Al2O3 on ash fusibility and carbothermal reaction

Fuel 255 (2019) 115846

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

The role of residual char on ash flow behavior, Part 2: Effect of SiO2/Al2O3 on ash fusibility and carbothermal reaction

T

⁎

Ji Wanga,b, Lingxue Konga, Jin Baia, , Huiling Zhaoa, Stefan Guhlc, Huaizhu Lia, Zongqing Baia, Bernd Meyerc, Wen Lia a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China University of Chinese Academy of Sciences, Beijing 100049, China c Institute of Energy Process Engineering and Chemical Engineering, Technische Universität Bergakademie Freiberg, Freiberg 09599, Germany b

A R T I C LE I N FO

A B S T R A C T

Keywords: Coal gasification Residual char Ash fusibility Carbothermal reaction SiO2/Al2O3

SiO2/Al2O3 weight ratio (Si/Al) in the ash content is always regarded as the key factor on ash fouling and sintering in the gasifier, and residual char is often found in gasification slag. The effect of Si/Al on the fusibility of ash containing different contents of residual char (5–15%) was investigated in this work. The residual char was added into five ash samples with different Si/Al from 1.0 to 3.0. The results show that the residual char content shows a significant effect on the AFTs and the fusion temperature range (ΔT = FT-DT) when the Si/Al of the ash is higher than 1.0, especially for the ash with more than 10% residual carbon. Carbothermal reaction products (SiC and Fe3Si) have obvious influence on hemispherical temperature and flow temperature of ash with a high Si/Al. For the ash with Si/Al of 1.0, anorthite, corundum and mullite are the major minerals at high temperatures, and formation of SiC and Fe3Si shows a slight effect on the ash fusion temperatures. However, there are less anorthite and SiO2 at high temperatures in the ashes with Si/Al of 2.0 and 3.0. Besides, Si/Al mainly affects the formation of SiC, while it does not influence formation of Fe3Si. The formation rate of SiC in ash with Si/Al = 2.0 is slower than that of the ash with Si/Al = 1.0 and Si/Al = 3.0. Furthermore, it demonstrates that a high amorphous content and less SiO2 in the ash are detrimental to carbothermal reaction (3C + SiO2 = SiC + 2CO).

1. Introduction Coal gasification is an efficient clean coal technology to transform coal into syngas. The quantity and quality of the mineral matter in the coal determines the operating temperature of the gasifiers [1]. The ash fusibility is the most important criteria and directly determines the limit of operating temperature to maintain a stable ash discharge condition [2]. Fluidized-bed gasifiers usually operate at temperatures well below the ash fusion temperatures of the fuels (900–1050 °C) to avoid ash melting and operational difficulties such as agglomeration, sintering [3]. Entrained-flow gasifiers require that flow temperature (FT) of the coal ash should be below 1350 °C to transform the ash/slag into a liquid slag [4]. Therefore, it is necessary to study the ash fusibility of the coal [5]. The ash fusibility at high temperatures is predominantly dependent on the major mineral phases of coal ash and the interaction between them [5,6]. The four major components: silica (generally 20–80 wt% of ash weight), alumina (5–45 wt%), ferric oxide (0.2–30 wt%) and

⁎

calcium oxide (0.2–32 wt%) usually comprise about 90 wt% of coal ash [7]. Generally, the coal ash components can be divided into acid and basic oxides. Acid oxides (SiO2, Al2O3 and TiO2), which easily combine with oxygen, form polymer structure and increase AFTs, while basic oxides (Fe2O3, CaO, MgO, Na2O and K2O) serve to prevent the formation of the polymers and decrease ash fusion temperatures (AFTs) [8]. Since silica and alumina are the most abundant components in the coal ash, it is often regarded as a silicoaluminate system [9]. As a non-negligible component in gasification slag, coal char has complex interaction with molten ash during coal gasification process, which leads to the residual char in gasification slag [10]. Shimizu et al. [11] proposed a model of char captured by molten ash surface under high-temperature gasification conditions. Montagnaro et al. [12,13] investigated the occurrence of near-wall segregation of carbon particles during entrained-flow gasification of coal in the slagging regime. Nearwall segregation of carbon was beneficial to carbon conversion as far as permanent carbon entrapment inside the molten ash could be ruled out. The effect of residual char on the ash fusibility has also been

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

https://doi.org/10.1016/j.fuel.2019.115846 Received 17 June 2019; Received in revised form 11 July 2019; Accepted 18 July 2019 Available online 22 July 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

