Fe2O3 ratio on slag viscosity behavior under entrained flow gasification conditions

Fuel 258 (2019) 116129

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Full Length Article

Effect of CaO/Fe2O3 ratio on slag viscosity behavior under entrained flow gasification conditions

T

Zefeng Gea,b, Lingxue Konga,c, Jin Baia, , Huiling Zhaoa, Xi Caoa,b, Huaizhu Lia, Zongqing Baia, Bernd Meyerd, Stefan Guhld, Ping Lic, 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 100049, PR China c State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, PR China d Institute of Energy Process Engineering and Chemical Engineering, Technische Universität Bergakademie Freiberg, Freiberg 09599, Germany b

ARTICLE INFO

ABSTRACT

Keywords: Entrained flow gasification Slag viscosity behavior CaO/Fe2O3 ratio Slag structure Crystallization behavior

The key factor for smooth slag tapping of entrained flow gasifiers was slag viscosity behavior. Shanxi coals can’t meet the requirement for discharging slag due to its high SiO2 + Al2O3 contents. The single fluxing agent (CaO or Fe2O3) was also inoperative for Shanxi coals. Hence, the development of binary composite flux was urgent and necessary. The effect of CaO-Fe2O3 binary composite flux on slag viscosity temperature behavior was investigated in this work. As the CaO/Fe2O3 ratio decreased, the slag viscosity at same temperatures decreased. Furthermore, a linear relationship between the slag viscosity and CaO/Fe2O3 ratio was established at high temperatures. Characterization of slag structure showed that the polymerization degree decreased with the decreasing CaO/Fe2O3 ratio. The calculated BO/(BO + NBO) (fraction of bridging oxygen) also presented an excellent linear relationship with CaO/Fe2O3 ratio. This indicated that CaO/Fe2O3 ratio affected the slag structure, leading to the variation of slag viscosity. During the cooling step, the crystallization activation energy Ec was used to describe the effect of CaO/Fe2O3 ratio on slag crystallization behavior quantitatively. Besides, the growth pattern of crystal phases transformed from the surface crystallization to the bulk crystallization with the decreasing CaO/Fe2O3 ratio, and the bulk crystallization was more easily to promote the crystallization behavior. In brief, slag viscosity and crystallization behavior could be modified via adjusting CaO/Fe2O3 ratio. These results can provide a guide for using CaO-Fe2O3 binary composite flux to improve the slag viscosity behavior in entrained flow gasification.

1. Introduction China has a huge demand for coal, which accounts for around 70% of primary energy supply for many decades [1]. The efficient and clean utilization of coal resource is increasingly urgent for economic growth. Coal gasification has become the dominant clean coal utilization technology to generate green energy and promote coal chemical process [2]. Many high value-added and eco-friendly chemical products could be produced from gasified syngas [3]. The entrained flow gasification is the first choice for coal gasification due to its high throughput, high carbon conversion and fuel feedstock flexibility [4]. In the entrained flow gasifiers, a high temperature which is usually higher than 1300 °C is employed to improve coal conversion [5]. Mineral matters in coal are completely melted at high temperatures and

⁎

flow down along the inner wall by gravity, and then drop into a water quenching system [6]. The continuous and smooth slag tapping is crucial for the long-term operation of the entrained flow gasifiers, which is dependent on slag viscosity behavior [7]. Slag viscosity behavior is the relationship between slag viscosity value and temperature. There are three aspects used to characterize slag viscosity behavior: the viscosity dependence on temperature, the temperature of critical viscosity (TCV) and the pattern of viscosity temperature curve [8]. For Shell gasifier, the slag viscosity is required seriously between 2.5 Pa·s and 25 Pa·s at slag tapping temperature [9]. It is generally accepted that TCV is recognized as the sharp break in the viscosity temperature curve, which reflects the division that whether the slag viscosity is affected by the presence of crystal phase. The fluctuation in temperature around TCV can cause the accumulation of

Corresponding author. E-mail address: [email protected] (J. Bai).

