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Optimizing coal water slurry concentration via synergistic coal blending and particle size distribution

  • Ming Liu EMAIL logo and Hanxu Li
Published/Copyright: March 11, 2025
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Green Processing and Synthesis
From the journal Green Processing and Synthesis Volume 14 Issue 1

Abstract

This study explores strategies to enhance coal utilization efficiency, enable on-site coal conversion, transition thermal coal into raw coal, and improve the energy efficiency of coal water slurry (CWS) gasification. By optimizing coal blending and particle size distribution, CWS concentration can be significantly increased. The results demonstrate that, under the constraints of liquid slag discharge technology, a maximum blending ratio of 10% high-rank Panji coal (CP coal) achieves a slurry concentration of 60.5%. This process elevates the coal ash flow temperature by 90°C and narrows the gasification operating range by 44°C, attributed to a notable increase in the relative content of CaAl2Si2O8. Further improvements are achieved by blending finely ground CP coal (CP*) with low-rank Shenhua coal (SH coal) at a 9:1 ratio and increasing CWS additive concentration to 3.0‰, resulting in a slurry concentration of 63.5%. A comprehensive technical and economic evaluation identifies the optimal configuration as incorporating 10% CP* and 0.2% CWS additives into SH coal. Compared to the SH CWS gasification process, Scheme D enhances the effective gas composition and flow rate, reduces costs by 4.19%, and significantly lowers CO2 emissions. The proposed approach offers substantial economic and environmental benefits, supporting the development of clean coal technology.

Keywords: coal water slurry; coal blending; particle size optimization

1 Introduction

The global environment is undergoing significant deterioration, driven by the continued reliance on traditional fossil fuels. In 2020, China announced its commitment to achieving “peak carbon emissions” and “carbon neutrality,” underscoring the urgency of carbon reduction initiatives. Despite a gradual decline in coal consumption, coal remains the dominant energy source, accounting for 56% of total primary energy consumption, with less than 10% utilized as a chemical raw material. Predominantly used as power coal, its direct combustion contributes substantially to CO2 emissions. Alternatively, when utilized as a chemical raw material, coal undergoes gasification, producing synthesis gas (i.e., CO), which serves as a precursor for various chemical products. This highlights the critical role of clean coal technology in supporting carbon reduction goals. Among these technologies, coal water slurry (CWS) gasification stands out as a pivotal development and has seen increasing adoption in China [1]. However, the application of CWS is hindered by challenges such as low slurry concentration, which negatively impacts gasification energy efficiency. Strategies such as coal blending and particle size optimization have shown significant potential to improve coal utilization efficiency, addressing these limitations effectively.

Recently, studies demonstrated that coal blending significantly reduces the concentration of oxygen functional groups, enhances the adsorption capacity of coal surfaces for anionic additives, and improves the overall performance of CWS [2]. The inclusion of high-grade coal enhances the slurryability of coal that is otherwise difficult to slurry, resulting in a substantial reduction in viscosity, an increase in concentration by 3–5%, and notable improvements in the stability of CWS [3]. However, while increasing CWS concentration through coal blending, the effects of new coal types on coal ash properties must be carefully evaluated [4]. Variations in blending ratios can alter the ash composition, which in turn affects the flow temperature and viscosity of coal ash slag [5]. Consequently, as blending ratios are adjusted, it is essential to promptly modify the operating load and gasifier temperature to accommodate changes in ash characteristics and ensure optimal performance [6]. Furthermore, the incorporation of low-quality, cost-effective coal into gasification feedstock has emerged as an economically viable approach for reducing operational costs [7,8].

Improving stacking efficiency through particle size grading is a key strategy for increasing the concentration of CWS [9]. Coarse coal particles significantly contribute to higher slurry concentration, while fine particles are essential for improving fluidity and stability. A continuous particle size distribution that integrates coarse components can optimize the ratio of fine particles, enhancing fluidity, stability, and overall slurry performance [10,11]. Studies indicate that increasing the mass ratio of coarse coal particles or the stacking efficiency reduces the pseudoplasticity of the slurry. Furthermore, coal samples with a bimodal particle size distribution consistently outperform those with a monodisperse distribution in CWS performance. In scenarios involving a broad particle size distribution, fine particles effectively fill the interstitial spaces between coarse particles, leading to higher volume fractions [12,13]. This characteristic is particularly evident in CWS derived from bimodal distributions, which show marked performance advantages. For instance, while the coal rank of Australian coal is slightly higher than that of Shenhua coal, its physical and chemical properties (e.g., greater porosity, specific surface area, and lower O/C ratio) yield significantly superior CWS performance [14]. Higher rank coal generally exhibits higher volume fraction values, further emphasizing the effect of coal rank on slurry behavior.