Fuel 255 (2019) 115846

J. Wang, et al.

2.2. Preparation of the blended ash and residual char

investigated by several researchers. Chen et al. [14] found that coal char in the ash has remarkable influence on the AFTs as well as the ash melting behavior. An infusible skeleton could be formed because of the sticking interaction between char and ash. Kong et al. [15] discovered that the effect of residual carbon on slag viscosity and ash fusibility was significant when the mass fraction of residual carbon exceeded 5 wt%, and “carbothermal reaction” occurred in the ash and slag. A refractory mineral, SiC, was formed due to the carbothermal reaction between minerals and residual char. SiO2/Al2O3 weight ratio (Si/Al) is always deemed as one important parameter, which influences the AFTs as well as mineral transformation [2,16,17]. Some researchers have studied the effect of Si/Al on the AFTs. Liu et al. [18] found that the AFTs of synthetic ash samples decreases as the Si/Al increases until it reaches 1.5, while it decreases slightly or even remains constant at higher Si/Al. Li et al. [19] found that a sharp decrease in AFTs of synthetic ashes was observed with the increasing Si/Al due to the decline of high-melting Al6Si2O13. Yan et al. [9] studied the effect of Si/Al on ash fusion process by thermomechanical analysis (TMA). The shrinkage of ash samples with low Si/ Al followed the trend of slag formation rate due to the low viscosity of liquid phase formed, while the samples with Si/Al higher than 3.5 can be considered as Newtonian fluid at FT due to low solid content. However, when residual carbon is presented in the ash, the effect of ash composition, especially Si/Al, on the ash fusibility and carbothermal reaction is still not clear. The effect of graphitization degree of residual char on ash fusibility and carbothermal reaction was studied in our previous work [20]. When the ash contains same content of residual char, the AFTs increase as the graphitization degree of residual char increases. In this part, we investigated the effect of Si/Al on the fusibility of the ash containing residual char under Ar atmosphere. X-ray diffraction (XRD) and FactSage calculation was used to characterize mineral transformation of the ashes with residual char. Thermogravimetry coupled with mass (TGMS) spectrometry were carried out to study the effect of Si/Al on carbothermal reaction during heating. The major objectives are (1) to clarify the effect Si/Al on the AFTs when residual char was presented in the ash; (2) to reveal the carbothermal reaction for the ash with different Si/Al.

A demineralized high temperature coal char was prepared and the detailed of the coal char can be found in our previous work [20]. Briefly, a demineralized Chinese anthracite coal was heated to 1400 °C under N2 and kept for 2 h in a horizontal tube furnace, and it was denoted as RC. In this work, RC was added into the above synthetic ashes based on the carbon content. The blended ashes were thoroughly grinded in an agate mortar and stored in the desiccator after heating at 110 °C for 2 h. The ashes with 5%, 10%, and 15% carbon were prepared, and labelled as -RC-5, -RC-10 and -RC-15, respectively. 2.3. Measurement of ash fusibility A Carbolite CAF 1600 ash fusion furnace (Carbolite Gero Limited, Britain) was used to measure the AFTs of the blended ashes under Ar atmosphere according to the American standard ASTM D1857-04. This procedure involved heating an ash with a specific geometry at a rate of 8 °C/min to 1560 °C. During this process, the initial deformational temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT) were identified according to the specific shapes of the ash. Owing to the man-induced factor and methodology, the accuracy of the AFTs is ± 30 °C. 2.4. Thermodynamic calculation Software FactSage is widely used to predict the equilibration of multiphase, liquidus temperature as well as the proportion of minerals and slag at high temperatures [21]. FactSage software packages 7.2 (Equilib module) together with the database FactPS and FToxid were applied to calculate the mineral compositions of the blended ashes from 800 to 1650 °C. The chemical compositions of the ash sample (SiO2, Al2O3, Fe2O3, CaO and graphite) by mass fraction were input under 1 atm under an Ar atmosphere. 2.5. Mineral transformation in the blended ash In order to illustrate the fusibility of ash containing RC, the synthetic ashes were treated in a horizontal electric tube furnace to investigate mineral transformation. The blended ashes were heated to the targeted temperature and kept for 10 min under Ar atmosphere, and then cooled at 8 °C/min in flowing Ar at 300 ml/min. The ash at high temperatures was ground to less than 0.074 mm. X-ray diffraction patterns of the ashes which were treated at high temperatures were recorded on a PANalytical X’Pert3 Powder diffractometer at a voltage of 40 kV and a current of 40 mA with Cu Kα radiation (1.541874 A), and the focusing (Bragg-Brentano) optics was used. Powder samples were mounted on a sample holder and scanned with a step size of 0.04° varying 2 Theta from 5° to 85°. A crystalline zinc oxide (ZnO) as spiking material was added into the samples to determine the content of crystalline minerals and amorphous matters using Siroquant. Generally, the error for each case is lower than 1.0%. More information about the software interpretation process have been given by Ward et al. [22].

