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Upgrading Chinese Shengli lignite by microwave irradiation for slurribility improvement
Fuel 159 (2015) 909–916
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Upgrading Chinese Shengli lignite by microwave irradiation for slurribility improvement Fan Zhou, Jun Cheng ⇑, Aiying Wang, Jianzhong Liu, Junhu Zhou, Kefa Cen State Key Lab of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 5 April 2015 Received in revised form 19 June 2015 Accepted 20 July 2015 Available online 26 July 2015 Keywords: Shengli lignite Upgrading Microwave Coal water slurry Solid concentration
a b s t r a c t Shengli lignite with high moisture content from the Inner Mongolia region of China was upgraded by microwave irradiation to prepare high-quality coal water slurry (CWS) fuel for combustion and gasification. The physicochemical properties of the upgraded lignite on the as-received basis were investigated with Fourier transform-infrared spectroscopy, X-ray diffraction spectroscopy, N2 adsorption porosimetry, scanning electron microscopy, contact angle analysis and rotary viscometry. Oxygen functional groups with high hydrophilicity decreased in the upgraded lignite. The decreased molar ratio of aliphatic hydrogen to aromatic hydrogen implied an increase in coalification degree. The particle size and contact angle of the upgraded lignite increased. When the upgraded lignite with a decreased hydrophilicity index was used to prepare CWS, the maximum solid concentration increased from 45.6 wt.% to 51.7 wt.%. Microwave irradiation was more efficient for removing external free moisture than internal bonded moisture in Shengli lignite. The unit energy consumption for external moisture removal and CWS concentration promotion was lower than that for internal moisture removal. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Coal is one of the world’s primary sources of energy and chemical feedstock. Coals are generally classified as having either high or low rank depending on their properties, such as heating value, moisture content, and coalification time. With the increasing worldwide demand for energy and the consumption of high-rank coals, the exploitation and utilisation of low-rank coals, given its abundant deposits, is predicted to play a vital role in supplying the primary energy needs in the foreseeable future [1]. Lignite, which is a typical low-rank coal, accounts for approximately 45% of the world’s coal reserves [2,3]. Thus, researchers should exert great effort to find ways of utilising lignite resources in an environmentally friendly and efficient manner [4]. Coal water slurry (CWS) fuel is widely used in various boilers and gasifiers, because similar to heavy oil, it can be conveniently and cleanly stored, transported, atomised and burned [5–8]. The process of producing CWS fuel from lignite for combustion and gasification is a promising technology in clean lignite utilisation. However, the high moisture content in lignite (20–60 wt.% as-received basis) results in lower calorific value, increased long-distance transportation cost, higher carbon dioxide emissions and risks of spontaneous combustion, which in turn limit the direct large-scale application of lignite with high efficiency [9,10]. The ⇑ Corresponding author. Tel.: +86 571 87952889; fax: +86 571 87951616. E-mail address: [email protected] (J. Cheng). http://dx.doi.org/10.1016/j.fuel.2015.07.060 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
characteristics of CWS would vary depending on the properties of coal [11,12]. High-quality CWS fuels with a high concentration (50–70 wt.%) and low viscosity (<1200 mPas at a shear rate of 100 s 1) are difficult to produce because of the high inherent moisture content, high oxygen content, poor grindability, low coalification degree, high wettability and abundant pore structures of lignite [13]. Therefore, upgrading lignite is the primary step in nearly all downstream utilisation processes. Microwave irradiation, which occurs through the direct conversion of electromagnetic energy into thermal energy within the material, offers several advantages over conventional heating, such as non-contact heating, rapid heating, energy transfer (instead of heat transfer), volumetric heating and selective heating [14,15]. Thus, microwave energy has been applied as an alternative to conventional convective and conductive heating, and it is used in many industrial processes [16]. Microwave irradiation is an effective and promising technology for upgrading coal [17,18]. The dewatering rate and efficiency of moisture removal from coal by microwave irradiation was found to be one to two orders of magnitude of that of conventional heating [19,20]. Microwave irradiation is superior to and more cost-efficient than conventional heating for improving the properties of CWS [21]. Sakoo [22,23] found that microwave treatment resulted in pyrite decomposition and in the conversion of a-silica into b-silica, which in turn enhanced the rheological properties of the high-ash Indian CWS (solid concentration of 30–50 wt.%, viscosity lower than
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200 mPas at a shear rate of 20 s 1). Meikap [24] indicated that the selective demineralisation effect of microwave irradiation reduced the slurry viscosity and increased the solid concentration of the high-ash Indian coal. The investigated CWS exhibited a low solid concentration that ranged between 20 wt.% and 40 wt.% and a viscosity that is lower than 300 mPas at a shear rate of 20 s 1. Cheng [21,25] employed microwave irradiation to improve the properties of CWS prepared from air-dried Shenhua coal and air-dried Philippine lignite. Cheng [26] also investigated the preparation of CWS from Indonesian lignite that was continuously modified in a tunnel-type microwave oven. The investigated CWS had a solid concentration of 30–54 wt.% and a viscosity of 900–1100 mPas at a shear rate of 100 s 1. The moisture in coal significantly influences the trading, transport and utilisation processes, whereby the total moisture content is a key indicator of its economic value evaluation (quality judgment, quantitative measurement and price estimation). The total moisture, which is also known as the moisture content based on the as-received basis, represent the moisture content of coals that have been just mined out, those that have been received by the industrial departments, and those to be put into use. The total moisture existing in lignite matrix can be classified into two forms: external moisture (i.e. moisture on particle surfaces or in large pores) and internal moisture (i.e. moisture adsorbed in capillary pores and chemically combined water). Cheng [21,25] used microwave irradiation for internal moisture removal to improve the properties of CWS prepared from air-dried lignite. The energy utilisation efficiency for moisture removal was low and the energy consumption was high. In addition, the amplitude for the improvement of the solid concentration of CWS was small. Cheng [26] did not consider the energy consumption differences between external and internal moisture in lignite during microwave irradiation. Lignite dewatering is a continuous, overlapping and complex process that is characterized first by the removal of external moisture and then by removal of internal water. In general, lignite has greater external moisture than internal moisture. Few reports have examined whether microwave irradiation is more effective for removing external free moisture than internal bonded moisture in lignite. Furthermore, few studies have investigated the effects of microwave irradiation on the physicochemical properties of lignite as received basis for slurribility improvement. Nonetheless, the more realistic utilisation of lignite resources requires that the upgrading of lignite as received basis be investigated. China has abundant lignite resources, and the proven lignite reserves in Inner Mongolia account for 77.55% of the total lignite reserves in China [27]. Investigating the Inner Mongolia lignite allows for the better utilisation of China’s lignite resources. In this study, we employed microwave irradiation to upgrade Shengli lignite as received basis with high total moisture content to improve its slurribility. Shengli lignite was obtained from the Shengli coalfield in the Inner Mongolia region of China, which holds 22.2 billion tons of lignite reserves and features of easy exploitation. To elucidate the improvement mechanisms of CWS, the microstructures of the modified lignite were investigated by oxygen functional group analysis, Fourier transform-infrared spectroscopy (FTIR), X-ray diffraction spectroscopy (XRD), N2 adsorption porosimetry, scanning electron microscopy (SEM) and contact angle analysis. 2. Material and methods 2.1. Material Shengli lignite obtained from the Inner Mongolia region of China was upgraded by microwave irradiation. Proximate analysis was measured in accordance with ISO 11722, ISO 1171 and ISO
