Study on the Occurrence Morphology and Combustion Characteristics of Carbon and Ash in Coal Gasification Coarse Slag with Different Granularity

Abstract

To investigate the morphological occurrence of carbon and ash in coal gasification coarse slag (CS) and their impacts on resource utilization, this study focused on Jinhua CS after grinding for 40 s. Samples with progressively decreasing particle sizes, ranging from CS60 (coarse fraction) to CS200 (fine fraction), were obtained through sieving. Combined with proximate analysis, LOI, XRF, FTIR, XRD, SEM-EDS, Raman, BET, and TG-DTG, the physicochemical properties of the samples were systematically analyzed. Results indicated that the particle size distribution was predominantly composed of intermediate fractions (CS60–200), with a total yield exceeding 90%. As the particle size decreased, the ash content reduced from 95.79% (CS60) to 82.72% (CS200), while the fixed carbon content correspondingly increased from 2.29 to 14.71%, illustrating the distribution pattern of ″residual carbon enrichment in fine fractions and inorganic mineral dominance in coarse fractions.″ CS200 exhibited a greater diversity of crystalline phases, with carbon and ash primarily existing as ″ash-encapsulated carbon,″ ″carbon-encapsulated ash,″ and cross-linked structures. This fraction demonstrated a surface carbon content of 54.74%, predominantly comprising stable C–C/C–H bonds (68.00%), along with a high specific surface area (104.35 m²/g), well-developed mesopores, abundant carbon defect sites, and excellent combustion activity (DTG_max = −4.69%·min^–1), making it suitable for preparing adsorbent materials or energy recovery as fuel. Conversely, CS60 was characterized by discrete and ″carbon-encapsulated ash″ distributions of carbon and ash, enriched with inorganic elements such as O, Si, and Al, exhibiting low combustion activity, and could be utilized for the preparation of geopolymers, construction aggregates, etc.

1. Introduction

Coal gasification technology represents a pivotal method for the clean and efficient utilization of coal, enabling the conversion of coal to high-value-added products such as syngas and fuel gas. This technology is extensively applied in critical areas, including power generation and chemical synthesis, providing essential support for the low-carbon utilization of coal resources. However, the process generates a significant amount of solid waste, specifically coal gasification coarse slag (CS), characterized by its complex composition. CS primarily consists of incompletely gasified residual carbon (a combustible phase) and aluminosilicate minerals (inorganic ash). , The direct stockpiling or inefficient disposal practices of CS not only occupy valuable land resources but also pose risks of soil and water pollution from heavy metal leaching and the leaching of soluble salts. This issue has become a major bottleneck constraining the large-scale deployment of coal gasification technology. Consequently, enhancing the resource utilization of coal gasification coarse slag has emerged as a central research focus aimed at reducing the waste volume and increasing efficiency, contributing to the green development of the coal chemical industry.

Elucidating the physicochemical properties of coarse slag and revealing the occurrence characteristics of its carbon and ash components are prerequisites to enable its high-value utilization. The structure and composition characteristics of coal gasification coarse slag are affected by multiple factors, including coal type, gasifier type, reaction temperature, and pressure. , Among these factors, particle size, as a key parameter influencing the properties of coarse slag, has attracted extensive attention owing to its inherent correlation with component distribution and performance. Zheng et al. demonstrated that coal particle size significantly affects the mechanical properties, pore structure, and permeability of coal-based materials, modulating the utilization efficiency of the materials. Chen et al. highlighted that the particle size of coal gasification slag is a primary determinant of the compressive strength of geopolymers, and the finer the slag particles, the higher the compressive strength of the resulting geopolymers. Miao et al. investigated coal gasification slag and found that samples with different particle sizes exhibit distinct crystalline structures, and the crystalline order of carbon exhibits a specific trend with particle size variation. Pan et al. further revealed that major elements such as Si, Al, and Ca are preferentially enriched in coarse slag with larger particle sizes, while elements such as Fe, S, and P are mostly distributed in fine-grained components. Li et al. investigated the composition, structure characteristics, and thermochemical properties of gasification slag with different particle sizes and found that particle size sieving facilitates the preliminary separation of ash and residual carbon. Moreover, as the particle size of the gasification slag increases, the ash content rises while the fixed carbon content declines correspondingly. Both the overall weight loss rate and the maximum weight loss rate decrease, indicating a marked improvement in thermal stability. These studies collectively indicate that during the gasification process, there are significant differences in the heating and cracking modes, volatile removal paths, and char-slag interactions of particles with different sizes, which ultimately lead to the differentiation of the structure and reaction characteristics of coarse slag with particle size. ,

Although existing studies have preliminarily confirmed the effect of particle size on the properties of coarse slag, there are still notable limitations. First, systematic characterization is lacking regarding the occurrence states of residual carbon and ash in coarse slag (such as interface bonding state and mutual encapsulation relationship) and their correlation with particle size. Second, the regulatory mechanism of carbon–ash interaction on the physicochemical properties of coarse slag (such as combustion reactivity and adsorption purification potential) remains poorly understood, which makes it difficult to support the precision and differentiated utilization of coarse slag.

