Numerical Simulation And Experimental Study Of NO_x And Fly-ash Distributions In Coal-fired Boiler

There are problems of low denitration efficiency, high ammonia escape rate, catalyst blockage, ash deposition, corrosion and wear of heating surface, low catalyst utilization rate in SCR denitration technology. The main parameters of flue gas denitration system (such as distribution of speed, temperature, NOx and fly ash, etc.) are poorly defined, simplified uniform inflow instead of non-uniform inflow are used, the influence of fly ash particles is ignored, which lead to unreasonable spraying ammonia and component design, resulting in the above problems. Computational fluid dynamics study is carried out in this paper, whole flow model is established for coal combustion and NOx formations process in both oppsed-firing boiler and tangential-firing boiler, to obtain combustion data in furnace and flue gas data of denitration system exit, especially NOx distribution, compare them with the field testing datas. In consideration of fly ash particles, gas-solid two phase flow numerical simulation is conducted in tangential-firing boiler for obtaining the motion characteristic of the fry ash particles, which can provide accurate gas-solid two-phase parameters for denitration system entrance.Simulation was performed for both a 660 MW opposed-firing boiler and a 330MW tangential-firing boiler, to calculate the flow, heat transfer, combustion and whole process flow under BMCR condition. The combustion simulation process provides fundamental data for the NOx prediction. Simulation results show that in opposed-firing boiler, jets flow generated by swirl burner recirculate near ports area, move to center of chamber and then gather to go up, the temperature symmetrically distribute along furnace depth. A large amount of CO is formed at burner zone, while CO2 generated at over-fired zone. Furnace and tail flue is filled of flue gas, mixing and heat transfer process of gas and particle phase exist through the entire flow. The results in tangential-firing boiler show that jets enter the furnace to form a rotating flow, then spiral rising, high temperature and high velocity area arrange in annular. O2 and CO distributions present a trend of first increasing then decrease along furnace height, while CO2 show the opposite. Each furnace corner form a vortex, prevented by over-fired air the swirl intensity of main flow is decreased, as for flow and temperature reducing by heating surface, average temperature and velocity are decreased deeply at economizer exit.Based on the combustion results, post-processing method is used to calculate furnace NOx formation for different types of boilers. By the method of characteristic equipotential surface and NOx flux analysis, NOx generation rule is studied, especially for a tangential boiler under variable load conditions, compared with the field test data, NOx distribution under actual operation is obtained. Results show that high temperature and high oxygen concentration range assume to be larger near burner nozzle in opposed-firing boiler, leads to more NOx generation, while less in recirculation zone. NOx distribute heterogeneously, especially for economizer exit, NOx flux distribution is extremely uneven along the depth direction. High temperature equpotential surface arrange in annular in tangential firing boiler, firing-facing side seems to produce abundant NOx, while inhomogeneous along furnace height. Boiler has higher NOx emissions under lower load.Experimental study is carried out for the tangential boiler, in consideration of fry ash, a gas-solid two phase flow process is simulated in the boiler furnace and tail flue. Results showed that flowing behavior of different diameter particles present a good consistence, spiral rise in furnace. Larger particles affected by inertia force partly exist in hopper, in tail flue mainly move to the side far away from rear wall. The motion characteristics of fry ash vary in different loads, diverse size and complex concentration of fry ash particles exist in economizer exit, residence time is longer under low load. Particle distribution is changed with load in tail flue, with high concentration in four corners.