Tall Stacks

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Tall stacks

The waste gases that escape from factories or power plants running on fossil fuels typically contain a variety of pollutants, including carbon monoxide , particulates, nitrogen oxides , and sulfur. The first of these pollutants, carbon monoxide, tends to dispense rapidly once it is released to the atmosphere and seldom poses a serious threat to human health or the environment . The same cannot be said of the last three categories of pollutants, all of which pose a threat to the well-being of plants, humans, and other animals.

During the 1960s, humans became increasingly aware of the range of hazards, posed by pollutants escaping from smokestacks. One of the most common events of the time was for residents in the area immediately adjacent to a factory or plant to find their homes and cars covered with fine dust from time to time. They soon learned that the dust was fly ash released from the nearby smokestacks. Particulates (fine particles of unburned carbon) also tend to settle out fairly rapidly and cover objects with a fine black powder.

It was clear that visible pollutants, such as fly ash and particulates, offered a hint that invisible pollutants, such as oxides of nitrogen and sulfur, were also accumulating in areas close to factories and power plants. As the health hazards of these pollutants became better-known, efforts to bring them under control increased.

Certain technical methods of air pollution control—electrostatic precipitation and scrubbing, for example—were reasonably well-known. However, many industries and utilities preferred not to make the significant financial investment needed to install these technologies. Instead, they proposed a simpler and less expensive solution: the construction of taller smokestacks.

The argument was that, with taller smokestacks, pollutants would be carried higher into the atmosphere. They would then have more time and space to disperse. Residents close to the plants and factories would be spared the fallout from the smokestacks. The "tall stack" solution met the requirements of the 1970 Clean Air Act as a "last-ditch alternative" to gas-cleaning technologies.

As a result, companies began building taller and taller smokestacks. The average stack height increased from about 200 ft (61 m) in 1956 to over 500 ft (152 m) in 1978. The tallest stack in the United States in 1956 was under 600 ft (182 m) tall, but a decade later, some stacks were more than 1,000 ft (305 m) high.

At first, this effort appeared to be a satisfactory way of dealing with air pollutants. The concentration of particulates and oxides of nitrogen and sulfur close to plants did indeed, decrease after the installation of tall stacks.

But new problems soon began to appear, the most serious of which was acid deposition . The longer oxides of sulfur and nitrogen remain in the air, the more likely they are to be oxidized to other forms, sulfur trioxide and nitrogen dioxide. These oxides, in turn, have the tendency to react with water droplets in the atmosphere forming sulfuric and nitric acids, respectively. These acids eventually fall back to earth as rain, snow, or some other form of precipitation. When they reach the ground, they then have the potential to damage plants, buildings, bridges, and other objects.

The mechanism by which stack gases are transported in the atmosphere is still not completely understood. Yet it is now clear that when they are emitted high enough into the atmosphere, they can be carried hundreds or thousands of miles away by prevailing winds. The very method that reduces the threat of air pollutants for communities near a factory or power plant, therefore, also increases the risk for communities farther away.

[David E. Newton ]