Na2O on fusion behavior of coal ash with high silicon and aluminum level

Fuel 265 (2020) 116964

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Full Length Article

Effect of K2O/Na2O on fusion behavior of coal ash with high silicon and aluminum level

T

Xiaoming Lia, Lifei Zhia, Wenju Shib,c, , Lingxue Kongb, Jin Baib, , Jianglong Yud,e, Markus Reinmöllerf, Stefan Guhlf, Bernd Meyerf,g, Wen Lib ⁎

⁎

a

School of Chemical and Biological Engineering, Taiyuan University of Science and Technology, Taiyuan, Shanxi, PR China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China c University of Chinese Academy of Sciences, Beijing 100049, PR China d Key Laboratory of Advanced Coal and Coking Technology of Liaoning Province, School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051, PR China e ICCCF & Chemical Engineering, University of Newcastle, Callaghan, NSW 2308, Australia f Institute of Energy Process Engineering and Chemical Engineering, Technische Universität Bergakademie Freiberg, 09599 Freiberg, Germany g Fraunhofer Institute for Microstructure of Materials and Systems (IMWS), Branch Circular Carbon Technologies, Walter-Hülse-Strasse 1, 06120 Halle, Germany b

ARTICLE INFO

ABSTRACT

Keywords: K2O/Na2O Ash fusibility Mixed-alkaline effect High silicon and aluminium

Ash fusion temperatures (AFTs) are widely used for evaluation of fusibility of coal ash for combustion and gasification. K2O and Na2O is regarded as the effective component to enhance the ash fusibility for slag tapping. The combination of K2O and Na2O may enhance the flux efficiency. In this work, the effect of K2O/Na2O mass ratio on fusion behavior of coal ash with high silicon and alumina level was evaluated by the height variation of ash cone during heating. The results showed that AFTs and theoretical fusion range (Tliq-Tini) decreased quickly and then increased slowly as the K2O/Na2O mass ratio in the coal ash was reduced. The sub-liquidus phase changed from mullite into feldspar, which was responsible for change of AFTs. Meanwhile, most of K element occurred in leucite (KAlSi2O6) and the rest existed in nepheline (KAlSiO4), but Na almost existed in nepheline (NaAlSiO4). The competition effect between K2O and Na2O on AFTs was reflected by KAlSiO4/NaAlSiO4 in nepheline, so the SiO2-KAlSi2O6-NaAlSi2O6 ternary phase diagram was constructed, which well explained the variation of AFTs caused by K2O/Na2O mass ratio. Meanwhile, the fusion process was divided into two stages based on variation rate of ash cone height. At first stage, the amount of liquidus phase is limited and the viscosity is high, so only a low shrinkage rate of ash cone exhibits. However, at the second stage, high shrinking rate is caused by not only the rapid increase of slag content but also the sharp decrease of viscosity besides, the addition of K2O/Na2O change ash fusion process from “melting-dissolving” into “softening-melting”.

1. Introduction With the extensive consumption of coal resource, the utilization of coal with high-efficiency and environmental friendly has been become a hotspot issue [1]. Among different coal utilization ways including combustion, liquefaction and gasification, gasification technology has been regarded as the most promising way for clean coal utilization. Now coal gasification is well developed for coal to chemicals and fuel in China [2,3]. Gasification technologies can be categorized into three types based on gasification furnace profiles, which includes fixed-bed, fluidized-bed and entrained-flow bed [4]. The entrained flow gasifier with large scale is the preferential gasification technique for the

advantages of flexibility in feedstock, high thermal efficiency and high quality syngas with low tar content [5]. The gasifiers can also be classified into slagging and non-slagging by the status of coal ash at the bottom of gasifier. Entrained flow gasifier is operated at the high temperature (> 1350 °C) and is typical slagging gasifier [6]. In the gasifier, organic matters almost transform into syngas and the minerals transform into melted slag at high temperature [7]. The slagging tapping at the bottom of gasifier and the slag layer on the membrane wall becomes the most significant issue for stable running the entrained flow gasifier [8]. However, the fusibility of most coal in China is not good enough for slag tapping, especially for the coal with high silicon and aluminum level from Shanxi Province. In hence, it is important to

