Study on thermal conversion and detoxification mechanism of fluorine during co-combustion of meager coal and spent cathode carbon block

2.4.1 Detection of fluorine

Gaseous fluoride and dust fluoride were detected by referring to the “Determination of fluoride in Air stationary Pollution Sources-ion Selective Electrode Method” (HJ/T67-2001). Soluble fluorine was detected by referring to “Solid waste leaching toxicity leaching Method Level shock Method” (HJ557-2010) and “Determination of solid waste fluoride ion Selective electrode method” (GB/T15555.11–1995). The total fluoride in the samples was determined by using a Shanghai Yidian PXSJ-216F ion meter with reference to “Determination of Water Soluble fluoride and total Fluoride in Soil” (HJ 873–2017).

2.4.2 Heavy metal detection

Lead, nickel, cadmium and chromium were detected by inductively coupled plasma atomic emission spectrometry (iCAP7000SERIES) based on Appendix A of Leaching Toxicity Identification Standard for Hazardous Waste (GB 5085.3–2007). Mercury and arsenic were detected by Beijing Jitian AFS-933 atomic fluorescence photometer with reference to “Determination of Mercury, Arsenic, Selenium, bismuth and antimony in Solid wastes by microwave digestion/atomic fluorescence method”. Each sample was tested 3 times and then averaged.

2.4.3 Characterization

At the end of the experiment, the ash generated by combustion in some working conditions was characterized and analyzed. The phase analysis of the sample was carried out by the SmartLab (3KW) X-ray diffractometer (XRD) of Nippon Shiko Corporation. The surface morphology of corroded samples was observed by Zeiss SUPRATM55 electron scanning microscope (SEM), and the distribution and content of elements in the corroded layer were tested by Oxford INCA sight X energy dispersive spectrometer (EDS).

3.2.1 Influence of combustion temperature

The conversion rate of fluorine in the sample α is calculated by Eq. (6):

where $\alpha$ (%) is the fluoride conversion rate; ${\mathrm{m}}_{\mathit{TF}}$ , ${\mathrm{m}}_{\mathit{GF}}$ and ${\mathrm{m}}_{\mathit{DF}}(\mathrm{\mu }\mathrm{g}/\mathrm{g})$ are the mass of total fluoride, gaseous fluoride and dust fluoride precipitated by the thermal decomposition of fluorine in the sample, respectively; ${\mathrm{m}}_F(\mathrm{\mu }\mathrm{g}/\mathrm{g}$ ) is the mass of the sample's initial fluorine.

The influence of combustion temperature on the thermal conversion of fluorine is shown in Fig. 4 and Table 6.

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Fig. 4

Influence of combustion temperature on thermal conversion of fluorine. (a) Total fluoride, (b) Fluoride conversion.

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As can be seen from Fig. 4 and Table 6, with the increase of temperature, both the production and the gas-phase conversion of fluorine are increasing. The gas phase conversion of fluorine in coal is the highest under the same temperature. When the mixing ratio of SCCB is 3 %, the gas phase conversion of fluorine is the lowest. And the conversion rate of the remaining mixed samples is in between. At the combustion temperature of 950–1400 ℃, the gas phase conversion rate of fluorine in coal ranges from 31.82 to 87.60 %, with an average of 69.26 %, which is 7.68 % lower than that reported in the literature (Qi et al., 2003). This may be because the samples in the porcelain ship are piled too thick, leading to the change in the heat transfer and diffusion characteristics of the samples, which affects the experimental results to some extent (Godbeer and Swaine, 1987). The gas phase conversion rate of fluorine in the mixed sample (SCCB-3) mixed with 3 % spent cathode carbon ranges from 28.53 to 58.80 %, with an average of 46.85 %. This may be because, at low temperatures (950–1100 ℃), Ca in coal reacts with F to form CaF₂, which is dominant. The metal complex formed at high temperature (1250–1400 ℃) affected the gas phase conversion of fluorine. However, with the increase of mixing ratio, the reaction diminishes, and the gas phase conversion of fluorine increases.

