Cite as: 540 U. S. 461 (2004)
Opinion of the Court
operational cap that allowed all but one generator to run full time. Ibid.
In 1996, Cominco initiated a project, with funding from the State, to expand zinc production by 40%. Brief for Petitioner 10; Reply Brief for Petitioner 11, n. 9. Anticipating that the project would increase nitrogen oxide emissions by more than 40 tons per year, see supra, at 472, Cominco applied to ADEC for a PSD permit to allow, inter alia, increased electricity generation by its standby generator, MG-5. App. 107-108; App. to Pet. for Cert. 33a. On March 3, 1999, ADEC preliminarily proposed as BACT for MG-5 the emission control technology known as selective catalytic reduction (SCR),5 which reduces nitrogen oxide emissions by 90%. App. 72, 108. In response, Cominco amended its application to add a seventh generator, MG-17, and to propose as BACT an alternative control technology—Low NOx 6—
that achieves a 30% reduction in nitrogen oxide pollutants. Brief for Respondents 5, and n. 1; App. 84.
On May 4, 1999, ADEC, in conjunction with Cominco's representative, issued a first draft PSD permit and preliminary technical analysis report that concluded Low NOx was BACT for MG-5 and MG-17. Id., at 55-95. To determine BACT, ADEC employed EPA's recommended top-down methodology, id., at 61:
"In brief, the top-down process provides that all available control technologies be ranked in descending order of control effectiveness. The PSD applicant first examines the most stringent—or 'top'—alternative. That al-5 SCR requires injections of "ammonia or urea into the exhaust before the exhaust enters a catalyst bed made with vanadium, titanium, or platinum. The reduction reaction occurs when the flue gas passes over the catalyst bed where the NOx and ammonia combine to become nitrogen, oxygen, and water . . . ." App. 71.
6 In Low NOx, changes are made to a generator to improve fuel atomization and modify the combustion space to enhance the mixing of air and fuel. Id., at 75.
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