Effect of Sodium Oxides in Ash Composition on Ash Fusibility

Article pubs.acs.org/EF

Effect of Sodium Oxides in Ash Composition on Ash Fusibility Yang Wang, Yu Xiang, Dongxu Wang, Changqing Dong,* Yongping Yang, Xianbin Xiao, Qiang Lu, and Ying Zhao National Engineering Laboratory for Biomass Power Generation Equipment, School of Renewable Energy Engineering, North China Electric Power University, Beijing 102206, China ABSTRACT: Ash fusibility is closely associated with ash slagging, which has great significance on efficiency and cleanliness of thermal conversion and coal utilization. Not being a dominant component of coal ash, Na2O is believed to be an inducement for slagging problems. In this study, for the realization of better understanding of the underlying mechanisms, the influence of Na2O on the ash fusibility was investigated from the perspectives of content variation and temperature rising. Experimental approaches, such as ash fusion temperature (AFT) test, X-ray diffraction (XRD), and scanning electronic microscope (SEM), were applied to quantify the ash fusibility and detect minerals’ transformation and surface morphology in ash melting. The ash melting process was simulated by a multicomponent system based on the FactSage database, which includes SiO2−Al2O3−Fe2O3−CaO−Na2O. This study also used Gibbs free energy theory to analyze the chemical reactions, mineral behaviors, and phase diagram and record the sodium migration in the melting process. In general, Na2O reduces the fusion temperatures of ash samples, which can be related to the performances of minerals albite, nepheline, and anorthite, as well as the eutectics they form.

1. INTRODUCTION With the further depletion of coal resources, efficient and clean utilization of coal resources become more important around the world.1 Among various coal utilization means, slagging has been a significant problem which causes safety concerns and economic losses.2,3 Therefore, avoiding slagging has become an important topic in the fields of both science and engineering. Characteristics affecting coal slagging include fusibility, flow property, and rheological property of coal ash, etc. Among these factors, fusibility is the most significant one. The melting process of coal ash can be described by the ash cone heating (in Figure 1). Four characteristic temperatures are used to describe

it is of significance to explore the ash fusibility in terms of the different roles of ash components and the mineral behaviors in the heating process. The influence of high-content oxides such as SiO2, Al2O3, and CaO on the ash fusibility has been investigated by many researchers.17−20 However, the influence of alkaline oxides in ash components, especially Na2O, has not won enough attention, although it also has significant effect.21 With increasing attention to Zhundong coal of China,22−24 whose Na2O can reach a content of more than 6%,25 the understanding of sodium behaviors in combustion and the effects of Na2O in ash on ash fusibility needs to be clarified. Zhang et at.24 investigated the transformation of sodium during the ashing of Zhundong coal, observing NaCl, albite, and nepheline separately with the rising temperature, but the transformation of sodium is not related to ash fusibility. Several studies have related sodium in ash to ash fusibility. Van Dyk et al.26 studied the effect of Na2O percentage on ash fusion temperature in coal blending gasification. They pointed out that by increasing the ash Na2O percentage from 0.5% to 10%, the liquid temperature decreases from1500 to 1200 °C. Gruber and Kalmanovitch27 indicated that, in the fixed bed gasification, the sodium percentage must be maintained below 8%. Weidong et al.28 studied the ash fusion temperatures of coal and sewage sludge mixtures. They found that the main reason for the reduction of coal ash fusion temperature mixed with sewage sludge was that, in the melting process, the additive sodium in sodium hydroxide generated mineral nepheline. After studying the performance of sodium feldspar in ash fusion test, Reifenstein et al.29 indicated that albite and quartz can form eutetics when the temperature reaches about 1062 °C. Based on the studies on the effect of Na2CO3 on coal ash fusibility, Yao et al.30 found

Figure 1. Ash melting process.

