Study on the Drying Characteristics of Moist Fine Lignite in a Dense Gas–Solid Separation Fluidized Bed

Abstract

Coal serves as a cornerstone and stabilizer for China’s energy security; utilizing it in a clean and efficient manner aligns with the current national energy situation. The moisture content of coal is a crucial factor affecting its calorific value and separation efficiency. Therefore, enhancing the drying rate while simultaneously reducing the moisture content in coal is essential to improve separation efficiency. This paper primarily investigates the drying and separation characteristics of wet fine coal particles within a gas–solid fluidized bed system. A hot gas–solid fluidized bed was employed to study the particle fluidization behavior, heat–mass transfer, and agglomeration drying properties under varying airflow temperatures. The results indicate that as the airflow temperature increases, the minimum fluidization velocity tends to decrease. Additionally, with an increase in bed height, the particle temperature correspondingly decreases, leading to weakened heat exchange capability in the upper layer of the bed. Faster heating rates facilitate rapid moisture removal while minimizing agglomeration formation. The lower the proportion of moisture and magnetite powder present, the less force is required to break apart particle agglomerates. The coal drying process exhibits distinct stages. Within a temperature range of 75 °C to 100 °C, there is a significant enhancement in drying rate, while issues such as particle fragmentation or pore structure collapse are avoided at elevated temperatures. This research aims to provide foundational insights into effective drying processes for wet coal particles in gas–solid fluidized beds.

1. Introduction

As a low-rank coal with abundant global reserves, lignite plays a vital role in the global energy scenario owing to its wide availability and relatively low cost [1,2,3,4,5]. However, its inherent high moisture content, typically ranging from 30% to 50%, together with a high proportion of oxygen-containing functional groups and a highly porous structure, severely limits its direct utilization efficiency [6,7,8]. Excessive moisture in lignite not only reduces its calorific value, but also increases transportation costs, impairs combustion stability, and complicates subsequent processing [9]. In addition to the intrinsic moisture, operational factors during mining, handling, and transportation—such as water spraying for dust suppression—can further elevate feed moisture levels. With the increasing mechanization and scale-up of coal mining equipment, the proportion of fine coal particles has risen substantially, making the upgrading of fine lignite a subject of both scientific interest and industrial significance [10,11,12]. Improving the quality of fine, high-moisture lignite through integrated drying and beneficiation technologies offers considerable potential for enhancing its energy utilization and economic value.

Various thermal and non-thermal dewatering methods have been developed for lignite, including rotary drum drying, mechanical–thermal dewatering, and microwave-assisted drying [13,14,15,16]. Hulston et al. [17] investigated the influence of temperature and pressure variations on moisture removal during mechanical–thermal dewatering and found that up to 85% of the inherent moisture in raw coal could be removed at 250 °C and 12.7 MPa. Seehra et al. [18] designed a laboratory-scale microwave dewatering device and conducted dehydration experiments on fine coal slurry samples containing approximately 52% moisture, demonstrating that the moisture content could be reduced to 10% under conditions of 2.45 GHz and 800 W. Despite their effectiveness in terms of moisture removal, these techniques primarily target water reduction without improving coal quality, meaning that a separate cleaning step is often required to upgrade the coal [19]. This separation step increases process complexity, energy consumption, and operating costs. In contrast, fluidized bed drying–beneficiation technology integrates moisture removal with density-based separation, offering a promising route for simultaneous lignite drying and upgrading. This approach can exploit the excellent gas–solid contact, uniform temperature distribution, and high heat and mass transfer efficiency of fluidized beds [20,21,22]. Nevertheless, when high-moisture lignite is fed into a dense medium fluidized bed, water-induced agglomeration between the dense medium particles and lignite fines can occur [23,24,25]. This phenomenon leads to reduced bed fluidization quality, the deterioration of product separation accuracy, and increased medium consumption. Such challenges necessitate a more in-depth understanding of the fluidization hydrodynamics and drying kinetics of high-moisture lignite under thermal fluidization conditions.

