3168 Chapter 26 



The technology for densifying particulate wood and bark has been around for 

 over 50 years (Sprout Waldron and Co. 1961; Hausmann 1974; Currier 1977; 

 Famsworth 1977). The famous "pres-to-logs" used for hand-firing fireplaces 

 and stoves were first made about 1933. In the 1950's machines were developed 

 to extrude wood residue rods suitable for fueling coal stoker furnaces. General- 

 ly, the rods are about 1 inch in diameter and can be cut to desired lengths. In 

 1959 a company in Tennessee first pelletized bark for fuel. These oak bark 

 pellets were used with coal to fire a steam boiler and reportedly gave burning 

 rates equivalent to soft-coal. Since the late 1960's extensive research on pelleti- 

 zation of wood and bark residues for fuel and other uses has been caried out by 

 Currier (1977) at Oregon State University and Steffensen (1973) at Georgia- 

 Pacific. 



Pelletization can be accomplished with standard agricultural pellet mills (Cur- 

 rier 1977). The product is generally 3/16- to 1/2-inch in diameter and l/2-inch 

 long. The compression rate in pelletizing is usually about 3 to 1 . Because of their 

 uniform density, fuel pellets are easier than hogged fuel to meter into a furnace 

 and can be burned at more closely controlled rates. Wet fuel cannot be pelletized 

 unless it is first dried to 6-10 percent moisture content, at a considerable energy 

 cost. Because of this, those industries whose wood wastes are dry, such as 

 furniture and cabinet manufacturers, may find pelletizing most economically 

 attractive. 



A company in Alabama pelletizes sawdust and bark after pre-drying it in a 

 rotary dryer fired by some of the pelletized product. The pellets are sold to a 

 power plant. Estimated total investment is about one-half million dollars includ- 

 ing land. The pelletizer sells for around $32,000. Similar estimates of cost have 

 been reported by Farnsworth (1977). A company in Oregon markets pelletized 

 Douglas-fir bark (Blackman 1978). Reportedly, the product sells for about half 

 the price of coal of equivalent Btu's. Plant operations use the equivalent of 12 

 percent of the energy contained in the pelletized product. Costs of densification 

 are dependent on degree of comminution and drying required (estimated at $2 to 

 $6 per ton in 1978). 



North American manufacturers of equipment to make densified pellets or logs 

 include the following: 



Extruders Agnew Environmental Products, Inc., Grants Pass, Ore. Califor- 

 nia Pellet Mill Co., San Francisco, Calif. Universal Wood Ltd., 

 San Diego, Calif. 



Pellet mills California Pellet Mill Co., San Francisco, Calif. Sprout Wal- 

 dron, Muncy, Pa. 



Wood and wax extruders Bonnot Co., Kent, Ohio 



Cubers Warren & Baerg Manufacturing, Dinuba, Calif. 



Residues. — Ash and incompletely combusted carbon are the solid residues 

 from burning wood. In boiler furnaces, where high burning temperatures are 

 achieved, slag and clinker are formed from the melting and fusion of ash. Ash 

 content of wood and bark of the 22 principal hardwoods found on southern pine 

 sites are shown in tables 6-1 and 6-18. Data from table 6-18, which describes 

 trees 6 inches in dbh, are summarized as follows: 



