劣质烟煤低氮燃烧模式下水冷壁高温腐蚀与硫化物沉积形成分析

In the transformation of ultra-low emissions for large coal-fired units, the technology of in-furnace air staged combustion + low-nitrogen burner transformation is widely adopted to significantly reduce the generation of nitrogen oxides in the furnace. However, as the quality of the coal fed into the furnace continuously deteriorates, with the ash content and sulfur content in the coal being on the high side and deviating seriously from the designed coal type, the risks of coking and high-temperature corrosion of the water wall under the low-nitrogen combustion mode increase sharply. Samples of the corrosion layer and deposition layer on the surface of the water wall tubes between the separated over fire air (SOFA) nozzle and the F-layer burner of a 330 MW thermal power unit were collected. It was found that the deposits adhered to the tube wall had a layered structure. After physically peeling off each layer, the element content determination and mineral phase characterization were carried out respectively, as well as the morphology and elemental energy spectrum analysis of the corrosion layer. The results showed that the corrosion layer was mainly composed of various iron sulfides, oxides and PbS, and also contained a small amount of elements such as As, Ge, Ga, Se and Zn; the inner and middle layers of the deposits on the water wall tubes were enriched with Fe, S, Pb and Zn elements, mainly existing in the forms of FeS, FeS2, PbS, ZnS and ZnAl1.04S2.13, but Pb was only enriched in the inner layer; the outer layer was mainly composed of aluminosilicates and aluminates, with a small amount of sulfides. Combined with thermodynamic calculations, it can be known that the enrichment of Pb and Zn elements in the deposition layer is mainly derived from the reaction of the gaseous Pb- and Zn-containing components precipitated during the combustion of bituminous coal with H_2S, and they reach the wall surface mainly by condensation and thermophoresis deposition. The enrichment amounts of Zn and Pb in the deposits are about 3 to 4 orders of magnitude higher than those in the coal and can be used to evaluate the corrosion degree of sulfides of the water wall tubes in the furnace.

大型燃煤机组超低排放改造中普遍采用炉内空气分级燃烧+低氮燃烧器改造技术,以显著降低炉内氮氧化物的生成。但随着入炉煤品质不断变差,煤中灰分与硫含量偏高,严重偏离设计煤种,低氮燃烧模式下水冷壁发生结焦与高温腐蚀的风险骤增。对某台330 MW热电机组炉内分离燃尽风(seperated over fire air, SOFA)喷口与F层燃烧器之间水冷壁管表面的腐蚀层与沉积层样品进行了收集,发现管壁粘附的沉积物呈层状结构,物理剥离各层后分别进行了元素含量测定和矿物相表征,以及腐蚀层的形貌与元素能谱分析。结果表明:腐蚀层主要为多种铁的硫化物与氧化物和PbS,还含有少量As、Ge、Ga、Se和Zn等元素;水冷壁管沉积物内层和中层富集Fe、S、Pb和Zn元素,主要以FeS、FeS2、 PbS,ZnS和ZnAl1.04S2.13形式存在,但Pb仅在内层富集;外层以硅铝酸盐、铝酸盐为主,含有少量硫化物。结合热力学计算可知,沉积层中Pb和Zn元素的富集,主要源于烟煤燃烧中析出的气态含Pb和Zn组分同H_2S反应生成的,它们主要以冷凝和热泳沉积的方式到达壁面。沉积物中Zn和Pb富集量约是煤中Zn和Pb含量的3~4个数量级,可以用于评估炉内水冷壁管材硫化物的腐蚀程度。