⁎ Corresponding authors at: State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China (W. Shi). E-mail addresses: [email protected] (W. Shi), [email protected] (J. Bai).

https://doi.org/10.1016/j.fuel.2019.116964 Received 15 November 2019; Received in revised form 16 December 2019; Accepted 26 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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design the efficient flux based on the fusion mechanism. Ash fusion temperatures (AFTs) are widely used to describe ash fusibility. However, four characteristic temperatures lack of scientific definition are not sufficient to reveal the effect of flux component on ash fusion behavior. Wall [8] et al. introduced the thermomechanical analysis (TMA) into ash fusibility study. TMA curve was divided into three stages and the new fusion mechanism as “sintering-primary fusion-dissolving” is raised [9]. Besides, TMA curve well explained the effect of SiO2/AlO3 and CaO/Fe2O3 on the ash fusion behavior, but TMA method is more complex than the traditional ash fusion temperatures by ash cones [10,11]. In hence, if more information on ash fusion behavior can be obtained from AFTs test, the traditional method will better support the understand of ash fusion process. Chemical composition is a significant factor to decide AFTs, which mainly consist of several oxides including SiO2, Al2O3, Fe2O3, Na2O, K2O, MgO, CaO, TiO2. These oxides can be divided into acidic oxides (ex. SiO2, Al2O3 and TiO2) and basic oxides (ex. Fe2O3, CaO, MgO, K2O and Na2O) based on their roles on the ash fusibility [8]. SiO2, Al2O3, CaO and Fe2O3 are the primary oxides in coal ash, so the effect of SiO2, Al2O3, CaO and Fe2O3 on ash fusibility was investigated extensively [12]. SiO2 and Al2O3 generally increases AFTs, so the coal ash with high silicon and aluminum level requires flux to obtain good fusibility for slag tapping. Basic oxides work as the network modifier and form low melting point minerals to drop AFTs and improve the fusibility. The effect of individual basic oxides, CaO, Fe2O3, Na2O and K2O, and also the ratio of Base/Acid, SiO2/AlO3 and CaO/Fe2O3 on ash fusibility has been widely discussed in the last 30 years [1,10,13]. Sodium and potassium showed good flux effect on coal ash. However, the effect of K2O/Na2O on the fusibility has not been investigated and the interaction between K2O and Na2O in coal ash is unknown. The interactions between K2O and Na2O in glass was called “mixed-alkaline effect” [14]. The mixed alkaline effect attributed to the enhanced stability of the intermediate compound and thereby leads to the development of maxima in enthalpy, free energy, electrical conductivity and chemical durability when another alkali component is added at a definite optimum ratio [15]. In addition, the dependences of ionic conductivity and dielectric loss on composition are extremely nonlinear and go through minima [16]. In other words, mixed alkaline effect would change the original variation trend of some specified physical and chemical properties. Considering the similarity of glass and coal ash, the synergism between K2O and Na2O should also influence the fusibility of coal ash. In this work, the height of ash cone during AFT test was recorded to reveal the effect of K2O/Na2O mass ratio on ash fusion behavior. Mineral transformation in coal at high temperatures and the interactions between alkalis and coal ash were investigated by FactSage. The liquid phase content and viscosity of liquid phase was calculated to interpret the influences of K2O/Na2O on ash fusion behavior.

