Effect of Fe₂O₃ on the Formation of Micro Glass Beads under Air Staged Combustion

Author NameAffiliation
Yuanjun ZhangSchool of Energy and Power Engineering, Shandong University, Jinan 250061, China
Dakai WangSchool of Energy and Power Engineering, Shandong University, Jinan 250061, China
Xingxing ChengSchool of Energy and Power Engineering, Shandong University, Jinan 250061, China
Zhiqiang WangSchool of Energy and Power Engineering, Shandong University, Jinan 250061, China

Abstract:
The effect of Fe₂O₃ on the formation of micro glass beads (MGBs) under air staged combustion was studied. The experimental temperature was 1450 ℃, and Hegang bituminous coal was used as the experimental object. X-ray diffractometer (XRD), ash fusion tester, viscosity formula and scanning electron microscopy (SEM) were used to analyze the fly ash. Nano Measurer 1.2 software was used to measure the diameter of MGBs. The results showed that with the increase of Fe₂O₃ in Hegang coal, the glass phase in fly ash first increased and then decreased. When the amount of Fe₂O₃ was 15%, the content of the glass phase was the highest, which was 51.26%. The ash melting point first decreased and then increased, while the viscosity gradually decreased and the particles gradually became spherical. With the increase of Fe₂O₃, the proportion of MGBs with particle size less than 10 μm increased gradually. From the above results, it can be concluded that the addition of Fe₂O₃ is conducive to the formation of MGBs and the reduction of particle size.
Key words:micro glass beadsfly ashFe₂O₃air staged combustion
DOI：10.11916/j.issn. 1005-9113.21049
Clc Number:TK01+9
Fund:

Yuanjun Zhang, Dakai Wang, Xingxing Cheng, Zhiqiang Wang. Effect of Fe₂O₃ on the Formation of Micro Glass Beads under Air Staged Combustion[J]. Journal of Harbin Institute of Technology (New Series), 2022, 29(3): 46-56. DOI: 10.11916/j.issn.1005-9113.21049
Fund Sponsored by the Natural Science Foundation of Shandong Province (Grant No. ZR2020ME190) and the Shandong Key Research and Development Plan (Grant No. 2019GSF109004) Corresponding author Zhiqiang Wang, Ph.D., Professor.E-mail: jackywzq@sdu.edu.cn Article history Received: 2021-09-07



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Effect of Fe₂O₃ on the Formation of Micro Glass Beads under Air Staged Combustion
Yuanjun Zhang, Dakai Wang, Xingxing Cheng, Zhiqiang Wang
School of Energy and Power Engineering, Shandong University, Jinan 250061, China
Received: 2021-09-07; Available online: 2022-02-08
Sponsored by the Natural Science Foundation of Shandong Province (Grant No. ZR2020ME190) and the Shandong Key Research and Development Plan (Grant No. 2019GSF109004)
Corresponding author: Zhiqiang Wang, Ph.D., Professor.E-mail: jackywzq@sdu.edu.cn.

