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Flow properties of ash and slag under co-gasification of coal and extract residue of direct coal liquefaction residue
Fuel 264 (2020) 116850
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Full Length Article
Flow properties of ash and slag under co-gasification of coal and extract residue of direct coal liquefaction residue
T
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Xi Caoa,b, Baozi Pengc,1, Lingxue Konga, Jin Baia, , Zefeng Gea,b, Huaizhu Lia, Zhen Liuc, Ziyang Fengc, Dapeng Bia, Zongqing Baia, Andrzej Szlękd, Wen Lia a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China University of Chinese Academy of Sciences, Beijing 102211, PR China c National Institute of Clean-and-Low-Carbon Energy, Beijing 100049, PR China d Institute of Thermal Technology, Silesian University of Technology, 44-100 Gliwice, Poland b
A R T I C LE I N FO
A B S T R A C T
Keywords: Co-gasification Extract residue of direct coal liquefaction residue Ash fusibility Slag viscosity
Efficient utilization of the extraction residue (ER) of direct coal liquefaction residue is a bottle neck for the efficiency of direct coal liquefaction process. The co-gasification of the ER and coal is a promising way for largescale utilization of ER. Flow properties of the feedstock including ash fusibility and slag viscosity are important parameters for the gasification process. To optimize co-gasification of the ER and coal, the ash fusibility and slag viscosity behavior of ER and coal under gasification conditions were studied. The results show that the ash fusion temperatures (AFTs) of the blending were lowered with the increasing blending ratio of ER due to the high content of calcium and iron in ER. The content of quartz and anorthite in the blended ashes decreased with the increasing ER blending ratio. The slag viscosities at high temperatures also decreased as the blending ratio of ER increased. The high content of calcium and iron in ER resulted in the decrease in the slag polymerization degree because of the break of Si-O structure and transformation from [AlO4]5− to [AlO6]9−. Besides, the slag presented the behavior of a crystalline slag when the ER blending ratio was increased up to 25% for the formation of anorthite during cooling. For the entrained flow gasification, the ER addition can effectively lower the operation temperature of the gasifiers, improve the gasification efficiency and avoid the slag tapping problems. The optimal ER addition should be in the range of 10–20%, and the corresponding tapping temperature was 1258–1575 °C.
1. Introduction Direct coal liquefaction (DCL) is an important process to produce desired chemicals and liquids from coal. The DCL process converts coal into a wide range of products, including gas, liquid and solid. However, a large amount of direct coal liquefaction residue (DCLR) is formed as a by-product [1,2], which includes carbonaceous materials and mineral matter generated from raw coal and catalyst added in the DCL process [3]. China Shenhua Coal to Liquid and Chemical Company Limited has built the world’s largest DCL plant, 1.08 million t/a direct liquefaction coal to oil project, in Ordos, Inner Mongolia. The amount of DCLR is about 0.5–0.7 million t/a and it accounts for about 30 wt% of the raw coal used in the DCL process. The economic performance of DCL is therefore largely dependent on the effective utilized of DCLR [4,5]. At present, the major utilizations of DCLR are used as the feed for
pyrolysis, gasification, combustion, and coking, etc. [6–8]. There are a large number of polycyclic aromatic compounds in DCLR, which are the products of hydro-liquefaction and a series of cycloalkyl and side chains [9]. Compared with coal tar pitch, DCLR is prone to thermal condensation polymerization, and it is a good precursor to prepare mesophase pitch. Therefore, it is an alternative way for further processing coal liquefaction residue to extract heavy oil and asphalt from solvent oil [10,11]. About 50% of extraction residue (ER) will be produced after extracting heavy oil and asphalt [12,13], which mainly consists of the unreacted coal, mineral matters in coal and additional catalyst. As the ER has high carbon content, high calorific value and low water content, it is considered to be a good feedstock for gasification. However, the ER also has high ash and sulfur contents, as well as iron catalyst [14]. Thus, the co-gasification of the ER and coal is deemed as an effective utilization way of the ER.
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Corresponding author. E-mail address: [email protected] (J. Bai). 1 Co-first author. https://doi.org/10.1016/j.fuel.2019.116850 Received 6 November 2019; Received in revised form 4 December 2019; Accepted 7 December 2019 Available online 14 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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(denoted as MC), was selected as the raw coal sample blended with the ER. The chemical compositions of ash were characterized by X-ray fluorescence (XRF) (Bruker S8 Tiger, Germany), as listed in Table 2. In order to obtain the “in-situ” minerals in coals, the low temperature asher (Quorum Technologies, K1050) was used to remove the organic matter in the sample. The experimental process is as follows: weigh about 0.5 g coal (≤75 μm) and put it in the porcelain boat (120 mm × 60 mm × 15 mm). Weigh it every 2 h until the masses difference between two times is less than 0.0010 g.
The ash fusibility and slag viscosity are two important parameters for the gasification [15,16]. Ash fusion temperatures (AFTs) are widely used to evaluate the ash fusibility, which usually act as the first criteria for coal selection and optimization of operation parameters of the gasifiers. For all gasifiers, the ash fusibility is also an important variable [17]. For the fluidized-bed gasifiers, the operating temperature should be lower than the ash fusion temperatures of the feedstocks (900–1050 °C), which can avoid the operational problems such as agglomeration and sintering [18]. For the entrained flow gasifier, the operating temperature is required to be in the range of 100–150 °C higher than the flow temperature (FT) to enable the smooth slag tapping [19]. Moreover, in order to avoid the blockage of slag on furnace, the FT of ash should be lower than 1350 °C for refractory bricks, and lower than 1450 °C for membrane wall [20]. The slag viscosity behavior is another important parameter for the slagging gasifiers, such as Shell, Siemens GSP, BGL, OMB gasifiers [21,22]. Criteria for adequate slag tapping are process specific. For the BGL gasifier, a viscosity of 5 Pa·s at bed temperature is required, whereas for the slag to flow from the entrained flow gasifiers, it is generally accepted that the slag viscosity should be less than 25 Pa·s at the tapping temperature [23]. However, if the slag viscosity was lower than 2 Pa·s, the slag becomes too fluid and potentially cause excessive wear of refractory walls (e.g. GE and ConocoPhillips E-gas technologies) or excessive heat loss through water-cooled walls (e.g. Shell or Siemens technologies) [24]. Therefore, the slag viscosity at slag tapping temperature should ideally be greater than 2 Pa·s and less than 25 Pa·s [25]. The optimal range of the slag viscosities for the operation of the slagging gasifiers is generally considered to be between 5 and 15 Pa·s at the slag tapping temperature [26,27]. However, there is no published work has attempted to characterize the flow properties of ash and slag from the ER and coal. In this work, the ash fusibility and slag viscosity behavior of the ER and coal under gasification conditions were carried out. Two coals and an ER from China Shenhua Coal to Liquid and Chemical Company Limited were used in the experiment. X-Ray diffraction (XRD) and Factsage software were applied to study the mineral transformation of the ash at high temperatures. The slag structures were characterized by Nuclear Magnetic Resonance Spectrometer (NMR) and Fourier Transform Infrared Spectroscopy (FTIR). Solid phases of the slag during cooling were investigated by scanning electron microscope and energy dispersive X-ray spectroscopy (SEM-EDX).
2.2. Ash fusion temperature test The AFTs of blended ash with different ratio of ER were measured under mild reducing atmosphere (CO/CO2 = 3/2, volume fraction) by the Carbolite CAF 1600 ash fusion point meter (Carbolite Gero Limited, Britain) based on the American standard ASTM D1857-04. The pyramid ash cone was heated to 1560 °C at a rate of 8 °C/min. Four characteristic temperatures, deformation temperature (DT), softening temperature (ST), hemisphere temperature (HT), and flow temperature (FT), were determined according to the shape of ash cone. To verify the results of the experiment, the tests were repeated three times, and the results exhibited a good reproducibility under the same conditions. Table 3 gives the AFTs of SM, GL, MC, and ER under mild reducing atmosphere. 2.3. Slag viscosity measurement The viscosity measurements were conducted with a Theta hightemperature rotating viscometer (Theta Industries, Inc., USA) using molybdenum rotors and crucibles. To obtain a homogeneous slag, the ashes were firstly melted in an electric furnace at a temperature which is 150 °C higher than the liquidus temperature under reducing atmosphere simulated by graphite. Then about 50 g pre-melted slag was crushed to less than 2 mm for the viscosity measurement. The start temperature for test should be higher than liquidus temperature to make sure the sample totally melted. The cooling rate was 3 °C/min and the viscosity and temperature were recorded continuously at an interval of 0.1 °C, then a viscosity-temperature curve was obtained. The parameters of the rotor and crucible were calibrated with a standard reference material 717A glass [28]. The tests were also repeated several times and the results showed a good reproducibility under the same conditions.
