Investigation of the mechanisms and control measures for fireside corrosion in eskom fossil fuel boilers

Moloko, Kediemetse Gloria
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Fireside corrosion in fossil-fired boilers is still amongst the damage mechanisms that contribute toward the most boiler tube failures. It was first identified in South African power generation utilities in the early 1990s when a number of coal-fired subcritical boilers (600 MW power generation per Unit) experienced tube failures on the furnace walls. Molten alkali sulphate corrosion and sulphidation have been identified as possible mechanisms through several failure investigations, but not ascertained. This study aims to close this gap by conducting an in- depth investigation into the mechanism underwhich fireside corrosion occurs on the furnace wall tubes, and the conditions of the furnace that enable this type of attack in Eskom coal fired 600 MW subcritical boilers. This research describes the process followed in sampling the furnace wall tubes from three coal-fired power stations (A, B and C) subjected to fireside corrosion, as well as outlining techniques employed in analyzing the fireside deposits and the findings thereof. SEM-EDS analysis revealed high concentrations of oxygen, iron and sulphur. QEMSCAN and XRD revealed coexistence of Fe-oxide and Fe-sulphide in a form of Fe3O4 and Fe2O3, FeS, and FeS2. DTA showed thermal activities at temperatures of 500-600°C, 900-1100°C and 1100-1250°C which are associated with FeS2 oxidation into FeS and Fe2O3, which occurs between 475°C and 525°C, formation of aluminosilicates such as mullite at 925°C to 1100°C and the melting of FeS around 1190°C. The coexistence of FeS and iron oxide in the fireside deposit of the tubing is indicative of the substoichiometric conditions in the furnace and on the surface of the boiler tubes. The detection of pyrite in the fireside deposit suggests that the pyrite is not completely burnt which points to poor combustion process. This was confirmed by the gas analysis of Unit 1 of station A where very high levels of CO were measured at the furnace wall (≥ 14000 ppm) and furnace exit (≥ 3500 ppm). The high CO concentrations are indicative of limited combustion caused by limited O2. These reducing conditions promote formation of FeS rich deposits, which are evident on all analysed tubes. The absence of primary elements that form alkali sulphates, viz. sodium and potassium, eliminates the involvement of molten alkali sulphates but affirms that the type of fireside corrosion taking place on furnace walls from power stations A, B and C is by sulphidation due to iron-sulphide-rich deposition. The mitigation of sulphidation fireside corrosion can be achieved through better control of the combustion process. In addition, thermal spray coatings and weld overlays have proven to protect the tubes from fireside corrosion for a specific period of time
Submitted to School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa October 2019
Moloko, Kediemetse Gloria (2019) Investigation of the mechanisms and control measures for fireside corrosion in Eskom Fossil Fuel Boilers, University of the Witwatersrand, Johannesburg, <>