b) were approximately 6.0, 7.0 and 8.0 m/s for the particle size groups smaller than 1.5 (), 2.0 () and 3.0 mm (), respectively. The lower at the gas velocities lower than the critical UPC was resulted from the insufficient entrainment of small-sized particles. On the contrary, the lower at the Up higher than the critical UPC was caused by carrying too many coarse particles into the entrainment product. Velocity UPC leading to max , is, therefore, the optimal gas velocity to realize the expected classification for the particles referred to by max . For example, Up should be approximately 8.0 m/s to classify the particles smaller than 3.0 mm into the entrained fraction with the possibly maximal amount (i.e., UPC = 8.0 m/s for 3.0 mm). Fig. 5 shows the variation of the classification performance with coal feeding rate F at UP = 8.0 m/s, clarifying a rather indistinct effect as compared to varying the gas velocity Up shown in Fig. 4. The amount of particles entrained to the bed top decreased only slightly from 87.5% to 84.0% upon raising the coal-feeding rate from 1.8 to 5.3 t/(m2 h). The nearly overlapping curves for the AF in Fig. 5 further verified that the size distribution of the entrained product did not significantly change, although a higher raw coal-feeding rate would lead to more particles with intermediate sizes (i.e., 1.0–3.0 mm) in the entrained fraction at the bed top. The estimated for fractions smaller than 3.0 mm present in the entrained product varied but slightly from 0.80 to 0.86 corresponding to the above-mentioned variation in F, illustrating little change in the classification performance. Therefore, it was the gas velocity, not the coal-feeding rate, that dominated the classification performance and particle conveying.
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Fig. 5. Classification performance at different coal feeding rates.
3.2. Prediction of classification gas velocity Compared to the data in Fig. 2, the critical (optimal) UPC determined in Fig. 4(b) for realizing max was approximately 1.0–2.0 m/s lower than Ut , the theoretical velocity for carrying a single particle of specified size, reflecting the influence of the particle mixture on the velocity for conveying particles. In gas-solid mixture flow, the actual conveying gas velocity is higher than the superficial gas velocity, implying the presence of the other particles to suppress the entrainment (Xu, Nomura, Nakagawa, & Kato, 1999). According to Fayed and Otten (1997), the effect of the presence of particles on the entrainment velocity can be predicted by the Richardson and Zaki equation (Fayed & Otten, 1997; Innocentini et al., 2009): Ut,a = Ut εn ,
(5)
where n is usually 4.7; ε is the voidage of the gas-solid flow; and Ut,a and Ut represent the actual entrainment velocities in a gas-solid mixture flow and of a single particle, respectively. For the tests shown in Fig. 4, the voidage ε in the pneumatic conveying top section for the different gas velocities could be roughly estimated from the differential pressure drop P5–6 across the top conveying section (Sun, Jiang, Liu, Sun, & Xu, 2008): ε=1−
P5−6 p gH5−6
Fig. 6. (a) Axial profiles of pressure for the tests in Fig. 4 and (b) corresponding pressure drops and voidages in the top conveying section as a function of Up .
the tested gas velocities, the voidages plotted in Fig. 6(b) can then be used in Eq. (5) to predict the actually required entrainment velocity, Ut,a , from the single-particle entrainment velocity, Ut , shown in Fig. 2. The resulting Ut,a is compared in Fig. 7 with the experimental UPC from Fig. 4. The corresponding max are also plotted in Fig. 7 versus the particle size dp . For all the size groups considered for classification (i.e., <1.0 mm, <1.5 mm, <2.0 mm and <3.0 mm), the realized max were all higher than 0.8, and the max tended to be slightly higher for classifying the smaller particles. For example, to classify the fractions of <1.0 mm and <3.0 mm, their max were 0.79 and 0.88, respectively. This refers to the fact that it is relatively easy to obtain the better classification for particles in a relatively
(6)
where H5–6 is the difference in height between the taps P5 and P6 (see Fig. 3); and p is the particle density. Fig. 6(a) shows the axial profiles of the pressure corresponding to all the tested Up in Fig. 4, indicating that for the tested bed configuration the pressure drop across the entire column was caused mainly by the fluidized bottom. The pressure difference P5–6 and its corresponding explicit particle holdup (1 − ε) estimated from Eq. (6) are shown in Fig. 6(b). As expected, the particle holdup (1 − ε) in the conveying section and the pressure drop P5–6 across this bed section were both elevated by raising Up . Under the tested conditions, the particle holdup was small (0.009–0.014), implying dilute flow in the top conveying section of the experimental bed. As shown in Fig. 4(b), the critical or optimal gas velocities, UPC , for all the identified max were 5.0, 6.0, 7.0 and 8.0 m/s, which corresponded to the classifications of particles smaller than 1.0, 1.5, 2.0 and 3.0 mm, respectively. Because these velocities are just equal to
Fig. 7. Comparison of the UPC determined from Fig. 4 and the Ut,a calculated via Eq. (6) based on Fig. 5 to classify the particles smaller than 1.0 to smaller than 3.0 mm.
