Study on Slagging Characteristics of Co-Combustion of Meager Coal and Spent Cathode Carbon Block

Abstract

The harmless disposal of spent cathode carbon blocks (SCCBs) has become an urgent issue in the primary aluminum industry, and the disposal of SCCBs by co-combustion in pulverized coal boilers is expected to be the most effective treatment method. A muffle furnace at 815 °C was used in this study to perform a co-combustion experiment of meager coal and SCCBs. The ash fusion characteristics (AFTs), microscopic morphology, and minerals composition of co-combustion ash were characterized. The interaction mechanism of different mineral components and the change in AFTs and viscosity-temperature characteristics were investigated using FactSage software. Results show that the change in the ash deformation temperature (DT) is correlated linearly with the SCCB addition ratio, whereas other characteristic temperatures exhibit a nonlinear relationship. The contents of SiO₂, Al₂O₃, and Na₂O collectively determine the DT in the ash, and the influence degree from high to low is in the order of SiO₂, Na₂O, and Al₂O₃. The phase diagram of Na₂O–Al₂O₃–SiO₂ is used to accurately predict the changing trend of the melting point of co-combustion ash. The ratio changes between refractory and fusible minerals in the ash, as well as the degree of low-temperature eutectic reaction between sodium- and calcium-containing minerals, are the main factors affecting the melting point of ash. When the blending amount of SCCBs is 5%, mostly complete combustion is achieved, and slagging does not occur easily. The optimal blending ratio of SCCBs is obtained using the co-combustion method from the aspect of AFTs and viscosity-temperature characteristics. This work lays a theoretical foundation for industrial application.

1. Introduction

Aluminum is an important basic raw material for national economic development. In 2021, the global primary aluminum production capacity was 67.24 million tons, whereas China’s production capacity was 38.5 million tons, which constituted approximately 57.26% of the total production capacity. Meanwhile, spent cathode carbon blocks (SCCBs) amounted to approximately 346.5 thousand tons, which were one of the highest hazardous wastes in the aluminum industry [1]. Currently, SCCBs are primarily disposed of using conventional methods such as landfill and stockpiling, which necessitates a significant amount of land resources; more seriously, the harmful substances such as soluble fluoride and cyanide contained in SCCBs cause irreversible pollution to water, air, and soil [2,3,4]. The treatment methods for SCCBs are primarily categorized into wet and thermal methods. Wet methods include leaching [5,6,7] and floatation [8] to achieve the recovery of carbon and fluorine. However, none of them has been utilized industrially because of their complex process flow, high energy consumption, and difficulties in secondary pollution control. Thermal methods are combustion methods that use SCCBs which are abundant in carbon and have a relatively high calorific value as an alternative fuel or a reductant, mainly include high temperature roasting [9], collaborative disposal [10], and reduction [11], which are considered as the most promising treatment methods for SCCBs. However, this disposal is not adopted broadly owing to high treatment costs and uneven regional distributions of reuse enterprises. This issue has become a bottleneck in the development of the aluminum industry. As a simple and effective method, co-combustion is worthy of further study.

Combustion is the most typical method to dispose of hazardous waste, and it is an effective method for utilizing resources [12,13]. Although scholars have conducted a lot of research on the co-combustion of carbonaceous materials [14,15] as a hazardous waste with complex composition and characteristics, the co-combustion of SCCB with coal is still relatively novel and worth studying. Zhang et al. investigated the combustion characteristics and kinetic behavior of co-combustion coal with SCCBs at different blending ratios and proposed the co-disposal of SCCBs using pulverized coal boilers [16]; however, they did not study the ash fusibility, corrosion during the co-combustion of coal and SCCBs. SCCBs are abundant in elements such as Na and F. Their direct combustion releases corrosive gases such as HF and SiF₄, as well as generates low-melting-point sodium salt, thereby causing problems such as equipment corrosion and easy slagging [17,18]. Considering the safety and stability of long-term boiler operation, an in-depth study regarding the ash accumulation, slagging, and corrosion of SCCB co-combustion with coal is necessitated; so far, only a few relevant studies have been reported.

