Abstract

MOLOKO, K.G.  and  VAN DER MERWE, J.W.. Investigation of the mechanism for fireside corrosion in coal-fired boilers in South Africa. J. S. Afr. Inst. Min. Metall. [online]. 2021, vol.121, n.6, pp.305-316. ISSN 2411-9717.  http://dx.doi.org/10.17159/2411-9717/951/2021.

Furnace wall tubes from 600 MW subcritical boilers at three coal-fired power stations were sampled and the fireside deposits examined to determine the mechanism of fireside corrosion. This involved an in-depth investigation into the morphology and composition of the fireside deposits and the conditions of the furnace that enable this type of attack. SEM-EDS analysis revealed high concentrations of oxygen, iron, and sulphur, QEMSCAN and XRD analyses identified the presence of Fe₃O₄, Fe₂O₃, FeS, and FeS₂. Differential thermal analysis showed thermal activities at temperatures of 500-600°C, 900-1100°C, and 1100-1250°C, which are associated, respectively, with FeS₂ oxidation to FeS and Fe₂O₃, at 475-525°C, formation of aluminosilicates at 925-1100°C, and melting of FeS around 1190°C. The absence of sodium and potassium eliminates the contribution of molten alkali sulphates to the corrosion. The consistent coexistence of iron sulphide and iron oxide is indicative of the substoichiometric conditions in the furnace, while the detection of pyrite suggests that the coal is not completely combusted, which points to a poor combustion process. These observations were affirmed by gas analysis at one of the stations, where very high levels of carbon monoxide were measured at the furnace wall (> 14 000 ppm) and furnace exit (> 3500 ppm). The high CO concentrations are indicative of limited combustion caused by limited O₂. These reducing conditions promote the formation of FeS-rich deposit, which is the corrosive species responsible for degradation.

Keywords : fireside corrosion; sulphidation; coal-fired boiler; furnace wall tubes.

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