NUMERICAL AND EXPERIMENTAL CHARACTERIZATION OF A SELF-REGENERATIVE FLAMELESS OXIDATION BURNER OPERATION IN A PILOT-SCALE FURNACE

Diluted combustion technologies, also called 'flameless oxidation', have been introduced as a very effective way to abate NOx production in high temperature gas fired furnaces using preheated combustion air. Besides NOx production, these techniques also modify flame characteristics as well as heat transfer inside the furnace. This study presents experimental data and results of numerical modeling of a pilot-scale furnace fired with flameless oxidation.
Tests are carried out on a 200 kW natural gas FLOX REGEMAT® 350 burner manufactured by WS-Essen (Germany). It is a self-regenerative burner through which gas and highly preheated air are injected at high momentum into the combustion chamber. The entrainment of flue gases at high rate by air and gas jets produces a high level of dilution of the reactants before mixing. The burner fires a rectangular fiber-lined pilot furnace of about 3m³, in which the temperature can be varied from 1100 to 1300°C thanks to water cooled tubes.
Burner characterization is performed by measuring flue gas temperature and composition (O2, CH4, CO, CO2, NOx) at the outlets and in the median plane of the furnace with a suction thermocouple probe combined with a sampling probe connected to a gas analysis unit. Profiles of radiative heat flux incident on the roof are measured with 4 elliptical radiometers and CCD imaging of OH self-emission in UV is performed to show the evolution of the combustion zone within the furnace for various furnace temperature conditions. The temperature fields measured in the furnace are uniform and the low-NOx performance is remarkable (4 to 35ppm@3%O2). In flameless combustion mode, the reaction zone is more extended, and located farther downstream from the burner as furnace temperature decreases.
The burner and the pilot scale furnace have been modeled using FLUENT 6 with a 340,000 hexahedral cells grid. Different combustion models are assessed. The 'PDF-equilibrium' model resolves a transport equation for the mixture fraction with a beta-shaped probability density function to integrate the turbulence/chemistry interaction. The 'Eddy-Dissipation/Finite Rate' and 'Eddy-Dissipation' models solve transport equations for each species involved in the reaction mechanism and differ by the way they compute average reaction rates (by taking into account chemical kinetic or not).Thermal and prompt NOx are predicted with FLUENT in a post-processing computation.
All combustion models give good prediction outside the reaction zone, but there is an overestimation of temperature in the near burner zone, especially with standard 'diffusion controlled' combustion models ('PDF' and 'Eddy-Dissipation'). The 'Finite Rate/ Eddy-Dissipation' model improves prediction because it takes chemical kinetic into account. However, the smooth increase of temperature and the location of the reaction zone do not agree with experimental observations. Computed NOx values well reproduce measurements although corresponding temperature fields do not seem totally correct.
Further work will focus on the integration of detailed reaction mechanisms and on the assessment of NOx models.