Characterization of Aluminosilicates in Fly Ashes with Different Melting

Aug 2, 2017 - Knowledge of the effect of aluminosilicates in fly ash on slagging is very important for the prevention and control of slagging in coal-...
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Characterization of Aluminosilicates in Fly Ashes with Different Melting Points Using 27Al Magic-Angel Spinning Nuclear Magnetic Resonance Sida Tian,*,† Zhizhong Kang,† Lei Chen,‡ Yongxu Fang,† Yuqun Zhuo,§ and Hong Xu† †

Key Laboratory of Condition Monitoring and Control for Power Plant Equipment of Ministry of Education, School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China ‡ National Center for Magnetic Resonance in Wuhan, State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan, 430071, China § Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Thermal Engineering Department, Tsinghua University, Beijing 100084, China ABSTRACT: Knowledge of the effect of aluminosilicates in fly ash on slagging is very important for the prevention and control of slagging in coal-fired boilers. To understand hydrochloric acid dissolution of the aluminum element of fly ash aluminosilicates at a chemical coordination level and establish a reliable method to characterize fly ash aluminosilicates that easily cause slagging, we performed hot hydrochloric acid separation for fly ashes with different melting points obtained from four power plants in China and measured the acid-solubility of major elements in the fly ashes. The fly ashes and their acid separation residues were analyzed using X-ray diffraction and 27Al magic-angle spinning nuclear magnetic resonance (27Al MAS NMR). The results show that the 6-fold coordinated aluminum (denoted as Al(VI)) in the fly ashes does not easily dissolve in hot hydrochloric acid. One part of the 4-fold coordinated aluminum (denoted as Al(IV)) easily dissolves in hydrochloric acid. The acid-soluble Al(IV) in ash has stronger quardapolar interactions than the Al(IV) of mullite, and the excess negative charges of the AlO4 tetrahedron are compensated by active metal cations. The aluminosilicates containing a large amount of acid-soluble Al(IV) are easily slagged. In the fly ashes with high melting point, the mullite and corundum contents are high; the Al(IV) content is low, and the acid-soluble Al(IV) is minor. In the fly ashes with low melting point, the aluminosilicates are mainly amorphous substances; the Al(IV) content is high, and the acid-soluble Al(IV) content is more. Therefore, the acid-soluble aluminum fraction of fly ash can be used to characterize the slagging propensity of aluminosilicates in fly ash.

1. INTRODUCTION The deterioration of fly ash deposition in coal-fired boiler furnaces causes boiler slagging, which seriously threatens the safe and economic operation of the boilers. Fly ash is a complex mixture of various substances, of which the aluminosilicates are the most abundant. In addition to mullite, most aluminosilicates in fly ash are amorphous substances.1 Aluminosilicates are also the main component of boiler slagging materials.1−4 Thus, research on the effect of aluminosilicates in fly ash on slagging is very important for the prevention and control of boiler slagging. Substantial research on the mineral species of fly ash has been conducted to predict the slagging characteristics of coal.4 Zhao et al.5,6 classified the iron- and calcium-bearing aluminosilicates in fly ash. Recently, to understand the role of mullite in preventing slag formation, Huang et al.7 studied the effect of additives on the formation of mullite in fly ash. Wu et al.8 studied the structural characteristics of mullite. Because of the complex composition of amorphous aluminosilicates in fly ash,9 it is difficult to characterize the easily slagged fly ash aluminosilicates. The fly ash aluminosilicates are basically the thermal reaction products of clay minerals in coal. In the glass research field, the ratio of nonbridging oxygen atoms to tetrahedrally coordinated cations (NBO/T) is a parameter that characterizes the degree of polymerization in a melt or glass. On the basis of the silicate © XXXX American Chemical Society