https://doi.org/10.1016/j.fuel.2019.116129 Received 4 August 2019; Received in revised form 23 August 2019; Accepted 30 August 2019 Available online 10 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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ash slag inside the gasifier, resulting in slag blockage [10]. Therefore, for entrained flow gasifiers, the TCV should be below the slag tapping temperature to avoid slag tapping problems. Besides, the crystalline slag of which a sharp increase in the slag viscosity is observed when the temperature is lower than TCV is normally unfavorable for the entrained flow gasifiers [11]. The slag viscosity behavior is influenced mainly by the chemical compositions of the slag. SiO2 and TiO2, are network formers which are prone to form the polymers, and increase the viscosity. CaO, Na2O, K2O, and MgO can be classified as network modifiers which tend to terminate the formation of the polymers, and therefore the addition of these oxides usually leads to a decrease in viscosity. Al2O3 and Fe2O3 are amphoteric, which can be served as both network formers and network modifiers depending on the overall composition of the melt [12,13]. For most of coals in Shanxi Province, China, the viscosity behavior of the coal ash slag can’t meet the requirement of slag tapping due to high SiO2 + Al2O3 content (> 80%, weight percentage) and low SiO2/ Al2O3 ratio (< 2.0, weight ratio). To improve the slag viscosity behavior that can be processed in entrained flow gasifiers, a common practice is to blend the coals with flux to produce a feedstock with appropriate slag viscosity behavior [14]. Previous studies demonstrated that the flux, such as CaO or Fe2O3, seemed to be necessary and effective for the coal ashes with high SiO2 + Al2O3 content [15,16]. Kong et al. [16] found that the increasing addition of CaO can decrease the SiO-Si content above the liquidus temperature, leading to a low viscosity value. Xuan et al. [17] illustrated that the increase in Fe2O3 content tended to decrease slag viscosity, while a high Fe2O3 content could enhance the crystallization behavior of ash slag, leading to a higher TCV. Although the viscosity of the coal slag can be lowered by a high addition of CaO and Fe2O3, CaO or Fe2O3 cannot be used to improve the slag viscosity behavior of Shanxi coals directly. Li et al. [18] reported that the high iron content of Fe2O3 in the ash would result in the formation of metallic iron in slag. The metallic iron in the slag can induce the rapid crystallization of minerals, which led to the sharp increase of viscosity, and the serious iron agglomeration in the slag. Kong et al. [19] further studied the effect of CaO on the pattern of viscosity temperature curve. They found that the slag tended to behave as a crystalline slag at high CaO content, and the CaO content at which the slag exhibited as the crystalline slag decreased with the increasing SiO2/ Al2O3 ratio. The slag with Si/Al = 1.68 behaved as the crystalline slag when the content of CaO was 14.25 wt%, and the TCV was 1375 °C. Wang et al. [20] founded that the effect of CaO-Fe2O3 binary composite flux on ash fusion behavior of the coals was superior to CaO or Fe2O3 single flux. It implied that the flux with CaO and Fe2O3 can be used to improve the slag viscosity behavior. However, the effect of CaO/Fe2O3 ratio on the slag viscosity behavior was not reported so far. In this work, the slags with same CaO + Fe2O3 content, different CaO/Fe2O3 ratio were prepared, and the effect of CaO/Fe2O3 ratio on the slag viscosity behavior was investigated. Structure of molten slags was studied by solid-state nuclear magnetic resonance (SS-NMR) and Raman. Differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Scanning electron microscope-Energy dispersive spectrometer (SEM-EDS) were used to study crystallization behavior of the slags and mineral compositions.

Table 1 Chemical compositions of the synthetic ash samples. Sample

%

SiO2

Al2O3

CaO

Fe2O3

Na2O

CaO/Fe2O3

Tliq (°C)

A-10C0F*

mass mole mass mole mass mole mass mole mass mole mass mole

42.00 50.77 42.00 51.20 42.00 52.17 42.00 52.07 42.00 52.52 42.00 52.98

28.00 29.86 28.00 30.12 28.00 30.69 28.00 30.63 28.00 30.90 28.00 31.17

29.00 18.78 23.20 15.15 17.40 11.58 11.60 7.70 5.80 3.89 0.00 0.00

0.00 0.00 5.80 2.95 11.60 4.97 17.40 8.99 23.20 12.09 29.00 15.24

1.00 0.58 1.00 0.59 1.00 0.60 1.00 0.60 1.00 0.61 1.00 0.61

10:0

1448

8:2

1459

6:4

1448

4:6

1400

2:8

1419

0:10

1491

A-8C2F** A-6C4F A-4C6F A-2C8F A-0C10F

*: A-10C0F: 29% (mass fraction) CaO in the synthetic ash without Fe2O3. **: A-8C2F: mass ratio of CaO and Fe2O3 is 8:2.