ash melting behaviors and ash fusion temperatures (AFTs) when being heated, namely, initial deformation temperature (IDT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT).4 Various methods have been employed to investigate the AFTs of coal ash.5−16 Coal ash fusibility is closely related to the ash components, which mainly consist of several oxides including SiO2, Al2O3, Fe2O3, Na2O, K2O, MgO, CaO, and TiO2 and some other trace oxides such as SO3 and P2O5. To distinguish them from the influence on fusibility, these oxides should be categorized into acidic oxides (increase the melting temperature) and alkaline oxides (decrease the melting temperature). In different stages of the ash melting process, different minerals may be produced by the reactions of these oxides and affect the ash fusibility in different ways. Therefore, © XXXX American Chemical Society

Received: November 17, 2015 Revised: January 7, 2016

A

DOI: 10.1021/acs.energyfuels.5b02722 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Ash Composition of a Real Zhundong Coal components

SiO2

Al2O3

CaO

Fe2O3

MgO

Na2O

K2O

TiO2

contents

42.36

22.32

15.76

8.56

2.72

6.35

0.76

0.88

Table 2. Chemical Composition and AFTs of Synthetic Ash Samples content of oxides (wt %)

AFTs (°C)

ash sample

SiO2

Al2O3

S/A

CaO

Fe2O3

Na2O

IDT

ST

HT

FT

1 2 3 4 5 6 7 8 9 10

50.00 49.90 49.70 49.50 49.25 49.00 48.50 47.50 46.00 45.00

25.00 24.95 24.85 24.75 24.63 24.5 24.25 23.75 23.00 22.50

2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00

17.00 16.97 16.90 16.83 16.75 16.66 16.49 16.15 15.64 15.30

8.00 7.98 7.95 7.92 7.88 7.84 7.76 7.60 7.36 7.20

0.00 0.20 0.60 1.00 1.50 2.00 3.00 5.00 8.00 10.00

1240 1240 1270 1240 1240 1200 1200 1160 1110 1100

1260 1270 1280 1270 1270 1270 1270 1220 1160 1120

1270 1290 1290 1280 1280 1280 1280 1240 1170 1130

1290 1290 1300 1300 1300 1300 1300 1290 1200 1170

that fluxing agent Na2CO3 can lower the coal ash fusion temperatures before its percentage reaches 20%. These studies are mainly conducted from the industrial application and focused on the fluxing effect of sodium in ash. However, in order to give a better understanding of the sodium fluxing effect, the reason why sodium can lower the ash fusion temperatures, the reactions and mineral behaviors in the ash melting process need to be further studied. In this work, a fundamental study is carried out to explore the influences of Na2O on ash fusion temperatures, reactions, and minerals behaviors in the ash melting process, which can lay a solid foundation for a better explanation of the Na2O effects on ash fusibility. In order to control the variables accurately, synthetic ash samples were taken as the research subjects, with the main components of SiO2, Al2O3, Fe2O3, CaO, and Na2O. The content of Na2O was settled as a variable while the ratios of other oxides were held constant. During the ash melting process, the high-temperature ash fusibility tester was used to measure the characteristic temperatures (IDT, ST, HT, and FT), the atmospheric high-temperature tube furnace which can be heated to 1700 °C was used to realize necessary conditions (e.g., temperature, pressure, and atmosphere), and the characterization methods of X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to identify the mineral types, structure, and morphological characteristics. In the meanwhile, the thermodynamic database FactSage was used to establish multiple components of SiO2− Al2O3−Fe2O3−CaO−Na2O for simulating the synthetic ash melting process. The Gibbs free energy theory and the multiple phase equilibrium theory were used to analyze the chemical reactions and mineral behaviors in the ash melting process, and a phase diagram was used to visually describe the ash melting process.31,32 This work first studied the variation of Na2O content. Synthetic ash samples with different Na2O contents were heated at the same temperature (1000 °C) to explain the diversity of AFTs of different ash samples. Then, the temperature increasing would be investigated. Three typical ash samples were selected for obtaining the mineral behaviors with the increasing temperature in the ash melting process. Finally, through combination of the preceding two tasks, the

mechanisms of Na2O’s effects on ash fusibilities would be concluded.

