On two-shaft DLE (Dry Low Emissions) gas turbines compressor air is often bled off the combustor housing to control the combustor flame temperature for part load emissions management. The bleed air is typically directed into the exhaust ducting, bypassing the turbine, causing engine efficiency to decrease. A prototype bleed air recirculation system was fitted onto a compressor package in the field that redirects bleed flow from the engine exhaust to the air inlet to raise the air temperature entering the engine. This system is designed to reduce the amount of bleed air required at a given ambient operating condition, while not compromising emissions, to increase part load efficiency.

The bleed air recirculation system was designed, built, installed, and tested on a Mars 16000S compressor package at a station in northern Pennsylvania. A control algorithm was developed and validated to control bleed air recirculation so that package durability was not impacted. Over an evaluation period of 12 months, no significant durability issues or degradation were noted.

The on-site evaluation indicated that the engine’s heat rate is significantly improved by using bleed air recirculation. The heat rate was reduced by as much as 7% during testing at site with a strong dependency on the engine operating load and ambient temperature. As predicted the best case was demonstrated at 50% load and at colder ambient conditions. The NOx emissions were also improved with bleed air recirculation. In addition, using this system extended the range of low emissions operation to lower loads. No effects on compressor surge or sound emissions were detected.

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