Organic Compounds

Organic compounds are unburned gaseous combustibles that are emitted from coal-fired boilers but generally in very small amounts [37]; however, for brief periods, unburned combustible emissions may increase significantly such as during system startup or upsets. The organic emissions from pulverized coal-fired or cyclone-fired units are lower than from smaller stoker-fired boilers, where operating conditions are not as well controlled [28].

Organic emissions are due to constituents present in the coal or are formed as products of incomplete combustion. Polycyclic organic matter (POM) has also been referred to as polynuclear or polycyclic aromatic compounds (PACs). Nine major categories of POM have been identified by the EPA [38]. The most common organic compounds in the flue gas of coal-fired boilers are polycyclic aromatic hydrocarbons (PAHs). Hydrocarbons as a class are not listed as criteria pollutants, although a large number of specific hydrocarbon compounds are listed among the 188 hazardous air pollutants under Title III of the Clean Air Act Amendments of 1990 (see Chapter 4 for a discussion of regulations). Furthermore, the EPA has identified 16 PAH compounds as priority pollutants: naphthalene, acenaphthylene, acenaphthene, fluorene, phenan-threne, anthracene, fluoranthene, pyrene, chrysene, benzo[a]anthracene, benzo[b]fluoranthene, benzo[A]fluoranthene, benzo[a]pyrene, indeno[1,2,3-c,d]pyrene, benzo[g,h,i]perylene, and dibenzo[a,h]anthracene.

Environmental Effects

Gaseous hydrocarbons as a broad class do not appear to cause any appreciable corrosive damage to materials [28]. Of all the hydrocarbons, only ethylene has adverse effects on plants at known ambient concentrations, including inhibition of plant growth and injury to orchids and cotton.

Health Effects

Studies of the effects of ambient air concentrations of many of the gaseous hydrocarbons have not demonstrated direct adverse effects upon human health [28]. Certain airborne PAHs, however, are known carcinogens. Also, studies of the carcinogenicity of certain classes of hydrocarbons indicate that some cancers appear to be caused by exposure to aromatic hydrocarbons found in soot and tars. An extreme example is the occurrence of highly elevated incidences of lung cancer in China from PAH exposure [39]. PAHs are released during unvented coal combustion of "smoky'' coal in homes, resulting in lung cancer mortality that is five times the national average in China.

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