3192 Chapter 26 



In equation 26-1 1 the sum of (a) through (d) equals losses. Losses due to (a) 

 through (c) can be readily calculated if the moisture content, ultimate analysis 

 (table 26-3), and higher heating value (table 9-12) for the fuel are known. The 

 ultimate analysis is required to calculate the amount of air needed for combus- 

 tion. Corder (1973) estimates that item (d) accounts for a 4 percent heat loss, but 

 the loss could be substantially greater in poorly insulated boilers. 



A procedure outlined by Wiley (1976) was used to calculate overall boiler 

 efficiency for pine-site hardwoods at various moisture contents (table 26-9). It 

 was assumed that the furnace used 40 percent excess combustion air and that the 

 stack gas temperature was 500°F (Corder 1973); these are values that might be 

 expected for efficient burning of hogged fuel. The overall efficiency for green 

 stemwood of average moisture content (42 percent wet basis, table 8-2) is 63 

 percent, i.e., 63 percent of the heat content of a pound of green wood is 

 converted to steam heat content. 



High fuel moisture content is the most important factor in loss of boiler 

 efficiency. The loss is a function of both the amount of moisture in the fuel and 

 the stack gas temperature. Heat is lost both in vaporizing the water and in raising 

 the temperature of the steam to that of the stack gases. At 50 percent moisture 

 content, 16 percent of the fuel energy is lost up the stack (table 26-9). If the 

 moisture content can be reduced to 10 percent, less than 1 percent of the fuel 

 energy is lost and overall efficiency is 74 percent. 



Table 26-9. — Heat losses and overall boiler efficiency related to pine-site hardwood 



moisture content^ 



Moisture content^ 



Heat loss from 10% 25% 42% 50% 



Percent loss 



Moisture in wood^ 1 5 12 16 



Moisture from hydrogen in fueP 9 9 9 9 



Dry stack gases^ 12 12 12 12 



Incomplete combustion, radiation and unaccounted"^ A_ 4_ 4_ 4_ 



Total heat loss 26 30 37 41 



Overall boiler efficiency 74% 70% 63% 59% 



^Boiler operating with 40 percent excess air and stack temperature of 500°F. 

 ^Wet basis. 



^Calculated by Wiley's method (1976); average stemwood value of 7,827 Btu/lb used (table 9- 

 12). 

 "^Estimated (Corder 1973). 



Evaporation of moisture formed from the combustion of hydrogen in wood also 

 causes loss in boiler efficiency. One pound of dry wood, of which 6.4 percent is 

 H will produce 0.58 lb of water on combustion (equation 26-2) this results in a 9 

 percent loss in boiler efficiency. 



Dry stack gas heat loss depends on both the amount of excess air and stack 

 temperature. As either excess air or stack gas temperature increases, heat loss 

 increases (Corder 1973). When pine-site hardwoods are used as fuel, the loss is 



