Simulation And Experimental Study On Deep Air-staged Low No_x Combustion Of Pulverized Coal

In 2018,China's total coal consumption was 3.68 billion tons,of which about80%was used for direct combustion.Although the“Double Hundred”coal-fired emission standard is generally implemented.The ultra-low emission standard is carried out in the developed areas because of the dense source and limited air pollutant capacity.Specifically,the emissions of the NO_x should be less than50mg/Nm³.The flue gas post-treatment technology can achieve the ultra-low emission standard of NO_x with disadvantage of high investment and operating cost.Therefore,it is difficult and drawn great attention to reducing the initial emission of NO_x and the cost of flue gas post-treatment for the coal-fired industrial boiler.The air-staged combustion is one of major technologies to control NO_x emission for coal-fired boilers.In order to reduce the initial NO_x emissions of the pulverized coal industrial boiler,the combustion characteristics and NO_x emissions of deep air-staged combustion are studied by means of numerical simulations and experiments.Firstly,a modified combustion-gasification model coupled with traditional pulverized coal combustion and char gasification is established,and the NO_x prediction parameters are improved.Secondly,the experiment of the deep air-staged combustion of the drop tube furnace?DTF?is carried out and the corresponding numerical simulation is implemented by the modified model.The modified model is verified by comparing the numerical data and the experiment results.Thirdly,the simulation is carried out for a 40t/h pulverized coal industrial boiler to study the influence of the over-fire air rate and the distribution height on the combustion characteristics and NO_x formation.Finally,an engineering test was carried out in a preferable deep air-staged condition to verify the accuracy of the modified model and the effect of deep air-staged combustion on the NO_x emission.The research content and research results of this dissertation include the following aspects:?1?Numerical simulation and experimental study of deep air-staged based on high temperature DTF.The char gasification reaction was coupled with the traditional pulverized coal combustion model,and a pulverized combustion modified model was established.The numerical calculation was carried out for the DTF.In addition,the deep air-staged experiment of the high temperature DTF was carried out.The results show that the modified model can accurately descript the characteristics of the reduction zone under deep air-staged conditions and quantitatively predicts the concentration of CO and H₂.The improved numerical model is reliable to predict deep air-staged combustion.?2?Numerical simulation of deep air-staged of a 40t/h pulverized coal industrial boiler.Firstly,the geometric model was established,the computational domain is meshed,and grid independence test were carried out.Secondly,the numerical calculation is implemented by using the modified model to study the influence of the over-fire air rate and the over-fire air distribution height on the combustion characteristics and NO_x formation.The results show that the initial NO_x emission concentration of the boiler decreases from 692mg/m³ to 421mg/m³ with over-fire air rate increasing from 0%to 45%.The average temperature and the flow velocity of the outlet gas of the burner increases by 317K and decreases by 27m/s,respectively.Moreover,a more uniform distributions of temperature and velocity are achieved.In addition,the recirculation zone in the burner is not affected by the decrease of the secondary air volume of the swirling.The ignition and stable combustion of the pulverized coal in the burner are not affected.?3?Experimental study on air-staged combustion of pulverized coal industrial boiler.The air-staged combustion test was carried out on a 40t/h pulverized coal industrial boiler.Firstly,the effects of boiler load and oxygen content on NO_x emissions under normal combustion?without air-staged combustion technology?were studied.Secondly,the effects of over-fire air rate on NO_x emissions under air-staged combustion conditions were studied.The variation law of the temperature and composition of the flue gas along the burner and the furnace was analyzed.The results show that the initial NO_x emission concentration of the boiler gradually decreases with the increase of the over-fire air rate under full load.When the over-fire air ratio is 30%,the initial NO_x emission concentration of the boiler is reduced from 630mg/m³to 409mg/m³with a decrease of 35%.The velocity of the flue gas in the burner is lowered,and the temperature of the upper part of the furnace is gradually increased by 40K.The temperature of the central part of the furnace is not change.The temperature and composition change of the flue gas in the burner reveals the process of conversion of pulverized coal combustion from pyrolysis and combustion reactions to gasification reactions.The deep air-staged combustion technology has an obvious effect of reducing the initial NO_x emissions of the boiler.The modified model has high accuracy,and the deep air-staged combustion technology can effectively reduce the initial NO_x emission of the 40t/h pulverized coal industrial boiler.