Tall Stacks

If concentration impacts are excessive, constructing a tall stack is one approach to reduce them. A tall stack dilutes ambient ground-level concentrations near the emission source. However, this approach does not reduce emission levels or total pollution loadings in a region; it merely provides greater initial dispersion at the source. The EPA regulates stack height under its tall stacks policy to encourage better control technology application.

Tall stacks describes stacks that are greater than the GEP stack height. For stacks which are taller than GEP guidelines, dispersion modeling is conducted as if the emission source has a GEP stack height. When the stack height is at the GEP level, downwash is not likely to occur, and modeling can proceed without further changes. If the proposed or existing stack is less than the GEP stack height, surrounding structures must be investigated as possible downwash sources for modifying dispersion parameters in the air dispersion model. Under GEP guidelines, however, a source that has a GEP stack height less than 65 m can raise it to that level (and still be considered GEP).

A consideration for building a tall stack is the cost of construction; investments for tall stacks usually start at $1 million, and costs of $4-5 million are not uncommon (Vatavuk 1990). Stacks of this size typically consist of an outer shell and a liner. The outer shell is usually constructed of concrete, while the liner is usually steel or acid-resistant brick. The choice of liner depends on whether the exhaust gas is above the acid dew point (steel above it, brick below). Given these costs, building a stack taller than GEP is rare; however, constructing a stack below GEP and conducting the additional downwash modeling required may be worthwhile. In some cases, if the environmental engineer is involved early in the design phase of the pro-

1.75H - GEP Stack Height Based on Wind Direction 3

FIG. 5.8.14 GEP determination of projected structure width and associated region of adverse influence for a structure in four different wind directions. (Reprinted from U.S. Environmental Protection Agency (EPA), 1985, Guideline for determination of good engineering practice stack height [Technical support document for the stack height regulations] [Revised], EPA-450/4-80-023R, Research Triangle Park, N.C.: U.S. EPA, Office of Air Quality Planning and Standards [June].)

1.75H - GEP Stack Height Based on Wind Direction 3

FIG. 5.8.14 GEP determination of projected structure width and associated region of adverse influence for a structure in four different wind directions. (Reprinted from U.S. Environmental Protection Agency (EPA), 1985, Guideline for determination of good engineering practice stack height [Technical support document for the stack height regulations] [Revised], EPA-450/4-80-023R, Research Triangle Park, N.C.: U.S. EPA, Office of Air Quality Planning and Standards [June].)

ject, designing the height and shape of buildings and nearby structures to lessen the need for a taller stack is possible.

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