What are the causes of coal erosion in boiler

The causes of coal erosion as distinct from all the other types of erosion are many but from a theoretical point of view are simply high velocity particles impacting and rubbing along the surface of the tubes.
The boiler designer minimizes this by providing a volume in the furnace and a direction of travel of the coal such that it is burned before it can touch the tubes. This can be defeated by increasing the velocity reducing the combustibility or increasing the mass flow. All of these parameters occur if you reduce the calorific value of the fuel or overload the boiler. If you had no erosion before changing your fuel that is the cause. If you have never had design fuel you don't know if it would have eroded anyway. If it would the cause will be a different reason such as aerodynamic flows and aiming of the burner or size of the target fireball center. This is a serious problem and should be dealt with by an experienced expert.