2. Experimental 2.1. Preparation of synthetic ash sample In order to simplify the system and eliminate effect of trace elements on the AFTs, four major components in the ash, SiO2, Al2O3, Fe2O3 and CaO, were used to prepare synthetic ashes. The purity of SiO2, Al2O3, Fe2O3, and CaO was 99.7%, 99.5%, 99.8%, and 98.0%, respectively. The chemical compositions of the synthetic ashes are listed in Table 1. In this study, the contents of Fe2O3, CaO and SiO2 + Al2O3 in the ashes were kept constant, and the SiO2/Al2O3 ratio varied from 1.0 to 3.0, which almost covered the ratio range of Shanxi coal ashes. The synthetic ash samples with different SiO2/Al2O3 were denoted as SA-1.0, SA-1.5, SA-2.0, SA-2.5, and SA-3.0.

2.6. TG-MS analysis

Table 1 Chemical compositions of the synthetic ashes (wt%).

SA-1.0 SA-1.5 SA-2.0 SA-2.5 SA-3.0

SiO2

Al2O3

Fe2O3

CaO

Si/Al

37.50 45.00 50.00 53.60 56.25

37.50 30.00 25.00 21.40 18.75

8.00 8.00 8.00 8.00 8.00

17.00 17.00 17.00 17.00 17.00

1.0 1.5 2.0 2.5 3.0

Carbothermal reaction of the ash with different Si/Al ratios was characterized by thermogravimetric analysis (TGA, SETSYS Evolution 16/18, France) coupled with mass spectrometry (Omnistar, Switzerland). About 10 mg sample were placed in a platinum holder under Ar atmosphere (purity, > 99.999%) at a flowing rate of about 50 ml/min. The samples were heated at a rate of 8 °C/min to 1560 °C, which was the same as for the AFTs test. 2

Fuel 255 (2019) 115846

J. Wang, et al.

1600

(a) DT

ST

HT

(b)

1600

FT

DT

ST

HT

FT

ΔΤ=56°C ΔΤ=55°C

1500

Temperature(°C)

Temperature(°C)

1500

1400

1300

1200

C=5%

C=10%

C=0%

C=15%

(c) DT

ST

HT

FT

C=5%

DT

ΔΤ=333°C

ST

HT

Temperature(°C)

ΔΤ=162°C ΔΤ=120°C

1200

C=15%

FT

ΔΤ=310°C

1500

1400

C=10%

(d)

1600

1500

Temperature(°C)

1300

1100

C=0%

1300

ΔΤ=200°C

1200

1100

1600

ΔΤ=178°C 1400

1400

ΔΤ=151°C

1300

ΔΤ=92°C 1200

1100

1100 C=0%

C=5%

C=10%

1600

C=15%

C=0%

C=5%

C=10%

C=15%

(e) DT

ST

HT

FT

ΔΤ=305°C

Temperature(°C)

1500

1400

ΔΤ=161°C 1300

ΔΤ=125°C

1200

1100 C=0%

C=5%

C=10%

C=15%

Fig. 1. AFTs of the ashes with different contents of RC Note: The arrow means the AFT is higher than 1560 °C. ΔT = FT-DT. (a): SA-1.0; (b): SA-1.5; (c): SA-2.0; (d): SA-2.5; (e): SA-3.0.

3. Results and discussions

the Si/Al of the ash is 2.0 (Fig. 1(c)), the FTs of ash with RC-5 and RC10 are elevated to 1319 °C and 1544 °C from 1266 °C, respectively. When the Si/Al of the ash increases up to 3.0 (Fig. 1(e)), the addition of RC causes a larger increase, over 200 °C, of FT. The fusion temperature range (ΔT = FT-DT) is usually used to characterize sensitivity of ash fusion to the temperature [23]. It can be seen that the ΔT of ash with different Si/Al increases with the increasing RC. Taking the ash with Si/Al of 2.0 as an example, the ΔT increases from 120 °C to 162 °C to 333 °C when the RC is from 0% to 5% to 10%. Besides, the presence of RC also shows an obvious influence on

3.1. Effect of Si/Al on ash fusibility As shown in Fig. 1, the AFTs of ash with RC are higher than those of the raw ash when 5–15% RC is added in the raw ash, and the increase of AFTs of the ash with Si/Al higher than 1.0 is more obvious when the ash has the same RC content. For example, when the Si/Al of the ash is 1.0, FT of the ash without RC is over 1400 °C, and it increases less than 100 °C when the ash contains 5%-15% RC (Fig. 1(a)). However, when 3

