The Reduction of Gas Turbine Idle Emissions by Fuel Zoning for Compressor Bleed

Two methods of reducing the idle emissions of gas turbine engines have been investigated. The methods were (1) fuel zoning, whereby a portion of the fuel nozzles were shut down and all of the fuel passed through the remaining nozzles and (2) larger than normal compressor overboard bleed. Both methods operate on the fact that a combustor’s efficiency increases as the fuel/air ratio is increased from idle to full power conditions. Fuel zoning increases the local fuel/air ratio making those portions of the combustor which are operating more efficient. This method has been shown to reduce the idle emission of total hydrocarbon by 5 to 1 in a double annular combustor sized for a large augmented turbofan engine. Operating with a larger than normal compressor overboard bleed allows increasing fuel/air ratio without increasing idle thrust. By using this method in a P&WA™ JT3C-7 engine a reduction of 2 to 1 in the emission of total hydrocarbon was demonstrated.