562. Elemental analysis was measured in accordance with ISO 625, ISO 333 and ISO 334. Calorific value analysis was conducted using bomb calorimetric method in accordance with ISO 1928. 2.2. Microwave upgrading process Microwave upgrading experiments were performed in a microwave oven with a maximum power output of 700 W, frequency of 2450 ± 50 MHz and multimode cavity dimensions of 206 mm 309 mm 334 mm. Experiments were conducted by loading 15 g lignite (as-received basis) into the quartz reactor (transparent to microwave) with highly pure nitrogen gas inlet and gas outlet placed in the centre of the cavity. Lignite was upgraded for four time levels of 1, 3, 5 and 10 min at a set output power of 700 W. Lignite was upgraded for four power levels of 126, 406, 567 and 567 W for a set time of 10 min. The upgraded lignite was pulverised to less than 150 lm. The CWS was prepared from the pulverised lignite, deionised water and NDF chemical additive [28] (0.8 wt.% based on the dried lignite weight) through a mixing process. 2.3. Methods 2.3.1. Oxygen functional groups of the upgraded lignite The contents of phenolic hydroxyl, carboxyl and total acidic oxygen functional groups in the lignite were measured by chemical titration method [28]. (a) Pulverised coal (0.2 g) reacted with 25 mL of 0.125 mol/L Ba(OH)2 solution and then with 30 mL of 0.25 mol/L HCl solution. The acid product was titrated with 0.1 mol/L NaOH solution, and the consumption was used to calculate the amount of total acidic oxygen functional groups. (b) Pulverised coal (0.2 g) reacted with 50 mL of 0.25 mol/L Ca(CH3COOH)2, and the product was titrated with 0.1 mol/L NaOH solution. The consumption was used to calculate the amount of carboxyl. (c) The carboxyl was subtracted from the total acid groups to obtain the phenolic hydroxyl content. 2.3.2. FTIR of upgraded lignite Lignite (1 mg) was pulverised with 100 mg of KBr and then pressed into a pellet. The pellets were analysed on an FTIR spectrophotometer (Nicolet 5700, Thermo, USA) with an infrared spectral range of 4000–400 cm 1. 2.3.3. XRD of the upgraded lignite The crystals of the mineral compositions in the lignite were investigated by using a X-ray diffraction spectrophotometer (ARL X’TRA, Thermo, USA). 2.3.4. Porosity structures of the upgraded lignite The specific surface area and pore structures of the pulverised lignite were determined on a nitrogen adsorption analyser (ASAP 2010, Micromeritics Instrument Corp., USA). 2.3.5. SEM of the upgraded lignite The surface morphology of the pulverised lignite was determined on a field-emission scanning electron microscope (SIRON-100, FEI Corp., Netherlands) at 10,000 magnifications. 2.3.6. Contact angle of the upgraded lignite Distilled water was deposited on the cylinder lignite sample with 20 mm diameter and 2 mm thickness. The contact angle of the lignite was measured on a contact angle analyser (JC2000C, Shanghai Zhongchen Ltd., China.)
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3. Results and discussion 3.1. Chemical compositions of the upgraded lignite Table 1 shows the proximate and ultimate analyses of the upgraded lignite. As microwave power and time increased, moisture content gradually decreased, whereas carbon content and calorific value increased. The moisture in Shengli lignite was efficiently removed by microwave irradiation, thus solving the problem of lignite having high moisture and low heating value as well as being economically unfeasible for long-distance transportation and storage. The moisture content of the upgraded lignite was reduced from 28.46 wt.% to 3.82 wt.% at a microwave power of 700 W for an irradiation time of 10 min. In addition, the carbon content and calorific value of the upgraded lignite was increased from 36.87 wt.% to 50.38 wt.% and from 16,343 J/g to 22,656 J/g, respectively. The upgraded lignite provided a greater amount of energy per unit mass. These chemical properties of upgraded lignite were beneficial to improve the solid concentration of CWS, which was discussed in detail in Section 3.8.
1.10
7.4
total acidic groups phenolic hydroxyl
7.2 7.0
carboxyl
1.05
6.8
1.00
6.6 6.4
0.95
6.2 6.0
0.90
5.8 5.6
0.85
5.4 5.2 5.0
(a) 0
1
2
3
4
5
6
7
8
9
10
Carboxyl content (mmol/g-coal)
Total acidic groups and phenolic hydroxyl contents (mmol/g-coal)
2.3.7. Determination of CWS properties The apparent viscosity of CWS was measured on a rotary viscometer (HAAKE VT550, Thermo, USA) in accordance with the Chinese National Standard GB/T18856.4-2008. The solid concentration of CWS was determined from the weight difference upon drying in an oven at 105 °C for 4 h.
0.80
3.2. Oxygen functional groups of the upgraded lignite The oxygen functional group contents of the upgraded lignite are shown in Fig. 1. As microwave power and irradiation time increased, the total acidic groups, phenolic hydroxyl and carboxyl contents of the upgraded lignite drastically decreased from 7.16, 6.16 and 1.00 mmol/g-coal to 6.11, 5.27 and 0.84 mmol/g-coal at a microwave power of 700 W for an irradiation time of 10 min. This result can be attributed to the breakdown and removal of the active oxygen functional groups as the microwave directly penetrated into the lignite particles, causing progressive temperature rise of the lignite matrix. The hydrophilic oxygen functional groups were weakened as microwave pre-treatment time increased [17,29,30]. Thus, after microwave irradiation, the properties of Shengli lignite were almost similar to that of high-rank coal (anthracite and bituminous coal). This finding was helpful for improving the slurribility of the upgraded lignite.
1.10
7.4
total acidic groups phenolic hydroxyl
7.2 7.0
carboxyl
1.05
6.8 1.00
6.6 6.4
0.95
6.2 6.0
0.90
5.8 5.6
0.85
5.4 5.2 5.0
Carboxyl content (mmol/g-coal)
Total acidic groups and phenolic hydroxyl contents (mmol/g-coal)
Microwave time (min)
(b) 0
100
200
300
400
500
600
700
0.80
Microwave power (W) Fig. 1. Oxygen functional groups of Shengli lignite upgraded through microwave irradiation: (a) Oxygen functional groups contents versus microwave time; (b) oxygen functional groups contents versus microwave power.
1500–1600 cm 1 to the peaks at a range of 2800–3000 cm 1, is a defined parameter related to aromaticity and coal rank [30]. As shown in Fig. 2(b), as microwave power and irradiation time increased, Har/Hal gradually increased from 0.55 to 0.62 at a microwave power of 700 W for an irradiation time of 10 min. Tahmasebi [17] indicated that methylene and methyl structures were converted into the aromatic rings of the lignite during microwave drying. During the microwave process, the breakdown of aliphatic bridge bonds and side chains with high reactivity resulted in the loss of aliphatic hydrogen in the macromolecular structures of the lignite. In addition, the generated hydrogen may be adducted to the unsaturated sites in some aromatic rings, thereby increasing the aromatic hydrogen. Thus, Har/Hal ratio was increased, suggesting improvements in aromaticity and the maturation of the lignite.