Abstract: The effect of Fe₂O₃ on the formation of micro glass beads (MGBs) under air staged combustion was studied. The experimental temperature was 1450 ℃, and Hegang bituminous coal was used as the experimental object. X-ray diffractometer (XRD), ash fusion tester, viscosity formula and scanning electron microscopy (SEM) were used to analyze the fly ash. Nano Measurer 1.2 software was used to measure the diameter of MGBs. The results showed that with the increase of Fe₂O₃ in Hegang coal, the glass phase in fly ash first increased and then decreased. When the amount of Fe₂O₃ was 15%, the content of the glass phase was the highest, which was 51.26%. The ash melting point first decreased and then increased, while the viscosity gradually decreased and the particles gradually became spherical. With the increase of Fe₂O₃, the proportion of MGBs with particle size less than 10 μm increased gradually. From the above results, it can be concluded that the addition of Fe₂O₃ is conducive to the formation of MGBs and the reduction of particle size.
Keywords: micro glass beadsfly ashFe₂O₃air staged combustion
0 Introduction As the main source of electricity, coal plays an important role in the world energy consumption structure^[1]. However, the fly ash produced by coal combustion has brought great challenges to environmental protection^[2-3].
Fly ash consists of particles of different shapes, most of which are spherical particles called micro glass beads (MGBs), and a few are unburned carbon particles and amorphous glass^[4]. According to the microstructure, micro glass beads are classified into solid micro glass beads, cenospheres, and plerospheres^[5]. Cenospheres have many excellent characteristics- lower specific gravity, smooth surface, high compressive strength, good fluidity, thermal stability, high specific resistance, and excellent chemical inertness^[6-7]. Therefore, cenospheres, as a high value-added product, are widely used in photocatalysts^[8], adsorbents^[9], light- weight composite materials^[10], insulation materials^[11], concrete^[12], etc. The main minerals and phases in MGBs are quartz, aluminosilicate glass, calcite, mullite, calcium silicate, iron oxide, and sulfate^[13-14]. In recent decades, some scholars have studied the properties of MGBs in detail.
Kolay and Buhsal^[15] found that the main minerals of cenospheres with higher density (1.282 g/CC) were quartz, graphite, cristobalite, and ferrosilicon, while the cenospheres with lower density (0.857 g/CC) were mainly composed of quartz, graphite, brucite, and mayenite. Li et al.^[16] and Ngu et al.^[17] found that with the increase of the diameter of ash cenospheres, the SiO₂/Al₂O₃ ratio decreased, while the total contents of TiO₂ and Fe₂O₃ increased. Research by Ghosal and Self^[18] showed that up to half of Fe₂O₃ in ash was concentrated in spherical particles with a diameter greater than 20 μm. These studies show that the properties of MGBs are affected by mineral composition. In addition to mineral composition, the characteristics of coal and the type of boiler combustion also affect the formation of MGBs^[19]. With the increasing demand for coal in power plants, blended coal is often used by power plants to solve the problem of insufficient supply of single coal. However, the combustion of blended coal will bring changes in mineral composition, which will affect the formation and emission characteristics of MGBs in fly ash, the control of particulate matter and the utilization of fly ash. The effect of mineral composition on MGBs formation has been studied by some scholars. Zhou et al.^[20] found that after adding 3.5% CaO to coal, the spherical particles produced by combustion increased from 49.4% to 55.2%, and the Brunauer-Emmett-teller specific surface area of particles decreased from 15.732 m²/g to 9.7052 m²/g. It was considered that this phenomenon was due to the formation of the fused phase after the decomposition of calcium sulfate crystals. After adding 20% and 25% CaO to Yanzhou coal, Jiao et al.^[21] found hydrolic minerals of 2CaO·SiO₂ and hydraulic minerals of CaO·Al₂O₃ in fly ash, respectively. Fomenko et al.^[22] found that the increase of iron concentration in magnetic cenospheres resulted in the increase of ferrospinel phase content and crystallite size, and the substitution degree of Mg and Al to iron decreased. This shows that the influence of mineral composition on MGBs cannot be ignored.
Air staged combustion technology with low transformation cost is widely used in coal-fired power plants in China to reduce NO_x emission^[23-25]. The use of air staged combustion technology has played an important role in suppressing NO_x emissions. Through this technology, NO_x emission can be reduced by more than 40%^[26-27]. However, air staging will change the combustion atmosphere in the furnace. Different atmospheres in the furnace have a great influence on the valence state of iron. Under oxidizing atmosphere, iron in coal ash is mainly trivalent (Fe₂O₃); under a weak reducing atmosphere, iron in coal ash is divalent (FeO); under a strongly reducing atmosphere, iron is metallic^[28].
Different content and valence states of iron will affect the ash fusion temperature and viscosity^[29-30], and then affect the formation of MGBs. Higher ash fusion temperature and viscosity are not easy to melt the ash, which is not conducive to the formation of MGBs, but conducive to the formation of amorphous particles. However, there are few studies on the effect of Fe₂O₃ on MGBs in fly ash under the condition of air staged combustion. This study continues our previous research^[31]. It is expected to provide a reference for the formation of MGBs in coal-fired power plants with air staged combustion.
1 Experiment Overview 1.1 Experimental Coal Hegang coal was selected as experimental material. The basic properties of Hegang coal and ash are shown in Table 1 and Table 2, respectively.
表 1
Table 1 The ultimate and approximate analyses of Hegang coal Ultimate analysis (%) Proximate analysis (%)
C_ad H_ad N_ad O_ad S_ad M_ad A_ad V_ad FC_ad
50.910 3.230 0.561 5.729 0.280 1.110 38.180 23.400 37.310

