贴壁风方式对锅炉壁面气氛及燃烧特性的影响

Influence of Near-wall Air Mode on Boiler Wall Atmosphere and Combustion Characteristics

深度空气分级燃烧作为一种高效的低氮燃烧技术已被超临界机组普遍采用,然而由于还原区过大,导致锅炉水冷壁及高温受热面频繁发生高温腐蚀。为了研究贴壁风对高温腐蚀的抑制作用,比较空气和烟气贴壁风对锅炉在不同负荷下的燃烧及NO_(x)排放特性的差异,针对某电厂660 MW超临界机组炉内燃烧流动问题,建立了考虑贴壁风标量方程的CFD(computational fluid dynamics)数值模型,研究了不同的贴壁风设计参数在BMCR(boiler maximum continous rating)工况与50%THA(turbine heat acceptance)工况下对水冷壁高温腐蚀区域受热面燃烧环境的影响,并将仿真结果与电厂对应数据进行对比验证。在前后墙中间截面处对CO浓度进行定性分析,发现两种工况下采用空气贴壁风和烟气贴壁风均使侧墙对应区域CO浓度降低,同时将颗粒燃尽率、出口NO_(x)浓度在同样运行功率间进行对比,发现变化不明显,可认为两者对炉内燃烧的影响差异较小,且均能控制壁面区域的还原范围。在50%THA工况下,由于负荷降低,该负荷下工况的出口处烟气温度均低于BMCR负荷工况下的出口烟气温度,但使用再循环烟气作为贴壁风比使用二次风作为贴壁风时的出口烟气温度可以提高6 K以上,在高负荷功率下出口烟气温度提高更明显一些。综合来看,在较宽的锅炉运行功率范围内使用再循环烟气均可以良好替代二次风作为贴壁风,同时再循环烟气原有的降低NO_(x)与提高出口温度的优势也可以起到作用。

As an efficient low-NO_(x) combustion technology,deep air staging has been widely used in supercritical boiler units.However,due to the large reduction area,high-temperature corrosion occurs frequently on the boiler water wall and high-temperature heating surface.In order to study the inhibition effect of near-wall wind on high temperature corrosion and compare the differences of using air or flue gas as the near-wall wind on combustion and NO_(x) emission characteristics under different boiler loads,a CFD(computational fluid dynamics)computational fluid dynamics numerical model considering the scalar equation of near-wall wind was established to simulate the combustion and flow behaviors in a 660 MW supercritical boiler.The effects of different design parameters of near-wall wind on the combustion environment were studied under the BMCR(boiler maximum continous rating)condition and the 50%THA(turbine heat acceptance)condition,especially for the areas where the high temperature corrosion may occur on the water wall.The simulation results were compared and verified with the corresponding data from the power plant.Through qualitative analysis of the CO concentration at the middle section of the front and rear walls,it is found that under the two working conditions,the use of near-wall wind can reduce the CO concentration in the corresponding area of the side wall,regardless of whether air or flue gas is used.Meanwhile,at the same boiler load,there is no obvious difference in the fuel burnout rate and the outlet NO_(x) concentration among these two conditions.Hence,it can be considered that applying air and flue gas as the near-wall wind can both control the reducing atmosphere of the wall area,and the in-furnace combustion state is similar.Under 50%THA working condition,since the boiler load decreases,the flue gas temperature at the outlet is higher than that under BMCR working condition.However,compared to the situation using secondary air,when using recycled flue gas as the near-wall wind,the outlet flue gas temperature can increase by more than 6 K.This difference is more obvious under higher boiler load.In general,the recycled flue gas can well replace the secondary air as the near-wall wind in a wide range of boiler load.At the same time,the advantages of recycled flue gas in reducing NO_(x) emission and increasing outlet temperature can also play a role.