polymerization theory, Tian et al. deduced that the aluminosilicates that easily cause slagging in fly ash should be substances with a high NBO/T and conducted hot hydrochloric acid separations for the ash samples of two coals prepared at different temperatures in a drop tube furnace (DTF) system.10 Previous studies have analyzed the clay minerals in coals and phase species in the DTF ash samples using X-ray diffraction (XRD).10,11 The results show that when the DTF combustion temperature is up to 1250 °C, the soluble fraction of aluminum in the DTF ash sample is positively correlated with the amount of high-NBO/T products of the clay minerals. In this article, we conducted hydrochloric acid separations for fly ashes with different melting points obtained from four power plants in China and measured the solubility of the major elements in the fly ashes. XRD and 27Al magic-angle spinning nuclear magnetic resonance (27Al MAS NMR) are employed to analyze the fly ash samples before and after acid separation. The aluminum coordination characteristics of the fly ashes with different melting points are studied. At the aluminum coordination level, we discuss the correlations between the Received: May 22, 2017 Revised: July 25, 2017 Published: August 2, 2017 A

DOI: 10.1021/acs.energyfuels.7b01466 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Chemical Compositions of Fly Ash Samples (%) DH DZ SH DL

SiO2

Al2O3

CaO

SO3

Fe2O3

MgO

Na2O

K2O

TiO2

P2O5

38.28 47.95 49.56 59.65

50.10 34.60 24.32 19.46

2.40 6.58 12.03 5.48

0.18 0.60 0.92 0.21

1.59 4.88 5.90 7.62

0.20 0.91 1.06 1.60

0.17 0.50 1.22 0.92

0.36 0.78 1.51 2.04

1.94 1.43 1.14 0.98

0.18 0.28 0.18 0.32

acid solubility of aluminum in fly ash and the easily slagged aluminosilicates in fly ash. MAS NMR is widely used in the analysis of amorphous solid structures.12,13 When the NMR spectra of solid powder samples are collected, the MAS technique is capable of eliminating the anisotropic contributions of chemical shift anisotropy and dipolar interaction to the spectra. However, 27Al is a halfinteger quadrupolar nucleus with a spin of 5/2 and possesses an electric quadrupolar moment. Its interaction with inhomogeneous electric fields is called quadrupolar interaction, which has an important effect on 27Al NMR. MAS can only partially average the second-order quadrupolar interaction of 27Al. The residual second-order quadrupolar interaction can still lead to spectral line broadening and quadrupolar displacement, often leading to the formation of spinning sidebands; this makes the 27 Al MAS NMR spectra complicated and difficult to identify.12,13 Fly ash is a complex mixture of various substances. In the research fields of materials and catalysis, 27Al NMR is mainly used to analyze the chemically treated fly ash (usually via alkali treatment).14−16 In this article, by comparing the NMR spectrum patterns of the fly ash samples, the centerbands and spinning sidebands of different aluminum coordinations for the fly ash samples were distinguished, and the reliability of aluminum coordination analysis was improved.

Table 2. Comparison of Ash Fusion Points (°C) of the Fly Ash Samples and Their Boiling Acid Separation Residuesa samples

DT

DH ash DZ ash SH ash DL ash DZ residue SH residue DL residue

>1548 1521 1366 1292 1548 1462 1386

ST

HT

FT

1548 1373 1313

1374 1326

1381 1358

1518 1442

1529 1471

1548 1515

a

DT, deformation temperature; ST, soft temperature; HT, hemispherical temperature; FT, fluid temperature.