Unlike previous studies that mainly focused on elemental distribution or combustion characteristics, the present work systematically elucidates the occurrence morphology of carbon and ash and establishes the relationship between the occurrence modes and resource utilization potential. On the basis of this, this study focuses on coal gasification coarse slag from a Jinhua furnace, following 40 s of grinding. Different particle-size samples (CS60 to CS200) were obtained through sieving. Combined with multidimensional characterization methods, the influence patterns of particle size on the occurrence forms, composition, structure, and physicochemical properties of carbon and ash in coarse slag were systematically investigated, aiming to provide a theoretical basis for the graded and targeted utilization of coarse slag.

2. Experiments

2.1. Sample Source

The gasification process flow diagram of the Jinhua furnace is shown in Figure . Coal slurry and oxygen from the air separation system are fed into the water-cooled wall gasifier through the process burner and undergo reactions in the gasification combustion chamber to produce syngas. The crude syngas and molten slag flow downward, with heat recovered in the radiant waste heat boiler chamber prior to cooling, and enter the scrubbing and purification system through the syngas pipeline. The cooled molten slag is discharged into the slag pool via the slag discharge system, yielding coal gasification coarse slag (CS).

1.

Gasification process flow diagram of Jinhua furnace.

After drying in a fume hood for 24 h, 100 g of CS samples were weighed. Each sample was crushed with an FW-100 crusher for 30 s, homogenized by shaking, cooled, and ground for an additional 10 s. This procedure was optimized via preliminary experiments to obtain well-dispersed and homogeneous particles and to prevent overgrinding-induced changes to the morphology and structure of the residual carbon and mineral phases. Graded sieving was performed using standard sieves of 60, 80, 100, and 200 mesh to isolate gasification coarse slag fractions of varying particle sizes. The sieving outcomes are summarized in Table , and the resulting samples were sealed and stored for subsequent analyses.

1. Visual Characteristics of Samples within Five Different Particle Size Ranges.

2.2. Analytical Methods

2.2.1. Proximate Analysis and Loss on Ignition

According to GB/T212-2008, a sample mass of 1 g was used for proximate analysis to determine the moisture, ash, and volatile matter in the residue samples, and the fixed carbon content was calculated. Ultimate analysis was performed using an Elementar Vario EL cube elemental analyzer (Elementar UNICUBE, Germany) according to GB/T 476-2008.

2.2.2. Composition and Crystal Structure Analysis

The crystal structure of the samples was characterized by an X-ray diffractometer (XRD, Bruker D8 Advance, Germany). The ash composition of the sample was analyzed by an X-ray fluorescence spectrometer (XRF, ZSX Primus III+, Japan), and the test method and mode were the tablet method and oxide method, respectively. Functional group structure types were analyzed by using infrared spectroscopy (FTIR, Thermo Scientific Nicolet iS20). The degree of graphitization of the carbon was characterized by a Raman spectrometer (Horiba LabRAM HR Evolution, Japan).

2.2.3. Surface Morphology Analysis

Scanning electron microscopy-energy spectrometry (SEM-EDS, ZEISSigma 300, Germany) was used to analyze the surface morphology and elemental distributions of the samples. The composition and chemical valence of the surface elements were determined by X-ray photoelectron spectroscopy (Thermo Scientific K-α). The pore structures of the particles were analyzed by an automatic specific surface area and porosity analyzer (BET, Micromeritics ASAP 2460).

2.2.4. Thermogravimetric Analysis

Pyrolysis and combustion behaviors were evaluated using a SDT Q600 thermogravimetric analyzer. Samples were heated from ambient temperature to 1000 °C at 10 °C/min under a carrier gas flow of high-purity nitrogen or air. The test sample mass was 8–10 mg, an alumina crucible was applied, and the gas flow rate was maintained at 50 mL/min. To facilitate comparison of combustion performance, the comprehensive combustion index (S) was calculated as follows:

$$S=({{\mathrm{DTG}}_{\max }}^{*}{\mathrm{DTG}}_{\mathrm{mean}})/{T_{\mathrm{i}}}^{2*}{T}_{\mathrm{f}}$$

where DTG_max denotes the maximum mass loss rate, DTG_mean represents the average mass loss rate, T _i is the ignition temperature, and T _f is the burnout temperature.