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Synthetic Ash Samples. The main ash composition of a real Zhundong coal is shown in Table 1, based on which the synthetic ash samples were prepared. The synthetic ash samples were composed of analytical reagents and were mixed in deionized water to make water slurry. Then, the muddy water was magnetically stirred for 4 h before being dried to ensure sufficient mixing. Similar to the coal ash, SiO2, Al2O3, Fe2O3, and CaO are major components in synthetic ash. In order to simplify the theoretical calculations and experiments to make useful conclusions clearer, low-content oxides in coal ash such as K2O, MgO, and TiO2 are not included in the synthetic ash samples to simplify the components. S/A means the mass ratio between SiO2 and Al2O3 (wt %). In order to control the variable, it is held at a constant value of 2.00, which is a common ratio in coal ash. After settling Na2O as variable, 10 groups of five-component synthetic ashes were prepared, as shown in Table 2. 2.2. AFT Test. In measuring the characteristic temperatures, the high-temperature ash fusibility tester can describe the softening and melting behavior of ash in heating. According to Chinese standard GB/T219-2008,33 The AFT test was carried out in a weakly reducing carbon atmosphere. Therefore, we filled the testing corundum boat with enough graphite powder and enclosed the furnace to ensure that ash cones were protected by carbon in the whole testing procedure. The AFT results are presented in Table 1. 2.3. Melting Process Conditions. The conditions required in the ash melting process, such as temperature, pressure, and atmosphere, were provided by a high-temperature tube furnace which can be heated to 1700 °C. The synthetic ash samples were filled in corundum crucibles which can withstand high temperatures and then heated to target temperatures in an air atmosphere for 4 h, ensuring that the reactions were sufficient. The pressure was maintained at 100 Pa below atmosphere to simulate furnace draft. All experimental data were recorded electronically to ensure the condition reliability. After reaching the scheduled temperature, the samples were quenched in water and collected for later testing. 2.4. Characterization and Testing. The mineral compositions of ash samples were detected by XRD (XRD-6000, SHIMADZU, Tokyo, Japan). The scanning angle was from 2° to 100°. The XRD peaks were analyzed by the computer software package MDI Jade 5.0. In addition, SEM (JSM-6510, JEOL, Tokyo, Japan) was used to observe the surface morphology of the minerals and make microanalysis of the ash particles. The ash samples were magnified 200 and 3000 times separately to show different levels of surface morphology. B


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Energy & Fuels 2.5. Global Equilibrium Analysis and Thermodynamic Database. The Gibbs free energy theory and the multiple phase equilibrium theory were applied to analyze the chemical reactions and mineral behaviors in ash melting process. A global equilibrium analysis has been performed to calculate the thermodynamic stable physical and chemical forms as functions of temperature, pressure, and ash compositions. The thermodynamic database FactSage7.0, which is based on the Gibbs free energy minimization theory, was applied to establish multiple components of SiO2−Al2O3−Fe2O3−CaO−Na2O to simulate the melting process of synthetic ash. FactSage was developed jointly by the FACT and ChemSage thermochemical package. The FactSage package consists of a series of information, database, calculation, and manipulation modules, which can ensure access and manipulation of pure substances as well as solution databases. When the Gibbs free energy in this system is minimized, the system and all possible reactions reach a thermodynamic equilibrium. According to Gibbs free energy theory, reactions with negative Gibbs free energy can happen and the lower the Gibbs free energy means the higher the priority of the reaction.34 In terms of the components, FToxid database was chosen to calculate the reactions of Gibbs energy, through which all thermodynamic data can be acquired. FToxid database includes a solution database which contains oxide solutions and a compound database which contains all stoichiometric solid and liquid oxide compounds. Both of them are evaluated or optimized by the FACT group and are thermodynamically consistent. FToxid-SLAGA, FToxidWOLLA, FToxid-Mull, FToxid-CORU, and FToxid-Neph in solution species were applied in this case. Although thermodynamic calculation is a powerful tool to predict the stable species during the chemical process, there are some shortcomings when applied to combustion. Thermodynamic calculations do not take kinetics into account, so the temperature must be high enough and species residence time must be long enough to reach the thermodynamic equilibrium. Temperature and composition gradients are not considered in the equilibrium calculation, either. In general, the thermodynamic calculation can be used to illustrate the material equilibrium distribution and the reaction mechanisms.35