Table 1 The ultimate and approximate analyses of Hegang coal


表 2
Table 2 Chemical composition and ash fusion temperature of Hegang coal ash Chemical composition of Hegang coal ash (%) Melting point of coal ash (℃)
SiO₂ Al₂O₃ Fe₂O₃ CaO MgO K₂O TiO₂ Others DT ST HT FT
57.78 28.93 2.77 2.60 3.53 2.59 0.61 1.19 1264 1342 1363 1383
(DT=temperature of deformation, ST=softening temperature, HT=hemispherical temperature, FT= flowing temperature)

Table 2 Chemical composition and ash fusion temperature of Hegang coal ash


Vario El Ⅲ element analyzer was used to determine C, H, N and S elements in Hegang coal. The content of the oxygen element was calculated according to GB/T476-2001. SDTGA 5000 proximate analyzer was used to measure the proximate analysis of Hegang coal. The instrument supports all kinds of coal analysis. It conformed to GB/T212-2008 and MT/T1087-2008. The precision of proximate analyses is shown in Table 3.
表 3
Table 3 Precision of proximate analyses Composition of analysis Mass fraction(%) Repeatability limit(%) Critical difference of reproducibility(%)
Moisture < 5.00 0.20 —
5.00-10.00 0.30 —
> 10.00 0.40 —
Ash < 15.00 0.20 0.30
15.00-30.00 0.30 0.50
> 30.00 0.50 0.70
Volatile < 20.00 0.30 0.50
20.00-40.00 0.50 1.00
> 40.00 0.80 1.50

Table 3 Precision of proximate analyses


PW4400 X-ray Fluorescence Spectrometer (XRF) was used to measure the chemical composition of fly ash. SDAF2000d Ash Fusion Tester was used to measure ash fusion temperature. The test standard of the instrument conformed to GB/T219-2008. The precision of ash fusion tester was shown in Table 4.
表 4
Table 4 Precision of ash fusion tester Fusion temperatures Precision
Repeatability limit(℃) Critical difference of reproducibility(℃)
DT 60 —
ST 40 80
HT 40 80
FT 40 80

Table 4 Precision of ash fusion tester


1.2 Experimental Equipment The experimental system of MGBs formation under air staged combustion is shown in Fig. 1. It is mainly composed of the following parts: furnace tube, temperature controller, micro feeder, mass flowmeter, computer, air supply, extracting pump, sample collector, and slag port.
Fig.1
Fig.1 Experimental system diagram of MGBs formation


