大型生物质直燃锅炉高温受热面腐蚀问题分析

Analysis of High-temperature Heating Surface Corrosion Problems in Large-scale Biomass-fired Boiler

随着人们对全球变暖问题的关注,生物质能源利用引起了广泛的重视。我国南方地区用于发电的生物质主要来源于甘蔗渣、桉树叶、桉树枝等农林废弃物。此类生物质燃料都含有大量的碱金属,尤其是钾,存在形态及其热化学利用过程中的迁移、转化直接影响到受热面的积灰,床料的结渣及烟道管壁的腐蚀等多种问题。某大型生物质直燃锅炉在运行3个月后发现屏式过热器定位管发生严重腐蚀问题,腐蚀外观表现外壁氧化膜破裂剥落,腐蚀沿着晶界向内延伸。通过对管材的金相分析及对管外垢样进行能谱分析发现,腐蚀产物中较多的Cl-离子、S2-离子和碱金属是造成过热器管壁严重腐蚀的重要原因。其中,沿海生物质高氯含量导致燃烧气相中产生氯气及含氯化物,与管壁接触发生复杂物理化学变化,加速金属合金的氧化是高温腐蚀的主要因素。

With the concerns over energy shortage and CO2emission,biomass is now being considered as an inexhaustible and clean energy resource all over the world.The biomass material for power generation inSouth China mainly comes from agricultural and forestry wastes,such as sugar cane bagasse,eucalyptus leaves,eucalyptus branches,etc.This kind of biomass fuels contain inherent inorganic elements,including sulfur and chlorine,as well as minor quantities of Si,Al,Ca,Fe,Mg and P,especially contain high content of potassium element.The corrosion problem in the high temperature heating surface of a large scale biomass-fired boiler in South China was investigated in the work.The super-heater positioning tube had been corroded seriously after running just for three months.By observation,the corroded surface was characterized by the rupture and peeling of the oxide film of the outer wall.And the corrosion extends inward along the grain boundary by metallographic analysis.The morphology and elements existing in the corrosion sample was studied by scanning electron microscope(SEM),combined with Bruker EDX Microanalysis. Results show that a high content of Cl,S and Alkali metal exists in the corrosion products. It was considered that under high temperature combustion condition,alkali metal-related compounds might release to the gas phase,then react with the metal oxide film and even emit Cl2 which can penetrate the oxide film and react with the alloy,causing more serious corrosion.The high chlorine and alkali content of biomass coming from the coastal was thought to be the main reasons caused high temperature corrosion of super-heater tube wall.