2. Experimental 2.4. Preparation of quenched slag 2.1. Sample preparation The quenched samples were prepared in a horizontal tube furnace under mild reducing atmosphere as the AFTs test. When the target temperature was reached, the corundum boat containing sample was pulled out quickly and quenched into the cold water [29]. The slags prepared at 1000–1400 °C during heating were used for XRD test, and the slags obtained at 1200 °C during cooling were analyzed by SEM.
Two bituminous coals, Shenmu coal (denoted as SM) and Gaoliu coal (denoted as GL), and an ER from Ordos DCL plant were used in this work. All the samples were crushed and ground to the sizes smaller than 75 μm. The proximate and ultimate analyses of the samples were performed according to GB/T212-2008 and GB/T476-2001, which was listed in Table 1. The coal ash was prepared at 815 °C in a muffle furnace based on GB/T 1574-2007. As the current blending ratio of Shell gasification in China Shenhua Coal to Liquid and Chemical Company Limited was 10:1, the same blending ratio, 10:1 of SM coal and GL coal
2.5. Characterization of quenched slags The mineralogical analysis was conducted by an X-ray powder
Table 1 Proximate and ultimate analyses of SM, GL, and ER (wt%). Sample
SM GL ER
Proximate analysis (wt%, ad)
Ultimate analysis (wt%, daf)
M
A
V
FC
C
H
Oa
N
St
6.31 0.54 1.64
10.02 37.08 37.52
30.75 20.52 21.01
52.92 41.86 39.83
80.97 81.82 88.35
4.60 5.11 3.65
12.92 10.55 0.76
1.02 1.17 0.67
0.49 1.35 6.57
ad: air dry base; a: by difference; M: moisture; A: ash; V: volatile; FC: fixed carbon, C: carbon; H: hydrogen; O: oxygen; N: nitrogen; St: total sulfur. 2
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Table 2 Ash chemical compositions of SM, GL, and ER (wt%). Samples
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
K2O
Na2O
TiO2
P2O5
SM GL ER
50.97 51.33 29.35
17.04 37.17 10.35
7.27 3.04 22.75
13.29 3.29 21.37
1.14 0.68 0.91
3.68 0.49 10.35
0.77 1.11 0.77
1.81 0.96 0.53
1.81 0.29 1.85
0.97 0.33 0.35
(CaSO4). The ER ash was mainly composed of calcite and quartz. For the ash prepared at 815 °C, the minerals of SM coal and ER coal were quartz, anhydrite, lime and hematite, and the main minerals in GL coal were quartz and hematite. It demonstrated that the mineral in the coal and ER reacted as Equation (1)-(2). For the ER, it was rich in calcium and iron and showed strong crystallization tendency [30].
Table 3 AFTs of SM, GL, MC, and ER. Samples
DT
ST
HT
FT
SM GL MC ER
1131 > 1550 1289 1110
1145 > 1550 1309 1124
1162 > 1550 1317 1129
1202 > 1550 1342 1137
2[Al2Si2 O5 (OH)4] → 2(Al2O3 ·2SiO2) → Al2O3 (γ ) + 3Al2O3 ·2SiO2 (mullite)
diffract meter (BRUKER D2, Germany), which equipped with Cu Kα radiation (40 kV, 100 mA). The powder samples were mounted on a sample glass and scanned with a step size of 0.02° over a 2θ range of 5–90°. FTIR spectra were recorded with a Bruker Vertex 70 Fourier transform infrared spectrometer (Bruker, Germany). KBr DRIFTs technique was used in the experiment. The powder sample was accurately weighted and mixed with KBr with the ratio of 1:200. The mixture was finely grinded for 30 min, and then pressed into a thin tablet under 12 MPa. The spectra were recorded over the range of 4000–400 cm−1 with a resolution of 0.4 cm−1. The aluminum nuclei were obtained by a Bruker Avance III 600 MHz Wide Bore spectrometer, which was supplied by Kratos Analytical Inc. (Spring Valley, NY), which has a 14.1 T magnetic field and a 4 mm CPMAS probe under 13 kHz MAS speed in zirconia rotors. The radio frequency field strength was verified by a 1.0 M aqueous AlCl3 solution and the chemical shift of 27Al signal were referenced to the AlCl3 powder signal. All samples were measured in powdered form. The microstructural analysis was determined by a JSM-7001F scanning electron microscope (SEM). Before SEM observations, the bulk slags were mounted in liquid epoxy resin, pelletized, polished, dried and finally coated with Au vapor to make them conductive. The slag was observed in back-scattering mode (BSE) at 15 kV, and Energy Dispersive X-ray (EDX) Spectroscopy was performed to analyze the chemical compositions of the crystals in the slag.
CaCO3 → CaO + CO2
(1) (2)
3.2. AFTs of the blended ash As shown in Fig. 2, the AFTs decreased with the increasing blended ratio of ER. When the blended ratio of ER was less than 10%, the AFTs decreased linearly with the increasing ER blended ratio. However, they decreased slowly when the blended ratio of ER was less than 10%. For example, when the blended ratio of ER was 0%, DT, ST, HT and FT were 1254 °C, 1273 °C, 1285 °C and 1316 °C, and they dropped to 1160 °C, 1182 °C, 1200 °C and 1253 °C as the ER blended ratio increased to 10%. When the blended ratio of ER was 25%, DT, ST, HT and FT were 1111 °C, 1123 °C, 1137 °C and 1170 °C, respectively. It illustrated that the addition of ER was favorable for the decrease in AFTs, and the influence of the ER on the AFTs was weakened as the ER blended ratio was higher than 10%. 3.3. Effect of ER on mineral evolution As shown in Fig. 3, the main minerals in MC at 1000 °C were quartz and anorthite. The minerals in SM coal ash, such as anhydrite, lime and hematite etc., can react with quartz and mullite, which are mainly refractory minerals in GL coal ash, to form the low fusion temperature minerals anorthite at above 1000 °C. When the temperature was 1100 °C, anorthite was the dominant phase in blended ash. However, the intensity of anorthite decreased above 1200 °C due to the partial dissolution of the phase assemblage. As the temperature increased to 1300 °C, the quartz no longer existed. Above 1300 °C, the blended ash was completely molten. The relative contents of minerals in the blended ashes were determined by the peak intensity of XRD [31–33], as shown in Fig. 4. The main minerals and the minerals transformation in the range of 1000–1400 °C were similar, while the content of quartz and anorthite at the same temperature was decreased due to the addition of the ER. With the increasing ER blended ratio, the content of quartz and anorthite decreased. When the temperature was 1200 °C, the intensity of quartz decreased from 2163 to 1344 with the increasing of ER mass fraction, and the intensity of anorthite decreased from 1917 to 1259. As the temperature was above 1100 °C, the quartz disappeared and the primary mineral was anorthite. The decrease in quartz and anorthite with the addition of the ER explained why the AFT of the blended ash lowered [31]. Fig. 5 showed the ternary phase diagram of SiO2-Al2O3-Fe2O3-CaO system, and the color lines indicate the liquidus temperatures (Tliq) at which the last solid phase disappeared. Because the range of Si/Al ratio (SiO2/Al2O3 weight ratio) in the mixed ash is in the range of 2.3–2.5, Si/Al ratio of 2.5 was selected to show the effect of ER addition on
2.6. Thermodynamic equilibrium calculation Factsage software packages 7.2 was employed to calculate the multiphase equilibration and phase diagram of SiO2-Al2O3-CaO-Fe2O3FeO system under a weak reducing atmosphere at 1 atm. In this work, the Phase Diagram module together along with FactPS and FToxid databases was applied for calculation. According to the oxygen partial pressure of the experimental atmosphere (CO/CO2 = 3/2, volume fraction), the ratio of Fe2+/Fe3+ is 8:2 in the database. In order to eliminate the influence of atmosphere on iron valence, the Fe2O3 replaced by FeO8.89Fe2O3 (Fe2+/Fe3+ = 8/2, mass ratio) was inputted in phase diagram. 3. Results and discussion 3.1. Sample analysis The minerals composition of the ashes from SM coal, GL coal and ER ashed at 150 °C and 815 °C are shown in Fig. 1. For the SM coal ash, the minerals at 150 °C were calcite, quartz, and a small amount of kaolinite. For the GL coal, they were calcite, quartz, and kaolinite, and the main minerals in MC were quartz (SiO2), hematite (Fe2O3) and anhydrite 3
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In te n s ity (a .u .)