The length of the furnace tube was 2000 mm, the diameter was 90 mm, and the limit temperature was 1600 ℃. The heating system of the drop tube furnace was composed of eight U-shaped silicon molybdenum rods. In order to ensure uniform heating of all areas in the furnace tube, thermal insulation bricks were arranged around the furnace tube, and insulated cotton with a thickness of more than 300 mm was wrapped around the thermal insulation brick and the drop tube furnace. By this method, the temperature difference in the furnace could be controlled within 30 ℃. A temperature controller was used to set furnace temperature, heating program, and heating power. Thermocouples were used to measure furnace temperature. Mfev-10 micro feeder produced by Sankyo piotech company in Japan was used for the supply of pulverized coal. Three-stage air was used in the experiment. The primary air was fed through the blanking pipe, the secondary air was sent from the bypass beside the blanking pipe, and the reburning air was sent from the position 500 mm away from the furnace tube inlet. The theoretical air volume flux was calculated by the following formula:
$ \begin{gathered}V_{0}=0.0889\left(\mathrm{C}_{\mathrm{ar}}+0.375 \mathrm{~S}_{\mathrm{ar}}\right)+ \\0.265 \mathrm{H}_{\mathrm{ar}}-0.033 \mathrm{O}_{\mathrm{ar}}\end{gathered} $ (1)
The air required for the experiment was provided by the LUD22-8 screw compressor manufactured by China Liuzhou Fuda Machinery Co., Ltd. The maximum instantaneous temperature that the filter bag in the sample collector can bear was 403 K, and the overall efficiency of sample collector was 99%.
1.3 Experimental Conditions and Analytical Methods In this experiment, the experimental temperature was 1450 ℃, and the pulverized coal was sent to the furnace at 3.0 g per minute. The excess air coefficient and primary air coefficient were 1.4 and 0.8, respectively. Hegang coal was mixed with Fe₂O₃ with mass fractions of 5%, 10%, 15%, 20%, and 25%, respectively. The addition amount of Fe₂O₃ was calculated according to the following formula:
$M_{\mathrm{Fe}_{2} \mathrm{O}_{3}}=M_{\text {coal }} \cdot W_{\text {ash }} \cdot \frac{W_{\mathrm{Fe}_{2} \mathrm{O}_{3}}}{\theta_{\mathrm{Fe}_{2} \mathrm{O}_{3}}}$ (2)
In this equation, M_Fe2O3 is the mass of Fe₂O₃ added to pulverized coal with "g" as its unit; M_coal is the mass of pulverized coal with "g" as its unit; W_ash is the mass fraction of ash with "%" as its unit; W_Fe2O3 is the mass fraction of Fe₂O₃ added to pulverized coal with "%" as its unit; θ_Fe2O3 is the purity of Fe₂O₃ with "%" as its unit.
X-ray diffractometer (XRD) was used to analyze mineral phases in fly ash. The glass phase content was determined by full spectrum fitting of XRD spectrum through the fitting function of Jade software. The micro morphology of fly ash was observed by scanning electron microscope (SEM). The particle size data of MGBs were obtained by Nano Measurer 1.2 software. In order to make the results convincing, six pictures of each sample were randomly selected for analysis, and it was ensured that the number of particles on each picture was greater than 300. Since the experimental temperature was constant, according to the calculation formula of viscosity, the changes of viscosity were only affected by chemical composition. The changes in the viscosity of coal ash droplets will directly affect the formation and particle size distribution of MGBs. The viscosity was calculated using the modified Urbain model^[32] as follows:
First,
$\begin{gathered}X=X_{\mathrm{CaO}}+X_{\mathrm{FeO}}+X_{\mathrm{Mg}}+ \\X_{\mathrm{Na}_{2} 0}+X_{\mathrm{K}_{2} \mathrm{O}}+2 X_{\mathrm{TiO}_{2}}\end{gathered}$ (3)
$ \theta=X /\left(X+X_{\mathrm{Al}_{2} \mathrm{O}_{3}}\right) $ (4)
Second,
$B_{0}=13.8+39.9355 \cdot \theta-44.049 \cdot \theta^{2}$ (5)
$ B_{1}=30.481-117.1505 \cdot \theta+129.9978 \cdot \theta^{2} $ (6)
$ B_{2}=-40.9429+234.0486 \cdot \theta-300.04 \cdot \theta^{2} $ (7)
$ B_{3}=60.7619-153.9276 \cdot \theta+211.1616 \cdot \theta^{2} $ (8)
$ B=B_{0}+B_{1} \cdot X_{\mathrm{SiO}_{2}}+B_{2} \cdot X_{\mathrm{SiO}_{2}}^{2}+B_{3} \cdot X_{\mathrm{SiO}_{2}}^{3} $ (9)
Third,
$\ln (A)=-(0.2812 \cdot B+11.8279)$ (10)
$\begin{aligned}&\ln (\text { viscosity })=\ln (A)+ \\&\quad \ln (T)+(1000 \cdot B / T)\end{aligned}$ (11)
In order to ensure the reliability of viscosity change trend, Urbain model^[33] was also used to calculate viscosity. θ and ln(A) were calculated according to the following formula:
$\theta=X_{\mathrm{CaO}} /\left(X_{\mathrm{CaO}}+X_{\mathrm{Al}_{2} \mathrm{O}_{3}}\right)$ (12)
$\ln (A)=-(0.2693 \cdot B+11.6725)$ (13)
In the formula, X_CaO, X_FeO, X_MgO, X_Na2O, X_K2O, and X_TiO2 are mole fraction with "%" as its unit (The mole fraction was determined according to the chemical oxide composition, and the iron oxide was converted to the equivalent ferrous oxide); T is furnace temperature with "K" as its unit; θ, A, and B are dimensionless numbers. The unit of viscosity is Pa·s. It is worth noting that the calculation method by the above formula is not completely accurate, but a reference value can be given easily and effectively.
2 Results and Discussion 2.1 Mineral Phase Analysis of Fly Ash The collected fly ash was screened by a 100 mesh screen and then put into a muffle furnace for burning at 700℃. The burned fly ash was analyzed by XRD, and the image was shown in Fig. 2.
Fig.2
Fig.2 XRD of fly ash from coal samples with different Fe₂O₃ ratios (Q: quartz, M: mullite, Li: lime, H: hematite, F: fayalite, Ma: magnetite)