When the hydrochloric acid separation was conducted, 0.2 g of fly ash sample was mixed with 20 mL of hydrochloric acid in a 100-mL beaker; then, the beaker was covered with a glass sheet and heated on an electric heating plate at 200 °C for 1 h. The hydrochloric acid used in the chemical separation was prepared with deionized water and analytical grade hydrochloric acid in a volume ratio of 1:1; the molar concentration is approximately 6.3 mol/L. After heating, the mixture was filtered with a quantitative filter paper; the filtrate volume was set to 100 mL. The amounts of the major elements dissolved in hydrochloric acid were determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). After being dried in a drying box at 80 °C for 30 min, the solid residues of the fly ash were collected for XRD and NMR analysis. In this study, the hot hydrochloric acid separation method is referred to as “boiling acid separation”. The XRD spectra of the ash samples and solid residues were collected using a D8-Advance diffractometer equipped with a LYNXEYE array detector, both produced by German Bruker Company. The measurement accuracy of the 2θ diffraction angle is 0.002°. The working parameters of the diffractometer are as follows: copper target, continuous scanning at a scanning speed of 4° (2θ)/min; CPS counting mode, a tube voltage of 40KV, and a tube current of 40 mA. In the phase-identification process, all the diffraction peaks of the XRD spectra were matched. To highlight the characteristics of the phase distribution on the XRD spectrum, only the major diffraction peaks of minerals are labeled in the XRD spectra. The 27Al MAS NMR spectroscopic experiments were performed using a 600 MHz NMR spectrometer with a 4 mm double-resonance rotor produced by Swiss Bruker Company. The MAS NMR spectra were recorded at ambient temperature with a single pulse width of 4.60 μs and accumulated 20,000 times with 1.0-s of cycle delay; the spectral width was 108696 Hz. The resonant frequency of 27Al was 156.4 MHz. The MAS speed for all the samples was 12 kHz. The Al3NO3 aqueous solution was used in the chemical shift calibration; its chemical shift is set to 0 ppm.

2. MATERIALS AND METHODS The four fly ashes studied herein were obtained from four power plants in China. The four power plants use different coals: the Daihai power plant (DH) uses a Junggar lignite, the Dingzhou power plant (DZ) uses a blend of bituminous coal produced by Shenhua Company, the Sanhe power plant (SH) uses a bituminous coal from the Shendong colliery, and the Dalian power plant (DL) uses a Pingzhuang lignite from Inner Mongolia. The four boilers being sampled are all subcritical. The capacities of the first three boilers are all 600 MW, and that of the DL boiler is 350MW. The temperatures of flame centers of these boilers are all higher than 1400 °C. Slagging problems often occur in DL and SH power plants. These ash samples were taken from the hoppers of the first field of the electrostatic precipitators. The contents of the major elements in the ash were measured according to the China national standard GB/T 1574-2007,17 as listed in Table 1. The DH fly ash has high aluminum content, which is a common feature of Junggar lignite. Calcium and sulfur contents are high in the SH fly ash. Melting points of the four fly ashes were determined according to the China national standard GB/T 219-2008.18 Ash samples were molded into triangular pyramids and heated up in a weakly reducing atmosphere. The heating rate is 20 °C/min at a temperature less than 900 °C, and 5 °C/min at a temperature greater than 900 °C. The melting points of ash samples were in the following order: DH > DZ > SH > DL (see Table 2); the order of the silica-to-alumina ratio of ash samples from low to high was the same. Additionally, the SH ash showed the highest base-to-acid ratio among the four fly ashes. B

DOI: 10.1021/acs.energyfuels.7b01466 Energy Fuels XXXX, XXX, XXX−XXX

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3. RESULTS AND DISCUSSION 3.1. XRD and NMR Analyses of Original Fly Ash Samples. The XRD analyses show that the four fly ashes contained mullite but no other aluminosilicate minerals (see Figure 1). The mullite contents in the DH, DZ, SH, and DL fly

Figure 1. XRD patterns of fly ash samples.