The gasification of CWS relies on liquid slag discharge technology, which necessitates stringent coal quality requirements. Enhancing the concentration of CWS in large-scale applications through coal blending poses considerable challenges. While optimizing particle size distribution can significantly improve CWS concentration, the raw coal used for gasification is often low-rank and characterized by a low hardgrove grindability index (HGI). Although the grinding of fine particles enhances slurry concentration, it also leads to higher energy consumption and accelerated equipment wear, complicating the process further.

This study presents a method that integrates coal blending and particle size grading to enhance the concentration of CWS, leveraging insights from current research. The approach involves finely grinding local power coal, which exhibits high slurry concentration and easy grindability, while processing gasification raw coal, characterized by low slurry concentration and greater grinding difficulty, to standard particle size. By adhering to the requirements of liquid slag discharge technology, the combination of two coal samples with different particle size distributions creates a synergistic effect between coal blending and particle size optimization. This method not only transforms local thermal coal into a viable raw material but also facilitates the on-site conversion of diverse local coal types, advancing coal utilization efficiency.

2 Methods

2.1 Coal samples

Two coal samples of differing ranks were selected for the experiment (e.g., low-rank coal from Shenhua [SH coal] and high-rank coal from Panji [CP coal]). Table 1 summarizes the results of the coal quality analysis conducted on these samples.

Table 1

Proximate and ultimate analysis of coal samples

Samples Proximate analysis (%) Ultimate analysis (%) Q b,ad (MJ·kg−1) FT (°C) HGI
M ad A d V daf FCad C ad H ad N ad S t,ad
SH 11.05 9.71 38.84 49.13 66.1 3.92 0.89 0.51 25.89 1,166 57
CP 1.74 19.06 23.56 60.80 71.56 2.90 0.76 0.33 24.74 >1,500 68

Note: M ad is the moisture content in coal under air-dried conditions; A d is the ash content in the coal after the removal of moisture; V daf is the volatile matter calculated based on organic matter alone, after excluding moisture and ash; FCad is the fixed carbon content in the coal sample under air-dried conditions.

SH coal is distinguished by its high internal water content and volatile matter, along with a low ash content. In contrast, CP coal features low water and volatile matter content and a significantly higher ash and carbon content. The flow temperature of SH coal ash is 1,166°C, making it suitable for direct application in the CWS gasification process. Conversely, the flow temperature of CP coal ash exceeds 1,500°C, which limits its use as a blending material and precludes its standalone application in CWS gasification [15]. Additionally, the HGI of CP coal is higher than that of SH coal, indicating that CP coal is more amenable to grinding.

The coal sample was subjected to drying at a temperature of 105°C for a duration of 2 h. Subsequently, it was combined with KBr in a mass ratio of 1:100. The resulting mixture was ground and formed into thin pellets, which were promptly analyzed using a Fourier transform infrared spectrometer (FTIR, Thermo Scientific) over a wavenumber range of 400–4,000 cm−1.

After ashing the coal sample at 815°C, it was subsequently ground to a particle size of 300–400 mesh and formed into circular discs. X-ray fluorescence spectroscopy was employed for the quantitative analysis of the coal ash composition (XRF2510, Changsha Weipu Technology Co., Ltd), and the results of this analysis are detailed in Table 2.

Table 2

Composition of coal ash samples

Composition
Samples SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 MnO2 P2O5 SO3
SH 39.95 15.26 9.50 20.45 1.81 1.30 0.93 0.68 0.33 0.05 9.74
CP 51.43 34.63 4.43 4.77 0.87 0.72 0.90 1.49 0.21 0.32 0.23

There exists a significant difference between the two types of coal ash. The SiO2/Al2O3 ratio for SH coal ash is 2.62, whereas for CP coal ash, it is 1.48. Additionally, the combined contents of SiO2 and Al2O3 for the two samples are 55.21% and 86.06%, respectively, while the combined contents of Fe2O3 and CaO, which act as fluxing agents, are 29.95% and 9.20%. Consequently, the low SiO2/Al2O3 ratio, the high contents of SiO2 and Al2O3, and the low concentrations of Fe2O3 and CaO are the primary factors contributing to the elevated flow temperature of CP coal ash [16].

