Does it make sense to model combustion of coal using a "fluid" approach just like combustion of methane into air, or is it better to use a DPM approach to model the coal particles?

I've noticed the Tutorial No. 13 "Modelling Species Transport and Gaseous Combustion", and I wondered if I could use coal instead of Methane leaving anything else unchanged?

It depends on what you want to evaluate. Quite often we are modeling a biomass boiler that has some supplemental pulverized coal, oil, or gas firing. We are interested in how the biomass burns, but can't ignore the coal burners from a heat input, oxygen demand, and fluid flow point of view. In these cases, we have modeled the coal as an equivalent amount of methane.

Certainly if you wanted to look at something like NOx emissions from the coal you couldn't use this approach. Perhaps you could try modeling the coal both ways, and let us know how it turns out.