3. Results and Discussion

3.1. Particle Size Distribution Characteristics of CS

As illustrated in Figure a, the particle size distribution range of the CS sample is from CS60 (>0.425 mm) to CS200 (<0.075 mm), and there are significant differences in yield and loss on ignition (LOI) among different particle size fractions. Among them, the CS100–200 fraction has the highest yield, reaching 45.7%, which is the dominant constituent of the sample. The yields of CS60–80 and CS80–100 fractions follow, accounting for 23.37 and 23.7%, respectively, with the combined proportion of these three fractions exceeding 90%. In contrast, the yields of CS60 and CS200 fractions are relatively low, at 1.76 and 5.46%, respectively. This indicates that the content of fine-grained (CS60) and coarse-grained (CS200) fractions in the sample is small, and the intermediate particle size (CS60–200) dominates. Except for the CS100–200 fraction, the LOI of the sample gradually increases with the decrease of particle size, indicating that volatile or decomposable components are enriched in the fine-grained fractions.

2.

Particle size distribution characteristics of CS.

Combined with the data in Table and Figure b, the overall CS sample exhibits the characteristics of high ash content (A_ad = 93.21%) and low fixed carbon content (FC_ad = 4.86%), and the variations among different particle size fractions are significant. With the decrease of particle size, the ash content generally decreases, and the fixed carbon content increases correspondingly, showing an obvious negative correlation between the two. Specifically, the coarse-grained CS60 exhibited the highest ash content of 95.79% and the lowest fixed carbon content (2.29%), indicating that this fraction has a very high proportion of inorganic minerals and a few combustible components. When the particle size decreases to the CS80–100 range, the ash content drops to 87.81%, and the fixed carbon content surges to 9.58%, approximately 3.2-fold higher relative to the CS60 fraction. The fixed carbon content (6.45%) of the CS100–200 fraction did not increase significantly, likely because of insufficient comminution. At this stage, particles primarily consisted of mechanical mixtures of inorganic minerals and residual carbon with incomplete component separation. Further reduction of the CS200 fraction resulted in a fixed carbon content of 14.71% and an ash content of 82.72%. This fixed carbon level was approximately 5.4-fold higher than that of CS60 and 2.0-fold higher than the parent CS, confirming significant enrichment of combustible components (fixed carbon) in fine-grained fractions.

2. Proximate Analysis of CS and Size-Fractionated CS.

	proximate analysis (ad, %)
sample	M	A	VM	FC
CS	0	93.21	1.93	4.86
CS60	0.23	95.79	1.69	2.29
CS60–80	0.33	90.68	2.43	6.56
CS80–100	0.61	87.81	1.80	9.58
CS100–200	0.38	91.13	2.04	6.45
CS200	0.57	82.72	2.00	14.71

3.2. Compositional and Structural Characteristics

3.2.1. Crystalline and Amorphous Structures

The ash composition was characterized by XRF, with results summarized in Table . Analysis reveals that the ash of all fractions is predominantly composed of SiO₂ (51.00–57.38%), Al₂O₃ (17.56–18.61%), Fe₂O₃ (9.91–15.30%), and CaO (6.26–7.59%), with the cumulative content of these four oxides exceeding 90% and with fluctuations confined to within ± 6.5%. The combined content of SiO₂+Al₂O₃+Fe₂O₃ surpasses 85%, while minor components (e.g., K₂O, MgO) exhibit similar levels across fractions, conforming to the criteria for low-calcium fly ash (Class F ash) and rendering the material suitable for building materials and construction applications. , In addition, the total proportion of SiO₂+Al₂O₃ exceeds 68%, with a high Si/Al ratio, making it a potential raw material for the preparation of aluminosilicate molecular sieves. Regarding the content of alkaline and alkaline earth metals (AAEM), the CS60–80 fraction displays the highest AAEM value (∼16.07%), whereas CS200 exhibits the lowest (∼12.96%), with a difference of ∼3.11%. These observations highlight discernible qualitative and quantitative variations in ash composition among size fractions.

3. Ash Composition Analysis of CS and Size-Fractionated CS (%) .

sample	SiO2	Al2O3	Fe2O3	CaO	K2O	MgO	Na2O	other	SiO2+Al2O3	AAEM
CS	57.38	18.61	9.91	6.71	3.03	1.91	0.72	1.73	75.99	13.76
CS60	53.08	17.93	13.58	7.32	2.75	2.15	0.71	2.48	71.01	15.18
CS60–80	51.00	17.56	15.30	7.59	2.72	2.05	0.71	2.35	68.56	16.07
CS80–100	55.83	18.49	11.10	6.65	2.90	1.93	0.71	2.39	74.32	13.80
CS100–200	52.40	17.67	13.97	7.40	2.89	1.96	0.68	2.63	70.07	15.66
CS200	53.89	18.25	12.86	6.26	2.41	2.09	0.68	2.87	72.14	12.96

^aNote: AAEM = Ash×(CaO+Na₂O+K₂O)/(SiO₂+Al₂O₃).

