On November 10, 2010, the Environmental Protection Agency (EPA) released guidance  to be used in implementing “best available control technology” (BACT) requirements for greenhouse gas (GHG) emissions from major new or modified stationary sources of air pollution. Under the Clean Air Act (the Act), major new sources or major modifications to existing sources must employ technologies aimed at limiting emissions from these sources.
Under the Act, the BACT requirements for a given facility are to be established in a way that addresses the specific conditions of the facility and reflect the maximum degree of emission reduction that has been demonstrated through available methods, systems, and techniques, while accounting for the economic, energy and environmental considerations of the facility. In most states, the state environmental agency, rather than US EPA, will be issuing the permit to the facility.
The use of BACT to limit emissions of regulated pollutants from facilities has been part of the Clean Air Act for decades. Its first application to GHG emissions occurred in February 2010, when Calpine Corporation voluntarily agreed to an air permit  that included a BACT determination for GHGs at a new power plant in California. The approved power plant included a slightly more efficient generation unit than had been initially proposed.
The new EPA guidance itself is technical in nature. Most importantly, under the guidance, covered facilities will generally be required to use the most energy efficient technologies available – much as was the case with the Calpine facility – rather than be required to install particular pollution control technologies. Among other things, carbon capture and sequestration technology will not be considered BACT except in extremely rare circumstances, such as when a facility is located next to an operating oil field whose operator wants to purchase carbon dioxide for enhanced oil recovery. Nor will the guidance require that specific types of fuels be used. In particular it does not require that proposed coal burning power plants switch to natural gas. The guidance also includes particular guidance for biomass facilities, stating that biomass itself could be considered BACT. EPA has indicated that it intends possibly to pursue additional rulemaking next year that may eliminate biomass burning facilities altogether from this permitting process. Overall, the BACT guidance maintains the same steps for individual BACT determination for GHGs that have long been used for BACT determination for traditional air pollutants.
In an earlier rulemaking, EPA established the threshold limits for which major new or modified sources would be required to meet BACT requirements. In its “tailoring” rule , EPA specified that beginning January 2, 2011, only sources that were already subject to BACT for “criteria” air pollutants (such as sulfur dioxide and nitrogen oxides) and had emissions of GHGs that exceeded 75,000 tons per year would have to meet BACT for GHGs. In July 2011, these requirements will be extended to apply also to any new source with GHG emissions above 100,000 tons per year and any modified source that increases GHG emissions by more than 75,000 tons per year.
More information from C2ES:
Clean Air Act Regulation for Greenhouse Gases overview  from EPA
BACT Guidance  from EPA
Fact Sheet on Proposed Rules on Clean Air Permits  from EPA