Fly Ash Erosion In Boilers

Fly ash erosion and erosion due to unburned coal particles are two modes of erosion failure that belong to the category of solid particle erosion.

Erosion due to solid particles is a serious issue for thermal power plants because it results in forced outages, loss of efficiency, and additional expenditure due to unwarranted repairs.

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Solid Particle Erosion

Solid particle erosion results in localized or general loss of material from the tube surface due to the mechanical impact of fly ash or coal particles. Fly ash erosion is a matter of concern especially when dealing with low grade coal with high ash content. Fly ash erosion is mainly observed in primary superheater and low-temperature superheater bends and tubes, screen tubes, the inlet section of steam reheater tubes, and areas of economizer bends and tubes. The extent of damage due to fly ash erosion multiplies if there is a nonuniform flow of flue gases in such areas. The impact by particles of fly ash in the flue gases causes abrasive erosion of screen tubes or superheater tubes.

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Typical features of fly ash erosion include polishing of the impacted surface, formation of flat spots, tube wall thinning, and tube rupture. Occasionally irregular flow marks and grooving are also observed. Coal used in coal-based thermal power plants mostly comprises combustible and noncombustible mineral constituents, normally known as ash.

Critical Factors

Hard quartz (SiO2) besides alumina (Al2O3), alkali feldspars, (K, Na) AlSi3O8, and traces of a garnet minerals, all of these constituents of ash contribute to erosion of the boiler tubes when the fly ash particles present in the flue gases impact the tube surface.

The extent of damage due to fly ash erosion depends on variables including:

• Velocity of flue gas or fly ash particles,

• Their angle of impact with respect to the tube surface,

• Shape and size of the particles,

• Hardness and abrasion resistance of the tube surface,

• Flue gas temperature.

Of all the variables, fly ash particle velocity is the most important. There is an exponential relation between the rate of erosion and the particle velocity. It is worth noting here that the particle velocity is governed by the local flow velocity of the flue gases at any particular location and not by the bulk velocity of the flue gases. The other two most important variables are the temperature of the flue gas and the mineral content of the coal. When high ash coals are used, it is not possible to combat fly ash erosion completely; rather, it can only be minimized.

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