Effect of lime addition on slag fluidity of coal ash

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 39, Issue 6, June 2011 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2011, 39(6), 407411

RESEARCH PAPER

Effect of lime addition on slag fluidity of coal ash KONG Ling-xue1,2, BAI Jin1,* , LI Wen1, BAI Zong-qing1, GUO Zhen-xing1 1

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

2

Graduate University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: The ash fusibility temperature (AFT) and slag fluidity of three different coal ash samples through addition of CaO with different amounts were studied. Especially the variation of temperature at critical viscosity was examined at different viscosities. The results show that the fusibility temperatures of coal ashes decrease and then increase with increasing addition amount of CaO, which is consistent with the change of liquids temperature with CaO content calculated by FACTsage. Slag viscosity also decreases with increasing amount of CaO addition above the temperature of critical viscosity (Tcv). The temperature of critical viscosity firstly decreases with increasing addition of CaO, and then reaches a minimum value when the content of CaO is around 15%. FCATsage was employed to calculate the liquid composition at the temperature of critical viscosity. It indicates that high content of FeO of liquid leads to the low temperature of critical viscosity. Key words: slag fluidity; AFT; CaO; temperature of critical viscosity; FACTsage

Coal gasification is the predominant technology for clean coal utilization[1]. Concerning about greenhouse gas emissions and the need for greater efficiency, entrained-Àow gasi¿ers was utilized in a number of integrated gasi¿cation combined-cycle (IGCC) plants for improving thermal conversion ef¿ciency[2]. In the gasi¿er, organic material in coal is completely combusted, and then the ash exposed to high-temperature conditions (1300–1700ºC) becomes liquid slag owing to the melting and reactions among mineral matters[3]. Slag viscosity must be in a certain range for both slag tapping and membrane wall[4]. Studies have reported that viscosity must be typically less than 25 Pa·s between 1300ºC and 1500ºC[5]. However, most Chinese coals with high ash fusion temperatures which are in excess of the optimum operating temperature of entrained flow gasi¿ers, and thus were considered unsuitable to be gasified without using a Àux to adjust the ash melting temperature and slag viscosity. Hence, it is necessary to study the effect of flux agent on ash fusion temperatures and slag viscosity. Pulverized limestone, with the effective ingredient of CaO, is expected to be used in entrained flow gasi¿ers to improve slag Àow properties because of its abundance and low cost. There have been a number of works on reducing ash fusion temperature and ash slag viscosity by adding limestone. Song et al[6] found that ash fusion temperature drops with increasing amount of CaO until 35%.

Lu et al[7] studied the effect of mineral matters on fusion temperature and figured out the formation of anorthite is the main reason for low ash fusion temperature. Ren et al[8] investigated the effects of different fluxes on the AFTs of coal ash and slag viscosity, and concluded that limestone have good effect on lowering AFTs and slag viscosity. Hurst et al[9] used the ternary equilibrium phase diagrams to study the effects of CaO on ash flow properties of Australian coal, and found that the amount of flux required for coals is different. The temperature at critical viscosity indicates a point of abrupt change in viscosity-temperature curve because solid phase in liquid slag begins to crystallize and to separate out from the liquid phase. Usually Tcv marks the division of viscosity affected by the presence of crystals or not. The Tcv is usually used as boundary between Newton fluid and non-Newtonian fluid. For ash with fixed compositions slag performed as Newton fluid when temperature is above Tcv, and the viscosity is only related with temperature. While slag performed as non-Newton fluid below Tcv, and the viscosity was influenced by both temperature and solid in melt slag[10]. Viscosity quickly increases when temperature is below Tcv, and thus lowers the operational temperature range for entrained flow gasification. It is important to know the change of Tcv with different additions of flux. Seggiani et al[11] studied the relation between ash fusion temperature and Tcv from ash chemical composition, and provided a method to predict the

