3216 



Chapter 26 



Figure 26-31. — Missouri-type batch kiln. (Photo courtesy off Bob Mossengole, Missouri 

 Department of Conservation.) 



supply of wood. The smallest of these furnaces requires about 100 tons of wood 

 per day (dry basis); with wood that has moisture content of 40 percent (wet 

 basis), production of charcoal is about 1 ton per hour. 



The Herreshoff furnace consists of several hearths stacked one on top of the 

 other and enclosed in a cylindrical, refractory- lined steel shell (Nichols Engi- 

 neering and Research Corporation 1975; Bliss and Blake 1977). Passsing up 

 through the center of the furnace is a shaft which carries arms with rabble teeth. 

 As the shaft turns, the teeth constantly plow through and turn the fuel over while 

 moving it in a spiral path across the hearth floors toward drop holes connecting 

 with the next lower hearth. The drop holes alternate on each hearth floor between 

 the inner periphery (around the shaft) and the outer periphery of the hearth. The 

 rabble teeth are alternately angled so that the material moves in the direction of 

 the next set of drop holes. In this way material fed at the top of the furnace is 

 spread over and across each hearth floor in a radial flow that alternates inwardly 

 and outwardly until the charcoal is finally discharged from the bottom of the 

 furnace. Material is dried, heated, and carbonized as it passes downward 

 through the furnace while hot combustible gas produced passes upward in a 

 current counter to the flow of the material (fig. 26-33). 



Heat for startup is provided by gas- or oil-fired burners mounted on the sides 

 of the hearth. When the proper temperature has been attained, the auxiliary fuel 

 is turned off. To sustain pyrolysis, sufficient combustion air is allowed into the 

 furnace to bum some of the volatile gases produced. Air entry is regulated to 

 maintain furnace temperatures at 900 to 1200°F. The evolving combustible 

 gases are more than that needed for drying and carbonizing the wood and can 

 also be burned in a boiler after they exit the furnace. With a heating value of 

 about 200 Btu per sdcf plus the sensible heat of the exhaust temperature, the 

 pyrolysis gases from the production of 2 tons of charcoal per hour can generate 