600 0 1200
(a)
600
SM Coal
1
400
3 2
31
1
1 1 1
11
11
2
GL Coal 1
600
2
0
1 3 2
21 1
ER
1
400
3 2
3
0 20
5 3
1 1 1
40
11
30000
5 43 5
6 54
5
200
4
GL Coal
3
400 4
3
4 4
0 400
3
ER
5 4 3 3 5
0
60
5 5 3
0
1
2θ (°)
6
5 6 64 4
3 54
200 1
1
SM Coal
(b)
200
In te n s ity (a .u .)
1200
4
5
4
5
6
20
80
5
40
3
6 4 5 5 5
2θ (°)
5 4
60
80
MC
(c)
3
Intensity (a.u.)
25000 20000 15000 10000 3
5000
5 6
54
5 4 43
5
5 3
3 6
3
3
0 10
20
30
40
50
2θ (°)
60
70
80
90
Fig. 1. XRD patterns of the ashes from SM, GL and ER. (a) 150 °C; (b) 815 °C; (c) ash of MC prepared at 815 °C. 1-calcite (CaCO3); 2-kaolinite (Al2Si2O5(OH)4); 3quartz (SiO2); 4-hematite (Fe2O3); 5-anhydrite (CaSO4); 6-lime (CaO).
Fusion Temperature (°C)
1350
phase diagram can be used to predict the effect of ER addition on the AFTs [34].
DT ST HT FT
1300
3.4. Slag viscosity behavior
1250
The viscosity temperature curves of the slags with different blended ratio of ER were shown in Fig. 6. As the ER blended ratio increased, the slag viscosity of the fully molten slags at the same temperature decreased monotonously. For example, the slag viscosities were 1.27 Pa·s, 1.70 Pa·s, 3.26 Pa·s, 4.21 Pa·s, 8.17 Pa·s and 13.26 Pa·s at 1500 °C when the ER blended ratio was 0%, 5%, 10%, 15%, 20% and 25%, respectively. The dote line at 25 Pa·s is the upper viscosity limit for slag tapping. It is noticed that the temperature at 25 Pa·s was lowered with the addition of the ER. The viscosity-temperature curves also indicated the effect of ER on slag viscosity behavior. When the ER blended ratio was between 0% and 20%, the slags exhibited the behavior of a glassy slag, of which the viscosity gradually decreased with the decreasing temperature. However, as the ER blended ratio increased up to 25%, the slag presented the behavior of a crystalline slag with a temperature of critical viscosity (TCV) of 1257 °C.
1200 1150 1100 0
5
10
15
20
M ass fraction of ER (wt% )
25
Fig. 2. AFTs of blended ashes.
primary phase of the mixed ash. It is can be seen that the ash composition of MC was located in the anorthite primary phase. With the increasing ER blended ratio, the bulk ash chemical composition of the blended ash was still located in the anorthite primary phase. The Tliq were 1427 °C, 1424 °C, 1419 °C, 1405 °C, 1388 °C and 1367 °C as the ER blended ratio was 0%-25% and the Tliq lowered with the increasing of ER content. As the liquidus temperatures were similar to the AFTs, the
3.5. Effect of ER addition on slag structure at high temperature 3.5.1. Effect of ER addition on Si-O structure The FTIR results of the slags with different blended ratio of ER and a typical fitted FTIR spectra were shown in Fig. 7(a) and (b). The FTIR 4
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Fig. 3. Mineral compositions of the blended ashes. 1-SiO2 (quartz); 2-CaAl2Si2O8 (anorthite).
(Q0 + Q1)/(Q2 + Q3 + Q4) (denoted as R) was calculated. The higher R implied a lower polymerization degree. As shown in Fig. 8, the R value increased from 0.85 to 3.57 with the increasing ER blended ratio. In addition, R increased rapidly when the ER blended ratio was 25%. This demonstrated that the polymerization degree significantly decreased with the increasing ER blended ratio.
spectra of silicate slags were typically focused within the wavenumber region between 1200 cm−1 and 800 cm−1, which was related to the [SiO4] tetrahedra symmetric stretching vibration [35]. Q-species was introduced to distinguish the characteristic stretching vibrations of SiO4 tetrahedra, and which indicates the slag polymerization degree. The index n represents the number of Si-O-Si bridging oxygen, and the more polymerized the melt, the more the Qn with higher n. The values of n assigned from 0 to 4 are structurally defined as monomers (880–850 cm−1), dimers (920–900 cm−1), chains (1000–950 cm−1), sheets (1100–1050 cm−1) and three-dimensional network (1200–1060 cm−1) [36]. With ER content increasing, the peak shifted towards low wavenumber, which indicated that the slag polymerization degree decreased due to the addition of ER. According to the results from Fig. 8, the content of Q1 and Q2 increased slightly with the increasing ER blended ratio when the ratio was lower than 20%. As the ER blended ratio was higher than 20%, the Q1 and Q2 species increased obviously and Q4 disappeared. In order to characterize the vibration of SiO4 tetrahedra, the area percentage of 5000
Quartz
3.5.2. Effect of ER addition on Al-O structure The influence of ER addition on Al-O structure at high temperatures was studied by 27Al-NMR spectra. Generally, two kinds of coordinate can be classified: six-coordinate Al ([AlO6]9−) and four-coordinated Al ([AlO4]5−). Al3+ in [AlO6]9− unit act as a network modifier, which can weaken the slag polymerization degree, while Al3+ in [AlO6]9− act as a network former to enhance the slag polymerization degree. Therefore, [AlO6]9− unit was prone to decrease the viscosity and its resonance centered in 10–20 ppm. [AlO4]5− species tend to increase the viscosity and the resonance at 50–100 ppm [37]. A typical deconvolution of 27Al NMR spectra and the relative
2100
1100°C
1100°C 1200°C
Intensity (a.u.)
4000
Intensity (a.u.)