The XRD patterns of parent CS and size-fractionated CS are presented in Figure . All samples exhibit a dominant amorphous phase, alongside minor crystalline phases. The amorphous phase primarily consists of aluminosilicate glass and amorphous carbon, indicating that the inorganic minerals in coal have formed stable mineral aggregates dominated by aluminosilicates after high-temperature oxidation, melting, and rapid cooling condensation. The crystalline phases are identified as quartz (SiO₂) and calcite (CaCO₃), which are attributed to incomplete melting of crystalline phases during gasification or crystallization upon rapid cooling. Notably, the diffraction peaks assigned to quartz and calcite are most pronounced in CS200, and this trend diminishes progressively with an increase in particle size, demonstrating that finer slag fractions possess more abundant crystalline structures.

3.

XRD patterns of CS and size-fractionated CS.

3.2.2. Micromorphology

During coal gasification, inorganic minerals in coal typically aggregate into spherical or irregular blocky particles with smooth surfaces upon melting and cooling, driven by turbulence and surface tension, whereas incompletely gasified residual char predominantly exhibits a dispersed flocculent morphology. SEM images of parent CS and size-fractionated CS are presented in Figure . Three distinct characteristic particle morphologies are observed: dense/smooth irregular blocky/flaky particles (type A), porous irregular flocculent particles (type B), and dense/smooth spherical particles (type C). The surface micromorphologies of the size-fractionated CS samples varied markedly, generally exhibiting intermixed, adhered, or encapsulated states. The CS60 fraction (Figure b1–3) predominantly comprised large-sized type A and C particles, indicating high inorganic ash content. Pores within type B particles were filled with spherical type C particles of varying sizes, reflecting ash embedded in residual char pores. With decreasing particle size, the size and abundance of type A and C particles progressively diminished, while the abundance of type B particles increased alongside more developed pore structures. This trend reflects decreasing inorganic mineral content and increasing residual char content, aligning with the proximate analysis result of an increasing fixed carbon content at smaller particle sizes. The CS100–200 (Figure e1–3) and CS200 fractions (Figure f1–3) primarily consisted of type B and C particles. Type B particles formed a spider-web-like porous skeleton with multiscale pores, displaying morphologies such as flower-like, rod-like, and needle-like structures. Numerous type C particles agglomerated into microspheres, interspersed, embedded, or attached to the type B surface/skeleton, forming a sintered body with an enamel luster and abundant microbeads. This reflects ash-encapsulated and cross-linked residual carbon, further explaining the relatively lower fixed carbon content in the CS100–200 fraction.

4.

Apparent morphology of CS and size-fractionated CS: CS (a1–3), CS60 (b1–3), CS60–80 (c1–3), CS80–100 (d1–3), CS100–200 (e1–3), and CS200 (f1–3).

To clarify the elemental composition patterns and distribution of typical particles across size fractions, EDS analysis was conducted on characteristic particles (types A, B, and C) from each size-fractionated CS sample, with results shown in Figure . In randomly selected regions, the elemental composition patterns of the porous type B matrix across size fractions were analogous, predominantly comprising C with minor O, indicating a carbonaceous skeleton core bearing minor oxygen-containing functional groups or adsorbed oxides on the surface. EDS results for type A (blocky/flaky) and type C (spherical) particles revealed complex yet highly analogous elemental compositions, both predominantly comprising C, O, Na, Mg, Al, Si, Ca, and Fe. Al, Si, and O were most abundant (totaling >70%), alongside minor alkali metals (Na, K), alkaline earth metals (Ca, Mg), and transition metals (Fe).

5.

Mapping diagrams of CS and size-fractionated CS.

Combined with the XRF and XRD results, we infer that both particle types are amorphous aluminosilicate inorganic phases. They formed as inorganic minerals in coal that melted and decomposed under high-temperature gasification, disrupting their original crystal structures, and rapidly solidified upon cooling to form amorphous glass. Spherical type C particles originated from surface tension-driven shrinkage of molten inorganic substances, whereas irregularly shaped type A particles likely adopted their forms due to uneven cooling rates and particle collisions/aggregations. Elements like Fe and Ca likely originated from residues of original iron-bearing minerals (e.g., hematite, pyrite) and calcium-containing minerals (e.g., calcite, gypsum) in coal or their transformation products. These elements may serve as active sites for catalytic reactions.

