3178 Chapter 26 



the grate as more fuel is pushed up from below. An automatic gas burner ignites 

 the pile, but shuts down after it is burning. Preheated air is forced up through the 

 pile as well as through swirl air plenums above it. Air supplied is insufficient for 

 complete combustion so that the fuel is mostly dried and gasified. Swirling air- 

 flows near the top of the primary combustion chamber subject the hot distillates 

 to a scrubbing action that separates particulates to the outer wall by centrifugal 

 action. The fixed carbon particulates complete oxidation as they are swept away. 

 As the volatiles rise to the upper combustion chamber, they are met by cooler air 

 that is vortexing downward. This action mixes and combusts the gases in a 

 cyclonic air flow. Excess combustion air is limited to 10-15 percent by this 

 mechanism. Exhaust gases leave the burner at 1,700 to 2,300°F and can fire a 

 boiler or can be blended with cooler air to use in kilns and dryers. 



For another example of a fuel cell burner, see figure 24-25. 



Inclined grate furnaces. — This type of furnace usually has a water-cooled 

 grate and water-cooled furnace walls (fig. 26-10). Fuel enters the furnace at the 

 top of the grate and is dried as it slides down to the lower horizontal section 

 where it is burned. Ash is removed intermittently through discharge doors. The 

 inclined grate furnace is one of the more popular types in Europe (Astrom and 

 Harris 1975). 



Spreader stokers. — The spreader stoker is probably the most commonly 

 used wood and bark burning furnace (Fuller 1976; Bliss and Blake 1977). These 

 furnaces can bum large amounts of wood and bark alone or in combination with 

 coal, oil, or gas with little difficulty. 



In spreader stokers, fuel is spread either pneumatically or mechanically into a 

 thin even bed across the grates. When the fuel is added above the grate, smaller 

 particles and volatiles bum in suspension while the large pieces of fuel fall to the 

 grate and bum in the fuel bed. Flames from the particles suspended above the 

 grate radiate heat that aids in combustion of the fuel bed. Fumace walls are 

 normally lined with heat exchange tubes (water walled). Because there is no 

 refractory surface to reflect heat back to fuel, combustion air is sometimes 

 preheated (Junge 1975a). 



Figure 26-11 shows a closeup sectional view of a traveling grate spreader 

 stoker with front ash discharge; in figure 26-12 the stoker is attached to a large 

 steam generator. Fuel is conveyed continuously and distributed into the furnace 

 through an air-swept spout. High-pressure overfire air jets provide turbulence to 

 aid the suspension burning of small particles and volatiles. Air for combustion in 

 the fuel bed is admitted from the plenum chamber through holes in the grate 

 bars. The fuel bed moves with the grate at a rate adjusted to allow complete 

 combustion of the fuel before the ash is dumped into a hopper. Other types of 

 grate are also used with spreader stoker fumaces (Fuller 1976). These include a 

 vibrating-grate where the fuel bed moves toward the ash hopper as the bed is 

 intermittently vibrated by an eccentric drive mechanism (fig. 26-13). In dump- 

 ing-grate stokers (fig. 26-14) the grate bars are rotated 90° to dump the ash into a 

 combined ash hopper-air plenum. 



