Over 35 Years of Providing Metallurgical Analysis

Falling Slag Damage

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. 

Probable causes:
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.

Corrective actions:
Corrective actions depend on the severity of the erosion problem.  If erosion is severe, the reduction in boiler availability due to tube failures must be included in costs for burning a higher slagging fuel.  If a change in fuel is not justified, increasing tube wall thickness or installing wear bars can provide additional protection from failures. 

Slag erosion can occur at the lower furnace wall which directs ash into the bottom ash hopper. The sloping wall tubes within 3 to 4 feet of each sidewall near the bottom will experience the greatest erosion. Falling slag erosion produces flat surfaces from metal removal.  A longitudinal thin edged fracture results when the wall thickness can no longer restrain the internal pressure.

Determine the important factors that result in slagging problems.  Analysis so far indicates that silica percentage and ash softening temperature can be used for categorizing the slagging tendency of coal.  Visual examinations and ultrasonic (UT) tube wall thickness measurements are methods to detect and monitor falling ash erosion.  UT surveys should be conducted during boiler overhaul outages to determine the extent of erosion and to provide data for planning corrective actions.

Common Locations: 
Lower Slope Area