Numerical Modelling of Combustion in 1.5 MW Low-NOx Burner

Results from CFD simulations of swirling non-premixed gas flame using a low-NOx gas burner are presented and compared to experimental data measured during a testing campaign in a semi-industrial water-cooled combustion chamber. The burner is both fuel-and air-staged with maximum capacity of 1.5 MW. Numerical simulations are performed with Ansys Fluent software using Reynolds-averaged Navier-Stokes (RANS) turbulence model coupled with eddy-dissipation chemistry model (EDM) and discrete ordinates radiative heat transfer model. This modelling approach is preferable for large-scale combustion applications such as process fired heaters, where employing advanced models (e.g. Large Eddy Simulation) is still computationally demanding. It is shown, how results are sensitive to the mixing rate constant of EDM, which is employed in a simulation of swirling turbulent diffusion flame. Specifically, temperatures are overestimated with the constant being set to 4 (the default value in Ansys Fluent), which leads to false prediction of NOx formation. Temperature peaks are reduced by lowering the mixing rate constant. An acceptable agreement with measured outlet and peak flame temperatures is achieved, when the constant is set to 0.6. With this value, however, highly increased concentrations of unburned species are reported at the outlet. Predicted values deviate from the measurements about three orders of magnitude. Based on the concentrations of unburned species and oxygen in the flue gas at the outlet, the best results are obtained with the constant 1.2.