At any time, the owner or operator of an existing electric powerplant may certify to the Secretary for purposes of this subsection whether or not it is technically and financially feasible to use a mixture of petroleum or natural gas and coal or another alternate fuel as a primary energy source in that powerplant. If an affirmative certification under this subsection is in effect with respect to such powerplant and if, after examining the basis for the certification, the Secretary concurs with the certification, the Secretary may prohibit, in accordance with section
8343
(a) of this title, the use of petroleum or natural gas, or both, in such powerplant in amounts in excess of the minimum amount necessary to maintain reliability of operation of the unit consistent with maintaining reasonable fuel efficiency of such mixture.