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Fig. 10. Classification performance comparison for the two bed configurations. Fig. 8. Classification performance as a function of the particle overflow height in the fluidized bottom section.
wide size range (0–3.0 mm is wider than 0–1.0 mm). The predicted actual entrainment velocity, Ut,a , using Eq. (5) agreed well with the experimentally determined UPC (Fig. 7). These results reveal that the actually required entrainment gas velocity, Ut,a = UPC , for classifying the particles smaller than a dp can be predicted via the Richardson and Zaki Eq. (5) based on the terminal velocity, Ut , for transporting a single particle with the size dp , and the voidage ε measured in the actual conveying column (approximately 0.01 here). 3.3. Optimization of bed configuration This section investigates how bed configuration affects classification performance. Fig. 8 shows the values for SF, AF and obtained at Up = 8.0 m/s, F = 5.3 t/(m2 h) and different heights Ho at the particle overflow outlet in the fluidized bottom section. Varying Ho from 0.28 to 0.84 m caused the amount of particles entrained to the bed top to increase from 84.0 to 90.6 wt%. The accumulative
Fig. 9. Classification performance with the adoption of a secondary airflow.
curve moves slightly down and to the right with increasing Ho , showing that more fine particles were entrained into the column top. With increasing Ho , the SF value decreases for the fine fraction <1.0 mm, but it was higher for the other large-sized fractions (>1.0 mm). Therefore, with a higher overflow outlet in the fluidized bottom section, the classification degree was lower, as shown in the suspended table of Fig. 8. The was 0.86, corresponding to Ho = 0.28 m, and decreased to 0.78 at Ho = 0.84 m. From the viewpoint of classification, the overflow outlet should be at the lowest possible position, although this configuration would slightly lower the entrainment capability of the gas and lead to somewhat lowered particle entrainment. This configuration is also necessary to lower the pressure drop across the fluidized section and reduce the energy consumption of the gas blower. Splitting the hot airflow supply provides a viable way to adjust the gas–solid interactions in the fluidized bottom. For the CCMC process, the split would be effective to control the amount of air interacting with the particles in the fluidized bottom. Fig. 9 shows the variation of the classification performance with the ratio RQP of the primary airflow (shown in Fig. 3) to the overall airflow at a fixed gas velocity of UP = 8.0 m/s in the top conveying section. As expected, the amount of entrained particles decreased from 84.0 to 77.0 wt% corresponding to the variation of RQP from 1.0 (all the air was primary) to 0.63. The particle size distribution did not vary greatly with RQP , indicating that in the tested rig and under the mentioned operating conditions, the fluidization state in the bottom did not significantly change the entrained particle sizes. Furthermore, the tested range of UF = 2.8–4.5 m/s can well fluidize all coal particles smaller than 2.0 mm (see Fig. 2), causing more than 80% of the entrained particles to be between 0 and 2.0 mm. This feature varied little with the changes in RQP under all the tested conditions in Fig. 9. However, when the amount of entrained particles decreased by reducing the primary airflow ratio from 1.0 to 0.63, the classification degree declined from 0.86 to 0.75. For RQP = 0.76, the was almost identical to that for RQP = 1.0. Consequently, it would be possible to split approximately 25% of the entire airflow into the conveying column as the secondary air. To reduce the airflow to the fluidized bottom section, the modified bed configuration shown in Fig. 3(b) was further tested. For the tests in this bed configuration, the adopted UP was 8.0 m/s,
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and the gas velocity in the intermediate expanded section was 4.5 m/s. Only 50% of the entire airflow was supplied from the primary gas inlet to maintain UF at 2.0 m/s. The result is compared in Fig. 10 with that obtained from testing the original bed configuration shown in Fig. 3(a). No obvious difference in the classification performance was observed from this change in bed configuration. The classification degree was maintained at approximately 0.75, and the conveyed particle amount remained at approximately 80.0 wt% of the coal fed. Therefore, to maintain the classification performance, it is feasible to adopt a column that integrates a small-sized bottom, a diameter-specified conveying top and an expanded section in-between. This new configuration would cause rather turbulent bottom fluidization to enhance entrainment of fine particles from the bed, and would reduce the gas supplied into the bottom, thus lowering the operating cost of the entire column. 4. Conclusions To develop a new coal classifying moisture control (CCMC) technology, this study investigated the classification behavior of coal particles with a wide size range in a column that integrated a fluidized or moving-bed bottom section and a pneumatic conveying top section, to maximize the separation of fine coal smaller than 3.0 mm from the other larger particles, leading to the following conclusions. (1) As theoretically anticipated, the classification behavior is subject primarily to the gas velocity in the pneumatic conveying section. A velocity of 8.0 m/s is required to maximize the degree of separation of the coal smaller than 3.0 mm from the large particles by entrainment in the tested tandem fluidized bed. This critical gas velocity was approximately 2.0 m/s lower than the entrainment velocity of a single particle of 3.0 mm and can be roughly predicted using the equation of Richardson and Zaki based on the voidage of the flow in the conveying section. In the tested 3.0-m-high column, the realized classification degree , a parameter newly defined in this article, was approximately 0.85, and only 4.0 wt% coarse coal particles with sizes larger than 3.0 mm was present in the entrained fine coal. Raising the coal feeding rate would reduce the classification degree, but this effect on the classification was obviously lower than that realized by changing the gas velocity, justifying the dominance of gas velocity over pneumatic classification. (2) Classification performance was slightly worse for increased particle bed height in the fluidized bottom. Thus, practical CCMC columns should be designed with the lowest possible particle bed height in the fluidized bottom (e.g., 300 mm). Splitting approximately 25% of the required overall gas flow to form the secondary gas stream was shown to be practically possible in order to increase the flexibility for controlling the performances of the fluidized bottom section and the conveying top section. Adopting a small-diameter bottom could increase the turbulence of the fluidized bottom to enhance particle classification as well as gas–particle interactions needed for preheating the coal and removing its moisture. This bed configuration was demonstrated to be capable of ensuring the expected performance for coal particle classification.
Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (21076217), the National Basic Research Program of China (2011CB201304), the National Key Technology Development Programs (2009BAC64B05 and 2010BAC66B01), the National High-Tech R&D Program of China (2009AA02Z209) and the cooperation of Henan Shuncheng Coal Chemical Group Company. References Baeyens, J., van Gauwbergen, D., & Vinckier, I. (1995). Pneumatic drying: The use of large-scale experimental data in a design procedure. Powder Technology, 83(2), 139–148. Chen, Z., Agarwal, P. K., & Agnew, J. B. (2001). Steam drying of coal, part 2. Modeling the operation of a fluidized bed drying unit. Fuel, 80(2), 209–223. Eswaraiah, C., Kavitha, T., Vidyasagar, S., & Narayanan, S. S. (2008). Classification of metals and plastics from printed circuit boards (PCB) using air classifier. Chemical Engineering and Processing: Process Intensification, 47(4), 565–576. Fayed, M. E., & Otten, L. (1997). Handbook of powder science and technology (2nd ed.). New York: Chapman & Hall. Felipe, C. A. S., & Rocha, S. C. S. (2007). Prediction of minimum fluidization velocity of gas–solid fluidized beds by pressure fluctuation measurements—Analysis of the standard deviation methodology. Powder Technology, 174(3), 104–133. He, X., & Liu, C. (2009). Dynamics and screening characteristics of a vibrating screen with variable elliptical trace (China). Mining Science and Technology, 19(4), 508–513. Innocentini, M. D. M., Barizan, W. S., Alves, M. N. O., & Pisani, R., Jr. (2009). Pneumatic separation of hulls and meats from cracked soybeans. Food and Bioproducts Processing, 87(4), 237–246. Kwauk, M., & Li, H. Z. (Eds.). (2008). Hand book of fluidization. Beijing: Chemical Industry Press, 170–172 (in Chinese). Li, J. (2005). Development of coal moisture control and its application in coking plant. Coal Chemical Industry, (1), 34–36 (in Chinese). Li, S., Tan, F., & Lan, R. (2010). Application of split bustle pipe steam rotary dryer in coal moisture control. Drying Technology & Equipment, 8(1), 16–19. Liu, S., Li, Y., Lü, W., Li, Z., Wang, L., & Zhang, Y. (2009). A new process of winnowing & moisture control for coke-making coal. Fuel & Chemical Processes, 40(3), 20–22, 24 (in Chinese). Liu, Z., Wang, Y., Tian, Y., Xie, Y., Ji, W., & Xu, G. (2010). Basic study for coal moisture control integrating pneumatic classification technique. In The 13th international conference on fluidization May, Daejeon, Gyeongju, Korea, (pp. 621–628). Mito, Y., Komatsu, N., Hasegawa, I., & Mae, K. (2005). Slurry dewatering process for biomass October, ICCS&T, Okinawa, Japan. Mohanty, M. K. (2003). Fine coal screening performance enhancement using the Pansep screen. International Journal of Mineral Processing, 69(l–4), 205–220. Mohanty, M. K., Palit, A., & Dube, B. (2002). A comparative evaluation of new fine particle size separation technologies. Minerals Engineering, 15(10), 727–736. Morimoto, H., & Shakouchi, T. (2003). Classification of ultra fine powder by a new pneumatic type classifier. Powder Technology, 131(1), 71–79. Palappan, K. G., & Sai, P. S. T. (2008). Studies on segregation of binary mixture of solids in continuous fast fluidized bed. Part II. Effect of particle size. Chemical Engineering Journal, 139(2), 330–338. Potter, O. E., & Keogh, A. J. (1981). Drying high-moisture coals before liquefaction or gasification. Fuel Processing Technology, 4(2–3), 217–227. Rogers, R. S. C. (1982). A classification function for vibrating screens. Powder Technology, 31(1), 135–137. Shapiro, M., & Galperin, V. (2005). Air classification of solid particles: A review. Chemical Engineering and Processing: Process Intensification, 44(2), 279–285. Sun, G., Jiang, G., Liu, X., Sun, G., & Xu, G. (2008). Buildup of high particle flux circulating conveying flow. Journal of Chemical Industry and Engineering (China), 59(11), 2774–2980 (in Chinese). Xu, G., Nomura, K., Nakagawa, N., & Kato, K. (1999). Hydrodynamic dependence on riser diameter for different particles in circulating fluidized beds. Powder Technology, 113(1–2), 80–87. Yang, G. (2003). A new process for coal blending combined with size classification of coal. Powder Engineering, 23(3), 2397–2399, 2396 (in Chinese). Yang, G., Zheng, D., Zhou, J., Zhao, Y., & Chen, Q. (2002). Air classification of moist raw coal in a vibrated fluidized bed. Minerals Engineering, 15(8), 623–625. Zheng, W. (2002). Coal preheating and moisture control in muroran coking Plant, Japan. Fuel and Chemical Processes, 33(1), 1–3 (in Chinese).