2.2 Preparation and characterization of CWS

2.2.1 Grinding of coal samples

Following natural drying, the coal sample was subjected to fine grinding utilizing a grinder. The resulting coal powder was then placed in an ultrasonic disperser, where it was dispersed prior to being introduced into the sample detection pool. As the laser beam from the device traverses the particle sample, the particles will scatter the laser light. By measuring the intensity distribution of the scattered light, one can infer the particle size distribution of the sample (MS2000, Malvern Instruments Limited, UK). The particle size distribution of the coal sample is illustrated in Figure 1.

Figure 1 Particle size distribution of coal samples.
Figure 1

Particle size distribution of coal samples.

Under identical grinding conditions, the particle size distribution of the fine coal produced from the two coal types exhibits slight variations. This discrepancy can primarily be attributed to the significant differences in the degree of coalification between the two coal samples, which results in variations in HGI. The D 50 value for SH coal is 88.64 μm, while the D 100 value is 465.2 μm. In contrast, the D 50 value for CP coal is 73.75 μm, with a D 100 value of 456.3 μm. Notably, the particle size distributions of both coal powders conform to the specifications required for the preparation of CWS [17].

2.2.2 Preparation of CWS

The additive for CWS was synthesized in the laboratory, utilizing a dosage of 1.5‰ of dry coal. Following the uniform mixing of water and the additive at a low stirring speed (the volume of water added was determined through Eq. 1), 70 g of coal sample was gradually introduced. The stirring speed was then incrementally increased to 1,500 rad·min−1. After a duration of 5 min of stirring, the preparation of CWS was complete

(1) C = 70 × ( 1 − M ad ) × ( 1 + 1.5 ‰ ) 70 + m H 2 O + 70 × ( 1 − M ad ) × 1.5 ‰ ,

where C is the concentration of CWS (%) and m H 2 O is the amount of water added (g).

2.2.3 Characterization of CWS performance

2.2.3.1 Concentration of CWS

The viscosity of CWS was tested six consecutive times at a shear rate of 100 s−1, with the average value calculated to determine the apparent viscosity (NSX-4CP, Chengdu Instrument Factory). Subsequently, the apparent viscosity of CWS at various concentrations is measured and plotted to create a line graph. The concentration corresponding to an apparent viscosity of 1,000 mPa·s was designated as the fixed viscosity concentration of the slurry.

2.2.3.2 Stability of CWS

CWS was poured into a flat-bottomed test tube up to a height of 2/3 and sealed with plastic wrap. After allowing it to stand for 24 h, the percentage of the supernatant height relative to the total height was measured, representing the water separation rate (WSR) of CWS. A higher WSR indicates poorer stability of CWS at the same concentration.

2.3 Coal ash flow temperature

A 20 mm high and 7 mm long equilateral triangular ash cone was made from coal ash and placed on a corundum tray. About 6 g of graphite and 6 g of activated carbon are mixed and placed in a tube furnace to create a weak reducing atmosphere at high temperature. The temperature was raised at a rate of 15°C·min−1 to 700°C, then adjusted to 5°C·min−1 up to 1,500°C [18]. The flow temperature of the ash was determined when the ash cone melts and unfolds into a thin layer with a height of less than 1.5 mm [2] (5E-AF3000, Changsha Kaiyuan Instruments Co., Ltd).

2.4 Analysis of coal ash viscosity–temperature characteristics

About 70 g of ash was placed in a corundum crucible, which was then placed into a tube furnace. The tube furnace was heated to 600°C at a rate of 20°C·min−1, and a mixed gas (CO 60%, CO2 40%) was introduced at a flow rate of 800 mL·min−1. Once the temperature exceeds 1,000°C, it was further increased to 1,500°C at a rate of 10°C·min−1. After the coal ash was completely melted, the measuring rod of a rotary viscometer was inserted into the slag, and the furnace temperature was gradually reduced at a rate of 2°C·min−1. The experiment was halted when the viscosity exceeded the equipment’s range, allowing for the measurement of coal ash viscosity at different temperatures (WKH-1.3, Beijing Huayixing New Technology Development and Research Institute).

2.5 Analog computation

Factsage was used to simulate the transformation of crystal minerals in coal ash at different temperatures, within a simulation range of 1,000–1,500°C and at intervals of 50°C. The simulation was conducted under a weakly reducing atmosphere (CO:N2 = 4:6).