It can also be seen from Fig. 4 that when the combustion temperature increases from 950 ℃ to 1100 ℃, the gas phase conversion gradient of fluorine is the largest. When the combustion temperature exceeds 1100 ℃, the gas phase conversion of fluorine tends to be gentle. This is mainly because fluorine in SCCB exists in the form of NaF, Na₃AlF₆, AlF₃, CaF₂ and other combined states, and the combustion decomposition temperature are high, which has a significant impact on the production of fluorine in the gas phase (Han et al., 2023).

3.2.2 Relationship between gaseous fluoride and dust fluoride

Fig. 5 and Table 7 show the comparison of fluorine forms in the flue gas of meager coal and SCCB.

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Fig. 5

Comparison of occurrence forms of fluorine thermal conversion in combustion flue gas. (a) 1250 ℃, (b) 1400 ℃, (c) Fluoride percentage.

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As can be seen from Fig. 5 and Table 7, in the gas phase fluorine precipitated by combustion of test samples, the gas fluorine is higher than the dust fluoride, and the lower the temperature, the greater the proportion of gas fluorine. When the combustion temperature is 1250 ℃, the proportion of fluorine in gas is 69.54-93.06 %. When the temperature is 1400 ℃, the proportion of fluorine in gas is 57.27-73.98 %. The greater the proportion of gas fluorine, the greater the pressure of flue gas defluorination. The dust and fluorine in flue gas can be well removed by cloth bag filter, which has been fully proved in the removal of particulate matter in power plant. However, it is still necessary to further study whether the limestone-gypsum wet desulphurization method can meet the emission standards and bring about new environmental problems.

3.2.3 Thermal decomposition kinetics of fluorinated minerals

Based on the experimental data of the sample at the combustion temperature of 950 ∼ 1400 ℃, the reaction kinetics curve of fluorine precipitation of the sample was obtained by linear fitting of $\text{- ln}\left(\text{1 -}X\right)$ and $\raisebox{1ex}{$\text{1}$}\!\left/ \!\raisebox{-1ex}{$T$}\right.$ , as shown in Fig. 6.

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Fig. 6

-lnk-1/T diagram of fluorine precipitation during combustion of mixed samples.

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According to the slope and intercept of the fitting curve in Fig. 6, the activation energy of reaction E and the pre-exponential factor A can be obtained.

As shown in Table 8, the correlation coefficient R² of the first-order kinetic curve of fluorine emission from meager coal used in the experiment is greater than 0.95, and the fitting result is linear. The activation energy of the reaction is 42.97 kJ/mol, lower than 77 kJ/mol of the thermal decomposition of fluorapatite (Moreno da Costa et al., 2023), indicating that fluorine in coal can be released at a lower temperature than that of fluorapatite.

After adding SCCB, the fluorine emission of the mixed fuel no longer accords with the first-order kinetic model, and the correlation coefficient R² of the fitting result of fluorine emission of SCCB-3 is greater than 0.98, which may be related to the low fluorine emission rate and low mixing ratio. However, from the obtained activation energy, adding a small amount of SCCB can promote the combustion of coal and the emission of fluorine, which is related to the high oxygen content of SCCB.

3.3.1 Migration and transformation characteristics of fluorine

Due to the limitation of experimental conditions, the amount of dust fluoride collected by filter membrane is very small, which cannot meet the requirements of detection. This paper will not analyze the influence of adding CaO on the dust fluoride thermal conversion. See Table 9 and Fig. 7 for the effect of CaO addition on thermal conversion of gaseous fluoride.

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Fig. 7

Effect of CaO amount on thermal conversion of gaseous fluoride. (a) gaseous fluoride content, (b) fluorine fixation rate.

