Energy, fuels, and chemicals 3203 



ENERGY PRODUCTION 



Gasifier types. — Reed (1980) described four general types as updraft, down- 

 draft, fluidized-bed, and suspension gasifiers (fig. 26-26). In updraft gasifiers 

 (fig. 26-26 top left) air or oxygen is injected under a grate supporting charcoal 

 causing combustion and reduction of the gases. The resulting hot gases then rise 

 through the incoming biomass at the top of the furnace, producing oils and water 

 by pyrolysis and drying. The resulting gas must be close-coupled to the burner 

 and burned directly because suspended tars are difficult to remove. Updraft 

 gasifiers are especially appropriate for retrofitting existing gas- or oil-fired 

 boilers. 



In the downdraft gasifier (fig. 26-26 top right) air or oxygen is injected above 

 the char mass, causing pyrolysis of incoming biomass and producing char and 

 oils. These oils then pass over the hot char and are cracked to gases; as a result 

 very little oil is produced. For this reason, downdraft gasifiers are particularly 

 suitable for internal combustion engines. 



Although not as well developed, fluidized beds (fig. 26-26 center) for bio- 

 mass gasification have some theoretical advantages over updraft and downdraft 

 gasifiers. Because of their high recirculation rates, fluidized beds have high heat 

 transfer rates and high throughputs. Also, they are able to process wood in a 

 wide range of sizes. Because contact time is short, however, they are not as 

 efficient in consuming char or cracking oils and tars. In fluidized beds there is a 

 tendency for light biomass fractions to separate from the bed prematurely. Less 

 well developed is suspended gasification (fig. 26-26 bottom), even though it 

 holds promise for high throughput with a feedstock of small wood particles 

 (Reed 1980). 



Readers needing a compendium of gasifier types that are in operation or 

 undergoing research are refered to a survey by Solar Energy Research Institute 

 (1979b). Research on gasifiers is active; Graham's (1980) bibliography of bio- 

 mass pyrolysis/gasification contains 274 references. See also: Reed, T.B. (Ed.). 

 1981. Biomass gasification: Principles and technology. Energy technology re- 

 view No. 67, Park Ridge, New Jersey: Noyes Data Corp. 



Yields and efficiencies. — Thermal efficiencies of gasifiers are widely vari- 

 able. Beck (1979) and Beck et al. (1980) found that the fluidized bed gasifier 

 developed at Texas Tech University yielded 20-22 sdcf of gas per pound of dry 

 ash-free wood; the gases contained significant amounts of hydrocarbons which 

 led to high heating values. In tests using air and oak sawdust, the gas produced 

 had a heating value of 250-350 Btu/sdcf, a very high value for gasification with 

 air in a single vessel. These data suggest recovery of about 6,300 Btu in gas from 

 each pound of dry oak wood, which typically has a higher heat of combustion of 

 about 7,800 Btu/ovendry pound. Other researchers have reported thermal effi- 

 ciencies as low as 60 percent. When wood gasifiers are used to drive motor 

 vehicles with internal combustion engines, 12 to 20 percent of the energy 

 contained in the wood is converted to mechanical power (Datta and Dutt 198 1). 



On-site combustion of producer gas. — Most of the gasifiers under develop- 

 ment are air blown and produce a low Btu gas suitable to be burned on site so that 