As shown in Fig. 2, the increase of Fe₂O₃ in Hegang coal led to the decrease of diffraction peak intensity of mullite and quartz in fly ash, while the diffraction peak intensity of iron-containing phase increased. When the addition amount of Fe₂O₃ was 5%, the phase composition containing iron was only hematite. When the addition of Fe₂O₃ increased to 10% and 15%, fayalite was found in the XRD pattern. But when the Fe₂O₃ content increased to 20% and 25%, the fayalite disappeared and magnetite appeared.
The glass phase content was shown in Fig. 3. According to Fig. 3, it can be found that the glass phase in the fly ash collected under all conditions was less than 55%. However, with the increase of Fe₂O₃ content in Hegang pulverized coal, the content of glass phase first increased and then decreased. The glass phase content reached the highest when the addition of iron oxide was 15%, which was 51.26%.
Fig.3
Fig.3 Diagram of glass phase content in fly ash of coal samples with different Fe₂O₃ ratios


Since air staged combustion technology was used in this experiment, there was a weak reducing atmosphere in some furnace sections. Fe₂O₃ was reduced to FeO in a weak reducing atmosphere. FeO easily produced low-temperature eutectics with SiO₂ and Al₂O₃ in a weak reducing atmosphere, such as fayalite and almandine as shown in Eq. (14), (15), (16) and (17), which caused part of the quartz to melt and inhibited the formation of mullite. The low-temperature eutectics formed a glass phase after cooling, which caused the glass phase to increase with the increase of iron oxide. When the amount of Fe₂O₃ added was greater than 15%, Fe₂O₃ in the fly ash began to remain and magnetite was formed in a high temperature and weak reducing atmosphere, as shown in Eq. (18). With the increase of iron oxide, the content of magnetite would also increase, which would increase the crystallinity of the fly ash to a certain extent. The increase of crystallinity led to the decrease of glass phase content.
$\mathrm{FeO}+\mathrm{SiO}_{2} \rightarrow \mathrm{FeO} \cdot \mathrm{SiO}_{2}$ (14)
$ \left.\mathrm{FeO}+\mathrm{FeO} \cdot \mathrm{SiO}_{2} \rightarrow 2 \mathrm{FeO} \cdot \mathrm{SiO}_{2} \text { (Fayalite }\right) $ (15)
$ \begin{gathered}\mathrm{FeO}+3 \mathrm{Al}_{2} \mathrm{O}_{3} \cdot 2 \mathrm{SiO}_{2} \rightarrow 2 \mathrm{FeO} \cdot \mathrm{SiO}_{2}(\text { Fayalite })+ \\\mathrm{FeO} \cdot \mathrm{Al}_{2} \mathrm{O}_{3}(\text { Hercynite })\end{gathered} $ (16)
$ \mathrm{FeO}+\mathrm{SiO}_{2}+\mathrm{Al}_{2} \mathrm{O}_{3} \rightarrow \mathrm{Fe}_{2} \mathrm{Al}_{4} \mathrm{Si}_{5} \mathrm{O}_{18} \text { (Almandine) } $ (17)
$ \mathrm{Fe}_{2} \mathrm{O}_{3}+\mathrm{CO} \rightarrow \mathrm{Fe}_{3} \mathrm{O}_{4}+\mathrm{CO}_{2} $ (18)
2.2 Analysis of Coal Ash Melting and Viscosity Characteristics As shown in Fig. 4, the addition of Fe₂O₃ in pulverized coal led to a decrease in ash melting point. When the addition of Fe₂O₃ was 15%, the ash fusion temperatures of fly ash was the lowest (DT 1106 ℃, ST 1176 ℃, HT 1206 ℃, FT 1249 ℃). Then, with further addition of Fe₂O₃, the ash melting point increased gradually.
Fig.4
Fig.4 Ash melting point and viscosity under different Fe₂O₃ content


