Energy, fuels, and chemicals 3201 



The constitution of synthesis gas (hydrogen and carbon monoxide), which 

 can be used to produce several important chemicals, is closely approximated by 

 medium Btu gas from wood gasification. 



The basic processes in gasification are similar to those in combustion except 

 that complete oxidation of carbon to carbon dioxide is avoided. Only sufficient 

 air or oxygen is provided to the gasifier bed for ( 1 ) gasification of the carbon char 

 by partial oxidation to carbon monoxide and (2) for generation of enough heat to 

 support drying and pyrolysis of the fuel (Eggen and Kraatz 1976). Drying and 

 pyrolysis of the fuel occur much as they do in combustion except the volatile 

 gases produced are not oxidized. In pyrolysis, water vapor, carbon monoxide, 

 and carbon dioxide are formed first. Then further decomposition gives tars 

 which finally yield hydrogen and hydrocarbons leaving only a carbon char. The 

 carbon char is partially oxidized to carbon monoxide by oxygen, water, or 

 carbon dioxide. The carbon dioxide then is reduced to carbon monoxide. While 

 the addition of air or oxygen to the fuel bed is necessary to sustain this complex 

 oxidation-reduction scheme, no air or oxygen can be allowed to mix with the 

 gases produced as they would readily combust. 



Researchers are investigating the use of catalysts in gasification of wood 

 (Feldman 1978; Walkup et al. 1978; Garten et al. n.d.). A primary reason for 

 catalyzing gasification would be to lower gasification temperature and to im- 

 prove reactivity. The best catalysts found so far are calcium oxide and wood ash. 

 Catalysis may also be used to selectively form one product. If such a catalysis 

 system can be developed, gases that are predominantly methane, hydrogen, or 

 carbon monoxide or optimum mixtures for a specific purpose might be pro- 

 duced. These products would be of much higher value than producer gas and 

 would make gasification a more competitive conversion process. 



Gasification is not new; coal was gasified in the nineteenth century." Early 

 gasification units operated by blowing air up through the fuel bed and allowing 

 gases to exit the top (updraft operation). The producer gas was used to fire 

 boilers and furnaces. 



After cooling and cleaning, producer gas has been used to fuel internal 

 combustion engines. In fact, about 600 Crossley gas plants (fig. 26-24) were 

 constructed for that purpose between 1912 and 1940 (Levelton and O'Connor 

 1978). Reportedly some are still in working order. 



Portable gasification units were developed during the 1940's." About 

 700,000 vehicles were adapted to use producer gas in Europe during World War 

 II. Coal, coke, and charcoal were commonly gasified; wood and crop residues 

 had limited use. Although development of portable gasifiers mostly ceased after 

 World War II, Swedish researchers produced a successful wood-fueled down- 

 draft gasifier for use on agricultural vehicles. For an extensive account of the 

 Swedish experience in developing generation gas for motor vehicles, see Solar 

 Energy Research Institute (1979a). See section 28- 1 for discussion of the cost of 

 owning and operating a diesel tractor operated on wood gas. For description of 

 producer-gas-fueled spark-ignition engines for low power systems (to 20 kW) 

 using charcoal, seeHoUingdaleetal. (1983). See also: Kaup, A. andJ.R. Goss. 



