(b) Such determination of the commissioner will establish that compliance with the provisions of this Part must be accomplished within 30 months after the effective date of this Part. Such a determination extending the final compliance date beyond that otherwise required by subdivision (a) of this section can only be issued to an affected source for which an application for a permit to construct was received by the commissioner before October 24, 1974, and when the source owner or operator can demonstrate:

1. (1) unreasonable cost of control due to plant age, location or process design;
2. (2) physical impossibility of installing necessary control equipment within the allowable time; or
3. (3) other factors specific to the facility that make application of such a compliance schedule unreasonable.