Study on lignite dewatering by vibration mechanical thermal expression process

Fuel Processing Technology 130 (2015) 101–106

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Study on lignite dewatering by vibration mechanical thermal expression process Yixin Zhang, Jianjun Wu ⁎, Jiang Ma, Binbin Wang, Xiaoling Shang, Chongdian Si Chinese National Engineering Research Center of Coal Preparation and Purification, School of Chemical Engineering, China University of Mining and Technology, No. 1 Daxue Road, Xuzhou 221116, China

a r t i c l e

i n f o

Article history: Received 6 June 2014 Received in revised form 24 September 2014 Accepted 25 September 2014 Available online xxxx Keywords: Vibration Mechanical thermal expression Lignite Dewatering

a b s t r a c t A new dewatering process — vibration mechanical thermal expression (VMTE) process developed from mechanical thermal expression (MTE) process was studied in this paper. The enhancement of moisture content reduction of Zhaotong lignite by vibration in VMTE process under different temperatures and pressure was determined. Compared to MTE process, the dewatering efficiency was enhanced by 10% by VMTE process with a vibration force of 5 kN in tested range. The moisture content decreased from 1 to 0.19 (g/g) (db) under the most severe processing conditions. The investigation was carried out to identify how variations in vibration force affect the dewatering during the VMTE process. The vibration was proved to enhance the volume reduction of lignite samples which is closely related to the decrease of moisture content by accelerating particle compaction and the change of coal structure. The increase of vibration force in a special range resulted in more severe reduction of moisture content than that out of the range. The reduction of moisture content caused by the vibration force increase in the range from 1 kN to 4 kN was 3.6 times that in the range from 0 to 1 kN. A limitation of dewatering caused by the increase of vibration force was expected to exist. © 2014 Elsevier B.V. All rights reserved.

1. Introduction In China there were large deposits of lignite that were or could be used for power generation. Therefore, the efficient utilization of lignite was significant for coal-based energy systems in China. However, lignite were generally charactered with high moisture content which was sometimes greater than 60% (wet basis) on a mass fraction basis which exerted strong influences on their utilization processes, such as combustion, gasification and liquefaction [1,2]. But lignite did have certain advantages over black coal, such as low mining cost, high reactivity, high amount of volatiles, and low pollution-forming impurities including sulfur, nitrogen, and heavy metals [1,3,4]. Effective dewatering technologies were needed for the preparation of lignite prior to utilization. Mechanical thermal expression (MTE) was a process developed for the energy efficient dewatering from high moisture content lignite. A significant amount of researches on many different aspects of the MTE process had been carried out by researchers in CRC for Clean Power from Lignite and Monash University in Australia and Dortmund University in Germany [5–13]. During the MTE process, lignite was typically heated to between 100 and 250 °C and the water was removed without evaporation by the application of mechanical pressure up to ⁎ Corresponding author. Tel.: +86 13951350506. E-mail address: [email protected] (J. Wu).

http://dx.doi.org/10.1016/j.fuproc.2014.09.032 0378-3820/© 2014 Elsevier B.V. All rights reserved.

12 MPa. Lignite was compacted and the pore volume where water existed in was reduced due to the combined application of thermal and mechanical energy. Importantly, it had been illustrated that a significant reduction in water content can be obtained by MTE process for Australian, Greek, German and Indonesian lignite and biomaterials [6,14,15]. But the costs of time were always more than 100 s in the experimental tests which would be a barrier between lab study and industry utilization. The mechanism study of MTE process also pointed out that the time needed for “constant-rate process” (also called “primary consolidation”), which dewatering mainly occurred in, is proportional to the square of the initial height of the coal filling [16,17]. If the MTE process was enlarged to industry scale the cost of time would be out of acceptable. So, it would be significant to find out a way to improve the dewatering efficiency of MTE process. Researches in particle compaction showed that vibration could accelerate the compaction process [18,19]. The vibration forced particle relative motions by reducing the friction between particles which resulted in the improvement of compaction compared with static compaction [19]. The density achieved in vibration compaction was higher than that in static compaction under the same conditions. The vibration was expected to accelerate the dewatering in MTE process because the MTE process was also a compaction process. In this paper, lignite dewatering by vibration mechanical thermal expression (VMTE) process improved from MTE was tested.