Received: 18-Oct-2010; Revised: 28-Jan-2011 * Corresponding author. 86-351-4048967, E-mail: [email protected] Foundation item: Supported by the Major State Basic Research Development Program of China (973 Prpgram, 2010CB227005-02), the National Natural Science Funds (21006121), the Youth Foundation of Shanxi Province (2010021008-2), and State Key Laboratory of Coal Combustion (FSKLCC0909). Copyright ” 2011, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

KONG Ling-xue et al. / Journal of Fuel Chemistry and Technology, 2011, 39(6): 407411

Ash content was usually controlled below 35% for entrained flow gasification. Based on this consideration and the ash content in coal samples, the addition ratios of CaO are listed in Table 3.

effect of adding minerals (such as CaO) on Tcv from AFTs. Song et al[1] found that Tcv of slag decreases with increasing CaO content, whereas it increases rapidly when CaO content is higher than 35%. The amount of CaO addition varies greatly in literatures, some of them are markedly higher than those used in practice. At the same time, the change of viscosity is also out of the operation range in entrained flow gasifier. Such results are difficult to guide the practical operation of gasifier. Hence, it is necessary to understand the change of slag viscosity and Tcv when the content of CaO addition is low. In this work, the ash fusion temperature auto detecting system and THETA high-temperature rotational viscometer were used to investigate the AFTs and slag viscosity of 3 coal ashes through addition of CaO with different amount up to 22%. The thermodynamic software package FACTsage was used to calculate slag liquid composition at each Tcv.

1 1.1

1.3

In this work, we used inductively coupled plasma-atomic emission spectrometry (ICP-AES) to characterize ash composition according to ASTM D6349. The chemical composition of coal ashes is given in Table 4. 1.4

Viscosity measurements

The THETA high-temperature rotational viscometer was used to test slag viscosity under weak reducing atmosphere (CO/CO2=6:4). Viscosity measurements were started at the temperature at which the melt was Newtonian Àuid, and held for 30 minutes. The temperature was lowered at 1ºC/min, the rotor speed and the torque on the rotor were recorded. NIST standard reference material 717a glass was used to check the test methods and to calibrate equipment for the determination of the viscosity of glass in accordance with ASTM Procedure C 965-81.

Experimental Samples

Three Chinese coal samples were used in the study. The ultimate and proximate analyses are given in Table 1. The ash samples were prepared in a muffle furnace at 815ºC according to the Chinese standard GB/T 1574-2007. 1.2

Ash characterization

1.5

Ash fusion temperature test

Thermodynamic equilibrium calculations

The thermodynamic software package FACTSage allows calculating and predicting multiphase equilibria, liquidus temperatures, and the proportions of the liquid and solid phases in a specified atmosphere from chemical composition. For ash and slag, phase formation data for these oxides and their combinations were selected from the FToxid database. FACTsage was used in this study to calculate the liquid temperature with different additions of CaO, as well as liquid composition of slag at each Tcv obtained by tangent of viscosity-temperature curve.

The fusion temperature tests were performed as the Chinese standard procedures (GB/T219-2008). This test involves heating a sample cone of specified geometry at 5ºC/min in weak reducing atmosphere (CO/CO2=6:4). The following temperatures are recorded for each sample corresponding to the specific shapes of the ash cones: initial deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT). The coal ash melting temperatures are listed in Table 2.

Table 1 Proximate and ultimate analysis of three coals Samples

Proximate analysis w /%

Ultimate analysis wdaf /%

Ad

Vdaf

FCdaf

C

H

St,d

N

Dt

25.43

31.51

68.49

83.77

4.76

1.45

1.08

Yq

23.34

11.66

88.34

86.48

3.58

2.48

1.30

Yc

20.42

34.57

65.43

82.89

4.53

0.67

1.30

Table 2 Coal ash melting temperatures Samples

Temperature t / ºC DT

ST

HT

FT

Dt

1288

1328

1354

1405

Yc

1404

1427

1443

1496

Yq

1486

>1500

>1500

>1500

KONG Ling-xue et al. / Journal of Fuel Chemistry and Technology, 2011, 39(6): 407411 Table 3 Addition ratios of CaO Samples

