Changing fuels and operating conditions such as retrofitting low-NOx burners can cause the formation of clinkers in the high-temperature regions of the boiler. The addition of over-fire air registers (OFAs) and secondary over-fire air registers (SOFAs) can promote flame impingement and secondary combustion issues, resulting in localized overheating issues and higher furnace exit-gas temperature (FEGT). The rise in FEGT may lead to several issues such as plugging of vertical pendants with ash deposits. These deposits can detach from the heating surfaces and cause significant damage in the lower slope area. 

External Appearance:

Tubes impacted by falling slag can appear dented, gouged, cracked, and polished. 

Collection of refuse on the furnace walls and the pendant Superheater (slagging) ultimately falls free from weight/vibration resulting in falling slag erosion or impact damage.  Coal properties and boiler design are important factors when evaluating slagging problems.  The root cause of falling slag erosion can be verified by evaluating the slagging potential of the fuel. Various coal ash properties are used in the evaluation, but ASTM Standard D 1857 is the best representation.