China’s reliance on coal as an essential energy source and chemical raw material is expected to persist until 2050, making the clean and efficient utilization of coal resources crucial for the country’s scientific and sustainable development (Lin et al. 2018). Coal gasification technology serves as a fundamental approach to achieving this objective and is imperative for the advancement of modern coal chemical industries (Wang. 2022; Akanksha et al. 2023; Guo et al. 2020). During the entrained flow gasification process, the majority of organic matter is converted into syngas, while mineral matter undergoes a high-temperature transformation into slag or ash. The behavior of coal minerals in terms of melting, migration, deposition, and slagging at high temperatures can significantly impact the stable and continuous operation of gasification equipment (Wang et al. 2012). Failure to maintain the appropriate fluidity of the slag in accordance with gasifier operating conditions can result in various issues, including abnormal shutdowns of the gasification system (Nikolay et al. 2023; Tang et al. 2016). Consequently, it is necessary to study the gasification ash at high temperature.

Achieving a smooth discharge of slag within the desired viscosity range is crucial for ensuring the stable operation of entrained flow gasification systems (Shen et al. 2021). The crystallization behavior of the slag directly influences its fluidity. As the crystal content increases, the slag transitions from a Newtonian fluid to a non-Newtonian fluid, causing a substantial increase in viscosity (Xuan et al. 2019). Excessive viscosity prevents proper slag discharge, leading to blockages in the gasifier’s outlet (Dong et al. 2006). Schwitalla et al. (2017) have established a correlation between slag crystallization and its critical viscosity-temperature relationship, emphasizing the significant impact of crystal growth on slag viscosity. It is worth noting that the crystal particles are not entirely solid but can entrap liquid residues. Seebold et al. (2017) have demonstrated the significant influence of crystallization time and temperature on slag viscosity below the liquid phase line, as well as the effect of these parameters on the crystallization range, thereby highlighting the complexity of high-temperature slag crystallization. He et al. (2019) employed FactSage to simulate the impact of iron content in coal ash on slag crystallization and viscosity. Their findings indicate that iron content affects the presence of melilite and spinel in the slag. Utilizing model predictions enables the minimization of solid phases in the slag, thereby controlling slag viscosity. Consequently, investigating the crystal phase composition of slag at high temperatures is essential for comprehending and optimizing liquid slag discharge in gasifiers.

Residual carbon, an integral component of gasification ash, interacts with other slag components at high temperatures, thereby influencing ash fusibility, viscosity-temperature characteristics, and slag fluidity (Wang et al. 2021). During the gasification process, the conversion of organic matter in coal is incomplete due to coal type and gasification conditions, leading to a significant amount of residual carbon in the slag (Montagnaro et al. 2011). Under high temperatures, residual carbon undergoes intricate physicochemical reactions with slag minerals, forming substances with high melting points (Kong et al. 2015). The presence of residual carbon profoundly affects the crystallization and solidification behavior of the slag, necessitating an investigation into its impact on the gasification process. Kong et al. (2015) have demonstrated that residual carbon substantially influences slag viscosity and fusibility, and the carbothermal reaction between minerals and residual carbon generates refractory mineral silicon carbide (SiC), thereby affecting slag fusibility. Wang et al. (2019; 2018a, b; 2020) conducted simulations using coal ash compositions with varying degrees of carbon graphitization and observed that higher graphitization levels of residual carbon corresponded to higher melting temperatures of residual carbon slag. Notably, when residual carbon content exceeded 5%, the effect of carbon graphitization on slag melting temperature became more pronounced.

In the process of gasifier slagging, the temperature of the flowing slag will drop, which may lead to the precipitation of some crystals. Therefore, there are many studies on the crystallization behavior of slag during the cooling process. Wang et al. (2018a, b) simulated slag using a five-component oxide system of Ca–Mg–Si–Al–Fe to examine the influence of temperature and cooling rate on slag crystallization during isothermal processes. Their findings revealed that higher cooling rates resulted in reduced crystallization ratios, with distinct crystal morphologies forming at different temperature ranges. Xuan et al. (2014) simulated coal gasification slag using SiO₂–Al₂O₃–CaO–Fe₂O₃–MgO and studied the crystallization characteristics of synthetic slag at isothermal temperatures and continuous cooling rates. The studies indicated that higher cooling rates inhibited crystal growth, and exceeding the critical cooling rate even led to the appearance of a glassy substance. However, there are few studies on the effect of residual carbon on slag under different cooling conditions, so the influence of residual carbon on slag crystallization and solidification behavior under different cooling conditions is discussed by comparing quenching and natural cooling conditions.