In this study, the drying characteristics of moist fine lignite in a thermal fluidized bed were systematically investigated. The effects of operational parameters—including gas temperature and bed height—on particle fluidization behavior, heat and mass transfer performance, and drying kinetics were examined in detail. Special attention was given to identifying the effective moisture diffusion coefficient and establishing an empirical fitting equation for its dependence on process variables. By elucidating the interplay between hydrodynamic conditions and drying kinetics, this work aims to provide a theoretical foundation for the development of efficient, integrated dry beneficiation processes for fine lignite. The findings not only contribute to the scientific understanding of lignite drying in fluidized beds but also support the design and optimization of industrial-scale systems for the simultaneous upgrading and moisture reduction in low-rank coals.

2. Materials and Methods

2.1. Experimental Setup

The experimental system consists of a hot-state gas–solid fluidized bed, including a gas supply unit, a heating module, and a fluidization chamber (Figure 1a,b). The bed is cylindrical, constructed from plexiglass, and has a 100 mm inner diameter, 350 mm height, and 10 mm wall thickness. A stainless-steel porous distributor with 3 mm apertures and 31.39% porosity is placed at the base, above a windbag equipped with an umbrella-shaped gas diffuser. To minimize heat loss, all piping is insulated with thermal cotton. A high-temperature vortex flowmeter replaces conventional rotameters, and thermocouples are used to monitor bed temperature.

2.2. Determination of Minimum Fluidization Velocity

To determine the minimum fluidization velocity (U_mf), magnetite powder with 10% moisture content is added to the bed. Air is heated before entering the bed through the blower and heater. The bed is initially fixed at a static height of 100 mm. The airflow is gradually increased until visible bubbling indicates fluidization, at which point the superficial velocity and corresponding pressure drop are recorded. The airflow rate is then decreased stepwise, and the pressure drop is recorded at each step until zero velocity is reached. A plot of pressure drop vs. velocity is used to determine U_mf via linear regression, defined by the intersection point on the velocity axis.

2.3. Temperature Measurement

To assess the internal thermal behavior of the bed, the column is divided into three horizontal layers, with thermocouples positioned at the center of each (Figure 2). Temperature readings are recorded at regular intervals to observe changes over time (T1, T2, and T3 represent the lower, middle, and upper layers, respectively).

2.4. Coal Drying Tests

After preheating the magnetite in the bed, lignite samples are introduced. At fixed intervals, samples are withdrawn, magnetite is removed, and the coal is dried to constant weight. Moisture content (M_t) is calculated based on weight loss. The moisture ratio (M_R) at time t is defined as

$$M_R={\displaystyle \frac{M_t}{M_0}}$$

(1)

where M_t is the moisture at time t and M₀ is the initial moisture.

The drying rate is defined as

$${\displaystyle \frac{d{M}_t}{dt}}={\displaystyle \frac{M_{t_1}-M_{t_2}}{t_2-t_1}}$$

(2)

where $M_{t_1}$ and $M_{t_2}$ are moisture contents at times t₁ and t₂, respectively.

2.5. Sample Characteristics

The experiments utilize lignite (6.0–0.5 mm) and magnetite (0.300–0.074 mm), with bed density maintained at 1.65–2.00 g/cm³, which is suitable for dry separation. In addition, the average total particle size of the bed material is approximately 0.71 mm, and the overall bulk density is 2.16 g/cm³.The physical properties of the magnetite are listed in

Table 1. Physical properties of magnetite powder.

Sample	Size Range (mm)	Mean Particle Size (mm)	Content (%)	Particle Density (g/cm3)	Bulk Density (g/cm3)
Magnetite powder	−0.300	0.207	100	4.6	2.43
	0.300~0.150	0.247	80	4.6	2.44
	0.150~0.074	0.147	15	4.6	2.35
	−0.074	0.068	5	4.6	2.05

, and coal quality is shown in

Table 2. Results of coal quality analysis test.

Sample	Proximate Analysis				Ultimate Analysis
	Mad (%)	Ad (%)	Vdaf (%)	FCad (%)	Cdaf (%)	Hdaf (%)	Odaf (%)	Ndaf (%)	Sdaf (%)
Coal	6.13	16.93	36.34	49.64	62.51	3.36	0.79	10.64	0.71

.