Elemental distribution profiles of random surfaces are shown in Figure a–f, with integrated data in Figure g–i. Analysis revealed that the CS60 fraction had the lowest surface C content but the highest O and Ti contents; the CS60–80 fraction exhibited the highest surface K, Ca, and Fe contents (potentially providing catalytically active sites); and the CS200 fraction displayed the highest surface C content. Overall, the surface C content exhibited an increasing trend with decreasing particle size, whereas the O, Al, and Si contents displayed decreasing trends. This pattern aligns with the results from other analyses.

6.

EDS Elemental Composition Diagrams of CS (a), size-fractionated CS (b-f) and integrated data (g-i).

XPS full-spectrum analysis (Figure ) revealed that surfaces of parent CS and size-fractionated CS were predominantly composed of C, O, Si, and Al, with content distributions (Table ) aligning with prior analyses. Specifically, the surface C content increased with decreasing particle size: the CS80–100 and CS200 fractions had elevated C contents of 55.80 and 54.74%, respectively, whereas the coarse CS60 fraction had a lower C content (32.82%). Conversely, the O, Si, and Al contents progressively diminished with decreasing particle size. CS60 exhibited the highest O content (40.47%) and notably higher Si (8.68%) and Al (3.81%) contents than those of fine-grained samples. This further confirmed that residual char was enriched on fine-grained fraction surfaces, whereas inorganic minerals (e.g., aluminosilicates) dominated coarse-grained fraction surfaces. Notably, the surface C contents across all samples ranged from 32.82 to 55.80%. Combined with micromorphology, this implies a ″carbon-encapsulated ash″ structural feature in CS samples.

7.

XPS full spectra of CS and size-fractionated CS.

4. Surface Elemental Analysis of CS and Size-Fractionated CS.

sample	C 1s	O 1s	Si 2p	Al 2p	Fe 2p	Ca 2p	other
CS	52.02	32.67	8.08	3.69	0.53	1.56	1.45
CS60	32.82	40.47	8.68	3.81	1.71	1.63	10.88
CS60–80	36.52	37.30	8.58	3.71	1.31	1.54	11.04
CS80–100	55.80	26.62	6.40	2.81	0.79	1.04	6.54
CS100–200	45.44	30.67	7.89	3.35	1.06	1.17	10.42
CS200	54.74	25.23	6.19	2.81	0.73	0.99	9.31

Surface C 1s and O 1s spectra and their fitting curves (Figure , Table ) revealed pronounced variations in the chemical states of C and O across size fractions. C primarily existed as stable C–C/C–H bonds (284.8 eV), a characteristic signature of residual char reflecting aromatic/aliphatic carbon chain features of unburned carbon. The fine-grained CS200 had the highest C–C/C–H proportion (68.00%), indicating greater chemical stability of its residual char. The medium-coarse CS60–80 fraction exhibited a relatively high C–O–C bond proportion (34.19% at 286 eV), suggesting that its residual char surface likely contained elevated oxygen-containing functional groups (e.g., ether and epoxy). Additionally, the O–CO bond proportion (288.5 eV) was generally low (10.95–18.36%), indicating a sparse distribution of strong oxygen-containing groups (e.g., carboxyl and ester) on residual char surfaces.

8.

XPS C 1s and O 1s spectra and their fitting curves of CS and size-fractionated CS.

5. Distribution Forms of C and O in CS and Size-Fractionated CS.

	C 1s			O 1s
sample	C–C (284.8 eV)	C–O–C (286 eV)	O–CO (288.5 eV)	metal oxides (530 eV)	Al2O3 (532.4 eV)	SiO2 (533.1 eV)	organic O (534.1 eV)
CS	55.66	25.97	18.36	0.87	3.08	55.17	40.88
CS60	64.27	23.52	12.21	6.96	31.18	42.07	19.80
CS60–80	51.90	34.19	13.91	2.60	25.38	37.32	34.70
CS80–100	54.80	34.25	10.95	3.24	29.77	37.81	29.18
CS100–200	56.08	29.04	14.88	3.16	21.46	45.41	29.96
CS200	68.00	17.31	14.69	2.34	15.96	52.65	29.05

The chemical state of O correlated closely with the inorganic mineral composition and residual char oxygen-containing groups. SiO₂ (533.1 eV) was the predominant O form (37.32–55.17%), confirming the dominance of aluminosilicate inorganic phases. In the coarse CS60 fraction, proportions of metal oxides (530 eV) and Al₂O₃ (532.4 eV) were significantly higher (6.96 and 31.18%), reflecting Al/Fe oxide enrichment. In fine-grained fractions (e.g., CS200), the organic oxygen proportion (534.1 eV) reached 29.05%. Coupled with its high C–C/C–H proportion, this indicated that oxygen-containing groups (e.g., hydroxyl and ether) on residual char surfaces were primarily associated with the organic carbon skeleton rather than inorganic minerals.