This research aims to investigate the impact of residual carbon on the melting characteristics and crystallization behaviors of gasification slag obtained from an entrained flow gasifier, considering various cooling methods. The mineral composition and content of the slag at high temperatures will be analyzed using XRD and the K-value semiquantitative method. Additionally, the mineral and carbothermal reactions during DSC heating will be examined, and the morphology of the slag at elevated temperatures will be analyzed using SEM.

2.1 Raw materials

The coarse gasification slag used in the experiment came from the gasification section of Ningxia Coal to the Oil Company of the National Energy Group. After grinding of coarse dry residue into particles < 74 μm, it was recorded as reserve slag.

2.2 Experimental methods

Weigh 6 g of the reserve slag and place it in a cylindrical alumina crucible. The crucible was placed in the horizontal tube furnace (TSK-8-12) of Fernsen (Beijing) Electric Furnace Co., LTD. Under N₂ condition, and the sample was heated through a three-stage heating process: the first stage was increased from 10 °C /min to 300 °C; the second stage increased to 800 °C at 7.5 °C /min; the third stage was heated at 5 °C /min to different final temperatures (900 °C, 1000 °C, 1100 °C, 1200 °C, 1300 °C, 1400 °C), and kept for 30 min. After the heat preservation is finished, the sample is immediately removed and immersed in water to obtain the quenching sample (CQ). Natural cooling means that after the holding time is over, the sample is cooled to room temperature together with the tube furnace and then taken out to get the natural cooling sample (CN).

The reserve coarse slag (denoted as C) was decarburized at 815 °C, 3 h with air atmosphere. The decarburized coarse slag (denoted as DC) was treated at different final temperatures and with different cooling methods according to the experimental method in the above method. And the decarburized coarse slag samples with quenching and natural cooling samples were denoted as DCQ and DCN.

An X-ray fluorescence spectroscopy (XRF) was used to analyze the chemical composition of ash in coarse slag. The surface morphology of the sample was measured by a scanning electron microscope (ZEISS EVO18, Germany) at different temperatures and different cooling modes, and the test voltage was 15 kV. Differential thermal analysis (DSC) was used to determine physicochemical changes (crystallographic transformation, melting, and sublimation, etc.) in the sample during heating. X-ray diffraction (XRD) was used to analyze the mineral composition characteristics and crystallization behavior of samples at elevated temperatures. The mineral composition and crystallization behavior of the samples at high temperature were analyzed by X-ray diffraction (SmartlabSE, Japan). Diffraction conditions: Cu kα radiation (40 kV, 40 mA), scanned with a step size of 0.01° varying 2θ from 5° to 85°. The K-value method in XRD analysis was used to semi-quantify the crystalline minerals (Xu et al. 2015), which was defined as:

where w_i, I_i and K_i are the mass fraction, the diffraction intensity, and RIR value of the i phase, and n is the number of phases retrieved.

The calculation of the amorphous phase material was performed using MDI. Jade 6 software, which was defined as:

where x, I_a, and I_c are the mass fraction of the amorphous phase, the diffraction intensity of the amorphous phase, and the diffraction intensity of the crystalline phase. The diffraction intensity of a phase refers to the height or area of the diffraction peak that appears in XRD. In the XRD pattern, the sum of the diffraction intensity of the crystal (I_c) and the amorphous (I_a) is 100%, So the diffraction intensity of the amorphous phase is equal to I_a = 100% − I_c.

3.1 Analysis of chemical composition and melting characteristics of coarse slag

The ash in coarse slag is primarily composed of SiO₂, Al₂O₃, Cao, and Fe₂O₃. The existence of acidic oxides such as SiO₂ and Al₂O₃ can increase the melting point of coal ash (Li et al. 2012). On the other hand, The presence of basic oxides (CaO, Na₂O, MgO, and Fe₂O₃) can effectively reduce the melting point of coal ash (Wang et al. 2019). Table 1 presents the chemical composition analysis of the coarse slag, with acidic oxides accounting for 68% and basic oxides accounting for 29%, resulting in a B/A (base-acid) ratio of 0.44. Previous studies (Jia et al. 2007) have investigated the addition of residual carbon (> 5%) to five different ash samples with Si/Al ratios ranging from 1.0 to 3.0, exploring the impact of the Si/Al ratio on the fusibility of coal ash. Wang et al. (2020) found that when the Si/Al ash ratio exceeded 1.0, the content of residual carbon significantly affected the AFT and the fusion temperature range (ΔT = FT–DT). The presence of refractory minerals such as FeSi and Fe in the slag hindered their melting in the liquid slag, resulting in an increased interval between FT and DT. The coarse gasification slag used in this study had a Si/Al ratio of 2.48 and a residual carbon content of 1.7%. The melting points of the coarse slag and decarburized coarse slag were analyzed in Table 2. Although there were no significant differences in the deformation temperature, sphere temperature, and hemisphere temperature of coarse slag and decarburized coarse slag, the flow temperature of the coarse slag was higher than that of the decarburized coarse slag. This disparity may be attributed to the reaction between residual carbon and other minerals in the coarse slag at high temperatures, causing an increase in the flow temperature of coarse slag. ΔT represents the effect of temperature on the meltability of the ash. Furthermore, the variation of ΔT for coarse slag with residual carbon was higher compared to decarburized coarse slag. This indicates that residual carbon content below 5% still exerted some influence on the melting of coarse slag, particularly at high temperatures.