Fly ash, as the residue of coal after complex physical and chemical reactions and transformation in the combustion process, contained a variety of minerals and vitreous. In the furnace, the melting of fly ash was a complex mineral evolution process. In addition to the melting reaction between mineral components to form new inorganic compounds, there was also low-temperature eutectic between minerals, which affected the melting characteristics of ash.
Vorres^[34] proposed the concept of "ion potential" based on the correlation between acid-base components in coal ash and their ionic chemical structure characteristics. The ionic potential of Fe³⁺ was 4.69 and that of Fe²⁺ was 2.70. Cations with high ionic potential combined with oxygen to form polymers or complex ions. On the contrary, the alkaline component with low ionic potential could reduce the ash fusion temperatures because it could act as an oxygen donor to terminate the accumulation of polymers.
Under the condition of air staged combustion, Fe₂O₃ was converted to FeO in a weak reducing atmosphere. FeO easily formed low-temperature eutectics with SiO₂ and Al₂O₃ in a reducing atmosphere, such as fayalite and almandine, which caused part of the quartz to melt and inhibited the formation of mullite. After low-temperature eutectic cooling, amorphous glass was formed, so that the ash melting point was reduced. The addition of Fe₂O₃ promoted the melting of fly ash at high temperatures. By observing the XRD pattern in Fig. 2, it can be found that magnetite appeared when the addition amount of Fe₂O₃ was 20%. Because magnetite was a high melting point crystal (the melting point was 1595 ℃), the increase of magnetite was not conducive to the melting of fly ash, resulting in the increase of ash melting point.
It could be seen from Fig. 4 that with the increase of the proportion of Fe₂O₃ added to pulverized coal, the viscosity of coal ash melts calculated by the two models showed a decreasing trend.
According to the modified Urbain model, the viscosity of Hegang fly ash at 1450 ℃ was 739.21 Pa·s. The viscosity of fly ash decreased to the lowest 102.13 Pa·s when the addition ratio of ferric oxide was 25%. If the content of Fe₂O₃ in coal ash increased continuously in an oxidizing atmosphere, the increased Fe₂O₃ could act as a melt network forming agent to make the melt network more stable and increase the friction between particles in the melt. Therefore, the viscosity of coal ash melt would be increased. In this experiment, air staged combustion technology was adopted, and there was a weak reducing atmosphere area in the furnace. In this area, iron oxide in coal ash could be reduced to ferrous oxide. According to the network theory, Fe²⁺ as a network modifier was easy to connect with more O^2- containing unsaturated bonds in the melt network. It destroyed the cyclic Si-O covalent bond and prolonged the bond between two adjacent silicon atoms, which would destroy the melt network and reduce the stability of the melt network, as shown in Fig. 5. The melt was also easy to flow because the friction between internal particles was significantly reduced, which reduced the viscosity of coal ash melt. Indeed, in addition to SiO₂, there were mainly CaO, Al₂O₃ and other oxides in fly ash. According to the research of Vargas et al.^[35], Ca²⁺, like Fe²⁺, acted as a network modifier in silicate melts and reduced the viscosity of the melt. Al³⁺ was a typical amphoteric. According to the coordination number of Al³⁺ in the melt, it could act as a network former or as a modifier. When Al³⁺ combined with the charge-balancing modified ions, they formed a stable metal-oxygen anion group, which increased the viscosity of the melt. However, if there were insufficient modifier ions in the melt, Al³⁺ would act as a modifier ion to reduce the viscosity of the melt. Nevertheless, the main component that affects the viscosity is obviously silica, because it is the main component.
Fig.5
Fig.5 Effect of Fe²⁺ on silica network (Yellow: Si; Red: O; Black: Fe²⁺)


2.3 Morphology Analysis Fig. 6 shows the SEM image of fly ash collected after burning coal samples with different Fe₂O₃ content. In order to reflect the distribution and quantity of MGBs more clearly in the picture, the spherical particles were marked in the picture with yellow circles and displayed in the upper right of each picture. From Fig. 6(a), it can be seen that there were few MGBs in Hegang coal, most of which were amorphous particles. When 5% Fe₂O₃ was added, the number of MGBs increased and the formation of amorphous particles was inhibited. When 15% Fe₂O₃ was added, it can be found from Fig. 6(c) that MGBs occupied most of the area of the photo, which shows that the addition of Fe₂O₃ could promote the formation of MGBs. However, when the proportion of Fe₂O₃ exceeded 15%, especially when the amount of Fe₂O₃ reached 25%, although there were still many MGBs in the fly ash, the fine amorphous particles began to increase.
Fig.6
Fig.6 SEM image of fly ash collected after burning coal samples with different Fe₂O₃ content