Fuel 255 (2019) 115846

J. Wang, et al.

(a) SA-1.0-RC-10

(b) SA-2.0-RC-10

(c) SA-3.0-RC-10

Fig. 2. Pictures of ash cones after AFTs test.

the ΔT of the ashes with high Si/Al when the RC is the same. For example, when 5% RC is added into the ash, the ΔT of SA-1.0 is 56 °C, while it is more than 150 °C for the ash with Si/Al higher than 1.0. Moreover, the ΔT of the ash ascends sharply from about 150 °C to more than 300 °C when the RC of the ash increases to 10% (Fig. 1(c-e)). Pictures of ash cones after the AFTs test are shown in Fig. 2. The apex of non-molten ash cone is round and top of ash cone has already melted. As the Si/Al of the ash increases, height of the ash cone at high temperature decreases. Besides, an interesting observation is that there are a lot of holes in the ash cone, which implies that gas releases during heating, and the SA-2.0-RC-10 has more holes. Furthermore, some spherical iron particles are found in the ash cones from Fig. 2 (b) and (c). This implies that Fe2O3 is reduced to Fe by the RC [24]. However, the effect of iron precipitation on the AFTs is not considered in this work.

the RC increases. Therefore, the AFTs of the ash with more RC are higher (Fig. 1(a)). However, because the ash has abundant anorthite (53.8%) and corundum (35.2%), the effect of Fe3Si (3.5–8.3%) and SiC (2.7%) formation on AFTs is slight. These transformations contributed to the slight increase in the AFTs of SA-1.0 with RC. When the Si/Al of the ash increases to 2.0 (Fig. 5), the contents of anorthite and corundum decreases sharply, and the content of amorphous phase in slag without RC is 81%. The amorphous phase indicates the formation of liquid phase at high temperature [27]. Therefore, the AFTs of SA-2.0 are greatly lower than that of SA-1.0. When the RC is 5%, Fe3Si (2.8%) formed at 1400 °C, which contributes to the increase of AFTs. It also can be observed that the content of Fe3Si increases from 2.8% to 8.9% as the RC increases from 5% to 10%. SiC is also present in the ash with RC-10. Compared with the ash without RC and with RC-5, Fe3Si and SiC lead to a sharp increase of hemispherical temperature (HT) and fluid temperature (FT) of the ash with RC-10. Besides, SiC is detected in the SA-2.0-RC-10. This suggests that SiC is more easily formed in SA-2.0. In Fig. 6, more amorphous phase (90.5%) is formed at 1400 °C when Si/Al rises to 3.0, and anorthite is not found in the ash, which implies that more crystalline phase changes into amorphous phase [27]. Besides, graphite is found in the ash at 1400 °C which indicates that the carbothermal reaction between the minerals and RC is impeded. Dai et al. [28] explored the role of residual carbon on fusibility and flow properties of rice straw ash, and reported that the melting phase could cover the unreacted residual carbon and silica mineral to impede the carbothermal reaction. Because the melting temperature of carbon including crystalline (graphite) and amorphous is about 3500 °C, the unreacted carbon tends to form a skeleton in the ash and prevent the ash fusion. In addition, as the difference of amorphous phase and crystal phase between SA-3.0 and SA-2.0 is not obvious, the AFTs does not change greatly (Fig. 1(c) and (e)).

3.2. Mineral transformation of ash containing RC Mineral transformations of ashes with 10% RC were calculated by FactSage. Because there is no data for coal char in the database, graphite instead of RC was used in the calculation. In Fig. 3(a-b), the mineral of ash without graphite is mainly anorthite. Mullite and spinel are also found at low temperatures, which is favor of the increase of the DT of ash. When graphite is added into the ash, SiC, FeSi and Al2O3 are the minerals in the ash while the temperature is above 1382 °C. In addition, the melt also starts and slag forms at this temperature. The refractory minerals, SiC and FeSi, are attributed to the increase of the liquidus temperature at which temperature of the last solid phase disappeared [25]. When the Si/Al rises to 2.0 (Fig. 3(c-d)), the amount of anorthite is lower than that of SA-1.0 at the same temperature. Besides, the melting point of other minerals such as SiO2 (Melting point: 1650 °C) and andradite (Ca3Fe2Si3O8) (Melting point: 1300 °C) are lower than that of mullite (Melting point: 1850 °C) and spinel (Melting point: 1780 °C) in SA-1.0. The temperature at which the slag is formed for SA2.0 is lower than that of SA-1.0. When 10% graphite is added in the ash (Fig. 3(d)), the high-melting minerals, SiC and FeSi, highly rises the liquidus temperature of the ash. As the Si/Al increases to 3.0, there are more SiO2 and less anorthite at low temperature. However, it was found that the max contents of FeSi and SiC are 7.56% and 6.82% in Fig. 3. (b), (d) and (f), which demonstrates that formation of FeSi and SiC was not affected by Si/Al. The minerals in the ash at 1400 °C are used to illustrate the influence of Si/Al on the fusibility of ash with RC. As shown in Fig. 4, for SA1.0, anorthite (58.3%) and corundum (35.2%) are the dominant minerals in the ash without RC at 1400 °C. It is known that corundum (melting point = 2050 °C) and anorthite (melting point = 1553 °C) are refractory minerals, which lead to the high fusion temperature of ash [26]. As the addition of RC increases from 10% to 15%, the contents of amorphous phase and anorthite decrease while the content of corundum increases. This explains why the DT of RC-10 slightly decreases compared to the ash with RC-15. Besides, the contents of Fe3Si (formed when 5% RC in ash) and SiC (formed when 15% RC in ash) ascend as

