Analysis of NOX Formation in an Axially Staged Combustion System at Elevated Pressure Conditions

Abstract

The objective of this investigation was to study the effect of axially staged injection of methane in the vitiated air cross flow in a two stage combustion chamber on the formation of NOX for different momentum flux ratios. The primary cylindrical combustor equipped with a low swirl air blast nozzle operating with Jet-A liquid fuel generates vitiated air in the temperature range of 1473–1673 K at pressures of 5–8 bars. A methane injector was flush mounted to the inner surface of the secondary combustor at an angle of 30 deg. Oil cooled movable and static gas probes were used to collect the gas samples. The mole fractions of NO, NO2, CO, CO2, and O2 in the collected exhaust gas samples were measured using gas analyzers. For all the investigated operating conditions, the change in the mole fraction of NOX due to the injection of methane (ΔNOX) corrected to 15% O2 and measured in dry mode was less than 15 ppm. The mole fraction of ΔNOX increased with an increase in mass flow rate of methane and it was not affected by a change in the momentum flux ratio. The penetration depth of the methane jet was estimated from the profiles of mole fraction of O2 obtained from the samples collected using the movable gas probe. For the investigated momentum flux ratios, the penetration depth observed was 15 mm at 5 bars and 5 mm at 6.5 and 8 bars. The results obtained from the simulations of the secondary combustor using a RANS turbulence model were also presented. Reaction modeling of the jet flame present in a vitiated air cross flow posed a significant challenge as it was embedded in a high turbulent flow and burns in partial premixed mode. The applicability of two different reaction models has been investigated. The first approach employed a combination of the eddy dissipation and the finite rate chemistry models to determine the reaction rate, while the presumed JPDF model was used in the further investigations. Predictions were in closer agreement to the measurements while employing the presumed JPDF model. This model was also able to predict some key features of the flow such as the change of penetration depth with the pressure.

Autoignition flame dynamics in sequential combustors

“…An important technology step change is the development of constant pressure sequential combustion systems, also known as axial staging concepts [11,18,21]. In these combustors, the first stage is based on lean premixed turbulent combustion, with swirled flames anchored on conventional burners.…”

Time-Averaged Characteristics of a Reacting Fuel Jet in Vitiated Cross-Flow

“…The figure shows that NO x emissions increases linearly with the ΔT across the jet and is insensitive to the fuel composition for CH 4 and H 2 /CH 4 blends. The approximately linear relationship between NO x emissions levels and secondary stage ΔT shown in Figure 12 has been observed in previous studies [6,28] where JICF fuel jets were injected into high temperature vitiated crossflows. In order to bound these data, detailed chemical kinetic calculations were performed for two limiting cases.…”

“…Subsequent reduction in the equivalence ratio led to a sharp increase in NO x formation as the mixture approached stoichiometric ratios of fuel and oxidizer. Prathap et al [6] studied NO x formation from a reacting jet fuel jet injected into a vitiated crossflow and found that varying the momentum flux ratio and Reynolds number of the jet could significantly move the jet trajectory but had relatively little impact on NO x emissions for a given temperature rise due to the jet combustion. These observations suggest that NO x emissions are not particularly sensitive to detailed fluid mechanical or reaction zone features of JICF in low O 2 environments with sufficiently high temperatures.…”

“…Recent efforts to design next generation staged combustion devices capable of delivering fuel-flexible, low NOx performance are also driving a renewed interest in reacting jets in crossflow [5][6][7]. Reheat combustion concepts utilizing jet in crossflow configurations have also garnered interest based on their ability to increase specific work or thrust at a fixed turbine inlet temperatures [6]. Historically, turbine inlet temperatures have limited the combustor firing temperatures, but NO x emissions may soon provide the upper limit on firing temperatures [8].…”

“…Common application areas include gas turbine premixers, film cooling jets, dilution air jets, and fluidic flame stabilization [3,4]. Recent efforts to design next generation staged combustion devices capable of delivering fuel-flexible, low NOx performance are also driving a renewed interest in reacting jets in crossflow [5][6][7]. Reheat combustion concepts utilizing jet in crossflow configurations have also garnered interest based on their ability to increase specific work or thrust at a fixed turbine inlet temperatures [6].…”