2.2. AFTs test The ash fusion temperatures were tested by 5E-AF4115 with a maximum test temperature of 1550 °C under weak reducing atmosphere (CO/CO2 = 3/2, volume fraction). The ash fusion temperatures experiments were done following the Chinese standard GB/T 219-2008. The pyramid ash cone was heated at a rate of 17.5 °C/min to 900 °C and then at 5 °C/min to flow temperature. During this process, four characteristic temperatures of ash (deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT) were identified on the basis of the specific shapes of ash cones. Besides, the height of ash cone was recorded ranging from 900 °C to flow temperature. The data was applied to describe the height variation (denoted as (H0-H)/H0) curve of ash cone. 2.3. Thermodynamic modeling FactSage was used to calculate the multiphase equilibration, initial liquidus temperature (Tini) and liquidus temperature (Tliq) of multicomponent for the system of SiO2-Al2O3-Fe2O3-CaO-K2O-Na2O under weak reducing atmosphere (CO/CO2 = 3/2, volume fraction). In the calculation, FT-oxides database was selected. Equilibrium calculations could automatically predict the proportion of minerals species and contents [17,18]. The viscosity of liquid phase is also calculated by FactSage [19]. 3. Results and discussion 3.1. Effect of K2O/Na2O on AFTs and Tliq As shown in Fig. 1, the AFTs of coal ash samples decreased with addition of K2O/Na2O. It was attributed to low ionic potential of K+ and Na+ (Na:10.5 nm−1, K+:7.5 nm−1), which was inclined to break the original bond of Si-O in coal ash and tended to decrease the AFTs [20,21]. Meanwhile, the fluxing ability increased with decreasing ionic potential. In hence, AFTs should decrease with decreasing K2O/Na2O based on network structure theory, because the ionic potential of K+ is smaller than Na+. However, AFTs decreased quickly and then increased as the K2O/Na2O mass ratio in the coal ash is reduced. The deviation of variation trend of AFTs between network structure theory and AFTs measurements was caused by mixed alkaline effect [16,22]. When the presence of K+ and Na+, the dependences of viscosity on compositions was obviously nonlinear and goes through minimum [23]. The theory from glass can well explain the synergism by K2O/Na2O on the fusibility of coal, which helps to design the highly effective flux for improving the fusibility. The fusibility by AFTs is influenced by the mineral reaction and diffusion of liquid phase. The effect of K2O/Na2O on theoretical fusion range by thermodynamic calculation was also evaluated and was shown in Fig. 2. The initial liquidus temperature (Tini) and liquidus temperature (Tliq) was introduced to describe the whole fusion range. Tini is the temperature at which the liquid slag start to appear, and Tliq is the temperature at which the solid phase disappear [24]. The lowest Tini and Tliq occurred at the K2O/Na2O mass ratio of 4/6. The theoretical fusibility also supports the existence of “mixed-alkali effect” in the coal ash slag. However, the lowest fusion temperature by experiments was at K2O/Na2O mass ratio of 6/4, which attributed to the thermodynamic calculations do not take kinetics, mass and heat transfer into consideration. Meanwhile, Tliq was paralleled with Tini as well as theoretical fusion range as the K2O/Na2O mass ratio changed. K2O/Na2O mass ratio showed obvious influence on ΔT of coal ash samples, the ΔT decreased as the K2O/Na2O mass ratio decreased until it dropped to 6/4 and then increased quickly. However, it is interesting that the change of fusion range fits the variation of fusion temperatures by K2O/Na2O. In hence, the transformation of potassium and sodium is investigated to reveal the ash fusibility.

2. Experimental 2.1. Ash preparation As typical anthracite in China, Sihe (SH) coal, with high silicon and aluminum level, was used in this work. The sample was heated to 500 °C at a rate of 6 °C/min in a muffle furnace and held for 30 min, then increased to 815 °C at a rate of 3 °C/min and held at this temperature for 2 h. Finally, the ash was moved out and cooled to room temperature. The synthetic ash samples were prepared by putting K2CO3 or/and Na2CO3 into SH raw coal ash and keep K2O/Na2O mass ratio as 10/0, 8/2, 6/4, 4/6, 2/8, 0/10 respectively. The compositions of SH coal ash were shown in Table 1. The compositions of ash mixture with addition of K2O/Na2O were shown in Table 2.