of the synthetic ash samples in mass and mole fraction are shown in Table 1, and the samples were denoted as A-10C0F, A-8C2F, A-6C4F, A4C6F, A-2C8F and A-0C10F. The ashes had same Si/Al ratio (mass ratio Si + Al content (total content of of SiO2/Al2O3 = 1.5), SiO2 + Al2O3 = 70%), and CaO + Fe2O3 content (total content of CaO + Fe2O3 = 29%), while CaO/Fe2O3 ratio (mass ratio of CaO/ Fe2O3) varied from 10:0 to 0:10. The liquidus temperature (Tliq) at which the last solid phase disappeared was calculated by FactSage, and also given in Table 1. 2.2. Viscosity test The synthetic ashes for viscosity measurement were firstly premelted in an electric furnace. About 100–110 g ash sample was put into a corundum crucible and heated to 150 °C higher than the Tliq, and hold for 30 min to reach the equilibrium. Graphite was used to simulate a reducing atmosphere. After that, the pre-melting slag was crushed to 2 mm. A high temperature rotational viscometer (Theta Corporation, USA) was used to measure the slag viscosity at high temperatures. A corundum crucible with approximately 50 g pre-melted slag was fixed in the center of the sample holder. The apparatus was evacuated for 10 min. A mixture of CO/CO2 (60/40, volume fraction) was injected from the top of the protect tube at 300 ml/minute. The furnace was gradually heated to 1200 °C at 10 °C/min and then to the temperature 150 °C higher than the liquidus at 5 °C/min. The temperature was hold for 30 min to obtain the fully molten slag. The rotor was lowered to find the top of the melt with a constant shear rate of 2.5 s−1, and then immerged into the liquid slag. The viscosity was measured under a constant shear rate of 2.5 s−1 with a cooling rate of 3 °C/min, which was recorded at the interval of 0.1 °C. The parameters of the rotor and crucible for rheology were determined by the calibration with standard reference material 717A glass at high temperatures [21]. 2.3. Preparation of quenched slag Quenching experiment was processed in a horizontal electric tube under the same reducing atmosphere with viscosity test (CO/ CO2 = 60:40, volume fraction). A corundum boat with 1 g pre-melting slag was pushed into the high temperature zone of the horizontal tube. The furnace was heated to 1200 °C at 10 °C/min, and then to 1550 °C at 5 °C/min and hold for 30 min to ensure that the sample was fully molten. After that, the temperature was decreased with a cooling rate of 3 °C/min. When the temperature reached to target temperature, the sample was pulled out quickly and quenched into an ice water. The sample was dried and crushed to analyze mineral composition, morphology and slag structure.

2. Experimental 2.1. Synthetic ash samples In this work, the synthetic ashes were prepared to simplify the system and to eliminate the effect of the trace elements. Because the oxides of SiO2, Al2O3, Fe2O3, CaO and Na2O accounts for more than 90% (mass fraction), the synthetic ash samples were prepared by analytic reagents SiO2, Al2O3, Fe2O3, CaO and Na2CO3. These five reagents were grounded to finer than 75 μm after heated at 815 °C, and then thoroughly mixed for 30 min in a ball milling. Chemical compositions 2

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2.4. Characterization of slag structure

Kissinger equation is obtained via taking the derivative of Eq. (4) with respect to time at the first peak temperature of DSC curve (Tp), then taking the logarithmic form [25].

Quenched slag at 1525 °C which is above Tliq was used to characterize slag structure at high temperatures. A Horiba LabRAM HR800 Raman spectrometer coupled with a CCD detector was applied to characterize the coordination of silicon atoms. The sample excitation was preceded under Ar+ laser with 532 nm line. The range was between 100 and 2200 cm−1, and the quantitative results were acquired from 600 cm−1 to 1400 cm−1 by the mathematical method of statistical curve-fitting. 27 Al-Nuclear magnetic resonance (NMR) spectra was carried out to study the Al coordination of the slag samples. The data was acquired by a Bruker Avance III 600 MHz Wide Bore spectrometer, which has a 14.1 T magnetic field and a 4 mm CPMAS probe under 13 kHz MAS speed in zirconia rotors. The radiofrequency field strength and the chemical shift of 27Al signal were verified and referenced by using a 1.0 M aqueous AlCl3 solution and AlCl3 powder respectively.

ln

where

Ec KR + ln 0 RTp Ec

Tp2

n=

2.5RTp2 Tf × Ec

(6)

3.1. High-temperature viscosity behavior From Fig. 2(a), as the CaO/Fe2O3 ratio decreased, the slag viscosity value at same temperatures decreased, and the difference of the slag viscosity value was enlarged at low temperatures (Fig. 2(b)). For example, when the temperature was 1500 °C, the viscosity value was 1.27, 1.1, 0.91, 0.79, 0.66, and 0.59 Pa·s, respectively for the sample from A10C0F to A-0C10F. This was because that Fe2O3 had a higher effect on decreasing the slag viscosity compared with CaO for its low ionic potential. The difference of viscosity value at 1600 °C was 0.39 Pa·s between with A-10C0F and A-0C10F, while it increased to 0.68 Pa·s at 1500 °C. This demonstrated that the decreasing CaO/Fe2O3 ratio was prone to decrease the slag viscosity, which was benefit to the slag tapping of the entrained flow gasifiers. Besides, the A-10C0F behaved as a glassy slag, and the slag viscosity showed a gradually increase as the temperature decreased. However, other samples exhibited the behavior of a crystalline slag, of which the slag had a rapid increase in slag viscosity when the temperature was

(1)

where X stands for the volume fraction of crystallized phase at a constant temperature; t means the reaction time; m is the reaction order; K is effective overall reaction rate, which can be obtained by Arrhenius equation:

Tp

DSC / (W/g)

×

(2)

where K0 is the frequency factor; Ec is activation energy of crystallization; R is gas constant; T is temperature. Differentiating Eq. (1), then substituting Eq. (1) into the differential result gives Eq. (3). m -1 m

Tf

(3)

The Kissinger method assumes crystallization as a first-order reaction, which means that the index m = 1 [24]. Hence, Eq. (3) combined with Eq. (2) can be simplified as Eq. (4).