When the proportion of iron oxide was 15%, the SEM image was mainly in the form of MGBs. It was because the viscosity of ash droplets decreased due to the addition of iron oxide, and the molten droplets were more likely to become spherical under the action of surface tension. When the spherical molten ash droplets entered the low-temperature zone of the furnace, they were cooled to form MGBs. When the proportion of iron oxide exceeded 15%, with the increase of iron oxide content, excessive iron oxide precipitated and transformed into magnetite at high temperature, as shown in Eq(18). These magnetite particles had a high melting point and were difficult to melt to form spherical particles to further form fine amorphous particles.
As shown in Fig. 7, there are mainly six kinds of particles in fly ash: cenospheres, amorphous particles, plerospheres, perforated beads, fragments, and ferrospheres. The viscosity and surface tension of molten coal ash directly affect the formation of MGBs. In the high-temperature area of the furnace, pulverized coal particles begin to melt. With the increase of time, the solid phase decreases and the liquid phase increases. When the solid-liquid ratio exceeds a certain value, the particles begin to have fluidity. Due to the effect of flow field and droplet surface tension in the furnace, the particles melt into a closed molten ash film. The increase of furnace temperature further reduces the viscosity of the refractory matter in the particles, promotes its movement from the inside to the shell, and finally forms a sphere with a fixed curvature and thickness shell^[36], as shown in Fig. 7(a). If two or more droplets contact each other, they will form a larger sphere under the action of higher temperature and liquid film surface tension, which is beneficial to the collection of fly ash. If the melting temperature of coal ash is too high, the particles will not fully melt into spheres, resulting in the formation of amorphous particles^[37]. In this process, some spherical or nearly spherical particles will inevitably adhere to the surface of amorphous particles, as shown in Fig. 7(b). There is a case that the secondary beads are wrapped in the beads (as shown in Fig. 7(c)), which is called "plerospheres". The reason for this complex sphere may be that the solid particles in coal ash do not melt into liquid at the same time in the furnace, but exist in the state of solid-liquid mixing. Firstly, the outer surface of the mineral particles melts, and a series of chemical reactions such as carbonate decomposition, silicate dehydration, pyrolysis, and carbonaceous combustion produce gas, resulting in the separation of the molten part of the mineral particles from the internal solid core. Then, the solid core further melts at high temperatures to form one or more fine beads, thus forming plerospheres^[38]. For plerospheres, if the number of encapsulated fine beads is increased, the emission of fine beads in the atmosphere will be reduced. Others are notched microspheres (as shown in Fig. 7(d)). Because during the cooling process, part of the shell is damaged due to the excessive pressure of the gas inside the sphere, resulting in the formation of incomplete spheres. If the cooling speed is too fast, the molten droplets will suddenly shrink during the cooling process, and the internal and external pressure difference is too large, resulting in the fragmentation of the sphere and the formation of fragments^[39](Fig. 7(e)). There is also a special kind of particle in fly ash-ferrospheres, as shown in Fig. 7(f). The iron oxide phase in the ferrospheres mainly includes hematite and magnetite, and the iron content ranges from 20wt% to 88wt%^[40]. Iron-containing minerals (mainly FeS₂) exist in coal powder in both internal and external forms. Internal and external iron-containing minerals work together to promote the formation of ferrospheres during coal combustion^[41]. In this experiment, iron oxide was added to Hegang coal. A small amount of iron oxide was reduced to ferrous oxide in a weak reducing atmosphere, and produced low-temperature eutectics with SiO₂ and Al₂O₃, such as fayalite and almandine. Since iron oxide was an extrinsic mineral, when the iron oxide was excessive, the iron oxide particles were transformed into magnetite, forming the fine amorphous particles in Fig. 6(d).
Fig.7
Fig.7 Morphology of different particles in fly ash


2.4 Particle Size Distribution of MGBs The Nano Measurer 1.2 software was used to measure the size of the MGBs in the SEM image. After the data was processed, the particle size distribution of MGBs was obtained. The results are shown in Fig. 8.
Fig.8
Fig.8 Particle size distribution of MGBs