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As can be seen from Fig. 7 and Table 9, the lower the combustion temperature, the better the fluorine fixation effect. When the combustion temperature is 1100 ℃ and 3 % CaO (Ca:F = 3.3:1) is added to the mixture sample mixed with 5 % spent cathode carbon (SCCB-5), the fluorine fixation rate reaches 42.67 %. The rate of fluoride fixation decreases rapidly with increasing temperature. CaO is the main absorbent of fluoride fixation. CaO can react with fluoride to form CaF₂. However, CaF₂ is unstable at high temperature. CaF₂ starts to decompose at the temperature of more than 1150 ℃, and decomposes completely at 1350 ℃. Therefore, the increase in temperature promotes the decomposition of CaF₂, and fluorine is released resulting in a decrease in the rate of fluoride fixation. On the one hand, the negative fluorine fixation rate is caused by the decomposition of CaF₂ generated in coal. On the other hand, it is also caused by the addition of CaO, which improves the production amount of particulate matter in flue gas and the production amount of dust fluoride associated with particulate matter. When the combustion temperature is 1250 ℃, Ca/F increases with the development of CaO addition, and the fluorine fixation rate increases gradually from negative to positive. When Ca/F is 5.5, the generation rate of CaF₂ is greater than the decomposition rate. But the production of dust fluoride is large, and the fluorine fixation rate basically approximates the equilibrium point. When Ca/F is less than 5.5, the fluorine fixation by combustion not only has no effect but also intensifies the emission of total fluoride. However, with the further increase of Ca/F ratio, the effect of fluoride fixation gradually appeared. When the Ca/F reaches 11, the fluorine fixation rate turns 23.37 %. When the combustion temperature is 1400 ℃, adding CaO not only has no effect on fluorine fixation but also increases the emission of dust fluoride.

According to the above analysis, the effect of fluorine fixation in low-temperature combustion (T < 1100 ℃) furnace is obvious. But the fluorine fixation through high temperature (T > 1250 ℃) combustion furnace is inefficient, which should not be a way of defluorination. This is consistent with the conclusion that the optimal temperature for the combustion of calcium-based fluoride fixation agents is 800-1000 ℃ (Qi et al., 2003).

3.3.2 Release feature of SO₂

The influences of combustion temperature and CaO amount on the release characteristics of SO₂ are shown in Fig. 8. In this paper, the integral of the curve of SO₂ production concentration over time is used to estimate the sulfur fixation rate, and the calculation results are shown in Table 10.

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Fig. 8

Influence of CaO on release characteristics of SO_2. (a) SCCB-0–1100 ℃, (b) SCCB-5–1100 ℃, (c) SCCB-5–1250 ℃, (d) SCCB-5–1400 ℃.

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As can be seen from Fig. 8(a-b) and Table 10, when the combustion temperature is 1100 ℃, the sulfur fixation rate increases gradually with the increase of CaO addition, from 21.56 % to 31.89 %. The sulfur fixation rate of the sample (SCCB-5) mixed with 5 % spent cathode carbon increased from 0.21 % to 35.66 % with the increase of CaO. In the mixed samples with 3 % and 5 % CaO, the higher the fluoride content, the lower the sulfur fixation rate, indicating the competitive removal of fluorine and sulfur. When adding 10 % CaO, Ca/S reaches about 4.5/1, which has little effect on the sulfur fixation rate. In Fig. 8(c-d), no obvious sulfur fixation effect can be observed in SCCB-5 samples under two combustion temperatures of 1250 ℃ and 1400 ℃.

Fig. 9 shows the effect of CaO amount on the collaborative removal of sulfur and fluorine.

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Fig. 9

Effect of CaO amount on the collaborative removal of sulfur and fluorine. (a) sulfur, (b) fluorine.

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As can be seen from Fig. 9, at the combustion temperature of 1100 ℃, during coal combustion, with the addition of CaO, the fluorine fixation rate first decreases and then increases, with a small range of 52.56 %-57.64 %. But the fluorine fixation rate is significantly higher than the sulfur fixation rate. With the increase of CaO amount in the combustion process of the mixed sample with 5 % spent cathode carbon (SCCB-5), the fluorine fixation rate increases first and then decreases from 42.67 % to 46.72 % and then to 36.72 %. But the fluorine fixation rate is still much higher. The first reason is that Ca/F is much higher than Ca/S. Secondly, CaF₂, with a higher decomposition temperature, is more stable than CaSO₄. Meanwhile, CaF₂ (molecular weight 76) has a much lower degree of pore plugging on CaO than CaSO₄ (molecular weight 136).