ashes decreased in that order, whereas the quartz contents increased successively. The order of the corundum (α-Al2O3) content in the four fly ashes is the same as that of the mullite contents. DH fly ash contains little quartz; however, it has the highest corundum content. No corundum is found in DL fly ash. In addition, SH ash with the highest calcium content contains lime, and DL ash with the highest iron content contains hematite. The diffuse diffraction humps in the XRD spectra originate from the amorphous substances. The ignition losses of the fly ash samples are less than 2%, indicating that the amounts of unburned carbon are very small in the fly ashes. Therefore, these diffuse diffraction humps are basically formed by amorphous aluminosilicates. Formation of spinning sidebands is a common factor that causes problems in the 27Al MAS NMR spectrum analysis. However, the spinning sidebands are symmetrically distributed on both sides of their centerband, and their distance to the centerband (frequency difference) is denoted as nνR, where νR stands for the MAS frequency, and n stands for the order of spinning sidebands (n = 1, 2···). The stronger the spinning sidebands of the observed nucleus, the greater are its anisotropic interactions.12 In this study, based on the properties of spinning sidebands, all the peaks of the 27Al MAS NMR spectra were confirmed to be either centerbands or spinning sidebands. In the calculation of the fraction of a certain coordinated aluminum, not only the centerband but also the spinning sidebands were taken into account to improve the reliability of the calculation. Among the four fly ashes used herein, the NMR spinning sidebands of DH fly ash are the weakest (see Figure 2). Referring to the XRD analysis and the relevant literature,19,20 it can be determined that there exist three types of aluminum coordinations in DH fly ash. The spectral peak with a chemical shift of 14.7 ppm is the centerband of the 6-fold coordinated aluminum (denoted as Al(VI)) in corundum (α-Al2O3). Another type of Al(VI) produced a spectral peak with a chemical shift of 3.1 ppm; the high-field side (the direction of

Figure 2. 27Al MAS NMR spectra of fly ash samples. IV, VI 1, VI 2 denote Al(IV), corundum Al(VI), and high-field Al(VI), respectively. * denotes the first-order spinning sideband of Al(IV) ; Δ denotes the first-order spinning sideband of corundum Al(VI).

Figure 3. XRD patterns for boiling acid separation residues of fly ashes.

the chemical shift reduced) of this peak decreases more gradually. Such a line shape is caused by the combined action of quadrupolar interaction and inhomogeneous chemical shielding.20 Al(VI) with a chemical shift of 3.1 ppm is called highfield Al(VI) in order to distinguish it from the Al(VI) in corundum. The spectral peak whose peak is at a chemical shift of 60 ppm is the centerband of 4-fold coordinated aluminum (denoted as Al(IV)), and this spectral peak has a larger halfpeak width compared with the centerband of high-field Al(VI). Although the chemical formula of phase-equilibrium mullite is C

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which is the same value as that observed for amorphous silicon oxide.10 This indicates that the boiling acid treatment increased the polymeric degrees of the amorphous aluminosilicates in the SH and DL samples. The 27Al MAS NMR spectrum of the DH ash residue shows that after boiling acid separation, only small dents occurred on the centerband of Al(IV), whereas the centerbands of corundum Al(VI) and high-field Al(VI) were unchanged (see (Figure 4)). For DZ ash, the spinning sidebands of Al(IV) fall