Aspen Plus was employed to simulate the gasification process of CWS, allowing for the analysis of gasification results under different pulping schemes. A comprehensive evaluation of the technical and economic feasibility of the CWS gasification scheme was conducted based on actual industrial raw material costs.

3 Results and discussion

3.1 Slurry performance assessment of individual coal sample

The dispersibility and stability of CWS are critical evaluation indicators in the gasification process. Various gasification methods impose different requirements regarding the apparent viscosity of CWS. This article is based on the specific technical requirements of a CWS gasification process employed by Sinopec [19]. The upper viscosity limit for CWS is established at 1,000 mPa·s, while the WSR of CWS, measured after 24 h of standing, serves as the evaluation criterion for the stability of the slurry. The experimental results are presented in Figures 2 and 3.

Figure 2 Fixed viscosity concentration of CWS prepared from individual coal samples.
Figure 2

Fixed viscosity concentration of CWS prepared from individual coal samples.

Figure 3 WSR of CWS prepared from individual coal samples.
Figure 3

WSR of CWS prepared from individual coal samples.

In Figures 2 and 3, the maximum concentration of CWS prepared from SH coal is observed to be only 59.1% with a WSR of 7.95%. This indicates that SH coal exhibits characteristics of low slurry concentration and poor stability. In contrast, the highest concentration of CWS derived from CP coal reaches 69.1%, with a significantly lower WSR of 2.48%. The concentration and stability of CWS produced from CP coal are markedly superior to those obtained from SH coal. Therefore, the incorporation of CP coal into SH coal not only enhances the concentration of CWS but also improves the stability of the slurry.

To further investigate the significant differences in CWS concentrations between the two types of coal, FTIR analysis was employed to examine the structural variations between SH coal and CP coal (Figure 4).

Figure 4 FTIR spectra analysis of coal samples.
Figure 4

FTIR spectra analysis of coal samples.

The types and compositions of functional groups in coal significantly impact its hydrophilicity, which in turn affects its slurryability [20,21]. According to FTIR analysis, both SH and CP coal samples exhibit complex characteristics comprising organic and inorganic components. FTIR spectra of SH and CP coals reveal a stretching vibration peak at 3,436 and 3,434 cm−1, respectively, which corresponds to the O–H bond. This bond is attributed to adsorbed moisture and hydrated hydroxyl groups present in the minerals. Additionally, the peaks observed at 2,922, 2,853, and 2,850 cm−1 indicate the vibrations of –CH2 groups in saturated carbon chains, suggesting the presence of long-chain aliphatic hydrocarbons. The peaks at 1,627 and 1,620 cm−1 represent the bending vibrations of H–O–H in water molecules. Furthermore, the peaks at 1,383 and 1,384 cm−1 are associated with nitrate vibrations, indicating a small amount of mineral content. The peak at 472 cm−1 corresponds to the vibration of the Si–O bond, suggesting the potential presence of silica or other silicate minerals. Furthermore, CP coal exhibits a stretching vibration of C–O and Si–O bonds at 1,035 cm−1, which can be attributed to ether, ester, and silicate minerals, while the peak at 542 cm−1 further indicates the presence of additional mineral components.

In summary, both samples are characterized by the presence of aromatic compounds, long-chain alkanes, and a degree of mineral impurities. However, CP coal demonstrates a greater degree of aromaticity, enhanced hydrophilicity, and a more complex mineral composition.

In Table 1, CP coal exhibits characteristics typical of high-rank coal, such as low moisture content, low volatile matter, and high ash content. Research on the fractal dimensions D1 and D2 suggests that D2 has a negative impact on the slurryability of coal. High-rank coals, characterized by elevated ash content and reduced volatile matter, tend to possess higher D1 values and lower D2 values [22]. The pronounced aromaticity of CP coal contributes to its enhanced hydrophilicity. The attributes of high-rank coal, including low moisture and low volatile matter, correlate with a reduced fractal dimension D2. Additionally, the lower M ad value further increases the free water content during the preparation of CWS, resulting in a higher concentration of CWS when utilizing CP coal.