2

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Table 1 Chemical compositions of raw coal ash. Compositions

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

TiO2

K2O

Na2O

P2O5

%

40.04

30.90

9.58

8.11

0.38

7.25

1.09

0.75

0.22

0.03

containing K or/and Na include feldspar (NaAlSi3O8 and KAlSi3O8), nepheline (NaAlSiO4 and KAlSiO4) and leucite (KAlSi2O6) and shown in Fig. 3. The low melting point minerals formed by the reaction between alkalis and acidic compositions (ex. SiO2 and Al2O3), which was responsible for low ash fusion temperature of ash with K2O or/and Na2O fluxing addition. As can be seen, the content of feldspar almost kept constant with K2O/Na2O mass ratio ranging from 10/0 to 2/8 when the temperature was below 1000 °C and then decreased slightly. However, it decreased with the K2O/Na2O mass ratio decreasing when the temperature was higher than 1000 °C and finally disappeared at 1300 °C. The influence of temperature on content of feldspar was limited when the K2O/Na2O mass ratio was less than 4/6 but became dominant factor as the K2O/Na2O mass ratio surpassed 4/6. The content of nepheline increased linearly with the K2O/Na2O mass ratio decreasing (shown in Fig. 3B). Besides, the nepheline increased with the temperature increasing when the temperature was less than 1100 °C and then decreased with the temperature increasing, but K2O/Na2O mass ratio was the primary factor. Leucite (KAlSi2O6) dropped with the K2O/Na2O mass ratio decreasing as well as temperature increasing. Additionally, the disappeared temperature of leucite moved towards low fusion temperature as the K2O/Na2O mass ratio decreased. From the above discussion we can induce that high K2O/Na2O mass ratio was likely to produce feldspar and leucite but nepheline was more inclined to form in low K2O/Na2O mass ratio samples. The influence of K2O or Na2O on ash fusibility attributed to minerals containing K or Na (former discussed). However, the charge number and ionic radius of K+ and Na+ were very close, so two ions would be appeared in one crystal. The competition between K+ and Na+ would be similar to Ca2+ and Fe2+ in slag. In the structure of the silicates melts, Ca2+ and Fe2+ were regarded to be stabilized in the network structure by Si4+ and Al3+ [17]. As shown in Fig. 4, the mineral species with K and/or Na include Feldspar, nepheline, KAlSiO4, KAlSi2O6 and slag which were normalized into 100%. Most of K occurred in leucite and remained portion existed in nepheline and feldspar at 1000 °C. However, the feldspar content is limited compared to nepheline and therefore a small influence on AFTs. In addition, little of K appeared in feldspar when the temperature surpasses 1200 °C. As discussed above, the competition effect between K2O and Na2O on AFTs was mainly reflected in nepheline. The nepheline was the exclusive mineral which large number of K and Na co-contained, so the proportion of NaAlSiO4 and KAlSiO4 in coal ash at high temperatures might be one of the key factors influencing ash fusibility. In hence, the SiO2-NaAlSiO4-KAlSiO4 ternary phase diagram was constructed to clarify the effect of K2O/ Na2O on AFTs. Many studies found that the parallel variation trend between the ash fusion temperatures and liquidus temperature, which was proved in previous study [27,28]. Hence, the change of liquidus temperature line in SiO2-NaAlSiO4-KAlSiO4 ternary phase diagram was able to describe the ash fusion temperatures variation. As shown in phase diagram (Fig. 5), the lowest liquidus temperature appeared in nepheline primary phase area. In this area, the content of NaAlSiO4 was higher than KAlSiO4, which proved that fluxing ability of N2O is superior to K2O. However, when the primary phase was changed, the continuous increase of NaAlSiO4 would result in the increase of liquidus temperatures. In hence, there must be an optimal KAlSiO4/NaAlSiO4 at which a minimum liquids temperature occurred. As show in Fig. 6, the lowest Tliq occurred at KAlSiO4/NaAlSiO4 of 7/3, because the KAlSiO4 and NaAlSiO4 may form solid solution at this ratio, so the optimal AFTs between the ratio 8/2 and 6/4 of K2O/N2O is well explained.