Contents lists available at SciVerse ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Tandem fluidized bed elutriator—Pneumatic classification of coal particles in a fluidized conveyer Zhouen Liu a , Yimin Xie b , Yin Wang a , Jian Yu a , Shiqiu Gao a , Guangwen Xu a,∗ a b
State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China School of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China
a r t i c l e
i n f o
Article history: Received 24 March 2011 Received in revised form 16 December 2011 Accepted 14 March 2012 Keywords: Tandem fluidized bed elutriator Coking Coal classification Moisture control Pneumatic bed Fluidized bed
a b s t r a c t Coal moisture control (CMC) in coking process, which reduces coal moisture before loading the coal into the coke oven, allows substantial reduction in coking energy consumption and increase in coke productivity. The technology is seeking to integrate the coal classification, thus calling it the coal classifying moisture control (CCMC), to separate the fine and coarse coal fractions in the CMC process so that the downstream coal crushing can only treat the coarse fraction. CCMC adopts a reactor that integrates a fluidized bottom section and a pneumatic conveying top section. The present work investigates the pneumatic classification behavior in a laboratory CCMC reactor with such a configuration by removing the coal fraction below a given size (e.g., 3.0 mm) from a 0 to 20.0 mm coal feed. The results show that the coal classification were dominated by the gas velocity in the top conveying section, and the required gas velocity for ensuring the maximal degree of removing a fine coal fraction could be roughly predicted by the Richardson and Zaki equation. The effect of bottom fluidization on the performance of CCMC is also examined. © 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction To improve coke quality, to save energy, and to reduce emission (e.g., wastewater) in the coal coking process, it is desirable to limit the coal feedstock for coking ovens (i.e., the tamping coking oven) to sizes smaller than 3.0 mm and to moisture contents between 6.0 wt% and 9.0 wt% (Liu et al., 2009; Yang, 2003). However, actual raw coal feedstock used in coking usually has a wide size distribution of, e.g., 0–20.0 mm, and an average moisture content of 11.0–20.0 wt%, thus calling for both coal crushing and coal moisture control (CMC) before loading into the coking oven. If all the coal particles, including those already smaller than 3.0 mm, are crushed, higher energy consumption is inevitable. As to coal moisture content, it was found that the coal fraction (e.g., <3.0 mm) is dominant in forming the moisture content of the raw coal, and the moisture content of the coarse fraction (e.g., >3.0 mm) is usually below 10.0 wt% (Liu et al., 2010). Therefore, in coking process the coal crushing should primarily be used for the coarse fraction, whereas the moisture control should be mainly applied to the fine fraction. This justifies the combination of coal size classification and moisture content control so that coal is classified into fine (e.g.,
∗ Corresponding author. Tel.: +86 10 82629912; fax: +86 10 82629912. E-mail addresses: [email protected] (Z. Liu), [email protected] (G. Xu).
<3.0 mm) and coarse (e.g., >3.0 mm) fractions to allow the former for moisture removal and the latter for further crushing. A great deal of work has been conducted on developing CMC technology in Japan, China and European countries, resulting in three generations of technologies, which are oil CMC, steam CMC and fluidized bed CMC (Chen, Agarwal, & Agnew, 2001; Li, Tan, & Lan, 2010; Li, 2005; Mito, Komatsu, Hasegawa, & Mae, 2005; Potter & Keogh, 1981; Zheng, 2002). Of these, the former two generations were based on indirect heat exchange of the coal with oil and with steam in the contactors, e.g., the rotary oven. The fluidized bed CMC was first developed in Japan and recently further updated in China, by using the sensible heat of the coking oven flue gas in direct contact with wet coal in a fluidized bed. The first fluidized bed CMC in Japan was placed downstream of the coal crusher, whereas in more recent development in China the CMC was set before the coal crusher. In the latter case, the preheating, moisture control and pneumatic classification for coal are implemented simultaneously in a single process, which represents a fresh development of the CMC technology. The fluidized bed CMC adopts either a vibration fluidized bed (VFB) (Yang, Zheng, Zhou, Zhao, & Chen, 2002) or a fluidized bed with an internal grate chain to move non-fluidized large particles (Palappan & Sai, 2008). In these cases, pneumatic classification is possible only for the fine particles, e.g., smaller than 1.0 mm. While separate treatment of these fine particles may cause serious
1674-2001/$ – see front matter © 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.partic.2012.03.005
Z. Liu et al. / Particuology 10 (2012) 600–606
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Nomenclature AF C1 C2 dp F Ho RQP SF UF Umf UP Ut Ut,a Wi,Pr Wi,Fs ε f p
accumulative fraction (wt%) coefficient, C1 = 33.7 coefficient, C2 = 0.048 particle diameter (m) coal feeding rate (t/(m2 h)) height of overflow outlet (m) ratio of primary airflow to total airflow fraction of a sieving size range (wt%) superficial gas velocity in fluidized bottom (m/s) minimum fluidization velocity (m/s) superficial gas velocity in pneumatic top (m/s) transport velocity for a single particle (m/s) entrainment velocity in a gas–solid flow (m/s) mass (weight) of a fraction i in product (kg) mass (weight) of a fraction i in feedstock (kg) voidage viscosity (Pa s) fluid density (kg/m3 ) particle density (kg/m3 ) classification degree
Fig. 1. Distributions of particle size and moisture content of tested coal. (AF = accumulative fraction; SF = sieving fraction.)
