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Coal is the prime fuel for power generation across the globe. More than fifty percent of electricity is produced by coal. Coal has in it organic material, an inorganic portion, and moisture. There are other components like sulphur, trace metals like arsenic, etc. which all form a very small percentage. The inorganic portion in coal varies highly making it difficult to predict any standard behavior. As coal formed under the earth is a heterogeneous material, it makes it still difficult.
Coal burners, both in wall fired boilers and in tangential fired boilers, are subjected to wear during their operation. The rate of wear on the burner depends on many factors.
- Percentage of ash (Inorganic portion): As the percentage of ash in coal increases, the wear increases, but the rate of wear need not be the same for same percentage of ash coals.
- The chemical composition of the ash: Knowing the constituents which make up the ash in coal facilitates our understanding of the reason for the difference in rate of wear of the burner using the same percentage of ash in coal. It is seen that the silica in coal plays a major role in erosion of coal burners.
- Velocity adopted for fuel air mixture: It is seen that the velocity adopted has an influence on the wear rate on the coal burner. The higher the velocity, the higher the rate of wear, but this cannot be varied much as it depends on the burner requirement, the flame front anchoring point, etc. The effect of velocity is more predominant in the case of pressure parts erosion in the boiler.
- The pulverized coal fineness: Coal fineness has an influence on the rate of wear depending upon the segregation of ash from organic portion of the coal
- The number of hours operated
- Material used for burner construction: As burners are subjected to heat radiation from burning fuel, they are made of very high alloy steels. The more often preferred material is stainless steel. However to contain wear and improve life a lot of methods like weld overlay, cladding, or metal spraying are adopted.
- The flame profile and distance from burner tip: The flame intensity and flame distance from the burner tip has two influences on the wear of the burner. One, it leads to warping of the burner tip and second, it creates eddies which speeds up wear.
- The load carried by the burner, which is basically the quantity of coal flow through the burner
Many of the factors like percentage ash, chemical composition, the number of hours of operation, etc. cannot be tailored to contain the wear of the burner components. The designers adopt mainly weld overlay or cladding or metal spraying to improve life of the coal burner. The method chosen will depend on the ash percentage and composition of coal being used in the boiler, the cost, and the life enhancement period.
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About the Author
Dr V T Sathyanathan is a boiler consultant with 35 years of experience in various areas of high pressure boiler trouble shooting. He holds a PhD in coal combustion in boilers.
Coal is one of the main fuels for power production. Coal quality deterioration over the years has created challenges for boiler designers the world over to compact and minimize erosion in pressure parts. Fly ash erosion is a major factor for pressure parts damage in high ash coal fired boilers.
Coal Formation Theories - Coal may be defined as a compact stratified mass of plant debris which has been modified chemically and physically by natural agencies, interspersed with smaller amounts of inorganic matter. In situ and Drift are the two major theories of coal formation.
Coal blending in power stations is mainly adopted to reduce the cost of generation and increase the availability of coal. The low-grade coals can be mixed with better grade coal without deterioration in thermal performance of the boiler, thus reducing the cost of generation.