3.3. Effect of Si/Al on carbothermal reaction To further investigate the effect of Si/Al on carbothermal reaction, the TG-DTG-MS results are shown in Fig. 7. In our previous work, weight loss during 1100–1450 °C which is caused by the carbothermal reaction can be divided into two stages according to the DTG and MS curves. Stage I is assigned to the formation of Fe3Si (Eq. (1)), and Stage II is the formation of SiC (Eq. (2)).

3Fe + SiO2 + 2 C= Fe3 Si + 2CO (g)

(1)

3 C+ SiO2 = SiC + 2CO (g)

(2)

It can be seen that weight loss of the ash decreases evidently from 8.91% to 5.5% when Si/Al rises from 1.0 to 2.0, and then it increases slightly from 5.5% to 6.16% as the Si/Al rises from 2.0 to 3.0. The weight loss of the ashes with different Si/Al is listed in Table 2. The weight loss of Stage I (Eq. (1)) is almost the same, which implies that formation of Fe3Si is not affect by the Si/Al. this is due to the Si/Al does not alter the content of Fe2O3 in the ash and there are enough SiO2 and 4

Fuel 255 (2019) 115846

J. Wang, et al.

100

Ca(Al,Fe)12O19

80

Melitite

19

CaAl2Si2O8

(b)

Gas

Fe3C

SiC

80

Mass / %

Slag

40

Fe

Mullte Al2O3

Spinel

60

12

100

(a)

Mullite

Fe2O3

Mass /%

Ca(Al,Fe) O

Fe3Si Al2O3

C

60

Slag CaAl2Si2O8

40

CaAl12O19 20

20

SiC

FeSi

0 800

Al2O3

0

1000

1200

1400

1600

800

1000

(c)

Ca3Fe2Si3O12

Fe2O3

Fe3C

Fe

80

Mass / %

Slag

CaAl2Si2O8

SiO2

Fe3Si

C

60

(d)

Gas

CaSiO3

SiO2

60

Mass /%

1600

100

100

40

1400

Temperature /°C

Temperature /°C

80

1200

Slag

SiC

FeSi

40

CaAl2Si2O8 20

20

0

0 800

1000

1200

1400

800

1600

1000

Temperature /°C

1600

Temperature / ºC

(e)

Fe3C

Fe

Ca3Fe2Si3O12

SiO2 Mass / %

Slag 40

(f)

Gas

CaSiO3

80

Fe2O3

60

Mass /%

1400

100

100

80

1200

SiO2

60

Fe3Si

Slag

C 40

SiC

FeSi 20

0 800

CaAl2Si2O8

CaAl2Si2O8

20

0 1000

1200

1400

800

1600

1000

1200

1400

1600

Temperature / ºC

Temperature /°C

Fig. 3. Mineral transformation of the ashes at high temperatures. (a): SA-1.0; (b): SA-1.0-RC-10; (c): SA-2.0; (d): SA-2.0-RC-10; (e): SA-3.0; (f): SA-3.0-RC-10.

Stage I, the ashes with different Si/Al have similar max reaction rate, which suggests that the reaction of Eq. (1) is not affected by the Si/Al of ash. However, the max reaction rate of Stage II decreases from 0.0891 mg/min to 0.0571 mg/min, and then ascends to 0.0708 mg/min as the Si/Al of the ash increases from 1.0 to 3.0. When the Si/Al of the ash is 2.0, it has the low carbothermal reaction rate. This demonstrates that the carbothermal reaction of Eq. (2) is influenced by the Si/Al of the ash.