Al6Si2O13, natural mullite is defective, and its structure may vary in a certain range. The chemical formula of mullite with a defective structure can be written as Al(VI)2(Al(IV)2+2xSi2−2x)O10‑x, where x represents the missing oxygen atom number, which ranges from 0.2 to 0.9.21−23 A large amount of mullite exists in DH fly ash, which should mainly comprise Al(IV) and high-field Al(VI). DH fly ash also contains some amorphous aluminosilicates, as shown in the XRD analysis; therefore, the aluminum in the amorphous materials also contributes to the two spectral peaks. A comparison of the 27Al MAS NMR spectra of DZ and DH ashes shows that the above-mentioned three types of aluminum coordinations are also present in DZ ash. However, the spectral peak of Al(IV) in DZ ash is the highest, whereas that of the Al(VI) of corundum is the lowest. The spinning sidebands of Al(IV) in DZ ash are stronger than those in DH ash. A bump is present on the high-field flank of the peak of high-field Al(VI) in DZ fly ash. On the basis of the properties of the spinning sidebands, the bump is confirmed to be the first-order spinning sideband of Al(IV) on the high-field side. On the basis of the 27Al NMR analyses of DZ and DH ashes, it can be concluded that Al(IV), corundum Al(VI), and highfield Al(VI) are also present in SH fly ash. However, the fraction of Al(IV) in the total Al content of SH ash is obviously higher than those of the other two types of aluminum; moreover, its first-order spinning sideband on the high-field side (−20 ppm) is higher even than the centerband of highfield Al(VI). The spinning sidebands of Al(IV) are stronger in DL ash than in SH ash; moreover, the spinning sideband at the −20 ppm chemical shift is significantly higher than the centerband of high-field Al(VI). Corundum Al(VI) may be ignored. The mullite contents are low in SH and DL ashes, and no other mineral species containing Al(IV) exist in the two ashes. Therefore, the high Al(IV) contents in the two lowmelting-point ashes originate from amorphous aluminosilicates. Moreover, the spinning sidebands of Al(IV) for SH and DL ashes are much stronger than those for DH and DZ ashes, indicating that stronger anisotropic interactions occur at the Al(IV) nuclei of amorphous aluminosilicates in the two lowmelting-point ashes. 3.2. XRD and NMR Analyses of the Acid Separation Residues of Fly Ashes. Figure 3 shows that mullite, quartz, and corundum are still retained in the ash residues after boiling acid separation and that the order of contents of these minerals in the four ash residues is the same as that observed in the original ashes; this indicates that these minerals are chemically stable during boiling acid separation. The lime in SH fly ash and the hematite in DL fly ash are dissolved. The XRD pattern exhibits a small amount of kyanite (a mineral containing a large amount of aluminum) in the DZ ash residue. Minor changes in the diffuse diffraction hump of the DH sample are observed after boiling acid separation. However, significant changes are observed in the profile of the diffuse diffraction humps of SH and DL ashes before and after boiling acid separation; this indicates that the constituents of amorphous aluminosilicates in the two low-melting-point ashes changed significantly after boiling acid separation. For the residues of the four fly ashes, a contrast in the areas of the XRD diffuse diffraction humps is observed; this demonstrates that the amorphous aluminosilicate contents in SH and DL ash residues were higher than those in the DH and DZ ash residues. In the case wherein the instrument background of XRD is deducted, the peaks of the diffuse diffraction humps of all the residues are at 22.7° 2θ,

Figure 4. 27Al MAS NMR spectra of boiling acid separation residues of fly ashes. IV, VI 1, and VI 2 denote Al(IV), corundum Al(VI), and high-field Al(VI), respectively. * denotes the first-order spinning sideband of Al(IV).

markedly after boiling acid separation, and the bump on the centerband of high-field Al(VI) disappears. The centerband of Al(IV) also decreases slightly. A comparison of the NMR spectra of the samples shows that although the same type of coordinated aluminum is present in different amounts in the DZ and DH ash residues, its NMR pattern features are identical; this indicates that every type of coordinated aluminum is in the same local environment for the two ash residues. For SH ash, a more obvious change is observed, i.e., the spinning sidebands of Al(IV) in the ash residue drop dramatically compared with those of the original sample. The centerband of high-field Al(VI) not only displays the high-field tail but also possesses a sharp resonance peak at the chemical shift of −0.5 ppm. The centerband of Al(IV) also decreases; however, it is larger than that of the high-field Al(VI). Consequently, most of the dissolved Al(IV) generates the spinning sidebands in the NMR spectrum. Similarly, the spinning sidebands of Al(IV) in the DL ash residue also decreased substantially when compared with those of the original ash. Similar NMR patterns were observed for high-field D

