udf for partical surface reaction
 

I'm trying to use the Eulerian granular model to simulate the combustion of solid carbon and I've just started with a simple case and found that the rate of surface reaction needs to be defined using udf and the reaction is c(s) + o2 > co2. but I don't know how to write a udf so I appreciate anyone who can help me in doing so.

many thanks and your corporation is highly appreciated.