3.3. FTIR spectra of the upgraded lignite FTIR spectra of the upgraded lignite are shown in Fig. 2(a). The aromatic hydrogen/aliphatic hydrogen (Har/Hal) ratio, which was calculated by the area ratio of the peaks at a range of
Table 1 Chemical compositions of Shengli lignite upgraded through microwave irradiation (as received). Microwave power (W)
700 700 700 700 700 567 406 126
Microwave time (min)
0 1 3 5 10 10 10 10
Proximate analysis (wt.%) Mar
Aar
Var
FCar
28.46 21.83 12.89 7.60 3.82 7.67 10.55 20.66
7.60 7.90 8.88 9.64 9.98 9.33 8.66 7.82
27.07 30.20 32.83 34.65 35.82 35.30 34.07 30.87
36.87 40.07 45.40 48.11 50.38 47.70 46.71 40.65
Qnet,ar (J/g)
Ultimate analysis (wt.%) Car
Har
Nar
Sar
16,343 17,754 20,688 21,622 22,656 22,138 21,170 18,580
47.07 51.99 57.92 60.74 62.96 59.09 58.04 51.31
5.57 5.40 4.84 4.63 4.44 4.53 4.59 5.06
0.68 0.66 0.72 0.73 0.77 0.78 0.78 0.70
1.80 1.44 1.60 1.70 1.81 1.72 1.68 1.53
Note: Mad refers to moisture on an air dry basis; Mar, Aar, Var and FCar refer to moisture, ash, volatile matter and fixed carbon on an as received basis; Qnet,ar refers to net calorific value on an as received basis; ultimate analysis was conducted on an as received basis.
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Microwave power=700W Microwave time:
2 1 3
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Microwave power=700W Microwave time:
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3200
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4000
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80
Fig. 3. X-ray diffraction spectroscopy analysis of Shengli lignite upgraded through microwave irradiation.
Microwave power (W) 100
2
2
2θ (deg)
Wave number (cm-1)
0
2
1- Kaolinite, 2- Quartz, 3-Hematite
(a) 400
10min 21
600
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0.64
Har/Hal vs. microwave time Har/Hal vs. microwave power
0.63
microwave power and time increased. When microwave time was increased from 0 min to 10 min, the total specific surface area initially decreased and then increased from 3.84 m2/g to 6.46 m2/g, and the total specific pore volume initially decreased and then increased from 0.0111 cm3/g to 0.0135 cm3/g, respectively. The valley point appeared at a microwave time of 5 min. Conversely,
0.62
Har/Hal
0.61 0.60 0.59 0.58 0.57 0.56
Microwave power (W)
(b)
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700
7.0
0.54 0
1
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9
10 2
Specific surface area (m /g)
Microwave time (min) Fig. 2. Fourier transform-infrared spectroscopy (FTIR) analysis of Shengli lignite upgraded through microwave irradiation: (a) FTIR spectra of upgraded lignite; (b) effects of microwave power and time on molar ratio of aromatic hydrogen to aliphatic hydrogen (Har/Hal) in upgraded lignite.
3.4. XRD spectra of the upgraded lignite
3.5. Pore structures of upgraded lignite
6.0 5.5 5.0 4.5 4.0 3.5 3.0
(a) 0
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Microwave time (min) Microwave power (W) 0
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0.014
specific pore volume vs. microwave time specific pore volume vs. microwave power
3
Specific pore volume (cm /g)
The XRD spectra of the upgraded lignite are shown in Fig. 3. The mineral crystals that existed in Shengli lignite were mainly composed of kaolinite Al2(Si2O6)(OH)4, quartz SiO2 and Hematite Fe2O3, and the other mineral crystals with less than 0.5 wt.% can hardly be detected by XRD. When microwave time delayed was to 5 min, the full width at the half-maximum of kaolinite, quartz and hematite decreased from 0.28, 0.26 and 0.31 to 0.15, 0.22 and 0.22, respectively, indicating that crystallinity was increased. This conclusion is drawn in accordance with the suggestion of Sahoo [31] that the crystallinity of coal increased after microwave treatment. As the moisture content gradually decreased, the ash content of the upgraded lignite was increased from 7.60 wt.% to 9.64 wt.% at a microwave power of 700 W for an irradiation time of 5 min. These mineral crystals mainly existed in ash of the lignite. Thus, the relative contents of mineral crystals in the upgraded lignite were increased, leading to an increase in crystallinity after microwave irradiation.
specific surface area vs. microwave time specific surface area vs. microwave power
6.5
0.013
0.012
0.011
0.010
(b) 0
1
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5
6
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8
9
10
Microwave time (min) The specific surface area and pore volume of the upgraded lignite are shown in Fig. 4. The total specific surface area and total specific pore volume initially decreased and then increased as
Fig. 4. Pore structures of Shengli lignite upgraded through microwave irradiation: (a) Effects of microwave on specific surface area; (b) effects of microwave on specific pore volume.
F. Zhou et al. / Fuel 159 (2015) 909–916
913
Fig. 5. Scanning electron microscopy images (10,000) of Shengli lignite upgraded through microwave irradiation: (a) Raw lignite; (b) microwave power = 700 W, microwave time = 1 min; (c) 700 W, 3 min; (d) 700 W, 5 min; (e) 700 W, 10 min; (f) 567 W, 10 min; (g) 406 W, 10 min; (h) 126 W, 10 min.
moisture was rapidly heated into steam when microwave directly penetrated into the lignite particles, resulting in the quick rise in pressure difference inside and outside the pores. The escape of steam dramatically decreased the inner pressure and created an irreversible collapse of the pores. Lignite exhibited a gel-like behaviour, shrinking in response to water loss [32]. Upon moisture removal, the strong hydrogen bond bridges between micelles caused the open gel structure to collapse. Meanwhile, the exposure of the lignite core to microwave generated new fractures and cracks and raised the existing cleat apertures for the unstressed
core [33–35]. During lignite drying, the surface area of macropores considerably increased, whereas that of mesopores partially increased. Some of the oxygen functional groups of the modified lignite were removed by microwave irradiation. The generation of small-molecule gases as a result of the decomposition of some functional groups of the modified lignite may lead to the creation of new pore structures. Overall, the collapse and shrinkage of the pores primarily caused the decrease in specific surface area and pore volume at low microwave power and time. However, the higher microwave strength achieved by higher microwave power
F. Zhou et al. / Fuel 159 (2015) 909–916
and time may exert a more significant effect on the new formation of pore structures. Thus, the specific surface area and the specific pore volume increased again.
Microwave power (W) 0
The contact angles of the upgraded Shengli lignite are shown in Fig. 6. As microwave power and time increased, the contact angle gradually increased from 62.82° to 106.32° at a microwave power of 700 W for an irradiation time of 10 min. The contact angle is geometrically defined as the angle formed by a liquid at the three-phase boundary where a liquid, gas and solid intersect [36,37]. The contact angle of lignite largely depends on the oxygen functional group content and the complex pore structure. Water molecules interact with the surfaces of lignite by forming hydrogen bonds with the oxygen functional groups [38]. Oxygen functional groups have strong hydrophilicity, which can enhance the water-absorbing capacity of coal. The reduced oxygen functional group content of the upgraded lignite resulted in an increased contact angle. Thus, the properties of the upgraded lignite are similar to that of high-rank coal, which is always hydrophobic.