3.2 Experimental analysis on coal blending between SH and CP

This study investigates the effects of varying proportions of CP coal blended with SH coal on the preparation of CWS. Specifically, it analyzes the effects of the coal blending ratio on both the maximum concentration and the stability of CWS. The gasification process of CWS employs liquid slag removal technology, which necessitates that the flow temperature of the coal ash does not exceed 1,300°C. Consequently, while incorporating CP coal to enhance the concentration of CWS, it is essential to consider the melting characteristics of the coal ash comprehensively [23]. This research aims to determine the maximum allowable blending ratio of CP coal by examining the variations in ash-flow temperature and the viscosity–temperature characteristics of the ash.

3.2.1 Effect of coal blending on CWS performance

SH coal and CP coal were blended in ratios of 9:1, 7:3, 5:5, and 3:7. M ad of the mixed coal samples was calculated by weighting the values of the two individual coals (Table 1). The preparation method for CWS was conducted in accordance with Sections 2.2.2 and 2.2.3. The effects of coal blending on the fixed viscosity concentration and WSR of CWS are illustrated in Figures 5 and 6.

Figure 5 Effect of coal blending ratios on viscosity concentration for CWS.
Figure 5

Effect of coal blending ratios on viscosity concentration for CWS.

Figure 6 Effect of coal blending ratios on WSR for CWS.
Figure 6

Effect of coal blending ratios on WSR for CWS.

In Figure 5, the concentration of CWS increases progressively to 63.3% with the addition of CP coal. In Figure 2, the fixed viscosity concentrations of SH coal and CP coal are 59.1% and 69.1%, respectively. When preparing CWS using different blending ratios of these two coals, only the 9:1 mixing ratio achieves a fixed viscosity concentration of 60.5%, which exceeds the weighted average value of 60.1% calculated for the two individual coals at the same ratio. In Figure 6, the WSR of CWS decreases significantly as the ratio of CP coal addition increases. WSR associated with the highest CWS concentrations across various blending ratios remains consistently below 4%, demonstrating a significant enhancement in slurry stability.

3.2.2 Effect of coal blending on flow temperature for coal ash

The flow temperature of coal ash is a critical parameter in the gasification process of CWS [24]. The typical operating temperature for CWS gasification is approximately 1,350°C, which is 50–100°C higher than the flow temperature of coal ash [7]. This elevated temperature is necessary to ensure that the coal ash remains in a molten state following gasification. The flow temperature of coal ash, after the incorporation of varying proportions of CP coal into SH coal, is depicted in Figure 7.

Figure 7 Variation of coal ash flow temperature with blending ratios.
Figure 7

Variation of coal ash flow temperature with blending ratios.

The incorporation of CP coal in increasing proportions leads to a significant rise in the flow temperature of coal ash. Specifically, the addition of 10% CP coal results in a flow temperature of 1,256°C. When the blending ratio exceeds 30%, the flow temperature of coal ash surpasses 1,300°C, which poses a challenge to the efficient discharge of slag from the gasifier [25]. The flow temperature of coal ash is observed to increase as the SiO2/Al2O3 ratio decreases [16]. Following the blending of 10% CP coal with SH coal, the SiO2/Al2O3 ratio of the resulting coal ash decreased from 2.62 to 2.22. This reduction in the ratio is identified as the primary factor contributing to the increase in the flow temperature of the coal ash.

According to the analysis of the effect of coal blending on slurry performance and ash melting temperature, a 9:1 mixture of SH coal and CP coal (SC91) represents the optimal coal blending configuration.

A broader temperature range associated with a viscosity of 5–25 Pa·s is advantageous for the stable operation of the gasifier. It is essential that this temperature range does not fall below 50°C to prevent complications in slag discharge resulting from temperature fluctuations [26]. Figure 8 presents the comparative temperature characteristics of coal ash viscosity between SC91 and SH.

Figure 8 Viscosity–temperature relationship of coal ash.
Figure 8

Viscosity–temperature relationship of coal ash.

For SH coal ash, the temperatures corresponding to viscosities of 5 and 25 Pa·s are 1,385°C and 1,236°C, respectively, resulting in a temperature range of 149°C. In comparison, for SC91 coal ash, the temperatures at 2 and 25 Pa·s are 1,387°C and 1,282°C, yielding a narrower range of 105°C. The viscosity of SH slag demonstrates a nearly linear increase as temperature decreases, reflecting characteristics typical of glass slag. In contrast, the viscosity of SC91 slag begins to rise approximately 1,285°C with decreasing temperature, indicating properties characteristic of crystalline slag [27]. This shift suggests that incorporating 10% CP coal transitions the slag type from glass to crystalline, increasing the temperature at 25 Pa·s by 46°C while reducing the viscosity range between 5 and 25 Pa·s by 44°C. To ensure efficient slag discharge from the gasifier, the operating temperature for SC91 gasification must be raised above 1,285°C. SiO2 serves as the principal component in the formation of the network structure of the melt [28,29]. An increase in the SiO2 content ratio correlates with a heightened degree of network polymerization within the melt at elevated temperatures, which in turn results in an increase in the viscosity of the melt. CP coal ash is characterized by a high SiO2 content. Following the incorporation of 10% CP coal with SH coal, SiO2 content in the coal ash rose from 39.95% to 42.18%, thereby enhancing the viscosity of the coal ash.

