Erosion is caused when material in components is removed due to the collision of particles and gases at high velocity.  Erosion due to fly ash can damage the boiler tubes. 

In boilers, erosion can be caused by coal as well as by fly ash.  When fly ash and coal particles hit piping at high velocities, material is removed. 

Erosion can be minimized by choosing coal with lower fly ash content.  Reducing the velocity of the gases by effective flow modification techniques such as the use of baffles, perforated plates and metallic screens can also help reduce erosion. 

Coal Erosion
Coal erosion is caused when unburnt particles of coal hit the boiler tubes.  This can be controlled by reducing the velocity of the coal air mixture and designing the direction such that complete combustion occurs before the particle hits the tubes.

http://www.powermag.com/fly-ash-erosion-control-and-prevention/