E dX = K 0(1 - X )exp - c dt RT

according to Eq. (5). Ec is the

3. Results and discussion

Crystallization behavior of the slags with different CaO/Fe2O3 ratio was studied by a simultaneous thermal analyzer (STA 449 F3, NETZSCH, Germany). A sample of approximately 25 mg was placed in a Pt crucible with an empty crucible as reference under the same atmosphere of the viscosity measurement. Ash samples were heated to 1200 °C at 10 °C/min, then to 1500 °C at 5 °C/min and kept for 20 min. The samples were also cooled to 1000 °C under cooling rates of 3, 5 and 10 °C/min to study cstallization kinetics. The crystallization activation energy was calculated by Kissinger method, which can be deduced by classical Johnson-Mehl-Avrami (JMA) theory [22,23]:

dX = nK (1 - X ) ln(1 - X ) - 1 dt

1 RTp

where Tf stands for the full width at half maximum of the first exothermic peak. The definition of Tp and Tf is depicted in Fig. 1.

2.6. Crystallization analysis of the slags

Ec RT

versus

slope of the straight line. There are two typical crystallization growth pattern: surface crystallization and bulk crystallization [26]. The Avrami parameter, n, is defined as the crystallization index, which is widely used to characterize the mechanism of crystal growth. When n is 0 to 3, it is surface crystallization; while n is 3 to 4, it is bulk crystallization [27]. Generally, the high n value is favor of the crystallization growth. The Arvami parameter could be calculated by Augis-Bennett method [28].

The quenched slag prepared at the temperature lower than the TCV was ground to < 75 μm, which was used to study mineral composition and morphology of crystal phases. A PANalytical X’pert3 powder diffractometer with Cu Kα radiation (40 kV, 40 mA) was introduced to investigate the mineral phase of the slag samples during cooling. The range of 2θ was scanned from 5° to 90° at 4°/min at the interval of 0.02°. The solid phases in the slags were analyzed by a JSM-7001F scanning electron microscope (SEM) under black-scattering mode (BSE, 15 kV, 10 mA). Besides, the chemical compositions of the solid phases were characterized by energy dispersive spectrometer (EDS).

K = K 0exp -

(5)

represents the cooling rate of DSC test. A straight line can be

obtained by plotting ln

2.5. Characterization of minerals and solid phases in slags

X = 1 - exp [ - Kt m]

= -

Tp2

1400

1300

1200

1100

Temperature / oC

(4)

Fig. 1. The definition of Tp and Tf . 3

1000

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200

(b)

A-10C0F A-8C2F A-6C4F A-4C6F A-2C8F A-0C10F

2.0

Viscosity / Pa·s

160

Viscosity / Pa·s

2.5

A-10C0F A-8C2F A-6C4F A-4C6F A-2C8F A-0C10F

(a)

120

80

1.5

1.0

40

0 1200

Tcv 1250

0.5

Magnify in (b) 1300

1350

1400

1450

1500

1550

1450

1470

Temperature / oC

1450

1490

1510

1530

1550

Temperature / oC

(c)

Tcv / oC

1400

1350

1300

1250

1200 10:0

8:2

6:4

4:6

2:8

0:10

CaO/Fe2O3 Fig. 2. (a) Viscosity-temperature curves of the ash slags with different CaO/Fe2O3 ratio; (b) Slag viscosity-temperature curves at high temperatures; (c) TCV of the ash slags with different CaO/Fe2O3 ratio.

lowered below TCV. As the decreasing CaO/Fe2O3 ratio, the Tcv of the slags increased from 1225 °C to 1383 °C, and then decreased to 1313 °C, and then increased to 1451 °C (Fig. 2(c)). Because the fluctuation in temperature around the TCV can cause the accumulation of slag inside the gasifier, the crystalline slag was usually unfavorable in the entrained flow gasifier. It illustrated that the slags were not favor of the slag tapping with the decreasing CaO/Fe2O3 ratio. In order to further illustrate relationship between the slag viscosity and CaO/Fe2O3 ratio, a mathematic method of linear fitting was applied. As shown in Fig. 3(a), there was a good linear relationship between CaO/Fe2O3 ratio and the slag viscosity when the temperature was higher than the maximum Tliq (A-0C10F, 1491 °C), namely, 1500 °C, 1525 °C and 1550 °C. In this case, the slag viscosity at high temperature depended on the slag structure, which implied that the slag structure was not influenced due to the change of CaO/Fe2O3 ratio. When A-0C10F was not included, the good linear relationship was also observed when the temperature lower than the Tliq. As the temperature was lowered below Tliq, solid phases were formed in slag, which affected the slag viscosity. Because the fraction of solid phases was lower than 1% when the CaO/Fe2O3 ratio was higher than 0:10, the slag viscosity was not affected by the formation solid phase (Fig. 3(b)). However, for A-0C10F, the solid fraction was 2.35%, 3.95% and 7.78%