3.2.3. Carbon Structure

Figure a presents the Raman spectra of CS and its size-fractionated samples. The G band corresponds to the stretching vibration mode of the ideal graphite lattice, whereas the D band is generally associated with structural disorder and defects in carbon materials, including edge sites, lattice distortions, and amorphous carbon domains. , Except for the CS100–200 fraction, the Raman peak intensity gradually increased with decreasing particle size, which is generally consistent with the enrichment trend of the fixed carbon.

9.

Raman Spectra (a), Fitting Peak Data of CS and size-fractionated CS­(b-g) and the calculated Raman peak ratios­(h, i).

Peak fitting was performed using PeakFit 4.0 software, and the fitted spectra are presented in Figure b–g. The corresponding fitting parameters are summarized in Table , and the calculated Raman peak ratios are shown in Figure h,i. All ID₁/IG ratios exceeded 1 (1.27–1.35), indicating that the residual carbon in all particle size fractions predominantly consisted of highly disordered carbon structures formed during the rapid gasification and quenching processes. This disordered carbon is generally characterized by abundant edge carbon atoms, lattice distortions, vacancies, and ungraphitized sp² carbon clusters, reflecting the incomplete structural ordering of the residual char.

6. Fitting Peak Ratios of Raman Spectra.

		ratio
band	center (cm–1)	CS	CS60	CS60–80	CS80–100	CS100–200	CS200
G	1580	29.46	28.84	27.90	27.81	27.97	28.64
D1	1350	37.99	36.59	36.53	36.80	36.43	38.55
D2	1620	12.96	14.19	16.63	14.97	14.35	12.75
D3	1500	11.90	13.54	13.37	13.36	12.61	12.96
D4	1200	7.69	6.84	5.57	7.05	8.64	7.10
ID1/IG	/	1.29	1.27	1.31	1.32	1.30	1.35
ID1/D1+D3+D4	/	0.66	0.64	0.66	0.64	0.63	0.66
ID3/ID1	/	0.31	0.37	0.37	0.36	0.35	0.34

CS200 exhibited the highest ID₁/IG (1.35) and ID₁/(D₁+ D₃+ D₄) (0.66) values, indicating that the finest fraction contained the largest proportion of defect-rich amorphous carbon. The abundant structural defects provide active sites for oxygen adsorption and oxidation reactions, resulting in enhanced combustion reactivity, which is consistent with the TG-DTG results (Section ). In addition, the defect-rich carbon framework, together with the well-developed pore structure revealed by BET analysis, is favorable for adsorption and surface reactions, suggesting that CS200 has promising potential for the preparation of adsorbents, catalyst supports, and other carbon-based functional materials. In contrast, CS60 exhibited a lower ID₃/ID₁ value (0.31), indicating a relatively lower degree of structural disorder than that of the finer fractions.

3.2.4. Pore Structure

To further evaluate the adsorption performance and potential application value of CS with different particle sizes, the pore structure was systematically characterized by nitrogen adsorption–desorption isotherms (Figure ) and pore size distribution curves (Figure a). The specific surface area and pore structure parameters are summarized in Table . As shown in Figure , the adsorption–desorption isotherms of size-fractionated CS are largely consistent in shape, exhibiting a characteristic inverted S shape. According to the IUPAC classification, it can be classified as a type IV isotherm with a typical type H2 hysteresis loop. This characteristic indicates that all samples have a well-developed mesoporous structure, and the pore type is mainly slit pores or ink-bottle pores.

10.

Adsorption–desorption curves of CS with different particle sizes.

11.

Pore size distribution curves (a), specific surface area (b), and pore volume (c) of CS and size-fractionated CS.

7. Specific Surface Area and Pore Structure Parameters of CS and Size-Fractionated CS.

sample	SBET (m2/g)	V T (cm3/g)	D p (nm)	Smic (m2/g)	V mic (cm3/g)	V mic/V T (%)
CS	34.2585	0.0274	3.2047	22.7473	0.0100	36.50
CS60	13.7553	0.0142	4.1427	12.6210	0.0106	74.65
CS60–80	34.3697	0.0270	3.1373	33.2431	0.0233	86.30
CS80–100	57.3398	0.0454	3.1659	55.1351	0.0383	84.36
CS100–200	44.8576	0.0338	3.0124	43.3741	0.0290	85.80
CS200	104.3485	0.0863	3.3090	101.0781	0.0757	87.72

