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As an alternative to usual combustion processes with air as oxidant, oxy-fuel or oxygen-enhanced processes are attractive in high temperature thermal or thermochemical processes, novel power plant concepts or in gasification processes. Especially for production of basic chemicals or synthetic fuels, high-purity synthetic gas without nitrogen dilution is required for further industrial processing. In case of oxy-fuel combustion, the absence of nitrogen leads to higher flame temperatures and larger concentration of major species as well as intermediate species. Because of the increased reactivity, most combustors are designed in non-premixed configuration. In the present work, the extinction limits of non-premixed CH4/N2/O2 flames were numerically calculated using the CHEMKIN Pro opposed flow code together with the GRI 3.0 mechanism. The inlet compositions were varied from high nitrogen dilution to pure oxy-fuel conditions. Additionally, the stoichiometric mixture fraction was varied by shifting the (inert) N2-fraction stepwise from the oxidizer to the fuel side. Therewith, the equilibrium temperature of the mixture is constant for a fixed N2-dilution. The results show, that as expected, the extinction occurs at higher strain rates with increasing the flame temperatures by reducing the N2-diluation. Additionally, the extinction limits strongly increase for a fixed N2-dilution with increasing the stoichiometric mixture fraction. The shift of the N2 from the oxidizer to the fuel side leads to a movement of the stagnation plane towards the fuel side and hence, a significant change of the temperature field and the flame structure. Thus, the radical chain branching occurs at higher temperature level, which strengthen the flame resistance against strain. The increase of the stoichiometric mixture leads to a shift of the flame towards the fuel side and crosses at high values the stagnation plane.

As an alternative to usual combustion processes with air as oxidant, oxy-fuel processes are attractive in high temperature thermal or thermochemical processes, novel power plant concepts or in gasification processes. Especially for production of basic chemicals or synthetic fuels, high-purity synthetic gas without nitrogen dilution is required for further industrial processing. In case of oxy-fuel combustion, the absence of nitrogen leads to higher flame temperatures and larger concentration of major species as well as intermediate species. In the present work, freely propagating methane-oxygen-nitrogen flames were numerically calculated using a 1D model from lean to rich conditions in order to investigate the appearance of super-adiabatic flame temperatures (SAFT). The calculations were performed for equivalence ratios of 0.5 The SAFT phenomenon was identified based on temperature and species profiles. Additionally, a detailed analysis of convection, diffusion and chemical source of both, temperature and species, were performed in order to investigate the role of physical transport processes on SAFT. The results showed that the maximum flame temperature exceeds the equilibrium temperature for equivalence ratios Φ > 0.9. Two different regimes were identified, where SAFT phenomenon appears. The first regime was found in slightly rich conditions (1.0 2.0). A first maximum of temperature difference is observed at an equivalence ratio of Φ = 1.5. Here, an exaggeration of approximately 120–180 K, depending on the applied reaction mechanism, was found for standard conditions. The first maximum at Φ = 1.5 correlates with the maximum concentration of the H-radical, which plays a key role in the first SAFT regime. A minimum temperature difference of 50 K was identified at an equivalence ratio of Φ = 2.1. While increasing the equivalence ratio further, the maximum flame temperature exceeded the equilibrium up to almost 400 K at Φ = 3.0 in the second SAFT regime. An increased preheating temperature enhanced the occurrence of SAFT in the first regime and degraded it in the second regime. Elevated pressure leads to the opposite effects with decreased SAFT in the first and increased SAFT in the second regime.

The structure of fuel-rich CH4/O2-flames at equivalence ratios in the range of 2.5 ≤ ϕ ≤ 2.9 , a preheating temperature of T P = 573 K and pressure in the range of 1 b a r a ≤ p ≤ 5 b a r a was studied experimentally and compared with numerical results using detailed reaction mechanisms. Two measurement techniques, invasive gas sampling (GC/MS) and optical in-situ TDLAS measurements (tunable diode laser absorption spectroscopy), were used to determine temperature and major combustion species (CH4, O2, H2, H2O, CO and CO2). The results showed that the composition of the synthesis gas in the post flame zone strongly depends on ϕ , while p has only a minor influence. Furthermore, the influence of ϕ and p on the phenomena of super-adiabatic flame temperatures (SAFT) was determined by experimentally and numerically investigating temperature and H2O concentration. The results showed similar trends as for the synthesis gas composition: a strong dependence of ϕ and a minor influence of pressure on the degree of significance of the SAFT phenomena.