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certain type of Al(IV) that produced the spinning sidebands on the 27Al NMR spectra. 3.3. Structural Differences of Amorphous Aluminosilicates in Fly Ashes. Compared with SiO4 tetrahedra in aluminosilicates, the AlO4 tetrahedra formed by Al(IV) possess more negative charges that need to be balanced because the valency of silicon is +4 and that of aluminum is +3. The excess negative charge generated by the Al(IV) in mullite is balanced by oxygen vacancies.21−23 In geological research, the mechanism of charge deficit of Al(IV) in amorphous aluminosilicates has been a concern. Toplis et al.24 proposed the existence of two types of mutual transitional Al atoms in amorphous aluminosilicates: (1) charge-balanced, 4-fold coordinated aluminum and (2) any structural role of aluminum that does not require a charge-balancing cation, which often combines with an oxygen tricluster structure. Le Losq et al.25 comprehensively studied Na2O−Al2O3−SiO2 glasses and melts and confirmed the role change of Na+ cations from network modifiers to charge compensators in the presence of Al3+ ions. These previous research works indicate that excess negative charges caused by Al(IV) are balanced basically by two ways: (1) oxygen vacancies or oxygen tricluster structure; (2) active metal cations as charge compensators. In this study, the differences of the 27Al MAS NMR spectra of the four ashes and the changes in them after boiling acid separation reveal that excessive negative charges in the acid-soluble Al(IV) are balanced by the active metal cations in the aluminosilicates. When a MAS NMR spectrum is recorded, spinning sidebands are generated because the MAS speed/frequency is lower than the intensity/frequency of the inhomogeneous anisotropic interactions of the observed nucleus at a certain atom site in substances. The anisotropic interactions of halfinteger quadrupolar nuclei consist mainly of quadrupolar interaction and anisotropic chemical shielding.20,26,27 27Al nuclei hold a spin of 5/2. The quadrupolar interaction can reach the order of MHz, whereas the chemical shielding anisotropy is generally much weaker than the quadrupolar interaction.20,27 The more distorted the electric field gradient tensor at the 27Al nucleus, the stronger the generation of the quadrupole effect and spinning sidebands. In this study, the MAS speed for all the samples is 12 kHz. The intensities of the spinning sidebands in the 27Al NMR spectra reflect the differences in quadrupolar interactions of the aluminum nuclei. In DH ash, which has the highest melting point among the four types of ashes used herein, the aluminosilicates consist mainly of mullite, whereas the spinning sidebands of the Al(IV) are the lowest. The results illustrate that the quadrupolar interaction of Al(IV) in mullite is weak, i.e., the anisotropy of the electric field gradient caused by the oxygen vacancy is weak. Moreover, the amount of dissolved aluminum is low in DH ash (see Figure 7). Only a slight change is observed in the NMR pattern of Al(IV) after boiling acid separation. Consequently, most Al(IV)’s in the amorphous aluminosilicates contained in the DH ash have local environments similar to that of the Al(IV) in mullite. The analysis conducted for the DZ sample indicates that the aluminosilicates in the DZ ash residue have the same aluminum-coordination distribution as those of the DH ash; moreover, the acid-dissolved Al(IV) nuclei that generate spinning sidebands have stronger quadrupolar interactions than the nuclei of the Al(IV) remaining in the ash residues. Furthermore, Al(IV) has much stronger spinning sidebands in SH ash than in DZ ash, as shown in Figure 6. Therefore, in

Al(VI) in both the DL and SH ash residues. The features of the centerband of high-field Al(VI) are the same in both the DL and SH ash residues; moreover, a spike is also present at the chemical shift of −0.5 ppm. This indicates the similarity in their distribution in the two fly ash residues. Figure 5 compares the Al(IV) fractions in the original ashes and their respective residues. For the four original ashes, the

Figure 5. Comparison of fractions of Al(IV) in total Al for four fly ashes and their boiling acid separation residues.

