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Chapter 26 



WET WASTE WOOD 



250»F 



FINE FUEL TO BURNER 



Figure 26-3. — Typical rotary dryer system. (Drawing after Johnson 1975. 



Rotary drum dryers are suited for handling large quantities of wet fuel. The 

 inlet temperature can go as high as 1600°F if the moisture in the fuel is high 

 enough to absorb the heat without scorching the wood. A major environmental 

 concern for the forest products industry, however, is blue haze, caused by 

 distillation of organic material such as terpenoids into the air while drying wood. 

 Douglas-fir dried above 750°F creates blue haze. Optimum drying conditions for 

 pine-site hardwoods are not known. 



Bark or hogged wood for fuel can also be dried in a vertical dryer (fig. 26-4) 

 in which flue gases from combustion provide required heat. Before entering the 

 dryer, the flue gas is divided. One part enters the unit through a centrally located 

 slot in the bottom. The other part is supplied thorugh peripheral slots in the 

 conical bottom. It conveys the bark from the periphery into the center. Here a 

 continuous cascade of flue gas and bark goes up from the bottom and reverses at 

 the deflector in the dryer's top. 



After deflection, the bark drops and rejoins the cascade while the flue gases, 

 now cooled, leave the dryer through the outlet above the deflector. Wet bark is 

 supplied through a rotary air lock located in the upper part of the unit. An 

 adjustable outlet slot in the conical bottom of the dryer discharges bark to 

 another rotary air lock. By adjusting the slot area, the bark retention can be 

 controlled and adjusted to various operating conditions. 



The flue gas leaving the dryer carries both ash from the boiler and the 

 combustible dry fines from the bark which must be collected. Collection and 

 separation takes place in two stages. First, combustible material is extracted in a 

 precollector, then pneumatically conveyed and either mixed with the bark or 

 burned separately in a suspension burner. Second, ash is collected in a multi- 

 cyclone unit and then conveyed for deposit. 