2. Experimental 2.1. Material characterization
dusting, the coke oven allows the direct use of the 0–3.0 mm coal fines from classification. This (having 0–3.0 mm fines) consequently reduces the working load required for the coal crushing (saving more energy) and avoids the dust elutriation as well. Therefore, a coal classifying moisture control (CCMC) process was proposed to implement simultaneously the classification for 0–3.0 mm particles and coal moisture control (CMC). Based on the concept of tandem fluidized bed elutriator (TFBE), the proposed process adopts an integrated bed consisting of a dense fluidized or moving-bed bottom section and a dilute, pneumatic conveyer top section to entrain coal particles below a given size (e.g., <3.0 mm), whereas the large particles, with low moisture content, are withdrawn from the bottom of the bed. By interacting with hot coke oven flue gas supplied from the bottom, moisture removal for the fine and coarse fractions occurs in the top and bottom sections, respectively. The present article investigates the pneumatic classification of 0–20.0 mm raw coal feed in a test rig simulating the bed structure for the CCMC process. Numerous prior studies described pneumatic classification (Baeyens, van Gauwbergen, & Vinckier, 1995; Eswaraiah, Kavitha, Vidyasagar, & Narayanan, 2008; He & Liu, 2009; Innocentini, Barizan, Alves, & J, 2009; Mohanty, Palit, & Dube, 2002; Mohanty, 2003; Morimoto & Shakouchi, 2003; Rogers, 1982; Shapiro & Galperin, 2005; Yang et al., 2002). Innocentini et al. (2009) pneumatically separated hulls and meats from cracked soybeans, and Eswaraiah et al. (2008) studied the pneumatic separation of metal and plastic particles. Morimoto and Shakouchi (2003) tested ultra-fine powder elutriation in an air-blown classifier. However, most of the particles tested in these studies had narrow size distributions and were small (e.g., <1.0 mm). For feedstock with sizes of up to several tens of millimeters (e.g., 20.0 mm in this work), there have been but limited studies. Integrating a fluidized bed or moving bed bottom and a pneumatic conveying top represents a special structure for particle classification that has rarely been studied. This study intends to clarify the suitable conditions and bed configuration that ensure the maximal separation of coal particles smaller than 3.0 mm from coarser particles. The results will provide a basis for designing CCMC employing such integral configuration.
The coal tested was a raw feedstock taken before crushing from the Henan Shuncheng Coking Chemical Corporation. The size distribution of the coal was determined by using a vibratory sieve consisting of sieve sizes (di ) of 1.0, 1.5, 2.0, 3.0, 4.0, 6.0, 8.0, 10.0 and 12.0 mm, and the Sauter mean diameter (dmn ) for the fraction between the sieve sizes dm and dn was estimated by
n Wi dmn = n i=m+1 i=m+1
(Wi /di )
, n ≥ m + 1,
m = 0, 1, 2, . . . , N − 1
(1)
where Wi is the mass fraction of particles in sizes between di−1 and di ; m and n represent the series number of the selected sieves; N is the total number of sieves used. The moisture content of the coal was measured according to the GB/T211-1996 method (drying at 105–110 ◦ C for approximately 2 h). The size distribution and the corresponding moisture content of the tested coal samples are shown in Fig. 1, where SF is the sieving fraction, and AF is the accumulative fraction. The overall size range was 0–20.0 mm, and approximately 60.0 wt% was smaller than 3.0 mm (see the AF) and did not need crushing for use in coking. The Sauter mean diameters for the coal fractions smaller than and larger than 3.0 mm were approximately 1.0 and 6.0 mm, respectively (calculated by Eq. (1)). Fig. 1 shows that the coal moisture decreased with increasing coal particle size. The moisture content for the fraction smaller than 3.0 mm was as high as 10.5–13.0 wt%, but for the fraction larger than 6.0 mm, it was less than 8.0 wt%, illustrating again the necessity of CMC based on size classification so that moisture control is implemented primarily for the fine coal fraction (i.e., <3.0 mm). Using helium pycnometry (AccuPyc 1330), the coal particle density was found to be 1398 kg/m3 . The minimum fluidization velocity Umf and the entrainment velocity Ut for a given size fraction were calculated using the following Eqs. (2) and (3), respectively (Kwauk & Li, 2008):
Umf =
C
1/2
+ C2
dp3 f (p − f )g 2
1/2 − C1
, dp f
(2)
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Fig. 2. Minimum fluidization and entrainment velocities, Umf and Ut , for differently sized particles determined by both calculation and experimental measurements.