respectively at 1485 °C, 1475 °C, and 1454 °C. The slag viscosity was affected by the formation of solid phases. Activation energy for viscous flow, E I ·, was also calculated to evaluate the resistance of slag movement, which is shown in Fig. 4. The calculation of E I · is based on Arrhenius equation in fully molten region, which was presented in the previous work [29]. The slag with a high E I · value always had a high viscosity value at high temperature. From Fig. 4, the E I · decreased with the decreasing CaO/Fe2O3 ratio, indicating that the ash slags with low CaO/Fe2O3 ratio tended to show a high flow ability and a low viscosity at high temperatures. In addition, it showed a significant decrease in the E I · when the CaO/Fe2O3 ratio decreased to 8:2. This implied that the flow ability of the ash slags was intensely influenced by the additive of Fe2O3. 3.2. Effect of CaO/Fe2O3 ratio on slag network structure 3.2.1. Effect of CaO/Fe2O3 ratio on Si-O structure It was widely accepted that the Raman shift of 400–600 cm−1 was defined as SieOeSi bond, which presented high [SiO4] tetrahedron polymerization degree. The range of 800–1200 cm−1 was classified as SieOeM (Al, Ca, Fe, Na) bonds, and it mainly contained non-bridging oxygen (NBO), which decreased the polymerization degree of silicate 4

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1454

=a× +b

=CaO/(CaO+Fe2O3)

1475 1485

Viscosity ( ) / Pa·s

1500 1525

1.6

1550

=-1.33× +0.63 R2=0.985 =-0.98× +0.58 R2=0.991

(a)

1454 1475 1485

8

=-0.86× +0.55 R2=0.989 =-0.69× +0.54 R2=0.976 =-0.61× +0.44 R2=0.942 =-0.43× +0.37 R2=0.961

Solid Fraction / %

2.0

1.2

0.8

(b)

6

4

2

0

0.4 8:2

10:0

4:6

6:4

2:8

0:10

10:0

CaO/Fe2O3

8:2

6:4

4:6

2:8

0:10

CaO/Fe2O3

Fig. 3. (a) Fitting curves of slag viscosities at high temperatures above TCV; (b) Solid fraction of the slags at 1454 °C, 1475 °C and 1485 °C calculated by FactSage.

melts [30,31]. Fig. 5 showed the Raman spectra and the calculated SiO-Si proportion with different CaO/Fe2O3 ratio. The quenched slags at 1525 °C had two asymmetric peaks, and Si-O-Si ratio decreased with the decreasing CaO/Fe2O3 ratio. The proportion of SieOeSi bond was 43.06%, 37.25%, 33.58%, 32.60%, 24.57% and 23.47% for samples from A-10C0F to A-0C10F respectively. This demonstrated a low polymerization degree of molten slag, leading to a low slag viscosity value at high temperatures. The parameter for depicting Si-O structure, Qn, was introduced to show effect of CaO/Fe2O3 ratio on the content of Qn [32]. The index n is number of the bridging oxygens (BO) in a [SiO4] tetrahedron structure (n = 0, 1, 2, 3, 4). Briefly, Q4 is tetrahedron skeleton, SiO2; Q3 indicates a planar layered structure, [Si2O5]2−; Q2 means a chain structure, [SiO3]2−; Q1 presents as ditetrahedron structure, [Si2O7]6−; Q0 effects as a monomer, [SiO4]4− [12,33]. A high n results in a more intense aluminosilicate framework, leading to a high slag viscosity of the fully molten slag. The deconvolution method was used to calculate the relative contents of Q0, Q1, Q2 and Q3 by Gaussian method for the Raman range between 800 and 1200 cm−1. Fig. 6(a) showed a typical deconvolution result of Raman spectra, and the relative contents of Qn. Four Raman shift parameters were

130

E (KJ/mol)

120

110

100

90

80 10:0

8:2

6:4

4:6

2:8

0:10

Fig. 4. Activation energy (E I ·) as a function of CaO/Fe2O3 ratio.

Si-O-Si Area

45

Si-O-M Area

Si-O-Si proportion in Si-O bond / %

(a)

Intensity / a.u.

A-0C10F

A-2C8F A-4C6F A-6C4F A-8C2F A-10C0F 200

400

600

800

1000

Raman Shift / cm

1200

1400

1600

(b)

42 39 36 33 30 27 24 21

10:0

8:2

6:4

4:6

CaO/Fe2O3

-1

Fig. 5. (a) Raman spectra of the quenching ash slags at 1525 °C; (b) SieOeSi proportion among all SieO bonds. 5

2:8

0:10

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Fig. 6. (a) A typical deconvolution of Raman spectra; (b) Contents of Qn units based on deconvolution results.

selected around 865, 924, 997, and 1117 cm−1, which correspond to Q0, Q1, Q2 and Q3 [34]. The variation of Si-O-M was characterized by R value, which was calculated by (Q0 + Q1)/(Q2 + Q3). Therefore, the slag with a high R value had a low polymerization degree. As shown in in Fig. 6(b), the R value increased from 0.33 to 76.52 when the slag varied from A-10C0F to A-0C10F. Besides, a sharp increase was observed when the CaO/Fe2O3 ratio was lower than 4:6. This suggested the slag structure was broken as the CaO/Fe2O3 ratio decreased. As a result, the slag viscosity at high temperatures decreased with the decreasing CaO/Fe2O3 ratio.

