Chemistry in oxygen enriched flames and the effect on CFD modeling in combustion chambers

Oxygen enriched combustion (OEC) is an opportunity for process intensification in several industrial sectors especially where high temperatures are needed (e.g. aluminum, steel, glass, cement industry). The removal or reduction of nitrogen in OEC, which absorbs a significant amount of the heat of reaction, leads to an increase of the flame temperature. As a consequence, the heat transfer in furnaces, boilers etc. can be improved so that heat losses and fuel consumption can be reduced. However, for the construction of new furnaces for OEC or retrofitting existing systems detailed information of the fluid flow, temperature, species concentrations and other transport phenomena is required. Computational fluid dynamics (CFD) represents a method to analyze these issues without expensive test runs on existing industrial systems. For accurate prediction of the heat transfer a deep knowledge of chemistry and radiation is inevitable. In the present paper the main emphasis is the investigation of different reaction mechanisms for use in air-fuel combustion and OEC.
For this purpose a number of detailed reaction mechanisms with up to 53 species and 325 reactions were tested in 1D simulations. The mechanisms were numerically investigated using the setup of a laminar counter-flow diffusion flame for air-fuel combustion and combustion with pure oxygen. Differences between the reaction sets were examined with sensitivity analyses and consideration of the main reactions involved in the conversion of fuel to the reaction products. The applicability of the reaction mechanisms in OEC were further tested in 3D CFD simulations of two lab-scale furnaces for different N2/O2 mixture as oxidant. For the CFD simulation the steady laminar flamelet model was applied. Each furnace was equipped with two different burners types resulting in a non-premixed jet flame and a flat flame with a high swirl and streamline curvature. The jet flames were investigated for 21, 25, 30, 45 and 100 % oxygen in the N2/O2 mixture. In the second case the oxygen content in the oxidant was only increased to 37 % due to instability of the flat flame at higher enrichment levels.
It was found by analyzing the reaction sets that all detailed reaction mechanisms were in close accordance for the predicted temperature in a wide range of the scalar dissipation (strain rate). However, for combustion with pure oxygen, a chemical reaction set with 17 species and 25 reversible reactions showed an over-prediction of the temperature near the chemical equilibrium (low scalar dissipation). Additionally, this mechanism showed a higher H2O content in the high temperature region. A detailed sensitivity analysis revealed that two elementary reactions are responsible for the dissociation of H2O into radicals. The 3D simulation of both flames revealed that the aforementioned reaction mechanism was suitable to predict the flames in the full range of oxygen enrichment and also for air-fired conditions. Measured temperatures in the furnaces were in close accordance to the calculated results. The other reaction sets were not suitable to predict the flame with the steady laminar flamelet model at high oxygen enrichment levels. Therefore the reaction set with 17 species and 25 reversible reactions is able to predict the combustion in furnaces for jet flames and flat flame burner.