1000°C
Anorthite
1000°C
3000
2000
1800
1300°C 1400°C
1500 1200 900
1000 0
5
10
15
20
M ass fraction of ER (wt% )
600
25
0
5
Fig. 4. Quartz and anorthite content of blended ashes. 5
10
15
20
M ass fraction of ER (wt% )
25
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Fig. 5. Phase diagram of SiO2-Al2O3-Fe2O3-CaO system with SiO2/Al2O3 weight ratio of 2.5.
content of [AlO4]5− was presented in Fig. 9. [AlO4]5− was the main AlO structure, and the content of [AlO4]5− were all above 90% in the range of 0%–25% ER content. With the increasing of ER blended ratio, the [AlO4]5− content decreased, while the variation was slight. For the slags with 0%–20% ER, the fractions of [AlO4]5− were 99.32%, 97.83%, 95.70%, 94.58% and 94.32%, and it decreased to 92.41% as the ER blended ratio was 25%. This indicated that Al3+ transformed from [AlO4]5− into [AlO6]9− due to the ER addition, which results in the decrease in slag viscosity.
the glassy behavior [38]. When the ER blended ratio increased to 25%, a crystalline phase, anorthite, was detected. The formation of crystalline phases was usually attributed to the formation of the crystalline slag [39]. Therefore, the slag with 25% ER showed the behavior of the crystalline slag, which was mainly due to the formation of anorthite. 3.7. Determination of optimal addition ratio of ER In Fig. 12, T25 and T2 are the temperature at which the slag viscosities were 25 Pa·s and 2 Pa·s, respectively, which were the slag tapping temperature of the entrained flow gasifiers. The area between two lines represented the tapping temperature range at certain ER addition ratio, which can be used to determine the tapping temperature with the ER addition. The tapping temperature range increased with the increasing ER blended ratio. For the slag of MC, the tapping temperature
3.6. Effect of ER addition on solid phases during cooling The XRD and SEM-EDS results of quenched slags at 1200 °C are shown in Figs. 10 and 11. The slags with 0%-20% ER addition have mainly an amorphous/vitreous character. Therefore, the slags exhibited
25
(a)
MC 5% ER 10% ER 15% ER 20% ER 25% ER
75
MC 5% ER 10% ER 15% ER 20% ER 25% ER
20
Viscosity (Pa·s)
Viscosity (Pa·s)
100
(b)
50
25
15
10
5 0 1100
1200
1300
1400
Temperature ( °C)
1500
0 1400
1600
1450
1500
Fig. 6. Viscosity-temperature curves of the slags with different ER blended ratios. 6
1550
Temperature ( ° C)
1600
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0.16
(a)
(b)
0.12
Absorbance
25% ER 20% ER 15% ER 10% ER
Q 2 (939) Q 3 (1017)
0.08
Q 1 (903)
0.04
Q 4 (1130)
Q 0 (855)
5% ER
0.00
MC 750
900
1050
1200
1350
600
W avenumber (cm -1)
800
1000
1200
W avenumber (cm -1)
1400
Fig. 7. (a) FTIR spectra of slags; (b) Typical fitted FTIR spectra.
Q0
100
Q1
Q2
Q3
Q4
3
60 2 40 1
20
-5
0
5
10
15
20
Q 0 +Q 1 +Q 2 )/(Q 3 +Q 4 )
Area ratio (%)
80
0
operating temperature and improve the efficiency of the gasification [40]. Therefore, the temperature at which the slag viscosity was 25 Pa·s was usually less than 1350 °C. The purple solid line was drawn at 1350 °C. It can be seen that for the coal without ER, the operating temperature will be too high. The ER addition can significantly lower the tapping temperature, and energy consumption. However, when the ER addition was 25%, the slag exhibited the behavior of the crystalline slag which is not favorable for the slag tapping. Thus, based on the slag viscosity temperature characteristics of the coal with ER, the ER blended ratio should be 10%-20%, and the corresponding slag tapping temperature range was 1258–1575 °C, as shown the areas with slash character. This can guarantee a low operating temperature, and avoid the slag tapping problem of the entrained flow gasifiers.
4
4. Conclusions
0 30
25
M ass fraction of ER (wt% )
In this work, we investigated the ash fusibility and slag viscosity of the blended coal with extract residue of direct coal liquefaction residue. The mineral transformation behavior and influence of ER addition on slag structure and solid phase during cooling were discussed in detail. The main conclusions were summarized as following:
n
Fig. 8. Contents of Q units based on deconvolution results.
range was 1443–1600 °C, and the width of temperature window was 157 °C. When the ER addition was 10%, the range was from 1575 °C to 1336 °C, and the width of temperature window was increased to 240 °C. For the entrained flow gasifiers, the operating temperature is 100–150 °C higher than the slag tapping temperature to lower the
(1) The main minerals in ER were CaCO3 and SiO2 and kaolinite. ER has high ash content, and the ash was rich in calcium oxide and iron
100
5% ER 10% ER 15% ER 20% ER
[AlO 4]5-
MC
content (%)
(a)
(b)
98
96
94
92
25% ER -20
0
20 27
40
60
80
100
90
120
Fig. 9. (a) A typical deconvolution of
27
0
5
10
15
20
M ass fraction of ER (wt% )
Al chem ical shift (cm -1 )
Al NMR spectra; (b) [AlO4]5− content as a function of ER content. 7
25
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1
25000
20% ER
10% ER
20000
Intensity (a.u.)
15% ER
2θ (°)
60
10000
1 1 11 1 1 11
1
0
MC 40
15000
5000
5% ER
20
25% ER
20
80
40
2θ (°)
60
80
Fig. 10. XRD patterns of the slags quenched at 1200 °C (1-anorthite).
transformation of [AlO4]5− to [AlO6]9−. As the extraction residue blended ratio was increased to 25%, the slag showed the behavior of a crystalline slag with TCV of 1257 °C due to the crystallization of anorthite during cooling. (4) From the present work, the addition of ER can lower the operation temperature of the gasifiers, improve the gasification efficiency and avoid slag tapping problems of the entrained flow gasifiers. The optimal blended ratio of extraction residue in co-gasification should be 10–20%, and the corresponding tapping temperature range was 1258–1575 °C.
oxide, which indicates ER has strong crystallization tendency, so ER was not the suitable feedstock for entrained flow gasifier alone. (2) The addition of extraction residue decreased the AFTs of the blended ashes. Quartz and anorthite were the minerals in the blended ashes at high temperatures, while their content lowered by the addition of extraction residue, which was the main reason for the decrease in the AFTs. (3) The slag viscosities at high temperatures were also decreased by the addition of extraction residue. The high content of calcium and iron in extraction residue resulted in the break of Si-O structure and
Fig. 11. SEM-EDS results of slag quenched at 1200 °C (♥-Anorthite). 8
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1650 1600
Tem perature ( ° C )
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1575 o C
1550
1482 o C
1500
T2
T 25
1450 1400 1350 1300
1336 o C
1250 1200
1258 o C 0
5
10
15
20
M ass fraction of ER (wt% )
25
Fig. 12. Slag tapping temperature at different ratio of ER.
CRediT authorship contribution statement Xi Cao: Writing - original draft, Data curation. Baozi Peng: Conceptualization, Resources. Lingxue Kong: Writing - review & editing. Jin Bai: Supervision. Zefeng Ge: Data curation. Huaizhu Li: Formal analysis. Zhen Liu: Project administration. Ziyang. Feng: Resources. Dapeng Bi: Investigation. Zongqing Bai: Methodology. Andrzej Szlęk: Methodology. Wen Li: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Key R&D program of China [grant numbers 2017YFB0602603], Science and Technology Program of China and Beijing [grant number Z181100005118006], Science and technology innovation project of CHN Energy [grant number GJNY-18-72], Joint Foundation of Natural Science Foundation of China and Xinjiang [grant number U1703252], Joint Foundation of Natural Science Foundation of China and Shanxi Province [grant number U1510201], NSFC-DFG [grant number 21761132032], Youth Innovation Promotion Association, CAS, International Partnership Program of Chinese Academy of Sciences [grant number 122214KYSB20170020]. References [1] Wang Y, Niu Z, Shen J, Bai L, et al. Extraction of direct coal liquefaction residue using dipropylamine as a CO2-triggered switchable solvent. Fuel Process Technol 2017;159:27–30. [2] Yang J, Wang Z, Liu Z, Zhang Y. Novel use of residue from direct coal liquefaction process. Energy Fuels 2009;23(10):4717–22. [3] Cui H, Yang J, Liu Z, Bi J. Characteristics of residues from thermal and catalytic coal hydroliquefaction. Fuel 2003;82(12):1549–56. [4] Lv D, Bai Z, Yuchi W, Bai J, et al. Properties of direct coal liquefaction residue water slurry: effect of treatment by low temperature pyrolysis. Fuel 2016;179:135–40. [5] Zhou Y, Xiao N, Qiu J, Sun Y, et al. Preparation of carbon microfibers from coal liquefaction residue. Fuel 2008;87(15–16):3474–6. [6] Wen H, Kong L, Bai J, Bai Z, et al. Transformation of minerals in direct coal liquefaction residue under gasification atmosphere at high temperatures. J Fuel Chem Technol 2015;43(3):257–65.
