Perturbation analysis of a catalytic combustor

Abstract

Lean premixed combustion (LPC) of natural gas is widely employed in stationary gas-turbine combustion systems to minimize pollutants formed at high temperature, such as nitrogen oxides, but can lead to secondary problems such as CO/UHC emissions at part load, lean flame blowout, and strong pressure oscillations approaching limit cycles. Catalytic combustion is an attractive option to LPC. A range of pressures from one to above ten atmospheres has been investigated, with inlet temperatures and velocities appropriate for operation under gas-turbine conditions. Temperature profiles have been obtained from images provided by an infrared camera mounted in the postflame gas, looking forward at the exit face of the catalyst. Dark relatively low temperature regions, analogous to sunspots, are sometimes apparent on the exit face. The temperature variation appears to be spatially periodic with a fairly small amplitude, on the order of less than 100 K. As long as the entrance conditions are held constant, these dark spots have been observed to survive for periods of time (minutes to tens of minutes) which are much longer than the fluid mechanical or chemical time scales in the flow. The present linear analysis yields conditions under which periodic temperature fields are possible. A nonlinear analysismore » becomes necessary if the amplitude becomes too large; however, the latter regime is not of operational interest since it corresponds to flame extinction on a large scale.« less