3.2 XRD analysis of coarse slag and decarburized coarse slag

The results of the CQ XRD analysis are presented in Figs. 1 and 2. In XRD analysis, the variation in the diffraction intensity of the same mineral can roughly reflect the variation in its content (Wu et al. 2018). As observed in the figures, the original coarse slag sample (C) primarily exists in an amorphous state, with only weak quartz diffraction peaks detected. When the heating temperature ranged from 900 °C to 1300 °C, CQ exhibited quartz (Melting Point = 1610 °C) diffraction peaks, as well as the presence of anorthite (Melting Point = 1553 °C) and hematite (Melting Point = 1550 °C). The content of anorthite was highest at 1000 °C, gradually decreasing with increasing temperature. Furthermore, The crystalline diffraction peak of iron appears in CQ at 1100 °C, and the iron content gradually increases with increasing temperatures by 1300 °C, and transforms to Fe₃Si at 1400 °C. Concurrently, the diffraction peak of hematite gradually diminished, indicating a substantial involvement of hematite in the reaction. At a heating temperature of 1400 °C, an amorphous steamed bun diffraction peak appears in CQ, accompanied by the appearance of Fe₃Si (Melting Point = 1360 °C).

XRD patterns of coarse slag at 900 °C –1400 °C under quenching (CQ). An: Anorthite (CaAl₂Si₂O₈); I: Iron (Fe); H: Hematite (Fe₂O₃); Q: Quartz (SiO₂); S: Fe₃Si

Contents of minerals of coarse slag under quenching conditions of 900 °C –1400 °C

For the formation of Fe (Eq. (1)) and Fe₃Si (Eq. (2)) (Wang et al. 2020):

The calculation of Gibbs free energy at different temperatures yields results presented in Table 3. The reaction (1) can take place at 900 °C, while in the results the XRD analysis, the appearance of Fe diffraction peaks is at 1100 °C. This is due to the fact that in this temperature range, the reaction is between solid phases. Mass transfer is not significant during this stage, resulting in a low probability of reaction occurrence. As the sample starts to melt with increasing temperature, the mass transfer is gradually enhanced and the chance of reaction occurring increases. According to the results of Gibbs free energy calculation in Table 3, it can be found that the carbothermal reaction begins to occur at about 1300 °C, and the XRD analysis results in diffraction peaks of the carbothermal reaction products appearing at 1400 °C, which is consistent with the calculated results. Wang et al. (Wang et al. 2019) explored the ash melting temperature of slag will rise when the residual carbon is between 0% and 5%, which is due to the presence of refractory mineral phases (SiC, Fe₃C, Fe₃Si, etc.) in the slag, and because there is still unreacted residual carbon (Melting Point = 3500 ℃) in the slag, the unreacted carbon will form a skeleton in the slag, preventing the melting of slag.

The XRD analytical results of CN are illustrated in Figs. 3 and 4. For CN, the same crystalline diffraction peaks as CQ appeared at heating temperatures of 900 °C –1200 °C. However, iron crystalline diffraction peaks began to emerge at 1000 °C. Meanwhile, crystalline diffraction peaks of hematite did not disappear at 1300 °C, and crystalline diffraction peaks of quartz and anorthite were also present in the CN at 1400 °C.