3.3.3 Analysis of the mechanism of fluorine fixation and sulfur in combustion

CaO reacts with SO₂, HF and SiF₄ produced by the combustion of mixed samples to produce stable sulfide or fluoride. The basic equations are as follows:

Fluorination reaction:

Sulfuration reaction:

The combustion flue gas contains H₂S, CO and other gases, so the following reactions will occur:

On the basis of the previous study on the melting characteristics of ash (Liu et al,2000), the composition of ash is quite complex, containing a large number of acid oxides, such as Al₂O₃ and SiO₂. Under the catalytic action of acid oxides, the added CaO tends to form relatively stable metal complexes, such as 3CaO·3Al₂O₃·CaF₂, 3CaO·3SiO₂·CaF₂, 3CaO·3Al₂O₃·CaSO₄, Ca₄F₂Si₂O₇. The decomposition temperature of these complexes is higher, reaching more than 1400 ℃, which is also the reason why F is still contained in high-temperature ash. As seen in Table 10 and Fig. 9, there is a competitive relationship between desulfurization and defluoridation of CaO. In the reaction process, CaO preferentially reacts with HF to form CaF₂.The increase of CaO content has a little effect on the rate of fluorine fixation, while it has a larger effect on the rate of sulfur fixation. This indicates that the formation of CaSO₄ has a weak effect on the fluoride fixation rate of CaO. The combustion temperature is the key factor affecting the fluoride fixation rate when the CaO content is certain.

3.4.1 Fluorine footprint

The conversion behavior of fluorine during the combustion of the samples is presented in Fig. 10.

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Fig. 10

Transformation behaviors of F species during the (co–)combustion. (a)SCCB-0, (b)SCCB-10, (c)HF(g), (d)NaF.

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From Fig. 10(a-b), it can be seen that the fluorine in the fuel is mainly emitted to the outside in the form of HF gas after combustion. Flue gas defluoridation is the key to ensure that fluorine emissions meet the standards. Fluorine in coal mainly exists in the form of fluorapatite minerals, organic fluorine and inorganic fluorine decomposed at low temperature. In the combustion temperature of 640-800 ℃, fluorine apatite (Ca₅(PO₄)₃F) decomposition or carbon reduction, generating CaF₂. CaF₂ reacts with the steam produced by combustion to produce HF (g). Coal combustion process fluorine conversion reaction formula as shown in R6-R8.

Fluorine in the SCCB mainly exists in the form of NaF and Na₃AlF₆. Na₃AlF₆ decomposes to produce NaF and AlF₃ at a combustion temperature of 500-600 ℃. The fluorine conversion reaction equations in the combustion of SCCB are listed as R9-R11.

The decomposition (R9) and hydrolysis (R10-R11) phenomena were not observed in Fig. 10(b) when 10 % of SCCB was mixed with coal due to the low molar fraction of Na₃AlF₆. However, it can be seen from Fig. 10(c-d) that the content of NaF grows with increasing the amount of SCCB mixed. The molar fraction of HF gas produced gradually rises, which also explains the conversion process of fluorine in the SCCB.