increased gradually. However, the intensity of the peak for Al(IV) structure decreased. The relative content of Al(IV) unit was presented in Fig. 8. The proportion of Al(IV) decreased with the decreasing CaO/Fe2O3 ratio, and it decreased sharply when the CaO/Fe2O3 ratio was lower than 6:4. For instance, the proportion of Al(IV) was 99%, 96%, and 86%, respectively for A-10C0F, A-8C2F, and A-6C4F, and it decreased to 39% (A-4C6F), 22% (A-2C8F) and 0% (A-OC10F). Caurant et al. [38] found that both alkali metal ions and alkaline earth metal ions had strong charge compensation effect on Al3+, and this leads to the transformation of Al(VI) to Al(IV). In reducing conditions, the majority of the iron will be in the Fe2+ state [15]. Wu et al. [9] found that the charge compensation effect of Ca2+ on Al3+ was stronger than that of Fe2+ on Al3+. Therefore, the Al(IV) content decreased with the decreasing CaO/ Fe2O3 ratio, which resulted in the decrease in the viscosity of ash slag. To reveal the effect of CaO/Fe2O3 ratio on slag structure, nonbridging oxygen (NBO) fraction as a parameter was used to characterize the polymerization degree of the slag. The method which was proposed by Chen et al. [21] was used to calculate the bridging oxygen (BO) and non-bridging oxygen (NBO), and both SiO2 and Al2O3 were considered. The correction factor λ was obtained by SiO2/Al2O3 molar ratio, where λ = 2.55/(2.55 + 1). Therefore, BO fraction of the molten slags can be concluded as follow:

3.2.2. Effect of CaO/Fe2O3 ratio on Al-O structure Previous study showed that Al3+ would substitute for Si4+ in aluminosilicate system, enhancing slag polymerization [35]. Because the content of Al2O3 in the slag was high, it was necessary to study effect of CaO/Fe2O3 ratio on Al-O structure. Fig. 7 showed 27Al NMR spectra of the slags with different CaO/Fe2O3 ratios. The range around 50–90 ppm can be classified as Al(IV) ([AlO4]5−) coordination for tetrahedron structure, which enhanced slag polymerization degree. The range around 10–20 ppm was responsive for Al(VI) ([AlO6]9−) for octahedral structure, which decreased slag polymerization degree [7,36,37]. It can be seen that the chemical shift of Al(IV) moved to lower frequency with the decreasing CaO/Fe2O3 ratio, and the intensity of the peak also

100

80

Al(IV) Content / %

Intensity / a.u.

A-10C0F A-8C2F A-6C4F A-4C6F A-2C8F A-0C10F

Al(IV) Al-VI

60

40

20

0 0

10

20

30

40

50

60

70

80

10:0

90

Al Chemical Shift / ppm

Fig. 7.

27

8:2

6:4

4:6

2:8

CaO/Fe2O3

27

Al NMR spectra of quenching ash slags at 1525 °C.

Fig. 8. Al(IV) content as a function of CaO/Fe2O3 ratio. 6

0:10

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60

y = -49.68× +11.80

R2=0.994

Al(IV) proportion

y = -36.56× +11.53

R2=0.894

Si-O-Si proportion y = -12.73× +0.67

50

Relative Contents / %

BO/(BO+NBO)

R2=0.920

=CaO/(CaO+Fe2O3)

40 30 20 10 0

8:2

10:0

6:4

4:6

2:8

0:10

CaO/Fe2O3 Fig. 9. Correlation between CaO/Fe2O3 ratio and Al(IV) proportion, Si-O-Si proportion, BO/(BO + NBO) (considering both Si-O-Si and Al(IV)) at 1525 °C.

×

Si O Si

+ (1

)×

Al(IV)

Tcv=1226 oC

Exo

BO = BO + NBO

Fig. 10. Phase diagram with different CaO/Fe2O3 ratio.

(7)

A-10C0F

Tcv=1323 oC A-8C2F

Tcv=1383 oC

DSC (W/g)

As shown in Fig. 9, the three curves showed a same decreasing tendency with the decreasing CaO/Fe2O3 ratio. The Si-O-Si proportion decreased from 26.03% to 11.80% when the CaO/Fe2O3 ratio decreased from 10:0 to 0:10. This explained why the slag viscosity decreased with the decreasing CaO/Fe2O3 ratio. Besides, the correlation coefficient (R2) was 0.894, 0.920 and 0.994, respectively for Al(IV), Si-O-Si and BO/(BO + NBO). Therefore, slag structure parameter BO/(BO + NBO) had higher linear relationship with CaO/Fe2O3 ratio, indicating the variation of slag viscosity at high temperatures was attributed to the effect of CaO/Fe2O3 ratio on slag structure. Moreover, the slope of the three curves were −12.73, −36.56 and −49.68 for Si-O-Si, Al(IV) and BO/(BO + NBO), respectively. The slope of BO/(BO + NBO) was close to that of Al(IV) proportion. This indicated that the slag structure was affected mainly by Al-O structure.