fraction of Al(IV) increases successively in the DH, DZ, SH, and DL ashes; this is also the order of ash-melting-point from high to low for these ashes. The Al(IV) fractions in the DH and DZ samples undergo a minor change after boiling acid separation. The Al(IV) fractions in the SH and DL ash residues decreased considerably and are similar to the value obtained for the DZ ash residue. This suggests that compared with the DZ ash, a higher amount of Al(IV) dissolved in the two low-melting-point ashes during the boiling acid separation. Figure 6 shows that for the four ash residues, the intensity ratios of the first-order spinning sideband to the centerband of the Al(IV) tend to be consistent. This indicates that the aluminum dissolved during boiling acid separation is mainly a

Figure 6. Comparison of the intensity of the spinning sidebands of Al(IV) for four fly ashes and their boiling acid separation residues. Here, the value for the spinning sideband is the ratio of the first-order spinning sideband to the centerband of Al(IV). E

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Figure 7. Major-element concentrations in fly ash samples and their soluble fractions determined by ICP-AES (four bars in every group from left to right represent element concentrations of DH ash, DZ ash, SH ash, and DL ash, respectively. The gray part of every bar is the element concentration of the boiling acid separation residue of the ash).

spectra and is chemically stable; the other type of Al(IV) relies on the metal cations to balance the excess negative charges in the AlO4 tetrahedron, which produces strong spinning sidebands in the NMR spectra. A distorted electric-field implies the local structure is unstable; the second type of Al(IV) easily dissolves during boiling acid separation. Because the AlO4 tetrahedron has more electronegativity than the SiO 4 tetrahedron, the aluminosilicates with the latter type of Al(IV) should contain more active cations than the silicates with the same NBO/T value. According to the polymerization theory,10,24 aluminosilicates with a larger amount of acidsoluble Al(IV) react more easily with other substances under high temperature conditions and can be more easily melted. They easily cause slagging in boiler furnaces. The ash melting points of the residues of the low-melting-point fly ashes increased significantly (see Table 2). It signifies that the substances removed by boiling acid separation are the key materials that affect the ash melting point of fly ashes. The NMR and XRD analyses of the fly ashes and the residues show that either corundum Al(VI) or high-field Al(VI) does not easily dissolve in hot hydrochloric acid. For the DH and DZ samples, the fractions of Al(IV) in the residue are nearly the same as those in the original ashes (see Figure 3); this indicates that some part of the dissolved aluminum originates from Al(VI) in the two high-melting-point fly ashes. However, these ashes contain a lesser amount of acid-soluble aluminum than SH and DL ashes (see Figure 7). Therefore, the acid-soluble aluminum content in one fly ash can still reflect the content of the easily slagged fly ash aluminosilicates. Since the high-melting-point ashes have high aluminum contents, their acid-soluble aluminum fractions (the ratio of the acid-soluble aluminum to the total aluminum) are much lower than those of SH and DL ashes (see Figure 8). Consequently, the acidsoluble aluminum fraction of one fly ash can sufficiently characterize the fraction of easily slagged substances in the aluminosilicates of the fly ash. Among these four fly ashes, the acid-soluble aluminum fraction is the highest in SH ash. This is not consistent with the melting-point sequence of these fly ashes. This is because the acid-soluble aluminum fraction indicates the easily melting aluminosilicates in fly ashes, and the ash melting point is also influenced by other substances of fly ash.28,29 Either investigations of utility boiler operations or slagging experiments in laboratories have proven that there exist a considerable number of coals whose slagging propensities are not consistent with their ash melting point.30,31 The differences in thermal reactivity of fly ash