3.4.2 Effect of CaO addition on fluorine conversion

The effect of the CaO addition on the fluorine conversion is shown in Fig. 11. From Fig. 11(a), it can be seen that the production of HF gas increases and then decreases with increasing combustion temperature after the addition of CaO. CaO obviously inhibits the rate and amount of HF gas production. The release of HF gas from the SCCB-5 sample is 0.0343 kmol at 890 °C. After mixing 5 % and 10 % CaO, the release of HF gas is 0.0316 kmol and 0.0287 kmol, respectively. When the temperature is 1020 °C, the production of HF gas for the mixed samples mixed with 5 % and 10 % CaO is basically the same and reaches the maximum value (about 0.0336 k mol). Subsequently, the amount of HF gas production began to decrease, and at 1400 °C, the amount of HF gas production is about 0.0265 kmol.This is due to the fact that NaF and AlF₃ from the decomposition of Na₃AlF₆ react with CaO to form CaF₂, which inhibits the rate of HF generation. In addition, CaO reacts with acidic oxides (Al₂O₃, SiO₂) from combustion to form more stable metal complexes (3CaO·3Al₂O₃·CaF₂, 3CaO·3SiO₂·CaF₂,Ca₄F₂Si₂O₇), which leads to a decrease in the amount of HF. As can be seen from Fig. 11(b), the decomposition of Na₃AlF₆ is not affected by the addition of CaO.

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Fig. 11

Effect of CaO addition on fluorine conversion. (a) HF(g), (b) Na₃AlF₆.

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3.5.1 Influence of combustion temperature on soluble fluorine in ash

The influence of combustion temperature on soluble fluorine in ash is shown in Fig. 12. As can be seen from Fig. 12(a), the higher the combustion temperature, the lower the content of soluble fluorine in ash. When the combustion temperature is 950 ℃, the maximum value of soluble fluorine in the SCCB-10 cinder is 33.4 mg/L. When the combustion temperature rises to 1400 ℃, the content of soluble fluorine is only 0.51 mg/L. The content of soluble fluorine in ash residue at other mixing ratios also showed a similar rule, all of which were far lower than the requirement of 100 mg/L concentration limit of inorganic fluoride (excluding CaF₂) in the leaching solution in Table 1 of the Standard for Identification of Hazardous Waste Leaching Toxicity (GB5085.3–2007).

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Fig. 12

Effect of combustion temperature on soluble fluorine in ash. (a) soluble fluorine, (b) conversion rate.

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Fig. 12(b) shows that incineration is the most effective way to remove the toxicity of SCCB. When the combustion temperature exceeds 1100 ℃, the thermal conversion rate of soluble fluorine in the mixed sample is as high as 99.68 %-99.98 %, while the thermal conversion rate of soluble fluorine is slightly lower at 950 ℃, also reaching 98.55 %-99.67 %. The thermal conversion rate of soluble fluorine in coal is lower than that of mixed samples at the same temperature, ranging from 86.53 % to 95.91 %, which is related to the occurrence form of fluorine in coal and is consistent with the thermal conversion rate of fluorine in coal obtained by pulverized coal boiler (Swain and Satapathy, 2023).

3.5.2 Influence of mixing ratio on soluble fluorine in ash

Fig. 13 indicates the influence of the mixing ratio on soluble fluorine in ash. With the rising mixing ratio, the fluorine content in the mixed sample and a mean value of soluble fluorine in the ash after combustion both increase. The soluble fluorine in coal ash ranges from 0.71 mg/L to 2.28 mg/L, with an average value of 1.30 mg/L. The soluble fluorine in the combustion ash of SCCB-10 ranges from 0.51 mg/L to 33.44 mg/L, with an average value of 9.82 mg/L, which is consistent with the previous research (Sun et al., 2021).

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Fig. 13

Influence of mixing ratio on soluble fluorine in ash.

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3.5.3 Heavy metals in ash

Fig. 14 and Table 11 show the content of heavy metals in the combustion ash of mixed samples. As is shown in Fig. 14, combustion temperature has little effect on heavy metal content in ash. The melting points of Ni, Cr, Pb, Cd, As and Se are 1453 ℃, 1907 ℃, 327 ℃, 321 ℃, 814 ℃ and 221 ℃, respectively. When the temperature is higher than the melting point of Pb, Cd, As and Se, it is easy to sublimate, which belongs to volatile heavy metals. Ni and Cr are volatile heavy metals. The combustion temperature of this experiment is between 950 ℃ and 1400 ℃. The temperature has little influence on the content of heavy metals in ash.

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Fig. 14

Concentration of heavy metals in mixed sample combustion ash ( ${\mathrm{C}}_0$ --Concentration limits of hazardous substances in leaching solution). (a) Ni, (b) Pb, (c) Cr, (d) Cd, (e) As, (f) Se.