Ut =
⎧ 2 dp (p − f )g ⎪ ⎪ , Ret < 0.4 ⎪ ⎪ 18 ⎪ ⎪ ⎪ ⎨ 0.5 2dp (p − )gRe
f t , 0.4 < Ret < 500 ⎪ 15f ⎪ ⎪ ⎪ ⎪ ⎪ d ( − f )g ⎪ ⎩ 4/3 p p , Ret > 500
(3)
0.43f
In the equations, p and f denote the particle and fluid densities; g is the acceleration of gravity; is the fluid viscosity; dp is the average particle diameter for a sieve size range; and C1 = 33.7 and C2 = 0.048 are two constant coefficients. Meanwhile, both Umf and Ut were experimentally measured in a 60.0-mm column in I.D. and 3.0 m high to verify the calculated values of the two velocities, considering that the employed large particle sizes of the coal might not be accurately predicted by Eqs. (2) and (3). The Ut was measured according to the approach reported by Innocentini et al. (2009), and the Umf was determined using the standard velocity reduction method (Felipe & Rocha, 2007). From the comparison in Fig. 2, one can see good agreement between measurement and empirical calculation. Therefore, to classify particles smaller than 3.0 mm via elutriation, the required superficial gas velocity in the conveyer may be approximately 10.0 m/s, and the fluidization velocity for the dense bottom of the proposed CCMC technology should be over 1.5 m/s (the minimum fluidization velocity based on the Sauter diameter of the coarse coal larger than 3.0 mm) to fully fluidize the coal fraction larger than 3.0 mm. 2.2. Apparatus and procedure The experimental apparatus had two different configurations, which are schematically shown in Fig. 3(a) and (b). The configuration shown in Fig. 3(a) refers to the normal design featured in a fluidized dense bottom and a conveying column above it. The top conveying column had an I.D. of 60.0 mm and a height of 3.0 m, and the bottom fluidized bed section had an 80.0 mm I.D. and a height of 1.5 m. Between these two sections there was a conical transition connection. Three particle overflow points were set in the fluidized bed section, with heights from the bottom of the fluidized bed at 28.0, 56.0 and 84.0 cm. Coal was continuously fed into the bed via a screw feeder from a position just at the bottom of the conveying section. Fig. 3(b) shows a modified configuration with a narrow bottom section to enable turbulent fluidization at the bottom for large
particles. In this case, the bottom was a column with 50.0 mm I.D., and an expanded section with a diameter of 80.0 mm and a length of 400 mm was mounted between this fluidized bottom and the top conveyer. Coal was fed into the bed at the same position as shown in Fig. 3(a). For both configurations, the entrained fine coal was collected in a cyclone. Compressed air was used for pneumatic classification. The test was conducted by first setting the gas velocity in the test column and then continuously feeding a given amount of coal into the column via the screw feeder. Both the gas velocity in the column and the coal-feeding rate were varied with the tests. The coarse fraction included both the overflow coal particles and those remaining in the bed bottom. The mass and size distribution of the collected coal at the bed bottom and top were measured to characterize the size classification performance. Of the tests shown here, almost all, unless otherwise specified, were conducted with the primary gas flow and the coal overflow in the bottom section. To evaluate the classification of particles with sizes smaller or larger than di (e.g., 3.0 mm) from the raw coal feedstock, we considered not only the percentage of the entrained particles with respect to the total fed particles but also the fraction of the desired particle size in the product obtained at the top. This leads to the definition of the classification degree : =
2 Wi,Pr
Wi,Fs WPr
,
(4)
where Wi,Fs and Wi,Pr are the mass of the coal particles smaller or larger than di (3.0 mm) in the feedstock and the mass of the product obtained from the column top (
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Fig. 3. Schematic diagram of the experimental columns.
Fig. 4. Classification performance as a function of superficial gas velocity in the conveying top section. (SF = sieving fraction and AF = accumulative fraction.)
reached 77.0 and 96.2 wt% at a UP of 5.0 and 8.0 m/s, respectively. These demonstrate that a UP of 8.0 m/s is sufficient to convey nearly all the particles smaller than 3.0 mm into the top of the tested conveying bed. Under this condition, the fraction of the particles larger than 3.0 mm in the entrained product was 13.5 wt%. Corresponding to the data in Fig. 4(a), the estimated with respect to the entrainment product at the bed top for particles smaller than 1.0–3.0 mm are presented in Fig. 4(b). Within the tested range of the gas velocity Up (5.0–9.0 m/s) in the top conveying section, the value of for the particles smaller than 1.0 mm () decreased with increasing UP , whereas for the other three sizes (, , and ), a peak value of , denoted as max , appeared at a critical velocity UPC . The velocities of UPC corresponding to all max in Fig. 4(b) were approximately 6.0, 7.0 and 8.0 m/s for the particle size groups smaller than 1.5 (), 2.0 () and 3.0 mm (), respectively. The lower at the gas velocities lower than the critical UPC was resulted from the insufficient entrainment of small-sized particles. On the contrary, the lower at the Up higher than the critical UPC was caused by carrying too many coarse particles into the entrainment product. Velocity UPC leading to max , is, therefore, the optimal gas velocity to realize the expected classification for the particles referred to by max . For example, Up should be approximately 8.0 m/s to classify the particles smaller than 3.0 mm into the entrained fraction with the possibly maximal amount (i.e., UPC = 8.0 m/s for 3.0 mm). Fig. 5 shows the variation of the classification performance with coal feeding rate F at UP = 8.0 m/s, clarifying a rather indistinct effect as compared to varying the gas velocity Up shown in Fig. 4. The amount of particles entrained to the bed top decreased only slightly from 87.5% to 84.0% upon raising the coal-feeding rate from 1.8 to 5.3 t/(m2 h). The nearly overlapping curves for the AF in Fig. 5 further verified that the size distribution of the entrained product did not significantly change, although a higher raw coal-feeding rate would lead to more particles with intermediate sizes (i.e., 1.0–3.0 mm) in the entrained fraction at the bed top. The estimated for fractions smaller than 3.0 mm present in the entrained product varied but slightly from 0.80 to 0.86 corresponding to the above-mentioned variation in F, illustrating little change in the classification performance. Therefore, it was the gas velocity, not the coal-feeding rate, that dominated the classification performance and particle conveying.
