3188 Chapter 26 



Fuel moisture contents can be up to 65 percent. Exhaust gas temperatures are 

 normally up to 2,200°F and bed temperatures around 1 ,750°F. The largest units 

 being constructed will handle 250 tons per day of hogged fuel and generate 

 55,000 pounds of steam per hour. Some authorities do not expect large units 

 comparable to the spreader stokers to be built in the near future (DeArmond et al. 

 1975; Keller 1975), but this opinion is not unanimous (Daman 1979). 



Jasper-Koch burner. — The Jasper-Koch burner (fig. 26-20) takes a new 

 approach to suspension burning. The unit is designed to bum wet wood or bark 

 efficiently in a small furnace with output of about 6 million Btu per hour that 

 costs less than grate-type furnaces of comparable capacity. 



As noted previously, bark or sawdust that is half water by weight bums very 

 poorly and must be partially dried before combustion occurs. This drying can 

 take place in a separate dryer before buming, in a pile on the floor of the fumace 

 combustion chamber, or in an integral dryer that pases through the combustion 

 zone. The Jasper-Koch bumer works on the latter principle. 



In the Jasper-Koch design, the combustion chamber is an annular space 

 between two concentric vertical cylinders; particulate fuel bums in suspension in 

 this chamber (fig. 26-21). In the commercial prototype, a stainless steel inner 

 cylinder 29 inches in diameter with walls !/4-inch thick houses a vertical down- 

 feeding screw that introduces fuel into the bottom of the chamber. The outer 

 stainless steel cylinder that confines the upward-moving combustion gases is 49 

 inches in diameter and also has !/4-inch thick walls. This cylinder is flared to 72 

 inches at the top of the combustion zone. The fuel is nearly ovendried in its 15- 

 minute transit to the combustion zone. Steam from the drying fuel exits through 

 the combustion zone. 



Surrounding the outer cylinder, along the entire 7-foot-high buming zone, is a 

 heat exchanger that preheats air to about 500°F and introduces it at high velocity 

 (1,000-1,300 fpm) into the bottom of the bumer. This air conveys fuel particles 

 upward into the combustion zone, where the temperature is about 1 ,600°F. A 30 

 hp blower is required for the preheated air to assure proper airfllow and velocity. 



The bumer has neither grate nor fuel bed. Combustion occurs throughout a 

 zone in which particles are suspended in the air stream. The outer cylinder is 

 flared at the top of the combustion zone. Since the inner cylinder has constant 

 diameter, the flare increases flow area for upward-moving combustion gases, 

 slowing their velocity. This causes particulate matter (other than fine ash) to fall 

 back into the buming zone for continuous recirculation until completely bumed. 

 Neither the laboratory model (Jasper and Koch 1975) nor the commercial proto- 

 type (Koch et al. 1978) formed slag since combustion temperatures do not 

 exceed 1,800°F. Ash formed during combustion is discharged upwards along 

 with hot combustion gases for later separation. The bumer is equipped with a gas 

 jet of one million Btu/hour capacity to facilitate startup. 



Nonuniformity of fuel particle size is a problem for designers of wood buming 

 fumaces. Most grate-type fumaces operate best with particles of pulp-chip size 

 and cannot tolerate more than 50% sawdust or fines in the fuel mixture. Because 

 wood particles can be hammermilled to smaller size, but cannot be increased in 

 size (unless pelletized), the operators of grate-type fumaces have difficulty with 

 an oversupply of fines and sawdust. 