Table 2 Compositions of synthetic ash.

1-1 1-2 1-3 1-4 1-5 1-6

SH (w %)

K2O (w %)

Na2O (w %)

SiO2/Al2O3

90.0

10.00 8.00 6.00 4.00% 2.00% 0.00%

0.00 2.00 4.00 6.00% 8.00% 10.00%

1.30

1400

DT ST HT FT

AFTs (oC)

1300

1200

1100

0/0

10/0

8/2

6/4

4/6

2/8

0/10

K2O/Na2O Fig. 1. The influence of K2O/Na2O on ash characteristic temperatures.

Tini Tliq

T (°C)

1500

450 400

1400

350

1300

300

1200

250

1100

200

1000

0/0

10/0

8/2

6/4

4/6

2/8

0/10

ΔT (oC)

1600

150

K2O/Na2O

Fig. 2. ΔT (Tliq-Tini) of samples with K2O/Na2O mass ratio.

3.2. The influence of K2O/Na2O on the migration of K and Na in mineral phases Alkalis in coal can be divided into four categories: water-soluble portion, ammonium acetate-soluble portion, hydrochloric acid-soluble portion and hydrochloric acid-insoluble portion [25,26]. The watersoluble and ammonium acetate-soluble portion would volatilize or transform into acid-insoluble portion in process of ash preparation, but the minerals species of acid-insoluble portion was still more important for the ash fusibility in coal ash at high temperatures. The minerals 3

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900 oC 1000 oC 1100 oC 1200 oC 1300 oC

60

Nepheline (%)

20

Feldspar (%)

80

900 oC 1000 oC 1100 oC 1200 oC 1300 oC

A

10

B

40

20

0

0 10/0

8/2

6/4

4/6

2/8

0/10

10/0

8/2

6/4

K2O/Na2O 60

2/8

0/10

900 oC 1000 oC 1100 oC 1200 oC 1300 oC

C

45

KAlSi2O6 (%)

4/6

K2O/Na2O

30

15

0 10/0

8/2

6/4

4/6

2/8

0/10

K2O/Na2O Fig. 3. The influence of K2O/Na2O on minerals containing K or Na.

3.3. Minerals transformation during ash fusion process

mechanism inclined to form abundant active melt which can quickly dissolve the residual refractory minerals, such as mullite and some SiO2, in the system. The “Softening-melting” mechanism was related to slow softening and subsequent melting of minerals [31]. In hence, it is easy to conclude that the fusion mechanism of SH coal ash belongs to “Melting-dissolution” based on its fusion properties. To illustrate the effect of K2O/Na2O mass ratio on ash fusibility behavior, the phase assemblage of ash samples for different K2O/Na2O mass ratio as a function of temperature were also calculated by FactSage (Fig. 8). Observations showed that the sub-liquidus phase changes from high-melting mullite into low-melting feldspar or spinel as the fluxing addition, which may be responsible for the fact that K2O/ Na2O fluxing addition decreased AFTs of SH raw coal ash. Compared to raw coal ash, mullite and cordierite disappeared and feldspar dropped quickly in the product as the K2O/Na2O fluxing added. Meanwhile, nepheline, leucite and mellite generated. Preceding analysis indicated that the K2O/Na2O mass ratio influence the nepheline and leucite content significantly in the ash melting process. Therefore, several reactions can be deduced according to the production analysis and showed as below:

