Experimental and theoretical investigation on relationship between structures of coal ash and its fusibility for Al2O3-SiO2-CaO-FeO system

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 47, Issue 6, June 2019 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2019, 47(6), 641648

RESEARCH PAPER

Experimental and theoretical investigation on relationship between structures of coal ash and its fusibility for Al2O3-SiO2-CaO-FeO system DAI Xin1,2, BAI Jin3,* , LI Dong-tao1,2, YUAN Ping4, YAN Ting-gui5, KONG Ling-xue3, LI Wen3 1

Shougang Group Research Institute of Technology, Beijing 100043, China;

2

Beijing Key Laboratory of Green Recyclable Process for Iron & Steel Production Technology, Beijing 100043, China;

3

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China;

4

Shanxi Research Institute for Clean Energy, Tsinghua University, Taiyuan 030032, China;

5

School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China

Abstract:

The molecular dynamics simulation, thermal dynamic calculation and experimental investigation were combined to

illustrate ash slag viscosity variation mechanism for Al 2O3-SiO2-CaO-FeO system. The viscosity declines and the viscosity curve are transformed from crystalline slag to glassy slag with increasing mass ratio (C/F) of calcium to ferrous oxide in Al2O3-SiO2-CaO-FeO system. There is an inflexion point when the C/F is equal to 2. When the C/F is below 2, there are mainly crystalline minerals in the system. While the C/F is above 2, there are mainly amorphous minerals in the system. With the increase of C/F, six-coordinated Al ([AlO6]9–) is transformed to four coordinated Al ([AlO4]5–) microscopically. Besides, the content of bridging oxygen decreases while that of non-bridging oxygen increases. Quantified function between base composition and viscosity are constructed based on the stability coefficients defined by oxygen bond species. Key words:

molecular dynamics simulation; thermal dynamic calculation; coal ash structure; oxygen bond species; calcium and

ferrous oxide ratio

Coal is the most important fossil fuel in China and with the strategic demands for energy evolution development and harsher environmental requirement. More attention has been put on the highly efficient and clean technology for coal utilization. Integrated gasification combined-cycle (IGCC) technology whose kernel part is coal gasification has long-term stable operability, high power generation efficiency and low greenhouse gas emission. High-pressure and high-temperature entrained-flow gasifier has many advantages including easy scale-up, wide coal flexibility and high carbon conversion, which has become a main coal conversion technology[1,2]. The reactivity and reaction velocity of organic matter in various coals is similar under high temperature, and thus the fluidity of mineral matter is key issues for determining operation parameters in gasification process [3].

Coal ash fluidity is significant to characterize fusion variation during heating process, which is important to identification of gasification temperature and operation stability[4]. Ash viscosity-temperature properties quantitatively reflect the slag viscosity and temperature variation under high temperature. Besides, it is also of importance to determine the operation temperature for slag tapping gasifier[5]. For entrained-flow gasifier with slag tapping, the expected viscosity range is 2.5–25 Pa·s under the operation temperature. Viscosity higher than that range may cause blockage at the bottom of the reactor near slag tapping hole and, consequently, leads to unscheduled emergency shutdown of the process. When the viscosity is below 2.5 Pa·s, however, the problem of refractory wear may arise[6,7]. Currently, investigation of ash viscosity temperature properties mainly focuses on effect of chemical

Received: 14-Jan-2019; Revised: 25-Mar-2019. Foundation items: Supported by Joint Foundation of the Natural Science Foundation of China and Xinjiang Province (U1703252), Joint Foundation of the Natural Science Foundation of China and Shanxi Province (U1510201), NSFC-DFG (21761132032) and National Natural Science Foundation of China (21808045), National Key R&D Program of China (2017YFB0304300 & 2017YFB0304303). *Corresponding author. Tel/Fax: 0351-4040289, E-mail: [email protected]. Copyright  2019, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

DAI Xin et al / Journal of Fuel Chemistry and Technology, 2019, 47(6): 641648

composition and external factors [8–10]. The major coal ash composition is CaO, SiO2, Fe2O3 and Al2O3, and thus many researchers studied the influence of composition on fluidity by simplifying coal ash to Al 2O3-SiO2-CaO-FeO system[11], but microscopic structural variation effect on fluidity still need further investigation [12–15]. The molecular dynamics simulation was adopted to illustrate influence of alkaline components on oxygen bond structure and fluidity of Al2O3-SiO2-CaO-FeO quaternary system. Destruction ability of various lattice structures is different according the alkaline components. Furthermore, there is competition between similar chemical bonds such as Ca–O and Al–O. Therefore, Ca–O and Fe–O have competitive effect in the quaternary system. In this work the molecular dynamics simulation, thermal dynamic calculation and experimental investigation were combined to illustrate mechanism and synergism of alkaline components and their competition effect on fluidity for Al2O3-SiO2-CaO-FeO quaternary system based on the calcium and ferrous oxide mass ratio (C/F).