a continuous and impressive decline until the Na2O content reaches 10%. The IDT declines obviously after the Na2O content reaches 1.5%, lower than those of the other AFTs (ST, HT, and FT), which decline rapidly after the Na2O content reaches 3%. The slight rise cannot be explained by current knowledge, and experimental error is one of the probable reasons because the rise is very small. The probable reasons of the continuous and impressive decline might be as follows: Na2O could react with other oxides to form minerals, which may further create eutectics with other minerals under an appropriate content at high temperatures. The generation of minerals and eutectics with low melting temperatures and the reduction of minerals with high melting temperatures may decrease the AFTs. 3.2. Analysis for Different Na2O Contents. Synthetic ash samples listed in Table 1 were heated at 1000 °C. The contents of minerals and slag liquids vary with the increasing Na2O content, whose trend can be calculated by the multicomponent system SiO2−Al2O3−Fe2O3−CaO−Na2O in the thermodynamic database FactSage. As shown in Figure 3, with the

3. RESULTS AND DISCUSSION 3.1. Effect of Na2O Content on AFTs. Figure 2 presents the experimental AFTs of the synthetic ash samples as a Figure 3. Effects of Na2O content on minerals and slag liquids at 1000 °C.

increasing of Na2O content, anorthite (CaAl2Si2O8) in the product decreases continuously, from nearly 70% of weight fraction (Na2O content is 0%) to about 15% (Na2O content is 10%). The content of albite (NaAlSi3O8) initially increases (Na2O content less than 3.5%) and then decreases until it disappears (Na2O content is about 6%); nepheline (NaAlSiO4) is generated and increases when albite (NaAlSi3O8) disappears. Andradite (Ca3Fe2(SiO4)3) increases slightly before Na2O content reaches 7% and then remains almost constant. In the meanwhile, wollastonite (CaSiO3) is generated when Na2O content is about 7%. Quartz (SiO2) and hematite (Fe2O3) decrease with the increasing Na2O content and disappear when the Na2O content is about 3.5% and 7%, respectively. At 1000 °C, liquid slag is generated when Na2O content is around 3% and gradually increases with the increasing Na2O content until to its peak. According to the preceding analysis, the Na2O content influences the mineral behaviors in the ash melting process greatly, which may contribute to the declining of AFTs. Along with the increasing Na2O content, sodium changes from albite to nepheline gradually. Meanwhile, calcium containing minerals

Figure 2. Effects of Na2O content on the AFTs.

function of Na2O content. Considering the Na2O content of coal is relatively low, the Na2O content of synthetic ash samples in this study is up to 10%. As shown in Figure 2, the AFTs of the synthetic ash samples change in a similar way: there is a small rise compared with the synthetic ash sample without Na2O before the Na2O content reaches 0.6%, and then follows C


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Energy & Fuels also change with the increasing Na2O content: anorthite declines continuously, while andradite and wollastonite increase in different stages. The critical point of andradite increase and wollastonite increase lies on the Na2O content of 7%, which may be due to the depleting of hematite. Along with the increasing in Na2O content, several reactions can be concluded according to the preceding analysis: 970 − 1130 ° C

Na 2O + CaAl 2Si 2O8 + 4SiO2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2NaAlSi3O8 + CaO ΔG /(kJ/mol) = −265.7 + 0.00159T

(1)

NaAlSi3O8 + CaAl 2Si 2O8 + Na 2O 970 − 1130 ° C

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 3NaAlSiO4 + CaO + 2SiO2 ΔG /(kJ/mol) = −215.3 + 0.01021T

(2)

970 − 1130 ° C

Figure 4. Effects of Na2O content on the liquid formation temperatures in ash samples.