A-6C4F o

Tcv=1343 C

A-4C6F

Tcv=1313 oC

A-2C8F

o

Tcv=1451 C A-0C10F

1450

3.3. Crystallization behavior of slag during cooling

1400

1350

1300

1250

1200

1150

1100

Fig. 11. DSC curves of ash slag during cooling.

FactSage 7.2 was used to show the effect of CaO/Fe2O3 ratio on primary phase of the slags, as shown in Fig. 10. The compositions of the slags, A-10C0F, A-8C2F, A-6C4F and A-4C6F, were located in anorthite primary phase, indicating that anorthite was the solid phase which was formed during cooling. For A-2C8F and A-0C10F, the compositions of the slags were located in corundum and spinel primary phases, respectively. This implied that corundum and spinel will be formed. Fig. 11 presented DSC curves of the slags with different CaO/Fe2O3 ratios, and the TCV was also given. Apart from A-10C0F, there were obvious exothermic peaks. The peak temperature (Tp) of each crystalline slag was 1320 °C (A-8C2F), 1385 °C (A-6C4F), 1339 °C (A-4C6F), 1323 °C (A-2C8F), 1454 °C (A-0C10F). Besides, the TCV of the slags was in the temperature range of crystallization. The Tp trend was similar to that seen in the TCV of the slags with the change in CaO/Fe2O3 ratio. For the crystalline slag, the rapid increase in the viscosity of slag was usually caused by formation of crystalline phase [19]. It demonstrated that the decrease in CaO/Fe2O3 ratio was favor of the slag crystallization. The slags were also characterized by XRD and SEM-EDS, and the results were given in Figs. 12 and 13. And the EDS data and deteced mineral phases were presented in Table 2. The A-10C0F showed an amorphous characterization (Fig. 12a), indicating that the crystalline phases were not formed during cooling, and solid phase was not found in Fig. 13(a). Therefore, the slag showed the behavior of the glass.

:Anorthite *:corundum #:mullite :spinel

(a)

Intensity (a.u.)

(b) (c) (d) (e)

*

* (f) 5

## #

15

25

* # #

35

* #

* #

#

45

55

65

75

85

Fig. 12. XRD pattern of ash slags quenching below TCV. (a) A-10C0F; (b) A8C2F; (c) A-6C4F; (d) A-4C6F; (e) A-2C8F; (f) A-0C10F.

7

Fuel 258 (2019) 116129

Z. Ge, et al.

Fig. 13. SEM analysis of quenched slags below TCV. (a) A-10C0F; (b) A-8C2F; (c) A-6C4F; (d) A-4C6F; (e) A-2C8F; (f) A-0C10F.

Si

Al

Ca

Fe

Na

O

Phase

1 2 3 4 5

13 16 – 10 10

14 16 42 30 31

14 8 – – –

– – – – 1

1 – – – –

58 60 58 60 60

Glass Anorthite Corundum Mullite Mullite + Spinel

at%: atom proportion.

However, for A-8C2F, A-6C4F and A-4C6F, many strong diffraction peaks were detected, which correspond to mineral phase: anorthite (Fig. 12(b–d)). The sharp increase in the viscosity was attributed to the formation of anorthite. Besides, particle sizes of anorthite increased from 40 μm to 150 μm and 90 μm for A-8C2F, A-6C4F, and A-4C6F, respectively (Fig. 13(b–d)). This indicated that the decrease in CaO/ Fe2O3 ratio facilitated the growth of anorthite. The crystal size was closely related to the TCV and a TCV event usually occurred when the size of the crystals significantly increased [7]. Thus, an increase in the TCV occurred with the decreasing CaO/Fe2O3 ratio. For A-2C8F, corundum was detected, while intensity of the XRD peak was not as strong as that of anorthite (Fig. 12(e)). The slag was filled with thin column crystals (Fig. 13(e)). For the slag was without CaO (A-0C10F), the crystalline phases were confirmed to be spinel and mullite, and mullite was the dominant crystalline phase (Fig. 12(f)). SEM-EDS showed that the diamond-shaped small crystal was recognized as mullite (Al6Si2O13) and the short stick-liked crystal was confirmed as both mullite and FeAl spinel (FeAl2O4) (Fig. 13(f)). The formation of mullite and spinel accounted for the high TCV of the slag.

Exo

Area

3 oC/min

DSC (W/g)

Table 2 EDS results of quenched slags below TCV (at%).