1 1.1

according Table 1. 1.2

Ash samples were all pre-melted in an electric-furnace at corundum crucibles to obtain a uniform slag sample. Adequate active carbon was placed in the furnace to attain reducing atmosphere during the pre-melting process. The pre-melting temperature was 80 K higher than the ash fusion temperatures and kept for 10 min at that temperature. Samples were crushed for viscosity measurement as naturally cooling down to room temperature. Programmed cooling method was used to measure viscosity temperature properties under weak reducing atmosphere (CO: CO2 = 4:6) with 1 K/min cooling rate[8]. 1.3

Mineral composition analysis

Bruke-AXS D2 X-ray analyzer (Bruker, Germany) with Cu (40 kV, 40 mA) radiation was adopted to analyze mineral matters in coal ash quantitatively and qualitatively at various temperatures. All the samples were grounded less than 200 mesh and prepared by pressing-tablets method. Scanning range was 10°–90°with scanning step of 0.02°. EVA software was used to identify the mineral composition.

Experimental Sample preparation

1.4

Synthetic ashes constituted of SiO2, Al2O3, CaO and Fe2O3 pure oxide were adopted for viscosity measurement and all the analytical reagents were produced by J&K Scientific LTD. All the reagents were burned from room temperature to 1123 K and kept for 2 h, and then heated to 1823 K and kept for 10 min to eliminate burning loss. Finally, the samples were cooled naturally at room temperature. All the burned samples were ground to less than 200 mesh and blended in a ball mill Table 1

Viscosity measurement

Coal ash structure analysis

The spectra were collected on a Bruker AVANCE III 600 MHz spectrometer (Bruker, Germany) with magnetic field of 14.09 T, 5 mm CPTCI, high resolution BBO, 4 mm Bruker magic angle spin probe. The 27Al NMR spectrum was recorded at a spinning rate of 14 kHz, 27Al chemical shift was referred at 0.0 of Al(NO3)3.

Composition of coal ash samples Composition w/%

C/F

SiO2

Al2O3

CaO

Fe2O3

0.66 (R4)

56.67

28.33

6.00

9.00

1.14 (R5)

56.67

28.33

8.00

7.00

2.00 (R6)

56.67

28.33

10.00

5.00

4.00 (R7)

56.67

28.33

12.00

3.00

14.00 (R8)

56.67

28.33

14.00

1.00

Table 2

Potential function coefficients fitted by Bertrand et al for various quaternary systems [16,17]

Atom type

Q/e

A/(kJ·mol–1)

ρ/nm

C/(106nm6kJmol–1)

Al

1.4175

2753544

0.0172

3336.26

Ca

0.945

15019679

0.0178

4077.45

O

–0.945

870570

0.0265

8210.17

Si

1.890

4853816

0.0161

4467.07

Fe2+

0.945

1257488

0.0190

0

DAI Xin et al / Journal of Fuel Chemistry and Technology, 2019, 47(6): 641648

Fig. 1 Viscosity variation for Al2O3-SiO2-FeO-CaO quaternary system as a function of calcium oxide and ferrous oxide ratio

2

Simulation and calculation

2.1

Simulation method

Molecular dynamics simulation was conducted by the LAMMPS software package (https://lammps.sandia.gov/). Simulation process was as follow: All the initial structures were heated to 3000 K in the isothermal-isobaric ensemble (NPT) and kept for 200 picoseconds (ps) for thermodynamic equilibrium. Then, structures were cooled down to 300 K with cooling rate of 10 K/s. The structures relaxed in the canonical ensemble (NVT) at 300 K for 200 picoseconds (ps). Finally, the relaxed structures were heated from 300 K to 3000 K at rate of 1 K/ps in the NPT ensemble. Nose-Hoover thermostat and barostat for the temperature and pressure control was used, respectively. The cutoff of short range interaction was 1.2 nm, while the electrostatic interaction was calculated by the Ewald method with cutoff of 0.8 nm. The Born-Mayer-Huggins (BMH) function was adopted for inter-atomic interaction, and corresponding potential coefficients are listed in Table 2 [16,17]. 2.2

Thermodynamic calculation

Equilib and Phase Diagram models of FactSage were adopted to calculate ash composition variation at various temperatures for multi-composition coal ash systems[18–20]. The FactPS and FToxid databases included most of thermodynamic property datum of ash composition (SiO 2, Al2O3, CaO, FeO, Fe2O3, MgO, Na2O, K2O and TiO2) could support all the information to study ash structures and fluidity[21].