RC to generate Fe3Si. However, the weight loss of Stage II ((Eq. (2)) decreases until the Si/Al of the ash increases to 2.0, and then increases. The weight loss of Stage II is caused by release of CO, and this also determines the content of carbothermal reaction product (SiC). Thus, it can be assumed that the variation trend of AFTs with Si/Al is due to the extent of carbothermal reaction (Eq. (2)). The carbothermal reaction rate can also be known from DTG curves. The max reaction rates of Stage I and Stage II are listed in Table 2. For 5

Fuel 255 (2019) 115846

J. Wang, et al.

(a) 1

1

3

4

1 1 2

1

22

1

10%Carbon

1

1 2

80

11

1

22

1 2 22

15%Carbon

1

4

1

11 1

1 1 2

1

1

1

4

1

11

60

40

5%Carbon

1

20

1

1

(b)

100

1

Mass / %

22

1 2

11

0%Carbon 0

10

20

30

40

50

60

70

0% carbon

80

2θ

Fe3Si

5% carbon

SiC

10% carbon

CaAl2Si2O8

15% carbon

Al2O3

Amorphous

Fig. 4. XRD patterns (a) and content of major minerals (b) in SA-1.0 at 1400 °C. 1: Corundum (Al2O3); 2: Anorthite (CaAl2Si2O8); 3: SiC; 4: FeSi3.

(b)

(a) 3 5

3

3

3 3 14 24

1 3 23

3

3 3 6

53

3

3

3 3

1 3 4

3

3

6 3

3

33

15%Carbon

80

3

Mass / %

1 3 4 24

100

6

33

10%Carbon

3 3

60

40

3

5%Carbon 20

3 3

33

0%Carbon 0 0% carbon

10

20

30

40

2θ

50

60

70

80

Fe3Si

5% carbon

SiO2

SiC

10% carbon

CaAl2Si2O8

15% carbon

Al2O3

Amorphous

Fig. 5. XRD patterns (a) and content of minerals (b) in SA-2.0 at 1400 °C. 1: Cristobalite (SiO2); 2: Quartz (SiO2); 3: Corundum (Al2O3); 4: Anorthite (CaAl2Si2O8); 5: SiC; 6: FeSi3.

(a) 1

(b)

5

3 62

4 3 3

100

3 4

3 3

15%Carbon 33 4

1 3

80

5 3

3

10%Carbon

3

33

1 3 62

3

5%Carbon

35

3

Mass / %

362

60

33

1 32

3

3

3

0%Carbon 0

10

20

30

40

50

60

70

0% carbon

80

Graphite

2θ

5% carbon

Fe3Si

SiO2

10% carbon

SiC

15% carbon

Al2O3

Amorphous

Fig. 6. XRD patterns (a) and contents of mineral (b) in SA-3.0 at 1400 °C. 1: Cristobalite (SiO2); 2: Quartz (SiO2); 3: Corundum (Al2O3); 4: SiC; 5: FeSi3; 6: Graphite. 6

Fuel 255 (2019) 115846

J. Wang, et al.

(a)

-18 -0.10 -21

-15

Stage Ι

-0.10 -21

-24

-0.15 1200

1300

1400

1000

MS-28 -0.15 1100

1200

1400

(d)

(c)

-10

TG

weight loss=5.5%

-12

-12

0.00

DTG -0.05

Stage Ι

Stage ΙΙ

-18 -20

-0.10

Mass loss / %

-16

DTG / (mg/min)

Mass loss / %

1300

Temperature / °C

Temperature / °C

-14

-0.05

Stage ΙΙ

-18

MS-28 -24 1100

0.00

DTG / (mg/min)

-0.05

Mass loss / %

Stage Ι

1000

weight loss=8.16%

DTG

Stage ΙΙ

DTG

-15

TG

-12

0.00

DTG / (mg/min)

Mass loss / %

weight loss=8.91%

-15

TG

weight loss=5.7%

0.00

Stage ΙΙ

DTG Stage Ι

-0.05

-18 -0.10

DTG / (mg/min)

TG

-12

(b)

-21

MS-28

MS-28

-24 1000

-24

-0.15 1100

1200

1300

1000

1400

-0.15 1100

1200

-8

1400

(e) TG

weight loss=6.16%

-10 -12

Mass loss / %

1300

Temperature / °C

Temperature / °C

0.00

Stage ΙΙ

DTG Stage Ι

-14

-0.05

-16 -18

-0.10

-20 -22

MS-28

DTG / (mg/min)

-22

-0.15

-24 1000

1100

1200

1300

1400

Temperature / °C Fig. 7. TG-DTG-MS curves of the ash with different Si/Al containing 10% RC (Stage I: formation of Fe3Si, Stage II: formation of SiC). (a): SA-1.0; (b): SA-1.5; (c): SA2.0; (d): SA-2.5; (e): SA-3.0.

