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.