XRD patterns of coarse slag at 900 °C –1400 °C natural cooling (CN). An: Anorthite (CaAl₂Si₂O₈); I: Iron (Fe); H: Hematite (Fe₂O₃); Q: Quartz (SiO₂); S: Fe₃Si

Contents of minerals of coarse slag under natural cooling conditions of 900 °C –1400 °C

By comparing the results of the XRD and its semiquantitative analysis of CQ and CN, it is evident that the intensity of crystalline peaks for each substance at 1300 °C is significantly lower under quenching conditions compared to natural cooling conditions. This may be due to the fact that under quenching conditions at 1300 °C, the ions in the melt are in a relatively free and disordered state. After quenching, the sample remains almost in the same state as at high temperatures, so the crystal content of the sample is very low. In contrast, under natural cooling conditions, the ions in the melt have relatively sufficient time to transfer and arrange themselves in a certain way, thus forming crystals. Therefore, the crystallization content under natural cooling conditions is higher than that under quenching conditions.

The XRD analysis results of DCQ are presented in Figs. 5 and 6. From the figures, it can be found that the crystalline diffraction peaks of quartz, anorthite, and hematite appear in DCQ at a heating temperature of 900 °C. With the increase in the temperature, the intensity of this diffraction peak also gradually increased. This shows that with the increase of temperature, the mass transfer between various minerals in the slag is enhanced, and the more intense the chemical reaction leads to the increase of mineral content in the slag and the enhancement of diffraction peak. When the temperature increases to 1200 °C –1300 °C, the intensity of the crystalline diffraction peaks starts to gradually decrease. This may be due to the gradual increase of the liquid phase at increasing temperatures. Under the quenching conditions, the crystallization time of the minerals is insufficient, which leads to the increase of the amorphous phase.

XRD patterns of decarbonized coarse slag at 900 °C–1400 °C under quenching (DCQ). An: Anorthite (CaAl₂Si₂O₈); H: Hematite (Fe₂O₃); Q: Quartz (SiO₂)

Contents of minerals of decarbonized coarse slag under quenching conditions of 900 °C –1400 °C

The XRD analysis results of DCN are displayed in Figs. 7 and 8. In comparison to DCQ, DCN exhibits consistent crystalline minerals. However, the intensity of the crystalline diffraction peak in DCN is notably stronger than that of DCQ at 1300 °C, which can be attributed to the different cooling methods employed.

XRD patterns of decarbonized coarse slag at 900 °C –1400 °C under natural cooling (DCN). An: Anorthite (CaAl₂Si₂O₈); H: Hematite (Fe₂O₃); Q: Quartz (SiO₂)

Contents of minerals of decarbonized coarse slag under natural cooling conditions of 900 °C –1400 °C

Compared with the analysis of coarse slag and decarburized coarse slag at the same temperature and cooling method, it can be found that when residual carbon is present, the hematite in the coarse slag would be reduced to iron, which then reacts with SiO₂ and residual carbon in the coarse slag in a “carbothermal reaction” to form a high melting point crystalline mineral, Fe₃Si. Conversely, since the decarburized coarse slag lacks residual carbon, no crystalline diffraction peaks of Fe₃Si appear in the XRD analysis as the heating temperature increases.

We also calculated the content of the amorphous phase (Fig. 9). However, due to errors in the calculations, we analyzed only the cases with large variations in the noncrystalline content. From the figure, it can be found that the amorphous phase content in the CQ and DCQ samples gradually increases with the increase of heating temperature. Furthermore, under quenching conditions, coarse slag and decarburized coarse slag exhibit higher amorphous phase content than natural cooling conditions at the same temperature. Under quenching conditions, the content of the amorphous phase increased significantly when the temperature increased from 1200 °C to 1300 °C. This indicates that the slag has a large increase in its liquid phase with increasing temperature, when the slag is quenched and the crystals have not yet formed an ordered structure, leading to an increase in the content of the amorphous phase (Li et al. 2011). Therefore, the cooling mode of the slag is an important factor in affecting its crystallization and solidification behavior, and the cooling mode of the slag of the gasifier is an aspect that we should pay attention to in the actual production process.

Amorphous phase content of coarse slag and decarbonized coarse slag under different cooling conditions

3.3 DSC analysis of coarse slag and decarburized coarse slag

To further investigate the influence of carbon residue on the reaction of individual substances in coarse slag, the DSC results are shown in Fig. 10. From the DSC curves in the figure, it can be seen that when the temperature rose to 1000 °C, the baseline of the DSC curve appeared to fall. The reason for the decrease in the baseline of the sample was mainly that the volume of the sample started to shrink during the heating process and the heat capacity changed (Sudo 1981).

