Energy, fuels, and chemicals 



3175 



Basically there are two classes of furnace design in use for wood burning — 

 grate burners and suspension burners. Dutch ovens, inclined grate furnaces 

 and spreader stokers bum the fuel on a grate, either in a pile or spread into a thin 

 bed. In suspension burning, the fuel is supported by air during its combustion. 

 Within these two classes of furnace design are a variety of furnace types which 

 can differ markedly in basic operation, largely in adjusting to the various forms 

 that wood fuel can take. A few of the most important types are described in this 

 text. Readers needing additional information on hardware for industrial burning 

 systems will find useful the proceedings on this subject published by the Forest 

 Products Research Society (1979). 



Dutch ovens. — Historically, Dutch ovens (fig. 26-7) provided steam for 

 many industries in this country. They are now considered obsolete for new 

 installations, because they are expensive to maintain and respond poorly to load 

 changes. 



TO 



STACK 



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Figure 26-7. — Dutch oven furnace and boiler. (Drawing after McKenzie 1968.) 



The Dutch oven is a two-stage refractory (firebrick-lined) furnace. Fuel is fed 

 through a chute in the oven roof and forms a conical pile on the grate in the oven 

 or primary furnace. Here the fuel is dried and partially combusted. The system is 

 designed so that there is insufficient air for complete combustion in the primary 

 furnace. Combustible gases emerging from the fuel pile pass into the secondary 

 furnace, or combustion chamber, where air entering through the overfire ports 

 completes combustion. On their way to the stack, the hot combustion gases pass 

 through water-carrying heat exchangers to generate steam. Removal of ash is 

 done by either manual rake-out or dumping grates. Combustion rates of about 

 600,000 Btu per ft^ of grate per hour have been attained with air and wood 

 having 45 percent moisture content, but combustion rate drops off rapidly above 

 that level. Excess air at the stack is usually between 30 and 40 percent (Babcock 

 and Wilcox Company 1972; Junge 1975a). 