3.2.3 Thermodynamic simulation of coal ash

The composition of SH and CP coal ash is outlined in Table 2. The composition of SC91 coal ash is determined by blending SH coal and CP coal in a ratio of 9:1 and aggregating their respective ash contents. The mineral transformation of coal ash at varying temperatures is illustrated in Figure 8.

In Figure 9a, SH coal ash is predominantly composed of FeS, CaSiO3, CaMgSi2O6, and CaAl2Si2O8, with a minimal presence of CaAl2Si2O8 in the slag at temperatures exceeding 1,150°C. In Figure 9b, the composition of CP coal ash is more complex, containing SiO2, Mg2Al4Si5O18, CaAl2Si2O8, Fe2Al4Si5O18, and mullite. At temperatures above 1,250°C, the slag is primarily constituted of mullite, and at 1,500°C, approximately 20% of the slag remains as mullite. Consequently, mullite is identified as the principal factor contributing to the elevated flow temperature of CP coal ash [23]. Following the incorporation of 10% CP coal into SH, there was no notable alteration in the mineral composition of SC91 and SH coal ash. However, the relative content of CaAl2Si2O8 increased significantly, resulting in an elevation of the melting temperature of the coal ash to approximately 1,250°C [15,30,31].

Figure 9 Thermodynamic simulation of coal ash composition: (a) SH coal ash facesage simulation, (b) CP coal ash facesage simulation, and (c) SC91 coal ash factsage simulation.
Figure 9

Thermodynamic simulation of coal ash composition: (a) SH coal ash facesage simulation, (b) CP coal ash facesage simulation, and (c) SC91 coal ash factsage simulation.

3.3 Synergistic effects of particle size grading and coal blending on CWS performance

The analysis presented above indicates that the direct blending of CP coal with SH coal has a minimal effect on enhancing the concentration of CWS. This limited effect can be attributed to two primary factors. First, the constraints imposed by liquid slag discharge technology permit the incorporation of only 10% CP coal. Second, the particle size distribution of both coal types exhibits an unimodal distribution, with negligible differences in particle size, which consequently leads to a low efficiency in coal particle stacking [32].

Based on the characteristic that CP coal exhibits superior grindability, an additional 10% of CP coal is subjected to further grinding and subsequently mixed with SH coal to prepare a CWS (SC91*). Following the fine grinding of the coal sample, there is an increase in the specific surface area [33]. Consequently, it is essential to augment the quantity of CWS additives to ensure the complete dispersion of the coal particles. This approach aims to achieve an optimal synergy in coal blending and particle size distribution. The experimental results are presented as follows.

3.3.1 Optimization of particle size distribution in CWS

CP coal is ground to a D 50 of approximately 10 μm (CP*). The particle size of CP* exhibits a “unimodal distribution,” whereas the particle size of SC91* displays a “bimodal distribution.” The particle size distributions of CP* and SC91* are presented in Figure 10.

Figure 10 Particle size distribution curves for CP* and SC91*.
Figure 10

Particle size distribution curves for CP* and SC91*.

In Figure 10, the incorporation of 10% CP* led to the emergence of a bimodal distribution in SC91*. The peak corresponding to the coarse particles is observed at 179 μm, indicating no significant variation when compared to SC91. Conversely, the peak for the fine particles is noted at 22 μm. Assuming that coal particles are spherical and arranged in regular tetrahedrons with a diameter of 88.64 μm, the maximum diameter of particles that can occupy the voids is given by φ 2 = 0.2247 φ 1 = 19.92 μm. However, actual coal particles are not spherical; nonspherical particles are less effective at filling the voids between larger particles, resulting in lower volume fractions [34]. Additionally, the particles are not entirely stacked in tetrahedral formations, and the proportion of CP coal that can be mixed is limited to 10%. Therefore, CP coal is further ground to approximately 10 microns in D 50 to effectively fill the gaps between SH coal particles.