SH ash, there is a considerable amount of Al(IV) nuclei undergoing strong quadrupolar interactions; moreover, the electric field gradients on these nuclei are strongly distorted. Such distorted electric-field gradient at these Al(IV) sites should be formed not by oxygen vacancies but via electrostatic interactions of the neighboring metal cations; that is, the excess negative charges of these AlO4 tetrahedrons are balanced by other metal cations. For DL ash, hematite exists in the ash, and the iron content in the fly ash reaches 7.62% (calculated in the form of Fe2O3). The significant broadening of the 27Al MAS NMR pattern may have resulted partly from a trace of paramagnetic substances in the ash. Nevertheless, among the four ashes, the original DL ash has the strongest spinning sidebands of Al(IV), and boiling acid separation removed most of the spinning sidebands. The excess negative charges of the tetrahedrons of acid-soluble Al(IV) must be balanced by other metal cations. The differences in the contents and acid solubility of the major elements in the four fly ashes also support this argument (see Figure 7). Compared with DH and DZ ashes, SH and DL ashes contain more silicon and less aluminum; however, the amount of dissolved silicon in SH and DL ashes increases less and that of dissolved aluminum increases more. Mullite and corundum cannot be removed from fly ash by the hot hydrochloric acid. Consequently, the dissolved amorphous substances in the low-melting-point ashes are mainly aluminosilicates rather than silicates. Moreover, SH ash lacks iron-bearing minerals, and DL ash lacks calcium-containing minerals (see Figure 1). Both the low-melting-point fly ashes contain large amounts of iron and calcium, and most of the two elements dissolved in the hot hydrochloric acid. Therefore, the acid-soluble iron in SH ash and the acid-soluble calcium in DL ash mainly originate from amorphous aluminosilicates. In addition, SH and DL ashes contain more potassium, magnesium, and sodium compared with DH and DZ ashes. As cations, these metal elements may balance the excessive negative charges in the AlO4 tetrahedral and also cause the distorted electric-field gradients at the 27Al nuclei. During boiling acid separation, with the dissolution of the other metal cations, the AlO4 tetrahedrons are destroyed and dissolve into the hot acid. In summary, the Al(IV) in amorphous aluminosilicates of fly ash may be further divided into two types depending on the local environments. The first type of Al(IV) relies on the oxygen vacancy to eliminate the excess negative charges as in mullite, which generates weak spinning sidebands in the NMR F

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Energy & Fuels Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation of China (No. 51376061). We also gratefully acknowledge Dr. Zhonghua Zhan for the initial review of the manuscript.

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Figure 8. Acid-soluble aluminum fractions in fly ash samples.

aluminosilicates probably elucidate the inconsistency between the slagging performances of the coals and their ash melting points.

4. CONCLUSIONS Boiling acid separation was performed for four fly ashes with different melting points obtained from power plants in China. These fly ashes and their solid residues were analyzed using XRD and 27Al MAS NMR. The solubility of the major elements in the fly ashes and the melting points of the solid residues were measured. The characteristics of aluminum-coordination in the fly ashes are studied. The structure differences in the aluminosilicates in the fly ashes are discussed. The following conclusions are drawn. According to the chemical shift of 27Al MAS NMR, the aluminum element in fly ash can be divided into three categories: Al(IV) (chemical shift of the peak: 60 ppm), corundum Al(VI) (chemical shift: 14.7 ppm), and high-field Al(VI) (chemical shift of the peak: 3.1 ppm). The Al(VI) in the fly ashes does not easily dissolve in hot hydrochloric acid. One part of Al(IV) easily dissolves into the hydrochloric acid. The acid-soluble Al(IV) generates the NMR spinning sidebands and has strong quadrupolar interactions. The excess negative charges of the AlO4 tetrahedron are compensated by other metal cations, and the aluminosilicates containing a large amount of acid-soluble Al(IV) are easily slagged. For the fly ashes with high melting point, the mullite and corundum contents are high; the Al(IV) content is low, and the acid-soluble Al(IV) content is minor. For the fly ashes with low melting point, the aluminosilicates are mainly amorphous substances; the Al(IV) content is high and the acid-soluble Al(IV) content is more. Therefore, the amount of the acidsoluble aluminum in fly ashes reflects the easily slagged aluminosilicates in the fly ashes. The acid-soluble aluminum fractions can be used to characterize the slagging propensity of the aluminosilicates in the fly ashes.

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Sida Tian: 0000-0001-8763-0854 G

DOI: 10.1021/acs.energyfuels.7b01466 Energy Fuels XXXX, XXX, XXX−XXX