In order to explain the weight loss of SA-2.0 is less than that of SA1.0 and SA-3.0, the major mineral contents of the ash with Si/Al ranging from 1.0 to 3.0 containing 10% RC are shown in Fig. 8. As the Si/ Al increases, the amorphous content increases in the ash, while the crystal phase such as anorthite decreases. When Si/Al of the ash is 1.0, the high content of SiO2 prefers to react with CaO and Al2O3 to form anorthite at low temperature [29]. Although SiO2 content decreases above 1200 °C, anorthite can react with RC to generate SiC [15]. When the Si/Al is 3.0, the fraction of amorphous phase is nearly almost 80%. However, surplus SiO2 in slag which caused by the increasing Si/Al can react with RC to release CO. When the Si/Al is 2.0, the crystalline phase

Table 2 Weight loss and DTG of carbothermal reaction. Sample

SA-1.0 SA-1.5 SA-2.0 SA-2.5 SA-3.0

Stage I

Stage II

Weight loss (%)

DTG (Max) (mg/min)

Weight loss (%)

DTG (Max) (mg/min)

2.29 2.52 2.60 2.72 2.49

8.23*10−2 9.00*10−2 8.60*10−2 9.33*10−2 10.12*10-2

5.36 5.26 2.79 3.27 3.35

8.91*10−2 7.01*10−2 5.71*10−2 6.72*10−2 7.08*10-2

Total weight loss (%)

8.91 8.16 5.50 5.70 6.16

7

Fuel 255 (2019) 115846

J. Wang, et al.

Content / %

80 70 60 50

Chinese Academy of Sciences (grant number 122214KYSB20170020). References [1] Hotchkiss R. Coal gasification technologies. Proc. Inst. Mech. Eng. Part A: J. Power Energy 2003;217:27–33. [2] Bryant GW, Browning GJ, Emanuel H, Gupta SK, Gupta RP, Lucas JA, et al. The fusibility of blended coal ash. Energy Fuels 2000;14:316–25. [3] Minchener AJ. Coal gasification for advanced power generation. Fuel 2005;84:2222–35. [4] Li F, Yu B, Wang G, Fan H, Wang T, Guo M, et al. Investigation on improve ash fusion temperature (AFT) of low-AFT coal by biomass addition. Fuel Process Technol 2019;191:11–9. [5] Vassilev Vassilev S, Takeda S, Tsurue T. Influence of mineral and chemical composition of coal ashes on their fusibility. Fuel Process Technol 1995;45:27–51. [6] Song W, Tang L, Zhu X, Wu Y, Zhu Z, Koyama S. Effect of coal ash composition on ash fusion temperatures. Energy Fuels 2010;24:182–9. [7] Ilyushechkin AY, Hla SS, Roberts DG, Kinaev NN. The effect of solids and phase compositions on viscosity behaviour and TCV of slags from Australian bituminous coals. J Non-Cryst Solids 2011;357:893–902. [8] Chen X, Kong L, Bai J, Bai Z, Li W. Study on fusibility of coal ash rich in sodium and sulfur by synthetic ash under different atmospheres. Fuel 2017;202:175–83. [9] Yan T, Bai J, Kong L, Bai Z, Li W, Xu J. Effect of SiO2/Al2O3 on fusion behavior of coal ash at high temperature. Fuel 2017;193:275–83. [10] Zhao X, Zeng C, Mao Y, Li W, Peng Y, Wang T, et al. The surface characteristics and reactivity of residual carbon in coal gasification slag. Energy Fuels 2010;24:91–4. [11] Shimizu T, Tominaga H. A model of char capture by molten slag surface under hightemperature gasification conditions. Fuel 2006;85:170–8. [12] Montagnaro F, Brachi P, Salatino P. Char-wall interaction and properties of slag waste in entrained-flow gasification of coal. Energy Fuels 2011;25:3671–7. [13] Montagnaro F, Salatino P. Analysis of char–slag interaction and near-wall particle segregation in entrained-flow gasification of coal. Combust Flame 2010;157:874–83. [14] Chen D, Tang L, Zhu Y, Wang W, Wu Y, Zhu Z. Effect of char on the melting characteristics of coal ash. J Fuel Chem Technol 2007;35:136–40. [15] Kong L, Bai J, Li W, Wen X, Liu X, Li X, et al. The internal and external factor on coal ash slag viscosity at high temperatures, Part 2: Effect of residual carbon on slag viscosity. Fuel 2015;158:976–82. [16] Hu H, Zhou K, Meng K, Song L, Lin Q. Effects of SiO2/Al2O3 ratios on sintering characteristics of synthetic coal ash. Energies 2017;10:242. [17] Xuan W, Whitty KJ, Guan Q, Bi D, Zhan Z, Zhang J. Influence of SiO2/Al2O3 on crystallization characteristics of synthetic coal slags. Fuel 2015;144:103–10. [18] Liu B, He QH, Jiang ZH, Xu RF, Hu BX. Relationship between coal ash composition and ash fusion temperatures. Fuel 2013;105:293–300. [19] Li J, Wang X, Wang B, Zhao J, Fang Y. Effect of silica and alumina on petroleum coke ash fusibility. Energy Fuels 2017;31:13494–501. [20] Wang J, Kong L, Bai J, Li H, Bai Z, Li X, et al. The role of residual char on ash flow behavior, Part 1: The effect of graphitization degree of residual char on ash fusibility. Fuel 2018;234:1173–80. [21] Bale CW, Bélisle E, Chartrand P, Decterov SA, Eriksson G, Gheribi AE, et al. FactSage thermochemical software and databases, 2010–2016. Calphad 2016;54:35–53. [22] Ward Colin R, Taylor John C, Matulis CE, Dale LS. Quantification of mineral matter in the Argonne Premium Coals using interactive Rietveld-based X-ray diffraction. Int J Coal Geol 2001;46:67–82. [23] Yan T, Bai J, Kong L, Li H, Wang Z, Bai Z, et al. Improved prediction of criticalviscosity temperature by fusion behavior of coal ash. Fuel 2019;253:1521–30. [24] Wei Y, Li H, Yamada N, Sato A, Ninomiya Y, Honma K, et al. A microscopic study of the precipitation of metallic iron in slag from iron-rich coal during high temperature gasification. Fuel 2013;103:101–10. [25] Shi W, Dai X, Bai J, Kong L, Xu J, Li X, et al. A new method of estimating the liquidus temperature of coal ash slag using ash composition. Chem Eng Sci 2018;175:278–85. [26] Vassilev SV, Baxter D. Vassileva CG. An overview of the behaviour of biomass during combustion: Part II. Ash fusion and ash formation mechanisms of biomass types. Fuel 2014;117:152–83. [27] Li F, Huang J, Fang Y, Wang Y. The effects of leaching and floatation on the ash fusion temperatures of three selected lignites. Fuel 2011;90:2377–83. [28] Dai X, Jin B, Lu P, Wang X, Huang Y. The role of residual carbon on fusibility and flow properties of rice straw ash. Fuel 2019;253:1512–20. [29] Song W, Tang L, Zhu X, Wu Y, Rong Y, Zhu Z, et al. Fusibility and flow properties of coal ash and slag. Fuel 2009;88:297–304.