The AFTs are one of the most important parameters to determine the tendency of the fuel to slag and accumulate ash [19] and are usually characterized by the DT, softening temperature (ST), hemispheric temperature (HT), and flow temperature (FT). Previous studies have concluded that AFTs are closely related to the composition of ash and mineral, as well as mineral changes during melting [18,20,21]. However, the ash melting characteristics of blended coals are non-linear and cannot be discerned entirely by the slagging of the component single coals [22,23,24]. Therefore, it is particularly important to study the melting characteristics of co-combustion ash of SCCBs and coal. In this study, two samples of meager coal and SCCB were used to perform ash melting experiments based on different blending ratios. The slagging index and characteristic temperature were calculated, and the relationship between the ash melting point and ash content was investigated. X-ray fluorescence (XRF), X-ray diffractometer (XRD), scanning electron microscopy (SEM), and other methods were used to analyze the AFTs of the mixed samples. The FactSage7.0 thermodynamic software was used to establish a co-combustion ash system of Na₂O–Al₂O₃–SiO₂–CaO–Fe₂O₃ and investigate the mineral composition changes of different co-combustion ash with respect to temperature. The variable temperature phase diagram of Na₂O–Al₂O₃–SiO₂ and the flow temperature obtained from the experiment were compared and analyzed. Subsequently, the viscosity of the ash was calculated. The effects of additives on mineral composition and viscosity-temperature characteristics were also considered. This study reveals the mechanism of ash agglomeration of SCCBs and meager coal during the co-combustion process, which provides an important reference for the large-scale disposal of SCCBs using the co-combustion method in coal-fired boilers.

2. Experiment

2.1. Preparation of Ash Sample

The meager coal was purchased from Taiyuan, Shanxi Province, China. The SCCBs were produced from an aluminum electrolytic cell used by Shandong Weiqiao Aluminum and Electricity Co., Ltd. for 5 years. The meager coal and SCCBs were pulverized and dried and then grounded evenly using a grinding pestle. After sieving, powder measuring less than 0.125 mm was obtained. Subsequently, meager coal and SCCBs of different qualities were placed in the experimental mixer and mixed thoroughly for 30 min, after which fully mixed samples were obtained and stored in a glass desiccator. The mass of each sample was 10 g. The mass proportions of the SCCBs in the mixture were 0%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, and 100%. The prepared sample was named SCCB-X (where X represents the proportion of SCCBs, and 100-X represents the proportion of meager coal).

First, 2 ± 0.01 g of the sample was obtained from the dryer and placed in an ash pan; subsequently, it was put into the muffle furnace. The temperature of the furnace was increased to 500 °C at a rate of 5 °C/min and maintained for 30 min. Subsequently, it was increased to 815 °C at a rate of 5 °C/min and then burned for 1 h. Finally, the ash pan was removed after it was cooled down to room temperature and then placed in a desiccator on a heat-resistant plate to obtain ash samples of different mixing ratios.

2.2. Determination of Ash Melting Point

The ash samples were formed into ash cones with a base length of 7 mm and a height of 20 mm, and the ash melting points of the 12 groups of ash samples were measured under a weak oxidizing atmosphere using a ZDHR-3 ash melting point tester. Subsequently, the characteristic melting temperature of the ash was measured according to the Chinese National Standard GB/T219-2008.

2.3. Characterization of Ash

A PANalytical Axios sequential wavelength dispersive X-ray fluorescence spectrometer (XRF) was used to determine the composition and content of each element in the ash sample, and an X-ray diffraction analyzer (XRD, Rigaku Corporation, Tokyo, Japan) was used to detect minerals contained in the meager coal, SCCB, and ash, based on a scanning angle of 5°–90°, scanning speed of 5°/min, and step size of 0.02°. Furthermore, scanning electron microscopy (SEM, JEOL, Tokyo, Japan) was performed to observe the micromorphology of the ash samples.

2.4. Analysis Method

In general, the chemical composition of ash can be categorized into two groups: acidic components with high melting points ($\mathit{Si}{O}_2+{\mathit{Al}}_2{O}_3+\mathit{Ti}{O}_2$) and alkaline components that serve as fluxes to lower the melting points ($F{e}_2{O}_3+CaO+MgO+N{a}_{2}O+K_{2}O$) [25]. The effective prediction of the slagging characteristics of coal combustion has aroused the interest of domestic and foreign scholars, and many methods have been proposed to predict slagging [26,27,28].

Table 1. Summary of some existing coal slagging indices.

