Study of Alkali Metal Corrosion on Heating Surfaces and Bed Material Agglomerate in Biomass-fired Fluidized Bed Boiler

摘要

The bed material agglomeration and heating surface high-temperature Corrosion Problems of biomass-fired boiler in South China were studied in this work. The inner and outer surfaces of the corrosion sample were investigated by scanning electron microscope (SEM) with Bruker EDX and XRD. Results showed that the outer side of the corrosion sample was mainly composed of alkali chloride deposited ash, sulphide and a small amount of eutectoid;while the inner side of the corrosion sample was still mainly made up of the composition of SUS316, but added with alkali metal, oxygen, chlorine and sulphur elements, appearing as the corrosion products and eutectoid. It was thought that alkali chloride deposit and the reaction with pipe metal to generate low melting point eutectoid on the outer surfaces, or the corrosion reaction through the alkali metal sulphatization process was the main reasons leading to the damage of metal surface oxide film. Chlorine plays a role as haptoreaction in the corrosion process, and transports metal material as the form of chloride from the inner side to the outer side of the pipe surfaces by diffusion, accelerating the corrosion process. Meanwhile, the slag was studied by scanning electron microscope (SEM) with Bruker EDX, and the transformation process of slage was computationally analyzed by FACTSAGE. Results showed that the amount of alkali metal in the agglomerates was little, however, caused a great impact on severe agglomerates. The increase of temperature enhanced the conversion process of alkali metal to molten oxide, especially when the temperature was higher than760℃, the amount of molten product increased sharply. Thus, the temperature control of fluidized bed plays an important role in solving the problem of alkali metal agglomerates;it also reliefs the volatile of alkali metal into gas phase, benefiting the control of heating surface corrosion.