The global fuel-to-air ratio varies greatly with load because the fuel flow at part load is reduced at a higher rate than the airflow if no control measures are taken. The full-load lean flame thus becomes even leaner at part load, and, at some point, combustion in the main flame can no longer be sustained and the flame would be
extinguished. The remedy for this unstable combustion situation is fuel staging. To maintain a stable combustion at all gas-turbine loads from idle to full load, pilot fuel is used. Pilot flames are more fuel rich and therefore more stable, although the downside is their contribution to the NOx emissions. Typical expected emissions from a 50 MWel single-shaft gas turbine is shown in Figure 10.3. The NOx emission is about 12 ppm@15%O2 at full load and down to 50% load corresponding to a lean premixed combustion generating also low emissions of CO and unburnt hydrocarbons. Below 50% load, it is necessary to support the main flames with the more fuel-rich pilot flames leading to an increase in NOx emissions. At the same time, the emissions of CO and unburnt increase because overall combustion temperature gets colder and the residence time and/
or mixing in the combustor is no longer sufficient to achieve a complete combustion of the fuel.