Energy, fuels, and chemicals 



DRYER 



3167 



FURNACE 



Figure 26-4. — Bark dryer in which flue gases from combustion supply required heat. (1) 

 Wet bark conveyed to dryer. (2) High velocity gas stream. (3) Main gas stream. (4) 

 Stone and scrap discharge. (5) Gas and combustible fines leave dryer. (6) Dried bark 

 conveyed to boiler. (7) Pre-collector separates combustible fines. (8) Secondary cy- 

 clone for combustible fines from pre-collector. (9) Combustible fines join dried bark 

 enroute to boiler. (1 0) Multicyclone for final separation. (1 1 ) Secondary cyclone for fly 

 ash from multicyclone. (12) Fly ash removed from system. (13) Stack emission cleaned 

 to code. (Drawing from BAHCO Systems, Inc.) 



Some burners will operate only on dry fuel. Others are designed to burn wet 

 fuel; opinion is not unanimous on the economic virtues of drying fuel in a 

 separate operation before admitting it to wet-fuel burners. Some furnace design- 

 ers contend that the most economical place to dry wet fuel is in the combustion 

 chamber of a wet-fuel burner; others disagree. 



Bulkiness of wood fuel can be circumvented to some extend by compressing it 

 into more dense material of various shapes and sizes. Such compressing makes 

 storing, handling, and shipping easier and promotes uniform burning. 



There are several ways to densify wood for fuel. Tops and limbs can be baled 

 (fig. 16-53). Compression bailing of wood chips can reduce volume to one-fifth 

 their loose bulk while squeezing out 12 to 15 percent of their moisture (American 

 Hoist and Derrick 1976). Dry particulate wood and bark can be formed into log 

 or pellet shape by compressing in a die under heat and pressure sufficient to 

 break down the elasticity of wood or bark, so that the densified form will have 

 permanence (Hausmann 1974). It is possible that polyphenolic compounds 

 (especially in bark) play a role in successful densification. The compounds may 

 act as natural binding agents or glues when subjected to heat and pressure. Fire 

 logs for home use can employ a wax or other binder (Braun 1975). 



