3198 



Chapter 26 



An alternative way to bum wood cleanly is to bum it at very high temperature 

 in a small refractory-lined combustion chamber with heated, turbulent, forced- 

 flow combustion air. Heat from resulting complete combustion can be captured 

 by a heat exchanger which transfers it to an insulated water storage tank with 

 sufficient storage capacity to heat a home for 2 days or more. The homeowner 

 monitors the water temperature and reheats it with a hot furnace fire when 

 necessary. Such fumaces, developed by Hill (1979), are manufactured in Maine 

 for firing with round or split wood. 



Chips can be automatically stoker-fed to furnaces in response to thermostat 

 signals and they are cheaper per ton than firewood in stick form. '° These factors 

 stimulated Riley et al. (1979) to develop at the University of Maine a residential 

 furnace (fig. 26-23) fueled by wood chips via a top-feed stoker. In their design 

 chimney temperatures are usually 250-300°F to allow natural draft to provide 

 sufficient combustion air, eliminating need for safety chimneys or emergency 

 blowers in the event of a power failure. The small chip bin holds a 1- to several- 

 day fuel supply. A short conveyor carries chips to the firebox on call of a 

 thermostat, with feed rates from 8 to 16 pounds of wood (ovendry weight basis) 

 per hour resulting in heat outputs of 50,000 to 100,000 Btu per hour. Fuel is fed 

 into the top of the firebox to provide a break in the fuel supply line and thus 

 eliminate the possibility of pyrolysis occurring in the fuel supply line. An oil 

 burner (with 6-second time delay to allow a chimney draft inducer to come up to 



ZX 



STACK 



FAN SWITCH 



HEAT 

 EXCHANGER 



Figure 26-23. — Chip burner retrofitted to a conventional residential oil furnace; output 

 is 50,000 to 100,000 Btu per hour. (Drawing after Riley et al. 1979.) 



'^American Fyr-Feeder Engineers. (N.d.) Wood chips — the rediscovered energy fuel source. 

 American Fyr-Feeder Engineers, Des Plaines, 111. 



