what is the difference between timestep and interval?
 

I am doing a coal combustion project,but it is hard to ensure convergence ,the cfx help tells that if you are running High Res and are having convergence difficulty, try reducing your timestep.I don't know what does the timestep mean.
I hope somebody can tell me .Thank you.

You have to reduce your timescale in the solver options. If it is set to automatic you have to change it to physical timescale and then make it smaller.

am also doing same kind of project.first try your simulation with ideal gas(with out coal combustion or heat transfer) and check the convergence first.if it is converging well then there is no problem with the mesh and try to do the simulation step by step and not whole things in single simulation.