3. Results and Discussion

3.1. Fluidization Characteristics in a Hot Gas–Solid Fluidized Bed

In a gas–solid fluidized bed system, the fluidization characteristics are significantly influenced by the temperature of the inlet gas. With the static bed height fixed at 100 mm, the relationship between gas velocity and bed pressure drop was experimentally investigated under varying gas temperatures (Figure 3a). The results indicate that as the gas temperature increases, the minimum fluidization velocity decreases, whereas the bed pressure drop increases gradually. At a constant gas velocity, elevating the gas temperature enhances the fluidization quality, leading to a more uniform distribution of the solid particles. The fluidization curves under different gas temperatures are presented in Figure 3b. As the gas temperature rises, thermal expansion of the gas initially causes a rapid reduction in the minimum fluidization velocity, followed by a slower rate of decrease. When the gas temperature increases from 25 °C to 100 °C, the reduction in the minimum fluidization velocity exceeds 10%.

The bed pressure drop (∆P) is determined by both viscous and inertial losses. At low Reynolds numbers (Re), viscous losses dominate, whereas inertial losses become significant at high Re. The overall pressure drop can be expressed as

$${\displaystyle \frac{\Delta P}{L}}=150{\displaystyle \frac{(1-{\epsilon }^2)}{{\epsilon }^3}}{\displaystyle \frac{\mu u}{{\left(\varphi d_p\right)}^2}}+1.75{\displaystyle \frac{1-\epsilon }{{\epsilon }^3}}{\displaystyle \frac{{\rho }_g{u}^2}{\varphi d_p}}$$

(3)

where ∆P is the pressure drop (Pa), L is bed height (mm), ε is bed porosity, μ is gas viscosity (Pa·s), u is superficial velocity (cm/s), dp is particle diameter (cm), and Φ is particle sphericity.

The empirical relationship between gas temperature and gas viscosity is given by Equations (4) and (5), where μ is the dynamic viscosity of gas (Pa·s), T is the absolute temperature (K), K is an empirical constant (Pa·s·K^2/3), C_μ is a dimensionless constant related to gas properties, and ${\rho }_g$ is the gas density (kg/m³). The values of the constants used are K = 1.458 × 10⁻⁶ Pa·s·K^2/3 and C_μ = 110.4 K.

$$\mu ={\displaystyle \frac{K{T}^{\frac{2}{3}}}{C_{\mathsf{\mu }}+T}}$$

(4)

$${\rho }_g={\displaystyle \frac{1.2\times 193}{T}}$$

(5)

As shown in the above equation, an increase in gas temperature leads to a decrease in gas density and an increase in gas viscosity, resulting in a higher bed pressure drop. The corresponding expression is given in Equation (6).

$$PV=nRT$$

(6)

With increasing temperature, the gas density decreases while viscosity increases; this enhances drag force on particles and thus increases pressure drop even at constant particle mass and airflow velocity. This behavior is consistent with the observed decrease in U_mf at higher temperatures.

3.2. Heat Transfer Behavior in the Bed

The particle temperature at various bed heights under different airflow temperatures is illustrated in Figure 4a,b. The findings indicate a decreasing trend in particle temperature as the bed height increases. Near the bottom of the bed, particles experience rapid heating due to their proximity to the hot airflow; however, the rate of temperature increase diminishes further from the bottom. The reduced temperature gradient observed in the upper layers leads to decreased heat transfer efficiency, suggesting a progressive weakening of gas–solid heat exchange as height increases. It should also be noted that under airflow temperatures of 50 °C and 75 °C, a completely stable state was not reached within the 60 min test period; thus, the curves mainly reflect the trend of temperature distribution.

Measuring the temperature at various bed heights throughout the fluidization process demonstrated that the average particle temperature within the bed varies with bed height, as depicted in Figure 5. The results indicate a decreasing trend in average particle temperature as bed height increases. At the bottom of the bed, particles absorb heat more rapidly; however, as height increases, the rate of temperature increase within the bed gradually diminishes and the temperature differences between the layers decrease. Consequently, the particle temperatures at the top of the bed become more uniform and are consistently lower. In comparison to the lower regions of the bed, the smaller gas–solid temperature difference in upper layers results in less efficient energy transfer, meaning gas–solid heat transfer performance declines as bed height increases.