The substitution of gasoline with bio-ethanol is an intended way to reduce the climate impact of the traffic sector. To extend the knowledge of fundamental flame properties of ethanol/iso-octane flames and improve the numerical predictions for the effect of ethanol blending in internal combustion engines, measurements of the laminar burning velocity of established blend ratios of ethanol and iso-octane were carried out and compared to existing numerical mechanisms. The measurements were carried out with the Heat Flux burner, with thermocouples of type E at the burner plate, which was adapted with an evaporation unit based on direct vaporization to investigate the liquid fuels. The preheating temperature ranges from 298 K to 373 K and the pressure is atmospheric. First measurements of the laminar burning velocity of ethanol/air and iso-octane/air flames were carried out for validating the system. A good agreement with available literature data could be achieved for the investigated equivalence ratios from 0.7 to 1.4. Furthermore laminar burning velocities of different iso-octane/ethanol/air blends, namely E10, E24, E40 and E85, are presented. Through variation of the preheating temperatures (298 K, 323 K, 348 K and 373 K) the temperature dependency could be analyzed. The uncertainty analysis of the measurements has been revealed. Numerical simulations were carried out using different chemical mechanisms for ethanol/air flames, iso-octane/air flames as well as various fuel blends and preheating temperatures and are compared to the experimental data. The agreement is evaluated through a classification of the discrepancy between both.

This work presents an experimental investigation to study the characteristics of combustion using a premixed methane–air mixture within a non-homogeneous porous inert medium (PIM) under high pressure and temperature. In order to obtain a stable flame under these operating conditions within PIM, a novel flame stabilization technique in porous inert media (PIM) combustion under high pressure and temperature has been developed and evaluated. The proposed technique avoids the draw backs of the hitherto developed techniques by properly matching the flow and flame speeds and, consequently, ensuring a stable combustion, for a wide range of operating pressure and temperature. The success of this technique permits the extension of PIM combustion to new applications such as gas turbines. The validity of this new technique has been assessed experimentally in detail by analyzing combustion inside a prototype burner. The effects of various operating conditions, such as initial preheating temperature and elevated pressure, have been examined for an output power range between 5 and 40 kW. The experiments covered a broad spectrum of operating conditions ranging from a mixture inlet temperature of 20 °C and pressure ratio of 1 up to a temperature of 400 °C and a pressure ratio of 9. Evaluation of the results revealed excellent flame stability with respect to both flashback and blow-out limits throughout all the operating conditions studied, including relative air ratios far beyond the normal lean limit. While the blow-out stability showed no significant dependence on pressure, it was strongly determined by the preheating mixture inlet temperature. A remarkable broadening of the stability range from 0.6 to 1.0 on preheating to 400 °C was observed. This reveals the potential of pre-heat temperature to improve the dynamic modularity of the burner.

This paper focuses on the numerical simulations of methane thermal partial oxidation reforming process within inert porous media and their comparison with experiments. In order to produce hydrogen rich mixtures and for the sake of reaction stability, the reformer consists on a diffuser filled with porous media. The validity of using a quasi-1D approach to model this system is explored based on 3D simulations of the isothermal fluid flow through the porous solid structure. Several fluid flow cases were taken into account as well as two different porous materials, Al2O3 fiber lamellae and SiSiC foam. The detailed fluid flow information obtained from the 3D study was used to provide the realistic cross-sectional area variation of the quasi-1D model. The quasi-1D 12-steps reduced chemistry model predictions are in very satisfactory agreement with the temperature and concentration fields measured within the diffuser porous reformer.