Contact angle (°)
100
400
500
600
700
95 90 85 80 75 70 65 60 0
1
2
3
4
5
6
7
8
9
10
Microwave time (min) Fig. 6. Contact angles of Shengli lignite upgraded through microwave irradiation.
hydrogen to aromatic hydrogen in the upgraded lignite decreased, implying that the coal rank was upgraded. The hydrophilicity of the upgraded lignite surfaces was reduced. In addition, the specific surface area of the upgraded lignite was less than that of raw lignite. The water-holding capacity of the pore structures in the lignite particle was also weakened. The holding-water was firmly bound on the lignite surfaces or trapped in numerous pores, consequently losing its fluidity. Thus, more free water as the flow media among lignite particles would be present in CWS prepared from the upgraded lignite. Therefore, the solid concentration of CWS was increased. 3.9. Energy consumption analysis The unit energy consumption for moisture removal and CWS concentration promotion by 1% are shown in Fig. 8. The unit energy consumption for moisture removal was calculated as follows: UECMR = Pt/[W0(M0 Mt)]. The unit energy consumption for CWS concentration promotion by 1% was calculated as follows: UECSC = Pt/[W0(Ct C0)]. Here, P was microwave power (KW), W0 was initial weight of lignite (kg), M0 was initial moisture content in lignite (wt.%), Mt was moisture content (wt.%) at microwave time of t (h), C0 was solid concentration of CWS prepared from raw lignite (%), Ct was solid concentration of CWS prepared from
3.8. CWS solid concentration of the upgraded lignite
Microwave power (W) 0
100
2.20
200
300
400
500
600
700
Hydrophiclicity index vs. microwave time Hydrophiclicity index vs. microwave power
2.18
Hydrophiclicity index
The maximum solid concentration of CWS (viscosity of approximately 1000 mPas at the shear rate of 100 s 1) of the upgraded Shengli lignite is presented in Fig. 7. The maximum solid concentration of CWS gradually increased from 45.6 wt.% to 51.7 wt.% at a microwave power of 700 W for an irradiation time of 10 min. The hydrophilicity index was calculated based on the equation [39]: Hydrophilicity index = [(–COOH) + 2(–OH)]/[(R–H) + (Ar–H)], where (–COOH), (–OH), (R–H) and (Ar–H) were the adsorption intensity values as expressed by the Kubelka–Munk function. The FTIR peaks appeared at 3400 cm 1 for hydroxyl groups (–OH), at 1620 cm 1 for carboxyl groups (–COOH), at 1430 cm 1 for aliphatic hydrogen (R–H), and at 3030 cm 1 for aromatic hydrogen groups (Ar–H). As microwave power and time increased, the hydrophilicity index of the upgraded lignite monotonously decreased, whereas the maximum solid concentration remarkably increased. This result is consistent with the increase in contact angle, indicating the hydrophobization of the upgraded lignite. Oxygen functional groups with high hydrophilicity and the molar ratio of aliphatic
300
contact angle vs. microwave time contact angle vs. microwave power
105
3.7. Contact angles of the upgraded lignite
200
110
3.6. SEM image of the upgraded lignite The SEM images of the upgraded lignite are shown in Fig. 5. The raw lignite particles rendered a multilayer structure. As microwave power and time increased, the particles exhibited trends of increasing particle size and hardening particle surfaces. During the microwave upgrading process, the pore structures of the lignite particles kept on changing with progressive moisture removal. On the one hand, the gradual expansion of the water vapour and the decomposition of some functional groups in the semi-closed or closed pores created new crosslinking surfaces, which in turn enlarged the size of the particles. On the other hand, the lignite particles shrunk as the pores collapsed after the removal of moisture and oxygen functional groups that existed in the newly created pore structures, thus causing the surface of the particles to harden. The microwave treatment of coal smoothens the surface, weakens the coal mineral matrix and alters the angularity and surface properties of the ground particles. Under the same lignite mass fraction, CWS prepared from the dense upgraded lignite reduced the volume fraction of the solid phase, thereby increasing the flowing space of the water in the suspension. Therefore, the solid content (per dry solids weight/volume) of CWS prepared from the upgraded lignite was increased.
100
52
2.16
51
2.14
50
2.12 49
2.10 2.08
48
2.06 47
2.04 2.02
46
CWS concentration vs. microwave time CWS concentration vs. microwave power
2.00
CWS concentration (wt.%)
914
45
1.98 0
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Microwave time (min) Fig. 7. Hydrophilicity index and CWS solid concentration of Shengli lignite upgraded with microwave irradiation.
0.35
Microwave power=700W 1.3
0.30
unit energy consumption CWS concentration promotion moisture removal
1.2
0.25
1.1
0.20
1.0
0.15
0.9
0.10
0.8 0.7
0.05
(a)
moisture content of lignite
35 30 25 20 15 10 5
0.00
0
1.4
0.35
Microwave time=10min unit energy consumption 1.3 CWS concentration promotion moisture removal 1.2
35
0.30
1.1
0.20
1.0
0.15
0.9
0.10
0.8
0.05
0
1
2
3
4
5
6
7
8
9
Moisture content of upgraded lignite (wt.%)
1.4
Unit energy consumption for moisture removal by 1 % (KW⋅h/kg-coal)
Unit energy consumption for CWS concentration promotion by 1% (KW⋅h/kg-coal)
F. Zhou et al. / Fuel 159 (2015) 909–916
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lignite. The unit energy consumption for external moisture removal and CWS concentration promotion was lower than that for internal moisture removal. Oxygen functional groups with high hydrophilicity in the upgraded lignite decreased. The decreased molar ratio of aliphatic hydrogen to aromatic hydrogen implied an increase in coalification degree. The particle size and contact angle of the upgraded lignite increased. In addition, the specific surface area of upgraded lignite was lower than that of raw lignite. When the upgraded lignite with a decreased hydrophilicity index was used to prepare CWS, the maximum solid concentration increased from 45.6 wt.% to 51.7 wt.%. In future works, microwave absorbers should be studied to strengthen the heat transfer and consequently improve the upgrading and slurribility of lignite.
10
(b) 0
moisture content of lignite 0.00 100
200
300
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30 25 20 15 10 5 0
Moisture content of upgraded lignite (wt.%)
0.7
0.25
Acknowledgements Unit energy consumption for moisture removal by 1 % (KW ⋅h/kg-coal)
Unit energy consumption for CWS concentration promotion by 1% (KW ⋅h/kg-coal)
Microwave time (min)
700
Microwave power (W) Fig. 8. Unit energy consumption for CWS concentration promotion and moisture removal of Shengli lignite upgraded with microwave irradiation: (a) Effects of microwave time; (b) effects of microwave power.
upgraded lignite at microwave time of t (h). The unit energy consumption increased as microwave power and time increased. Microwave heating is a type of selective heating in which the dielectric loss value of a material is the indicator of its heating ability in the presence of microwave. External water, such as the moisture on particle surfaces or that in large pores of lignite, which can easily absorb microwaves, was mostly removed at the initial dewatering rate of the lignite. Conversely, internal water, such as the adsorbed moisture in capillary pores of lignite and chemically combined water, which has difficulty in absorbing microwaves, was mostly removed subsequently at the following dewatering rate. Thus, with the decreased moisture content of the lignite and as microwave power and time increased, the mobility of moisture slowed down, and the microwave absorption capacity of moisture in lignite started to decrease at a critical specific value, resulting in the need for much more energy to remove the residual moisture of lignite. Microwave irradiation was more effective at removing more freely type water than more strongly bonded water in lignite. Thus, the low unit energy consumption for moisture removal and CWS concentration promotion by 1% at a low microwave power (less than 406 W) and time (lower than 5 min) was mainly caused by the removal of more external moisture than internal moisture in lignite. 4. Conclusions Microwave irradiation can effectively upgrade Shengli lignite from the Inner Mongolia region of China and thus improve its slurribility. Microwave irradiation was more efficient for removing external free moisture than internal bonded moisture in Shengli
This study was supported by the National Natural Science Foundation – China (51176163 and 51476141); the National High Technology R&D Program – China (2012AA050101); the Special Funds for Major State Basic Research Projects – China (2010CB227001); and the Program of Introducing Talents of Discipline to University – China (B08026).
