Flue gas recirculation in a gas-fired laboratory furnace: Measurements and modelling

This paper presents an experimental and numerical study of the effect of flue gas recirculation (FGR) on flame characteristics and pollutant emissions. The experimental study was performed in a small-scale laboratory furnace fired by a gas swirl burner of industrial type. The data reported include simultaneous flue gas concentrations of O-2, CO, CO2, unburnt hydrocarbons (UHC) and NOx. In addition, detailed in-flame data for major gas-phase species concentrations and gas temperatures were obtained in the near-burner region for two representative operating conditions. For these conditions, a mathematical model based on the numerical solution of the equations governing conservation of mass, momentum and energy and the transport equations for scalar quantities was used. The flue gas data show a marked decrease of NOx emissions with FGR without significant effects on flame stability, overall combustion efficiency and CO and UHC emissions. The transition between yellow and blue flame occurs at higher FGR rates as the excess air increases. The detailed in-flame data suggest that prompt NOx is an important mechanism of NOx formation for the present flow configuration without FGR and that FGR is an effective method for reducing it. These trends are correctly predicted by the mathematical model. However, discrepancies between the predicted and measured temperature and species concentrations, including NOx, were found, especially close to the burner. These may be due to the shortcomings of the turbulence model in the prediction of swirling flows. (C) 1997 Elsevier Science Ltd.