SiO2

20

CaAl2Si2O8 Amorphous

10

0

Si/Al=1

Si/Al=2

Si/Al=3

Fig. 8. Mineral contents of ash with different Si/Al containing 10% RC at 1400 °C.

decreases sharply and the content of SiO2 in the ash is very low. This implies that the high content of amorphous phase and less SiO2 in slag at high temperatures is detrimental to the carbothermal reaction. Thus, the weight loss of ash with Si/Al = 2.0 is less than that of the ash with Si/Al = 1.0 and Si/Al = 3.0. 4. Conclusions In this study, we investigated the effect of SiO2/Al2O3 ratio on the fusibility of the ash containing residual char. The ashes with different SiO2/Al2O3 ratios and contents of residual carbon were used to study the effect of SiO2/Al2O3 ratio on mineral transformation and carbothermal reaction. The main conclusions could be drawn from this work: (1) The effect of residual char on the ash fusibility becomes obvious when the Si/Al of ash is higher than 1.0. Besides, the effect is more when the content of residual char is more than 10%. (2) Ash fusibility is mainly affected by carbothermal reaction products (SiC and Fe3Si), which also depends on minerals in the ash without residual char. As Anorthite, mullite and corundum are the main minerals in the ash with Si/Al of 1.0, the AFTs are slightly affected by formation of Fe3Si and SiC. However, when the Si/Al is in the range of 2.0 to 3.0, SiC and Fe3Si greatly increases HT and FT of ash due to less anorthite and SiO2 in the ash. (3) Si/Al mainly influences the carbothermal reaction rate of SiC, while it does not affect formation of Fe3Si. The high content of amorphous phase and less SiO2 in the ash at high temperatures are not favorable to the carbothermal reaction. Acknowledgments This work was financial supported by 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), Bureau of International Cooperation,

8