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Table 1 Characteristics of raw coal sample. Analysis

Units

Value

Total moisture

% arb g/g db % db % daf % daf % daf % daf % daf % daf % daf g/cm3

50.06 1.00 20.08 60.27 39.73 66.58 3.98 1.83 1.92 25.69 1.23

Ash Volatile matter Fixed carbon C H N S Odiff True density

controlled electrical heating mantle surrounded the sample chamber. The water removed from the samples through a filter membrane located at the bottom of the sample chamber. VMTE tests were carried out at a variety of vibration forces, temperatures and pressures, ranging between 0 and 5 kN, 50 and 250 °C and 3.4 and 12.7 MPa, respectively. Experiments were begun by placing 30 g of raw coal into the sample chamber. The piston was inserted into the sample chamber and a pressure slightly higher than 2.0 MPa applied to expel any residual air trapped in the sample chamber and prevent water evaporation during heating. The pressure increased to the desired value at a rate of 10 MPa/min after the temperature reached the desired value. After the pressure reached the desired value, the vibration was performed for 10 s. After completion of the experiment, the MTE products were collected for subsequent tests (Fig. 1).

2. Experimental

2.3. Moisture content determination

2.1. Coal sample description

The moisture content was determined based on a method according to the GB/T 211-2007 standard which was similar to the Australian Standard AS 2434.1 [20]. Samples were dried under flowing nitrogen at 105–110 °C until no further mass decrease of samples. Oven dried samples were subsequently used for further tests.

The raw coal used in this study was obtained from Zhaotong, Yunnan, China. The characteristic of this raw coal sample was given in Table 1. The raw coal used in the test had a particle size of less than 1 mm.

2.4. True density determination 2.2. VMTE experiments The equipment used in VMTE experiments was developed from MTE rig reported by Hulston and Chaffee [6]. The experimental equipment was constructed with a vibration platform and a temperature controlled pressure dewatering rig. The vibration force was provided by two vibrating motors located on the vibration platform. These two vibration motors were run at the same speed but in opposite directions to eliminate the vibration force in horizontal direction. There was a stainless steel sample chamber (40 mm in diameter, 80 mm in height) fitted with filter membrane and a water collection chamber. Samples were compressed by a compression device, which applied a known force to sample chamber. The applied pressure was measured by a piezometer in the compression device. The samples were heated by a temperature

The true densities of the samples were determined by helium pycnometry using an Ultra PYC 1200e pycnometer (Quantachrome, USA) on dried samples. Prior to measurements, samples were purged under vacuum environment to ensure complete removal of air. 2.5. Total volume and pore volume determination The pore volume Vpore of V-MTE products was determined by the difference between the total volume and the volume occupied by the coal [6], such that Vpore ¼

π 2 m D H− coal ρHe 4

ð1Þ

Vibration Platform

Piston

Heating mantle Sample chamber Filter membrane Drainage valves

Compression device

Fig. 1. Schematic diagram of VMTE rig.

Y. Zhang et al. / Fuel Processing Technology 130 (2015) 101–106

2.6. Pore size distribution determination Pore size distribution was determined by Mercury Intrusion Porosimetry (MIP), using an AUTOPORE IV mercury porosimeter (Micromeritics). The instrument is capable of applying pressures (P) between 3.4 kPa and 207 MPa and testing diameters ranging between 0.006 and 358 μm. The boundary of inter- and intra-particle pore has been set at 60 μm [6]. The macropore and mesopore sizes ranged from 0.05 to 60 μm [6] and 0.006 to 0.05 μm, respectively. The lower bound to the mesopore size range was higher than the conventional IUPAC [21] boundary of 0.002 μm, as the instrument cannot intrude mercury into pores smaller than 0.006 μm in diameter.

1.0

5HVLGXDOPRLVWXUHFRQWHQW JJGE

where D and H represent the diameter and height of the wet pellet (determined by vernier caliper measurements, ±0.02 mm), and mcoal and ρHe were mass and helium density of the dry coal respectively.

103

MTE VMTE(FV=3kN) 0.8

VMTE(FV=5kN)

0.6

0.4

0

2

4

6

8

10

12

14

16

Pressure(MPa) Fig. 3. Effect of pressure on the residual moisture content of Zhaotong lignite during VMTE and MTE processes (150 °C).

