In the figure above NOx and CO emissions were plotted against primary zone’s temperature. This figure shows that large quantities of CO are formed at temperatures below 1680 K, whereas excessive NOx formation occurs at temperatures above 1900 K. Therefore, there is a temperature “window”, in which both NOx and CO emissions are kept to a minimum value. Through the years the emissions regulations become stricter and consequently the temperature “window” becomes narrower.
Variable geometry reduces the emissions on both high and low operating conditions without sacrificing combustor performance. It can increase combustion efficiency at low operating conditions and facilitate engine’s relight at altitude. However, it introduces more complicated mechanical and control issues. Since the range of environmental and operational conditions that combustors of gas turbines work is wide, the control of variable geometry is one of the effective methods to keep the combustors working stably, reliably, efficiently and with low emissions.
In order to sufficiently diminish emission levels the variable geometry concept should be applied on combustors in conjunction with pre-mixed/ pre-vaporized fuel injection systems. This combination could lead to avoidance of “hot spots” and “cold spots” that enhance NOx and CO emissions production respectively.
In radial staging (see the figure above), the simplest probably application is in double-banked annular combustors. In these combustors – at low fuel flows – it is a relatively simple matter to inject all the fuel into the inner or outer combustion zone correspondingly.
Finally, in axial staging the primary zone is designed for optimum performance at low power settings. When more power is required, extra fuel is injected in one or more locations downstream of the primary zone (additional burning zones).
In order to achieve a satisfactory performance over the whole engine’s cycle, the lean pre-mixed/pre-vaporized (LPP) combustion systems should be used in combination with variable geometry. However, the main drawback of these systems is that sometimes the air admittance control that is achieved with variable geometry may not be sufficient to avoid the operation close to the weak extinction limit.
Another disadvantage of LPP systems is their autoignition tendency. The main function that is responsible for this undesired situation is the residence time needed for fuel pre-vaporization. If it is high, then under high load/temperature operation autoignition may take place. Furthermore, the airflow rate required in the front of the main combustion chamber in order to achieve lean combustion at high power operation may lead to flame blowout tendencies at low power conditions.
The catalytic reactor is usually consisted of several sections, each of them being made from a different kind of catalyst. This occurs due to the necessity of using a catalyst that is active at low temperatures for the inlet, whereas in the following sections should provide good oxidation efficiency. Downstream of the reactor a thermal reaction zone can be used to provide the necessary heat for the continuity of reactions.
Similar to LPP combustors, catalytic combustors also suffer from autoignition, which appeared upstream of the catalyst although the fuel/air ratios may be below the lean flammability limit. The cause of autoignition is the existence of local rich mixtures near the fuel injector before complete mixing occurs. Hence, mixing must be finished in less than ignition delay time, so as to prevent autoignition.
The article was based on my M.Sc. thesis:
Samaras C., 2010, “Emissions estimation from industrial gas turbine combustors”, M.Sc. Thesis, Cranfield University, UK.
The figures were found from:
 Lefebvre, A. H., 1998, “Gas Turbine Combustion”, Taylor and Francis, 2nd Edition, London, UK.
 Li, Y. G., and Hales, R. L., 2002, “Gas turbine emissions control using variable geometry combustor and fuel staging”, 40th AIAA Aerospace Sciences Meeting & Exhibit, Reno, NV, US.
 Boyce, P. M, 2002, “Gas Turbine Engineering Handbook“, Gulf Professional Publishing, 2nd edition, Huston, Texas, USA.
 Rizk, N. K., and Mongia, H. C., 1990, “Lean low NOx combustion concept evaluation”, 23rd Symposium (International) on Combustion/The Combustion Institute, pp. 1063–1070.