DSC diagram of coarse slag (C) and decarbonized coarse slag (DC)

Distinct heat absorption peaks were observed at 1177 °C for coarse slag and 1176 °C for decarburized coarse slag. Considering the XRD results of coarse slag and decarburized coarse slag discussed earlier, it was observed that the anorthite content reached its maximum at 1000 °C and significantly decreased as the temperature rose to 1100 °C. This decline could be attributed to the formation of low-temperature eutectics between anorthite and other minerals in the samples, leading to an increase in the liquid phase (Cheng et al. 2017). This increase in the liquid phase absorbed a significant amount of heat, resulting in a heat endothermic peak. At this point, both coarse slag and decarburized coarse slag reached their deformation temperatures, indicating an increase in various reactions within the slag, accompanied by substantial heat absorption.

The DSC curves of the coarse slag and the decarburized coarse slag show a clear exothermic peak in the subsequent temperature range of 1225 °C. This may be caused by the significant appearance of the liquid phase in the sample at this time and the obvious reaction between the minerals. However, there were some small exothermic peaks (1226 °C, 1238 °C and, 1243 °C) in the DSC curve of coarse slag, which may be due to the formation of Fe₃Si generated by carbothermal reaction.

3.4 Analysis of the surface morphology of coarse slag and decarburized coarse slag

At high temperatures, both the composition and shape of slag undergo changes. The results of the SEM analysis of CQ and CN are illustrated in Figs. 11 and 12. From the figures, it can be seen from the figures that both CQ and CN show no sintering at 900 °C. When the heating temperature was increased to 1200 °C, CQ and CN started to show a sintering phenomenon and exhibited obvious agglomeration. According to the literature, with the increase in temperature, inorganic minerals will accumulate on the surface of carbon residues. The higher the temperature, the more inorganic minerals accumulate (Ma et al. 2013). However, as the coarse slag under natural cooling conditions grew, many flakes of crumbly material appeared on the surface. This could also indicate that the minerals in the coarse slag under natural cooling conditions had enough time to crystallize, which resulted in the precipitation of crystals attached to the surface of the agglomerated coarse slag. When the temperature increases to 1300 °C –1400 °C, small holes appear on the surface of the coarse slag. This was due to small holes formed by the release of gases generated by the “carbothermal reaction” that occurs in the coarse slag.

SEM of coarse slag under quenching conditions

SEM of coarse slag under natural cooling conditions

The SEM analysis results of DCQ and DCN are presented in Figs. 13 and 14. From the figures, it can be seen that DCQ and DCN did not show a sintering phenomenon at 900 °C. With the increase of temperature in 1200 °C –1400 °C, the decarburized coarse slag showed an obvious sintering phenomenon. Notably, the decarburized coarse slag did not show spheroidization but appeared as a smooth and dense surface material. For DCN, a large amount of flaky and crumbly material was formed on the surface as the temperature increased. It may be that a large number of inorganic minerals appear on the surface of the decarbonized coarse slag under natural cooling conditions.

SEM of decarbonized coarse slag under quenching conditions

SEM of decarbonized coarse slag under natural cooling conditions

The SEM image analysis of the coarse slag and the decarburized coarse slag reveals that because of the presence of residual carbon, the slag appeared to be spherical. Although the decarburized coarse slag had a sintering phenomenon after 1200 °C, and the sintering phenomenon was more obvious with the increase in temperature, it did not form a spherical slag. This indicates that the presence of residual carbon affects the evolution of minerals in the slag and may have an effect on the surface tension and structure of the slag. However, the effect of residual carbon on the surface tension of the slag has not been studied in depth, which will be the focus of subsequent work.

This study aims to examine the impact of residual carbon on the melting behavior of coarse slag and the crystallization and solidification characteristics of both coarse slag and decarburized coarse slag at different temperatures under varying cooling conditions. The main findings are summarized as follows:

1. (1)

   When residual carbon is present, the flow temperature of coarse slag is higher than that of decarburized coarse slag. When the temperature exceeds 1300 °C, the residual carbon will react with hematite and iron in the slag to form Fe₃Si, resulting in a higher flow temperature of coarse slag than that of decarburized coarse slag. Meanwhile, the DSC results also show that the carbothermal reaction occurs in the presence of residual carbon.

2. (2)

   For the coarse slag and decarburized coarse slag under different cooling conditions, the coarse slag has obvious spheroidization phenomenon when the residual carbon is present, on the contrary, the decarburized coarse slag does not appear, which indicates that the residual carbon has a certain influence on the surface tension of the slag during the cooling process.

3. (3)

   Whether the coarse slag or the decarburized coarse slag, the crystals content obtained under quenching condition is obviously lower than that under natural cooling condition.

These findings contribute to our understanding of the effects of residual carbon on slag properties and the importance of residual carbon on crystallization and solidification behavior of gasification slag under different cooling conditions.