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Fig. 5. Classification performance at different coal feeding rates.
3.2. Prediction of classification gas velocity Compared to the data in Fig. 2, the critical (optimal) UPC determined in Fig. 4(b) for realizing max was approximately 1.0–2.0 m/s lower than Ut , the theoretical velocity for carrying a single particle of specified size, reflecting the influence of the particle mixture on the velocity for conveying particles. In gas-solid mixture flow, the actual conveying gas velocity is higher than the superficial gas velocity, implying the presence of the other particles to suppress the entrainment (Xu, Nomura, Nakagawa, & Kato, 1999). According to Fayed and Otten (1997), the effect of the presence of particles on the entrainment velocity can be predicted by the Richardson and Zaki equation (Fayed & Otten, 1997; Innocentini et al., 2009): Ut,a = Ut εn ,
(5)
where n is usually 4.7; ε is the voidage of the gas-solid flow; and Ut,a and Ut represent the actual entrainment velocities in a gas-solid mixture flow and of a single particle, respectively. For the tests shown in Fig. 4, the voidage ε in the pneumatic conveying top section for the different gas velocities could be roughly estimated from the differential pressure drop P5–6 across the top conveying section (Sun, Jiang, Liu, Sun, & Xu, 2008): ε=1−
P5−6 p gH5−6
Fig. 6. (a) Axial profiles of pressure for the tests in Fig. 4 and (b) corresponding pressure drops and voidages in the top conveying section as a function of Up .
the tested gas velocities, the voidages plotted in Fig. 6(b) can then be used in Eq. (5) to predict the actually required entrainment velocity, Ut,a , from the single-particle entrainment velocity, Ut , shown in Fig. 2. The resulting Ut,a is compared in Fig. 7 with the experimental UPC from Fig. 4. The corresponding max are also plotted in Fig. 7 versus the particle size dp . For all the size groups considered for classification (i.e., <1.0 mm, <1.5 mm, <2.0 mm and <3.0 mm), the realized max were all higher than 0.8, and the max tended to be slightly higher for classifying the smaller particles. For example, to classify the fractions of <1.0 mm and <3.0 mm, their max were 0.79 and 0.88, respectively. This refers to the fact that it is relatively easy to obtain the better classification for particles in a relatively
(6)
where H5–6 is the difference in height between the taps P5 and P6 (see Fig. 3); and p is the particle density. Fig. 6(a) shows the axial profiles of the pressure corresponding to all the tested Up in Fig. 4, indicating that for the tested bed configuration the pressure drop across the entire column was caused mainly by the fluidized bottom. The pressure difference P5–6 and its corresponding explicit particle holdup (1 − ε) estimated from Eq. (6) are shown in Fig. 6(b). As expected, the particle holdup (1 − ε) in the conveying section and the pressure drop P5–6 across this bed section were both elevated by raising Up . Under the tested conditions, the particle holdup was small (0.009–0.014), implying dilute flow in the top conveying section of the experimental bed. As shown in Fig. 4(b), the critical or optimal gas velocities, UPC , for all the identified max were 5.0, 6.0, 7.0 and 8.0 m/s, which corresponded to the classifications of particles smaller than 1.0, 1.5, 2.0 and 3.0 mm, respectively. Because these velocities are just equal to
Fig. 7. Comparison of the UPC determined from Fig. 4 and the Ut,a calculated via Eq. (6) based on Fig. 5 to classify the particles smaller than 1.0 to smaller than 3.0 mm.
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Fig. 10. Classification performance comparison for the two bed configurations. Fig. 8. Classification performance as a function of the particle overflow height in the fluidized bottom section.
wide size range (0–3.0 mm is wider than 0–1.0 mm). The predicted actual entrainment velocity, Ut,a , using Eq. (5) agreed well with the experimentally determined UPC (Fig. 7). These results reveal that the actually required entrainment gas velocity, Ut,a = UPC , for classifying the particles smaller than a dp can be predicted via the Richardson and Zaki Eq. (5) based on the terminal velocity, Ut , for transporting a single particle with the size dp , and the voidage ε measured in the actual conveying column (approximately 0.01 here). 3.3. Optimization of bed configuration This section investigates how bed configuration affects classification performance. Fig. 8 shows the values for SF, AF and obtained at Up = 8.0 m/s, F = 5.3 t/(m2 h) and different heights Ho at the particle overflow outlet in the fluidized bottom section. Varying Ho from 0.28 to 0.84 m caused the amount of particles entrained to the bed top to increase from 84.0 to 90.6 wt%. The accumulative
Fig. 9. Classification performance with the adoption of a secondary airflow.