Further analysis of Figure a reveals that pore sizes in all samples are primarily concentrated within the 3–5 nm range, confirming that the internal pore system is dominated by mesopores. The complex and diverse pore architecture potentially provides ample channels for mass transfer during adsorption and catalytic processes. Combined with the quantitative analysis results in Table and Figure b,c, except for the CS100–200 fraction, the S_BET, S_mic, and V _T of other fractions exhibited a significant increasing trend with the decrease of particle size. Among these, CS200 exhibited significantly higher values, all exceeding those of other samples. This observation may be attributed to the higher specific surface area of fine particles, which exhibit enhanced adsorption of tar and hydrocarbon intermediates during gasification. This facilitates the deposition of amorphous carbon on their surfaces and within pores, thereby enriching the pore structure. The observed reduction in pore parameters for the CS100–200 fraction may relate to the cross-linked distribution of residual carbon and ash within this fraction. Notably, the micropore-to-total pore volume ratio (V _mic/V _T) remained stable within the 84–88% range across all fractions, indicating that micropores are the dominant component of the pore system. Mesopores (3–5 nm) serve as auxiliary structures to improve pore interconnectivity, collectively forming a hierarchical pore structure with synergistic micropore–mesopore interactions. Such a structure significantly enhances the material’s adsorption capacity and mass transfer efficiency.

3.2.5. Occurrence Modes and Distribution Patterns of Residual Carbon and Ash

The compositional and structural characteristics of CS and size-fractionated CS were systematically explored through multidimensional characterization. The analysis revealed that carbon and ash mainly coexist in four modes: discrete, ″ash-encapsulated carbon″, ″carbon-encapsulated ash”, and cross-linked structure distribution (Figure a–d). These four occurrence modes coexist in all size-fractionated CS samples, exhibiting notable variations across the fractions. Coarse-grained fractions (e.g., CS60) predominantly exhibit discrete distribution and ″carbon-encapsulated ash”, whereas fine-grained fractions (e.g., CS200) mainly display ″ash-encapsulated carbon”, ″carbon-encapsulated ash”, and cross-linked distribution. This variation stems from differences in heating intensity, reaction extent, and phase-transition behaviors of particles with distinct sizes during gasification. These findings provide structural-level theoretical support for the graded utilization and precision application of size-fractionated CSs.

12.

Four main occurrence modes of carbon and ash: (a) discrete structure, (b) ash-encapsulated carbon structure, (c) carbon-encapsulated ash structure, and (d) cross-linked structure.

3.3. Thermal Conversion Performance

3.3.1. Pyrolysis Characteristics

Figure a presents the TG curves of CS and size-fractionated CS under a nitrogen atmosphere. The weight loss percentages are 2.29, 0.88, 1.15, 1.89, 1.61, and 1.97%, respectively. Except for CS100–200, the weight loss percentage gradually increases with the decrease of particle size, which is consistent with the proximate analysis results. The DTG (derivative thermogravimetric) curve in Figure b reveals three distinct weight loss peaks for all size fractions. The first stage (30–300 °C) is attributed to the evaporation of moisture in the coarse slag and the desorption of low molecular weight and low boiling point organic components in the syngas. The second stage (500–700 °C) is mainly caused by the cracking of organic macromolecules and the devolatilization of organics. The peak temperatures of CS and CS200 are the highest, which may stem from their high ash content and crystalline phase structure, which hinder the release of organics. The third stage (850–1000 °C) may be caused by the decomposition of minerals in the coarse slag. For example, there are many carbonates in the ash, which decompose to generate CO₂ after 900 °C.

13.

TG-DTG curves of CS and size-fractionated CS under an inert atmosphere (a, b).

3.3.2. Combustion Characteristics

Figure a shows the TG curves of parent CS and size-fractionated CS under air, with weight loss percentages of 16.38, 1.29, 7.15, 11.01, 8.64, and 17.07%, respectively. Except for CS100–200, the weight loss percentage gradually increases with the decrease of particle size, which is consistent with the proximate analysis results. Figure b reveals that the primary weight loss occurs in the range of 500–700 °C. During this stage, macromolecules decompose intensely, the structure begins to break down, volatile matter is released, and fixed carbon (coke) undergoes violent high-temperature oxidation with O₂ to generate CO₂, causing significant weight loss.

14.

(a) TG; (b) DTG; and (c–h) tangent curves of CS and size-fractionated CS under an air atmosphere.

To compare combustion performance, ignition temperature (T _i) and burnout temperature (T _f) were determined via the tangent method (Figure c–h), and key combustion characteristic indices are summarized in Table . Parent CS exhibited the highest T _i (570 °C) and comprehensive combustion index (S = 1.12 × 10^–8 %²·min^–2·°C^–3), likely due to its wide particle size distribution. Coarse particles provide thermal stability, while fine particles offer high reactivity, collectively creating a ″gradient combustion″ effect that mitigates limitations of single-size fractions.