5 oC/min

10 oC/min 1450

1400

1350

1300

1250

1200

1150

1100

Temperature / oC Fig. 14. DSC curves of the A-4C6F under different cooling rates.

major crystalline phase of the A-4C6F. According to Kissinger equation, crystallization activation energy (Ec ) was determined by plotting ln

Tp2

versus

1 . RTp

Ec is an indication of

the energy barrier that must be overcome for precipitating crystal phases. Since sample A-10C0F behaved as a glassy slag, no exothermic peaks were detected by DSC test during cooling step. In Fig. 15, the A0C10F had the lowest Ec , indicating mullite had the strongest crystallization tendency. And corundum showed a weak crystallization tendency due to its high Ec . For the A-8C2F, A-6C4F and A-4C6F, the crystalline phase was anorthite, while the Ec decreased with the decreasing CaO/Fe2O3 ratio. It implied the crystallization of anorthite was easier for the slag with a low CaO/Fe2O3 ratio. In Table 3, the n value was 1.55, 2.54, 2.05, 1.06 and 3.18, respectively for A-8C2F, A-6C4F, A-4C6F, A-2C8F and A-0C10F. This demonstrated that the crystal growth pattern of A-0C10F was a bulk crystallization (n > 3), while other slags were surface crystallization

3.4. Crystallization kinetics analysis As shown in Fig. 14, two major exotherms can be observed from the DSC cooling curves of the A-4C6F in the range of 1400–1225 °C under cooling rates of 3, 5, and 10 °C/min. The peak temperature of exotherms shifted to lower temperature as the cooling rate increased. The Tp was 1340, 1311 and 1301 °C under 3, 5 and 10 °C/min, respectively. This suggested that the increase in cooling rate avoids the formation of 8

2000

anorthite. 4) The crystallization activation energy (Ec ) decreased firstly, then increased to the maximum value, and then decreased to the lowest value sharply. The crystallization growth pattern of the slags changed from the surface crystallization to the bulk crystallization with decreasing CaO/Fe2O3 ratio. 5) The usage of CaO-Fe2O3 binary composite flux was benefit to lower the slag viscosity, while the decrease in the CaO/Fe2O3 ratio preferred to the formation of the crystalline slag, which was usually avoided for the entrained flow gasifiers.

Corundum

Fuel 258 (2019) 116129

800

Mullite

Anorthite

1200

Anorthite

1600

Anorthite

Crystallization Activation Energy/ KJ/mol

Z. Ge, et al.

Acknowledgements

400

This work was supported by the Joint Foundation of Natural Science Foundation of China and Shanxi Province [grant number U1510201]; NSFC-DFG [grant number 21761132032]; Joint Foundation of Natural Science Foundation of China and Xinjiang [grant number U1703252]; Shanxi Province Science Foundation [grant number 201703D421033]; Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering [grant number 2017-K21].

0 8:2

6:4

4:6

2:8

0:10

CaO/Fe2 O3 ratio Fig. 15. The calculated activation energy as a function of CaO/Fe2O3 ratio.

References

Table 3 The calculated Avrami index (n) with different CaO/Fe2O3 ratio. Parameters

A-8C2F

A-6C4F

A-4C6F

A-2C8F

A-0C10F

Tp /K

1593.15

1658.15

1612.15

1596.15

1728.15

Ec /KJ·mol−1 n Growth mechanism

946.8 623.5 693.9 1.55 2.54 2.05 Surface Crystallization

1510.3 1.06

514.3 3.18 Bulk Crystallization

Tf /K

36

36

38

33

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(n < 3). Generally, for the slag with the bulk crystallization, the slag would grow fast [27]. The result showed that it was easier for mullite (A-0C10F) to be precipitated and grow up. Besides, for the slag with surface crystallization manner, a high Avrami index was beneficial for the growth of crystals. Therefore, it was hard to precipitate with a big particle size for corundum crystal (A-2C8F). For Anorthite phase (samples A-8C2F, A-6C4F and A-4C6F), the high n value resulted in the large crystal particle size with the variation of CaO/Fe2O3 ratio. 4. Conclusions Slag viscosity behavior was crucial for the smooth slag tapping process of the entrained flow gasifiers. In this work, the effect of CaO/ Fe2O3 ratio on slag viscosity-temperature behavior was investigated. The slag structure at high temperatures and crystallization behavior during cooling step were studied by viscosity measure, Raman, NMR, FactSage7.2, DSC, XRD and SEM-EDS. The conclusions can be drawn as follows: 1) As the CaO/Fe2O3 ratio decreased, the slag viscosity at the same temperature decreased due to the high decrease ability of Fe2O3 on slag viscosity. It showed an excellent linear relationship between the slag viscosity and CaO/Fe2O3 ratio. Besides, the slag transformed from a glassy slag to a crystalline one with the decreasing CaO/ Fe2O3 ratio. 2) The decrease in the CaO/Fe2O3 ratio resulted in the disruption of SiO-Si bridges and the reduce of Al(IV) proportion. The slag structure with the change in the CaO/Fe2O3 ratio was mainly dependent on Al-O structure. 3) Crystalline phases of the slags varied from anorthite to corundum, and to spinel and mullite with the decreasing CaO/Fe2O3 ratio. The decrease in the CaO/Fe2O3 ratio was also favor of growth of 9

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10