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The content of heavy metals in the slag of the mixed sample after combustion is very low. The concentrations of Ni, Cr, Pb and Cd range from 0.048 to 0.180 mg/L, 0.873 to 1.663 mg/L, 0.018 to 0.068 mg/L, and 0.042 to 0.114 mg/L, respectively. The concentrations of As and Se range from 1.515 to 1.628 ug/L and 2.283 to 3.070 mg/L, respectively, which are far lower than the concentration limits of hazardous substances in the leach solution in Table 1 of the Standard for Identification of Hazardous Waste Leaching Toxicity (GB5085.3–2007).

3.5.4 Properties of ash

According to previous analysis, SCCB are identified as hazardous waste due to containing a large amount of soluble fluoride and a small amount of soluble cyanide. At the combustion temperature of 950 ∼ 1400 ℃, the soluble cyanide in the SCCB can completely burn and decompose into CO₂ and NO_x. At low mixing ratio ( $\mathrm{X}\le 10\%$ ), the maximum value of soluble fluorine in ash residue is 33.44 mg/L, which meets the requirement of 100 mg/L concentration limit of inorganic fluoride (excluding CaF₂) in leaching solution of Hazardous Waste Identification Standard (GB5085.3–2007), not belonging to hazardous waste.

3.6.1 XRD analysis

Fig. 15 shows the XRD pattern of SCCB-5 and the ash after combustion with CaO. As SCCB co-combusts with coal, aluminosilicate minerals rich in NaAlSi₃O₈ and pyroxene (Na_0.72Ca₄(Al₁₀Si₂₆O₇₂)(H₂O)_29.12) appear in the phase composition of ash, corresponding to the high sodium content of SCCB. Such minerals are flux-auxiliating minerals. Diffraction intensity gradually decreases with the increase in temperature. When the temperature is 1400 ℃, diffraction peaks of NaAlSi₃O₈, Na_0.72Ca₄(Al₁₀Si₂₆O₇₂)(H₂O)_29.12 cannot be observed, which is consistent with the conclusion that the diffraction peak disappears when the temperature exceeds 1260 ℃. When the combustion temperature is 1100 ℃, the diffraction peak of CaF₂ appears, which corresponds to the high fluorine content of SCCB. When the temperature rises to 1250 ℃, the diffraction peak of CaF₂ disappears, which means that the CaF₂ content decreases. The physical property of CaF₂ decomposition over 1150 ℃ can also be explained. In the XRD pattern, Al₂O₃, Fe₂O₃ and Al₂O₃·SiO₂ are stable, and their diffraction intensity does not change significantly with the increase of temperature, indicating that these substances belong to the refractory minerals, acting as the skeleton.

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Fig. 15

XRD pattern of SCCB-5 and the ash after combustion with CaO.

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When the combustion temperature is 1100 ℃, the diffraction peak of plagioclase (Ca₂Al₂SiO₇) appears in the combustion ash of SCCB-5 samples with CaO, indicating that CaO reacts with SiO₂ and Al₂O₃ to form plagioclase ( $2\mathrm{C}\mathrm{a}\mathrm{O}+\mathrm{S}\mathrm{i}{\mathrm{O}}_2+{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3=\mathrm{C}\mathrm{a}{\mathrm{A}\mathrm{l}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_7$ ). The diffraction peak of CaF₂ (2θ = 46.93°) is more obvious, which also means that CaO has a certain effect on fluorine fixation at low temperatures.

3.6.2 Microstructure of ash

As can be seen from Fig. 16, great differences in the microscopic morphology of the four kinds of ash exist.

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Fig. 16

Microstructure of co-combustion ash of meager coal and SCCB at 1100 ℃. (a) MC, (b) SCCB-5, (c) SCCB-10, (d) SCCB-5-CaO-5.

