Energy, fuels, and chemicals 



3183 



Hamrick (1978, 1980) found that large-scale furnaces to bum wood in suspen- 

 sion at rates comparable to those achieved with powdered coal require finely 

 ground wood particles fired with primary combustion air at 500°F and secondary 

 combustion air at about 1 ,0(X)°F. 



In the system shown in figure 26-15, turbulence is provided by high velocity, 

 tangentially injected flows of preheated air through nozzles at various heights in 

 the furnace (Femandes 1976). In such tangential firing, the air flows create 

 spinning air masses or fire circles that hold the fuel particles in suspension while 

 they bum (fig. 26-16). Figure 26-17 reproduces a photograph of the fire circle in 

 a boiler fumace. 



In the suspension burners, a small dump grate is located at the bottom of the 

 fumace to catch and bum larger fuel particles that fall out of suspension (Fer- 

 nandes 1976). Usually, suspension-fired boilers are simpler than spreader stok- 

 ers, but fuel must be hogged to smaller sizes. Bark alone can be used as fuel but 

 usually an auxiliary fuel, such as oil, pulverized coal, or gas is co-fired in 

 suspension with the bark. Auxiliary fuel nozzels are usually located immediately 

 above and below the bark nozzle. Suspension fired boilers are often large units, 

 some capable of generating over 500,000 lb of steam per hour (Hall et al. 1976). 



BARK NOZZLE 

 AT BURNER 



TANGENTIAL AIR 



Figure 26-15. — Schematic of a suspension firing system used in large boilers. (Drawing 

 after Fernandes 1976.) 



