Heating of free-flowing particles by contact with the wall of a rotary drum without inserts has been simulated in two dimensions by means of the thermal version of the Discrete Element Method (DEM). The results are shown to be in qualitative agreement with existing experimental data and previous analysis by the penetration model for the limiting cases of heat transfer controlled by a contact resistance at the wall of the drum, or by the penetration of heat in the mechanically agitated bed. The ability of thermal DEM to assess influences that are not accessible to the penetration model, like temperature distributions, is discussed.