Study Of MILD Combustion Burner For NG-fired Boiler

In recent years,the heavy hazy weather appears in many places frequently.With the progress of coal-to-gas conversion,the use of NG-fired boiler increased sharply.Nitrogen oxides(NOx)is one of the major sources of pollution and its emission standards have become more stringent.How to reduce the NOx emission in the exhaust gas of gas boilers has become one of the important means to mitigate the haze pollution.Therefore,how to make the boiler directly burn to meet the latest NOx emission standards or even lower will have important significance.As a new combustion technology,the Moderate or Intense Low-oxygen Dilution(MILD)combustion has some obvious advantages such as combustion stabilization,uniform temperature distribution and low NOx emissions.In this paper,based on the MILD combustion,the axial fuel-staged combustion and air humidification are studied.The MILD combustion with low NOx emissions for NG-fired boiler is studied by numerical simulation and experiment.Firstly,the combustion and emission characteristics of the axial fuel-staged combustion are studied through software simulation.The results show that under given conditions the axial fuel-staged combustion can reduce the equivalence ratio of the primary combustion zone,make the peak flame temperature lower,and increase the downstream flame temperature,which will make the temperature field more uniform and suppress thermal NOx formation effectively.It shows that when the proportion of secondary fuel-staged range from 0%to 25%,the MILD combustion can be achieved by adjusting the flue gas recirculation ratio.And in the case of meeting the minimum reflow ratio of MILD combustion,the fuel-staged combustion can reduce the NOx formation and the effect will decrease as the secondary fuel increases.When the secondary fuel-staged ratio is 15%,it can obtain the best effect.Then,because of the decrease of the temperature of the reflux flue gas in the primary combustion zone,the critical reflux ratio of MILD combustion will increase slightly.When the secondary axial fuel-staged ratio is greater than 25%and the main equivalence ratio is lower than 0.7,it is not conducive to the realization of the MILD combustion.For the high-equivalent and high-heat-intensity furnace combustion,the axial fuel-staged is more effective for optimizing the OH distribution and uniform temperature field.And it can achieve lower NOx emissions.After the scheme of experiment was determined through numerical simulation,the experimental platform was set up.Then,the research on combustion characteristics and emission characteristics were carried out under different equivalence ratio,heat release intensity,and humidification.Under the experimental conditions in the text,the experimental results show that the outlet velocity of burner nozzle will affect the reflux ratio,which will make the temperature of the primary combustion zone have a slight increase when the speed is low,instead of decreasing with the increase of the secondary fuel-staged ratio.And it is not conducive to the control of NOx emissions in exhaust gas of the furnace.Under the experimental conditions of humidification given in the text,using hot water to humidify the air can reduce the peak flame temperature and optimize the temperature field.And it can make NOx emissions less than 10 ppm@3.5%O2.Moreover,the effect of using hot water humidification is more obvious than that using room temperature water.In a certain range of humidification,the amount of NOx reduction will increase with the increase of humidificition.The research content of this paper can provide reference for the design of low NOx MILD combustion burners used for NG-fired boilers.