Addition of CaO / w/%, ash basis

Dt

8.20

10.93

13.66

16.40

Yq

6.38

10.64

12.77

14.90

Yc

10.99

18.31

21.97

-

Table 4 Chemical composition of coal ash Samples

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

K2O

Na2O

Dt

60.56

21.99

9.24

3.05

0.83

1.87

1.32

0.24

Yc

56.98

27.33

4.92

5.19

0.98

1.05

1.06

0.43

Yq

56.81

29.39

6.09

2.97

0.50

1.13

0.77

0.75

Fig. 1 Liquidus temperature vs CaO content

Fig. 2 Fusion temperatures vs CaO content

: Dt; : Yq; : Yc

liquidus temperature: : Dt; : Yc; : Yq; FT: : Dt; : Yc; : Yq

2 2.1

Results and discussion Effect of CaO content on liquids temperature

Figure 1 shows the plots of liquidus temperatures against CaO content in A12O3-SiO2-CaO-FeO system calculated by FACTsage under weak reducing atmosphere. The figure can be divided into four zones A, B, C and D according to the changing trends. The liquidus temperature decreases with increasing CaO content in zone A and C, while it increases in zone B and D. Zone A lies in mullite and anorthite regions in phase diagram. Mullite (3Al2O3·2SiO2) reacts with the added CaO to form anorthite (CaO·Al2O3·2SiO2) with low melting temperature, resulting in the decrease in liquidus temperature[12]. Zone B locates in anorthite region. Gehlenite is formed with addition of CaO, thus the liquidus temperature increases. Zone C also lies in anorthite region in which gehlenite and CaO produce eutectic mixture reducing liquidus temperature. Zone D is in ps-wollastonite region. The rising liquidus temperature is due to ps-wollastonite (ps-3CaO·SiO2) with high melting temperature in the solid phase. The addition of CaO is more easily to promote the formation of

ps-wollastonite raising the liquidus temperature[13]. The reactions occurred in Fig. 1 is shown in the following. CaO+3Al2O3·2SiO2ĺ CaO·Al2O3·2SiO2 CaO·Al2O3·2SiO2+ CaO ĺ2 CaO·Al2O3·2SiO2 2CaO·Al2O3·2SiO2+ CaO ĺ SiO2- Al2O3- CaO SiO2+ CaO ĺ 3CaO·SiO2 2.2

Effect of CaO on AFTs

Figure 2 presented the AFTs (FT) of coal ash samples as a function of CaO content. The AFTs decrease with increasing CaO content and then increase slowly, whereas AFTs decreases at higher CaO content. This trend is similar with that of the liquidus temperatures. Meanwhile, the liquidus temperatures calculated by FACTsage are all higher than AFTs because ash was not fully melt during AFT test. Evgueni Jak et al[14] proved that the AFTs were parallel with each other, and also parallel with the liquidus temperature. The trend provides the method for AFTs prediction from calculating liquidus temperature. The gap between liquidus temperature and flow temperature is all about 150ºC for three samples. 2.3

Effect of CaO on slag viscosity

KONG Ling-xue et al. / Journal of Fuel Chemistry and Technology, 2011, 39(6): 407411

