15 U.S.C. § 3392: Natural gas for essential industrial process and feedstock uses

does not reduce the quantity of natural gas delivered for such use below the use requirement specified in subsection (c); is necessary in order to meet the requirements of high-priority users; or is necessary in order to meet the requirements for essential agricultural uses of natural gas for which curtailment priority is established under section 3391 of this title . The provisions of subsection (a) shall apply with respect to any curtailment of deliveries for any essential industrial process or feedstock use only if the Commission determines that use of a fuel (other than natural gas) is not economically practicable and that no fuel is reasonably available as an alternative for such use. The Secretary of Energy shall determine and certify to the Commission the natural gas requirements (expressed either as volumes or percentages of use) of persons (or classes thereof) for essential industrial process and feedstock uses (other than those referred to in section 3391(f)(1)(B) of this title ). The term “essential industrial process or feedstock use” means any use of natural gas in an industrial process or as a feedstock which the Secretary determines is essential. The term “high-priority user” has the same meaning as given such term in section 3391(f)(2) of this title .