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Upgrading Chinese Shengli lignite by microwave irradiation for slurribility improvement Fan Zhou, Jun Cheng ⇑, Aiying Wang, Jianzhong Liu, Junhu Zhou, Kefa Cen State Key Lab of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 5 April 2015 Received in revised form 19 June 2015 Accepted 20 July 2015 Available online 26 July 2015 Keywords: Shengli lignite Upgrading Microwave Coal water slurry Solid concentration
a b s t r a c t Shengli lignite with high moisture content from the Inner Mongolia region of China was upgraded by microwave irradiation to prepare high-quality coal water slurry (CWS) fuel for combustion and gasification. The physicochemical properties of the upgraded lignite on the as-received basis were investigated with Fourier transform-infrared spectroscopy, X-ray diffraction spectroscopy, N2 adsorption porosimetry, scanning electron microscopy, contact angle analysis and rotary viscometry. Oxygen functional groups with high hydrophilicity decreased in the upgraded lignite. The decreased molar ratio of aliphatic hydrogen to aromatic hydrogen implied an increase in coalification degree. The particle size and contact angle of the upgraded lignite increased. When the upgraded lignite with a decreased hydrophilicity index was used to prepare CWS, the maximum solid concentration increased from 45.6 wt.% to 51.7 wt.%. Microwave irradiation was more efficient for removing external free moisture than internal bonded moisture in Shengli lignite. The unit energy consumption for external moisture removal and CWS concentration promotion was lower than that for internal moisture removal. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Coal is one of the world’s primary sources of energy and chemical feedstock. Coals are generally classified as having either high or low rank depending on their properties, such as heating value, moisture content, and coalification time. With the increasing worldwide demand for energy and the consumption of high-rank coals, the exploitation and utilisation of low-rank coals, given its abundant deposits, is predicted to play a vital role in supplying the primary energy needs in the foreseeable future [1]. Lignite, which is a typical low-rank coal, accounts for approximately 45% of the world’s coal reserves [2,3]. Thus, researchers should exert great effort to find ways of utilising lignite resources in an environmentally friendly and efficient manner [4]. Coal water slurry (CWS) fuel is widely used in various boilers and gasifiers, because similar to heavy oil, it can be conveniently and cleanly stored, transported, atomised and burned [5–8]. The process of producing CWS fuel from lignite for combustion and gasification is a promising technology in clean lignite utilisation. However, the high moisture content in lignite (20–60 wt.% as-received basis) results in lower calorific value, increased long-distance transportation cost, higher carbon dioxide emissions and risks of spontaneous combustion, which in turn limit the direct large-scale application of lignite with high efficiency [9,10]. The ⇑ Corresponding author. Tel.: +86 571 87952889; fax: +86 571 87951616. E-mail address: [email protected] (J. Cheng). http://dx.doi.org/10.1016/j.fuel.2015.07.060 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
characteristics of CWS would vary depending on the properties of coal [11,12]. High-quality CWS fuels with a high concentration (50–70 wt.%) and low viscosity (<1200 mPas at a shear rate of 100 s 1) are difficult to produce because of the high inherent moisture content, high oxygen content, poor grindability, low coalification degree, high wettability and abundant pore structures of lignite [13]. Therefore, upgrading lignite is the primary step in nearly all downstream utilisation processes. Microwave irradiation, which occurs through the direct conversion of electromagnetic energy into thermal energy within the material, offers several advantages over conventional heating, such as non-contact heating, rapid heating, energy transfer (instead of heat transfer), volumetric heating and selective heating [14,15]. Thus, microwave energy has been applied as an alternative to conventional convective and conductive heating, and it is used in many industrial processes [16]. Microwave irradiation is an effective and promising technology for upgrading coal [17,18]. The dewatering rate and efficiency of moisture removal from coal by microwave irradiation was found to be one to two orders of magnitude of that of conventional heating [19,20]. Microwave irradiation is superior to and more cost-efficient than conventional heating for improving the properties of CWS [21]. Sakoo [22,23] found that microwave treatment resulted in pyrite decomposition and in the conversion of a-silica into b-silica, which in turn enhanced the rheological properties of the high-ash Indian CWS (solid concentration of 30–50 wt.%, viscosity lower than
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200 mPas at a shear rate of 20 s 1). Meikap [24] indicated that the selective demineralisation effect of microwave irradiation reduced the slurry viscosity and increased the solid concentration of the high-ash Indian coal. The investigated CWS exhibited a low solid concentration that ranged between 20 wt.% and 40 wt.% and a viscosity that is lower than 300 mPas at a shear rate of 20 s 1. Cheng [21,25] employed microwave irradiation to improve the properties of CWS prepared from air-dried Shenhua coal and air-dried Philippine lignite. Cheng [26] also investigated the preparation of CWS from Indonesian lignite that was continuously modified in a tunnel-type microwave oven. The investigated CWS had a solid concentration of 30–54 wt.% and a viscosity of 900–1100 mPas at a shear rate of 100 s 1. The moisture in coal significantly influences the trading, transport and utilisation processes, whereby the total moisture content is a key indicator of its economic value evaluation (quality judgment, quantitative measurement and price estimation). The total moisture, which is also known as the moisture content based on the as-received basis, represent the moisture content of coals that have been just mined out, those that have been received by the industrial departments, and those to be put into use. The total moisture existing in lignite matrix can be classified into two forms: external moisture (i.e. moisture on particle surfaces or in large pores) and internal moisture (i.e. moisture adsorbed in capillary pores and chemically combined water). Cheng [21,25] used microwave irradiation for internal moisture removal to improve the properties of CWS prepared from air-dried lignite. The energy utilisation efficiency for moisture removal was low and the energy consumption was high. In addition, the amplitude for the improvement of the solid concentration of CWS was small. Cheng [26] did not consider the energy consumption differences between external and internal moisture in lignite during microwave irradiation. Lignite dewatering is a continuous, overlapping and complex process that is characterized first by the removal of external moisture and then by removal of internal water. In general, lignite has greater external moisture than internal moisture. Few reports have examined whether microwave irradiation is more effective for removing external free moisture than internal bonded moisture in lignite. Furthermore, few studies have investigated the effects of microwave irradiation on the physicochemical properties of lignite as received basis for slurribility improvement. Nonetheless, the more realistic utilisation of lignite resources requires that the upgrading of lignite as received basis be investigated. China has abundant lignite resources, and the proven lignite reserves in Inner Mongolia account for 77.55% of the total lignite reserves in China [27]. Investigating the Inner Mongolia lignite allows for the better utilisation of China’s lignite resources. In this study, we employed microwave irradiation to upgrade Shengli lignite as received basis with high total moisture content to improve its slurribility. Shengli lignite was obtained from the Shengli coalfield in the Inner Mongolia region of China, which holds 22.2 billion tons of lignite reserves and features of easy exploitation. To elucidate the improvement mechanisms of CWS, the microstructures of the modified lignite were investigated by oxygen functional group analysis, Fourier transform-infrared spectroscopy (FTIR), X-ray diffraction spectroscopy (XRD), N2 adsorption porosimetry, scanning electron microscopy (SEM) and contact angle analysis. 2. Material and methods 2.1. Material Shengli lignite obtained from the Inner Mongolia region of China was upgraded by microwave irradiation. Proximate analysis was measured in accordance with ISO 11722, ISO 1171 and ISO
