Abstract
In practical operations, the carbon monoxide (CO) distribution in an opposite-wall-firing furnace (OWFF) is characterized by a high concentration near the side walls and a low concentration in the center, accompanied by a series of combustion-related issues. To find the reasons for the CO distribution, a numerical study was conducted on a 660 MWe OWFF. The CO concentration profiles, distribution coefficients of coal and air, mixing coefficients, and the aerodynamic characteristics were extracted for analysis. The CO distribution within the furnace greatly depends on the mixing of coal and air. A mismatch between the aerodynamic behaviors of coal and air causes the non-uniform distribution of CO. Taking into consideration that distinctive flow patterns exist within the different regions, the formation mechanisms of the CO distribution can be divided into two components: (1) In the burner region, the collision of opposite flows leads to the migration of gas and particles toward the side wall which, together with the vortexes formed at furnace corners, is responsible for unburned particles concentrated and oxygenized from the furnace center to the side wall. Thus, high CO concentrations appear in these areas. (2) As the over-fire air (OFA) jet is injected into the furnace, it occupies the central region of furnace and pushes the gas from the burner region outward to the side wall, which is disadvantageous for the mixing effect in the side wall region. As a consequence, a U-shaped distribution of CO concentration is formed. Our results contribute to a theoretical basis for facilitating the control of variation in CO concentration within the furnace.
概要
目 的
前后墙对冲燃烧锅炉在实际运行中普遍存在沿炉膛宽度 CO 体积分数呈中间低、 两边高的分布特点. 该分布特点往往伴随着锅炉效率降低、 炉膛出口 NOx 排放偏高、 侧墙结渣和高温腐蚀严重等一系列问题. 本文旨在分析前后墙对冲燃烧锅炉内 CO 分布规律的形成原因, 为机组运行调整与燃烧系统改造等工程实践提供理论依据.
创新点
1. 针对煤粉炉燃烧过程中存在剧烈气固反应流的特点, 定义了与气、 固相质量变化无关的风、 煤分布系数 ηa 和 ηp, 进而得到了风/煤混合系数 η. 2. 对比炉膛不同区域内的 CO 分布与风/煤混合系数分布, 结合炉膛的空气动力学特性, 解释了前后墙对冲燃烧锅炉内 CO 分布特征的形成过程.
方 法
1. 建立一台 660 MWe 前后墙对冲燃烧锅炉的数值模型, 并通过现场测量数据与模拟数据的比较, 验证模型的有效性 (表 5 和图 3). 2. 定义炉内风、 煤分布系数与风/煤混合系数 (公式(1)~(3)). 3. 对比不同特征截面上 CO 分布特性与风/煤混合特性, 探讨二者之间的关联性 (图 4 和 5). 3. 分析风的分布和煤的分布分别对风/煤混合特性的影响 (图 6 和 7). 4. 综合考虑炉内气流结构、 风和煤的分布及其混合特性, 揭示炉内 CO 分布规律的形成原因 (图4~9).
结 论
1. 对前后墙对冲燃烧锅炉而言, 燃烧器区域出口的 CO 大量聚集在炉膛中间至侧墙区域, 燃尽风区域出口的 CO 则主要集中于侧墙附近. 2. CO 分布与风/煤混合系数分布基本一致. 3. 在整个炉膛空间内, 风和煤粉皆有向侧墙流动的趋势, 但煤粉更加聚集在侧墙周围. 4. 在燃烧器区域, 对冲气流使得风与煤粉向侧墙流动, 然后在炉膛四角形成涡流; 四角涡流导致烟气在沿侧墙上升的过程中发生明显的气固分离, 使煤粉被甩至侧墙附近并在那里燃烧, 形成高浓度 CO; 5. 在燃尽风区域, 燃尽风与侧墙附近烟气的混合并不理想, 因此侧墙附近的高浓度 CO 难以消除, 最终导致沿炉膛宽度的 CO 体积分数呈中间低、 两边高的分布特点. 6. 基于上述结果, 本文分析了当前应用中存在的问题, 并提出了改进建议.
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Jian-guo YANG, Chao-yang ZHU, and Hong ZHAO proposed the research goal. Jian-guo YANG and Xiao-qiang XIE designed the research. Xiao-qiang XIE conducted the research and analyzed the corresponding data. Chao-yang ZHU and Chuan-huai LIU provided the technical assistance during the in-situ experiment. Xiao-qiang XIE wrote the first draft of the manuscript. Jian-guo YANG and Hong ZHAO helped to organize and revise the manuscript. Zhi-hua WANG offered valuable guidance for the fulfillment of the manuscript. Xiao-qiang XIE revised and edited the final version.
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Xiao-qiang XIE, Jian-guo YANG, Chao-yang ZHU, Chuan-huai LIU, Hong ZHAO, and Zhi-hua WANG declare that they have no conflict of interest.
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Xie, Xq., Yang, Jg., Zhu, Cy. et al. Numerical analysis of reasons for the CO distribution in an opposite-wall-firing furnace. J. Zhejiang Univ. Sci. A 21, 193–208 (2020). https://doi.org/10.1631/jzus.A1900363
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DOI: https://doi.org/10.1631/jzus.A1900363
Key words
- Opposite-wall-firing furnace (OWFF)
- Carbon monoxide (CO) distribution
- Distributions of coal and air
- Gas/particle flow
- Corner vortex
- Over-fire air (OFA)