Index	Index Expression	Slagging Potential
		Low	Medium	High	Severe
$R_{B/A}$	$R_{B/A}=\frac{\left(F{e}_2{O}_3+CaO+MgO+N{a}_{2}O+K_{2}O\right)}{\left(Si{O}_2+A{l}_2{O}_3+Ti{O}_2\right)}$	<0.206	0.206–0.4	>0.4	---
$S_R$	$S_R=100\times \frac{Si{O}_2}{\left(Si{O}_2+F{e}_2{O}_3+CaO+MgO\right)}$	>78.8	66.1–78.8	<66.1	---
$F_u$	$F_u=\frac{\left(F{e}_2{O}_3+CaO+MgO+K_{2}O+N{a}_{2}O\right)}{\left(Si{O}_2+A{l}_2{O}_3+Ti{O}_2\right)}\times \left(N{a}_{2}O+K_{2}O\right)$	<0.6	---	0.6-40	>40
$F_s$	$F_s=\frac{4\mathit{DT}+\mathit{HT}}{5}$	>1343 °C	1232–1343 °C	1149–1232 °C	<1149 °C

Note: Alkali-acid ratio ($R_{\mathit{B/A}}$), slag viscosity index ($S_R$), fouling index ($F_u$), and slagging index ($F_s$).

shows the indices used in this study for the qualitative analysis of the slagging characteristics [29,30].

2.5. Thermodynamic Calculation

Based on the chemical composition of ash, the Equilib module in the FactSage7.0 thermodynamic software was used, and the FToxid database was selected to establish the Na₂O–Al₂O₃–SiO₂–CaO–Fe₂O₃ system for the co-combustion ash. Using temperature as a variable, the changes in the mineral composition of different co-fired ash during the temperature increase from 900 °C to 1600 °C were investigated, and the temperature interval was 100 °C. Furthermore, In addition, the ternary phase diagrams of a Na₂O-Al₂O₃-SiO₂ system with constant CaO content and a CaO-Al₂O₃-SiO₂ system with constant Na₂O content are drawn by the phase diagram module. Finally, the viscosity-temperature characteristics of different co-combustion ashes were calculated using the viscosity module.

3. Results and Discussion

3.1. Ash Composition Analysis

The composition and content of each element in the SCCB-X ash are listed in

Table 2. Analysis of ash composition of meager coal, SCCB, and their mixed samples.

Sample	Na2O	MgO	Al2O3	SiO2	P2O5	SO3	K2O	CaO	TiO2	Fe2O3	Others
SCCB-0	0.62	0.83	32.91	49.56	0.19	1.03	0.98	3.17	1.03	6.28	3.40
SCCB-5	1.86	0.39	34.87	45.45	0.94	3.94	1.24	2.62	1.45	5.76	1.48
SCCB-10	4.31	0.38	34.39	42.94	0.88	5.12	1.16	3.52	1.33	5.43	0.54
SCCB-15	7.31	0.38	33.99	39.67	0.37	6.71	1.09	3.66	1.24	5.18	0.40
SCCB-20	7.70	0.44	33.83	38.92	0.93	5.78	1.47	3.84	1.29	5.27	0.53
SCCB-25	11.43	0.41	33.83	34.82	0.62	6.92	1.18	4.07	1.12	5.04	0.56
SCCB-30	13.36	0.36	34.04	32.83	0.55	7.16	0.93	4.23	1.11	4.88	0.55
SCCB-35	14.04	0.80	33.81	32.56	0.23	8.19	0.90	3.79	0.96	4.39	0.33
SCCB-40	16.87	0.27	34.77	31.30	0.67	4.57	1.71	4.02	0.95	4.46	0.41
SCCB-45	17.59	0.87	34.74	28.28	0.23	7.90	1.17	3.99	0.84	4.07	0.32
SCCB-50	17.95	0.87	34.72	27.38	0.11	8.77	0.89	4.10	0.80	4.07	0.34
SCCB-100	32.80	0.83	32.31	23.44	0.06	0.15	0.70	4.09	0.47	4.39	0.76

. As shown in Table 2, the SCCB ash is abundant in Na₂O (32.80%), Al₂O₃ (32.31%), and SiO₂ (23.44%), and the meager coal ash is abundant in Al₂O₃ (32.91%) and SiO₂ (49.56%) but low in Na₂O (0.62%), which is similar to the composition of other samples reported in the literature [31]. As the SCCB blending amount increased, the content of Na₂O increased, and the content of SiO₂ decreased gradually. However, the mass fraction remained beyond 38.92%. When the temperature is higher than 900 °C, SiO₂ can easily react with the alkaline components (Na₂O and CaO) to produce silicate containing sodium and calcium (Na₂SiO₃ and CaSiO₃). Furthermore, it can form a low-temperature eutectic of sodium- and calcium-containing minerals (Na₂CaSiO₄) [32], thereby reducing the AFTs. Generally, the acidic components have the function of increasing FT, while the basic components have the function of reducing FT [33,34,35,36]. However, when the SiO₂ content is extremely high, the monomeric SiO₂ with a high melting point (1600 °C) causes an increase in the ash melting point, which reflects the duality of the effect of SiO₂ on AFTs. The volatilization and release of sodium at high temperatures may reduce the effect of sodium on the melting temperature [37].