562. Elemental analysis was measured in accordance with ISO 625, ISO 333 and ISO 334. Calorific value analysis was conducted using bomb calorimetric method in accordance with ISO 1928. 2.2. Microwave upgrading process Microwave upgrading experiments were performed in a microwave oven with a maximum power output of 700 W, frequency of 2450 ± 50 MHz and multimode cavity dimensions of 206 mm 309 mm 334 mm. Experiments were conducted by loading 15 g lignite (as-received basis) into the quartz reactor (transparent to microwave) with highly pure nitrogen gas inlet and gas outlet placed in the centre of the cavity. Lignite was upgraded for four time levels of 1, 3, 5 and 10 min at a set output power of 700 W. Lignite was upgraded for four power levels of 126, 406, 567 and 567 W for a set time of 10 min. The upgraded lignite was pulverised to less than 150 lm. The CWS was prepared from the pulverised lignite, deionised water and NDF chemical additive [28] (0.8 wt.% based on the dried lignite weight) through a mixing process. 2.3. Methods 2.3.1. Oxygen functional groups of the upgraded lignite The contents of phenolic hydroxyl, carboxyl and total acidic oxygen functional groups in the lignite were measured by chemical titration method [28]. (a) Pulverised coal (0.2 g) reacted with 25 mL of 0.125 mol/L Ba(OH)2 solution and then with 30 mL of 0.25 mol/L HCl solution. The acid product was titrated with 0.1 mol/L NaOH solution, and the consumption was used to calculate the amount of total acidic oxygen functional groups. (b) Pulverised coal (0.2 g) reacted with 50 mL of 0.25 mol/L Ca(CH3COOH)2, and the product was titrated with 0.1 mol/L NaOH solution. The consumption was used to calculate the amount of carboxyl. (c) The carboxyl was subtracted from the total acid groups to obtain the phenolic hydroxyl content. 2.3.2. FTIR of upgraded lignite Lignite (1 mg) was pulverised with 100 mg of KBr and then pressed into a pellet. The pellets were analysed on an FTIR spectrophotometer (Nicolet 5700, Thermo, USA) with an infrared spectral range of 4000–400 cm 1. 2.3.3. XRD of the upgraded lignite The crystals of the mineral compositions in the lignite were investigated by using a X-ray diffraction spectrophotometer (ARL X’TRA, Thermo, USA). 2.3.4. Porosity structures of the upgraded lignite The specific surface area and pore structures of the pulverised lignite were determined on a nitrogen adsorption analyser (ASAP 2010, Micromeritics Instrument Corp., USA). 2.3.5. SEM of the upgraded lignite The surface morphology of the pulverised lignite was determined on a field-emission scanning electron microscope (SIRON-100, FEI Corp., Netherlands) at 10,000 magnifications. 2.3.6. Contact angle of the upgraded lignite Distilled water was deposited on the cylinder lignite sample with 20 mm diameter and 2 mm thickness. The contact angle of the lignite was measured on a contact angle analyser (JC2000C, Shanghai Zhongchen Ltd., China.)
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3. Results and discussion 3.1. Chemical compositions of the upgraded lignite Table 1 shows the proximate and ultimate analyses of the upgraded lignite. As microwave power and time increased, moisture content gradually decreased, whereas carbon content and calorific value increased. The moisture in Shengli lignite was efficiently removed by microwave irradiation, thus solving the problem of lignite having high moisture and low heating value as well as being economically unfeasible for long-distance transportation and storage. The moisture content of the upgraded lignite was reduced from 28.46 wt.% to 3.82 wt.% at a microwave power of 700 W for an irradiation time of 10 min. In addition, the carbon content and calorific value of the upgraded lignite was increased from 36.87 wt.% to 50.38 wt.% and from 16,343 J/g to 22,656 J/g, respectively. The upgraded lignite provided a greater amount of energy per unit mass. These chemical properties of upgraded lignite were beneficial to improve the solid concentration of CWS, which was discussed in detail in Section 3.8.
1.10
7.4
total acidic groups phenolic hydroxyl
7.2 7.0
carboxyl
1.05
6.8
1.00
6.6 6.4
0.95
6.2 6.0
0.90
5.8 5.6
0.85
5.4 5.2 5.0
(a) 0
1
2
3
4
5
6
7
8
9
10
Carboxyl content (mmol/g-coal)
Total acidic groups and phenolic hydroxyl contents (mmol/g-coal)
2.3.7. Determination of CWS properties The apparent viscosity of CWS was measured on a rotary viscometer (HAAKE VT550, Thermo, USA) in accordance with the Chinese National Standard GB/T18856.4-2008. The solid concentration of CWS was determined from the weight difference upon drying in an oven at 105 °C for 4 h.
0.80
3.2. Oxygen functional groups of the upgraded lignite The oxygen functional group contents of the upgraded lignite are shown in Fig. 1. As microwave power and irradiation time increased, the total acidic groups, phenolic hydroxyl and carboxyl contents of the upgraded lignite drastically decreased from 7.16, 6.16 and 1.00 mmol/g-coal to 6.11, 5.27 and 0.84 mmol/g-coal at a microwave power of 700 W for an irradiation time of 10 min. This result can be attributed to the breakdown and removal of the active oxygen functional groups as the microwave directly penetrated into the lignite particles, causing progressive temperature rise of the lignite matrix. The hydrophilic oxygen functional groups were weakened as microwave pre-treatment time increased [17,29,30]. Thus, after microwave irradiation, the properties of Shengli lignite were almost similar to that of high-rank coal (anthracite and bituminous coal). This finding was helpful for improving the slurribility of the upgraded lignite.
1.10
7.4
total acidic groups phenolic hydroxyl
7.2 7.0
carboxyl
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6.8 1.00
6.6 6.4
0.95
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0.90
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0.85
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Carboxyl content (mmol/g-coal)
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Microwave time (min)
(b) 0
100
200
300
400
500
600
700
0.80
Microwave power (W) Fig. 1. Oxygen functional groups of Shengli lignite upgraded through microwave irradiation: (a) Oxygen functional groups contents versus microwave time; (b) oxygen functional groups contents versus microwave power.
1500–1600 cm 1 to the peaks at a range of 2800–3000 cm 1, is a defined parameter related to aromaticity and coal rank [30]. As shown in Fig. 2(b), as microwave power and irradiation time increased, Har/Hal gradually increased from 0.55 to 0.62 at a microwave power of 700 W for an irradiation time of 10 min. Tahmasebi [17] indicated that methylene and methyl structures were converted into the aromatic rings of the lignite during microwave drying. During the microwave process, the breakdown of aliphatic bridge bonds and side chains with high reactivity resulted in the loss of aliphatic hydrogen in the macromolecular structures of the lignite. In addition, the generated hydrogen may be adducted to the unsaturated sites in some aromatic rings, thereby increasing the aromatic hydrogen. Thus, Har/Hal ratio was increased, suggesting improvements in aromaticity and the maturation of the lignite.