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Fig. 16 shows the microstructure of the co-combustion ash of meager coal and SCCB at the combustion temperature of 1100 ℃, from which it can be seen that great differences exist in different kinds of ash. Fig. 16(a) shows that when the combustion temperature is 1100 ℃, the coal ash is partially melted and presents a uniform small and agglomerative vitreous structure with a smooth surface. The EDS results show that the main components of ash are SiO₂ and Al₂O₃. According to the element composition and existing studies (Lim et al., 2023), the main solid phase of coal ash is mullite (3Al₂O₃·2SiO₂) with a high melting point. As shown in Fig. 16(b), when 5 % of the SCCB is added, the ash is stratified, changing from the original agglomeration into a loose cake structure, and the melting phenomenon is relieved. In Fig. 16(c), when the mixing ratio of SCCBs reaches 10 %, the ash presents a mixed structure of particles and sheets with obvious improvement effects, but some small particles still accumulate locally. This is because the SCCB contains Na alkali metal, and the ion potential of Na⁺ is very low, which can destroy the polymers in the ash, form low melting point sodium-containing minerals such as NaAlSi₃O₈, and reduce the melting temperature of ash. In Fig. 16(d), after adding 5 % CaO to the sample, the ash turns loose in texture and appears a uniform and flaky structure. This is because the added CaO reacts with SiO₂ and Al₂O₃ to produce calcite (CaAl₂Si₂O₈), calcite (2CaO·Al₂O₃·SiO₂), wollite (CaSiO₃) and other calcium-containing minerals, which inhibits the generation of mullite, changes the type of main solid phase, and reduces the percentage of main solid phase, making the melting temperature of ash further reduce.

Fig. 17 draws that the combustion temperature and melting characteristics of ash have significant effects on the surface morphology and microstructure of ash. It can be seen from Fig. 17 and Fig. 16(b) that with the increase in combustion temperature, the morphology of ash changes from sintered state to the molten state. The melting phenomenon is observable at the combustion temperature of 1250 ℃ and extremely strict when the temperature rises to 1400 ℃.

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Fig. 17

Microstructure and element analysis of co-combustion ash of SCCB-5 mixed samples at different temperatures. (a) 1250℃, (b) 1400℃.

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In Fig. 17(a), the ash at 1250 ℃ is fused into a dense massive solid, with a small amount of solid particles attached to the surface, forming a spherical cavity with a diameter of 0.3 µm and a cavernous cavity with an irregular length of 3.97 µm and width of 1.65 µm. The boundary between the two cavities is smooth and uniform, showing obvious traces of melting flow. The EDS analysis indicates that the main components of ash residue are SiO₂, Al₂O₃, CaO, Na₂O, Fe₂O₃ and a small amount of CaF₂. From the element composition, it can be inferred that the formation of the spherical cavity is mainly caused by the decomposition of CaCO₃ and the decomposition of a small amount of CaF₂ and the generation of gaseous HF. The irregular cavernous cavity is mainly caused by the liquid phase material flow generated by the cryoeutectic reaction of SiO₂, Al₂O₃, CaO and Na₂O at high temperatures, and the volume shrinkage caused by the precipitation of the generated anorthite and albite during the cooling process.

As shown in Fig. 17(b), serious melting occurred in the ash slag at 1400 ℃. The surface is smooth with almost no particles, forming a more dense massive solid. The number of spherical cavities on the surface is increasing, and the pore size varies from dozens of nanometres to 1 µm. The EDS results show that the main components of the ash are SiO₂, Al₂O₃, CaO, Na₂O, and Fe₂O₃ without the element F. From the element composition, it can be inferred that the formation of a spherical cavity is mainly caused by a low-temperature eutectic reaction of SiO₂, Al₂O₃, CaO, Na₂O and Fe₂O₃ at high temperature, resulting in liquid phase material flow and cooling contraction to form dense Na-Ca-Al-Fe-Si low temperature eutectic. At the same time, the CaCO₃ and CaF₂ in the ash are completely decomposed into gaseous CO₂ and HF, and the gaseous HF etches the surface of ash to form a spherical cavity during the precipitation process.