1. a) "Good engineering practice", with respect to stack height, means the greater of:

   1. 1) 65 meters, measured from the ground-level elevation at the base of the stack;

   2. 2) The following:

      1. A) For a stack in existence on January 12, 1979, and for which the owner or operator had obtained all necessary preconstruction approvals or permits required under 40 CFR Parts 51 and 52 (incorporated by reference in 35 Ill. Adm. Code 204.100):

         H_g = 2.5H,

         provided the owner or operator produces evidence that this equation was actually relied on in establishing an emission limitation;

      2. B) For all other stacks:

         H_g = H + 1.5L

         where:

         Hg	=	good engineering practice stack height, measured from the ground-level elevation at the base of the stack;
         H	=	height of nearby structure or structures measured from the ground-level elevation at the base of the stack;
         L	=	lesser dimension, height or projected width, of nearby structure or structures provided that USEPA or the Agency may require the use of a field study or fluid model to verify good engineering practice stack height for the source; or

   3. 3) The height demonstrated by a fluid model or a field study approved by USEPA or the Agency that ensures the emissions from a stack do not result in excessive concentrations of any air pollutant as a result of atmospheric downwash, wakes, or eddy effects created by the source itself, nearby structures, or nearby terrain features.
2. b) For purposes of this definition, "stack" means any point in a source designed to emit solids, liquids, or gases into the air, including a pipe or duct but not including flares.

(Source: Added at 49 Ill. Reg. 6237, effective April 23, 2025)