3.3. FTIR spectra of the upgraded lignite FTIR spectra of the upgraded lignite are shown in Fig. 2(a). The aromatic hydrogen/aliphatic hydrogen (Har/Hal) ratio, which was calculated by the area ratio of the peaks at a range of
Table 1 Chemical compositions of Shengli lignite upgraded through microwave irradiation (as received). Microwave power (W)
700 700 700 700 700 567 406 126
Microwave time (min)
0 1 3 5 10 10 10 10
Proximate analysis (wt.%) Mar
Aar
Var
FCar
28.46 21.83 12.89 7.60 3.82 7.67 10.55 20.66
7.60 7.90 8.88 9.64 9.98 9.33 8.66 7.82
27.07 30.20 32.83 34.65 35.82 35.30 34.07 30.87
36.87 40.07 45.40 48.11 50.38 47.70 46.71 40.65
Qnet,ar (J/g)
Ultimate analysis (wt.%) Car
Har
Nar
Sar
16,343 17,754 20,688 21,622 22,656 22,138 21,170 18,580
47.07 51.99 57.92 60.74 62.96 59.09 58.04 51.31
5.57 5.40 4.84 4.63 4.44 4.53 4.59 5.06
0.68 0.66 0.72 0.73 0.77 0.78 0.78 0.70
1.80 1.44 1.60 1.70 1.81 1.72 1.68 1.53
Note: Mad refers to moisture on an air dry basis; Mar, Aar, Var and FCar refer to moisture, ash, volatile matter and fixed carbon on an as received basis; Qnet,ar refers to net calorific value on an as received basis; ultimate analysis was conducted on an as received basis.
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Microwave power=700W Microwave time:
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3600
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4000
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Fig. 3. X-ray diffraction spectroscopy analysis of Shengli lignite upgraded through microwave irradiation.
Microwave power (W) 100
2
2
2θ (deg)
Wave number (cm-1)
0
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1- Kaolinite, 2- Quartz, 3-Hematite
(a) 400
10min 21
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Har/Hal vs. microwave time Har/Hal vs. microwave power
0.63
microwave power and time increased. When microwave time was increased from 0 min to 10 min, the total specific surface area initially decreased and then increased from 3.84 m2/g to 6.46 m2/g, and the total specific pore volume initially decreased and then increased from 0.0111 cm3/g to 0.0135 cm3/g, respectively. The valley point appeared at a microwave time of 5 min. Conversely,
0.62
Har/Hal
0.61 0.60 0.59 0.58 0.57 0.56
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(b)
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700
7.0
0.54 0
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10 2
Specific surface area (m /g)
Microwave time (min) Fig. 2. Fourier transform-infrared spectroscopy (FTIR) analysis of Shengli lignite upgraded through microwave irradiation: (a) FTIR spectra of upgraded lignite; (b) effects of microwave power and time on molar ratio of aromatic hydrogen to aliphatic hydrogen (Har/Hal) in upgraded lignite.
3.4. XRD spectra of the upgraded lignite
3.5. Pore structures of upgraded lignite
6.0 5.5 5.0 4.5 4.0 3.5 3.0
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specific pore volume vs. microwave time specific pore volume vs. microwave power
3
Specific pore volume (cm /g)
The XRD spectra of the upgraded lignite are shown in Fig. 3. The mineral crystals that existed in Shengli lignite were mainly composed of kaolinite Al2(Si2O6)(OH)4, quartz SiO2 and Hematite Fe2O3, and the other mineral crystals with less than 0.5 wt.% can hardly be detected by XRD. When microwave time delayed was to 5 min, the full width at the half-maximum of kaolinite, quartz and hematite decreased from 0.28, 0.26 and 0.31 to 0.15, 0.22 and 0.22, respectively, indicating that crystallinity was increased. This conclusion is drawn in accordance with the suggestion of Sahoo [31] that the crystallinity of coal increased after microwave treatment. As the moisture content gradually decreased, the ash content of the upgraded lignite was increased from 7.60 wt.% to 9.64 wt.% at a microwave power of 700 W for an irradiation time of 5 min. These mineral crystals mainly existed in ash of the lignite. Thus, the relative contents of mineral crystals in the upgraded lignite were increased, leading to an increase in crystallinity after microwave irradiation.
specific surface area vs. microwave time specific surface area vs. microwave power
6.5
0.013
0.012
0.011
0.010
(b) 0
1
2
3
4
5
6
7
8
9
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Microwave time (min) The specific surface area and pore volume of the upgraded lignite are shown in Fig. 4. The total specific surface area and total specific pore volume initially decreased and then increased as
Fig. 4. Pore structures of Shengli lignite upgraded through microwave irradiation: (a) Effects of microwave on specific surface area; (b) effects of microwave on specific pore volume.
F. Zhou et al. / Fuel 159 (2015) 909–916
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Fig. 5. Scanning electron microscopy images (10,000) of Shengli lignite upgraded through microwave irradiation: (a) Raw lignite; (b) microwave power = 700 W, microwave time = 1 min; (c) 700 W, 3 min; (d) 700 W, 5 min; (e) 700 W, 10 min; (f) 567 W, 10 min; (g) 406 W, 10 min; (h) 126 W, 10 min.
moisture was rapidly heated into steam when microwave directly penetrated into the lignite particles, resulting in the quick rise in pressure difference inside and outside the pores. The escape of steam dramatically decreased the inner pressure and created an irreversible collapse of the pores. Lignite exhibited a gel-like behaviour, shrinking in response to water loss [32]. Upon moisture removal, the strong hydrogen bond bridges between micelles caused the open gel structure to collapse. Meanwhile, the exposure of the lignite core to microwave generated new fractures and cracks and raised the existing cleat apertures for the unstressed
core [33–35]. During lignite drying, the surface area of macropores considerably increased, whereas that of mesopores partially increased. Some of the oxygen functional groups of the modified lignite were removed by microwave irradiation. The generation of small-molecule gases as a result of the decomposition of some functional groups of the modified lignite may lead to the creation of new pore structures. Overall, the collapse and shrinkage of the pores primarily caused the decrease in specific surface area and pore volume at low microwave power and time. However, the higher microwave strength achieved by higher microwave power
F. Zhou et al. / Fuel 159 (2015) 909–916
and time may exert a more significant effect on the new formation of pore structures. Thus, the specific surface area and the specific pore volume increased again.
Microwave power (W) 0
The contact angles of the upgraded Shengli lignite are shown in Fig. 6. As microwave power and time increased, the contact angle gradually increased from 62.82° to 106.32° at a microwave power of 700 W for an irradiation time of 10 min. The contact angle is geometrically defined as the angle formed by a liquid at the three-phase boundary where a liquid, gas and solid intersect [36,37]. The contact angle of lignite largely depends on the oxygen functional group content and the complex pore structure. Water molecules interact with the surfaces of lignite by forming hydrogen bonds with the oxygen functional groups [38]. Oxygen functional groups have strong hydrophilicity, which can enhance the water-absorbing capacity of coal. The reduced oxygen functional group content of the upgraded lignite resulted in an increased contact angle. Thus, the properties of the upgraded lignite are similar to that of high-rank coal, which is always hydrophobic.