curve moves slightly down and to the right with increasing Ho , showing that more fine particles were entrained into the column top. With increasing Ho , the SF value decreases for the fine fraction <1.0 mm, but it was higher for the other large-sized fractions (>1.0 mm). Therefore, with a higher overflow outlet in the fluidized bottom section, the classification degree was lower, as shown in the suspended table of Fig. 8. The was 0.86, corresponding to Ho = 0.28 m, and decreased to 0.78 at Ho = 0.84 m. From the viewpoint of classification, the overflow outlet should be at the lowest possible position, although this configuration would slightly lower the entrainment capability of the gas and lead to somewhat lowered particle entrainment. This configuration is also necessary to lower the pressure drop across the fluidized section and reduce the energy consumption of the gas blower. Splitting the hot airflow supply provides a viable way to adjust the gas–solid interactions in the fluidized bottom. For the CCMC process, the split would be effective to control the amount of air interacting with the particles in the fluidized bottom. Fig. 9 shows the variation of the classification performance with the ratio RQP of the primary airflow (shown in Fig. 3) to the overall airflow at a fixed gas velocity of UP = 8.0 m/s in the top conveying section. As expected, the amount of entrained particles decreased from 84.0 to 77.0 wt% corresponding to the variation of RQP from 1.0 (all the air was primary) to 0.63. The particle size distribution did not vary greatly with RQP , indicating that in the tested rig and under the mentioned operating conditions, the fluidization state in the bottom did not significantly change the entrained particle sizes. Furthermore, the tested range of UF = 2.8–4.5 m/s can well fluidize all coal particles smaller than 2.0 mm (see Fig. 2), causing more than 80% of the entrained particles to be between 0 and 2.0 mm. This feature varied little with the changes in RQP under all the tested conditions in Fig. 9. However, when the amount of entrained particles decreased by reducing the primary airflow ratio from 1.0 to 0.63, the classification degree declined from 0.86 to 0.75. For RQP = 0.76, the was almost identical to that for RQP = 1.0. Consequently, it would be possible to split approximately 25% of the entire airflow into the conveying column as the secondary air. To reduce the airflow to the fluidized bottom section, the modified bed configuration shown in Fig. 3(b) was further tested. For the tests in this bed configuration, the adopted UP was 8.0 m/s,
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and the gas velocity in the intermediate expanded section was 4.5 m/s. Only 50% of the entire airflow was supplied from the primary gas inlet to maintain UF at 2.0 m/s. The result is compared in Fig. 10 with that obtained from testing the original bed configuration shown in Fig. 3(a). No obvious difference in the classification performance was observed from this change in bed configuration. The classification degree was maintained at approximately 0.75, and the conveyed particle amount remained at approximately 80.0 wt% of the coal fed. Therefore, to maintain the classification performance, it is feasible to adopt a column that integrates a small-sized bottom, a diameter-specified conveying top and an expanded section in-between. This new configuration would cause rather turbulent bottom fluidization to enhance entrainment of fine particles from the bed, and would reduce the gas supplied into the bottom, thus lowering the operating cost of the entire column. 4. Conclusions To develop a new coal classifying moisture control (CCMC) technology, this study investigated the classification behavior of coal particles with a wide size range in a column that integrated a fluidized or moving-bed bottom section and a pneumatic conveying top section, to maximize the separation of fine coal smaller than 3.0 mm from the other larger particles, leading to the following conclusions. (1) As theoretically anticipated, the classification behavior is subject primarily to the gas velocity in the pneumatic conveying section. A velocity of 8.0 m/s is required to maximize the degree of separation of the coal smaller than 3.0 mm from the large particles by entrainment in the tested tandem fluidized bed. This critical gas velocity was approximately 2.0 m/s lower than the entrainment velocity of a single particle of 3.0 mm and can be roughly predicted using the equation of Richardson and Zaki based on the voidage of the flow in the conveying section. In the tested 3.0-m-high column, the realized classification degree , a parameter newly defined in this article, was approximately 0.85, and only 4.0 wt% coarse coal particles with sizes larger than 3.0 mm was present in the entrained fine coal. Raising the coal feeding rate would reduce the classification degree, but this effect on the classification was obviously lower than that realized by changing the gas velocity, justifying the dominance of gas velocity over pneumatic classification. (2) Classification performance was slightly worse for increased particle bed height in the fluidized bottom. Thus, practical CCMC columns should be designed with the lowest possible particle bed height in the fluidized bottom (e.g., 300 mm). Splitting approximately 25% of the required overall gas flow to form the secondary gas stream was shown to be practically possible in order to increase the flexibility for controlling the performances of the fluidized bottom section and the conveying top section. Adopting a small-diameter bottom could increase the turbulence of the fluidized bottom to enhance particle classification as well as gas–particle interactions needed for preheating the coal and removing its moisture. This bed configuration was demonstrated to be capable of ensuring the expected performance for coal particle classification.
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