Autoignition Limits of Hydrogen at Relevant Reheat Combustor Operating Conditions

Abstract

The use of highly reactive fuels in the lean premixed combustion systems employed in stationary gas turbines can lead to many practical problems, such as unwanted autoignition in regions not designed for combustion. In the present study, autoignition characteristics for hydrogen, diluted with up to 30 vol. % nitrogen, were investigated at conditions relevant to reheat combustor operation (p = 15 bar, T >1000K, hot flue gas, relevant residence times). The experiments were performed in a generic, optically accessible reheat combustor, by applying high-speed imaging and particle image veiocimetry. Autoignition limits for different mixing section (temperature, velocity) and fuel jet (N2 dilution) parameters are described. The dominant factor influencing autoignition was the temperature, with an increase of around 2% ieading to a reduction of the higiiest possible H2 concentration without "flame-stabilizing autoignition kernels" of approximately 16 vol. %. Furtiierniore, the onset and propagation of the ignition kernels were elucidated using the high-speed measurements. It was found that the ability of individual autoignition kerneis to develop into stable flames depends on the initial position of tire kernel and tiie corresponding axial velocity at that position. Whiie unwanted autoignition occurred prior to reaching the desired operating point for most investigated conditions, for certain conditions the reheat combtistor could be operated stably with up to 80 vol. % H2 in the fuei.

Autoignition of hydrogen/nitrogen jets in vitiated air crossflows at different pressures

“…[1]. This is likely due to the thermal expansion and stronger jet penetration associated with the burning jet that occurred rapidly after the onset of autoignition [21].…”

“…The reason that flame-stabilizing kernels occurred in this x-range is likely related to a low velocity region in the jet wake, which allowed the flame to propagate against the flow [1]. The broad range where ignition kernels originated and the overlap of regions with flame-stabilizing and non-stabilizing kernels are due to the strong dependence of autoignition on the local strain, temperature, and mixture fraction histories of specific fluid parcels.…”

“…[1], the conditions in the duct (p, T cf , X H2 ) at flame stabilization were defined to be those at the measurement immediately prior to the rise in temperature and pressure. For different measuring points, the instantaneous crossflow temperature at a stabilizing autoignition event differed from the set point value (T cf ) by a maximum of 1.1%.…”

“…Detailed understanding of autoignition in reactive mixtures is becoming increasingly relevant for modern low-NO x combustion technologies, such as lean premixed combustion (LPC) systems in stationary gas turbines (GT) [1], HCCI engines [2], and high speed propulsion devices [3]. In LPC systems, unwanted autoignition in mixing zones can lead to combustor damage.…”

“…The chosen operating conditions, having high crossflow temperatures (1143 and 1185 K) and velocities (200 and 300 m/s), along with the fuel jet being injected into vitiated air, are in the range which is relevant for mixing sections of gas turbine reheat combustors [1,[17][18].…”