As the SCCB blending amount increased, the Al₂O₃ content increased slightly. The Al₂O₃ content of the meager coal ash was the lowest at 32.91%, which was relatively high. It served as a skeleton in the ash melt and hindered the deformation of the melt, thereby reducing slagging. Simultaneously, the Fe₂O₃ content decreased gradually. The study by Zhao et al. [38] showed that in an oxidizing atmosphere, Fe₂O₃ with a high melting point barely participated in the reaction; however, when the content was low, it dissolved rapidly in the solution, causing it to rapidly reach the flow temperature. Fe₂O₃ is an alkaline oxide with low ionic potential energy, which can destroy the polymers in ash melt and play the role of solvent. It was reported that when the content of Fe₂O₃ was less than 20%, the softening temperature decreased by about 12.7 °C with every 1% increase in Fe₂O₃ content [39]. In addition, with the increase in the SCCB blending content, the content of CaO did not change significantly (2.62–4.23 wt.%), and the contents of MgO, K₂O, and TiO₂ were low (total of 2.0–3.08 wt.%). Therefore, they did not significantly affect the melting temperature of the ash.

3.2. Analysis of Ash Melting Characteristics

3.2.1. Characteristic Index Analysis

The results of the slagging index calculations for the combustion ash of the meager coal, SCCB, and their mixed samples are shown in

Table 3. Slagging index of mixed sample ash.

Sample	${\mathit{R}}_{\mathit{B}/\mathit{A}}$	${\mathit{S}}_{\mathit{R}}$	${\mathit{F}}_{\mathit{u}}$	${\mathit{F}}_{\mathit{s}}$
SCCB-0	0.142	82.82	0.088	1402
SCCB-5	0.145	83.83	0.270	1383
SCCB-10	0.188	82.15	0.810	1364.6
SCCB-15	0.235	81.14	1.718	1356.8
SCCB-20	0.253	80.30	1.948	1334.8
SCCB-25	0.317	78.53	3.623	1309
SCCB-30	0.350	77.61	4.676	1297.6
SCCB-35	0.355	78.38	4.984	1281.6
SCCB-40	0.408	78.15	6.883	1281
SCCB-45	0.434	76.00	7.634	1255.4
SCCB-50	0.443	75.18	7.952	1245.4

. Compared with the meager coal ash, the SCCB ash is abundant in Na, low in Si, and has a similar Al content. The $R_{B/A}$, $S_R$, and $F_u$ values of the SCCB ash were 0.598, 71.57, and 13.036, respectively, indicating that the slagging of the combustion process was relatively severe. This is due to the high Na content of the SCCB. The $R_{B/A}$, $S_R$, and $F_u$ of the meager coal ash were 0.142, 82.82, and 0.088, respectively. These values reflect the high Si content, which serves as a skeleton in the ash and exists primarily in the form of silicate crystals with a high melting point and less slagging.

As the content of SCCBs increased, $R_{B/A}$ and $F_u$ increased, whereas $S_R$ decreased. When the blending amount of SCCBs reached 10%, the $F_u$ value of ash was 0.810; when it reached 15%, the $R_{B/A}$ value of ash was 0.235; meanwhile, when it exceeded 20%, the $S_R$ value of ash was 78.53, which may be regarded as medium caking based on the corresponding standard. This is because when mixed with a small number of SCCBs, which contains less Na, the high content of Al₂O₃ easily reacts with SiO₂ to generate high-melting-point mullite (${3\mathrm{Al}}_2{\mathrm{O}}_3{+2\mathrm{SiO}}_2\stackrel{1100℃}{\to }{3\mathrm{Al}}_2{\mathrm{O}}_3\cdot {2\mathrm{SiO}}_2$); at a high temperature of 1600 °C, volatilization is not decomposed. When the Na content in the ash is sufficiently high, a eutectic phase occurs between Al₂O₃ and SiO₂, which resulted in the deposition of low-melting-point sodium feldspar (${\mathrm{Na}}_2{\mathrm{O}+\mathrm{Al}}_2{\mathrm{O}}_3{+6\mathrm{SiO}}_2\stackrel{970℃}{\to }{2\mathrm{NaAlSi}}_3{\mathrm{O}}_8$) on the walls of the heat exchanger tubes and intensified the accumulation of ash slagging [40]. It was shown that at high temperatures, the Si–Al in the ash trapped sodium and retained it in the ash slag [41,42]. Therefore, Na was the main cause of slagging affecting the co-combustion of SCCBs with coal, and similar conclusions have been reported in studies pertaining to biomass co-combustion with coal [43]. Based on an analysis of the slagging characteristics, the blending amount of SCCBs should not exceed 10%.