After iron oxide was added, the proportion of small-sized MGBs increased, and the proportion of large-sized MGBs decreased. MGBs with a particle size of 0-10 μm accounted for the highest proportion, and MGBs with a particle size greater than 30 μm accounted for the lowest proportion. It can also be found from Fig. 8 that the proportion of MGBs with a particle size of 0-10 μm was positively correlated with the proportion of iron oxide added.
When the proportion of Fe₂O₃ was 25%, the proportion of MGBs with a particle size of 0-10 μm reached the highest, which was 84.47%. After adding iron oxide, the proportion of MGBs with particle size larger than 10 μm decreased. When the proportion of iron oxide was 20%, there were almost no MGBs larger than 30 μm in the fly ash. The above analysis showed that the addition of iron oxide to Hegang pulverized coal increased the proportion of smaller MGBs, thus reducing the average particle size of MGBs.
Raask^[42-43] explained the formation principle of micro hollow spheres. As the combustion progresses, the carbon matrix in the particles was enveloped by an external molten liquid film. At this time, the carbon matrix underwent a series of chemical reactions to generate gas. The liquid film played a role in the transfer of oxidants in redox reactions, and some oxides in the liquid film also reacted. The following will take the iron oxide in the ash as an example to show the entire reaction process:
$\mathrm{CO}_{2}+\mathrm{C} \rightarrow 2 \mathrm{CO}$ (19)
$ \mathrm{Fe}_{2} \mathrm{O}_{3}+\mathrm{CO} \rightarrow 2 \mathrm{FeO}+\mathrm{CO}_{2} $ (20)
$ 4 \mathrm{FeO}+\mathrm{O}_{2} \rightarrow 2 \mathrm{Fe}_{2} \mathrm{O}_{3} $ (21)
Due to the weak reducing atmosphere produced by air staged combustion, the iron oxide on the surface of coal particles was reduced to ferrous oxide. The increase of Fe₂ O₃ prevented the movement of iron oxide inside the droplet to the surface and inhibited the occurrence of reactions (20) and (21). Since the above reaction occurred inside the droplet, the reduction atmosphere caused by air staged combustion could not promote the occurrence of reactions (19) and (20).
As shown in Fig. 4, with the increase of iron oxide addition ratio in Hegang coal, the viscosity of droplets gradually decreased. Low viscosity has two effects on droplets: a) at low viscosity, molten particles can expand more; b) the fluidity of oxygen carrier becomes larger, which can promote the occurrence of reaction (21). However, the reaction (21) was inhibited in a reducing atmosphere as an oxidation reaction. The low oxygen concentration limited the reaction rate, resulting in the reduction of the amount of oxygen carrier (Fe₂ O₃ ). The circulation of the whole chemical reaction was hindered, which hampered the generation and expansion of gas in molten droplets. Therefore, the increase of the proportion of iron oxide in coal under air staged combustion would inhibit the expansion of gas in coal ash molten droplets, resulting in the decrease of the average particle size of MGBs.
3 Conclusions Adding Fe₂O₃ to Hegang coal causes the glass phase content of fly ash to increase first and then decrease. The morphology of Hegang coal ash is mainly amorphous glass particles. With the increase of Fe₂O₃ in Hegang coal, the particle morphology in fly ash is gradually close to spherical, and the spherical particles are the most when the amount of Fe₂O₃ is 15%. When the amount of Fe₂O₃ is more than 15%, the fine amorphous glass particles begin to increase, but the spherical particles still account for the majority.
The particle size distribution of MGBs was analyzed by Nano Measurer 1.2 software. With the increase of Fe₂O₃, the proportion of MGBs with small particle sizes increases gradually. When the amount of Fe₂O₃ is greater than or equal to 20%, there are almost no MGBs more than 30 μm in fly ash. When the amount of Fe₂O₃ is 25%, MGBs of 0-10 μm were as high as 84.47%.
At the same time, Fe₂O₃ is reduced to FeO in a weak reducing atmosphere. FeO will form low-temperature eutectic with SiO₂ and Al₂O₃, which reduces the ash melting point. Fe²⁺ as a network modifier can reduce the viscosity of fly ash. The decrease of ash fusion temperatures and viscosity promotes the formation of MGBs and reduces the particle size of MGBs to a certain extent. This shows that Fe₂O₃ plays an important role in the formation and particle size distribution of MGBs.

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