9
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Full Length Article
Flow properties of ash and slag under co-gasification of coal and extract residue of direct coal liquefaction residue
T
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Xi Caoa,b, Baozi Pengc,1, Lingxue Konga, Jin Baia, , Zefeng Gea,b, Huaizhu Lia, Zhen Liuc, Ziyang Fengc, Dapeng Bia, Zongqing Baia, Andrzej Szlękd, Wen Lia a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China University of Chinese Academy of Sciences, Beijing 102211, PR China c National Institute of Clean-and-Low-Carbon Energy, Beijing 100049, PR China d Institute of Thermal Technology, Silesian University of Technology, 44-100 Gliwice, Poland b
A R T I C LE I N FO
A B S T R A C T
Keywords: Co-gasification Extract residue of direct coal liquefaction residue Ash fusibility Slag viscosity
Efficient utilization of the extraction residue (ER) of direct coal liquefaction residue is a bottle neck for the efficiency of direct coal liquefaction process. The co-gasification of the ER and coal is a promising way for largescale utilization of ER. Flow properties of the feedstock including ash fusibility and slag viscosity are important parameters for the gasification process. To optimize co-gasification of the ER and coal, the ash fusibility and slag viscosity behavior of ER and coal under gasification conditions were studied. The results show that the ash fusion temperatures (AFTs) of the blending were lowered with the increasing blending ratio of ER due to the high content of calcium and iron in ER. The content of quartz and anorthite in the blended ashes decreased with the increasing ER blending ratio. The slag viscosities at high temperatures also decreased as the blending ratio of ER increased. The high content of calcium and iron in ER resulted in the decrease in the slag polymerization degree because of the break of Si-O structure and transformation from [AlO4]5− to [AlO6]9−. Besides, the slag presented the behavior of a crystalline slag when the ER blending ratio was increased up to 25% for the formation of anorthite during cooling. For the entrained flow gasification, the ER addition can effectively lower the operation temperature of the gasifiers, improve the gasification efficiency and avoid the slag tapping problems. The optimal ER addition should be in the range of 10–20%, and the corresponding tapping temperature was 1258–1575 °C.
1. Introduction Direct coal liquefaction (DCL) is an important process to produce desired chemicals and liquids from coal. The DCL process converts coal into a wide range of products, including gas, liquid and solid. However, a large amount of direct coal liquefaction residue (DCLR) is formed as a by-product [1,2], which includes carbonaceous materials and mineral matter generated from raw coal and catalyst added in the DCL process [3]. China Shenhua Coal to Liquid and Chemical Company Limited has built the world’s largest DCL plant, 1.08 million t/a direct liquefaction coal to oil project, in Ordos, Inner Mongolia. The amount of DCLR is about 0.5–0.7 million t/a and it accounts for about 30 wt% of the raw coal used in the DCL process. The economic performance of DCL is therefore largely dependent on the effective utilized of DCLR [4,5]. At present, the major utilizations of DCLR are used as the feed for
pyrolysis, gasification, combustion, and coking, etc. [6–8]. There are a large number of polycyclic aromatic compounds in DCLR, which are the products of hydro-liquefaction and a series of cycloalkyl and side chains [9]. Compared with coal tar pitch, DCLR is prone to thermal condensation polymerization, and it is a good precursor to prepare mesophase pitch. Therefore, it is an alternative way for further processing coal liquefaction residue to extract heavy oil and asphalt from solvent oil [10,11]. About 50% of extraction residue (ER) will be produced after extracting heavy oil and asphalt [12,13], which mainly consists of the unreacted coal, mineral matters in coal and additional catalyst. As the ER has high carbon content, high calorific value and low water content, it is considered to be a good feedstock for gasification. However, the ER also has high ash and sulfur contents, as well as iron catalyst [14]. Thus, the co-gasification of the ER and coal is deemed as an effective utilization way of the ER.
⁎
Corresponding author. E-mail address: [email protected] (J. Bai). 1 Co-first author. https://doi.org/10.1016/j.fuel.2019.116850 Received 6 November 2019; Received in revised form 4 December 2019; Accepted 7 December 2019 Available online 14 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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(denoted as MC), was selected as the raw coal sample blended with the ER. The chemical compositions of ash were characterized by X-ray fluorescence (XRF) (Bruker S8 Tiger, Germany), as listed in Table 2. In order to obtain the “in-situ” minerals in coals, the low temperature asher (Quorum Technologies, K1050) was used to remove the organic matter in the sample. The experimental process is as follows: weigh about 0.5 g coal (≤75 μm) and put it in the porcelain boat (120 mm × 60 mm × 15 mm). Weigh it every 2 h until the masses difference between two times is less than 0.0010 g.
The ash fusibility and slag viscosity are two important parameters for the gasification [15,16]. Ash fusion temperatures (AFTs) are widely used to evaluate the ash fusibility, which usually act as the first criteria for coal selection and optimization of operation parameters of the gasifiers. For all gasifiers, the ash fusibility is also an important variable [17]. For the fluidized-bed gasifiers, the operating temperature should be lower than the ash fusion temperatures of the feedstocks (900–1050 °C), which can avoid the operational problems such as agglomeration and sintering [18]. For the entrained flow gasifier, the operating temperature is required to be in the range of 100–150 °C higher than the flow temperature (FT) to enable the smooth slag tapping [19]. Moreover, in order to avoid the blockage of slag on furnace, the FT of ash should be lower than 1350 °C for refractory bricks, and lower than 1450 °C for membrane wall [20]. The slag viscosity behavior is another important parameter for the slagging gasifiers, such as Shell, Siemens GSP, BGL, OMB gasifiers [21,22]. Criteria for adequate slag tapping are process specific. For the BGL gasifier, a viscosity of 5 Pa·s at bed temperature is required, whereas for the slag to flow from the entrained flow gasifiers, it is generally accepted that the slag viscosity should be less than 25 Pa·s at the tapping temperature [23]. However, if the slag viscosity was lower than 2 Pa·s, the slag becomes too fluid and potentially cause excessive wear of refractory walls (e.g. GE and ConocoPhillips E-gas technologies) or excessive heat loss through water-cooled walls (e.g. Shell or Siemens technologies) [24]. Therefore, the slag viscosity at slag tapping temperature should ideally be greater than 2 Pa·s and less than 25 Pa·s [25]. The optimal range of the slag viscosities for the operation of the slagging gasifiers is generally considered to be between 5 and 15 Pa·s at the slag tapping temperature [26,27]. However, there is no published work has attempted to characterize the flow properties of ash and slag from the ER and coal. In this work, the ash fusibility and slag viscosity behavior of the ER and coal under gasification conditions were carried out. Two coals and an ER from China Shenhua Coal to Liquid and Chemical Company Limited were used in the experiment. X-Ray diffraction (XRD) and Factsage software were applied to study the mineral transformation of the ash at high temperatures. The slag structures were characterized by Nuclear Magnetic Resonance Spectrometer (NMR) and Fourier Transform Infrared Spectroscopy (FTIR). Solid phases of the slag during cooling were investigated by scanning electron microscope and energy dispersive X-ray spectroscopy (SEM-EDX).
2.2. Ash fusion temperature test The AFTs of blended ash with different ratio of ER were measured under mild reducing atmosphere (CO/CO2 = 3/2, volume fraction) by the Carbolite CAF 1600 ash fusion point meter (Carbolite Gero Limited, Britain) based on the American standard ASTM D1857-04. The pyramid ash cone was heated to 1560 °C at a rate of 8 °C/min. Four characteristic temperatures, deformation temperature (DT), softening temperature (ST), hemisphere temperature (HT), and flow temperature (FT), were determined according to the shape of ash cone. To verify the results of the experiment, the tests were repeated three times, and the results exhibited a good reproducibility under the same conditions. Table 3 gives the AFTs of SM, GL, MC, and ER under mild reducing atmosphere. 2.3. Slag viscosity measurement The viscosity measurements were conducted with a Theta hightemperature rotating viscometer (Theta Industries, Inc., USA) using molybdenum rotors and crucibles. To obtain a homogeneous slag, the ashes were firstly melted in an electric furnace at a temperature which is 150 °C higher than the liquidus temperature under reducing atmosphere simulated by graphite. Then about 50 g pre-melted slag was crushed to less than 2 mm for the viscosity measurement. The start temperature for test should be higher than liquidus temperature to make sure the sample totally melted. The cooling rate was 3 °C/min and the viscosity and temperature were recorded continuously at an interval of 0.1 °C, then a viscosity-temperature curve was obtained. The parameters of the rotor and crucible were calibrated with a standard reference material 717A glass [28]. The tests were also repeated several times and the results showed a good reproducibility under the same conditions.