3.2.2. AFT Analysis

The AFT of ash can reflect its dynamic deposition characteristics in a boiler [44]. Figure 1 and Table S1 show the characteristic temperatures of the co-combustion ash of meager coal and SCCBs at different blending ratios. The DT of meager coal ash was the lowest (1384 °C), whereas the DT of SCCB ash was the highest (1444 °C); as the amount of SCCB blending increased, its deformation temperature decreased gradually. The ST exhibited a change law similar to the DT. The FT of mixed combustion ash exceeded 1400 °C, and it did not satisfy the requirements of liquid discharge [45]. However, pulverized coal furnaces generally use solid slag discharge, and the boiler heating surface does not form severe coking.

3.2.3. Correlation Analysis between DT and Characteristic Index

Figure 2 shows the relationship between DT and the characteristic index of co-combustion ash of SCCB-X. As shown in Figure 2, with the decrease of DT, the alkali-acid ratio ($R_{B/A}$) and fouling index ($F_u$) increase, while the slag viscosity index ($S_R$) and slagging index ($F_s$) decrease, indicating that DT and characteristic index are linearly correlated. This is due to the increase of alkali metal content, easy to produce low-melting-point of minerals, resulting in the decrease of ash melting point and the increase of slagging tendency.

3.3. Relationship between Ash Melting Point and Ash Content

3.3.1. Single Oxide Evaluation

A DT with a larger temperature range was selected as the research object to investigate the relationship between the ash melting temperature and the ash composition. Figure 3 shows the relationship between the single ash content and DT. Compared with meager coal, the SCCB exhibited a higher degree of carbonization, a more complex composition in ash, and a higher DT. Three typical oxides (Na₂O, Al₂O₃, and SiO₂) of the ash were selected. It was observed that the Al₂O₃ variation range of SCCB-X was only 5.46%, which was much smaller than 21.18% for Na₂O and 26.12% for SiO₂. Regarding the effect of a single component in the ash on the DT, the regularity of the effect of the Al₂O₃ content on the DT was insignificant and hence will not be explained herein. The change in the Na₂O and SiO₂ contents significantly affected the DT. The linear regression coefficient R² exceeded 0.95, which implied a close reflection of the real situation. The slope of the fitted line shows that the content of Na₂O was positively correlated with the DT, whereas SiO₂ was negatively correlated with the DT. Therefore, for the single oxide in the ash of SCCB-X, Na₂O increased the DT, whereas SiO₂ imposed the opposite effect. The absolute value of the slope of the fitted line segment indicates the degree of influence, where the higher the absolute value, the greater the effect of the change in the independent variable on the dependent variable. The oxide content was the independent variable, whereas the DT was the dependent variable. The degree of influence of the three oxides was in the following order: SiO₂ > Na₂O > Al₂O₃.

3.3.2. Phase Diagram Analysis

As shown in Table 1, the contents of Al₂O₃, CaO, Na₂O, and SiO₂ in the SCCB-X co-combustion ash exceeded 84%, and the content of CaO did not change significantly. Therefore, this system can be simplified to a Na₂O–Al₂O₃–SiO₂–CaO system with a fixed CaO mass fraction (4.0%). Meanwhile, a CaO-Al₂O₃-SiO₂-Na₂O system with a fixed mass fraction of Na₂O (2.0%) was constructed to investigate the effect of adding CaO on the melting characteristics of ash. The simulation results obtained using FactSage are presented in Figure 4 and Figure 5.

