The need for gas turbine combustion operations which meet air pollution requirements and maximize fuel utilization is of sufficient importance to have prompted a great deal of experimentation in the area. It is known that controlled mixing of excess air in the second stage of a two stage combustion system is the key to limiting NO.sub.x formation.
In a gas turbine engine, inlet air is continuously compressed, mixed with fuel and then burned in a combustor. Quantities of air greatly in excess of stoichiometric amounts are compressed and used to keep the combustor liner cool and to dilute the combustor exhaust gases so as to avoid damage to the turbine blades and nozzle. Generally, primary sections of the combustor are operated near stoichiometric conditions which produce combustor gas temperatures up to approximately 4,000.degree. F. Further down the combustor, secondary air is added which raises the air-fuel ratio and lowers gas temperatures so that the gases exiting the combustor are in the range of 2,000.degree. F. The fuel injection pressure varies and it is typically 600 PSI for full power and as low as 60-100 PSI for idle conditions.
It is known that NO.sub.x formation is thermodynamically favored by high temperatures. Kinetic studies indicate that the rate of NO formation has a high activation energy (approx. 115 k cal/mole) so that the major formation of NO must take place in the high temperature primary combustion zone of conventional turbines. Since NO formation reaction is so very highly temperature dependent, decreasing peak combustion temperatures provide an effective means of reducing NO.sub.x emissions from combustion equipment. Operating the combustion in a very lean condition (i.e., high excess air) is one of the simplest ways of achieving low temperatures and consequently, low NO.sub.x emissions. The problems of very lean ignition and combustion are ones that have been encountered and solved for many automotive emission control systems and for industrial fume-solvent incineration systems. In both of these cases, catalysts are used to promote and complete the combustion process. In a similar way, catalysts can be used with gas turbines to provide efficient combustion in lean systems. This invention, therefore, relates to methods of operating gas turbine combustors while minimizing the formation and discharge of pollutants such as NO.sub.x. More particularly, the invention describes the use of a series of two or more catalysts to effect fuel oxidation at temperatures below flame temperatures, which thereby will minimize NO.sub.x formation. In another embodiment of the invention, a staged catalytic combustor is operated wherein fuel is burned under fuel rich conditions in a noncatalytic zone, followed by catalytic oxidation of the partially burned fuel in a second zone in which a catalyst is employed to complete the fuel oxidation and minimize NO.sub.x and other emissions. The invention is also directed to the use of a novel primary combustion zone design in which fuel is partially burned with substoichiometric amounts of air and the partially burned primary zone effluent is thereafter mixed into the secondary air stream without continued high temperature combustion. This has the effect of both quenching the hot partially burned primary zone effluent and providing a sufficient mix of the partially burned fuel with secondary air so that complete combustion may be maintained under conditions which do not favor the formation of NO.sub.x. The operation of gas turbine combustors as per the above described embodiments provides, in addition to NO.sub.x reduction, the following benefits: improved fuel efficiency and minimization of CO and unburned hydrocarbon emissions.