Practical Considerations for the Development of Industrial Low Emissions Combustor Systems

 

A.Ateshkadi, B.Shlein, A.Veninger, Industrial Combustor Systems Pratt and Whitney, East Hartford, USA.

 

Industry wide, land based turbine combustion systems incorporate lean premixing techniques to control NOx and CO emissions.All premixing systems aim to limit the maximum flame temperature in the combustion process and thus the production of thermal NOx.Reduced flame temperatures tend to increase the levels of CO unless sufficient volume and time can be allowed for further oxidation.This becomes a balancing act that gets more difficult if the machine must hold emissions levels as power level and firing temperature is decreased.System complexity increases, usually through the implementation of modulating or staging the combustor process through the use of multiple zone combustors, variable geometry, air bleed systems, catalytic systems, or a combination thereof.With extensive development over the past decade, there are now a variety of aero-derivative gas turbines that have achieved NOx and CO below 25 ppm while state-of-the-art frame machine units, designed to operate at essentially constant firing temperature from intermediate to full load, have attained single digit NOx and CO.The development of all of these combustors have been met with operational, durability, and emissions attainment difficulties that required improved engineering understanding and methodologies applied to these complex systems. 

This paper summarizes Pratt & Whitney’s practical considerations and experience developing low emissions combustion systems in aero-derivative gas turbines for mechanical drive and power generation applications. Unlike the aero engine that is certified for emissions under international standards for a set operating cycle, the industrial derivative must conform to customer emissions permitting requirements that vary worldwide by region. Emissions compliance can be governed by rules for continuous attainment or total tons of pollutant per year. These permitting requirements and the duty cycle of the application will govern the best available control technology (BACT) to be incorporated in the engine and the site exhaust system to meet total package performance and competitive product cost.

The development of the production GG8 Dry Low NOx (DLN) system is discussed, including the tradeoffs to optimize NOx and CO emissions for baseload/peaking operation and a practical means to achieve turndown performance. Design robustness and sensing to prevent mixture flashback or autoignition is presented. Dynamic sensing and de-tuning through mechanical means and piloting fuel to enhance durability and ensure smooth throttle excursions is also described.

Lessons learned from previous industrial combustor systems developed are highlighted and discussed with a view to more demanding systems requirements for higher efficiency products that also attain lower emissions levels. Enabling technologies that include noise reduction methods, flame temperature control and advanced materials and cooling techniques are briefly described as some of the key challenges in developing these advanced industrial combustor systems.