Na 2O + CaAl 2Si 2O8 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2NaAlSiO4 + CaO ΔG /(kJ/mol) = −232.1 + 0.00734T

(3)

is higher than those of albite (FT 1118 °C) and nepheline (FT 1524 °C),. In order to verify the results of theoretical calculation, synthetic ash samples 8 (Na2O content is 5%) and 10 (Na2O content is 10%) were heated at 1000 °C and then analyzed by XRD to show the changes of crystalline structures. The XRD peaks were analyzed by the computer software package MDI Jade 5.0, as shown in Figure 5. Quartz and corundum exist in

867 − 1465 ° C

3CaO + Fe2O3 + 3SiO2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ca3Fe2(SiO4 )3 ΔG /(kJ/mol) = −323.9 + 0.04870T

(4)

The Gibbs energies of all four reactions are negative at 1000 °C, which indicates that the reactions are reasonable. At 1000 °C, reaction 1 has a lower Gibbs energy (−264.1 kJ/mol) than reaction 2 (−205.1 kJ/mol) and reaction 3 (−224.7 kJ/mol). According to the principles of Gibbs free energy, in the presence of free SiO2, reaction 1 has higher priority than reactions 2 and 3. In this way, anorthite transforms into albite. With the depleting of SiO2 in reaction 1, reactions 2 and 3 happen, in which Na2O can obtain Al2O3 and SiO2 from anorthite to form nepheline. This can well explain why quartz disappears and albite starts to decrease when Na2O content is about 3.5%. With anorthite decomposing continuously, free CaO produced by reactions 1, 2, and 3 can contribute to the slight increase of andradite and lead to the hematite decrease, in reaction 4. The SiO2 in this reaction when free SiO2 is depleted (Na2O content is higher than 3.5%) comes from the products of reaction 2. While hematite disappears (Na2O content is higher than 7%), andradite content remains constant and free CaO contributes to the generation and increasing of wollastonite. Reactions 1, 2, 3, and 4 provide a reasonable explanation of the mineral behaviors, shown in Figure 3. Liquid formation temperatures of the synthetic ash samples are calculated as the function of Na2O content by the multicomponent system SiO2−Al2O3−Fe2O3−CaO−Na2O, as shown in Figure 4. When the Na2O content increases from 0 to 3.5%, the liquid formation temperature decreases sharply (from about 1160 °C to about 994 °C) with the formation of albite. When the Na2O content increases from 3.5% to 5.5%, the liquid formation temperature has a slight drop (from about 994 °C to about 984 °C) with the quartz disappearance and the nepheline formation. When the Na2O content increases from 5.5% to 10%, the liquid formation temperature remains nearly constant with the formation of wollastonite. The figure coincides with the minerals behaviors in Figure 3, which illustrates that sodium minerals albite and nepheline can lower the liquid formation temperature of ash. The continuous decline of anorthite content in the entire process will also contribute to the AFTs decrease in synthetic ash samples, considering the melting temperature of anorthite (FT 1555 °C)

Figure 5. XRD analysis of synthetic ash samples with Na2O contents of 5% (a) and 10% (b) at 1000 °C. (CPS: counts per second).

both samples at 1000 °C, while the peaks of quartz are much weaker in ash sample 10 than in ash sample 8, which can be explained by the depletion of SiO2 in reaction 1. Albite exists in ash sample 8 and nepheline exists in ash sample 10 at 1000 °C, consistent with reactions 2 and 3, and the minerals behaviors in Figure 3. Lime is found in both ash samples 8 and 10 at 1000 °C, which can be well explained by the transformation of anorthite into albite and nepheline, in reactions 1, 2, and 3. Panels a−d of Figure 6 present micromorphology of ash samples 3, 8, and 10, which are quenched in water from 1000 °C with Na2O contents of 0.6%, 5%, and 10%, respectively. Meanwhile, 1000 °C is lower than any AFTs of all ash samples, so the micrographs can indicate the melting extent of the ash D


