440 t/h燃煤锅炉低氮燃烧模式下水冷壁高温腐蚀分析

分别采用X射线荧光光谱、X射线衍射光谱、电镜扫描和能谱分析等方法，对某440 t/h燃煤锅炉下层燃烧器附近水冷壁区域收集的腐蚀层进行详细表征，分析并讨论腐蚀产物的形成和痕量元素富集特点。结果表明：腐蚀内层主要含有FeS₂、Fe₂O₃、PbS和ZnS，但Pb比Zn富集程度高；腐蚀外层以ZnS、PbS、GaS、Fe_1-xS和Zn_1.0Al_1.04S_2.13为主，还含有少量Ga、Se、Bi等元素，但Zn与Ga元素富集程度显著高于内层；腐蚀层中Pb、Zn、Ga、Se、Ge、As、Bi、Th、Sn、Sb等元素主要通过气化-冷凝和黏附的未燃尽碳颗粒释放析出的方式到达管壁，部分元素仍存在二次气化现象并扩散至腐蚀层深处，以进一步增大腐蚀裂纹尺度。基体铁的氧化和硫化反应同时发生并相互竞争，靠近基体侧腐蚀层先发生裂缝后逐渐开始破碎。黄铁矿向外转移中会释放气态硫并生成多种晶型的磁黄铁矿，这说明气态硫在腐蚀过程中扮演重要作用。此外，沉积层中微细颗粒和气态痕量元素可能会向基体侧移动与扩散渗透，增加腐蚀内部应力进而加剧腐蚀。

Analysis on High Temperature Corrosion of Water-Wall in a 440 t/h Boiler Under Low-NOx Combustion Mode

The corrosive products that were collected from water-wall tubes close to burners at the lowest layer in a 440 t/h coal-fired furnace, were characterized by various technology methods including X-Ray Fluorescence spectrum (XRF), X-Ray power Diffraction (XRD), Scanning Electron Microscope and Energy Dispersive System (SEM-EDS), for the analysis and discussion of the formation of corrosive products and enrichments of trace elements. The results showed that inner corroded layer was mainly composed of FeS₂, Fe₂O₃, PbS and ZnS, with higher enrichment of Pb element than Zn. While in the outer corroded layer, ZnS, PbS, GaS, Fe_1-xS and Zn_1.0Al_1.04S_2.13 were the major compositions of the mineral phases with minor elements of Ga, Se and Bi, but more significant enrichment of Zn and Ga elements were observed than that in the inner corroded layer. Trace elements including Pb, Zn, Ga, Se, Ge, As, Bi, Th, Sn, Sb were enriched both in inner and outer corroded layer. They were formed mainly via the vaporization-condensation of gaseous species from coal combustion and the release from the unburned carbon particle that adhered to water-wall tube surfaces. Moreover, some of condensed trace elements may re-vaporize and then diffuse into deeper corrosion layer, which would further cause the growth of the crack opening size in corrosion. During the corrosive process, the oxidation and sulfidation reactions of iron matrix occurred simultaneously and competed with each other. The cracks were first formulated at the corroded layer adjacent to iron matrix and then as the cracks developed, it started to break into small patches. Various kinds of pyrrhotite with different crystal sizes would be generated and some gaseous sulfur would be released as the pyrite migrated toward the outer layer, which suggests that gaseous sulfur may play an important role in corrosive process. Additionally, the migration and diffusion of fine particles and gaseous trace elements into iron matrix side may further accelerate the rate of internal stress cracking, to promote the diffusion of corrosive gases and aggravate the corrosion.