2. Experimental 2.4. Preparation of quenched slag 2.1. Sample preparation The quenched samples were prepared in a horizontal tube furnace under mild reducing atmosphere as the AFTs test. When the target temperature was reached, the corundum boat containing sample was pulled out quickly and quenched into the cold water [29]. The slags prepared at 1000–1400 °C during heating were used for XRD test, and the slags obtained at 1200 °C during cooling were analyzed by SEM.
Two bituminous coals, Shenmu coal (denoted as SM) and Gaoliu coal (denoted as GL), and an ER from Ordos DCL plant were used in this work. All the samples were crushed and ground to the sizes smaller than 75 μm. The proximate and ultimate analyses of the samples were performed according to GB/T212-2008 and GB/T476-2001, which was listed in Table 1. The coal ash was prepared at 815 °C in a muffle furnace based on GB/T 1574-2007. As the current blending ratio of Shell gasification in China Shenhua Coal to Liquid and Chemical Company Limited was 10:1, the same blending ratio, 10:1 of SM coal and GL coal
2.5. Characterization of quenched slags The mineralogical analysis was conducted by an X-ray powder
Table 1 Proximate and ultimate analyses of SM, GL, and ER (wt%). Sample
SM GL ER
Proximate analysis (wt%, ad)
Ultimate analysis (wt%, daf)
M
A
V
FC
C
H
Oa
N
St
6.31 0.54 1.64
10.02 37.08 37.52
30.75 20.52 21.01
52.92 41.86 39.83
80.97 81.82 88.35
4.60 5.11 3.65
12.92 10.55 0.76
1.02 1.17 0.67
0.49 1.35 6.57
ad: air dry base; a: by difference; M: moisture; A: ash; V: volatile; FC: fixed carbon, C: carbon; H: hydrogen; O: oxygen; N: nitrogen; St: total sulfur. 2
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Table 2 Ash chemical compositions of SM, GL, and ER (wt%). Samples
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
K2O
Na2O
TiO2
P2O5
SM GL ER
50.97 51.33 29.35
17.04 37.17 10.35
7.27 3.04 22.75
13.29 3.29 21.37
1.14 0.68 0.91
3.68 0.49 10.35
0.77 1.11 0.77
1.81 0.96 0.53
1.81 0.29 1.85
0.97 0.33 0.35
(CaSO4). The ER ash was mainly composed of calcite and quartz. For the ash prepared at 815 °C, the minerals of SM coal and ER coal were quartz, anhydrite, lime and hematite, and the main minerals in GL coal were quartz and hematite. It demonstrated that the mineral in the coal and ER reacted as Equation (1)-(2). For the ER, it was rich in calcium and iron and showed strong crystallization tendency [30].
Table 3 AFTs of SM, GL, MC, and ER. Samples
DT
ST
HT
FT
SM GL MC ER
1131 > 1550 1289 1110
1145 > 1550 1309 1124
1162 > 1550 1317 1129
1202 > 1550 1342 1137
2[Al2Si2 O5 (OH)4] → 2(Al2O3 ·2SiO2) → Al2O3 (γ ) + 3Al2O3 ·2SiO2 (mullite)
diffract meter (BRUKER D2, Germany), which equipped with Cu Kα radiation (40 kV, 100 mA). The powder samples were mounted on a sample glass and scanned with a step size of 0.02° over a 2θ range of 5–90°. FTIR spectra were recorded with a Bruker Vertex 70 Fourier transform infrared spectrometer (Bruker, Germany). KBr DRIFTs technique was used in the experiment. The powder sample was accurately weighted and mixed with KBr with the ratio of 1:200. The mixture was finely grinded for 30 min, and then pressed into a thin tablet under 12 MPa. The spectra were recorded over the range of 4000–400 cm−1 with a resolution of 0.4 cm−1. The aluminum nuclei were obtained by a Bruker Avance III 600 MHz Wide Bore spectrometer, which was supplied by Kratos Analytical Inc. (Spring Valley, NY), which has a 14.1 T magnetic field and a 4 mm CPMAS probe under 13 kHz MAS speed in zirconia rotors. The radio frequency field strength was verified by a 1.0 M aqueous AlCl3 solution and the chemical shift of 27Al signal were referenced to the AlCl3 powder signal. All samples were measured in powdered form. The microstructural analysis was determined by a JSM-7001F scanning electron microscope (SEM). Before SEM observations, the bulk slags were mounted in liquid epoxy resin, pelletized, polished, dried and finally coated with Au vapor to make them conductive. The slag was observed in back-scattering mode (BSE) at 15 kV, and Energy Dispersive X-ray (EDX) Spectroscopy was performed to analyze the chemical compositions of the crystals in the slag.
CaCO3 → CaO + CO2
(1) (2)
3.2. AFTs of the blended ash As shown in Fig. 2, the AFTs decreased with the increasing blended ratio of ER. When the blended ratio of ER was less than 10%, the AFTs decreased linearly with the increasing ER blended ratio. However, they decreased slowly when the blended ratio of ER was less than 10%. For example, when the blended ratio of ER was 0%, DT, ST, HT and FT were 1254 °C, 1273 °C, 1285 °C and 1316 °C, and they dropped to 1160 °C, 1182 °C, 1200 °C and 1253 °C as the ER blended ratio increased to 10%. When the blended ratio of ER was 25%, DT, ST, HT and FT were 1111 °C, 1123 °C, 1137 °C and 1170 °C, respectively. It illustrated that the addition of ER was favorable for the decrease in AFTs, and the influence of the ER on the AFTs was weakened as the ER blended ratio was higher than 10%. 3.3. Effect of ER on mineral evolution As shown in Fig. 3, the main minerals in MC at 1000 °C were quartz and anorthite. The minerals in SM coal ash, such as anhydrite, lime and hematite etc., can react with quartz and mullite, which are mainly refractory minerals in GL coal ash, to form the low fusion temperature minerals anorthite at above 1000 °C. When the temperature was 1100 °C, anorthite was the dominant phase in blended ash. However, the intensity of anorthite decreased above 1200 °C due to the partial dissolution of the phase assemblage. As the temperature increased to 1300 °C, the quartz no longer existed. Above 1300 °C, the blended ash was completely molten. The relative contents of minerals in the blended ashes were determined by the peak intensity of XRD [31–33], as shown in Fig. 4. The main minerals and the minerals transformation in the range of 1000–1400 °C were similar, while the content of quartz and anorthite at the same temperature was decreased due to the addition of the ER. With the increasing ER blended ratio, the content of quartz and anorthite decreased. When the temperature was 1200 °C, the intensity of quartz decreased from 2163 to 1344 with the increasing of ER mass fraction, and the intensity of anorthite decreased from 1917 to 1259. As the temperature was above 1100 °C, the quartz disappeared and the primary mineral was anorthite. The decrease in quartz and anorthite with the addition of the ER explained why the AFT of the blended ash lowered [31]. Fig. 5 showed the ternary phase diagram of SiO2-Al2O3-Fe2O3-CaO system, and the color lines indicate the liquidus temperatures (Tliq) at which the last solid phase disappeared. Because the range of Si/Al ratio (SiO2/Al2O3 weight ratio) in the mixed ash is in the range of 2.3–2.5, Si/Al ratio of 2.5 was selected to show the effect of ER addition on
2.6. Thermodynamic equilibrium calculation Factsage software packages 7.2 was employed to calculate the multiphase equilibration and phase diagram of SiO2-Al2O3-CaO-Fe2O3FeO system under a weak reducing atmosphere at 1 atm. In this work, the Phase Diagram module together along with FactPS and FToxid databases was applied for calculation. According to the oxygen partial pressure of the experimental atmosphere (CO/CO2 = 3/2, volume fraction), the ratio of Fe2+/Fe3+ is 8:2 in the database. In order to eliminate the influence of atmosphere on iron valence, the Fe2O3 replaced by FeO8.89Fe2O3 (Fe2+/Fe3+ = 8/2, mass ratio) was inputted in phase diagram. 3. Results and discussion 3.1. Sample analysis The minerals composition of the ashes from SM coal, GL coal and ER ashed at 150 °C and 815 °C are shown in Fig. 1. For the SM coal ash, the minerals at 150 °C were calcite, quartz, and a small amount of kaolinite. For the GL coal, they were calcite, quartz, and kaolinite, and the main minerals in MC were quartz (SiO2), hematite (Fe2O3) and anhydrite 3
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In te n s ity (a .u .)
