3162 Chapter 26 



fuel.^ With the depletion of fossil fuels, there is renewed interest in using wood 

 and bark as fuel for residences (figs. 29-9AB), and industry. 



WOOD AS FUEL 



Heating value, ultimate analysis, proximate analysis, moisture content, and 

 size are some of the important characteristics that influence the value of wood as 

 a fuel. Some of these characteristics will not vary signficiantly, while others, 

 such as moisture content and size, can vary greatly. Often, fuel preparation will 

 be required to minimize variations and to provide a more homogeneous material 

 for efficient combustion. 



The combustion process. — Combustion is the rapid chemical combination 

 of oxygen with the elements of a fuel that will bum (Babcock and Wilcox 

 Company 1972). Chemically it is an oxidation process that results in the release 

 of heat energy. The major combustible elements in wood are carbon and hydro- 

 gen (table 26-3). The complete oxidation of carbon forms carbon dixoide (CO2), 

 and heat energy; complete oxidation of hydrogen (H2) yields water (H2O) and 

 heat energy. 



C + O2 — ►CO2 + 14,100 Btu/lb of C (26-1) 



2H2 + O2 — ►2H2O + 61,100 Btu/lb of H2 (26-2) 



Combustion of wood is not always complete, which results in the release of 

 lesser amounts of carbon monoxide (CO), hydrocarbons, and other substances. 



The burning of wood involves three consecutive stages. In the first stage the 

 moisture is evaporated to dry the wood, which requires about 1 , 100 Btu per lb of 

 water. In the second stage, the temperature of the fuel rises to the point where 

 volatiles are driven off and combusted. In the third stage, the fixed carbon 

 (carbon remaining after the volatiles are driven off) is combusted as fast as 

 oxygen can be brought in contact with it (Femandes 1976). 



In most combustion processes, air is the source of oxygen. For the purpose of 

 calculating the amount of air required for combustion, air can be considered to 

 be 23. 1 percent oxygen and 76.9 percent nitrogen by weight. Thus, for every lb 

 of oxygen required, 4.32 lb of air must be supplied. As carbon has an atomic 

 weight of 12 and oxygen a molecular weight of 32, it is seen from equation 26-1 

 that 12 pounds of carbon require 32 pounds of oxygen to produce 44 pounds of 

 carbon dioxide (molecular weight 44). To bum 1 lb of carbon will require 2.67 

 lb of oxygen. By similar calculations, it can be shown that 8 lb of oxygen are 

 required to bum 1 lb of hydrogen (equation 26-2). The amount of air required to 

 bum a fuel can be calculated from its ultimate analysis. The average hardwood 

 contains about 50.8 percent carbon, 41.8 percent oxygen, and 6.4 percent 

 hydrogen (table 26-3). The nitrogen and ash do not play a significant role in the 

 combustion calculations and can be left out. In calculating the minimum air 

 required to bum wood, the percentage of elements can be based on any weight; 

 here let us use 1 lb of wood. 



^Saeman, J. F. Energy and materials from the forest biomass. Institute of Gas Technology Symp. 

 on Clean Fuels from Biomass and Wastes. (Orlando, Fla., Jan. 1977.) 



