3200 Chapter 26 



Miller et al. (1982) also suggested that dry storage of chips under conditions that 

 do not allow fungal growth is important to avoid propagation of allergenic and 

 pathogenic fungi; chips subject to biological heating, if loaded into a home chip- 

 fuel furnace, may distribute fungal propagules throughout basement and upper 

 floors. 



26-4 GASIFICATION 



Water slurries of wood and bark, or whole-tree chips, can be converted by 

 anaerobic biodigestion to a gaseous fuel (methane); the process is not further 

 described because yields of gas are too low, and the process too slow, to warrant 

 economic consideration (National Aeronautics and Space Administration 1979; 

 Pfeffer 1978). 



Gasification, as discussed under this heading, is accomplished by thermal 

 decomposition of organic material in the presence of controlled and limited 

 amounts of air or oxygen to produce a combustible mixture of gases, often 

 referred to as producer gas. When air is used, the producer gas contains mostly 

 hydrogen, carbon monoxide, and nitrogen. Lesser amounts of carbon dixoide, 

 methane, and hydrocarbons are formed. The mixture is generally referred to as a 

 low Btu gas (table 26-1 1). Heating values range from 100 to about 200 Btu/sdcf 

 (standard dry cubic foot) (Levelton and O'Connor 1978; California State Energy 

 Commission''). 



When oxygen is used for gasification, the producer gas is termed medium Btu 

 gas, with heating values as high as 350 Btu/sdcf (Bliss and Blake 1977). 

 Nitrogen dilution is eliminated so that the two major components of medium Btu 

 gas are hydrogen and carbon monoxide. Lesser amounts of carbon dioxide, 

 methane, and hydrocarbons are also produced (Table 26-11). 



The exact chemical composition of either low or medium Btu gas depends on 

 variables that include gasification temperature, pressure, time, and presence of a 

 catalyst (Love and Overend 1978). In the crude state, both low and medium Btu 

 gas contain high percentages of moisture which originates from the partial 

 oxidation process and from moisture in the fuel. However, the moisture can be 

 readily removed (Bliss and Blake 1977). 



Table 26- 1 1 . — Typical composition for low and medium Btu gases produced on gasifi- 

 cation of biomass^- 



Low Btu Medium 



Constituent gas Btu gas 



Carbon monoxide (CO) 



Hydrogen (H2) 



Carbon dioxide (CO2) . 

 Hydrocarbons (CH^) . . . 

 Nitrogen (N2) 



'Data from Benemann (1978). 

 ^Composition varies. 



"California State Energy Commission, "Commercial Biomass Gasifier at State Central Heating 

 and Cooling Plant", Feasibility Study prepared by Fuels Office, Alternatives Division, April 1978, 

 63 p. 