8. Combustion Characteristic Indexes of CS and Size-Fractionated CS.

sample	T i (°C)	T f (°C)	T max (°C)	DTGmax (%·min–1)	DTGmean (%·min–1)	S (%2·min–2·°C–3)
CS	570	667	627	–1.82	–1.33	1.12 × 10–8
CS60	536	690	630	–0.27	–0.14	7.90 × 10–10
CS60–80	539	657	632	–1.47	–0.93	2.36 × 10–9
CS80–100	542	664	614	–2.29	–1.46	2.64 × 10–9
CS100–200	550	662	628	–1.92	–1.25	2.01 × 10–9
CS200	550	643	614	–4.69	–2.96	1.93 × 10–9

Ti increased gradually with decreasing particle size, suggesting that ignition behavior was not solely controlled by carbon content and specific surface area but was also affected by mineral composition and carbon–mineral interactions. As revealed by XRD analysis, CS200 exhibited more pronounced diffraction peaks of quartz and calcite, indicating an increased proportion of crystalline mineral phases in the fine fraction. These mineral components may influence heat transfer and oxygen accessibility during the initial oxidation stage, thereby partially offsetting the positive effects of higher carbon content and specific surface area on ignition. CS60 showed a higher T _f (732 °C), lower DTG_max, and lower S value, indicating a slow, prolonged combustion process. The S values of the remaining four samples were comparable. Notably, CS200 had the highest DTG_max (4.69%·min^–1), likely due to its high specific surface area, well-developed pores, and abundant amorphous carbon, as confirmed by BET and Raman analyses. These structural features facilitate oxygen diffusion and provide more active sites for oxidation reactions, leading to accelerated combustion after ignition. Although CS200 exhibited a slightly higher ignition temperature than CS60, it demonstrated significantly enhanced combustion kinetics once ignition occurred. Therefore, the combustion behavior of size-fractionated CS was determined by the combined effects of carbon content, pore structure, carbon defects, and mineral composition rather than by a single factor.

4. Conclusion

In this study, coarse coal gasification slag (CS) from a Jinhua gasifier was selected as the subject. A series of size-fractionated samples (CS60–200) were obtained via grinding and screening. Through proximate analysis, XRF, SEM, XPS, BET, and TG-DTG analysis, the effects of particle size on the occurrence modes, compositional structure, and combustion characteristics of residual carbon and ash in CS were systematically investigated, and the resource utilization potential of size-fractionated CS was assessed.

• (1)The majority of CS was of intermediate size (CS60–200), with a total yield exceeding 90%. As the particle size decreased, the ash content decreased, and the fixed carbon increased, showing a significant negative correlation. Coarse-grained CS60 had an ash content as high as 95.79% and was dominated by inorganic minerals. Fine-grained CS200 exhibited a fixed carbon content of 14.71%, approximately 5.4 times that of CS60 and 2.0 times that of the parent CS, demonstrating outstanding calorific value potential.
• (2)Significant differences were observed in the composition and structure of coarse slag with different particle sizes: in terms of chemical composition, the ash of each particle size is mainly composed of SiO₂, Al₂O₃, Fe₂O₃, and CaO (the total proportion is >90%), which meets the standard of low-calcium fly ash (grade F ash), with a stable Si/Al ratio, and has the basic conditions for building materials or molecular sieve raw materials. In terms of phase structure, the coarse slag is mainly amorphous aluminosilicate vitreous, containing a small amount of quartz and calcite crystal phase, and the fine-grained crystal phase is richer, and the diffraction peak is sharper; in terms of micromorphology and occurrence form of carbon and ash, coarse-grained (CS60) to dense blocky/spherical inorganic particles are mainly enriched with O, Si, Al, and other inorganic elements, and carbon and ash are mainly distributed in discrete distribution and ″carbon-encapsulated ash″ distribution; the results show that the fine-grained (CS200) is composed of porous flocculent residual carbon as the skeleton, with a surface C content of 54.74%, and mainly stable C–C/C–H bonds. The carbon and ash are mainly distributed in ″ash-coated carbon″, ″carbon-encapsulated ash”, and cross-linked structure, with strong chemical stability and rich defect sites; in terms of pore structure, the specific surface area of fine-grained is as high as 104.35 m²/g, showing a multilevel pore structure of ″micropore–mesopore″ coordination, while the coarse-grained pores are poorly developed.
• (3)Fine-grained fractions, with abundant residual char, well-developed pores, and excellent combustion activity (DTG_max = −4.69%·min^–1), are suitable for preparing adsorbents or recovering energy as fuel. Coarse-grained fractions, owing to their high inorganic mineral content, stable structure, and low combustion reactivity, can meet building material performance requirements and be utilized to produce geopolymers, construction aggregates, etc.

The authors declare no competing financial interest.