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In high ash coal fired boilers, fly ash erosion is a major concern and the tube failures due to fly ash erosion are almost 35% of the total tube failures. The amount of ash in coal and its velocity are major factors in the rate of pressure part erosion. Fly ash erosion is experienced in the economizer, primary SH, and inlet section of steam reheater tubes. When non-uniform flue gas flow distribution occur in these areas, the rate of erosion increases multifold.
Factors influencing fly ash erosion in coal fired boilers are
- The velocity of flue gas
- The temperature of flue gas
- The mineral content in coal
- The change in direction of flue gas
- The arrangement of pressure parts and
- The operation above the maximum conditions design rating or with excess airflow above design rate.
Of these factors, the velocity of flue gas, the temperature of flue gas (ash), and mineral matter in coal are the main influencing factors.
The velocity of flue gas
For low ash coals, the weight loss in pressure parts due to erosion is proportional to flue gas velocity to the power of 1.99. However for high ash Gondwana coals the erosion rate is velocity to the power of 3 to 5. The power depends upon the percentage of ash in coal, the percentage of silica in coal ash, the percentage of quartz in this silica, the percentage of alpha quartz in this quartz, and the structure of alpha quartz.
Temperature of flue gas
Higher temperature softens the minerals in the ash as well as reduces the strength properties of the material of pressure parts; due to this ash erosion is not predominant in high temperature zones like furnaces, final superheaters, exit reheaters, etc. The ash erosion mainly starts in the conventional two-pass boilers from the area where gas temperature is around 700 – 750 deg.C. The low temperature superheater (LTSH) and economizer are the areas where ash erosion is severe in a conventional two-pass boiler. The temperature of flue gas entry to LTSH can be around 650 to 700 degree C and leaving, the economizer can be around 350 – 300 degree C. The minerals, which mainly constitute the ash in flue gas at these temperatures, become hard and attain its full abrasiveness.
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Mineral matter in coal
The proportion and composition of the mineral matter in coal will determine the extent of fly ash erosion that can take place. All the mineral matter undergoes phase transformation during the process of combustion of coal in furnace. The phase transformation of the mineral matter is dependent on various factors like the presence of oxygen (oxidizing or reducing atmosphere) in the localized area of furnace, the temperature of the flame / furnace, the retention time, the composition of the minerals in question, etc.
Measures to Reduce Flyash Erosion
The following are the areas in boiler where coal ash erosion is normally experienced.
(i) economizer bends and tubes
(ii) LTSH bends and tubes
(iii) screen tubes
(iv) goose neck portion at furnace top
(v) soot blower openings in the water walls
(vi) wind box opening in the furnace
(vii) bottom hopper tubes
In the case of (i), (ii) and (iii) the erosion is due to ash in the flue gas stream directly impact and flow over the tube. In the case of (iv) and (vi) it is more due to ash collected in this region sliding over the tubes. In the bottom hopper impact of the water wall deposit is predominant. In the case of (v) and (vi) it is more due to entrained ash / fuel causing erosion due to eddies formed in this area.
To reduce the erosion in these areas
(a) Reduced gas velocity in second pass
(b) Use Inline arrangement for all second pass heat transfer surface
(c) Provide shield in places prone for higher erosion
(d) Provide cassette baffles for LTSH and economizer bends
(e) Go for refractory lining in areas of high erosion where shields cannot be provided.
As low grade coals are now emerging to be used in large quantity in boilers for power generation and process steam requirement, it has become necessary to protect the pressure parts from ash erosion. It can be said with confidence that in the case of high ash coals, erosion cannot be avoided; it can be only minimized to an optimum level. However data show that the boiler pressure parts in the second pass like LTSH and Economizer may need replacement in full from about 10-15 years of operation depending upon the nature of the ash, the type of operating regime maintained, etc.
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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.
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