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Figure 6. Micrographs of synthetic ash samples with different Na2O contents of 0.6% (a, b), 5% (c, d), and 10% (e, f) at 1000 °C. Panels a, c, and e are magnified 200 times; b, d, and f are magnified 3000 times.

samples at 1000 °C. As shown in panels b, d, and f, ash samples are magnified 3000 times. The micrograph of ash sample 3 shows a loose structure, whose surface is covered by micropores (panel b). The structures turn to being denser and smoother in the order of ash sample 3 (panel b), ash sample 8 (panel d), and ash sample 10 (panel f). The denser structure means the larger extent that ash samples are melted.36 Furthermore, as shown in panels a, c, and e, ash samples are magnified 200 times. There is an evident trend that the mineral blocks become larger from panel a, to panel c, to panel e. Meanwhile, in the same order, the AFTs of the ash samples become lower. This also proves that, at 1000 °C, the molten extent of the ash samples become lower from ash samples 10 to 8 to 3. These are all consistent with the AFTs’ trend in Figure 2 and the liquid formation temperatures trend in Figure 4. 3.3. Analysis for Rising Temperature. Ash melting is a high-temperature process. Relative liquid content of synthetic ash samples 1 (Na2O content is 0%), 3 (Na2O content is 0.6%), and 10 (Na2O content is 10%) with the rising temperature were calculated by the multicomponent system SiO2−Al2O3− Fe2O3−CaO−Na2O, as shown in Figure 7. When the Na2O content increases from 0 to 10%, the liquid formation temperature of ash declines considerably (from about 1150 °C to about 950 °C). It is consistent with the calculation in Figure 4. Line b is similar to line a in tendency: the liquid

Figure 7. Relative liquid content of synthetic ash samples with Na2O content of 0% (a), 0.6% (b), and 10% (c) with rising temperature.

content increases continuously with increasing temperature and minerals melt in the order of albite, andradite, corundum, hematite, and anorthite. Compared with line a and line b, the tendency of the liquid content in line c is steeper and the melting order of minerals is albite, wollastonite, nepheline, E


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Figure 8. Main minerals content in synthetic ash samples with Na2O content of 0% (a), 0.6% (b), and 10% (c) in the rising temperature.

andradite, anorthite, corundum, and hematite. As we can conclude, with the increasing Na2O content, the liquid content and the liquid producing rate at the same temperature in the melting process increase significantly. Anorthite melts at lower temperatures in high Na2O content ash than in low Na2O content ash, which may contribute to the decline of the AFTs. Figure 8 shows the main minerals’ relative content of synthetic ash samples 1, 3, and 10 with the rising temperature. As can be seen from the figure, the contents of all minerals decline with the increasing temperature. Compared with ash samples 1 and 3, nepheline is generated and the content of albite increases in ash sample 10, which may contribute to the reduction of AFTs. Furthermore, anorthite has a significant decline in the starting temperature of melting from ash sample 1 to samples 3 and 10, which is consistent with the conclusion in Figure 7. In ash sample 1, anorthite starts to melt at a temperature of about 1150 °C; in ash sample 3, it starts to melt at about 1100 °C, together with albite; in ash sample 10, it starts to melt at about 900 °C, together with albite and nepheline. The minerals’ starting temperatures of melting illustrate that sodium containing minerals albite and nepheline can react with anorthite and form eutectics, which can reduce the AFTs. XRD tests were applied on ash samples 1 and 10 after they were heated at their flowing temperatures (FT), and the peaks are shown in Figure 9. In ash sample 1, the flowing temperature is 1290 °C. Anorthite and quartz exist in this sample. In ash sample 10, the flowing temperature is 1170 °C, much lower than that in ash sample 1. However, anorthite and quartz both disappear in this sample. The disappearance of anorthite can be explained by the transformation from anorthite to albite and nepheline, in reactions 1, 2, and 3, and the reduction of the start melting temperature of anorthite in higher Na2O content ash, as illustrated in Figures 7 and 8. The disappearance of quartz may be due to the depletion in reaction 1. In ash sample 10 at 1170 °C, the minerals are almost the same as those at 1000 °C;

