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The water content of a coal reduces its heating value, causes handling difficulties, increases handling and transportation costs, and reduces yields in carbonization and other conversion processes. Reduction of the water content is often desirable. In fact, drying coal helps the coal to burn cleaner and more efficiently but because of the unique properties of each type of coal this drying process must be done differently.

Combined reserves of subbituminous coal and lignite (brown coal) make up approximately one-half of the world coal reserves and about one-half of the coal resources of the United States and these coals are rarely processed before shipment or use. However, the oxygen and moisture contents of low-rank coals are greater than those of bituminous coals, which reduces the heating value of the coal as mined, which increases the transportation cost on a heating value basis and reduces the thermal efficiency of the steam boilers that use these coals. One way to offset these disadvantages is to dry the coal before transportation or utilization. However, the characteristics of dried low-rank coal – it is friable, has a tendency to spontaneously heat, and readily reabsorbs moisture – constitute major obstacles that must be overcome to produce a saleable, transportable, dry coal product.

Water occurs in coals in three ways: (i) as inherent moisture contained in the internal pores of the coal substance, including water associated with the mineral impurities, (ii) as surface moisture wetting the external surfaces of the coal particles in which adsorption may play a small part, and (iii) as free water held by capillary forces in the interstices between the coal particles.

Inherent moisture is related to coal rank, being greatest for lignite and brown coals. However, the inherent moisture in coal can make a significant impact on the performance of low-rank coals (which are primarily used for electricity generation) and drying is generally restricted to (i) washed bituminous coals that are required to meet user specifications, and (ii) lignite or subbituminous coals that are employed for the manufacture of briquettes or of other specialty products such as absorbent carbon.

Surface moisture is related to the amount of available surface and the wettability of the coal but moisture contents are lower than might be expected from the available surface area because of the low wettability of coal compared with the surfaces of minerals. Surface moisture can be removed from washed higher rank (bituminous and anthracite) coals by drainage on a screen while the dewatering of washed small coal or coal fines can he accomplished by use of cyclones or centrifuges. If the moisture content must be reduced to lower levels there are two alternate methods which involve the use of rotary kilns or fluidized bed dryers. But 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).