During the ash fusion process, some minerals partially or fully melted into liquid phase and some transformed into other crystals through chemical reactions, which resulted in the variations of mineral species and contents at high temperatures [29]. Therefore, the fusibility of ash was mainly determined by the components and the quantities of minerals at high temperatures, which can be easily obtained by FactSage calculation [30]. Fig. 7 showed the phase assemblage temperature curve of SH coal ash calculated by FactSage software with SiO2-Al2O3Fe2O3-CaO-K2O-Na2O system. The main minerals include mullite (Al2Si2O6), feldspar (CaAl2Si2O8), spinel (FeAl2O4), cordierite (Fe2Al4Si5O18) and quartz (SiO2) and sub-liquids mineral was mullite. The process of slag formation consisted of three stages: the first stage (1100–1150 °C), slag mainly formed from the reaction of clinopyroxene and cordierite (Fe2Al4Si5O18); the second stage, the slag increased accompanying with feldspar dropping and the end of this stage was temperature that feldspar disappeared; the change in the last stage was the dissolve and melting of refractory mullite with high-melting point [11]. The ash fusion mechanism can be concluded into two types including “Melting-dissolution” and “Softening-melting” mechanism based on ash fusion process characters. The “Melting-dissolution”

CaAl2Si2O8 + K2O → 2KAlSiO4 + CaO

4

(1)

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KAlSi2O6

1000oC

100

Nepheline Feldspar

100

80

Na distribution (%)

K distribution (%)

80

60

40

60

40

20

20

0

10/0

8/2

6/4

4/6

2/8

0

0/10

10/0

8/2

Slag KAlSi2O6

1100 oC

100

Na distribution (%)

K distribution (%)

2/8

0/10

Slag Nepheline Feldspar

80

60

40

20

60

40

20

10/0

8/2

6/4

4/6

2/8

0

0/10

10/0

8/2

Slag KAlSiO4

1200 oC

100

6/4

4/6

2/8

0/10

K2O/Na2O

K2O/Na2O

80

Na distribution (%)

60

40

Slag KAlSiO4 Nepheline Feldspar

1300oC

100

KAlSi2O6

Nepheline Feldspar

80

K distribution (%)

4/6

1100 oC

100

Nepheline Feldspar

80

20

0

6/4

K2O/Na2O

K2O/Na2O

0

Nepheline Feldspar

1000 oC

60

40

20

10/0

8/2

6/4

4/6

2/8

0

0/10

10/0

8/2

K2O/Na2O

6/4

4/6

2/8

0/10

K2O/Na2O

Fig. 4. The distribution of K or Na on minerals.

K2O + Al2O3 + 4SiO2 → 2KAlSi2O6

(2)

CaAl2Si2O8 + Na2O → 2NaAlSiO4 + CaO

(3)

2CaO + Al2O3 + SiO2 → Ca2Al2SiO7

(4)

−163.8 kJ/mol respectively, which indicated that all of these reactions were feasible. Based on the principles of minimum Gibbs free energy, the reaction (2) was superior to reaction (1) when the K2O presented. This can well explain why most of K element occurred in leucite. Reaction (1), (3), (4) provided a reasonable explanation for the anorthite decreased and mellite formed, which finally resulted in a low ash fusion temperature.

The reaction order can be reflected by the value of Gibbs energy, so the Gibbs energy of above reactions at 1000 °C was calculated by reaction model of FactSage software. The Gibbs energy of above four reactions was −288.2 kJ/mol, −486.2 kJ/mol, −224.7 kJ/mol and 5

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Fig. 5. SiO2-NaAlSiO4-KAlSiO4 ternary phase diagram.

KAlSiO4/NaAlSiO4 1120

10/0

100

2.5/7.5 3.7/6.3 4.8/5.2 6.7/3.3 7.6.2.4 8.5/1.5

0/10

Mullite

1

3

2

Relative mass (%)

80

Tliq (oC)

1080

1040

1000

60

40

Spinel 20

Fe2Al4Si5O18 Clinopyroxene

SiO2 960 10/0

8/2

7/3

6/4

4/6

3/7

2/8

0 800

0/10

Slag

Feldspar

900

1000

1100

1200

1300

1400

1500

1600

T(C)

K2O/Na2O

Fig. 7. Phase assemblage-temperature curve for SH coal ash.

