Energy, fuels, and chemicals 3171 



When combustion is poor, hydrocarbon"^ emissions are increased. In extreme 

 cases, these emissions may be 55-85 lb/ton of fuel (Junge and Kwan 1974). High 

 hydrocarbon emissions could present a twofold pollution problem. First, poly- 

 cyclic aromatic compounds can be produced through the incomplete combustion 

 of solid carbonaceous fuels such as coal (Hall et al. 1976). Certain polyclyclic 

 aromatic compounds are suspected carcinogens. However, it is not certain to 

 what extent these compounds occur in the incomplete combustion of wood. The 

 second problem is the production of peroxy compounds that react with sunlight 

 as part of the complex process resulting in photochemical smog. Formation of 

 carcinogenic compounds or photochemical smog should not be problems in 

 wood burning if combustion is efficient (Hall et al. 1976). 



Nitrogen oxides (NO and NO2) are instrumental in smog formation. At pre- 

 sent, the automobile engine is by far the largest source of nitrogen oxides. In 

 high concentrations, these compounds not only harm life but destroy material as 

 well. Fortunately, the ambient concentration of nitrogen oxides is low enough in 

 most areas that emission control for them from wood burning should not be 

 required. However, if the need arises, modification in the combustion process 

 rather than the use of a cleaning device at the stack would be necessary (Hall et 

 al. 1976). 



Particulate emissions can be controlled by any of four basic devices. These are 

 cyclone separators, scrubbers, baghouse filters, and electrostatic precipitators 

 (Junge 1975a). Most wood-fueled boilers are equipped with cyclone separators; 

 however, some are equipped with scrubbers. 



Comparison to fossil fuels. — The higher percentages of oxygen in the mo- 

 lecular structures of wood and bark gives them lower heating values than coal 

 and oil (table 26-8). In comparison with coal, wood has a low ash content and 

 little or no sulfur. The chief drawback of woody fuels is their naturally high 

 moisture content and bulkiness. 



Arola (1975) has a graph (fig. 26-5) that is a useful tool for comparing the cost 

 of heat or steam produced from wood-based and fossil fuels. The graph has three 

 sets of lines which correspond to three classes of fuel considered: (1) oil; (2) 

 coal, bark, wood, and municipal solid refuse; and (3) gas. A fourth set of lines 

 represents efficiency values to allow for losses when converting heat energy to 

 steam. Heating values used for the solid fuels are "as fired", that is, the available 

 heat at the moisture content (wet basis) as fired (equation 26-7). 



'The term hydrocarbon is used rather loosely in the literature, sometimes denoting oxygen- 

 containing organic compounds as well as hydrocarbons. 



