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Steam Rotary Dryer

Process Description
The coal particle with size smaller than 1mm from section of crushing and screening is transferred to the drying system. The steam rotary dryer is adopted in this system. Its main body is a huge rotary cylinder, where heating tubular coils in 3 to 5 circles are concentrically run through inside. The steam is drawn into the cylinder to provide heat what drying needs. The heating tubes may be of light type or fin type or both, which depends on the characteristics of material and drying process requirements. The material to dry is fed into the dryer by hopper and then lifted, scattered along with rotation of the cylinder, accomplishing heat transfer by means of convection, conduction and radiation. The dehydrated material is further moved to the lower end from the higher end and discharged by air-lock valve at the end of dryer. Moisture is carried with air flow and vented out by fans. The airflow direction can be the same with the material flow or to the contrary, depending on process demand.

The system of drying/cooling can be briefly described as follows:
  N2 charging: Before a start-up, nitrogen enters the system from each charging port. A pneumatic valve behind the outlet of recycling fan will open when recycling fan is turned on and it will be closed when oxygen content inside reaches less than 8%. Meanwhile, each N2 sealing valve is opened, causing oxygen content inside to maintain within 8%.
  N2 circulating: During the process of normal operation, nitrogen at the initial state, coming from N2 storage tank, is heated by the heater and then transferred to the dryer, mixing thoroughly with the wet coal particles. It then leaves with the moisture and partial dry coal fines from the dryer to the dust collector I, through which most of coal fines is removed. Nitrogen and moisture as well as trace of coal fines enter the dust collector II, through which coal fines is further removed. After these two stages of dust removal, now the content of coal fines in system can be regarded as zero. The gas mixed with nitrogen and moisture is transferred by the circulating fan to the surface-cooler to be cooled down and then the moisture will be removed by mist eliminator. The clean nitrogen returns to the nitrogen storage tank, back to its initial state (with low temperature and humidity), thus accomplishing a closed circulation. 
  Flow of material processing: The wet coal particles is conveyed by the measuring conveyor to the rotary dryer, in which heat & mass transfer takes place between the wet material and steam heating coils. The moisture containing in the coal will be evaporated, coal fines being dehydrated with a rising temperature. When it arrives at the discharging port of the rotary dryer, the coal fines will be dehydrated to the rated value, with its surface temperature rising to the maximum value. The material is then discharged to the hopper and it further via air-lock valve fall onto the feeding conveyor, which transfers the hot and dry coal to the rotary cooler, by which the coal will be cooled down to the rated value. The coal fines will go out of the outlet of rotary cooler and fall into the air-lock hopper to the cooler, and then be transferred to the section of storage.

Safety: The advanced drying technology and reliable explosion-protected construction are adopted in process design. The steam rotary dryer relies itself on heat supply by heat exchanger equipped inside of the dryer. Nitrogen works as the carrier gas with moisture. The material can fulfill the whole dehydration process in nitrogen environment so as to avoid the accidents such as fire or explosion.
Reliability: The rotary-type equipment is not demanding for granularity class distribution of material so that the coal being crushed can be well dehydrated.
Low cost in operation: This indirect heat exchanging method can effectively reduce the losses of heat emission and greatly improve the thermal utilization rate of system.

Process Flow Diagram
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