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Nitrogen oxide (NOx ) emissions regulations started with passage of the Clean Air Act (CAA) in 1990. Compliance with CAA rules resulted in a 29 percent reduction in NOx emissions from 1990 to 2003, according to a government report, “A Review of DOE/NETL’s Advanced NOx Control Technology R&D Program for Coal-Fired Power Plants.” In 2005, the Clean Air Interstate Rule (CAIR) introduced another change for NOx compliance: It required 25 eastern states to reduce NOx emissions by 1.7 million tons in 2009 and 2.0 million tons by 2015. Since that time, a federal court has tossed out CAIR, leading EPA to introduce the Interstate Trading Rule, which attempts to resolve issues the court had with the CAIR program. It does this in part by proposing trading among states. The CAIR NOx emission caps are based on an equivalent emission rate of 0.15 pounds NOx per million Btu heat input (lb/MMBtu) for Phase I, which began in 2009 and 0.125 lb/MMBtu for Phase II beginning in 2015.

Another up-and-coming potential change for NOx control could come as a result of the Boiler MACT rules, which were released by EPA in February. The Boiler MACT rules are a game-changer for all emissions control and will re-emphasize the need for NOx control.   As an added concern for power producers, NO2 National Ambient Air Quality Standards(NAAQS) rules are also in effect, which set annual averages for NO2 at 0.053 parts per million (ppm). Coal-fired units are affected, as well as the Reasonably Available Control Technology (RACT) for the state of operation. Gas-fired units must abide by NO2 NAAQS rules and state RACT guidelines. (Read much more at Power Engineering, April 2011)