The problem of UDF in Non premixed combustion model
 

Hi everyone.
I am using Non pre-mixed combustion model to simulate the gasification of the coal water slurry. Only the reaction of char and oxider is included in this model. Other particle surface reaction such as the reaction of char and water, the reaction of char and carbon dioxide, are not included. But there is so much water and carbon dioxide in the gasifier that the two reaction can not be neglected. So I need to make a UDF to simulate the particle surface reaction ,but I do not know how to do it? Can anyone help me? Thank you!