Question

Justify the statement, “Tall stacks are no longer considered as an acceptable alternative for controlling emissions from electric power generating plants.”

Answer 1

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Tall stacks are closely associated with ICS programs, since increased stack height can yield decreased needs for intermittent emission reductions, assuming that control of ambient sulfur dioxide concentrations is the sole objective. Two basic types of tall stack-ICS programs are thus apparent. In the first, stacks of sufficient height are built so that ground-level concentrations do not abrogate standards, even without use of ICS measures. This approach is advocated in Great Britain (Stone and Clark 1967, Lucas 1974), and also by some utilities in this country for plants located in relatively flat terrain where no limitations on stack height (e.g., limitations imposed by flight safety requirements near airports) exist (Frankenberg 1970, Smith and Frankenberg 1974). In the second approach, applicable where ambient sulfur dioxide standards cannot be met simply by increasing stack height, tall stacks are coupled with ICS measures to control ground-level sulfur dioxide concentrations. Controversy has existed over whether tall stacks alone can adequately control ground-level sulfur dioxide concentrations. It seems clear, however, that as stack height increases, the required frequency of use of ICS measures decreases for most, if not all, given control situations. The frequency of ICS use is also dependent on emission rate, other stack parameters (stack gas velocity, temperature), meteorological conditions, and topography.

Regulations proposed by the Environmental Protection Agency (EPA) do not accept increases in stack height beyond levels of “good engineering practice” as acceptable air quality control measures, unless this is accomplished as part of an approved ICS program (FR 1973). These same proposed regulations impose stringent limitations on the types of sources and situations for which ICS programs are acceptable, and include the following constraints:

1. The ICS program must be a supplement to constant emission controls; the source must undertake research and development programs to accelerate development of applicable constant emission reduction technology.

2. A sulfur dioxide monitoring network sufficient to allow calibration of a dispersion model, as well as interpolation between samplers.

3. An operating model which relates meteorological inputs, emission rates, source data, terrain and location factors to current and future air quality.

4. Meteorological inputs suitable for use in air quality forecasting.

5. Objective rules for emission control, relating air quality predictions to controlled emission schedules.

6. A requirement for continuous evaluation and systematic improvement of ICS reliability (upgrade system).

The suggestion that tall stacks be used with or without intermittent controls to meet sulfur dioxide standards is a highly controversial one. On the one hand, these techniques have been shown to be effective, at relatively low costs, for reducing ground-level concentrations of sulfur dioxide in the vicinity of power plants burning high sulfur coal (Frankenberg 1970, Smith and Frankenberg 1974, Montgomery et al. 1973, Montgomery and Frey 1974, TVA 1974). On the other hand, the application of these techniques normally provides, over an extended length of time, only a negligible reduction in the amount of pollutants emitted. Primarily for this reason EPA “…considers constant emission reduction techniques, such as flue gas desulfurization, far superior to dispersion techniques and has proposed regulations that limit the use of such dispersion techniques to situations where constant emission reduction controls are not available” (EPA 1974). EPA further claims “…the concept is not compatible with the Clean Air Act requirement that constant emission limitations be used whenever possible” (EPA 1974b).

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