W火焰炉炉内流场偏斜与不对称燃烧的形成机制研究

Anthracite and lean coal account for more than 40% of power coal in China. Down-fired furnaces, designed specially for industrial firing of anthracite and lean coal, have popularized well in China in recent years. However, actual operating results revealed that asymmetric combustion phenomena existed widely in down-fired furnaces; this apparently deviates from the design ideals in which a symmetric W-shaped flame pattern should be established in these furnaces. Aside from resulting in poor combustion stability and low burnout, asymmetric combustion also creates large differences in volumetric heat load distribution in furnaces, thereby jeopardizing boiler safety operations because of large differences in wall temperatures and limits being exceeded. Previous investigations uncovered that with asymmetric combustion appearing, a deflected flow field, instead of a symmetric W-shaped one, developed in these furnaces. By performing (1) cold-modeling flow experiments at various parameters (i.e., the air distribution in the furnace, air-supplying angles, and furnace configuration referring to the furnace throat shringk, upper furnace height, and furnace-nose length), (2) industrial-size measurements at different operation conditions, and (3) the corresponding CFD simulations, characteristics of the flow-field deflection and asymmetric combustion within down-fired furnaces and the formation principles of the two phenomena are excepted to be disclosed within the forthcoming investigations. Moreover, a reasonable furnace design with both the flow-field deflection and asymmetric combustion being eliminated, will be finally established. Aside from providing information to deepen the understanding about the flow and combustion characteristics within down-fired furnaces, results of this investigation can be helpful as references for rationalizing designs and optimizing operations for down-fired furnaces.

无烟煤和贫煤占我国动力用煤的40%以上，针对燃用无烟煤和贫煤而引进的W火焰炉近年来在我国得到广泛应用。运行结果表明燃烧不对称现象广泛存在于W火焰锅炉内，这有悖于其形成对称W型火焰的设计理念。除导致燃烧稳定性差和燃尽低外，燃烧不对称还引起炉内热负荷分布不均匀、水冷壁壁温偏差过大和壁温超出允许值，进而危及锅炉安全运行。已有研究结果表明伴随燃烧不对称的是炉内形成偏斜流场，而非W型流场。本项目拟借助于冷态模化试验研究炉内配风、送风角度及炉膛结构参数(涉及喉口大小、上炉膛高度和折焰角大小)对炉内流动特性的影响，工业实验研究运行参数对炉内燃烧和NOx排放的影响，再配合数值计算，详尽揭示流场偏斜与不对称燃烧特性的同时确定其形成机制，并最终建立消除流场偏斜和不对称燃烧的锅炉合理参数设计。本项目的研究结果将助于深化对W火焰炉炉内流动和燃烧的认识，为优化W火焰炉设计与运行提供依据。

本项目选取两台英巴W火焰锅炉(即300MW和600MW锅炉)为研究对象，先是通过冷模试验、工业测量和数值计算相结合的方法，详尽揭示W火焰锅炉流场偏斜和不对称燃烧特性，确定了不对称燃烧对燃尽和NOx排放的影响，通过初调手段在实炉上探索了减轻不对称燃烧的运行模式；尔后以600MW锅炉为研究对象，通过系统性的冷模气固两相流模化试验和数值计算，得出流场偏斜和不对称燃烧形成机制及其主要影响因素，最终形成了构建对称流场和合理燃烧的锅炉成套参数设置。.在目前采用设计结构和配风参数下，炉内形成“前长后短、上行气流整体偏向前墙侧”型偏斜流场，流场偏斜导致炉内呈“后墙侧烟温明显高于前墙侧”的不对称燃烧特征；关小三次风可减轻但无法彻底消除流场偏斜；600MW锅炉因流场偏斜更为严重、燃用煤质更佳，其炉内烟温水平、燃尽率和NOx排放均较300MW锅炉高。不对称燃烧负面影响在于，引起炉内一侧燃烧状况较差且形成局部高温区，造成燃尽差的同时有促成更多NOx生成。.在300MW锅炉上开展的“在炉膛中心侧封堵部分二次风喷口”和“构建前后拱二次风不对称配风”初调手段均可有效减轻流场偏斜和不对称燃烧，鉴于封堵二次风喷口将使二次风箱压力显著升高且增加NOx排放，推荐优先考虑采用“拱部二次风不对称配风”运行模式来缓解不对称燃烧。.W火焰锅炉不对称燃烧的形成机制在于炉内形成偏斜流场，而偏斜流场的形成起因于“经短小上炉膛强化的上炉膛不对称结构效应”与“不合理燃烧器参数设计”的综合作用。在上炉膛固有不对称结构效应无法克服条件下，涉及上炉膛结构和燃烧器参数如上炉膛直段高度CH2、上下炉深比CW、拱部燃烧器位置CD和二次风喷口跨度CS是引发流场偏斜和不对称燃烧的主要因素，折焰角深度CL和三次风倾角是对流场偏斜影响较小的次要因素；加大CH2、加大CW、减小CD和减小CS均可有效减轻流场偏斜和不对称燃烧；在保持其它参数不变的情况下，对应CH2≥1.125、CW≥0.558、CD≤0.342和CS≤0.387中任一条件均可构建完好的W型流场。.考虑锅炉现场布置条件和锅炉造价成本，优选并向锅炉制造商推荐的组合参数设置为CH2=1.125、CL=0.298、CW=0.529、CD=0.396、CS=0.387、三次风倾角45°和二次风速41.47m/s，此时炉内燃烧对称、燃尽效果佳且NOx排放较低。