3

Results and discussion

3.1

Effect of C/F on viscosity-temperature properties

The viscosity and critical viscosity temperature (Tcv)

decreased with increasing C/F for five samples (R4–R8) in Al2O3-SiO2-CaO-FeO quaternary systems (the ratio of silicon and aluminum oxide is set as 2, S/A=2), as shown in Figure 1. Tcv of various samples was 1826, 1811, 1772, 1767 and 1762 K respectively. Structure is the internal factor that affecting the viscosity. The structural difference of various samples was minor when the temperature was higher than Tcv. Therefore, effect of temperature on structure and viscosity was minor. However, structures were further destroyed and led to a slight decline of viscosity with increasing temperature. When temperature was lower than Tcv, the system formed solid phase and liquid phase macroscopically while the structure significantly varied with temperature microscopically. Thus, the viscosity variation was attributed to effect of temperature on the structure. With the increase of C/F, sensitivity of structure to temperature enhanced and viscosity variation significantly increased as temperate decreased. From the macroscopic perspective, the viscosity curve transformed from crystalline slag type to glassy slag type. There were inflexions in viscosity curve of R4 (C/F=0.66) and R5 (C/F=1.14), and viscosity increased sharply when the temperature decreased to 1833 K and 1813 K. However, there were no inflexions in viscosity curve of R7 (C/F=4.00) and R8 (C/F=14.00) and viscosity increased smoothly with decreasing temperature. The viscosity variation of R6 (C/F=2.00) lied between the above two cases, and there was a transition section in the viscosity range between smooth enhancement and sharp increase. Equilib model in FactSage was used to calculate solid and liquid composition of five samples at various temperatures as shown in Figure 2. When the temperature was lower than liquidus temperature, mullite precipitated firstly for R4 (C/F=0.66) at 1862 K and quartz secondly at 1599 K. The precipitation temperature difference between the two minerals was large. With increasing C/F the content of mullite decreased generally and amorphous structures increased. When the C/F was 14, the first solid mineral was low-melting anorthite and its content was lower. The effect of temperature on viscosity was lowered and viscosity curve transformed from crystalline slag type to glassy one. The crystalline states of slag from viscosity measurement were characterized by XRD as shown in Figure 3. There was little crystal diffraction peak as C/F was higher than 2, indicating amorphous mineral was the majority in these samples and crystallizing tendency was weak. Besides, the viscosity curve belonged to glassy slag type. There was much more mullite when C/F was above 2, illustrating high crystallizing tendency and the viscosity curve belonging to crystalline slag type. Minor mullite and amorphous mineral appeared in the system as the C/F equaled to 2, and the viscosity curve was plastic slag type.

DAI Xin et al / Journal of Fuel Chemistry and Technology, 2019, 47(6): 641648

Fig. 2

Viscosity variation, solid and liquid phase contents for various C/F Al 2O3-SiO2-CaO-FeO quaternary systems as a function of temperature (a): C/F=0.66; (b): C/F=1.44; (c): C/F=2.00; (d): C/F=4.00; (e): C/F=14.00

Fig. 3

XRD patterns for various C/F Al2O3-SiO2-FeO-CaO quaternary systems after viscosity measurement

3.2

Effect of C/F on quaternary system structure

The structure has the decisive role on viscosity. Combination of experimental characterization and molecular dynamics simulation was used to investigate effect of C/F on ash structure microscopically. The XPS of Al 2p could reflect the coordination variation of Al ions and deconvolution of XPS could analyze coordination variation semi-quantitatively. Figure 4(a) was XPS results for various coal ashes, Figure 4(b) was deconvolution of XPS, and Figure 4(c) was the summary of deconvolution results. When C/F was lower than 2, the content of hexa-coordination ([AlO6]9–) decreased from 75% to 60% and tetra-coordination ([AlO4]5–) increased from 25% to 40% as the C/F increased. The competition between calcium and ferrous oxide was obvious and mainly reflected from effect of Ca and Fe on coordination variation, and further affected stability of Ca–O and Fe–O.

DAI Xin et al / Journal of Fuel Chemistry and Technology, 2019, 47(6): 641648

Fig. 4

XPS analysis for various C/F Al2O3-SiO2-FeO-CaO quaternary systems : [AlO4]5–;

: [AlO6]9–

hexa-coordination ([AlO6]9–) to tetra-coordination ([AlO4]5–) with increasing C/F, which was consistent with the result of XPS. Molecular dynamics simulation was performed to further understand the viscosity variation mechanism by analyzing oxygen bond species. In Figure 6, the bridging and non-bridging oxygen were major composition but cluster oxygen was minority. The content of bridging and non-bridging oxygen differed little as C/F was lower than 2. Thus, it reflected that R4 (C/F=0.66) and R5 (C/F=1.14) had similar structure. Fig. 5