600 0 1200
(a)
600
SM Coal
1
400
3 2
31
1
1 1 1
11
11
2
GL Coal 1
600
2
0
1 3 2
21 1
ER
1
400
3 2
3
0 20
5 3
1 1 1
40
11
30000
5 43 5
6 54
5
200
4
GL Coal
3
400 4
3
4 4
0 400
3
ER
5 4 3 3 5
0
60
5 5 3
0
1
2θ (°)
6
5 6 64 4
3 54
200 1
1
SM Coal
(b)
200
In te n s ity (a .u .)
1200
4
5
4
5
6
20
80
5
40
3
6 4 5 5 5
2θ (°)
5 4
60
80
MC
(c)
3
Intensity (a.u.)
25000 20000 15000 10000 3
5000
5 6
54
5 4 43
5
5 3
3 6
3
3
0 10
20
30
40
50
2θ (°)
60
70
80
90
Fig. 1. XRD patterns of the ashes from SM, GL and ER. (a) 150 °C; (b) 815 °C; (c) ash of MC prepared at 815 °C. 1-calcite (CaCO3); 2-kaolinite (Al2Si2O5(OH)4); 3quartz (SiO2); 4-hematite (Fe2O3); 5-anhydrite (CaSO4); 6-lime (CaO).
Fusion Temperature (°C)
1350
phase diagram can be used to predict the effect of ER addition on the AFTs [34].
DT ST HT FT
1300
3.4. Slag viscosity behavior
1250
The viscosity temperature curves of the slags with different blended ratio of ER were shown in Fig. 6. As the ER blended ratio increased, the slag viscosity of the fully molten slags at the same temperature decreased monotonously. For example, the slag viscosities were 1.27 Pa·s, 1.70 Pa·s, 3.26 Pa·s, 4.21 Pa·s, 8.17 Pa·s and 13.26 Pa·s at 1500 °C when the ER blended ratio was 0%, 5%, 10%, 15%, 20% and 25%, respectively. The dote line at 25 Pa·s is the upper viscosity limit for slag tapping. It is noticed that the temperature at 25 Pa·s was lowered with the addition of the ER. The viscosity-temperature curves also indicated the effect of ER on slag viscosity behavior. When the ER blended ratio was between 0% and 20%, the slags exhibited the behavior of a glassy slag, of which the viscosity gradually decreased with the decreasing temperature. However, as the ER blended ratio increased up to 25%, the slag presented the behavior of a crystalline slag with a temperature of critical viscosity (TCV) of 1257 °C.
1200 1150 1100 0
5
10
15
20
M ass fraction of ER (wt% )
25
Fig. 2. AFTs of blended ashes.
primary phase of the mixed ash. It is can be seen that the ash composition of MC was located in the anorthite primary phase. With the increasing ER blended ratio, the bulk ash chemical composition of the blended ash was still located in the anorthite primary phase. The Tliq were 1427 °C, 1424 °C, 1419 °C, 1405 °C, 1388 °C and 1367 °C as the ER blended ratio was 0%-25% and the Tliq lowered with the increasing of ER content. As the liquidus temperatures were similar to the AFTs, the
3.5. Effect of ER addition on slag structure at high temperature 3.5.1. Effect of ER addition on Si-O structure The FTIR results of the slags with different blended ratio of ER and a typical fitted FTIR spectra were shown in Fig. 7(a) and (b). The FTIR 4
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Fig. 3. Mineral compositions of the blended ashes. 1-SiO2 (quartz); 2-CaAl2Si2O8 (anorthite).
(Q0 + Q1)/(Q2 + Q3 + Q4) (denoted as R) was calculated. The higher R implied a lower polymerization degree. As shown in Fig. 8, the R value increased from 0.85 to 3.57 with the increasing ER blended ratio. In addition, R increased rapidly when the ER blended ratio was 25%. This demonstrated that the polymerization degree significantly decreased with the increasing ER blended ratio.
spectra of silicate slags were typically focused within the wavenumber region between 1200 cm−1 and 800 cm−1, which was related to the [SiO4] tetrahedra symmetric stretching vibration [35]. Q-species was introduced to distinguish the characteristic stretching vibrations of SiO4 tetrahedra, and which indicates the slag polymerization degree. The index n represents the number of Si-O-Si bridging oxygen, and the more polymerized the melt, the more the Qn with higher n. The values of n assigned from 0 to 4 are structurally defined as monomers (880–850 cm−1), dimers (920–900 cm−1), chains (1000–950 cm−1), sheets (1100–1050 cm−1) and three-dimensional network (1200–1060 cm−1) [36]. With ER content increasing, the peak shifted towards low wavenumber, which indicated that the slag polymerization degree decreased due to the addition of ER. According to the results from Fig. 8, the content of Q1 and Q2 increased slightly with the increasing ER blended ratio when the ratio was lower than 20%. As the ER blended ratio was higher than 20%, the Q1 and Q2 species increased obviously and Q4 disappeared. In order to characterize the vibration of SiO4 tetrahedra, the area percentage of 5000
Quartz
3.5.2. Effect of ER addition on Al-O structure The influence of ER addition on Al-O structure at high temperatures was studied by 27Al-NMR spectra. Generally, two kinds of coordinate can be classified: six-coordinate Al ([AlO6]9−) and four-coordinated Al ([AlO4]5−). Al3+ in [AlO6]9− unit act as a network modifier, which can weaken the slag polymerization degree, while Al3+ in [AlO6]9− act as a network former to enhance the slag polymerization degree. Therefore, [AlO6]9− unit was prone to decrease the viscosity and its resonance centered in 10–20 ppm. [AlO4]5− species tend to increase the viscosity and the resonance at 50–100 ppm [37]. A typical deconvolution of 27Al NMR spectra and the relative
2100
1100°C
1100°C 1200°C
Intensity (a.u.)
4000
Intensity (a.u.)