The phase diagram was used to convert the components of each ash sample, and dots were used in the figure to represent the location of the SCCB-X ash sample. Points 1 and 2 represent the ash samples of SCCB-0 and SCCB-5, respectively. The melting temperature can be determined at different locations. The flow temperature at Point 6 was the lowest, i.e., approximately 1420 °C, and the experimental value was 1492 °C. The flow temperatures at Points 1, 2, and 12 were some of the highest, i.e., exceeding 1600 °C. However, owing to the limitations of the experimental conditions, the experimental value could not be measured. It is noteworthy that the experimental results can be reflected in the phase diagram. Similar conclusions have been obtained from existing studies [32].

Figure 4 shows the phase change function of the Al₂O₃ content when the mass ratio of Na₂O/SiO₂ was 3/2. The first point in the upper left corner reflects the Al₂O₃ content of 17.5%, and it is located in the phase field of Na₂CaAl₄O₈; additionally, the liquidus temperature at this time is 1200 °C. As the Al₂O₃ content increased, the chemical composition of the mixed ash shifted toward the feldspar phase field, and the liquidus temperature increased gradually. When the Al₂O₃ content reached 31.9%, the liquidus temperature reached 1600 °C. When the Al₂O₃ content continued to increase, the liquidus temperature of the sample continued to increase. The trend shown in the phase diagram was similar to the flow temperature result of ash melting.

Figure 5 shows the ternary phase diagram of variable temperature in the Al₂O₃-SiO₂-CaO system when the mass fraction of Na₂O is 2.0%. The red line shows the phase change function of SiO₂ content when the mass ratio of CaO/Al₂O₃ is 13/7. The first point in the upper left corner indicates that when the content of SiO₂ is 32.0%, it is located in the phase field of Ca₂Al₂SiO₇, and the liquidus temperature at this time is 1500 °C. With the increase of SiO₂ content, the chemical composition of the mixed ash moves towards the CaAl₂Si₂O₈ or CaSiO₃ phase field, and the liquidus temperature decreases gradually. When the content of SiO₂ reaches 39.5%, the liquidus temperature drops to 1350 °C. This reflects the duality of the influence of SiO₂ on ash melting characteristics [37].

The blue line shows the phase change function of CaO content when the mass ratio of SiO₂/Al₂O₃ is 9/11. The first point in the lower middle corner indicates that when the content of CaO is 20.0%, it is located in the phase field of CaAl₂Si₂O₈, and the liquidus temperature is 1500 °C. With the increase of CaO content, the chemical composition of the mixed ash moves towards the Ca₂Al₂SiO₇ phase field, and the liquidus temperature firstly decreases and then increases. When the CaO content reaches 28.9%, the liquidus temperature drops to 1450 °C, which may be due to the low temperature eumelt formed by CaO and the high melting point minerals in the system, which reduces the melting temperature of ash. When the CaO content further increases to 34.8%, the liquidus temperature rises to 1500 °C, which is because the excessive CaO monomer in the ash raises the ash melting temperature.

3.4. Mineral Composition during Ash Melting

Changes in the mineral type, content, and liquid phase line can be used to predict changes in the ash melting temperature. It has been demonstrated that FactSage is an important tool for explaining the variation in ash flow properties and can accurately predict the mineral transformation process of ash [46,47]. As shown in Table 2, the contents of MgO, K₂O, and TiO₂ in the co-combustion ash were extremely low and varied only slightly; therefore, its effect on the ash melting characteristics was not considered in this study. The FactSage 7.0 thermodynamic software was used to calculate the mineral composition of combustion ash at different blending amounts.

Figure 6 shows that the meager coal ash has a liquid phase at 1200 °C. As the temperature increased, the liquid phase content increased significantly and then transformed completely into the liquid phase at 1534 °C. This explains the higher melting temperature of the meager coal ash. The main refractory components in the SCCB-X ash were composite oxides (Al₂O₃·Fe₂O₃), anorthite (CaAl₂Si₂O₈), albite (NaAlSi₃O₈), and quartz (SiO₂). As the SCCB content increased from 0% to 20%, the types of insoluble matter in the ash decreased. The liquidus temperatures were 1534 °C, 1540 °C, 1496 °C, 1450 °C, and 1432 °C for the abovementioned materials, respectively, showing a decreasing trend. This is because the increase in Na₂O promoted the production of sodium feldspar, which has a low melting point (1100 °C) and an excellent fluxing effect on fusible minerals such as quartz [48,49], resulting in a further decrease in the melting temperature. When the blending amount of SCCB was 5%, the melting rate of the solid phase was the lowest. When the temperature reached 1400 °C, 21.32% of the solid phase remained unmelted. As the SCCB blending amount increased, the content of albite (NaAlSi₃O₈) first increased and then decreased. When the blending amount was 10%, the albite content (NaAlSi₃O₈) was the highest. The content of high-melting anorthite (CaAl₂Si₂O₈) in the ash did not change significantly, which is consistent with the slight change in the CaO content in the ash. After the SCCB blending amount was increased to 15%, new calcium aluminate (Ca (Al, Fe)₁₂O₁₉) and nepheline (NaAlSiO₄) crystal phases formed in the slag, and the melting point was less than 1356 °C, which did not affect the changing trend of the liquidus temperature. At this time, the high-melting-point quartz (SiO₂) crystal phase disappeared. When the SCCB blending amount reached 25%, Na₂O reacted with Al₂O₃ and SiO₂ to generate a significant amount of nepheline (NaAlSiO₄). At this time, the crystalline phase was comprised primarily of nepheline (NaAlSiO₄) and calcium aluminate (Ca (Al, Fe)₁₂O₁₉), which increased the ash melting temperature of SCCB-25.