Figure 9. XRD analysis of synthetic ash sample 1 at 1290 °C (a) and synthetic ash sample 10 at 1170 °C (b). (CPS: counts per second).

as shown in Figure 5b, nepheline and lime still exist. However, pseudowollastonite is generated at this temperature, which is not predicted by the thermodynamic calculation. Figure 10 illustrates a pseudoternary phase diagram in SiO2− Al2O3−CaO−Na2O system, which can be used to visually express liquid temperatures of synthetic ash samples with a S/A ratio of 2 as a function of the Na2O content. This figure is called “projection”, in which different sections surrounded by black thick lines represent different minerals precipitating first from the liquid slags when temperature decreases. The lines of the same color are isotherms which represent all compositions that have the same given liquidus temperature. The red points in the figure represent the components of synthetic ash samples 3, 7, 8, 9, and 10 in Table 1. As shown in the figure, synthetic ash samples in this research mostly locate in the mullite section, F


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Figure 10. Liquidus surface in the SiO2−Al2O3−CaO−Na2O system with a SiO2/Al2O3 (S/A) ratio of 2.

which means mullite precipitates first from the liquid slags. It suggests that mullite (FT, 1850 °C) is a refractory mineral increasing the AFTs of ash samples. Ash sample 3 is located near the isotherm that mullite precipitates at a temperature of about 1750 °C while it is about 1650 °C in ash sample 10. Ash samples 7, 8, and 9 locate between ash samples 3 and 10, and their liquidus temperatures of mullite decrease with the rising of Na2O content. This can partly explain the reducing effect of Na2O in ash on ash fusion temperatures: refractory minerals melt at lower temperatures when Na2O content is higher in ash. The theory explains not only the experiments of this work but also those of other papers. The effect of CaO in ash on ash fusibility has been researched by Song et al.37 They suggested that CaO has an effect of reducing ash fusion temperatures. Blue points in Figure 10 represent ash components, whose CaO contents are nearly 40% (higher than ash samples in this study). The blue points with Na2O contents of 1% and 5% are located near the edge of the mullite region while the blue points with Na2O contents of 8% and 10% are located in the anorthite region. As mentioned in section 3.2, anorthite has a flowing temperature (FT) of 1555 °C, which is lower than that of mullite, thus partly proving the reducing effect of CaO on ash fusion temperatures. The fluxing effect of CaO can also be proved by the blue point with a Na2O content of 1%, which is located near the isotherm mullite precipitates at about 1600 °C. It is much lower than that of the ash sample 3 in this study (1750 °C), with a similar Na2O content. The blue point with a Na2O content of 12% is located near the isotherm anorthite precipitates at about 1450 °C, lower than the blue point with a Na2O content of 1% mentioned earlier. It proves that Na2O can reduce the ash fusion temperatures not only in this case but also in ash samples with higher CaO contents.

4. CONCLUSION Na2O has an effect of reducing the AFTs of ash samples. Sodium exists mainly in the forms of albite and nepheline in ash. With the rising content of Na2O, albite generates first and then transfers to nepheline gradually. Compared to sodium minerals, mullite and anorthite are refractory minerals that increase the AFTs. Na2O can lower refractory minerals’ liquidus temperatures and contents in ash by generating sodium minerals. Sodium minerals can replace refractory minerals and form eutectics (with low melting temperatures) with them, thus reducing the AFTs by several aspects. The XRD and SEM tests prove the experimental results and theoretical analysis well.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 10 61772992. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Basic Research Program of China (Grant 2015CB251501) and the Fundamental Research Funds for the Central Universities (Grants 2015ZD17 and 2015XS54).

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REFERENCES

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