Fig. 6. Effect of NaAlSiO4/KAlSiO4 on Tliq.

the further reaction among Na2O, SiO2 and Al2O3 to form feldspar. Meanwhile, the coal ash height variation traces were almost overlapped when the temperature was low, because the content of nepheline is almost not changed with K2O/Na2O variation when the K2O/Na2O mass ratio was lower than 4/6. Yan et al. [9] found that the whole fusion process can be divided into three stages based on shrinkage trace including sintering stage, primary fusion stage and free liquidus stage. The fusion process was divided into two steps based on average rate variation of ash cone height. Fig. 10 showed the variation of average shrinkage rate of ash cone. The average shrinkage rate is lower than 0.3%/°C at the first stage and not influenced by K2/O/Na2O, but all of them are higher than 0.75%/°C at second stage. The shrinkage rate not only influenced by slag content but also the viscosity of liquid phase. In hence, Fig. 9 also

3.4. Effect of K2O/Na2O on ash fusion process The ash fusion temperatures of SH coal ash were influenced by K2O/ Na2O mass ratio enormously, but four discontinuous characteristic temperatures were not sufficient to describe the whole fusion detail. Fig. 9 presented the height variation of ash cone with increasing temperature, which was used to describe the fusion process. When the K2O/ Na2O mass ratio was higher than 6/4, the shrinkage curve moved towards low temperature orientation as the K2O/Na2O mass ratio decreased. Besides, the shrinkage curve was smooth, which meant the disturbance caused by temperatures was limited. However, the shrinkage curve moved slowly towards high temperature direction when the K2O/Na2O mass ratio was lower than 4/6, which attributed to 6

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100

100

A

Spinel

80

60

KAlSi2O6

Relative mass (%)

80

Relative mass (%)

B

Spinel

Slag

40

Feldspar 20

KAlSi2O6

60

Nepheline

40

Feldspar

20

Nepheline

Mellite 0 800

900

1000

1100

1200

1300

1400

1500

Mellite

0 800

1600

Slag

900

1000

1100

o

Relative mass (%)

Relative mass (%)

80

Slag Nepheline

40

KAlSi2O6

60

Slag Nepheline

40

20

Spinel KAlSi2O6 Mellite

Mellite 900

1000

1100

1200

1300

1400

1500

0 800

1600

900

1000

1100

o

Relative mass (%)

Relative mass (%)

Spinel Feldspar

900

1000

60

1100

1200

1600

1300

F

1400

1500

0 800

1600

T (oC)

Slag

Nepheline

40

20

Mellite Olivine KAlSi2O6

0 800

1500

80

Slag

60

20

1400

100

E

Nepheline

1300

T ( C)

100

80

1200 o

T ( C)

40

1600

D

Feldspar

Feldspar

0 800

1500

100

C

Spinel

20

1400

T ( C)

100

60

1300

o

T ( C)

80

1200

Spinel Feldspar Mellite 900

1000

1100

1200

1300

1400

1500

1600

T (oC)

Fig. 8. Phase assemblage-temperature curves for SiO2-Al2O3-CaO-Fe2O3-Na2O-K2O with different K2O/Na2O mass ratio. A) K2O/Na2O = 10/0; B) K2O/Na2O = 8/2; C) K2O/Na2O = 6/4; D) K2O/Na2O = 4/6; E) K2O/Na2O = 2/8; F) K2O/Na2O = 0/10.

