Moisture Management in Low-Rank Coal Processing

Low-rank coals such as lignite and subbituminous grades contain high intrinsic moisture that undermines handling, transport and thermal efficiency in power generation and downstream conversion processes. Effective moisture management encompasses understanding water distribution within the porous coal matrix, designing dehydration strategies that minimise energy input, and preventing reabsorption during storage. Approaches range from mechanical dewatering and low-temperature fluidised-bed drying to advanced thermal methods including microwave and superheated-steam techniques. Fundamental studies of moisture migration and phase change in the coal particle reveal distinct drying stages—initial free-water evaporation, capillary-bound water removal and tightly bound water desorption—each requiring tailored thermal and mass-transfer control. Engineering developments have produced moving-bed and toroidal-bed dryers that exploit indirect heat transfer and interface renewal for rapid moisture release, as well as in-situ sensors for real-time moisture quantification. Integrating multi-physical simulation with experimental tracer analysis has refined the design of energy-efficient dryers and guided scale-up. Globally, improved moisture management reduces carbon emissions, lowers fuel costs and enhances the competitiveness of regions reliant on low-rank coal, while offering pathways to cleaner coal usage in coking, gasification and combustion applications.

Research from Nature Portfolio

Recent studies have applied coupled experimental and multi-physical field simulations to elucidate moisture migration mechanisms during microwave drying of coal slimes. By dividing the drying sequence into preheating, constant-temperature and reheating stages, researchers quantified vapourisation rates and pressure-driven moisture transport from interior regions to the surface. They demonstrated that over 74 % of initial water vapourises in the preheating stage, while the constant-temperature stage drives remaining liquid water outward via inter-particle pressure gradients. The reheating stage is dominated by steam diffusion through voids, with particle size and dough geometry shown to affect moisture migration pathways. These insights inform the optimisation of microwave-based dryers for high-viscosity, high-water-content coal residues, enhancing energy efficiency and throughput in coal-preparation circuits.

Moisture Management in Low-Rank Coal Processing publication trend

The graph below shows the total number of articles in moisture management in low-rank coal processing across all publications each year (not limited to Nature Index journals).

Technical terms

Low-rank coal: A coal category with high inherent moisture, high volatile matter and lower calorific value than higher-rank coals.

Microwave drying: A volumetric heating method using electromagnetic waves to rapidly remove moisture by dielectric loss.

Capillary water: Moisture held in pore networks by capillary forces, requiring higher energy input for removal than free water.

Effective diffusion coefficient: A parameter quantifying the rate at which moisture migrates through the coal matrix under thermal gradients.

Proton transverse relaxation (T2): NMR decay parameter sensitive to water mobility, used to quantify moisture in porous media.

Indirect drying: A process in which heat is transferred to the material via a heated surface or medium without direct contact of hot gas or steam.

References

1. Quantitative investigation of moisture migration during microwave drying of coal slime dough through simulation and tracer analysis. Scientific Reports (2025).
2. Effect of Temperature and Microwave Power Levels on Microwave Drying Kinetics of Zhaotong Lignite. Processes (2019).
3. Nondestructive quantification of moisture in powdered low-rank coal by a unilateral nuclear magnetic resonance scanner. International Journal of Coal Preparation and Utilization (2020).
4. Construction of a New Type of Coal Moisture Control Device Based on the Characteristic of Indirect Drying Process of Coking Coal. Energies (2020).
5. Drying of Lignite of Various Origins in a Pilot Scale Toroidal Fluidized Bed Dryer using Low Quality Heat. Energies (2019).