Study On Low NO_x Emission Optimization Of Low Swirl Burner

With the rapid development of natural gas industry,the emission of nitrogen oxides(NO_x)in the combustion process has gradually attracted people’s attention.Low swirl burner is a new type of fully premixed burner,which has the advantages of low NO_x emission and high combustion stability.In this paper,a typical vane type low swirl burner is taken as the research object,and the effects of structural parameters and air inlet conditions on the combustion characteristics are studied by numerical simulation.A three-dimensional grid including burner and combustor was established,and the appropriate mathematical model and chemical reaction mechanism were selected to verify the reliability of numerical simulation results.At the same time,the basic low swirl combustion characteristics were analyzed.Through the unique structure of the burner,the premixed gas is divided into two parts:the central direct flow and the surrounding swirling flow,and then forms the internal raised flame surface and the surrounding attached flame surface.The chemical reaction mostly occurs near the flame surface,while NO_x mainly occurs in the high-temperature reflux area above the internal raised flame surface.The influence of burner structure parameters on the flow and combustion characteristics was studied by using single factor analysis and response surface analysis.The results show that the angle of swirl blade,the diameter of orifice and the length of mixing section have great influence on the velocity distribution and flame shape;At the same time,with the angle of swirl blade increasing from 25° to 37° the NO concentration at the combustor outlet decreases by 18.93%;With the diameter of orifice increasing from 1.8mm to 2.2mm,NO concentration at combustor outlet increased by 13.98%;With the length of mixing section increasing from 10 mm to 50 mm,the NO concentration at combustor outlet increases by 8.75%.Based on the above results,the regression equation between the NO concentration at the combustor outlet and the structural parameters was obtained by response surface analysis,The order of the influence of the structural parameters on the NO concentration at the combustor outlet and the interaction among the factors are analyzed.The influence of combustor structure parameters on the flow and combustion characteristics was studied by using control variable method.The results show that the inclination angle of the combustor floor has a great influence on the velocity distribution and flame shape,while the diameter and height of the combustor have little influence.At the same time,with the inclination angle of the combustor floor increasing from 20° to 60°,the NO concentration at combustor outlet decreases by 35.24%;With the combustor diameter increasing from 120 mm to 180 mm,the NO concentration at combustor outlet first decreases and then increases,and the NO concentration at combustor outlet is the lowest when the combustor diameter is 135 mm;With the combustor height increasing from 200 mm to 400 mm,the NO concentration at combustor outlet increased by 28.36%.Through steady-state numerical simulation and unsteady-state numerical simulation,the influence of airflow inlet conditions on the flow and combustion characteristics was studied.The results show that the excess air coefficient has a great influence on the temperature distribution in the combustor,the inlet velocity has a great influence on the velocity distribution in the combustor,and the flame shape will be significantly changed by the introduction of wake air.At the same time,with the excess air coefficient increasing from 1.1 to 1.5,the NO concentration at the combustor outlet decreases by 99.63%;With the inlet velocity increasing from 6m/s to 14m/s,the NO concentration at combustor outlet decreases by 41.87%;With the inlet velocity of the wake air increasing from 0.2m/s to 0.4m/s,the NO concentration at the combustor outlet first increases and then decreases,and the NO concentration at the combustor outlet is the highest when the inlet velocity of the wake air is 0.25m/s.When the inlet gas velocity is unstable,the low swirl burner still maintains a stable combustion state,but the coupled oscillation will occur in the angular recirculation zone.