3.3. Analysis of the Drying Properties of Magnetite Powder–Coal Powder Agglomeration

To investigate the effect of magnetite powder on the drying and agglomeration behavior of fine coal particles, in situ high-temperature imaging was employed to monitor time-dependent changes in interparticle spacing under various heating rates. As shown in Figure 6, slower heating rates facilitate stronger particle agglomeration. At 10 °C/min, limited moisture evaporation leads to persistent liquid bridges and the tight packing of particles after 30 min. In contrast, a higher rate of 30 °C/min promotes rapid moisture removal, resulting in greater interparticle separation and reduced cohesion. This indicates that accelerated drying can effectively mitigate agglomeration.

Figure 7 illustrates a comparison of the morphology of magnetite powder under dry and hydrated conditions. In the dry state, particles are distributed loosely; however, water adsorption facilitates the formation of capillary bridges (highlighted in red), which significantly enhance interparticle cohesion. This hydrophilic characteristic of magnetite powder contributes to its potential for agglomeration during the drying process.

Figure 8 quantitatively presents the agglomeration strength of magnetite–coal blends with varying magnetite contents (0–90%) and moisture levels (0–40%), based on the force required to crush the agglomerates. The crushing force increases steadily with higher moisture and magnetite proportions. Notably, above 60% magnetite content, a sharp rise in force is observed, suggesting denser particle packing and enhanced cohesion.

3.4. Drying Kinetics of Fine Coal Particles

The diffusion coefficient D is a key parameter for evaluating the rate of diffusion as it represents the magnitude of the diffusion flux when the concentration gradient is unity. A larger D value indicates a faster diffusion rate. The effective moisture diffusion coefficient is an important indicator for assessing the rate of internal moisture migration within particles; it is influenced by various factors such as moisture content, porosity, and the physical state of water. According to Fick’s first law, the diffusion flux can be expressed as

$$J=-D{\displaystyle \frac{dC}{dx}}$$

(7)

where D denotes the diffusion coefficient (m²/s), J is the diffusion flux, C is the concentration of the diffusing species, and x represents the diffusion distance. The negative sign in the expression indicates that diffusion occurs in the direction opposite to the concentration gradient, i.e., substances always migrate from regions of higher concentration to regions of lower concentration. Based on this principle, by simplifying the moisture ratio and taking its logarithm, the following relationship can be derived:

$$\mathit{ln}{M}_R=\mathit{ln}{\displaystyle \frac{8}{{\pi }^2}}-{\pi }^2{\displaystyle \frac{D_{eff}}{4H^2}}t$$

(8)

where D_eff denotes the effective moisture diffusion coefficient, H represents the bed height of the material in the fluidized bed, and t is the drying time.

Figure 9 shows the moisture content of fine coal under different air flow temperatures. When the drying time is constant, the moisture content of coal decreases significantly as the temperature increases. In the initial stage of drying, the moisture content between particles is relatively high, and heat and mass transfer mainly occur between the coal, air, and heavy medium. As the drying process proceeds and most of the free water is removed, the residual moisture content decreases to around 6%. Later in the drying process, the influence of temperature is relatively weaker and the change tends to be less substantial.

Figure 10 illustrates the variation in the drying rate of fine coal under different air flow temperatures. The results demonstrate that the drying rate generally increases with rising air flow temperature, although distinct characteristics are observed at different drying stages. When the moisture content exceeds 40%, evaporation is primarily governed by surface free water, where moisture diffusion is relatively efficient, resulting in the highest drying rate at 125 °C. As the moisture content decreases to 20% < M_t < 40%, the drying rate under high-temperature conditions declines sharply, indicating that internal moisture migration becomes diffusion-limited, thereby slowing down the evaporation process. When the moisture content falls below 20%, the remaining water is predominantly bound water located within deeper structures, and its diffusion rate is significantly influenced by the pore structure. A further increase in the air flow temperature results in a leveling-off of the drying rate, suggesting temperature’s diminishing effect above a certain point. This phenomenon may be attributed to the collapse of pore structures under high temperatures, which hinders the effective release of bound water from the complex porous network and restricts further moisture diffusion.

As summarized in Figure 11 and

Table 3. Calculation results for the effective diffusion coefficient of water.

