Investigation On Gas/Particle Flows,Combustion And NO_x Formation Of Enhanced Stable Combustion FW Down-Fired Boiler

To achieve the goal of"carbon neutral",the installed capacity of solar and wind power generation in China has greatly increased in recent years.Thermal power units should meet the needs of flexible peak shaving.Low load（30%rated load or even lower）stable combustion technology urgently need to be developed.Down-fired boiler combustion technology is introduced from abroad for burning anthracite in China.However,at present,except for the American B&W down-fired boiler with cyclone burner,other furnace types cannot meet the national requirements for flexible peak shaving.In addition,down-fired boiler generally has the problem of excessive NO_xemission.In order to solve the above problem,the combustion organization mode of using secondary air to inject fuel-rich coal/airflow to penetrate downward for Foster Wheeler（FW）down-fired boiler is proposed,and the enhanced stable combustion technology of FW down-fired boiler is developed.The gas-solid two-phase flow characteristics,combustion and NO_xgeneration characteristics of the boiler are systematically studied by laboratory cold state experiment,industrial experiment and numerical simulation.An experimental method is proposed to quantitatively identify the airflow trajectory with smoke tracing.A 1:10 single-phase cold modeling test bench is set up,and single-phase cold experiment under 100%and 30%rated load are carried out using smoke tracing and hot wire anemometers to obtain the flow and mixing characteristics of pulverized coal/airflow under the structural and operating parameters such as oil-secondary air angle,fuel-lean coal/airflow declination angle,on arch secondary air ratio,oil secondary air ratio,and aerodynamic characteristics in the furnace.When the on arch secondary ratio is greater than 20%,the pulverized coal/airflow starts to flow to the near wall area after being injected into the furnace.The burnout and mixing distance between the pulverized coal/airflow and the secondary air are significantly prolonged.Under the structure of combining and backward arranging the fuel-lean coal/airflow nozzle,the dimensionless penetrating depth is relatively large under the low fuel-rich and fuel-lean coal/airflow ratio（5:5）,which is conducive to the ignition and stable combustion of pulverized coal.Under the parallel oil-secondary air structure,the mixing process of pulverized coal/airflow and oil-secondary air is relatively gentle and late.When the oil-secondary air ratio increases from 5%to 35%,the downward penetrating depth from the arch air increases gradually,and the mixing distance between the fuel-rich coal/airflow and the oil-secondary air increases first and then decreases.When the fuel-lean coal/airflow nozzle dip angle increases from 10°to 45°,the coal/airflow from arch moves to the near wall area,and the downward penetrating depth increases gradually.Finally the fuel-lean coal/airflow nozzle dip angle is recommended as 30°and the recommended oil-secondary air ratio is 25%.A 1:15 gas-solid two-phase cold modeling test bench is set up,and the gas-solid two-phase cold state experiments under 100%and 30%rated load are carried out using the three-dimensional laser Doppler dynamic particle analyzer to obtain the gas-solid two-phase flow characteristics in the furnace with or without the on arch secondary air,different fuel-lean coal/airflow declination angles and different oil-secondary air and F-layer secondary air distributions.After adding the on arch secondary air（air ratio of 20%）,the pulverized coal/airflow deflects to the near wall area during the downward penetrating process.The vertical gas-solid two phases dimensionless velocity attenuation slows down significantly.The burnout distance of pulverized coal is prolonged.When the fuel-lean coal/airflow angle is arranged at 10°,the downward penetrating depth of pulverized coal/airflow is shallow and the flow field is skewed.However,the peak particle volume flow rate appears on the rear wall when the fuel-lean cola/airflow angle is 45°,which is easy to cause slagging.When the fuel-lean coal/airflow angle is 30°with the oil-secondary air and F-layer secondary air distribution of 1:2,the downward penetrating depth of coal/airflow is large and its risk of scouring the water wall is low,which is recommended to be used in boiler operation.The enhanced stable combustion technology are applied to two 600 MW FW down-fired boilers and industrial hot state experiments are carried out to obtain the ignition,combustion and NO_xgeneration characteristics of pulverized coal/airflow under different loads,different oil-secondary air baffle openings,and different oil-secondary air and F-layer secondary air distributions.Under the new combustion technology,the boiler achieves stable combustion without oil support under 33%rated load,and the fuel-rich coal/airflow is ignited at1.4 m away from the burner nozzle outlet.Under 50%rated load,when the oil-secondary air baffle opening is increased from 0%to 40%,the ignition distance of pulverized coal/airflow is shortened,the combustion stability is enhanced.The carbon content in fly ash is reduced to 4.20%,and the NO_xemission is slightly increased which is recommended to be used in boiler operation.Under 90%rated load,when the oil-secondary air and F-layer secondary air distribution is increased from 4:6 to 6:4,the ignition distance is extended from 1.3 m to 1.6 m.And the flue gas temperature under the burner is decreased by about 50°C.When the secondary air distribution is 6:4,the NO_xemission and the carbon content in fly ash are both at a low level（853.86 mg/m³@6%O₂and 4.95%）,which is recommended to be used in boiler operation.Finally,the numerical simulation research under 100%and 30%rated load is carried out to obtain the thermal flow,combustion and NO_xgeneration characteristics of pulverized coal/airflow in the furnace under different oil-secondary air and over fire air distributions,burner operation modes and fuel-rich and fuel-lean coal/airflow ratios.When the oil-secondary air and over fire air distribution is increased from 0.24to 1.45,the carbon content in fly ash at furnace outlet gradually decreased from5.33%to 3.81%.When the secondary air distribution is 0.85,both NO_xemission and carbon content in fly ash are at a low level,which are respectively 835.60mg/m³@6%O₂and 4.62%.When the burner is centrally put into operation at low load,the combustion in the furnace is symmetrical,and the concentrated pulverized coal/airflow is ignited at 1.82 m away from the burner nozzle.In this mode,the flue gas temperature at the furnace outlet is relatively higher and the combustible carbon content in the fly ash is relatively lower（3.80%）when the fuel-rich and fuel-lean coal/airflow ratio is 1:1,which is recommended to be used in the boiler operation.