an indirect rotary dryer consists of a jacketed shell through which steam or other heating medium flows. At any one time, a small fraction of solids are exposed to the heated wall, resulting in low heat transfer rates and low drying efficiency. One way to improve the performance of an indirect dryer is to increase the area of contact between the heated wall and the particles. This is accomplished by introducing a series of tubes through the rotary shell and passing steam through the tubes. In the steam-tube dryer, wet solids are lifted and showered within the rotary shell in the usual sense, and heated by radiant heat and contact with the outer surfaces of the tubes.
Rotary dryers designed for continuous processing are usually slightly inclined so that as the main vessel rotates, feed material progresses from the higher end of the vessel to its lower end. In such a system, the particles are conveyed by repetitive lifting and falling action provided by the circumferentially mounted flights and the force of gravity. The periodic lifting and showering of the material creates a curtain of particles through which hot gas flows. This agitation leads to higher efficiencies, increased heat transfer rate, and reduced processing time compared to stationary units. Thus, feed material is heated and dried as it progresses through the dryer.
3.6.2 Fluidized Bed Dryers
Fluidized bed drying is ideal for a wide range of particulate or granular solids and has found widespread usage in various industries, including those dealing with chemicals, pharmaceuticals and bio-chemicals, food and dairy products, and polymers. This is mainly due to high temperatures and high rates of mass transfer as a result of vigorous gas-solid mixing. Fluidized bed dryers can compete successfully with more conventional dryer types (e.g., rotary, tunnel, conveyor) in the drying of powders, granules, agglomerates, and pellets, with particles averaging between 50 to 5000 microns. Both heat sensitive and non-heat sensitive products can be dried using one or more of the variations of fluidized bed dryers (Osman et al., 2011).
Each design has strengths and weaknesses and implementation is highly dependent on feed and product requirements. Other advantages include smaller footprint, relatively lower capital and maintenance cost, and ease of control. Among the major issues in fluidized bed drying are (i) high power consumption, (ii) increased gas handling requirements, (iii) tendency to cause product attrition, and (iv) low flexibility in terms of feed type (size and shape) that can be handled.
The performance of fluidized beds, usually characterized by the quality of fluidization, depends on the size and shape of the feed particles, which is apparent in coal drying. To facilitate fluidization of the bed, the most straightforward way is to grind and sieve raw coal before feeding into the drying vessel. Fluidization quality can also be improved by employing mechanical vibrations, agitation, or use pulsating flow of fluidizing gas (Osman et al., 2011).
3.6.3 Microwave Dryers
There has been, and continues to be, high interest in the utilization of microwave. Such overwhelming interest is understandable considering the advantages microwave-related drying systems offer over conventional ones. Conventional drying methods employ surface heating, and are generally a slow process since the rate of heat transfer from the surface to the core of the material is dependent on (i) the process parameters, (ii) the particle size of the coal, and (iii) the properties or type of the coal.
In microwave heating, volumetric heating is achieved and energy is preferentially transferred to moisture in the material without the need to heat the material first, resulting in shorter drying time. Capital and operating costs due to use of the highest form of energy (electricity) in microwave drying remain an impediment despite its technical advantages. It has been reported that the use of microwave energy for drying coal can also result in hot spots and, thus, local overheating of the coal and can be a disadvantage in the selection of this type of dryer for coal application (Osman et al., 2011).
In most microwave drying applications, the feed is usually not stationary – microwave heating is known to be uneven, and tends to form regions of underexposure (cold spots) and overexposure (hot spots). By keeping the material in constant motion relative to the microwave-guides, more even heating can be achieved. This relative movement is usually achieved by placing the material on a rotating plate or conveyor, and passing it under the microwave guides.
Advantages of microwave heating can be accrued in one of three ways: (i) as a pre-dryer, (ii) as a booster dryer, or (iii) as a post-dryer. When used as a pre-dryer, volumetric heating due to microwave quickly forces internal moisture to the surface, facilitating the optimal operation of a conventional dryer. In booster drying, microwave energy is added as the drying rate begins to fall off, thereby sustaining or even increasing the drying rate. When used as a post-dryer, the microwave system greatly improves drying efficiency of the conventional dryer since the last one-third of water is most difficult to remove by the conventional dryer alone.
Microwave drying also produces clean coal with low-sulfur content using the ability to preferentially direct the microwave energy at the pyrite (FeS2) in coal giving rise to localized thermo-desulfurization reaction between pyritic sulfur and other neighboring reactive compounds present in the solid (Weng and Wang, 1992). The polarization of microwave fields results in the cleavage of the iron-sulfur bonds, releasing sulfur in the form of hydrogen sulfide (H2S), carbonyl sulfide (COS), or sulfur dioxide (SO2).
3.6.4 Screw Conveyor Dryers
When there is need for simultaneous conveying and heating or cooling, a screw conveyor can be easily converted to a dryer or heat exchanger by providing the necessary heat to the moving solids either directly or indirectly and by removing the evaporated moisture by gentle gas flow or by application of vacuum (Osman et al., 2011).
Typically, a screw conveyor dryer consists of a jacketed vessel (generally cylinder or U-trough) in which material is simultaneously heated and dried as it is conveyed. The heating medium, usually hot water, steam, or any thermal fluid, may also flow through the hollow flights and shaft to provide high heat transfer area without the need for additional space or material.
The screw conveyor dryer is essentially a modified screw conveyor system. Therefore successful implementation of the screw conveyer dryer not only depends on the target output properties of the processed coal, but also on the screw dynamics and physical attributes. To determine a suitable screw configuration, physical characteristics of the material to be handled such as flow pattern (related to angle of repose), abrasiveness, and size must be known beforehand. Subsequently, the volumetric feed rate, screw speed, screw size, power requirement, heat requirement, and length of screw can be determined.
3.6.5 Superheated Steam Dryer
Although the concept of drying using superheated steam was conceived more than a century ago, serious interest in superheated steam drying has emerged in the last three decades (Mujumdar, 1990). Many benefits are associated with superheated steam drying, which include (i) the reduced risk of spontaneous combustion, (ii) the increased drying rates, (iii) the better energy efficiency, and (iv) the improved grindability of the coal (Osman et al., 2011).
In the process, the optimum pressure and drying time depend on the size of the coal particles and the resulting moisture content of the dried particle depends on the steam pressure and temperature, the particle size and moisture content of the coal feedstock.
Superheated steam drying requires less energy than hot gas dryer because there is no need to supply coal moisture with latent heat of vaporization. Drying in superheated steam also increases the apparent density of low-rank coal due to shrinkage of the particles on moisture removal. In addition, the decomposition of sulfur functional groups during steam drying process produces cleaner coal with high heating value. Because