Gas Temperature (°C)	Fitting Equation	R2	Deff (m2/s)
50	lnMR = 0.21496 − 0.04677t	0.9787	0.00020171
75	lnMR = 0.02926 − 0.06291t	0.9434	0.00025197
100	lnMR = −0.17198 − 0.06203t	0.9271	0.00025440
125	lnMR = −0.28033 − 0.07758t	0.9182	0.00031442

, D_eff increases with temperature, indicating faster moisture migration. At 125 °C, further increases in the drying rate become marginal, likely due to the structural collapse of pores and the stronger binding of residual water. Excessive heating may cause particle breakage and irreversible structural changes that ultimately hinder diffusion.

In the initial stage of drying, due to the high moisture content of coal particles, energy and mass exchange occur primarily through effective transfer between the particle surface, the surrounding air, and the added dense medium. Under high-temperature conditions, the gas markedly increases the temperature difference between the particle surface and both the air and dense medium, thereby accelerating the motion of water molecules and enhancing evaporation efficiency. As surface moisture is gradually depleted, the surface temperature rises steadily. At this stage, heat and mass transfer are no longer confined to the particle exterior but also occur in the interior, while moisture migration becomes significant both internally and externally under the driving force of a concentration gradient.

At higher airflow temperatures, the evaporation rate of moisture increases significantly, facilitating the formation of a moisture concentration difference between the particle core and surface. This difference drives the rapid migration of water from the core to the surface, thereby markedly accelerating the drying process. When the moisture content falls below 20%, further increasing the temperature—for example, from 100 °C to 125 °C—leads to a diminished drying rate improvement. This phenomenon often arises from intense chemical reactions under high temperatures, which can complicate the original chemical composition of fine coal and cause instability and collapse of its pore structure, as well as severe particle breakage. Meanwhile, the elevated temperature makes it more difficult to effectively release bound water from the complex porous network. Collectively, these processes exert a significant influence on the coal’s properties. Therefore, when drying moist particles, selecting an appropriate temperature is crucial to balance drying efficiency with the prevention of material solidification.

3.5. Analysis of Calorific Value, Ash Content and Total Moisture of Products

Table 4. Comparison of Ash, Moisture, Calorific Value, and Yield of Coal Products from Dense Gas–Solid Separation.

Product	Raw Coal	Cleaned Coal	Middling	Gangue
Ash/%	16.93	7.85	18.4	40.26
Moisture/%	30.54	6.36	8.15	4.35
Calorific value/cal·g−1	2180	4599	3124	-
Yield/%	100.00	63.38	12.74	23.88

presents the coal quality analysis results obtained from the integrated hot-state fluidized bed drying and separation process. The results demonstrate that the clean coal has an ash content of 7.85%, a moisture content of 6.36%, a calorific value of 4599 cal·g⁻¹, and a yield of 63.38%. These findings indicate that the process effectively reduces both ash content and moisture level, thereby significantly enhancing the calorific value of lignite.

4. Conclusions

This work investigates the drying characteristics of moist fine lignite in a gas–solid separation fluidized bed. The results indicate that an increase in temperature reduces gas density while simultaneously increasing gas viscosity. These concurrent changes amplify the resistance exerted on the particles, resulting in a significant rise in bed pressure drop under fixed bed conditions and constant superficial gas velocity. Consequently, the minimum fluidization velocity decreases, enhancing fluidization performance under appropriate operating conditions. The height of the bed has a significant impact on the particle temperature distribution. As the bed height increases, the internal heating rate of particles gradually decreases and the interlayer temperature difference diminishes. This trend suggests that heat transfer efficiency in the upper region declines, leading to a gradual decrease in gas–solid heat transfer performance with increasing distance from the bottom of the bed. Particle agglomeration occurs during coal drying; therefore, a faster heating rate facilitates rapid moisture removal and reduces agglomerate formation. Lower proportions of moisture and magnetite powder are associated with a reduced force being required to break apart particle agglomerates. Elevated temperatures can significantly reduce the moisture content in lignite, accelerate drying rates, and enhance effective moisture diffusion rates. Among the tested conditions, a temperature of 75 °C was found to achieve the optimal balance between drying efficiency and structural integrity, making it the most suitable drying temperature for this process. Excessively high temperatures may lead to pore rupture, which hinders the release of bound water. Thus, selecting an optimal temperature range during the fluidized bed drying and separation processes is crucial for achieving rapid dewatering while maintaining pore structure integrity.