Coking and slagging are two concepts, please do not confuse them. Next, let’s take a look at the specific reasons and solutions.
The material is burned in the furnace and when it achieved a high temperature, the materials forms a slag-like object. Most of the slag is glass, also known as silica.
There are two reasons for pellets slagging, one is the materials have many impurities and high sand content, and another one is incomplete combustion. For example, tree bark has a relatively high sand content, so it is easy slagging.