1000°C
Anorthite
1000°C
3000
2000
1800
1300°C 1400°C
1500 1200 900
1000 0
5
10
15
20
M ass fraction of ER (wt% )
600
25
0
5
Fig. 4. Quartz and anorthite content of blended ashes. 5
10
15
20
M ass fraction of ER (wt% )
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Fig. 5. Phase diagram of SiO2-Al2O3-Fe2O3-CaO system with SiO2/Al2O3 weight ratio of 2.5.
content of [AlO4]5− was presented in Fig. 9. [AlO4]5− was the main AlO structure, and the content of [AlO4]5− were all above 90% in the range of 0%–25% ER content. With the increasing of ER blended ratio, the [AlO4]5− content decreased, while the variation was slight. For the slags with 0%–20% ER, the fractions of [AlO4]5− were 99.32%, 97.83%, 95.70%, 94.58% and 94.32%, and it decreased to 92.41% as the ER blended ratio was 25%. This indicated that Al3+ transformed from [AlO4]5− into [AlO6]9− due to the ER addition, which results in the decrease in slag viscosity.
the glassy behavior [38]. When the ER blended ratio increased to 25%, a crystalline phase, anorthite, was detected. The formation of crystalline phases was usually attributed to the formation of the crystalline slag [39]. Therefore, the slag with 25% ER showed the behavior of the crystalline slag, which was mainly due to the formation of anorthite. 3.7. Determination of optimal addition ratio of ER In Fig. 12, T25 and T2 are the temperature at which the slag viscosities were 25 Pa·s and 2 Pa·s, respectively, which were the slag tapping temperature of the entrained flow gasifiers. The area between two lines represented the tapping temperature range at certain ER addition ratio, which can be used to determine the tapping temperature with the ER addition. The tapping temperature range increased with the increasing ER blended ratio. For the slag of MC, the tapping temperature
3.6. Effect of ER addition on solid phases during cooling The XRD and SEM-EDS results of quenched slags at 1200 °C are shown in Figs. 10 and 11. The slags with 0%-20% ER addition have mainly an amorphous/vitreous character. Therefore, the slags exhibited
25
(a)
MC 5% ER 10% ER 15% ER 20% ER 25% ER
75
MC 5% ER 10% ER 15% ER 20% ER 25% ER
20
Viscosity (Pa·s)
Viscosity (Pa·s)
100
(b)
50
25
15
10
5 0 1100
1200
1300
1400
Temperature ( °C)
1500
0 1400
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Fig. 6. Viscosity-temperature curves of the slags with different ER blended ratios. 6
1550
Temperature ( ° C)
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0.16
(a)
(b)
0.12
Absorbance
25% ER 20% ER 15% ER 10% ER
Q 2 (939) Q 3 (1017)
0.08
Q 1 (903)
0.04
Q 4 (1130)
Q 0 (855)
5% ER
0.00
MC 750
900
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1350
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W avenumber (cm -1)
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1000
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W avenumber (cm -1)
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Fig. 7. (a) FTIR spectra of slags; (b) Typical fitted FTIR spectra.
Q0
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Q1
Q2
Q3
Q4
3
60 2 40 1
20
-5
0
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Q 0 +Q 1 +Q 2 )/(Q 3 +Q 4 )
Area ratio (%)
80
0
operating temperature and improve the efficiency of the gasification [40]. Therefore, the temperature at which the slag viscosity was 25 Pa·s was usually less than 1350 °C. The purple solid line was drawn at 1350 °C. It can be seen that for the coal without ER, the operating temperature will be too high. The ER addition can significantly lower the tapping temperature, and energy consumption. However, when the ER addition was 25%, the slag exhibited the behavior of the crystalline slag which is not favorable for the slag tapping. Thus, based on the slag viscosity temperature characteristics of the coal with ER, the ER blended ratio should be 10%-20%, and the corresponding slag tapping temperature range was 1258–1575 °C, as shown the areas with slash character. This can guarantee a low operating temperature, and avoid the slag tapping problem of the entrained flow gasifiers.
4
4. Conclusions
0 30
25
M ass fraction of ER (wt% )
In this work, we investigated the ash fusibility and slag viscosity of the blended coal with extract residue of direct coal liquefaction residue. The mineral transformation behavior and influence of ER addition on slag structure and solid phase during cooling were discussed in detail. The main conclusions were summarized as following:
n
Fig. 8. Contents of Q units based on deconvolution results.
range was 1443–1600 °C, and the width of temperature window was 157 °C. When the ER addition was 10%, the range was from 1575 °C to 1336 °C, and the width of temperature window was increased to 240 °C. For the entrained flow gasifiers, the operating temperature is 100–150 °C higher than the slag tapping temperature to lower the
(1) The main minerals in ER were CaCO3 and SiO2 and kaolinite. ER has high ash content, and the ash was rich in calcium oxide and iron
100
5% ER 10% ER 15% ER 20% ER
[AlO 4]5-
MC
content (%)
(a)
(b)
98
96
94
92
25% ER -20
0
20 27
40
60
80
100
90
120
Fig. 9. (a) A typical deconvolution of
27
0
5
10
15
20
M ass fraction of ER (wt% )
Al chem ical shift (cm -1 )
Al NMR spectra; (b) [AlO4]5− content as a function of ER content. 7
25
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1
25000
20% ER
10% ER
20000
Intensity (a.u.)
15% ER
2θ (°)
60
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1 1 11 1 1 11
1
0
MC 40
15000
5000
5% ER
20
25% ER
20
80
40
2θ (°)
60
80
Fig. 10. XRD patterns of the slags quenched at 1200 °C (1-anorthite).
transformation of [AlO4]5− to [AlO6]9−. As the extraction residue blended ratio was increased to 25%, the slag showed the behavior of a crystalline slag with TCV of 1257 °C due to the crystallization of anorthite during cooling. (4) From the present work, the addition of ER can lower the operation temperature of the gasifiers, improve the gasification efficiency and avoid slag tapping problems of the entrained flow gasifiers. The optimal blended ratio of extraction residue in co-gasification should be 10–20%, and the corresponding tapping temperature range was 1258–1575 °C.
oxide, which indicates ER has strong crystallization tendency, so ER was not the suitable feedstock for entrained flow gasifier alone. (2) The addition of extraction residue decreased the AFTs of the blended ashes. Quartz and anorthite were the minerals in the blended ashes at high temperatures, while their content lowered by the addition of extraction residue, which was the main reason for the decrease in the AFTs. (3) The slag viscosities at high temperatures were also decreased by the addition of extraction residue. The high content of calcium and iron in extraction residue resulted in the break of Si-O structure and
Fig. 11. SEM-EDS results of slag quenched at 1200 °C (♥-Anorthite). 8
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1650 1600
Tem perature ( ° C )
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1575 o C
1550
1482 o C
1500
T2
T 25
1450 1400 1350 1300
1336 o C
1250 1200
1258 o C 0
5
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Fig. 12. Slag tapping temperature at different ratio of ER.
CRediT authorship contribution statement Xi Cao: Writing - original draft, Data curation. Baozi Peng: Conceptualization, Resources. Lingxue Kong: Writing - review & editing. Jin Bai: Supervision. Zefeng Ge: Data curation. Huaizhu Li: Formal analysis. Zhen Liu: Project administration. Ziyang. Feng: Resources. Dapeng Bi: Investigation. Zongqing Bai: Methodology. Andrzej Szlęk: Methodology. Wen Li: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Key R&D program of China [grant numbers 2017YFB0602603], Science and Technology Program of China and Beijing [grant number Z181100005118006], Science and technology innovation project of CHN Energy [grant number GJNY-18-72], Joint Foundation of Natural Science Foundation of China and Xinjiang [grant number U1703252], Joint Foundation of Natural Science Foundation of China and Shanxi Province [grant number U1510201], NSFC-DFG [grant number 21761132032], Youth Innovation Promotion Association, CAS, International Partnership Program of Chinese Academy of Sciences [grant number 122214KYSB20170020]. References [1] Wang Y, Niu Z, Shen J, Bai L, et al. Extraction of direct coal liquefaction residue using dipropylamine as a CO2-triggered switchable solvent. Fuel Process Technol 2017;159:27–30. [2] Yang J, Wang Z, Liu Z, Zhang Y. Novel use of residue from direct coal liquefaction process. Energy Fuels 2009;23(10):4717–22. [3] Cui H, Yang J, Liu Z, Bi J. Characteristics of residues from thermal and catalytic coal hydroliquefaction. Fuel 2003;82(12):1549–56. [4] Lv D, Bai Z, Yuchi W, Bai J, et al. Properties of direct coal liquefaction residue water slurry: effect of treatment by low temperature pyrolysis. Fuel 2016;179:135–40. [5] Zhou Y, Xiao N, Qiu J, Sun Y, et al. Preparation of carbon microfibers from coal liquefaction residue. Fuel 2008;87(15–16):3474–6. [6] Wen H, Kong L, Bai J, Bai Z, et al. Transformation of minerals in direct coal liquefaction residue under gasification atmosphere at high temperatures. J Fuel Chem Technol 2015;43(3):257–65.
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