3. Results and discussion

5HVLGXDOPRLVWXUHFRQWHQW JJGE

The VMTE and MTE processes significantly reduced the residual moisture content of Zhaotong lignite. Shown in Figs. 2 and 3 were the effects of temperature and pressure on the residual moisture content of lignite during VMTE and MTE process. Under the most severe processing conditions, the moisture content decreased from 1 to 0.19 (g/g) (db), corresponding to an 80% decrease of the amount of moisture originally present in the raw coal. The moisture content decreased almost linearly with the increasing temperature of up to about 200 °C in both VMTE and MTE processes. Bergins and Hulston [6,10] reported similar results in their studies. The residual moisture contents of coal samples dewatered by VMTE process were lower than those dewatered by MTE process in all the tested temperature ranges. Between 50 and 200 °C, the residual moisture content was reduced by 0.052, 0.049 and 0.047 (g/g) (db) for every 10 °C increase in temperature by VMTE process with a vibration force of 5 kN and 3 kN and MTE process, respectively. The dewatering efficiency of MTE process increased more than 10% by VMTE process with a vibration force of 5 kN. As a function of pressure, the residual moisture content reductions showed the same trend in both VMTE process and MTE process. The residual moisture content decreased significantly up to an applied pressure of about 5 MPa, beyond which further increases in pressure to about 12 MPa result in much slower reductions in moisture content. Beyond 12 MPa, a future increase in pressure only had slight impact. This was in agreement with the findings of Bergins and Hulston [6,10]. But compared with the MTE process, the VMTE process showed better performance in all the tested pressure ranges in coal dewatering. The

1.0

MTE VMTE(FV=3kN)

0.8

VMTE(FV=5kN)

0.6

0.4

0.2

residual moisture contents of lignite samples dewatered by VMTE process with a vibration force of 3 kN and 5 kN were 0.057 (g/g) (db) and 0.090 (g/g) (db) lower than that of lignite samples dewatered by MTE process (0.508 (g/g) (db)) at 5 MPa/150 °C, respectively. The further test on the effect of vibration on VMTE process was listed in Fig. 4. The moisture content decreased almost linearly when vibration force increased from 1 kN to 4 kN, and was reduced by 0.023, 0.021 and 0.024 (g/g) (db) for every 1 kN increase in vibration pressure under conditions of 5 MPa/150 °C, 5 MPa/175 °C and 5 MPa/200 °C, respectively. Beyond 1 kN to 4 kN, the increases in vibration force only resulted in more slight reductions in moisture content. The reduction of moisture content caused by the vibration force increase in the range from 1 kN to 4 kN was 3.6 times than that in the range from 0 to 1 kN. In Fig. 5, the residual moisture of Zhaotong lignite obtained by VMTE and MTE processes with the same operating time was plotted. With the same operating time, the residual moisture obtained by VMTE process was lower compared to MTE process in the tested range. In other words, the time required for dewatering to the same residual moisture content by VMTE process was shorter than MTE process. For example, the moisture was reduced to 0.415 (g/g) (db) by VMTE process in 10 s, only slightly higher than 0.408 (g/g) (db) which was obtained by MTE process in 30 s. The VMTE process showed great potential to improve the productivity of MTE process. 3.2. Effect of MTE and VMTE process on lignite volume Shown in Fig. 6 were the effects of MTE and VMTE processes on the reduction of lignite volume, which was taken as the volume difference

5HVLGXDOPRLVWXUHFRQWHQW JJGE ć

3.1. Effect of VMTE process conditions on dewatering

150ć 175ć 200ć

0.45 0.40 0.35 0.30 0.25 0.20

50

100

150

200

250

Temperature (ć) Fig. 2. Effect of temperature on the residual moisture content of Zhaotong lignite during VMTE and MTE processes (8.4 MPa).

0

1

2

3

4

5

Vibratiion Force (kN) Fig. 4. Effect of vibration on the residual moisture content of Zhaotong lignite during VMTE process (8.4 MPa).

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0.45

12

Volume loss (cm3)

5HVLGXDOZDWHUFRQWHQW JJGE

14

MTE VMTE(FV=3kN)

0.40

10 8 6 4 2

0.35

0 0

20

40

60

80

100

0

2

4

Time(s)

6

8

10

12

14

Moisture loss (g)

Fig. 5. Comparison of the residual moisture of Zhaotong lignite obtained by VMTE and MTE processes with the same operating time (150 °C/8.4 MPa).