Contact angle (°)
100
400
500
600
700
95 90 85 80 75 70 65 60 0
1
2
3
4
5
6
7
8
9
10
Microwave time (min) Fig. 6. Contact angles of Shengli lignite upgraded through microwave irradiation.
hydrogen to aromatic hydrogen in the upgraded lignite decreased, implying that the coal rank was upgraded. The hydrophilicity of the upgraded lignite surfaces was reduced. In addition, the specific surface area of the upgraded lignite was less than that of raw lignite. The water-holding capacity of the pore structures in the lignite particle was also weakened. The holding-water was firmly bound on the lignite surfaces or trapped in numerous pores, consequently losing its fluidity. Thus, more free water as the flow media among lignite particles would be present in CWS prepared from the upgraded lignite. Therefore, the solid concentration of CWS was increased. 3.9. Energy consumption analysis The unit energy consumption for moisture removal and CWS concentration promotion by 1% are shown in Fig. 8. The unit energy consumption for moisture removal was calculated as follows: UECMR = Pt/[W0(M0 Mt)]. The unit energy consumption for CWS concentration promotion by 1% was calculated as follows: UECSC = Pt/[W0(Ct C0)]. Here, P was microwave power (KW), W0 was initial weight of lignite (kg), M0 was initial moisture content in lignite (wt.%), Mt was moisture content (wt.%) at microwave time of t (h), C0 was solid concentration of CWS prepared from raw lignite (%), Ct was solid concentration of CWS prepared from
3.8. CWS solid concentration of the upgraded lignite
Microwave power (W) 0
100
2.20
200
300
400
500
600
700
Hydrophiclicity index vs. microwave time Hydrophiclicity index vs. microwave power
2.18
Hydrophiclicity index
The maximum solid concentration of CWS (viscosity of approximately 1000 mPas at the shear rate of 100 s 1) of the upgraded Shengli lignite is presented in Fig. 7. The maximum solid concentration of CWS gradually increased from 45.6 wt.% to 51.7 wt.% at a microwave power of 700 W for an irradiation time of 10 min. The hydrophilicity index was calculated based on the equation [39]: Hydrophilicity index = [(–COOH) + 2(–OH)]/[(R–H) + (Ar–H)], where (–COOH), (–OH), (R–H) and (Ar–H) were the adsorption intensity values as expressed by the Kubelka–Munk function. The FTIR peaks appeared at 3400 cm 1 for hydroxyl groups (–OH), at 1620 cm 1 for carboxyl groups (–COOH), at 1430 cm 1 for aliphatic hydrogen (R–H), and at 3030 cm 1 for aromatic hydrogen groups (Ar–H). As microwave power and time increased, the hydrophilicity index of the upgraded lignite monotonously decreased, whereas the maximum solid concentration remarkably increased. This result is consistent with the increase in contact angle, indicating the hydrophobization of the upgraded lignite. Oxygen functional groups with high hydrophilicity and the molar ratio of aliphatic
300
contact angle vs. microwave time contact angle vs. microwave power
105
3.7. Contact angles of the upgraded lignite
200
110
3.6. SEM image of the upgraded lignite The SEM images of the upgraded lignite are shown in Fig. 5. The raw lignite particles rendered a multilayer structure. As microwave power and time increased, the particles exhibited trends of increasing particle size and hardening particle surfaces. During the microwave upgrading process, the pore structures of the lignite particles kept on changing with progressive moisture removal. On the one hand, the gradual expansion of the water vapour and the decomposition of some functional groups in the semi-closed or closed pores created new crosslinking surfaces, which in turn enlarged the size of the particles. On the other hand, the lignite particles shrunk as the pores collapsed after the removal of moisture and oxygen functional groups that existed in the newly created pore structures, thus causing the surface of the particles to harden. The microwave treatment of coal smoothens the surface, weakens the coal mineral matrix and alters the angularity and surface properties of the ground particles. Under the same lignite mass fraction, CWS prepared from the dense upgraded lignite reduced the volume fraction of the solid phase, thereby increasing the flowing space of the water in the suspension. Therefore, the solid content (per dry solids weight/volume) of CWS prepared from the upgraded lignite was increased.
100
52
2.16
51
2.14
50
2.12 49
2.10 2.08
48
2.06 47
2.04 2.02
46
CWS concentration vs. microwave time CWS concentration vs. microwave power
2.00
CWS concentration (wt.%)
914
45
1.98 0
1
2
3
4
5
6
7
8
9
10
Microwave time (min) Fig. 7. Hydrophilicity index and CWS solid concentration of Shengli lignite upgraded with microwave irradiation.
0.35
Microwave power=700W 1.3
0.30
unit energy consumption CWS concentration promotion moisture removal
1.2
0.25
1.1
0.20
1.0
0.15
0.9
0.10
0.8 0.7
0.05
(a)
moisture content of lignite
35 30 25 20 15 10 5
0.00
0
1.4
0.35
Microwave time=10min unit energy consumption 1.3 CWS concentration promotion moisture removal 1.2
35
0.30
1.1
0.20
1.0
0.15
0.9
0.10
0.8
0.05
0
1
2
3
4
5
6
7
8
9
Moisture content of upgraded lignite (wt.%)
1.4
Unit energy consumption for moisture removal by 1 % (KW⋅h/kg-coal)
Unit energy consumption for CWS concentration promotion by 1% (KW⋅h/kg-coal)
F. Zhou et al. / Fuel 159 (2015) 909–916
915
lignite. The unit energy consumption for external moisture removal and CWS concentration promotion was lower than that for internal moisture removal. Oxygen functional groups with high hydrophilicity in the upgraded lignite decreased. The decreased molar ratio of aliphatic hydrogen to aromatic hydrogen implied an increase in coalification degree. The particle size and contact angle of the upgraded lignite increased. In addition, the specific surface area of upgraded lignite was lower than that of raw lignite. When the upgraded lignite with a decreased hydrophilicity index was used to prepare CWS, the maximum solid concentration increased from 45.6 wt.% to 51.7 wt.%. In future works, microwave absorbers should be studied to strengthen the heat transfer and consequently improve the upgrading and slurribility of lignite.
10
(b) 0
moisture content of lignite 0.00 100
200
300
400
500
600
30 25 20 15 10 5 0
Moisture content of upgraded lignite (wt.%)
0.7
0.25
Acknowledgements Unit energy consumption for moisture removal by 1 % (KW ⋅h/kg-coal)
Unit energy consumption for CWS concentration promotion by 1% (KW ⋅h/kg-coal)
Microwave time (min)
700
Microwave power (W) Fig. 8. Unit energy consumption for CWS concentration promotion and moisture removal of Shengli lignite upgraded with microwave irradiation: (a) Effects of microwave time; (b) effects of microwave power.
upgraded lignite at microwave time of t (h). The unit energy consumption increased as microwave power and time increased. Microwave heating is a type of selective heating in which the dielectric loss value of a material is the indicator of its heating ability in the presence of microwave. External water, such as the moisture on particle surfaces or that in large pores of lignite, which can easily absorb microwaves, was mostly removed at the initial dewatering rate of the lignite. Conversely, internal water, such as the adsorbed moisture in capillary pores of lignite and chemically combined water, which has difficulty in absorbing microwaves, was mostly removed subsequently at the following dewatering rate. Thus, with the decreased moisture content of the lignite and as microwave power and time increased, the mobility of moisture slowed down, and the microwave absorption capacity of moisture in lignite started to decrease at a critical specific value, resulting in the need for much more energy to remove the residual moisture of lignite. Microwave irradiation was more effective at removing more freely type water than more strongly bonded water in lignite. Thus, the low unit energy consumption for moisture removal and CWS concentration promotion by 1% at a low microwave power (less than 406 W) and time (lower than 5 min) was mainly caused by the removal of more external moisture than internal moisture in lignite. 4. Conclusions Microwave irradiation can effectively upgrade Shengli lignite from the Inner Mongolia region of China and thus improve its slurribility. Microwave irradiation was more efficient for removing external free moisture than internal bonded moisture in Shengli
This study was supported by the National Natural Science Foundation – China (51176163 and 51476141); the National High Technology R&D Program – China (2012AA050101); the Special Funds for Major State Basic Research Projects – China (2010CB227001); and the Program of Introducing Talents of Discipline to University – China (B08026).
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