3.3.2 Effect of technological collaboration on the dispersion and stability for CWS

D 50 of CP* is approximately 10 μm, and the incorporation of CP* will substantially enhance the specific surface area of SC91*. Consequently, the quantity of CWS additive introduced will progressively increase from 1.5‰ to 3.0‰. The properties of SC91* slurry are illustrated in Figures 11 and 12.

Figure 11 Effect of additive dosage on concentration for SC91* CWS.
Figure 11

Effect of additive dosage on concentration for SC91* CWS.

Figure 12 Effect of additive dosage on stability for SC91* CWS.
Figure 12

Effect of additive dosage on stability for SC91* CWS.

In Figure 11, the incorporation of CP* resulted in an increase in the concentration of SC91* CWS to 62.3%. Furthermore, as the quantity of CWS additive was gradually elevated to 3.0‰, the concentration rose to 63.5%. Concurrently, the WSR of CWS exhibited a further decline. Notably, when the additive concentration surpassed 0.20%, WSR fell below 1%, indicating that the stability of the slurry was significantly enhanced.

3.3.3 Effect of technological synergy on rheological properties for CWS

CWS is composed of coal particles, dispersants, and water, resulting in a relatively stable three-dimensional spatial structure. Within the voids of this three-dimensional arrangement, free water exists that has not been adsorbed or incorporated into the formation of a tightly bound hydration layer. As shear stress increases, the structural integrity is compromised, leading to the release of free water and a subsequent reduction in viscosity. Consequently, the slurry demonstrates shear-thinning pseudoplastic fluid behavior.

The rheological properties of CWS were characterized using a power-law function (Eq. 1). Figure 13 illustrates the relationship between shear rate (γ) and shear stress (τ) for CWS prepared with the highest concentration from three different coal blends (i.e., SH, SC91, and SC91*).

τ = τ 0 + K γ n

where τ is shear stress (Pa), τ 0 is the yield stress (Pa), K is the consistency coefficient (Pa·s), γ is the shear rate (s−1), and n is the rheological index (dimensionless). A smaller value of n corresponds to a more pronounced pseudoplastic behavior.

Figure 13 Relationship between shear rate and shear stress in CWS.
Figure 13

Relationship between shear rate and shear stress in CWS.

In Figure 13, the coefficient of determination (R 2) exceeds 0.99, indicating a robust correlation between the experimental data points and the fitted curve. For CWS formulated from SH and SC91 coals, both the consistency coefficient and the rheological index exhibit minimal variation. This can be attributed to the negligible difference in particle size between the two coal types, which results in a relatively comparable free water content within the three-dimensional structure of the slurry. Conversely, for CWS derived from SC91*, there is a marked increase in the consistency coefficient, accompanied by a notable decrease in the rheological index, which enhances the pseudoplastic behavior of the slurry. This phenomenon is primarily due to the small particle size of CP*, which effectively occupies the voids within the three-dimensional structure of the slurry, thereby significantly reducing the inter-particle distance. Consequently, the reduction in free water leads to an increase in both viscosity and the consistency coefficient. Under conditions of high shear stress, the free water present in the voids of the slurry structure is released more efficiently, thereby significantly augmenting the pseudoplastic behavior of CWS [35].

3.4 Technical and economic evaluation

The economic performance of various schemes will be simulated and calculated under optimal conditions utilizing Aspen Plus (Table 3).

Table 3

Technical and economic simulation scenarios

Scheme Raw coal Additive dosage (‰) Concentration (%)
A SH 1.5 59.1
B SC91 1.5 60.5
C SC91* 1.5 62.3
D 2.0 63.1
E 2.5 63.3
F 3.0 63.5

Based on actual industrial operating parameters, the simulated feed rate is set at 1,600 t·day−1. The gasification temperature is defined as the coal ash flow temperature plus 50°C, while the gasification pressure is maintained at 6.5 MPa [36]. The carbon conversion rate is achieved at 98%. The quality of SC91 coal is determined by combining SH and CP coal in a ratio of 9:1. The cost of SH coal is 890 RMB·t−1, whereas the cost of SC91 coal is 865 RMB·t−1. The price of the additive is 5800 RMB·t−1, the cost of industrial water is 3 RMB·t−1, and the oxygen cost is 0.76 RMB·Nm−3. The results of the gasification simulation, along with the effective gas consumption cost per 1,000 Nm3 for various schemes, are summarized in Table 4.

