such dried coal will reabsorb moisture on exposure to the atmosphere which may give rise to fire hazards as well as (explosion) hazards.
Furthermore, the water in the low-rank coals is progressively more strongly bound to the coal surface as the coal dries and equilibrium relative vapor pressure (or humidity) decreases. The water initially removed from the as-mined coal at close to the saturated vapor pressure (or 100% humidity) is water filling the large pores and inter-particle spaces in the coal. This water has the normal thermodynamic properties of free water. As the coal dries further, capillary water is removed with significant decrease in relative vapor pressure. Below 50% humidity, the water adsorbed in layers on the coal surface is progressively removed with increasing heat of vaporization required and lower vapor pressures due to the increasingly strong hydrogen bonding of the water molecules to the oxygen functional groups on the coal surface.
A distinction can also be made between water in larger pores and capillaries which passes through a liquid-solid freezing phase transition if the temperature is lowered in contrast to water in the smaller pores and surface layers which does not pass through this transition. However this is not significant to drying as the non-freezing water can still be evaporated if heat is applied or the vapor pressure lowered.
It is progressively more difficult to remove the more strongly bound water at lower moisture content. Also note that if a coal is dried to below the equilibrium moisture content (Chapter 5) and then exposed to a humid atmosphere it will re-adsorb moisture until it is in equilibrium with the ambient humidity. This can raise the temperature and exacerbate the propensity of low-rank coals and their upgraded products to spontaneous ignition and thence to spontaneous combustion during transport and storage (Chapters 3, 4).
Loss of volatile organic compounds (VOC) as a result of drying coal at high temperatures is another issue that must be addressed. The loss of useful volatile matter from coal reduces the calorific value while at the same time increases the risk of fire from the combustion of volatile organic compounds. Drying at lower temperature or by using a slight vacuum environment can minimize the loss of volatile organic compounds but such approaches result in a lower rate of drying.
Thermal drying, particularly of metallurgical coals, has found extensive application in some parts of the “coal” world and there is growing interest in the development of efficient methods for drying low-rank coals. Thermal drying entails contacting wet coal particles with hot gases, usually combustion products, under conditions that promote evaporation of the surface moisture without causing degradation or incipient combustion of the coal.
In the process, the clean coal from various wet cleaning processes is wet and requires drying to make it suitable for transportation and final consumption. Thermal drying is employed to dry the wet coal. Drying in the thermal dryer is achieved by a direct contact between the wet coal and currents of hot combustion gases. Various dryers marketed by different manufacturers work on the same basic principle.
The fluid-bed dryer operates under negative pressure in which drying gases are drawn from the heat source through a fluidizing chamber. Dryer and furnace temperature controllers are employed in the control system to readjust the heat input to match the evaporative load changes.
The multi-louver dryer is suitable for large volumes and for the coals requiring rapid drying. The coal is carried up in the flights and then flows downward in a shallow bed over the ascending flights. It gradually moves across the dryer, a little at each pass, from the feed point to the discharge point.
In the cascade dryer, wet coal is fed to the dryer by a rotary feeder; as the shelves in the dryer vibrate, the coal cascades down through the shelves and is collected in a conveyor at the bottom for evacuation. Hot gases are drawn upward through and between the wedge wire shelves.
In the flash dryer the wet coal is continuously introduced into a column of high- temperature gases and moisture removal is practically instantaneous.
Table 3.6 Different types of moisture in coal and methods for removal (Karthikeyan et al., 2009).
| Category | Location | Common name | Removal method |
| Interior adsorption water | Micropores and microcapillaries within each coal particle. | Inherent moisture | Thermal or chemical |
| Surface adsorption water | Particle surface. | Inherent moisture | Thermal or chemical |
| Capillary water | Capillaries in coal particles. | Inherent moisture | Thermal or chemical |
| Interparticle water | Small crevices found between two or more particles. | Surface moisture | Mechanical or thermal |
| Adhesive water | Film around the surface of individual or agglomerated particles. | Surface moisture | Mechanical or thermal |
Finally, understanding the composition of water in coal facilitates the effective removal of coal moisture: as the moisture exits in different states the corresponding method must be chosen for moisture removal (Table 3.6) (Karthikeyan et al., 2009). As a result, dryer type must be chosen according to the task at hand to produce the most effective feedstock for the power plant (Osman et al., 2011).
3.6.1 Rotary Dryers
The rotary dryer is the most established dryer type and one of the most common for general applications. The basic design consists of an insulated cylindrical shell that is mounted on rollers and rotates at a low speed. Rotary dryers allow direct and/or indirect contact between the drying medium and the wet particles, although the former is more common in industry.
In the direct rotary dryers, the wet material is in direct contact with the drying medium. Direct heat transfer is usually provided by a hot gaseous medium blown into the vessel from the gas inlet. For drying of low-rank coal, the drying medium must be free of oxygen to prevent spontaneous ignition and combustion during storage (Chapter 4). Flue gases or heated air are the most common drying medium and in principal suitable for application to low-rank coal. However, there are reports of fires and explosions from oxygen contacting hot coals especially during start-up and shut down from such a system (Wilver and Brumbaugh, 1985). To avoid such accidents, one must ensure that the coal is sufficiently cooled before exposure to the environment.
Gas can flow in the direction of feed progression (parallel flow), or in the opposite direction (counter-flow). Although counter-current flow offers higher thermal efficiency, parallel flow prevents the overheating of the coal near the exit of the dryer.
Typically,