Fig. 7. Relationship between the volume loss and moisture loss of lignite samples during VMTE process.

between the volume of raw lignite and MTE and VMTE products [6]. The raw lignite volume was determined after compacted (0.2 MPa/30 s) under environment temperature. As expected from the residual moisture content reductions, increasing temperature, pressure and vibration intensity resulted in the same trend lignite volume reductions. Under the most severe conditions (200 °C/8.4 MPa/5 kN), the lignite volume reduced by 45%, from 27.2 to 15.0 cm3. Fig. 7 showed that the loss of lignite volume was linearly correlated to the loss of moisture weight with slope nearly being 1 in VMTE process. So, the loss of lignite volume nearly equaled to the volume of lost moisture during the dewatering process. In other words, the pores of samples were almost entirely filled with water [10], though they did contain a little gas [5,22]. Moisture in lignite can be expelled by particle consolidation and the collapse of coal structure [6]. VMTE process which can accelerate particle consolidation and the collapse of coal structure by vibration would be a potential way to enhance the efficieny of MTE process.

In vibration compaction process, the configuration of particles changed from loose to compact and relative movement between particles happened. The internal friction was the shear resistance of the relative movements between particles. The shear resistance consisted of sliding friction, occlusal friction (caused by shear dilatancy effect) and particles breaking and realignment. Therefore, shear dilatancy and breaking effect can be neglected in shear resistance. Then, the shear resistance intensity τf can be expressed as τ f ¼ c þ tanφ

ð2Þ

where, c was the cohesion of particles, and φ was internal friction angle. The relative position of particle i under cyclic loads can be expressed as  π X i ¼ ai sin ωt þ 2

ð3Þ

3.3. Mechanism of vibration effect on VMTE process

where, ai was the amplitude of particle i, and ω was the frequency. So the vibration acceleration of particle i was

The VMTE process can be seen as a special vibration compaction process, and lignite can be seen as granular materials in VMTE process.

X i ¼ ai ω sin ðωt þ πÞ

2

ð4Þ

Reduction in sample volume Volume of final sample

30

20

07(

907()9 N1

03D

07(

907()9 N1

0 3  D 0  3D  03 D



3D 03 D 



3D 0

0





0 3  D 0  3D  03 D



ć



ć

ć





ć  ć



ć

15



Volume(cm3)

25

907()9 N1

ć

Fig. 6. Comparison of volume reduction of Zhaotong lignite during VMTE and MTE processes.

Y. Zhang et al. / Fuel Processing Technology 130 (2015) 101–106

Then, the inertial force of particles i can be expressed as Ii ¼ mi X

4. Conclusion ð5Þ

where, mi was the mass of particle i. Inertial force was different because of the different masses and shapes of particles in a certain vibration system. The inertial force of particles Ii was proportional to vibration force Fv.

 π 2 Ii ∝Fv ¼ mrω sin ωt þ 2

ð6Þ

Where, Fv was the vibration force, and m was the mass of eccentric. So, the difference of inertia force between particles was proportional to the vibration force, too. The difference of inertial force would cause new stress at the boundary which could break the connection and change the structure of particles. When vibration force was small, the difference of inertia force cannot overcome the shear resistance of particles. The relative position of particles was kept as they were. When the vibration force was higher than the critical value, the difference of inertia force can get over the cohesion force and constraint around. Then, the great decrease of occlusal function among particles happened, sliding internal friction angle and force were reduced significantly. As a result, the internal friction coefficient tanφ decreased. The initial relative position of particles was changed easily. The relative movement of particles always tended to fill inter-particle pores or shift inter-particle pores to smaller pores. The structure of particles also could be changed in this process. The test of pore size distribution in Fig. 8 showed an obvious decrease of inter-particle pores caused by the increase of vibration force. There was also a decrease of macropores which was considered to be caused by the change of particle structure. But when the vibration force was higher than a critical value, with the increase of vibration force, the shear resistance gradually approached to a fixed value and the no further compaction occurred [18]. This agreed with the results shown in Fig. 4. So, vibration could make VMTE process more efficient in compacting lignite sample. With the increase of vibration force in tested range, the lignite sample became more compact and the total pore volume of VMTE products decreased (Fig. 6). Since the pores of lignite sample were almost entirely filled with water, the changes in moisture content of VMTE product followed the same trends as the total pore volume and the compactness of lignite samples with increasing vibration force.

Log differential instrusion (cm3/g)

0.12 MTE VMTE (FV=3kN)

0.10

VMTE (FV=5kN)

0.08 0.06 0.04 0.02 0.00

100

10

1

0.1

105

0.01

Pore diameter (µm) Fig. 8. Pore size distribution of MTE and VMTE products prepared under 150 °C/8.4 MPa.

