Principles and System Requirements

In a conventional GT system, a fraction of the air delivered by the compressor and the fuel, typically natural gas, are mixed, then combusted in a flame and the hot gas expands and drives the turbine. 

Flame stability requires adiabatic combustion temperatures as high as 1600-1800 °C, which must be reduced to 1100-1450 °C by means of cooling bypass air before delivering the hot compressed gas to the turbine to avoid damaging the inlet blades. At such temperatures, within the tens milliseconds residence time required for complete burnout of fuel and CO, significant amounts of NO are produced, mostly by the Zeldovich thermal mechanism [2]. 

In a catalytic burner, the combustion is ignited and stabilized under ultra-lean conditions, which results in adiabatic temperatures close to those allowed for delivering the hot compressed gas to the turbine. Hence the need for by-pass air is minimized and the formation of thermal NOj, is almost prevented due to the absence of a hot combustion zone. Reduction of N emission has been reported to be even larger than expected from the lower combustion temperature if a significant fraction of the fuel is oxidized on the catalyst surface [3]. This effect has been attributed either to the reduction in the formation of prompt NO in view of the 

Design criteria Emission targets Pressure drops Catalyst durability NO , 5 ppm CO 10 ppm UHCs 10 ppm 5% 8000 h 

Operating constraints of GT systems (Table 12.1) pose severe requirements on the catalytic combustor. Air is delivered by the compressor at temperatures which typically range from 300 to 450 °C depending on the load conditions and nominal pressure ratio of the machine. 

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