Application of Coatings to Alleviate Fireside Corrosion on Heat Transfer Tubes during the Combustion of Low-Grade Solid Fuels: A Review

摘要

To address the energy and environmental issues related to the use of traditional fossil fuels, the thermal utilization of low-grade solid fuels such as low-rank coal, biomass, and municipal solid waste has gradually attracted much attention. During the combustion of the low-grade solid fuels, one of the major operation problems is the fireside corrosion on heat transfer tubes, which easily causes the frequent replacement of tubes and shutdown of boilers, largely reducing the energy and economic performances. In this review, the causes and mechanisms of corrosion were first clarified to provide a basis for mitigating the losses induced by the fireside corrosion, followed by the introduction of the preparation and properties of coatings, which show great potential to resist the invasion of corrosive species by attaching the reinforced surface to the tubes. To demonstrate the feasibility of the coating for high-temperature corrosion resistance, 94 kinds of coatings in more than 60 literature contributions were statistically analyzed. It was found that over 90% of anticorrosion coatings were prepared from thermal spraying techniques, and approximately three-quarters of the anticorrosion coatings were made of NiCr-based alloys. Many methods were reported to evaluate the corrosion resistance of coatings in literature, including mass gain/loss method, ash deposition method, contact angle method, and ash adhesion method. As for exploring the anticorrosion mechanisms of coatings, it was found that the protection was achieved by the formation of dense oxide scale, such as Cr2O3, NiO, MoO2, and so on. Furthermore, the application status of coatings in power plants was also summarized. According to the pilot-scale tests listed in literature, the obvious similarity is that the adopted technologies were mainly High velocity oxy-fuel (HVOF) spraying or cold spraying techniques, with almost all of the coated materials being NiCr-based alloys as well as a small portion of Fe/Co-based alloys. This indicates that the NiCr-based coatings prepared by HVOF spraying technique have been widely applied in power plants. In order to promote the large-scale application of anticorrosion coatings, it is prospective to focus on several key research aspects: (1) predicting the corrosion conditions from fuel compositions, (2) developing cost-effective coatings, (3) establishing the systematic evaluation methods by laboratory tests, (4) achieving in-depth understanding of the anticorrosion mechanisms, and (5) conducting a comprehensive economic analysis.