In this paper, a comparison of dewatering from lignite and volume shrinkage of lignite samples by VMTE and MTE processes in different temperatures and pressures was performed. The results showed that the VMTE process can effectively enhance the performance on dewatering. In detail, the dewatering efficiency of VMTE process with a vibration force of 5 kN increased more than 10% compared to MTE process in tested range. The increase of vibration force also caused the increase of moisture reduction, and there was a range where the vibration force varied and significantly affected the moisture reduction. The reduction of moisture content caused by the vibration force increase from 1 kN to 4 kN was 3.6 times than that from 0 to 1 kN. This paper also showed that there were close relationships between moisture content and total volume of VMTE and MTE products. The vibration that accelerated particle consolidation and the collapse of coal structure enhanced the volume reduction of lignite samples which directly resulted in the decrease of moisture content. The study of the mechanism of VMTE process showed that there was a critical value of vibration. If the vibration force was bigger than the critical value, the increase of vibration force would result in more severe changes of the structure and total volume of lignite samples which related to the moisture reduction. But there was also a limitation on the effect caused by the increase of vibration force. Acknowledgment This work was jointly supported by the Research and Innovation Project for Graduate Student of Jiangsu Province (No. CXZZ13_0940), the National Key Basic Research Program of China (No. 2012CB214900), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130095110004) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] J. Yu, A. Tahmasebi, Y. Han, F. Yin, X. Li, A review on water in low rank coals: the existence, interaction with coal structure and effects on coal utilization, Fuel Processing Technology 106 (2013) 9–20. [2] M. Karthikeyan, Z. Wu, A.S. Mujumdar, Low-rank coal drying technologies—current status and new developments, Drying Technology 27 (2009) 403–415. [3] W.G. Willson, D. Walsh, W. Irwinc, Overview of low-rank coal (LRC) drying, Coal Preparation 18 (1997) 1–15. [4] C.Z. Li, Some recent advances in the understanding of the pyrolysis and gasification behaviour of Victorian brown coal, Fuel 86 (2007) 1664–1683. [5] C. Bergins, Kinetics and mechanism during mechanical/thermal dewatering of ligniteq, Fuel 82 (2003) 355–364. [6] J. Hulston, G. Favas, A.L. Chaffee, Physico-chemical properties of Loy Yang lignite dewatered by mechanical thermal expression, Fuel 84 (2005) 1940–1948. [7] K. Strauß, S. Berger, C. Bergins, Mechanical/thermal brown coal dewatering, 20th International Mineral Processing Congress (IMPC), 1997, pp. 75–82. [8] C. Vogt, T. Wild, C. Bergins, K. Strauß, J. Hulston, A.L. Chaffee, Mechanical/thermal dewatering of lignite. Part 4: physico-chemical properties and pore structure during an acid treatment within the MTE process, Fuel 93 (2012) 433–442. [9] S.A. Clayton, R.A. Wheeler, A.F.A. Hoadley, Pore destruction resulting from mechanical thermal expression, Drying Technology 25 (2007) 533–546. [10] C. Bergins, J. Hulston, K. Strauss, A.L. Chaffee, Mechanical/thermal dewatering of lignite. Part 3: physical properties and pore structure of MTE product coals, Fuel 86 (2007) 3–16. [11] C.J. Butler, A.M. Green, A.L. Chaffee, MTE water remediation using Loy Yang brown coal as a filter bed adsorbent, Fuel 87 (2008) 894–904. [12] Y. Fei, Y. Artanto, L. Giroux, M. Marshall, W.R. Jackson, J.A. MacPhee, J.P. Charland, A.L. Chaffee, D.J. Allardice, Comparison of some physico-chemical properties of Victorian lignite dewatered under non-evaporative conditions, Fuel 85 (2006) 1987–1991. [13] R.A. Wheeler, A.F.A. Hoadley, S.A. Clayton, Modelling the mechanical thermal expression behaviour of lignite, Fuel 88 (2009) 1741–1751. [14] Y. Artanto, A.L. Chaffee, Dewatering low rank coals by mechanical thermal expression (MTE) and its influence on organic carbon and inorganic removal, Coal Preparation 25 (2005) 251–267. [15] S.A. Clayton, O.N. Scholes, A.F.A. Hoadley, R.A. Wheeler, M.J. Mclntosh, D.Q. Huynh, Dewatering of biomaterials by mechanical thermal expression, Drying Technology 24 (2006) 819–834. [16] B. Christian, Mechanical/thermal dewatering of lignite. Part 2: a rheological model for consolidation and creep process, Fuel 83 (2004) 267–276.

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