The analysis above indicates that the following reactions may occur during the co-combustion of meager coal and SCCBs at different proportions:

$${\mathrm{Fe}}_2{\mathrm{O}}_3\left(s\right){+\mathrm{Al}}_2{\mathrm{O}}_3\left(s\right)\to {\mathrm{Al}}_2{\mathrm{O}}_3\cdot {\mathrm{Fe}}_2{\mathrm{O}}_3\left(s\right)$$

(1)

$$\mathrm{CaO}\left(s\right){+\mathrm{Al}}_2{\mathrm{O}}_3\left(s\right){+2\mathrm{SiO}}_2\left(s\right)\to {\mathrm{CaAl}}_2{\mathrm{Si}}_2{\mathrm{O}}_8\left(s\right)$$

(2)

$${\mathrm{Na}}_2\mathrm{O}\left(s\right){+\mathrm{Al}}_2{\mathrm{O}}_3\left(s\right){+6\mathrm{SiO}}_2\left(s\right)\to {2\mathrm{NaAlSi}}_3{\mathrm{O}}_8\left(s\right)$$

(3)

$$\mathrm{CaO}\left(s\right){+6\mathrm{Fe}}_2{\mathrm{O}}_3\left(s\right){+6\mathrm{Al}}_2{\mathrm{O}}_3\left(s\right)\to \mathrm{Ca}{\left(\mathrm{Al},\mathrm{Fe}\right)}_{12}{\mathrm{O}}_{19}\left(s\right)+{9\mathrm{O}}_2\left(g\right)$$

(4)

$${\mathrm{Na}}_2\mathrm{O}\left(s\right){+\mathrm{Al}}_2{\mathrm{O}}_3\left(s\right){+2\mathrm{SiO}}_2\left(s\right)\to {2\mathrm{NaAlSiO}}_4\left(s\right)$$

(5)

3.5. Mineral Analysis of Ash

The XRD characterization of the ash from the co-combustion samples of meager coal and SCCBs after a complete combustion at 815 °C is shown in Figure 7. SiO₂ (2θ = 26.61°, 35.21°, 60.08°, and 68.11°) is the dominant substance in the co-combustion ash, which is consistent with the results of XRF. CaSO₄ is observed at 2θ = 25.54°, which is obtained by the further oxidation of CaSO₃ generated by the reaction of CaO with SO₂ released by coal combustion. In addition, Al₂O₃ (2θ = 20.71° and 22.98°), CaAl₂Si₂O₈ (2θ = 20.85° and 50.13°), and Ca₃Al₂O₆ (2θ = 35.20°) are observed. Al₂O₃ belongs to bridge oxide, which plays a link role to the substances in the ash. With the increase of mixing amount, the content of NaAlSiO₄ (2θ = 7.70°) in combustion ash increases and reaches the highest value when the mixing amount is 15%. This means that an increase in sodium content can accelerate the formation of low melting sodium-bearing minerals.

3.6. Analysis of Viscous Temperature Characteristics of Ash

Figure 8 shows that as the amount of SCCB blending increased, the viscosity first decreased and then increased, and the viscosity of the ash residue of SCCB-5 was the lowest under the same temperature condition. Figure 9 shows the critical viscosity temperature ($t_{cv}$) and the lowest operating temperature ($t_{lp}$) of SCCB-X. Under the condition of SCCB-5, the sample ash slag indicated the lowest $t_{cv}$ and $t_{lp}$, are 1368 °C and 1395 °C, respectively, lower than the flow temperature of coal ash (FT greater than 1400 °C). Thus, the heating surface of the boiler will not form a serious fusion.

