3214 Chapter 26 



the weight of the dry wood (Baker 1977). Currently, most activated carbon is 

 made from coal or coconut hulls. About one-third of the market is for water and 

 waste-water treatment (Walker 1980). Other uses are for air pollution control 

 systems, catalyst supports, sugar decolorization, and auto evaporation control 

 systems. Expanded demand is expected, however, if the Environmental Protec- 

 tion Agency (EPA) requires the use of activated carbon by all water treatment 

 plants to remove suspected carcinogens such as chloroform and carbon 

 tetrachloride. 



In the United States, most wood charcoal is now produced either in a batch 

 process using Missouri-type kilns or in a continuous process using a Herreshoff 

 multiple-hearth furnace. Recently, other continuous processes have been devel- 

 oped which can be used to produce liquid and gaseous fuels as well as charcoal 

 (Bliss and Blake 1977; Baker 1977). 



CHARCOAL FORMATION IN KILNS 



Ignition, coaling, and cooling, the three stages in the formation of charcoal 

 from wood (Hallett 1971), are most readily described in relation to batch 

 processes. 



Ignition is endothermic (requires heat) and requires considerable air (U.S. 

 Department of Agriculture, Forest Service 1961). The wood is first ignited with 

 kindling and kerosene and allowed to bum. Temperatures in the kiln rise rapidly 

 to about 1 ,000°F and then drop when most of the starter fuel has been consumed. 

 Coaling begins around 540°F, and at that temperature the process becomes 

 exothermic (gives off heat). When the coaling stage is reached, air intake is 

 restricted to control kiln temperature and to ensure that the wood will form 

 charcoal and not bum to ash. For the production of good quality charcoal, kiln 

 temperatures of 850° to 950°F are maintained during coaling. 



After coaling, the kiln is completely sealed to exclude all air and allowed to 

 cool. When temperatures are 150°F or less, it is generally safe to open the kiln. 



A complete cycle, from ignition through cooling, may require 6 to 7 days. 

 Typical charcoal produced from wood will have a higher heating value of about 

 12,000 Btu/lb. Proximate analysis will be about 20 to 25 percent volatile matter 

 and 75 to 80 pecent fixed carbon on a moisture free and ash free basis. Ash 

 content is generally about four times that of wood (Peter 1957; Baker 1977; U.S. 

 Department of Agriculture, Forest Service 1961). 



The transformation of wood into charcoal has been studied by simulating 

 commercial charring conditions in the laboratory (Slocum et al. 1978;. Most 

 work has been done on oak and hickory. Study of mass loss, shrinkage, and 

 physical properties of charcoal produced under various coaling conditions will 

 lead to a better understanding of this very complex and incompletely understood 

 process (McGinnes et al. 1971; Anonymous 1975; Slocum et al. 1978). Analyt- 

 ical techniques that have been found especially useful for these studies include 

 the use of scanning electron microscopy to follow shrinkage and small angle X- 

 ray analysis to evaluate micropore formation in the charcoal (von Bastian et al. 

 1972; Blankenhorn et al. 1972; Beall et al. 1974; McGinnes et al. 1974; 

 McGinnes et al. 1976). 