NMR analysis for various C/F Al2O3-SiO2-CaO-FeO quaternary systems

When C/F was higher than 2, enhancement of C/F had little influence on hexa-coordination ([AlO6]9–) and 5– tetra-coordination ([AlO4] ) content, suggesting the structure varied slightly with C/F variation. Besides, it also interpreted the viscosity variation difference was attributed to effect of C/F on structure during cooling process. NMR of 27Al was used to characterize Al coordination variation quantitatively in bulk phase as shown in Figure 5. Chemical shift (δ) of 27Al NMR at around 50 is belong to tetra-coordination ([AlO4]5–) while δ=0 stands for hexa-coordination([AlO6]9–). The increasing peak at δ=50 and decreasing peak at δ=0 demonstrated the transition from

Fig. 6

Oxygen bond species for various C/F Al2O3-SiO2-CaO-FeO quaternary systems : bridging oxygen;

: nonbridging oxygen

DAI Xin et al / Journal of Fuel Chemistry and Technology, 2019, 47(6): 641648

Fig. 7

Composition of bridging and nonbridging oxygen for various C/F Al2O3-SiO2-FeO-CaO quaternary systems : Al–O–Si;

Fig. 8

Relationship between viscosity and stability coefficient for Al2O3-SiO2-CaO-FeO quaternary systems

However, when C/F was higher than 2, the content of non-bridging oxygen increased and that of bridging oxygen decreased. The C/F of 2 was the inflection point. When it was above 2, the system structural stability was high and macroscopically reflected as crystalline slag. When it was below 2, the system structural stability was low and macroscopically exhibited as glassy slag. The bridging oxygen including Al–O–Al, Al–O–Si and Si–O–Si in Al2O3-SiO2-CaO-FeO quaternary systems, while non-bridging oxygen consisted of Al–O and Si–O. With increasing C/F the content of Al–O–Si decreased and content of Al–O–Al increased. Besides, content of Al–O increased and that of Si–O decreased. This was consistent with the Al coordination variation. The structural stability of Al–O–Si and Si–O–Si was higher than Al–O–Al. Thus, the system stability and viscosity decreased as content of Al–O–Si decreased.

: Al–O–Al;

: Si–O–Si

(1) SC = BO + TO  = (BO+TO) / (NBO+TO) (2) Where,  is polymerization coefficient; BO, TO and NBO are the content of bridging oxygen, cluster oxygen and non-bridging oxygen, respectively. The higher polymerization, the more stable the system is and thus the higher stability and viscosity and vice versa. The quantitative relationship between stability coefficient and coal ash viscosity was constructed by bridging oxygen. As shown in Figure 8, the function could be built for Al2O3-SiO2-FeO-CaO quaternary systems by fitting:  = e0.041×SC (3) Where,  is the viscosity, Pa·s; SC is the stability coefficient. Furthermore, variation of C/F is consistent with the content of calcium oxide. The quantitative relationship between SC and calcium oxide content is as follow and shown in Figure 9: SC = –1.68×CaO + 99.20 (4) Where, SC is stability coefficient and CaO is the content of calcium oxide, %. From above two equations, the quantitative relationship between viscosity and C/F could be obtained: ln = 4.07 – 0.069×CaO (5) Where,  is viscosity, Pa·s; CaO is the content of calcium oxide, %.

3.3 Fluidity prediction of Al2O3-SiO2-FeO-CaO quaternary systems In silicate system, cluster oxygen and bridging oxygen enhanced the structural stability, and thus definition of stability coefficient (SC) was as follow:

Fig. 9

Relationship between stability coefficient and C/F for Al2O3-SiO2-CaO-FeO quaternary systems

DAI Xin et al / Journal of Fuel Chemistry and Technology, 2019, 47(6): 641648

4

Conclusions

[9] Kong L X, Bai J, Li W, Wen X D, Li X, Bai Z Q, Guo Z X, Li H Z. The internal and external factor on coal ash slag viscosity at

In Al2O3-SiO2-CaO-FeO quaternary systems, the viscosity decreased and viscosity curve transformed from crystalline slag type to glassy one with increasing C/F. The inflection point of viscosity-temperature properties was C/F being 2. There were more crystalline minerals during cooling process when C/F was below 2, while there was much more amorphous minerals as C/F was above 2. From the microscopical perspective, enhancement of C/F led to transition from hexa-coordination ([AlO6]9–) to 5– tetra-coordination ([AlO4] ). Besides, there was competition between Ca and Fe atom for hexa-coordination ([AlO6]9–). Meanwhile, decline of polymerization and stability of the system was attributed to the decrease of bridging oxygen. The quantitative function relationship between alkaline composition (CaO) and viscosity for Al2O3-SiO2-CaO-FeO quaternary systems was constructed by oxygen bonds, namely ln = 4.07 – 0.069×CaO.

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