The ash viscosity was determined based on both the solid-phase and liquid-phase composition [50]. As the blending amount of the SCCBs increased, the SiO₂ content decreased, and the viscosity of the ash melt decreased. By contrast, when the content of Al₂O₃ increased beyond 24%, the melt viscosity increased with the Al₂O₃ content. When the decrease in viscosity caused by the decrease in SiO₂ was smaller than the increase in viscosity caused by the increase in Al₂O₃, the overall viscosity of the ash slag increased. In addition, the decrease in MgO contributed to the increase in the viscosity of the ash slag.

3.7. Analysis of Ash Surface Morphology

Figure 10 shows the microscopic morphology of the burnt ash. Figure 10a shows that when the combustion temperature is 815 °C, the coal ash is partially melted and presents an agglomeration granular structure with a large number of broken particles attached to the surface. The results of EDS analysis indicate that the main components of coal ash are SiO₂ and Al₂O₃. As shown in Figure 10b, when adding 5 wt.% SCCB, the agglomerated ash particles decrease in diameter and increase in quantity, the sheet structure attached to the surface reduces, and the melting phenomenon is slightly relieved. Figure 10c figures that when the mixing ratio of SCCB rises to 10%, the ash surface is coated with flake particles and the agglomeration phenomenon is aggravated. As shown in Figure 10d, when the mixing ratio of SCCB increases to 15%, the ash surface is coated with multilayer flake particles, and the agglomeration phenomenon is further deepened. The results of EDS analysis show that with the increase of the mixing ratio of SCCBs, the sodium content increases, while the silicon and aluminum content are low, which is consistent with the results of XPS. Because the combustion temperature is only 815 °C, no obvious melting and coking phenomenon is observed in the ash of the four samples.

3.8. Influence of Additive on the Co-Combustion Ash of SCCB-5

3.8.1. Effect of Additive on Mineral Composition

As shown in Figure 11, when the mixing ratio of CaO reaches 5%, the mineral phase in the slag changes greatly with temperature. The contents of composite oxides (Al₂O₃·Fe₂O₃), albite (NaAlSi₃O₈), and quartz (SiO₂) decrease, the content of anorthite (CaAl₂Si₂O₈) and mullite (3Al₂O₃·2SiO₂) increase. The temperature of the appearing liquid phase of ash slag increases from 1118 °C to 1190 °C, and the liquidus temperature increases from 1540 °C to 1590 °C. This is because the increase of CaO promotes the formation of anorthite [51], whose melting point is 1550 °C, and inhibits the destruction of the polymers in the ash by Na, which makes the main solid phase in the ash mainly mullite and anorthite, leading to the increase of liquidus temperature. This also means that adding CaO can improve the ash melting characteristics.

3.8.2. Effect of Additive on Viscosity Characteristics of Ash

As shown in Figure 12, when the mixing ratio of CaO reaches 5%, the viscosity of SCCB-5 ash increases, $t_{cv}$ and $t_{lp}$ increases to 1418 °C and 1436 °C, respectively. The increase of CaO content leads to the decrease of Al₂O₃ and SiO₂ content. However, when the decrease in viscosity caused by the increase in CaO content is smaller than that caused by the increase of Al₂O₃ and SiO₂ content, the viscosity of slag will increase on the whole, which also indicates that adding a small amount of CaO will not improve the viscosity of the slag.

4. Conclusions

In this study, the effect of different proportions of meager coal and SCCB co-combustion on ash fusion performance was investigated. It is found that the ash melting characteristics can be improved by mixing the proper proportion of SCCB in meager coal. When the mixing amount of SCCB is 5%, the slag viscosity index and slagging index are 83.83 and 1383 °C, respectively. In this condition, the co-combustion ash exhibits the lowest slagging tendency, and the ash particles produced by the co-combustion of meager coal and SCCB are fine and dispersed. In this case, the critical viscosity temperature and the minimum operating temperature of the ash residue are at 1368 °C, and 1395 °C, respectively, which are also the lowest, and the slag removal effect is better. If the mixing amount continues to increase, the tendency of cinder coking will increase, which is not conducive to slag discharge. Adding a suitable and small amount of CaO to SCCB-5 can improve the ash melting characteristics of co-combustion ash.