Fig. 3

Viscosity-temperature curves of coal ash slags : Dt; : Yc

The maximum tapping viscosity value is 25 Pa·s for continuous operation of entrained-Àow gasi¿er at 1300–1500ºC. Fig. 3 shows the viscosities of Dt and Yc without addition of CaO, and Yq was not given for its high fusion temperature. However, at 1500ºC the ash viscosity without fluxing agent is higher than 25 Pa·s because of the high proportion of solids particles in liquid slag formed by ash

at this temperature. Therefore, CaO should be added into ash to reduce the amount of solids in slag and to decrease liquid slag viscosity to the required values[15]. Figure 4 (a–c) represents the viscosities of ash versus temperature with different additions of CaO. The viscosity of ash decreases with the addition of CaO when temperature is above Tcv. The temperature range that viscosity is lower than 25 Pa s also increases. In network theory, the bigger the net is, the larger the viscosity is for much more internal friction when slag liquid flows. When adding CaO into slag, more O2– that appeared will break big net to smaller one, and the viscosity decreases[16]. Figure 4 also shows that coal ash has similar viscosity-temperature curves when CaO content is in a certain range. The curves with CaO addition of 8.20%, 10.93% and 13.66% are similar. While the content of CaO is beyond this range, it exhibits another shape. Generally, slag can be divided into vitreous slag, crystal slag and plastic slag from chemical composition, and the slags that have similar viscosity-temperature curves belong to same slag type.

Fig. 4 Viscosity-temperature curves of three coal ash slag with different additions of CaO (a): Dt

: raw; : 8.20%CaO; : 10.93%CaO; 1: 13.66%CaO; ]: 16.40%CaO (b): Yc : raw; : 10.99%CaO; : 18.31%CaO; 1: 21.97%CaO

(c): Yq : 6.38%CaO; : 10.64%CaO; : 12.77%CaO; 1: 14.90%CaO

Fig. 5 Temperature of critical viscosity with different CaO content

Fig. 6 FeO with different CaO content in coal ash slag liquid

: Dt; : Yq; : Yc

Temperature of critical viscosity: : Dt; : Yc; : Yq; FeO content:

: Dt; : Yc; : Yq

KONG Ling-xue et al. / Journal of Fuel Chemistry and Technology, 2011, 39(6): 407411

2.4

Effect of CaO on Tcv

properties of coal ash and slag. Fuel, 2009, 88(2): 297–304. [2] Bai J, Li W, Li B Q. Mineral behavior in coal under reducing

In order to know the minimum addition ratio of flux for proper AFTs and slag viscosity, it is necessary to know the trend of Tcv with addition of CaO, and then the maximum operation temperature can be known. The critical viscosity temperature of slags can be got from Fig. 4 (a–c). The relation between Tcv and CaO content is given in Fig. 5. When CaO content is in 4%–24% for three coal ash samples, Tcv of slags firstly decrease as CaO content increases, and then increase at higher CaO content. The minimum Tcv of these samples all appear at about 15% addition of CaO. This value is close to the boundary that separate zone A and B. The main reason of Tcv is due to crystallization, and the boundary between zone A and B can be thought as the critical point of composition change of crystal. At this point, the proper ration of flux can be known from change of liquidus temperature.

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2.5

Relations of Tcv and slag liquid composition

Chemistry and Technology, 2010, 38(2): 23–28. [8] Ren X G, Duan P P, Tong G L. Study on adding flux to reduce

Figure 6 shows the change of FeO content in slag liquid at each temperature with critical viscosity calculated by FACTsage under weak reducing atmosphere. It appears that the FeO content shows a different trend from Tcv change. The FeO content increases with increasing CaO content until reaching a maximum value occurring at minimum Tcv. In this view, one reason of Tcv change with CaO content may be the variation of FeO content in the slag liquid.

3

Conclusions

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The fusion temperature and liquidus temperature have the same changing trend when CaO content increases, which is consistent with phase diagram calculated by FACTsage. The slag viscosity is reduced with increasing CaO content when temperature is above Tcv because internal friction decreases as CaO breaks the large net structure of slag. The temperature of critical viscosity firstly decreases with increasing addition of CaO, and then reaches a minimum value when the content of CaO is about 15%. FACTsage was employed to calculate the liquid composition at the temperature with critical viscosity. FeO content in slag liquid increases with increasing CaO content. The maximum value of FeO appears at minimum Tcv. The change of FeO content in slag liquid may be the reason that leads to the change of temperature with critical viscosity.

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