3206 Chapter 26 



produced has a heat content of 150 to 165 Btu/standard cu ft and has composi- 

 tion, by volume, as follows: nitrogen and nitrogen compounds, 50 percent; 

 carbon monoxide, 30 percent; carbon dioxide, 7 percent; oils and tars, 3 percent; 

 hydrogen, 10 percent; water vapor, variable according to moisture content of 

 wood. 



Tests indicate that 75 tons per 24-hour day of hardwood or softwood chips 

 with average size of 3 by 3 by '/z-inch and maximum moisture content of 50 

 percent (wet basis) will produce 25 million Btu/hr. The system requires motors 

 totalling 110 hp and consumes water at 10 gallons/minute. Plant steam in the 

 amount of 500 Ib/hr is required to cool the grates and enhance Btu content of the 

 gas. Ash output is about 500 pounds per 24-hour day. 



Economics of retrofitting boilers for on-site combustion. — Wood gasi- 

 fiers, when commercially practical models are available, will be suitable for 

 retrofit to existing furnaces and boilers now fired by oil or natural gas (see sect. 

 28-6). The Solar Energy Institute (1979a) estimated that the cost of retrofitting 

 existing equipment with air gasifiers, having output in the range from 5 to 100 

 million Btu/hour, should be about two-thirds the cost of a new solid-fuel installa- 

 tion. Furthermore, gasifiers offer lower emissions and higher turndown ratios 

 (better ability to adjust to fluctuating loads) than solid-fuel boilers. The Solar 

 Energy Institute estimated that with a wood cost of $20/ton (green-weight basis), 

 total cost including capital, operating, and wood costs should be in the range 

 from $2.58 to $4.00/million Btu (1978 basis). 



Numerous other economic analyses of gasifier applications are in the litera- 

 ture, e.g., Eckert and Kasper (1978), Desrosiers (1979), and Crane and 

 Williams." 



Gas transportable in pipelines. — The medium Btu gas produced by oxygen- 

 blown gasifiers is comprised mainly of carbon monoxide and hydrogen (table 

 26-11) and if suitably cleaned of corrosive oils and tars can economically be 

 piped short distances to power turbines or synthesize chemicals (Reed 1980). A 

 pipeline-quality gas for transport over long distances can be produced by the 

 methanation reaction described in the next subsection (see equation 26-14), or 

 by the water shift reaction to make hydrogen (see equation 26-13). 



Fuel cells. — Generaton of electricity by fuel cells that use producer gas from 

 wood is an unproven process but potentially a viable one (Hodam 1978; Eggen 

 and Kraatz 1976). Fuel cells use hydrogen and oxygen (from air) to convert 

 stored chemical energy directly to electrical energy (figure 26-28). 



2H2 + O2 ^""' '^"" > 2H2O + heat + electrical power (26-12) 



The concept is not new. Fuel cells have provided electrical power for moon 

 landings and, on a demonstration basis, for apartments, commercial establish- 

 ments, and small industrial buildings. It is expected that electrical generation 

 from coupling wood gasifiers with fuel cells will be significantly more efficient 

 than using turbines or diesel engines (Hodam 1978). 



'^Crane, T.H., and R.O. Williams. 1979. Gasification of mill residues with a downdraft gasifier. 

 Paper presented at 33rd Ann. Meet.. For. Prod. Res. Soc, San Francisco. July 8-13, 1979. 



