Research On Numerical Simulation And Optimization Of Natural Gas Low-nitrogen Burner

In view of the low combustion efficiency of coal-fueled industrial boilers,high energy consumption,and many types of pollutant emissions,China has already introduced the policy of replacing coal with gas,so most of the new industrial boilers are changed to burning natural gas.In recent years,the government has become more stringent on nitrogen oxide emission control.Parts for the new gas boiler NOx emissions shall not be higher than80 mg/m~3（low nitrogen emissions）,nitrogen oxides are the most important pollutants produced by the combustion of natural gas and one of the main pollution sources in the atmosphere,for humans,animals and the environment has brought serious influence,thus reduce NOx emission is a hotspot of research on domestic and foreign scholars;At present,the low-nitrogen combustion technologies used in industrial burners include staged combustion,rich-lean combustion,low oxygen combustion,flue gas recirculation combustion technology,etc.Among them,flue gas recirculation technology can be divided into two kinds of flue gas internal circulation and external circulation,and flue gas external circulation technology is easy to transform for boilers in use.Therefore,flue gas recirculation technology is adopted.Recycling technology to achieve low nitrogen emissions will be of great significance.In this dissertation,the experimental test and numerical simulation are carried out for the gas boiler burner with an evaporation capacity of 25t/h;Solidworks is used to establish a3D model of the burner and furnace,and ICEM CFD is used to divide the 3D model into a hybrid mesh;Realizable k-（?）model to simulate the turbulent flow,using eddy dissipation model coupled finite rate model（FR/EDM）to simulate the situation inside the furnace,using methane-air-2steps as the chemical reaction mechanism,using P-1 The radiation model predicts the radiant heat inside the furnace,and finally the pollutant model is used to calculate the nitrogen oxides;The experimental and simulation comparison results show that the combustion load increases from 675 m~3/h to 1632 m~3/h,the temperature measured at the measurement point increases from 1533.21 K to 1893.56 K,and the nitrogen oxide concentration increases from18.36 mg/m~3 to 46.33 mg/m~3.Therefore,with the increase of load,both combustion temperature and nitrogen oxide emissions increased,and the maximum error between simulation and experiment was 12%,the average error is 6%.According to the numerical simulation,the maximum temperature increased from 1650.45 K to 1997.50 K,with an increase of about 20%,and the corresponding nitrogen oxide increased from 17.86 mg/m~3 to52.77 mg/m~3.Numerical simulation study of different excess air coefficients found that with the increase of excess air coefficient,the internal velocity of the furnace gradually increased,the high temperature area of furnace combustion increased first and then decreased,and the nitrogen oxide concentration trend also increased firstly and then decreased.Whenα=1.15,the high temperature combustion area is the largest,and the corresponding nitrogen oxide emission concentration is 65.21 mg/m~3.The concentration of carbon monoxide at the outlet decreased from 19.23 ppm to 4.02 ppm,and the residual oxygen increased from 3.4%to5.82%.It can be seen that if the excess air coefficient is too low,it will lead to incomplete combustion,too high will lead to combustion heat loss increasing.The excess air coefficient ofα=1.10 is the best choice for economic combustion.Through the simulation analysis of different external flue gas circulation rate,it is concluded that the external flue gas circulation rate increases from 0%to 20%,the maximum temperature of furnace decreases from 2047.83 K to 1703.52 K,and the nitrogen oxide emission decreases from 65.21 mg/m~3to 25 mg/m~3 by 61.7%.The external flue gas circulation rate can greatly reduce the nitrogen oxide emission.Through the numerical simulation analysis of the ejector tube with flat nozzle and tapered nozzle structure,it is concluded that the velocity in the tapered ejector tube reaches to the maximum of 117.98 m/s when the distance S=-5 mm.Comparing the flow field and combustion emission characteristics of three structures without ejector tube,flat nozzle ejector tube of S=10mm and tapered nozzle ejector tube with S=-5 mm,the nitrogen oxide emission of ejector tube burner with tapered type and spacing S=-5 mm is the lowest.The nitrogen oxide emission of the injector burner with reduced nozzle type and distance S=-5 mm is the lowest.The maximum combustion temperatures of the three structures without injector tube,flat nozzle type with injector tube of S=10 mm,and tapered nozzle type with injector tube of S=-5 mm are 2078.79 K,2047.83 K and 1980 K respectively,and the corresponding nitrogen oxide emissions are 78.79 mg/m~3,65.21 mg/m~3and 46.58 mg/m~3 respectively.