Energy, fuels, and chemicals 3165 



Fuel preparation. — High moisture content, bulkiness, dirt, and heteroge- 

 neous size are undesirable properties of some wood fuels. As with refined 

 petroleum fuels, wood can be upgraded in value for various burning situations 

 (Johnson 1975). Wood-burning furnaces, for example, are designed to burn chip 

 size or smaller material. This permits automation in feeding and more efficient 

 burning. But of course the size of pieces can range from slabs and logs to 

 sawdust and shavings. To reduce and size wood for easier handling and burning, 

 it usually is passed through an attrition mill or hog. In a typical hammer hog (fig. 

 18-277) large pieces enter at the top and are forced down and through spaced 

 bars by the rotating action of hammers. The size of the particles leaving the hog 

 depends on speed of the hammers and clearance between the hammers and the 

 spaced bars. Optimum particle size depends on the burner. In some grate-type 

 furnaces, for example, an excess of fines and sawdust inhibits efficient combus- 

 tion. See figures 18-276 and 18-278 through 18-280 for other types of hogs and 

 shredders. 



Hogged fuel is wood waste having no higher recovery value than that of fuel; 

 such fuel contains no municipal refuse (Johnson 1975). Under this broad defini- 

 tion, which is in common use today, a fuel would not have to be passed through a 

 hog to be called hogged fuel. Common hogged fuels include sander dust, 

 shavings, sawdust, bark, log yard clean-up, clarified sludge, forest residuals, 

 and fly carbon. 



Drying hogged fuel increases steam generation efficiency and reduces emis- 

 sions (Johnson 1975; Hall et al. 1976). Even furnaces designed for wet wood 

 operate best at fuel moisture of 50 percent or less on a wet basis. At moisture 

 contents exceeding 60 percent, most furnaces operate poorly, if at all. 



Wood fuel can be dried in several ways. Mechanical pressing squeezes water 

 out with simple equipment that requires no heat; moisture content can be reduced 

 to 50-55 percent by this method (Porter and Robinson 1976; Haygreen 1981). 

 Air drying (figs. 20-15 through 20-18) is slow but can reduce the moisture 

 content to about 20 percent and if climate permits, even less. Perhaps solar 

 dryers will be developed for this purpose. Today, fuel dryers which use hot gases 

 are in widespread use. The most common type is the rotary drum. Other types of 

 fuel dryers, such as the hot hog and hot conveyor system (American Logger and 

 Lumberman 1978; Johnson 1975) have not been as widely used.^ 



In a typical rotary drum dryer system, fuel that is to be dried is first screened 

 to remove and rehog oversized pieces. Hot gases and the fuel enter one end of the 

 drum or cylinder and are brought into contact with each other by cylinder 

 rotation and horizontal gas flow (fig. 26-3). An induced-draft fan draws the hot 

 gases through the dryer, and this gas flow pushes the dried fuel toward the outlet. 

 Two- to four-inch pieces can be adequately dried in 20 minutes or less. Fuel is 

 separated into different sizes at the dryer outlet. Airborne fines go to the cyclone 

 separator with the gas flow (Thompson 1975; Johnson 1975). 



Personal communication with David C. Junge, Oregon State Univ., Corvallis. 



