Energy, fuels, and chemicals 31b3 



The oxygen required to bum the carbon equals 



1 lb wood (0.508 lb C/lb wood)(2.67 lb 02/lb C) = 1.36 lb O2 

 The oxygen required to bum the hydrogen equals 



1 lb wood (0.064 lb H/lb wood)(8.00 lb Oj/lb H) = 0.512 lb O2 



Thus, total oxygen required equals 1.87 lb. 



The molecular stmcture of the average hardwood is already 41.8 percent 

 oxygen (table 26-3). This oxygen as well as oxygen from air will be involved in 

 the formation of CO2 and H2O and must be accounted for in the combustion 

 calculations. Thus, the oxygen already in 1 lb of wood = 0.418 lb. The oxygen 

 needed from the air = total oxygen required - oxygen already in wood = 1 .87 

 lb - 0.421b. = 1.451b. Therefore, total air required = (1.45 lb02)(4.321bair/ 

 lb O2) = 6.26 lb air. 



This minimum amount of air is called theoretical air and does not depend on 

 the moisture content of the wood. In theory, combustion of wood generally 

 requires about 6 lb of air per lb of wood (Wiley 1976). In practice, excess air is 

 provided to ensure complete combustion. This usually mns from 25 to 150 

 percent over the theoretical air. 



We have seen how the ultimate analysis can be used to calculate the air 

 required for buming wood. The proximate fuel analysis is an indication of the 

 relative amounts of volatile material that will be evolved and bumed in the 

 second stage of the combustion process. The amount of carbon left after the 

 volatiles leave is called the fixed carbon, and bums in the solid state. Seventy- 

 seven percent of the average hardwood will bum as volatile matter and 19 

 percent will bum in the solid state as fixed carbon (table 26-4). This kind of 

 analysis can suggest the best locations for combustion air inlets in a fumace. 



Moisture content and available heat. — The heat of combustion of wood 

 and bark can vary considerably with chemical content. Resin waxes, lignin, and 

 such compounds with high carbon and hydrogen contents have higher heating 

 values than carbohydrates which have a high oxygen content. Softwoods, be- 

 cause they generally have a higher resin and lignin content, have a higher heat of 

 combustion than hardwoods. For example, loblolly pine stemwood has a heat of 

 combustion of 8,600 Btu per ovendry pound (Howard 1973), while pine-site 

 hardwoods average 7,827 Btu per ovendry pound of stemwood and about 7,593 

 Btu per ovendry pound of stembark (table 9-12). 



Heats of combustion are determined with an oxygen bomb calorimeter. Val- 

 ues obtained are somewhat higher than those recovered in practice because the 

 calorimeter is closed and the products of combustion are contained. Thus, on 

 cooling, water vapor is condensed and releases its heat of vaporation. In an 

 industrial fumace, this heat is normally lost to the atmosphere. The heat combus- 

 tion measured by a bomb calorimeter is called the higher heating value. The 

 lower heating value is obtained by subtracting the heat of vaporization of the 

 water formed in combustion. The average lower heating value for pine-site 

 hardwoods is 7,261 Btu per pound for stemwood and 7,027 Btu per pound for 

 stembark. 



