Energy, fuels, and chemicals 



3213 



recent summary of the art of gasification, see: Overend, R.; 1982; Wood gasifi- 

 cation — Review of recent Canadian experience; National Research Council of 

 Canada, Ottowa, Paper no. 20094; p. 170-207. 



26-5 PYROLYSIS 



Pyrolysis is the thermal decomposition of organic matter in the absence of air, 

 generally at lower temperatures than gasification. Primary products are solid or 

 liquid fuels. The heat to initiate the process can be applied externally, or as is 

 common in charcoal manufacture, by burning part of the wood in air (U.S. 

 Department of Agriculture, Forest Service 1961). 



The process can be visualized by recalling what happens when external heat is 

 applied to a wood splint in a test tube. A clean combustible gas issues from the 

 mouth of the tube, a brown corrosive liquid (pyrolysis oil) collects near the cool 

 mouth, and charcoal remains in the lower portion. 



Goldstein (1980a) found that oil yields can range from 1 to 40 percent and 

 char yields from 10 to 40 percent. Overall thermal efficiency, about 45 percent, 

 was less than that of gasification. Higher values are claimed for certain propri- 

 etary byproduct recovery systems (see sec. 28-10 and figs. 28-13 and 28-14). 

 The yields of gaseous, liquid and solid products from wood pyrolysis are signifi- 

 cantly affected by particle size and reactor pressure, heating rate, temperature, 

 and residence time. Low temperatures favor liquids and char, low heating rates 

 favor gas and char, and short gas residence time favors liquids (Goldstein 

 1980a). If heat is applied fast enough, little or no char results (Diebold 1980'^). 



In the past, one method of pyrolysis was referred to as destructive distillation. 

 See Graham (1980) for a bibliography on biomas pyrolysis. Principal market- 

 able products from a hardwood distillation industry 30 years ago were acetic 

 acid, methanol, and charcoal (Beglinger 1948). Distillation was carried out in 

 externally heated ovens and retorts. While methanol and acetic acid are now 

 produced more cheaply by the petrochemical industry, wood charcoal remains 

 an economically viable product. 



Most charcoal produced in the United States today is processed into briquets 

 for cooking. Some is used as an industrial fuel, some for metallurgical process- 

 ing, and some to make activated carbon (Baker 1977; Walker 1980). In the near 

 future, wood charcoal may be slurried with high-sulfur oil or mixed with high- 

 sulfur coal before pulverization to make a fuel which would have lower sulfur 

 dioxide emissions and thus possibly comply with air pollution regulations (Bliss 

 and Blake 1977). Additionally, the charcoal-oil slurry could help reduce reliance 

 on imported oil. Technically, both processes appear feasible, but adequacy of 

 charcoal supplies for the proposed large, conventional oil- and gas-fired central 

 stations has been questioned. 



Wood is also suitable for making activated carbon. This can be done by steam 

 activation of wood charcoal; yield is generally around 12-15 percent based on 



'^Specialists' Workshop on Fast Pyrolysis of Biomass. Copper Mountain, CO; Oct. 20-22, 1980. 

 Golden, CO: Solar Energy Research Institute. 



