Abstract
SYNOPSIS 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.
Highlights
Fireside corrosion is the metal loss from tubes due to chemical attack on the fireside of heat exchanging surfaces in fossil fuel-fired furnaces
It is due to high temperatures and the unique environment encountered in the furnaces, which is caused by the complex interaction of the combustion gases at temperatures varying between 400 and 1200°C (Syed, Simms, and Oakey, 2011), the distribution of pulverized fuel, elements released from the fuel, the type and nature of the ash carried over in the gas stream, and the materials of construction of the boiler
Sampling During plant shut-downs at three power stations (A, B, and C), tubing samples were removed from the areas that are historically known to suffer from fireside corrosion
Summary
Fireside corrosion is the metal loss from tubes due to chemical attack on the fireside of heat exchanging surfaces in fossil fuel-fired furnaces. Fireside corrosion in boilers is commonly divided into two categories by location, namely furnace wall corrosion in the lower furnace and fuel ash corrosion of superheaters in the upper furnace This type of corrosion was first recognized as a serious problem in the early 1940s, when some of the power stations in the USA using pulverized coal-fired furnaces, experienced an increased number of furnace wall tube failures (Harb and Smith, 1990). Low excess air combustion and two-stage combustion are commonly applied as means of realizing low-NOx combustion (Bakker, 1998; Dooley and MacNaughton, 2007) These combustion methods can reduce NOx emissions from a boiler, a strong reducing atmosphere is formed in the region between the burners and air ports in the case of two-stage combustion.
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