Energy, fuels, and chemicals 3199 



speed), firing directly below the fire grate lights the fuel as it falls on the grate 

 and shuts off once the chips are ignited. Chip feed continues while the thermostat 

 calls for heat and ceases when the thermostat is satisfied. 



In the design of Riley et al. (1979) air supply is governed so that sufficient air 

 is always supplied for complete combustion but excess is limited to maintain a 

 firebox vacuum. As much of the combustion air as possible is passed through the 

 grate to: 



• Produce an intense hot fire which bums the chips, which must be pre- 

 viously dried to 30 percent, or less, moisture content 



• Produce a high ratio of energy release/grate area 



• Increase grate life by cooling the grate with air flow 



• Cool ash on the grate below fusing and slagging temperature, thus leav- 

 ing it particulate so it will fall through the grate into the ash pit. 



The refractory-lined firebox has two chambers, one containing the grate and the 

 other forming an afterburner tube to provide sufficient turbulence and retention 

 time at high temperature to insure complete combustion before flue gases leave 

 the firebox and commence cooling. Firebox operating temperatures are 1,400- 

 1 ,800°F with fairly dry chips and 1 , 100-1 ,300°F with moister chips. The small 

 amount of fuel remaining on the grate after the thermostat shuts off burns out in a 

 few minutes. The unit is designed for hot-air, hot- water, or steam central heating 

 systems. Wide application of chip-burning, top-feed central furnaces for homes 

 is somewhat inhibited by the need to pre-dry the chips to below 30 percent 

 moisture content and screen them to eliminate oversize chips. Also, distribution 

 systems must be established to deliver 1-ton loads of chips to home fuel bins. 

 Chip-fired residential stoves and furnaces (usually of under-feed design) are 

 common in Sweden; Thomqvist and Lundstrom (1980) found that airborne 

 fungal propagules were more numerous in Swedish homes so heated, than if 

 heated by oil or stick firewood. They found that risk of fungal attack is mini- 

 mized if chips are stored only short periods of time, if the roundwood from 

 which the chips are cut is stored a long time before chipping, if the porportion of 

 hardwood chips in the pile is reduced, and if trees are limbed and topped before 

 chipping (to decrease content of leaves, needles, and bark). They recommend 

 the following measures to avoid heavy air contamination by fungus propagules 

 to which many people are sensitive and which cause allergies: 



• Do not store chips in dwellings. 



• Wear a protective mask when working with chips. 



• Do not store clothes in a room where chips are stored. 



• Keep furnace room and storage room as clean as possible. 



• Remove old chips before introducing new chips to storage. 



• Build storage room of durable materials to prevent decay. 



• Make storage room spacious to allow large volumes of air above stored 

 chips. 



• Ventilate air from the storage room to prevent it from reaching living or 

 working areas. 