Table 4

Gasification simulation and effective gas cost per thousand standard cubic meters

Scheme Composition of crude syngas (%) Syngas flow rate (Nm3·h−1) Consumption Total cost (RMB)
CO H2 CO2 Oxygen (Nm3) Coal (kg) Additive (kg) Water (kg)
A 48.01 31.89 19.48 109,926 452.12 606.47 0.91 419.70 889.91
B 50.71 30.46 18.24 109,129 455.42 610.90 0.92 398.85 882.30
C 51.98 30.36 17.08 110,522 443.35 603.20 0.90 365.02 866.24
D 52.26 30.69 16.46 111,882 431.71 595.87 1.19 348.45 852.67
E 52.45 30.61 16.35 111,888 431.68 595.83 1.49 345.45 854.34
F 52.64 30.52 16.25 111,894 431.66 595.80 1.79 342.47 856.03

In Table 4, an increase in the concentration of CWS correlates with a gradual rise in the composition of effective gas components within crude synthesis gas. The effective gas flow rate in Scheme B is reduced, primarily due to the elevated gasification operating temperature associated with the increase in coal ash flow temperature. In contrast, Scheme D achieves the lowest cost of effective gas consumption per thousand standard cubic meters. This cost reduction is attributed to the higher coal slurry concentration enabled by additive inclusion, which enhances both the effective gas flow rate and composition. Although Schemes E and F yield higher effective gas flow rates and improved compositions, their greater additive requirements result in costs that outweigh the benefits. Based on this analysis, Scheme D (i.e., featuring the addition of 10% CP* and 0.2% additives to SH coal) provides the most advantageous technical and economic performance. Compared to SH CWS gasification, Scheme D delivers an effective gas composition of 82.95%, a 1.7% increase in effective gas flow rate, and a 4.19% reduction in production cost per thousand standard cubic meters. These results highlight the significant technical synergy achieved through the optimization of coal blending and particle size distribution.

Additionally, incorporating 10% CP coal into the gasification of raw coal enables the on-site conversion of thermal coal into raw coal. With a daily coal input of 1,600 t and an operational schedule of 300 days·year−1, this approach can reduce CO2 emissions by 34,349 t·year−1, delivering significant environmental benefits.

4 Conclusion

  1. The addition of CP coal to SH coal increases the concentration of CWS. However, the maximum allowable CP coal dosage is restricted to 10% due to the requirements of liquid slag discharge technology, with an additive dosage of 1.5‰. Under these conditions, the highest attainable CWS concentration is 60.5%, representing a 1.4% improvement compared to SH coal alone. This process results in a 90°C increase in the flow temperature of coal ash, elevates the minimum gasification operating temperature to 1,285°C, and reduces the gasification operating range by 44°C. The significant rise in the relative content of CaAl2Si2O8 is identified as the primary driver of the increased coal ash flow temperature.

  2. Maintaining the additive dosage at 1.5‰ and incorporating 10% CP* raises CWS concentration to 62.3%. Increasing the additive dosage to 0.2‰ or higher reduces WSR to less than 1%, indicating excellent slurry stability. At an additive dosage of 0.3‰, CWS concentration improves further, reaching 63.5%. The addition of CP* improves the pseudoplastic properties of CWS.

  3. Gasification simulations show that adding 10% CP* and 0.2% additives to SH coal increases the effective gas composition to 82.95% and raises the gas flow rate to 111,882 Nm3·h−1, while reducing the effective gas cost by 4.19% per thousand standard cubic meters. This strategy enables the transformation of local thermal coal into chemical raw material coal, delivering substantial economic and environmental advantages.

  1. Funding information: This study was supported by the Open Research Fund Program of the Anhui Provincial Institute of Modern Coal Processing Technology, Anhui University of Science and Technology (Grant No. MTY202308).

  2. Author contributions: Ming Liu: investigation, data analysis, and writing; Hanxu Li: conceptualization and supervision.

  3. Conflict of interest: The authors state no conflicts of interest.

  4. Data availability statement: All data generated or analyzed during this study are included in this published article.

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Received: 2024-09-19
Accepted: 2025-02-11
Published Online: 2025-03-11

© 2025 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/gps-2024-0202
Keywords for this article
coal water slurry; coal blending; particle size optimization
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