Abstract :
[en] Flameless Oxidation is a combustion technology developed in the early nineties to abate nitrogen oxides emissions in industrial furnaces, even when using highly preheated air to increase combustion efficiency. The principle is based on the entrainment of flue gas by the high velocity reactant jets in which the reactive species content is therefore reduced. As a consequence, temperature peaks decrease in the furnace and the thermal NOx formation is subsequently minimized.
This combustion mode has been rapidly applied to industrial burners, but its deep understanding and its numerical modeling is still under investigation. The present study deals with a laboratory scale furnace operating in Flameless Oxidation. The objective is to create an experimental database that will help to understand this combustion mode and that will be used for the validation of numerical models. The small volume of the combustion chamber (0,12 m³) allows detailed in-furnace measurements and ensures a shorter computational time.
The furnace temperature is controlled by varying the immersion depth of four sliding water cooled tubes which feature the load. The combustion air is preheated electrically (up to 1000°C) and is injected through the centre of the square base of the furnace. Two natural gas injectors are laid out on both sides of the air injector. The gas firing rate is around 30kW. The experimental characterization of the reaction zone is performed through measurements of wall temperature profile, OH self-emission in UV, as well as temperature and species concentrations (O2, CH4, CO, CO2 and NOx content) in the furnace vertical symmetry plane with intrusive probes. With a small scale furnace, the perturbations induced by the intrusive character of the probe are considerable, and the probe has therefore to be carefully designed to affect as little as possible the results. The paper presents the differences in the species concentration fields obtained with various configurations of the sliding suction probe (completely cooled or not, effect of the suction orifice diameter).
Moreover, the furnace is modeled using the CFD commercial code Fluent. Experimental boundary conditions are used in the model, and the numerical results are compared with measurements to validate the numerical results. The standard k-e model and the discrete ordinates model are used respectively for turbulence and radiative heat transfer modeling. Different sets of parameters are tested in the Eddy-Dissipation/Finite Rate combustion model, and their influence on the results is discussed. The objective is to find the model parameters able to predict the most accurately possible the shape of the reaction zone and its location in Flameless Oxidation mode.
