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Fireside Furnace Corrosion
Fireside corrosion remains a leading cause of failure in coal fired units. Fireside corrosion occurs via two modes. In the first mode of corrosion, tube wastage occurs by the formation of low melting species which dissolve the protective iron oxides on carbon or low alloy (Cr-Mo) steels. In the second mode, the tube wastage occurs when there are reducing conditions inside the furnace, and iron sulfide scales form instead of protective oxides. These iron sulfide scales are more porous and less protective than oxides. Reducing conditions may also promote carburization of Cr-Mo, ferritic and stainless steels, resulting in a loss of corrosion resistance. Unburnt carbon and carbon monoxide carry carbon to the steel surface under reducing conditions. Furnace wall tubes are also subject to fireside corrosion, but the low melting species differ from superheater/reheaters. Sodium and potassium pyrosulfate (Na2S2O7 or K2S2O7) have been responsible for furnace wall corrosion. Both of these species melt below 800oF, where the furnace wall tubes operate. The melting points of Na2S2O7 and K2S2O7 are 7500F and 570oF, respectively. Mixtures of these two compounds could melt at even lower temperatures. Melting points as low as 635oF-770oF have been measured.
Fireside corrosion produces wall thinning that eventually results in longitudinal, ductile rupture. Wall thinning is uniform across several tubes in a particular location. Circumferential grooving or cracking with deep finger-like penetrations into the tube wall has been found in super-critical boilers. Hard, dark ash deposits are usually found on the external surface.
Fire-side corrosion is caused by corrosive conditions in the combustion zone, which are due to inadequate oxygen supply, high concentration of sulfur and increased chlorides in the fuel, improper alignment of the fuel burners, and formation of molten ash on the tube surface.
Ultrasonic (UT) wall thickness measurements are taken to establish the corrosion rate and extent. Removal of the ash deposit and iron oxide scale by sandblasting or grinding is necessary In order to obtain accurate UT thickness data. The data can be input into computerized data analysis programs for plotting the location of the thinning, for trending periodic measurements, and for calculating the remaining service life of the water wall tubing.
Corrective actions depend on operating conditions, adjusting burner alignment, increasing coal fineness, spraying thermal corrosion-resistant coatings, and bleeding air into the sidewall area. Long-term actions include furnace modification to improve combustion conditions or installation of co-extruded tubing or other surface modified tubing to provide a corrosion-resistant material on the outside of the tube. An estimate of the remaining life of the tube should be performed based on the corrosion rate and the level of hoop stress.
Fireside corrosion can occur at locations that have:
(1) Incomplete combustion conditions and a reducing atmosphere at the water wall.
(2) Corrosive ash deposits.
(3) Flame impingement.
Visual observation through inspection ports may show a defect in combustion such as direct flame impingement or carbon particle impingement. Field testing may be required to determine the carbon monoxide level near the water wall and the amount of unburned carbon in the ash.