The efficiency of combined-cycle cogeneration plants makes them cleaner-in terms of air emissions per volume of fuel needed to produce the same power output-than other types of plants. But in areas of poor air quality, such inherent cleanliness may be insufficient to meet ever-tightening NOx emissions limits. Read why and how Calpine's Texas City Cogeneration plant, which is in such an area, retrofit one of its gas turbines with low-emissions combustors in advance of imminent new NOx limits-as opposed to taking the conventional approach of adding a selective catalytic reduction system.