Simulation Of Combustion Characteristics On W-shaped Boiler

Low-volatile coal occupies a large proportion of China’s coal resources and this type of coal is preferred for thermal power generation according to China’s coal policy.W-shaped flame boiler,with stable combustion and high combustion efficiency,is suitable for such coal combustion.In order to study the combustion characteristics and solve the problems occurring in a 600MW supercritical W-shaped flame boiler of certain plant,such as pipe expansion in high temperature surface,slagging,high NO_x emissions,numerical simulation on the combustion process of the boiler is conducted with the software of CFD.First,a mathematical model of numerical simulation of boiler combustion is created.According to the actual size,physical model is created and meshed,at the same time boundary conditions and wall conditions of the model also are set.In order to get the primary-air-coal ratio and provide data support for combustion simulation in the boiler,the numerical simulation of the cyclone separator burner is carried out.After that,numerical simulation is carried out on the basic working condition,variable coals working conditions and variable loads working conditions.Combustion characteristics,slagging,wall heat flux distribution and other issues are studied by analyzing the flow field,velocity field,temperature field,component concentration field,wall temperature and wall heat flux.The calculation results show that the symmetry of the flow field and the temperature field under the basic conditions is better and flame center height will become lower when it is close to the center of the furnace in the width direction.Temperature field symmetry becomes worse on low-load working condition and flame center height decrease with load decreasing.High-temperature gas flow even directly wash cold ash bucket area in360MW.The shape of flame does not change when the type of coal change,but the coal with high volatile content has a higher temperature in lower furnace and the coal with higher carbon content has a higher temperature in upper furnace.The slagging of the W-type flame boiler is caused by its own structure.Due to the high temperature gas scouring,the slagging location mainly concentrates on the side wall and the wing wall near the furnace arch.The slagging of the cold ash hopper is worse on low-load working condition and the drgree of slagging is influenced by the type of coal.On the basic working condition,the heat flux on the front wall and the rear wall along the width direction present a special M type distribution with two peaks near the 1/4 furnace width and 3/4 furnace width.The heat flux distribution present asymmetric distribution or bell type distribution and the heat flux distribution is worse because the temperature field in the furnace is no longer symmetrical on low-load working condition.Finally,optimized simulation for burning in the boiler is researched by changing the angle of secondary air under the arch.The unburned combustibles in fly ash and the concentration of NO_x emission both decrease with increasing angle of secondary air.At the same time,the drgree of slagging is better because the high temperature flue gas reduces the scouring of the side walls and wing walls.The high temperature flue gas directly wash cold ash hopper when the angle is too large.This result in a large area of cold ash bucket slagging which has a potential safety hazard on the operation of the boiler.So,the angle of secondary air should be controlled within the range of 20°~30°。The overall distribution of wall heat flux does not change much with the angle of secondary air increasing,but the overall distribution uniformity will be improved to some extent.