3 1 64 Chapter 26 



There are two ways to express moisture content of wood. The ovendry method 

 is most commonly used by people in the forest products industry. 



M.C. (dry) = (weight of water) ^ j^ ^26-3) 



(weight of ovendry wood) 



People in the field of fuels and combustion and most engineers use the wet basis 

 moisture content. 



M.C. (wet) = (weight of water) ^ ^^ ^26-4) 



(total weight of wood and water) 



A material that is half water and half wood would have M.C. (dry) = 100 

 percent and M.C. (wet) = 50 percent. The two moisture contents can be 

 converted by the following formula: 



M.C. (wet) = 100 M.C. (dry)/[100 + M.C. (dry)] (26-5) 



M.C. (dry) = 100 M.C. (wet)/[100 - M.C. (wet)] (26-6) 



In keeping with most of the literature referenced, the wet basis moisture content 

 will be used in the following discussion unless otherwise noted. 



The economic value of a fuel will depend both on its heating value and 

 moisture content. High moisture content is detrimental in two ways. First, it 

 reduces the available heat of the fuel. The higher heating value of a fuel does 

 not change with increasing moisture content, but the available heat does change 

 (equation 26-7) because there is simply less fuel per unit weight (table 26-5, Ince 

 1977). 



Available heat = (percent wood)(higher heating value) (26-7) 



Table 26-5. — Available heat versus moisture content for pine-site hardwoods^ 



^Average value for stem wood (table 9-12). 



High moisture content also reduces furnace efficiency, because heat energy is 

 lost up the stack in vaporizing the moisture in the wood. Vaporization of fuel 

 moisture is the first stage in combustion; thus heat energy must raise the moisture 

 temperature from ambient (70°F) to the boiling point (212°F). Additional heat 

 energy is required to vaporize the water to steam at 212°F. Heat energy is also 

 lost in raising the steam temperature to that of the furnace stack gases. Steam 

 tables can be readily used to calculate the amount of energy required to heat and 

 evaporate water. For example, if the stack temperature is 500°F, the calculated 

 heat energy lost in changing the water at 70°F to steam at 500°F and 14.7 psi is 

 1,250 Btu/lb of water (Wiley 1976). 