7

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0.8

K2O/Na2O=6/4

A

100

K2O/Na2O=8/2

K2O/Na2O=2/8

0.6

K2O/Na2O=0/10

Slag (%)

Stage 2

K2O/Na2O=4/6

80

0.4

0.2

60

K2O/Na2O=10/0

B

K2O/Na2O=8/2 K2O/Na2O=6/4 K2O/Na2O=4/6

stage 2

K2O/Na2O=10/0

K2O/Na2O=2/8

K2O/Na2O=0/10

40

20

0.0

stage 1

1.0

Stage 1

(H-Ho)/Ho

X. Li, et al.

0

1000

1050

1100

1150

1200

1250

1300

1350

950

1000

1050

T (oC)

1100

1150

1200

1250

1300

1350

1400

T (oC)

Fig. 9. The height variation of ash cone (A) and slag (B) with K2O/Na2O mass ratio. stage 1 stage 2 stage 1 stage 2

Viscosity (Pa·S)

25000

1.5

20000 1.0

15000 10000

0.5 5000 0

10/0

8/2

6/4

4/6

2/8

0/10

mass ratio in the coal ash is reduced. Meanwhile, theoretical fusion range (Tliq-Tini) showed the similar trend with the decreasing K2O/ Na2O mass ratio. (2) The competition effect between K2O and Na2O on AFTs can be reflected by proportion of KAlSiO4 and NaAlSiO4 in nepheline, which supported by SiO2-KAlSiO4-NaAlSiO4 ternary phase diagram. (3) Addition of K2O or/and Na2O changed the sub-liquidus phase from high-melting mullite into low-melting feldspar and nepheline, which was responsible for that K2O/Na2O decreases AFTs of coal ash. (4) Ash fusion process can be divided into two stage based on the variation of ash cone height. Limited slag content and large viscosity causes a low shrinkage rate at first stage. However, the combined effect by rapid increase of slag content and sharp dropped viscosity leads to a high shrinkage rate at the second stage.

2.0

Averagre Shrinkage rate (%/oC)

30000

0.0

K2O/Na2O

Fig. 10. The average shrinkage rate and viscosity of ash samples at different stages.

CRediT authorship contribution statement

presented the viscosity of coal ash at specified temperatures. The temperatures of slag content at 20% and 80% are close to the end temperature of first stage and second stage respectively, at which the corresponding slag content have reached maxima and the viscosity became the most important influential factor on flow. In hence, the temperature of slag content of 20% and 80% were selected for viscosity. The viscosity of ashes in first stage was higher than that in second stage. It was indicated that limited slag content and high viscosity was hard to make ash cone height dropped at first stage. However, when it entered into second stage, quickly decreased viscosity and sharply increased slag content sped the shrinkage of ash cone and thereby cause a high shrinkage rate. As discussed above, addition of K2O/Na2O flux not only made the refractory minerals disappeared but also leaded to a prolonged fusion process, which implied the SH ash with flux addition trended to be softening-melting mechanism. In other words, K2O/Na2O change ash fusion mechanism from “melting dissolving” to “softening melting”.

Xiaoming Li: Conceptualization, Methodology. Lifei Zhi: Visualization, Investigation. Wenju Shi: Data curation, Writing - original draft. Lingxue Kong: Visualization, Investigation. Jin Bai: Supervision, Validation. Jianglong Yu: Writing - review & editing. Markus Reinmöller: Visualization, Investigation. Stefan Guhl: Writing - review & editing. Bernd Meyer: Writing - review & editing. Wen Li: Supervision, Validation.

4. Conclusions

This work was supported by Natural Science Foundation of Shanxi Province [Grant numbers 201801D121050 and 201801D121051], Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province [Grant number 2017006], Doctoral Scientific Research Foundation of Taiyuan University of Science and Technology [Grant number 20162023], Joint Foundation of Natural Science Foundation of China and Shanxi Province [Grant number U1510201], and NSFC-DFG [grant number 21761132032].

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. Acknowledgments

In this work, the effect of K2O/Na2O mass ratio on fusibility behaviors of SH coal ash was explored under weak reducing atmosphere. The main conclusion presented as follow: (1) AFTs decreased quickly and then increased slowly as the K2O/Na2O 8

Fuel 265 (2020) 116964

X. Li, et al.

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